Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

8.11 Nonlinear Optical Properties of Metal NP-Attached ITO 315

Figure 8.17 (a) Cyclic voltammograms recorded using

(a) PtNC/GC and (b) PtNCF electrodes in a solution

containing 0.5 M H

2

SO

4

. Scan rate = 50 mV s

− 1

; (b) Cyclic

voltammograms obtained for 1.0 M methanol oxidation in

N

2

- saturated 0.5 M H

2

SO

4

recorded using (a) PtNC/GC and

(b) PtNCF electrodes. Scan rate = 50 mV s

− 1

. Reproduced with

8.10

Electrochemical Measurements of Biomolecules Using AuNP / ITO Electrodes

It has been shown that the AuNP/ITO electrodes can be used for the electrochemi-

cal measurements of biomolecules and, in collaboration with Professor Goyal

in India, we have acquired a host of data using our AuNP/ITO electrodes, albeit

with a differential pulse voltammetry technique. Examples include the electro-

chemical analysis of uric acid [38] , paracetamol (acetaminophen) [39] , atenolol

[40] , nandrolone [41] , methylprednisolone acetate [42] , and salbutamol [43] . In

addition, the simultaneous electrochemical determination of guanosine and gua-

nosine 5 ′ - triphosphate [44] , adenosine and adenosine 5 ′ - triphosphate [45] , and

dopamine and serotonin [46] have been successfully achieved with the AuNP/ITO

electrodes.

Although the AuNP/ITO electrodes are not robust, and the modiﬁ ed surfaces

are difﬁ cult to renew if damaged, Professor Goyal ’ s group used the AuNP/ITO

electrodes (which were sent from Japan) with great care and so obtained excellent

results. Yet, this may in fact provide evidence of the practical strength of the

AuNP/ITO electrodes prepared using the seed - mediated growth method.

8.11

Nonlinear Optical Properties of Metal NP - Attached ITO

Beyond application as electrodes, the metal NP - attached ITO materials exhibit

interesting nonlinear optical characteristics, and in order to investigate these

properties we are presently collaborating with Professor Kityk in Poland and

Professor Eboth é in France. To date, a variety of interesting nonlinear optical

316 8 Recent Advances in Metal Nanoparticle-Attached Electrodes

properties of AuNPs [47 – 51] and AgNPs [52, 53] , attached either on ITO or glass,

have been reported. In addition, PdNPs [54, 55] , PtNPs [54] , and NiNPs [56]

attached to ITO surfaces have also been investigated.

Because the seed - mediated growth technique includes a chemical reduction

process in solution, the doping of second ions can be easily carried out. Conse-

quently, the luminescence properties of not only the NPs of pure metals but also

of erbium ion - doped AgNPs have been reported [57] . In addition, due to the uni-

versal applicability of the seed - mediated method, a ZnO ﬁ lm was modiﬁ ed with

AuNPs and the nonlinear optical properties reported [58, 59] .

In addition to the nonlinear optical properties of metal NPs, the modiﬁ cation

of glasses with metal NPs has the potential for use as a new type of substrate for

spectroscopic detection. As an example, AuNP - attached glass plates were described

for use in surface plasmon resonance spectroscopy [60] .

8.12

Concluding Remarks

In this chapter, we have summarized the wet chemical preparation methods – notably

seed - mediated growth – for fabricating various metal NP (or nanostructured

crystal) - attached electrodes. It has been shown that small metal seed - NPs can be

attached to the surfaces of substrates (typically ITO and GC, but also other materi-

als) without the use of binding reagents, and the seed - NPs can then be grown via

chemical reductions while retaining a strong attachment. The simplicity of the

preparation method – that is, a two - step immersion ﬁ rst into the seed solution, and

second into the growth solution (both of which are aqueous and at ambient tem-

perature) – would be of signiﬁ cant merit in any proposed seed - mediated method

for nanofabrication.

The thus - fabricated metal NP - attached electrodes have demonstrated interesting

properties in terms of their electrochemical applications, which differ from those

of bulk metals. The lower charge transfer resistances should represent a key

advantage when constructing electrode materials with metal NPs. Consequently,

nanocomposites consisting of metal NPs and base substrates, when prepared

using the seed - mediated growth method, should be regarded as advanced - function

materials, with potential applications also in nonlinear optics.

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Opt. , 8 , 268 .

Mesoscale Radical Polymers: Bottom - Up Fabrication of

Electrodes in Organic Polymer Batteries

Kenichi Oyaizu and Hiroyuki Nishide

319

Advanced Nanomaterials. Edited by Kurt E. Geckeler and Hiroyuki Nishide

ISBN: 978-3-527-31794-3

9

9.1

Mesostructured Materials for Energy Storage Devices

Energy storage is one of the most important worldwide concerns of the twenty - ﬁ rst

century, and it is essential that new, low - cost, and environmentally benign

(i.e., sustainable) materials for energy conversion and storage purposes are found

[1, 2] . Energy - related nanostructured materials have attracted much attention

because of the unusual electrical properties that originate from their conﬁ ned

nanoscale dimensions and combined bulk/interfacial properties, which affect

their overall behavior [3] . On the other hand, a new trend in materials research

has begun to emerge recently, focusing on the fabrication of mesoscale structures

[4] , which is a broader term (or superordinate concept) of the ordered ﬁ ne

structures, characterized by dimensions larger than those in nanomaterials that

typically consist of nanoparticles and top - down nanoarchitectured structures.

The exploration of properties of materials on the mesoscale has led to new tech-

nological perspectives of electrochemical devices such as batteries and sensors,

which rely on ion transport and, thus, are strongly inﬂ uenced by the size effects

of composing materials on mass transfer, transport, and storage. In particular,

mesoscale materials are becoming increasingly important for electrochemical

energy storage applications, due to the limited distance of diffusion for the mass

transfer during charging/discharging processes. A typical example is that of lith-

ium - ion batteries, which consist of a Li - ion intercalating anode (e.g., graphite) and

a cathode (e.g., LiCoO

2

), separated by Li - ion - conducting electrolyte layers such as

a solution of LiPF

6

in an ethylene carbonate/diethylcarbonate ( EC/DEC ) mixture

(Figure 9.1 ) [5] .

The advantages of ordered mesostructures for these electrode - active materials

are the improved accommodation of the lattice transformation resulting from the

Li

+

intercalation/deintercalation to improve cycle life, the increased electrode/

electrolyte contact area leading to higher charging/discharging rates, and – most

importantly – the short path length for electronic and ionic transport which permit

operation with low electric conductivity and hence at high power [6] .

320 9 Mesoscale Radical Polymers: Bottom-Up Fabrication of Electrodes in Organic Polymer Batteries

Figure 9.1 Li - ion battery using mesoscale lithium metal oxide

cathodes for an improved rate performance.

Figure 9.2 (a) Ragone plot for batteries, fuel

cells, and supercapacitors. The dashed lines

represent the time parameter, τ , which is

characteristic of the rate performance;

(b) Characteristic length of electrode - active

materials deﬁ ned as the layer thickness of

diffusing ions and/or propagating charges

during the charging/discharging process,

showing the top - down and bottom - up

strategies to fabricate the target mesoscale

materials.

Ragone plots (Figure 9.2 a) of speciﬁ c power density versus speciﬁ c energy

density for various batteries, displayed with those of fuel cells and supercapacitors

as references, show their speciﬁ c time parameter, τ , that characterizes the period

of time required for charging/discharging operations [2] . The diffusion length

( D τ )

1/2

of ions and propagating charges within the electrode - active materials

corresponds to their characteristic dimensional scales, where D is the diffusion

coefﬁ cient for the mass transfer processes. For cathode - active materials in

Li - ion batteries, the lithium cobalt oxide (LiCoO

2

) is characterized by a relatively

large diffusion coefﬁ cient of D = 10

− 7

to 10

− 9

c m

2

s

− 1

[7] , but the recently investigated

less expensive and/or less toxic oxides show lower diffusivities for the Li

+

ion,

such as those in Li

1 −

δ

Ni

0.5

Mn

0.5 −

x

Ti

x

O

2

( x = 0 – 0.3, D = 10

− 9

to 10

− 14

c m

2

s

− 1

)

[8] , Li

7

BiO

6

( D = 5 × 1 0

− 13

c m

2

s

− 1

) [9] , LiFePO

4

( D = 1.8 × 1 0

− 14

c m

2

s

− 1

) [10] ,

and LiV

3

O

8

( D = 2 – 8 × 1 0

− 10

c m

2

s

− 1

) [11] . The resulting dimensional scale, or the

9.1 Mesostructured Materials for Energy Storage Devices 321

characteristic length, is typically in the order of 10

2

< ( D τ )

1/2

< 1 0

4

nm, thus reveal-

ing the importance of fabricating mesostructured materials (Figure 9.2 b), which

are in - between nanoscopic and macroscopic materials in dimension.

Fabrication of the mesostructured oxides has been accomplished by using

top - down methods, although mesoscale cathodes are less developed than the

mesostructured anodes. For example, a similar approach to the formation of

silicon nanopillars has been applied for cathode - active materials. By using suitable

templates such as porous alumina and porous polymeric materials, nanopillars

of V

2

O

5

and LiMn

2

O

4

have been successfully grown on metal substrates as the

current collectors [6] . The mesostructured materials have additional advantages

over the conventional electrode - active materials, to accommodate relatively large

volumetric changes during charging/discharging cycles, thus improving the

overall power - rate performance of the resulting batteries.

Organic molecule - based electrode - active materials are more advantageous than

inorganic materials in terms of their potential capability for the molecule - based

“ bottom - up ” fabrication of mesostructures (Figure 9.2 b), in addition to their inher-

ent ﬂ exibility, safety, and accessibility from unlimited resources [12] . The organic

polymers most typically examined for batteries are conducting polymers such as

polyacetylene, polyaniline, polypyrrole, polythiophene, and polyphenylene [13 – 17] .

Sulfur and redox polymerization electrodes have also been reported as electroactive

materials [13] . However, the low doping levels in conducting polymers result in a

small theoretical capacity. Moreover, the charging/discharging processes have

often been impeded by the slow kinetics of electrode reactions [13] . Another group

of molecules investigated for charge storage is a series of redox polymers with

nonconjugated backbones and redox centers localized in pendant groups, such as

tetrathiafulvalene [18] , ferrocene [19, 20] , and carbazole [21, 22] . In this case, the

redox centers, rather than the polymer backbones, govern the redox behaviors.

Conductivity arises when these redox centers can exchange charge, through for

example electron hopping between the redox isolated sites within the polymers

(Figure 9.3 ).

It has been found recently that organic robust radicals are potentially useful as

the redox centers due to their rapid one - electron - redox reactions [23, 24] . Moreover,

excellent ﬁ lm - forming properties of the organic radical polymers by simple wet

processes allow various mesoscale fabrication such as homogeneous layers with

precisely controlled thicknesses and ordered mesostructures supported on carbon

ﬁ ber matrices. This aim of this chapter is to provide a concise overview of recent

Figure 9.3 Charge propagation within redox polymer layers,

swollen in electrolyte solutions, by an electron self - exchange

(hopping) mechanism.

322 9 Mesoscale Radical Polymers: Bottom-Up Fabrication of Electrodes in Organic Polymer Batteries

approaches to fabricate mesoscale electrode - active materials, with emphasis placed

on the organic polymers for the radical polymer battery [25] , which is characterized

by an excellent rate performance and capability of fabricating ﬂ exible, paper - like,

and (semi)transparent rechargeable energy - storage devices.

9.2

Mesoscale Fabrication of Inorganic Electrode - Active Materials

The top - down fabrication of inorganic electrode - active materials has been moti-

vated by the possibility of producing a ﬁ lm - like, bendable lithium - ion battery,

which depends in turn on the development of soft electrode - active materials such

as those composed of nanoparticles and nanocoatings of metal oxides for the

cathodes, and lithium foil or nanocarbon materials for the anodes [1, 26] . The

top - down fabrication has also been driven by the recent challenge to maximize

transport rates by employing mesostructured inorganic - based materials. The elec-

trode reaction of inorganic - based materials is often rate - determined by the slow

kinetics of ion intercalation and migration that accompany lattice transformation

[6] . The power - rate properties have been improved by employing the top - down

strategy so as to produce a large surface area and suitable path length for the mass

transfer processes [27] . Recent interesting approaches have included the virus -

enabled synthesis and assembly of cobalt oxide nanowires (Figure 9.4 ). This has

been accomplished by incorporating gold - binding peptides into a ﬁ lament coat to

Figure 9.4 Virus - enabled synthesis of Au - Co

3

O

4

nanowire and

self - assembly strategy to form an ordered monolayer of Co

3

O

4

nanowires [28] .

9.3 Bottom-Up Strategy for Organic Electrode Fabrication 323

form mesoscale gold – cobalt oxide hybrid wires that improve battery capacity as a

new cathode - active material [28] . Indeed, engineered electrode mesostructures to

enhance electrode kinetics have been a target of intense research, including those

based on the electrochemically assisted template growth of Cu nanorods onto a

current collector, followed by electrochemical plating of Fe

3

O

4

to form the ordered

structure at a mesoscale level [27] . The signiﬁ cant effect of the mesoscopic size on

transport rates and ﬂ exibility has been demonstrated recently by Li - ion batteries

made from nanocomposite papers [26, 29] . These are obtained by inﬁ ltrating

cellulose into carbon nanotube s ( CNT s) grown on a silicon substrate and impreg-

nated with the electrolyte, allowing the combination of the cathode (the nanotubes)

and the separator (the cellulose) in a single unit [26] .

9.3

Bottom - Up Strategy for Organic Electrode Fabrication

9.3.1

Conjugated Polymers for Electrode - Active Materials

Research into organic electrode - active materials was prompted by the discovery of

the electric conductivity of doped polyacetylene, which in turn led to the explora-

tion of organic batteries by employing the redox capacity of polyacetylene. This

was based on the reversible p - type and n - type doping/de - doping processes for the

cathode and the anode, respectively, as the principal electrode reactions [30] . Elec-

trically conducting polymers such as polyaniline, polypyrrole, polythiophene, and

their related derivatives, have been similarly examined as electrode - active materi-

als, based on their reversible electrochemical doping behaviors. The limitations of

conducting polymers are based on their insufﬁ cient doping levels, the resulting

low redox capacities, and the chemical instability of the doped states that frequently

lead to the self - discharge and degradation of the rechargeable properties of the

resulting batteries.

Another concept of using conjugated polymers for batteries is to explore a

charge – storage conﬁ guration in which the electrolyte layer is sandwiched by thin

layers of even less conductive polymers incorporating redox - active groups, with a

view to increasing the overall redox capacity. In this case, the polymer backbones

provide a matrix and/or a conducting path to interconnect innumerable redox sites

for the hopping of electrons by a self - exchange mechanism, and this results in the

storage and transport of charge in a homogeneous solid. Along with this concept,

polythiophene - based conjugated polymers bearing redox sites such as tetrathiaful-

valene derivatives [31] and polypyrrole coupled with redox - active dopants such as

indigo carmine derivatives [32 – 35] , have been recently designed. However, the

chemical bond cleavage and the formation that are often accompanied by the redox

reaction of these closed - shell molecules are frequently electrochemically irrevers-

ible or nonreversible, and hence are unfavorable for application as electrode - active

materials. Simple methods to fabricate mesostructures by the bottom - up strategies

324 9 Mesoscale Radical Polymers: Bottom-Up Fabrication of Electrodes in Organic Polymer Batteries

have been also awaited, in order to bring out the performance of the organic

polymers.

9.3.2

Mesoscale Organic Radical Polymer Electrodes

A new concept, applicable to the mesostructure fabrication, has been proposed for

the organic electrode - active materials, based on the sufﬁ ciently large redox capacity

of aliphatic redox polymers – that is, organic polymers densely populated with

pendant redox - isolated sites. The principal ﬁ nding to permit the use of the non-

conjugated redox polymers is the capability of organic robust radicals, or open -

shell molecules, to allow fully reversible, one - electron redox reactions featuring

fast electrode kinetics and reactant recyclability. While the chemically inert organic

radicals constitute an important and extensively studied group of functional mate-

rials, such as organic ferromagnets [36 – 39] , metal - free redox mediators for syn-

thetic applications [40 – 44] , and electron and hole transport materials for organic

devices [45, 46] , the idea of using them as electroactive materials for rechargeable

batteries has never been proposed, except in our recent reports [47 – 52] and those

from other groups [53 – 64] . Hence, we have focused on organic radicals stabilized

thermodynamically by an effective delocalization of the unpaired electron and/or

kinetically by bulky substituents to employ them as the redox centers. Radical

groups such as the NO - centered nitroxides, N - centered triarylaminium cation

radicals, and diphenylpicrylhydrazyl derivatives, O - centered phenoxyls and

galvinoxyls, and C - centered trityls and phenalenyls, are typically examined as the

pendant group of the radical polymers (Scheme 9.1 ).

These radicals are persistent at ambient temperatures under air and are

characterized by a rapid and reversible one - electron - electrode reactions, typically

with heterogeneous electron - transfer rate constants of k

0

= 10

− 1

cm s

− 1

in the

Scheme 9.1 Chemical structures of organic robust radicals in the radical polymers.

Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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