Investigation of Relative Stability of Different Facets of Ag₂O Nanocrystals through Face-Selective Etching

Ag2O nanocubes, rhombicuboctahedra, octahedra, and extended hexapods were employed for the examination of the relative stability of different crystal planes to chemical etching through careful face-selective etching. Precise control of the amount of NH3 solution injected into a mixture of Ag2O nanocrystals and NaOH enables this face-selective etching. Ag(NH3)2+ formed from dissolved silver ions should drive the etching process while NaOH tunes the reaction equilibrium to control morphology of the etched nanocrystals. The order of facet stability in this reaction was found to be {111} > {110} > {100}. The {100} faces are most easily etched. By carefully adjusting the volume of NH3 solution introduced, novel Ag2O cubic nanoframes and rhombicuboctahedra with square depressions on all the {100} faces can be fabricated. The {111} facets contain significant terminal silver atoms, so hydroxide ions should interact strongly to maintain these surfaces. Hydroxide ions are less effective at adsorbing on the {100} faces with terminal oxygen atoms, so these faces are more susceptible to etching. ζ potential measurements support the argument of hydroxide ion adsorption. Interestingly, the {100} facets of Ag2O were found to be most stable in a weakly acidic HNO3 solution; octahedral nanocrystals can transform into particles consisted of {100} square terraces

Formation of Hexabranched GeO₂ Nanoparticles via a Reverse Micelle System

We report the first synthesis of GeO2 nanoparticles with six symmetrically arranged branches running along the long axis of each particle. The nanoparticles have a shape resembling that of a star fruit. A reverse micelle system with Triton X-100 serving as the capping surfactant for the aqueous phase and n-hexanol as the cosurfactant was adopted. Ge(OEt)4 was selected as the germanium source. Using the optimal synthesis procedure by reacting the mixture at room temperature for 3 h, hexabranched GeO2 particles with an average length of 185 nm were produced. The products have been examined by FE-SEM, TEM, X-ray diffraction, and FT-IR techniques. GeO2 nanoparticles with structually well-developed branches were gnereated only with solution pH values in the range of 0.9−1.1. At a low [H2O]/[Ge(OEt)4] molar ratio of 45, particles having a hexagonal bipyramidal shape but without branch formation were observed. Increasing this ratio to 90, branches begin to appear from the six side edges of the particles. By simply varying the reaction time, the sizes of the branched GeO2 nanoparticles can be adjusted from around 100 nm in length to as large as 300 nm in length

Formation of Indium Nitride Nanorods within Mesoporous Silica SBA-15

We report the first formation of arrays of InN nanorods inside the nanoscale channels of mesoporous silica SBA-15. In(NO3)3 dissolved in methanol was incorporated into SBA-15 powder without prior pore surface functionalization. Formation of InN nanorod arrays was carried out by ammonolysis at 700 °C for 8 h. The final products have been characterized by FT-IR spectra, 29Si MAS NMR spectra, Raman spectra, XRD patterns, TEM images, nitrogen adsorption–desorption isotherm measurements, and optical spectroscopy. The freestanding InN nanorods observed after silica framework removal with HF solution show diameters of 6–7.5 nm and lengths of 25–50 nm. Formation of a trace amount of In2O3 was also verified. The InN nanorods exhibit a broad band centered at around 550–600 nm, and a band gap energy of 1.5 eV was determined. No light absorption in the near-IR region was measured. The nanorods give a weak emission band centered at around 600 nm. These optical properties are believed to be related to the possible incorporation of oxygen during InN nanorod synthesis

Synthesis of Submicrometer-Sized Cu₂O Crystals with Morphological Evolution from Cubic to Hexapod Structures and Their Comparative Photocatalytic Activity

We report a facile method for the synthesis of cuprous oxide nanocrystals with systematic morphological evolution. Cubic, truncated cubic, cuboctahedral, truncated octahedral, octahedral, and short hexapod structures have been synthesized in an aqueous solution of CuCl2, NaOH, sodium dodecyl sulfate (SDS) surfactant, and hydroxylamine (NH2OH·HCl) reductant by simply varying the volume of hydroxylamine added to the reaction mixture. A slight modification in the volume of some reagents produced the extended hexapods. The order of the introduction of the reagents is important to the formation of these crystals with distinct morphologies and sharp faces. The sizes of these particles fall mostly in the range of 400−700 nm. Clear transition in the relative intensities of the (111) and the (200) reflection peaks in their XRD patterns was observed. Scattering bands dominate the UV−vis absorption spectra of these crystals. Crystal model analysis revealed that the {111} face contains surface copper atoms with dangling bonds, and is expected to interact more strongly with negatively charged molecules. Tests of photodegradation of negatively charged methyl orange showed that octahedra and the extended hexapods were catalytically active. The cubes with only the {100} faces were not active. On the contrary, both cubes and octahedra were not effective at photodecomposing positively charged methylene blue molecules. Surprisingly, octahedra and hexapods cannot be well suspended in the methylene blue solution; a significant amount of the crystals gradually moved to the surface of the solution with increasing stirring time. The results clearly demonstrate the dramatic differences in the catalytic activities of the {111} and {100} faces of Cu2O crystals for the first time

Formation of Hollow Gallium Nitride Spheres via Silica Sphere Templates

We report the formation of hollow GaN spheres using silica sphere templates. First, silica spheres with an average diameter of ∼130 nm were synthesized. A mixture of GaCl3, silica spheres, water, and urea in 2-propanol was prepared and heated to 100 °C for 24 h to generate silica spheres with γ-Ga2O3 nanoparticle shells. Decomposition of urea and its reaction with water slowly increased the solution pH to ∼8; this controlled reaction is the key to forming uniform γ-Ga2O3 shells with a thickness of about 13 nm. The amounts of urea and water have been varied to find the optimal conditions for the preparation of the oxide shells. Transfer of the colloidal particle solution to silicon substrates and ammonolysis at 850 °C for 6 h produced the SiO2−GaN core−shell nanostructures. Immersion of the silicon substrates in an HF solution removed the silica cores, and hollow GaN spheres with a shell thickness of around 8 nm were formed. The morphologies and crystal structures of the oxide and nitride shells have been carefully examined. The GaN shell materials show an absorption band at 350−360 nm and a broad defect-related emission band centered at around 570 nm

Fabrication of Truncated Rhombic Dodecahedral Cu₂O Nanocages and Nanoframes by Particle Aggregation and Acidic Etching

We report a simple approach for the fabrication of cuprous oxide (Cu2O) nanocages and nanoframes possessing an unusual truncated rhombic dodecahedral structure. An aqueous solution containing CuCl2, sodium dodecyl sulfate (SDS) surfactant, NH2OH·HCl reductant, HCl, and NaOH was prepared, with the reagents introduced in the order listed. Rapid seed-particle aggregation and surface reconstruction of the intermediate structure resulted in the growth of type-I nanoframes, which have only {110} skeleton faces and empty {100} faces, 45 min after mixing the reagents. Continued crystal growth for additional 75 min produced nanocages with filled {100} faces. The nanocages have diameters of 350−400 nm, and their walls are thicker than those of the nanoframes. Selective acidic etching over the {110} faces of the nanocages by HCl via the addition of ethanol followed by sonication of the solution led to the formation of type-II nanoframes, which have elliptical pores on the {110} faces. The morphologies of these nanoframes were carefully examined by electron microscopy. Without addition of ethanol, random etching of the nanocages can occur at a slow rate. Octahedral gold nanocrystals and high-aspect-ratio gold nanorods were successfully encapsulated in the interiors of these Cu2O nanocages by adding the gold nanostructures into the reaction solution. The formation process for such core−cage composite structures was studied. These composite materials should display interesting properties and functions

Facile Synthesis of Cu₂O Nanocrystals with Systematic Shape Evolution from Cubic to Octahedral Structures

We report a facile method for the synthesis of cuprous oxide nanocrystals with systematic shape evolution. Monodisperse truncated cubic, cuboctahedral, truncated octahedral, and octahedral nanocrystals can be synthesized directly in an aqueous solution of CuCl2, sodium dodecyl sulfate (SDS) surfactant, hydroxylamine (NH2OH·HCl) reductant, and NaOH by simply varying the volume of hydroxylamine added to the reaction mixture. SDS surfactant was found to be necessary for a precise control of the nanocrystal morphology. Adjustment of the volume of NaOH added provides a means to vary the particle size. In the case of octahedral nanocrystals, particles with sizes of 160−460 nm can be prepared. By examining the intermediate products formed, which resemble the final nanocrystal structures, a growth mechanism is proposed. Optical characterization of these Cu2O nanocrystals showed band gap absorption at 470−490 nm and strong light scattering bands extending from the visible to the near-infrared light region. All four samples displayed activity toward photodegradation of rhodamine B molecules, but truncated octahedral and octahedral nanocrystals exhibited a higher extent of the photodecomposition reaction, suggesting the {111} faces of Cu2O nanostructures are catalytically more active than the {100} faces. These nanocrystals should allow the examination of their various properties as a function of the particle shape

Synthesis of Branched Gold Nanocrystals by a Seeding Growth Approach

Synthesis of branched gold nanocrystals by a seeding growth approach is described. In this process, HAuCl4 aqueous solution was supplied stepwise to grow the gold seeds (∼2.5 nm) into larger nanoparticles with a highly faceted particle structure (∼15−20 nm in diameter). Sodium dodecyl sulfate (SDS) served as a capping agent to facilitate the formation of highly faceted nanoparticles, and ascorbic acid was used as a weak reducing agent. The highly faceted nanoparticles then transformed into branched nanocrystals (∼40 nm in length) by further addition of the SDS−HAuCl4 solution and ascorbic acid for particle growth. The branched nanocrystals show bipod, tripod, tetrapod, and pentapod structures and are composed of mainly (111) lattice planes. These multipods appear to grow along the twin boundaries of the initially formed highly faceted gold nanoparticles, as the twin boundaries on the pods originate from the centers of the branched nanocrystals. The concentration of ascorbate ions in the solution was found to have a profound influence on branch formation. These branched nanocrystals are stable to storage at low temperature (that is, 4 °C), but they may slowly evolve into a multitwinned faceted crystal structure (that is, pentagonal-shaped decahedral structure) when stored at 30 °C

High-Density Assembly of Gold Nanoparticles on Multiwalled Carbon Nanotubes Using 1-Pyrenemethylamine as Interlinker

In this article, we describe the formation of carbon nanotube (CNT)−gold nanoparticle composites in aqueous solution using 1-pyrenemethylamine (Py−CH2NH2) as the interlinker. The alkylamine substituent of 1-pyrenemethylamine binds to a gold nanoparticle, while the pyrene chromophore is noncovalently attached to the sidewall of a carbon nanotube via π−π stacking interaction. Using this strategy, gold nanoparticles with diameters of 2−4 nm can be densely assembled on the sidewalls of multiwalled carbon nanotubes. The formation of functionalized gold nanoparticles and CNT−Au nanoparticle composites was followed by UV−vis absorption and luminescence spectroscopy. After functionalization of gold nanoparticles with 1-pyrenemethylamine, the distinct absorption vibronic structure of the pyrene chromophore was greatly perturbed and its absorbance value was decreased. There was also a corresponding red shift of the surface plasmon resonance (SPR) absorption band of the gold nanoparticles after surface modification from 508 to 556 nm due to interparticle plasmon coupling. Further reduction of the pyrene chromophore absorbance was observed upon formation of the CNT−Au nanoparticle composites. The photoluminescence of 1-pyrenemethylamine was largely quenched after attaching to gold nanoparticles; formation of the CNT−Au nanoparticle composites further lowered its emission intensity. The pyrene fluoroprobe also sensed a relatively nonpolar environment after its attachment to the nanotube surface. The present approach to forming high-density deposition of gold nanoparticles on the surface of multiwalled carbon nanotubes can be extended to other molecules with similar structures such as N-(1-naphthyl)ethylenediamine and phenethylamine, demonstrating the generality of this strategy for making CNT−Au nanostructure composites

Hydrothermal Synthesis of Free-Floating Au₂S Nanoparticle Superstructures

We describe the formation of gold sulfide (Au2S) nanoparticle superstructures via hydrothermal synthesis approach at 175 °C using HAuCl4 and Na2S reagents and cetyltrimethylammonium bromide (CTAB) surfactant. Uniform 2−4 nm Au2S nanoparticles were found to assemble into a densely packed lamellar phase structure, and the resulting material displays a cross-sheet-like morphology. The sheets are 125−250 nm in length and can be suspended in solution. XRD, TEM, and EDS characterization of the nanoparticle samples confirmed the composition of the nanoparticles as Au2S. Some isolated faceted gold nanoparticles were also observed. XPS data also support the formation of Au2S nanoparticles. UV−vis absorption spectra showed only absorption features from the Au2S nanoparticles, and an indirect band gap value of 1.77 eV was obtained. Sufficiently high concentrations of Na2S and CTAB in the reaction mixture were concluded to be necessary to promote the growth of Au2S nanoparticles, and reduce the production of gold nanocrystals. Maintenance of the superstructures after boiling in water suggests the absence of CTAB molecules on the particle surfaces. FT-IR, XPS, and NMR results indicate thermolysis of CTAB, and long alkyl chains interact with the Au2S nanoparticles and direct their assembly into a lamellar phase structure