12 research outputs found
Energy-Storage Applications for a pH Gradient between Two Benzimidazole-Ligated Ruthenium Complexes That Engage in Proton-Coupled Electron-Transfer Reactions in Solution
The judicious selection
of pairs of benzimidazole-ligated ruthenium complexes allowed the
construction of a rechargeable proton-coupled electron-transfer (PCET)-type
redox battery. A series of rutheniumĀ(II) and -(III) complexes were
synthesized that contain substituted benzimidazoles that engage in
PCET reactions. The formation of intramolecular RuāC cyclometalation
bonds stabilized the resulting rutheniumĀ(III) complexes, in which
p<i>K</i><sub>a</sub> values of the imino NāH protons
on the benzimidazoles are usually lower than those for the corresponding
rutheniumĀ(II) complexes. As a proof-of-concept study for a solution
redox battery based on such PCET reactions, the charging/discharging
cycles of several pairs of ruthenium complexes were examined by chronopotentiometry
in an H-type device with half-cells separated by a Nafion membrane
in unbuffered CH<sub>3</sub>CN/H<sub>2</sub>O (1/1, v/v) containing
0.1 M NaCl. During the charging/discharging cycles, the pH value of
the solution gradually changed accompanied by a change of the open-circuit
potential (OCP). The changes for the OCP and pH value of the solution
in the anodic and cathodic half-cells were in good agreement with
the predicted values from the Pourbaix diagrams for the pairs of ruthenium
complexes used. Accordingly, the careful selection of pairs of ruthenium
complexes with a sufficient potential gradient and a suitably large
p<i>K</i><sub>a</sub> difference is crucial: the charge
generated between the two ruthenium complexes changes the OCP and
the pH difference between the two cells in an unbuffered solution,
given that the PCET reactions occur at both electrodes and that discharging
leads to the original state. Because the electric energy is stored
as a pH gradient between the half-cells, new possibilities for PCET-type
rocking-chair redox batteries arise
Electronic Band Structure of Exfoliated Titanium- and/or Niobium-Based Oxide Nanosheets Probed by Electrochemical and Photoelectrochemical Measurements
Exfoliated two-dimensional (2D) unilamellar nanosheets
of Ca<sub>2</sub>Nb<sub>3</sub>O<sub>10</sub><sup>ā</sup>,
TiNbO<sub>5</sub><sup>ā</sup>, Ti<sub>2</sub>NbO<sub>7</sub><sup>ā</sup>, and Ti<sub>5</sub>NbO<sub>14</sub><sup>3ā</sup> were deposited
layer-by-layer to produce multilayer films on indiumātināoxide
(ITO)-coated glass electrodes, and their electrochemical and photoelectrochemical
properties were explored. The layer-by-layer assembly process via
sequential adsorption with counter polycations was monitored by UVāvisible
absorption spectra and X-ray diffraction measurements, which confirmed
the successful growth of films, where nanosheets and polycations are
alternately stacked at a separation of 1.6ā2.4 nm. Exposure
to UV light totally removed polycations, producing inorganic films.
Cyclic voltammetry on Ti and/or Nb oxide nanosheet electrodes thus
fabricated showed reduction/oxidation (Ti<sup>3+</sup>/Ti<sup>4+</sup> and Nb<sup>4+</sup>/Nb<sup>5+</sup>) peaks associated with insertion/extraction of Li<sup>+</sup> ions into/from intersheet galleries of the films. The extent
of the redox reaction is found to be governed by the cation density
in the nanosheet gallery. Anodic photocurrents of the oxide nanosheet
electrodes were observed under UV light irradiation. These action
spectra showed close resemblance to optical absorption profiles of
the colloidal nanosheets, indicating that the photocurrent was generated
from the nanosheets. Their analysis indicates that the nanosheets
of Ca<sub>2</sub>Nb<sub>3</sub>O<sub>10</sub><sup>ā</sup>,
TiNbO<sub>5</sub><sup>ā</sup>, Ti<sub>2</sub>NbO<sub>7</sub><sup>ā</sup>, and Ti<sub>5</sub>NbO<sub>14</sub><sup>3ā</sup> are all indirect transition-type wide-gap semiconductors with bandgap
energies of 3.44, 3.68, 3.64, and 3.53 eV, respectively.
These values are larger than those for corresponding parent layered
oxide compounds before delamination, suggesting confinement effects
into 2D nanosheet structure. Furthermore, the value was invariable
for the films with a different number of nanosheet layers, indicating
that quantized nanosheets were electronically isolated with each other.
In addition, photocurrent generation was measured as a function of
applied electrode potential, and the flatband potential was estimated
from the photocurrent onset values as ā1.12, ā1.33,
ā1.30, and ā1.29 V vs Ag/Ag<sup>+</sup>, for Ca<sub>2</sub>Nb<sub>3</sub>O<sub>10</sub><sup>ā</sup>, TiNbO<sub>5</sub><sup>ā</sup>, Ti<sub>2</sub>NbO<sub>7</sub><sup>ā</sup>, and Ti<sub>5</sub>NbO<sub>14</sub><sup>3ā</sup> nanosheets,
respectively, providing a diagram of electronic band structure for
the nanosheets
Proton-Rocking-Chair-Type Redox Capacitors Based on Indium Tin Oxide Electrodes with Multilayer Films Containing Ru Complexes
A rechargeable
proton-rocking-chair-type redox capacitor was fabricated using scalable
layer-by-layer-(LbL)-assembled films composed of two dinuclear Ru
complexes that exhibit proton-coupled electron-transfer (PCET) reactions
with different RuĀ(II/III) redox potentials (<b>RuNHāOH</b> and <b>RuCHāOH</b>). <b>RuNHāOH</b> and <b>RuCHāOH</b> contain different coordination environments
that involve two phosphonate linker ligands at both ends and bridging
2,6,2ā²,6ā²-tetrakisĀ(benzimidazol-2-yl)-4,4ā²-bipyridine
or 1,3,1ā²,3ā²-tetrakisĀ(benzimidazol-2-yl)-5,5ā²-biphenyl
ligands, respectively. The molecular units were assembled onto indium
tin oxide (ITO) electrodes by complexation between the phosphonate
groups and zirconiumĀ(IV) ions. The LbL growing process of these multilayer
films was monitored by electrochemical or UVāvis spectroscopic
measurements. The thus obtained LbL films on the ITO electrodes showed
PCET reactions at different potentials, depending on the bridging
ligands. The introduction of cyclometalated RuāC bonds in the
bridging ligand of <b>RuCHāOH</b> led to the stabilization
of the rutheniumĀ(III) oxidation state, and therefore, <b>RuCHāOH</b> exhibited lower p<i>K</i><sub>a</sub> values for the imino
NāH protons in the bridging benzimidazole groups compared to
those of the corresponding <b>RuNHāOH</b> complex. The
proton movements that accompany the redox reaction in the Ru multilayer
films on the ITO electrode were confirmed using a pH indicator probe.
For the performance test of a proton-rocking-chair-type redox capacitor,
a two-electrode system composed of <b>RuNHāOH</b> and <b>RuCHāOH</b> multilayer films on ITO electrodes was examined
in an aqueous solution of NaClO<sub>4</sub>. Under galvanostatic conditions,
stable, reversible, and repeatable charging/discharging processes
occurred. The capacitance increased with an increasing number of LbL
layers. For comparison, a similar redox capacitor composed of two <b>RuNMeāOH</b> and <b>RuCMeāOH</b> analogues,
in which all four imino NāH protons on the benzimidazole moieties
are protected by NāMe groups, was constructed and examined.
In these complexes, the capacitance decreased by 77% compared to the
PCET-type capacitor composed of a cell containing <b>RuNHāOH</b> and <b>RuCHāOH</b>; this result strongly suggests that
the proton movement plays a more important role for the charge storage
than the anion movement. In such LbL films composed of Ru complexes
that exhibit PCET-type redox reactions, the capacitance is drastically
improved with an increasing number of layers and using protons as
charge carriers
Controlling the Adsorption of Ruthenium Complexes on Carbon Surfaces through Noncovalent Bonding with Pyrene Anchors: An Electrochemical Study
Surface modifications of carbon nanomaterials,
such as graphene
or carbon nanotubes, through noncovalent ĻāĻ interactions
between Ļ-conjugated carbon surfaces and pyrene anchors have
received much attention on account of the applications of these materials
in organic electronic and sensor devices. Despite the rapidly expanding
use of pyrene anchors, little is known about the number of pyrene
groups required in order to achieve a stable attachment of molecules
on nanocarbon surfaces. So far, systematic studies on such surface
modifications through adsorption isotherms and desorption behavior
of molecules still remain scarce. In this study, we have investigated
the effect of the number of pyrene anchors in redox-active Ru complexes
on their adsorption on carbon nanomaterials through noncovalent ĻāĻ
interactions. The RuĀ(II/III) couple was used as a redox marker in
order to determine the surface coverage on nanocarbon surfaces such
as highly oriented pyrolytic graphite (HOPG), single-walled carbon
nanotubes (SWCNTs), and multiwalled carbon nanotubes (MWCNTs). The
amount of surface coverage as well as the kinetic stability of the
Ru complexes was thereby observed to be directly proportional to the
number of pyrene groups present in the ligands. The desorption rate
from HOPG electrode increased in the order <b>Ru-1</b> with
eight pyrene groups (<i>k</i> = 2.0 Ć 10<sup>ā5</sup> s<sup>ā1</sup>) < <b>Ru-2</b> with four pyrenes
(4.1 Ć 10<sup>ā5</sup> s<sup>ā1</sup>) < <b>Ru-3</b> with two pyrenes (6.8 Ć 10<sup>ā5</sup> s<sup>ā1</sup>) āŖ <b>Ru-4</b> with one pyrene (4.1
Ć 10<sup>ā3</sup> s<sup>ā1</sup>). Furthermore,
the electrochemical polymerization of the Ru complex with four pyrene
groups proceeded more efficiently compared to complexes with one or
two pyrene groups. As a consequence, compounds having more than two
and/or optimally four pyrene groups revealed a stable adsorption on
the nanocarbon surfaces. The heterogeneous electron transfer rate
between the Ru complex, <b>Ru-2</b>, and the carbon nanomaterials
increased in the order SWCNTs (<i>k</i><sub>ET</sub> = 1.3
s<sup>ā1</sup>) < MWCNTs (Ļ = 5ā9 nm) (<i>k</i><sub>ET</sub> = 4.0 s<sup>ā1</sup>) < MWCNTs
(Ļ = 110ā170 nm) (<i>k</i><sub>ET</sub> = 14.9
s<sup>ā1</sup>) < HOPG (<i>k</i><sub>ET</sub> =
110 s<sup>ā1</sup>)
Long-Range Electron Transport of Ruthenium-Centered Multilayer Films <i>via</i> a Stepping-Stone Mechanism
We studied electron transport of Ru complex multilayer films, whose structure resembles redox-active complex films known in the literature to have long-range electron transport abilities. Hydrogen bond formation in terms of pH control was used to induce spontaneous growth of a Ru complex multilayer. We made a cross-check between electrochemical measurements and <i>IāV</i> measurements using PEDOT:PSS to eliminate the risk of pinhole contributions to the mechanism and have found small Ī² values of 0.012ā0.021 Ć
<sup>ā1</sup>. Our Ru complex layers exhibit long-range electron transport but with low conductance. On the basis of the results of our theoreticalāexperimental collaboration, we propose a modified tunneling mechanism named the āstepping-stone mechanismā, where the alignment of site potentials forms a narrow band around <i>E</i><sub>F</sub>, making resonant tunneling possible. Our observations may support Tuccito <i>et al</i>.ās proposed mechanism
Wisely Designed Phthalocyanine Derivative for Convenient Molecular Fabrication on a Substrate
An
axial-substituted silicon phthalocyanine derivative, SiPcĀ(OR)<sub>2</sub> (R = C<sub>4</sub>H<sub>9</sub>), that is soluble in organic
solvent is conveniently synthesized. This silicon phthalocyanine derivative
reacts with a hydroxyl group on a substrate and then with another
phthalocyanine derivative under mild conditions. The accumulation
number of the phthalocyanine molecules on the substrates is easily
controlled by the immersion time. On the basis of AFM (atomic force
microscopy) images, the surface of the phthalocyanine-modified glass
substrate has uneven structures on the nanometer scale. ITO electrodes
modified with the composition of the phthalocyanine derivative and
PCBM show stable cathodic photocurrent generation upon light irradiation
Simultaneous Formation and Spatial Patterning of ZnO on ITO Surfaces by Local Laser-Induced Generation of Microbubbles in Aqueous Solutions of [Zn(NH<sub>3</sub>)<sub>4</sub>]<sup>2+</sup>
We
demonstrate the simultaneous formation and spatial patterning of ZnO
nanocrystals on an indiumātin oxide (ITO) surface upon local
heating using a laser (1064 nm) and subsequent formation of microbubbles.
Laser irradiation of an ITO surface in aqueous [ZnĀ(NH<sub>3</sub>)<sub>4</sub>]<sup>2+</sup> solution (1.0 Ć 10<sup>ā2</sup> M at pH 12.0) under an optical microscope produced ZnO nanocrystals,
the presence of which was confirmed by X-ray diffraction analysis
and Raman microspectroscopy. Scanning the focused laser beam over
the ITO surface generated a spatial ZnO pattern (height: ā¼60
nm, width: ā¼1 Ī¼m) in the absence of a template or mask.
The Marangoni convection generated in the vicinity of the microbubbles
resulted in a rapid concentration/accumulation of [ZnĀ(NH<sub>3</sub>)<sub>4</sub>]<sup>2+</sup> around the microbubbles, which led to
the formation of ZnO at the solidābubbleāsolution three-phase
contact line around the bubbles and thus afforded ZnO nanocrystals
on the ITO surface upon local heating with a laser
Observation of an Orientation Change in Highly Oriented Layer-by-Layer Films of a Ruthenium Complex upon Oxidation Reaction
Layer-by-layer films composed of
redox-active ruthenium dimer and
ZrĀ(IV) ions were fabricated on an indium tin oxide electrode. The
fabricating behavior was monitored by cyclic voltammetry and UVāvis
absorption spectral measurements. The orientation of the film was
also monitored by grazing-incidence small-angle and wide-angle X-ray
scattering (GISAXS) measurements, and it has been clarified that this
film has a crystalline structure. The peaks obtained by GISAXS were
changed upon oxidation reaction, which indicates that a change in
the orientation of the ruthenium dimer occurred in the film
Glycine Crystallization in Solution by CW Laser-Induced Microbubble on Gold Thin Film Surface
We have developed a novel laser-induced crystallization
method
utilizing local heat-induced bubble/water interface. Continuous laser
beam of 1064 nm is focused on a gold nanoparticles thin film surface
covered with glycine supersaturated aqueous solution. Light absorption
of the film due to localized plasmon resonance caused local heating
at the focal position and produced a single thermal vapor microbubble,
which generated thermal gradient followed by convection flow around
the bubble and eventually induced glycine crystallization and growth.
The crystallization mechanism is discussed by considering gathering
and accumulating molecules around the bubble/water interface assisted
by convection flow and temperature jump
Tuning of Redox Potentials by Introducing a Cyclometalated Bond to Bis-tridentate Ruthenium(II) Complexes Bearing Bis(<i>N</i>-methylbenzimidazolyl)benzene or -pyridine Ligands
A series of asymmetrical bis-tridentate cyclometalated
complexes
including [RuĀ(Mebib)Ā(Mebip)]<sup>+</sup>, [RuĀ(Mebip)Ā(dpb)]<sup>+</sup>, [RuĀ(Mebip)Ā(Medpb)]<sup>+</sup>, and [RuĀ(Mebib)Ā(tpy)]<sup>+</sup> and two bis-tridentate noncyclometalated complexes [RuĀ(Mebip)<sub>2</sub>]<sup>2+</sup> and [RuĀ(Mebip)Ā(tpy)]<sup>2+</sup> were prepared
and characterized, where Mebib is bisĀ(<i>N</i>-methylbenzimidazolyl)Ābenzene,
Mebip is bisĀ(<i>N</i>-methylbenzimidazolyl)Āpyridine, dpb
is 1,3-di-2-pyridylbenzene, Medpb is 4,6-dimethyl-1,3-di-2-pyridylbenzene,
and tpy is 2,2ā²:6ā²,2ā³-terpyridine. The solid-state
structure of [RuĀ(Mebip)Ā(Medpb)]<sup>+</sup> is studied by X-ray crystallographic
analysis. The electrochemical and spectroscopic properties of these
ruthenium complexes were studied and compared with those of known
complexes [RuĀ(tpy)Ā(dpb)]<sup>+</sup> and [RuĀ(tpy)<sub>2</sub>]<sup>2+</sup>. The change of the supporting ligands and coordination environment
allows progressive modulation of the metal-associated redox potentials
(Ru<sup>II/III</sup>) from +0.26 to +1.32 V vs Ag/AgCl. The introduction
of a ruthenium cyclometalated bond in these complexes results in a
significant negative potential shift. The Ru<sup>II/III</sup> potentials
of these complexes were analyzed on the basis of Leverās electrochemical
parameters (<i>E</i><sub>L</sub>). Density functional theory
(DFT) and time-dependent DFT calculations were carried out to elucidate
the electronic structures and spectroscopic spectra of complexes with
Mebib or Mebip ligands