Understanding the Growth Mechanism of α‑Fe₂O₃ Nanoparticles through a Controlled Shape Transformation

The growth mechanism of α-Fe₂O₃ nanoparticles in solution has been elucidated from a comprehensive analysis on the shape and morphology of obtained particles. It is found that the hydrothermal synthesis of α-Fe₂O₃ nanoparticles from ferric chloride precursor follows two stages: the initial nucleation of α-Fe₂O₃ nuclei and the subsequent ripening of nuclei into various shapes. The initial nucleation involves the formation of polynuclears from hydrolysis of Fe³⁺ salt precursors, followed by the growth of β-FeOOH nanowires with an akaganeite structure, and then into two-line ferrihydrite nanoparticles through a dissolution–recrystallization process. In the subsequent ripening process, we suggest that the formation of large α-Fe₂O₃ particles follows the dissolution of two-line ferrihydrite and then precipitation and oriented aggregation of α-Fe₂O₃ nuclei rather than the oriented aggregation of ferrihydrite nanoparticles followed by phase transformation. The oriented attachment of {104} facets between α-Fe₂O₃ nuclei results in the formation of oblate spheroid nanocrystals (nanoflower-like particles) either in ethanol or in the beginning stage where the particles first undergo oriented aggregation. With the addition of water, Ostwald ripening process (dissolution–reprecipitation) will play an important role to convert the assembly of nanoflowers into a 3D rhombohedral shape with well-defined edges and surfaces. The proposed mechanism in this article not only allows us to better control the synthesis of iron oxide particles with designed shapes and structures but also provides guidance for theoretical simulations on the oriented attachment process for hematite formation

Understanding the Growth Mechanism of α‑Fe₂O₃ Nanoparticles through a Controlled Shape Transformation

The growth mechanism of α-Fe₂O₃ nanoparticles in solution has been elucidated from a comprehensive analysis on the shape and morphology of obtained particles. It is found that the hydrothermal synthesis of α-Fe₂O₃ nanoparticles from ferric chloride precursor follows two stages: the initial nucleation of α-Fe₂O₃ nuclei and the subsequent ripening of nuclei into various shapes. The initial nucleation involves the formation of polynuclears from hydrolysis of Fe³⁺ salt precursors, followed by the growth of β-FeOOH nanowires with an akaganeite structure, and then into two-line ferrihydrite nanoparticles through a dissolution–recrystallization process. In the subsequent ripening process, we suggest that the formation of large α-Fe₂O₃ particles follows the dissolution of two-line ferrihydrite and then precipitation and oriented aggregation of α-Fe₂O₃ nuclei rather than the oriented aggregation of ferrihydrite nanoparticles followed by phase transformation. The oriented attachment of {104} facets between α-Fe₂O₃ nuclei results in the formation of oblate spheroid nanocrystals (nanoflower-like particles) either in ethanol or in the beginning stage where the particles first undergo oriented aggregation. With the addition of water, Ostwald ripening process (dissolution–reprecipitation) will play an important role to convert the assembly of nanoflowers into a 3D rhombohedral shape with well-defined edges and surfaces. The proposed mechanism in this article not only allows us to better control the synthesis of iron oxide particles with designed shapes and structures but also provides guidance for theoretical simulations on the oriented attachment process for hematite formation

Experimental Evidence for Self-Assembly of CeO₂ Particles in Solution: Formation of Single-Crystalline Porous CeO₂ Nanocrystals

Single-crystalline porous CeO₂ nanocrystals, with sizes of ∼20 nm and pore diameters of 1–2 nm, were synthesized successfully using a hydrothermal method. Using electron tomography, we imaged the three-dimensional structure of the pores in the nanocrystals and found that the oriented aggregation of small CeO₂ nanoparticles resulted in the growth of CeO₂ nanocrystals with an irregular truncated octahedral shape and pores extending along the ⟨110⟩ directions. Oxygen vacancies were found on the crystal surfaces and internal walls of the pores by scanning transmission electron microscopy and electron energy-loss spectroscopy. The oxygen vacancies might play an important role in oxygen diffusion in the crystals and the catalytic activities of single-crystalline porous CeO₂ structures

Experimental Evidence for Self-Assembly of CeO₂ Particles in Solution: Formation of Single-Crystalline Porous CeO₂ Nanocrystals

Single-crystalline porous CeO₂ nanocrystals, with sizes of ∼20 nm and pore diameters of 1–2 nm, were synthesized successfully using a hydrothermal method. Using electron tomography, we imaged the three-dimensional structure of the pores in the nanocrystals and found that the oriented aggregation of small CeO₂ nanoparticles resulted in the growth of CeO₂ nanocrystals with an irregular truncated octahedral shape and pores extending along the ⟨110⟩ directions. Oxygen vacancies were found on the crystal surfaces and internal walls of the pores by scanning transmission electron microscopy and electron energy-loss spectroscopy. The oxygen vacancies might play an important role in oxygen diffusion in the crystals and the catalytic activities of single-crystalline porous CeO₂ structures

Hydrothermal Synthesis of Octadecahedral Hematite (α-Fe₂O₃) Nanoparticles: An Epitaxial Growth from Goethite (α-FeOOH)

Driven by the demand for shape-controlled synthesis of α-Fe₂O₃ nanostructures and the understanding of their growth mechanism and shape-dependent properties, we report the synthesis of octadecahedral α-Fe₂O₃ nanocrystals with a hexagonal bipyramid shape by introducing F^– anions in the solution. The hydrothermal growth process from hydrolysis of Fe³⁺ precursors involves three steps: the nucleation of akaganeite (β-FeOOH) nanorods, followed by the formation of goethite (α-FeOOH) crystals with acicular and twinned shapes, and a subsequent transformation into hematite (α-Fe₂O₃) nanoparticles. The phase transformation and growth of α-Fe₂O₃ particles from α-FeOOH follows dissolution of goethite and reprecipitation as hematite process. The initial nucleation of α-Fe₂O₃ particles was found to form epitaxially on goethite {001} surfaces due to a perfect lattice match between goethite {001} surface and hematite {001} planes. The structural relationship between goethite and hematite is G(020)//H(030) with G[100]//H[100]. The obtained α-Fe₂O₃ hexagonal bipyramid particles are enclosed by 12 {113} planes and six {104} facets. Since the twinned α-FeOOH particles are one of the typical shapes of intermediate goethite crystals, the nucleation of hematite particles on two twinned arms gives rise to the formation of twinned hematite particles. F^– anions play an important role in the formation of α-Fe₂O₃ particles with a hexagonal bipyramid shape because high concentration of F^– anions can stabilize the exposed {113} surfaces. The controlled synthesis of α-Fe₂O₃ nanoparticles with defined surfaces not only provides significant information on hematite surface structures and energies but also is critical to give the structure–property relationship for the application of hematite materials

NIR Schottky Photodetectors Based on Individual Single-Crystalline GeSe Nanosheet

We have synthesized high-quality, micrometer-sized, single-crystal GeSe nanosheets using vapor transport and deposition techniques. Photoresponse is investigated based on mechanically exfoliated GeSe nanosheet combined with Au contacts under a global laser irradiation scheme. The nonlinearship, asymmetric, and unsaturated characteristics of the <i>I</i>–<i>V</i> curves reveal that two uneven back-to-back Schottky contacts are formed. First-principles calculations indicate that the occurrence of defects-induced in-gap defective states, which are responsible for the slow decay of the current in the OFF state and for the weak light intensity dependence of photocurrent. The Schottky photodetector exhibits a marked photoresponse to NIR light illumination (maximum photoconductive gain ∼5.3 × 10² % at 4 V) at a wavelength of 808 nm. The significant photoresponse and good responsitivity (∼3.5 A W^–1) suggests its potential applications as photodetectors

Inverse Stellation of CuAu-ZnO Multimetallic-Semiconductor Nanostartube for Plasmon-Enhanced Photocatalysis

One-dimensional (1D) metallic nanocrystals constitute an important class of plasmonic materials for localization of light into subwavelength dimensions. Coupled with their intrinsic conductive properties and extended optical paths for light absorption, metallic nanowires are prevalent in light-harnessing applications. However, the transverse surface plasmon resonance (SPR) mode of traditional multiply twinned nanowires often suffers from weaker electric field enhancement due to its low degree of morphological curvature in comparison to other complex anisotropic nanocrystals. Herein, simultaneous anisotropic stellation and excavation of multiply twinned nanowires are demonstrated through a site-selective galvanic reaction for a pronounced manipulation of light–matter interaction. The introduction of longitudinal extrusions and cavitation along the nanowires leads to a significant enhancement in plasmon field with reduced quenching of localized surface plasmon resonance (LSPR). The as-synthesized multimetallic nanostartubes serve as a panchromatic plasmonic framework for incorporation of photocatalytic materials for plasmon-assisted solar fuel production

Fluorescence Concentric Triangles: A Case of Chemical Heterogeneity in WS₂ Atomic Monolayer

We report a novel optical property in WS₂ monolayer. The monolayer naturally exhibits beautiful in-plane periodical and lateral homojunctions by way of alternate dark and bright band in the fluorescence images of these monolayers. The interface between different fluorescence species within the sample is distinct and sharp. This gives rise to intriguing concentric triangular fluorescence patterns in the monolayer. The novel optical property of this special WS₂ monolayer is facilitated by chemical heterogeneity. The photoluminescence of the bright band is dominated by emissions from trion and biexciton while the emission from defect-bound exciton dominates the photoluminescence at the dark band. The discovery of such concentric fluorescence patterns represents a potentially new form of optoelectronic or photonic functionality

Reduced Graphene Oxide Conjugated Cu₂O Nanowire Mesocrystals for High-Performance NO₂ Gas Sensor

Reduced graphene oxide (rGO)-conjugated Cu₂O nanowire mesocrystals were formed by nonclassical crystallization in the presence of GO and <i>o</i>-anisidine under hydrothermal conditions. The resultant mesocrystals are comprised of highly anisotropic nanowires as building blocks and possess a distinct octahedral morphology with eight {111} equivalent crystal faces. The mechanisms underlying the sequential formation of the mesocrystals are as follows: first, GO-promoted agglomeration of amorphous spherical Cu₂O nanoparticles at the initial stage, leading to the transition of growth mechanism from conventional ion-by-ion growth to particle-mediated crystallization; second, the evolution of the amorphous microspheres into hierarchical structure, and finally to nanowire mesocrystals through mesoscale transformation, where Ostwald ripening is responsible for the growth of the nanowire building blocks; third, large-scale self-organization of the mesocrystals and the reduction of GO (at high GO concentration) occur simultaneously, resulting in an integrated hybrid architecture where porous three-dimensional (3D) framework structures interspersed among two-dimensional (2D) rGO sheets. Interestingly, “super-mesocrystals” formed by 3D oriented attachment of mesocrystals are also formed judging from the voided Sierpinski polyhedrons observed. Furthermore, the interior nanowire architecture of these mesocrystals can be kinetically controlled by careful variation of growth conditions. Owing to high specific surface area and improved conductivity, the rGO-Cu₂O mesocrystals achieved a higher sensitivity toward NO₂ at room temperature, surpassing the performance of standalone systems of Cu₂O nanowires networks and rGO sheets. The unique characteristics of rGO-Cu₂O mesocrystal point to its promising applications in ultrasensitive environmental sensors

Direct Patterning of Zinc Sulfide on a Sub-10 Nanometer Scale <i>via</i> Electron Beam Lithography

Nanostructures of metal sulfides are conventionally prepared <i>via</i> chemical techniques and patterned using self-assembly. This poses a considerable amount of challenge when arbitrary shapes and sizes of nanostructures are desired to be placed at precise locations. Here, we describe an alternative approach of nanoscale patterning of zinc sulfide (ZnS) directly using a spin-coatable and electron beam sensitive zinc butylxanthate resist without the lift-off or etching step. Time-resolved electron beam damage studies using micro-Raman and micro-FTIR spectroscopies suggest that exposure to a beam of electrons leads to quick disappearance of xanthate moieties most likely <i>via</i> the Chugaev elimination, and further increase of electron dose results in the appearance of ZnS, thereby making the exposed resist insoluble in organic solvents. Formation of ZnS nanocrystals was confirmed by high-resolution transmission electron microscopy and selected area electron diffraction. This property was exploited for the fabrication of ZnS lines as small as 6 nm and also enabled patterning of 10 nm dots with pitches as close as 22 nm. The ZnS patterns fabricated by this technique showed defect-induced photoluminescence related to sub-band-gap optical transitions. This method offers an easy way to generate an ensemble of functional ZnS nanostructures that can be arbitrarily patterned and placed in a precise way. Such an approach may enable programmable design of functional chalcogenide nanostructures