11 research outputs found
New Insights into the Early Stages of Nanoparticle Electrodeposition
Electrodeposition is an increasingly important method to synthesize supported nanoparticles, yet the early stages of electrochemical nanoparticle formation are not perfectly understood. In this paper, the early stages of silver nanoparticle electrodeposition on carbon substrates have been studied by aberration-corrected TEM, using carbon-coated TEM grids as electrochemical electrodes. In this manner we have access to as-deposited nanoparticle size distribution and structural characterization at the atomic scale combined with electrochemical measurements, which represents a breakthrough in a full understanding of the nanoparticle electrodeposition mechanisms. Whereas classical models, based upon characterization at the nanoscale, assume that electrochemical growth is only driven by direct attachment, the results reported hereafter indicate that early nanoparticle growth is mostly driven by nanocluster surface movement and aggregation. Hence, we conclude that electrochemical nulceation and growth models should be revised and that an electrochemical aggregative growth mechanism should be considered in the early stages of nanoparticle electrodeposition
In Situ Study of the Deposition of (Ultra)thin Organic Phosphonic Acid Layers on the Oxide of Aluminum
The interest in self-assembling monolayer deposition
on various
oxide substrate surfaces is steeply increasing in the last decades.
Although many studies are being performed, literature does not come
with a general insight in the adsorption of these layers on oxide
surfaces. Also for the deposition of phosphonic acids on aluminum
oxides, there is no global consensus. In this paper, we present an
original in situ analysis in order to eludicate the real layer formation
mechanism. First of all, the state of the phosphonic acid molecules
was determined using DOSY NMR, making sure that no structures other
than free molecules were present at the concentration used. With in
situ atomic force microscopy and in situ visual ellipsometry, multilayers
of phosphonic acids, showing 3D island growth, were determined. It
was shown that using the variation of the in situ obtained roughness
and bearing ratio, together with the equivalent thickness modeled
by ellipsometry, the growth of the layers occurs in situ in three
different stages. They consist of increasing number of islands growth,
followed by filling up the gaps between islands. At last, within the
adsorption time frame measured, the islands grow further in dimensions
but not in numbers. This closely corresponds with the behavior of
the octylphosphonic acid films analyzed by ex situ techniques
A Generalized Electrochemical Aggregative Growth Mechanism
The early stages of nanocrystal nucleation and growth are still
an active field of research and remain unrevealed. In this work, by
the combination of aberration-corrected transmission electron microscopy
(TEM) and electrochemical characterization of the electrodeposition
of different metals, we provide a complete reformulation of the Volmer–Weber
3D island growth mechanism, which has always been accepted to explain
the early stages of metal electrodeposition and thin-film growth on
low-energy substrates. We have developed a Generalized Electrochemical
Aggregative Growth Mechanism which mimics the atomistic processes
during the early stages of thin-film growth, by incorporating nanoclusters
as building blocks. We discuss the influence of new processes such
as nanocluster self-limiting growth, surface diffusion, aggregation,
and coalescence on the growth mechanism and morphology of the resulting
nanostructures. Self-limiting growth mechanisms hinder nanocluster
growth and favor coalescence driven growth. The size of the primary
nanoclusters is independent of the applied potential and deposition
time. The balance between nucleation, nanocluster surface diffusion,
and coalescence depends on the material and the overpotential, and
influences strongly the morphology of the deposits. A small extent
of coalescence leads to ultraporous dendritic structures, large surface
coverage, and small particle size. Contrarily, full recrystallization
leads to larger hemispherical monocrystalline islands and smaller
particle density. The mechanism we propose represents a scientific
breakthrough from the fundamental point of view and indicates that
achieving the right balance between nucleation, self-limiting growth,
cluster surface diffusion, and coalescence is essential and opens
new, exciting possibilities to build up enhanced supported nanostructures
using nanoclusters as building blocks
The Role of Nanocluster Aggregation, Coalescence, and Recrystallization in the Electrochemical Deposition of Platinum Nanostructures
By
using an optimized characterization approach that combines aberration-corrected
transmission electron microscopy, electron tomography, and in situ
ultrasmall angle X-ray scattering (USAXS), we show that the early
stages of Pt electrochemical growth on carbon substrates may be affected
by the aggregation, self-alignment, and partial coalescence of nanoclusters
of <i>d</i> ≈ 2 nm. The morphology of the resulting
nanostructures depends on the degree of coalescence and recrystallization
of nanocluster aggregates, which in turn depends on the electrodeposition
potential. At low overpotentials, a self-limiting growth mechanism
may block the epitaxial growth of primary nanoclusters and results
in loose dendritic aggregates. At more negative potentials, the extent
of nanocluster coalescence and recrystallization is larger and further
growth by atomic incorporation may be allowed. On one hand, this suggests
a revision of the Volmer–Weber island growth mechanism. Whereas
this theory has traditionally assumed direct attachment as the only
growth mechanism, it is suggested that nanocluster self-limiting growth,
aggregation, and coalescence should also be taken into account during
the early stages of nanoscale electrodeposition. On the other hand,
depending on the deposition potential, ultrahigh porosities can be
achieved, turning electrodeposition in an ideal process for highly
active electrocatalyst production without the need of using high surface
area carbon supports
XPS Analysis of the Surface Chemistry and Interfacial Bonding of Barrier-Type Cr(VI)-Free Anodic Oxides
In the transition to environmental
friendly pretreatment of aerospace
aluminum alloys, chromic acid anodizing (CAA) is being replaced by
sulfuric acid (SAA), phosphoric acid (PAA), or phosphoric-sulfuric
acid (PSA) anodizing. While generally the main concern is controlling
the film morphology, such as the pore diameter, oxide-, and barrier
layer thickness, little is known on how the anodic oxide chemistry
affects the interactions at the interface upon adhesive bonding. To
study the link between surface chemistry and interfacial bonding,
featureless oxides were prepared by stopping the anodizing during
the formation of the barrier layer. A model was developed to quantify
the relative amounts of OH<sup>–</sup>, PO<sub>4</sub><sup>3–</sup>, and SO<sub>4</sub><sup>2–</sup> by curve-fitting
the XPS data. Calculations showed that almost 40% of the surface species
in PAA oxide are phosphates (PO<sub>4</sub><sup>3–</sup>),
whereas about 15% are sulfates (SO<sub>4</sub><sup>2</sup>) in SAA.
When both anions were present in the electrolyte, phosphate incorporation
was inhibited. Studies of the interaction between this set of CrÂ(VI)-free
oxides and diethylenetriamine (DETA)î—¸an amine curing-agent
for epoxy resinî—¸showed that all oxides interact with the nitrogen
of DETA. However, larger ratios of Lewis-like acid–base bonding
between the amine electron pair and the acidic hydroxyl on phosphate
surface sites were observed
Comprehensive Study of the Electrodeposition of Nickel Nanostructures from Deep Eutectic Solvents: Self-Limiting Growth by Electrolysis of Residual Water
The
electrodeposition of nickel nanostructures on glassy carbon
was investigated in 1:2 choline chloride-urea (1:2 ChCl-U) deep eutectic
solvent (DES). By combining electrochemical techniques with ex situ
FE-SEM, XPS, HAADF-STEM, and EDX, the electrochemical processes occurring
during nickel deposition were better understood. Special attention
was given to the interaction between the solvent and the growing nickel
nanoparticles. The application of sufficiently negative potential
results in the electrocatlytic hydrolysis of residual water in the
DES, which leads to the formation of a mixed layer of Ni/NiÂ(OH)<sub>2(ads)</sub>. In addition, hydrogen bonds between hydroxide species
and the DES components could be formed, quenching the growth of the
nickel clusters favoring their aggregation. Due to these processes,
a highly dense distribution of nickel nanostructures can be obtained
within a wide potential range. Understanding the role of residual
water and the interactions at the interface during metal electrodeposition
from DESs is essential to produce supported nanostructures in a controllable
way for a broad range of applications and technologies
Chromatographic Properties of Minimal Aspect Ratio Monolithic Silica Columns
We
report on a study wherein we synthesized TMOS-based silica monolithic
skeletons in capillaries with an i.d. of 5 and 10 μm to produce
skeleton structures with very low capillary-to-domain size aspect-ratios.
These structures include the absolute minimal aspect-ratio case of
a monolithic structure whose cross-section only contains a single
node point. With domain-sized based reduced plate heights running
as low as <i>h</i><sub>min</sub> = 1.3–1.5 for retained
coumarin dyes providing a retention factor of <i>k</i> =
0.6–1.0, the study confirms the classic observation that ultralow
aspect ratio columns generate a markedly lower dispersion than columns
with a larger aspect ratio made in the past by Knox, Jorgenson, and
Kennedy for the packed bed of spheres, but now for silica monoliths.
The course of the reduced van Deemter curves, and more specifically
the ratio of <i>A</i>-term versus <i>C</i>-term
band broadening, could be interpreted in terms of the width and persistence
length of the velocity bias zones in the columns. Considering the
overall kinetic performance, it is found that the two best performing
structures are also the structures with the lowest number of domains
or node points, that is, with the lowest capillary-to-domain size
aspect-ratio and, hence, resembling closest to the open-tubular format,
which remains confirmed as the column format with the best kinetic
performance. This is quantified by the fact that the minimal impedance
values (order of <i>E</i><sub>min</sub> = 100) of the best
performing ultralow aspect ratio monolithic columns are still significantly
larger than the <i>E</i><sub>min</sub> values for the reference
open-tubular columns (order of <i>E</i><sub>min</sub> =
15–20)
A Shape-Recovery Polymer Coating for the Corrosion Protection of Metallic Surfaces
Self-healing
polymer coatings are a type of smart material aimed
for advanced corrosion protection of metals. This paper presents the
synthesis and characterization of two new UV-cure self-healing coatings
based on acrylated polycaprolactone polyurethanes. On a macroscopic
scale, the cured films all show outstanding mechanical properties,
combining relatively high Young’s modulus of up to 270 MPa
with a strain at break above 350%. After thermal activation the strained
films recover up to 97% of their original length. Optical and electron
microscopy reveals the self-healing properties of these coatings on
hot dip galvanized steel with scratches and microindentations. The
temperature-induced closing of such defects restores the corrosion
protection and barrier properties of the coating as shown by electrochemical
impedance spectroscopy and scanning vibrating electrode technique.
Therefore, such coatings are a complementary option for encapsulation-based
autonomous corrosion protection systems
A Green, Simple Chemical Route for the Synthesis of Pure Nanocalcite Crystals
A new, simple chemical route was
developed for the synthesis of
pure nanocalcite crystals by controlling the reaction of an aqueous
solution of CaO and CO<sub>2</sub> gas. Results revealed formation
of well-defined and pure nanocalcite crystals with controlled crystallite
and particle size, without additives or organic solvents. The crystallite
and particle size can be controlled, and smaller sizes are obtained
by decreasing the CaO concentration and increasing the CO<sub>2</sub> flow rate. The decrease of crystallite and particle size below a
certain threshold provides the nanocalcite crystal signature characteristics
that can be clearly observed in XRD patterns, TEM images, FTIR spectra,
Raman spectra, XPS spectra, and thermal stability measurements