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

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8.4 Improved Methods for Attachment and Growth of AuNPs on ITO 305

Figure 8.5 FE - SEM images of AuNP - attached ITO surfaces

prepared using the cast seed - mediated growth method. The

repeated cycles of casting were (a) three times and (b) 10

times. The insets show the higher - magniﬁ cation images.

surfaces having a higher density and a narrower size distribution. The FE - SEM

image is shown in Figure 8.5 a. On the other hand, a 10 - cycle cast seeding process

formed the connected or networked nanostructures of AuNPs (see Figure 8.5 b),

while the optical properties were also different from those of the dispersed AuNP -

attached ITO [28] . The cast seeding approach provided a facile and practical strat-

egy for attaching AuNPs on the ITO surfaces while controlling the amount of Au

loading and without using certain organic binder molecules.

Furthermore, by adjusting the concentration of citrate ions in the seed solution

from 1 m M to 50 m M by adding trisodium citrate after the preparation of the Au

nanoseed solution, dramatic changes could be observed in the SEM images and

in the actual colors of the ITO substrates, which in turn indicated changes in the

nanostructures of the AuNPs formed on the ITO surfaces [29] . The attachment

of smaller AuNPs with a higher density was observed when 25 m M citrate ions

were added in the seed solution (see Figure 8.6 ). In contrast, larger AuNPs were

seen to attach when the concentration of citrate ions was increased to 50 m M .

On the basis of this difference and the FE - SEM images observed just after seeding,

it was inferred that the citrate ions affected not only the growth process but also

the seeding process [29] .

Whilst the repulsive power expected from the increased negative charges of

citrate ions was not signiﬁ cant, a rather dense attachment was promoted as the

particular effect of citrate ions. Such control of the AuNP attachment on ITO

would be practically effective because the dense attachment could be achieved by

simply changing the composition of the seed solution.

306 8 Recent Advances in Metal Nanoparticle-Attached Electrodes

Figure 8.6 FE - SEM images of AuNP - attached ITO surface.

The sample was prepared by adding (a) 25 m M and

(b) 50 m M citrate ions into the seed solution before

seeding, followed by the growth treatment. Reproduced

8.5

Attachment and Growth of AuNPs on Other Substrates

The seed - mediated growth method for surface modiﬁ cation with AuNPs could be

applicable to the surface treatments of various types of substrate, mainly because

that the modiﬁ cation of AuNPs was found to be possible on all of the examined

materials, including glassy carbon ( GC ), mica, stainless, epoxy resin, phenol resin,

simple glass, and ZnO ﬁ lm. FE - SEM observations of these materials revealed that

the AuNPs attached and grew on the surfaces, although the grown size of the

AuNPs and the formation of rod - like particles varied depending on the substrates.

Thus, attachment of the Au nanoseed particles by simple immersion into the Au

colloid solutions is considered to proceed commonly via physisorption, and not by

speciﬁ c chemical bonding.

As an example, Figure 8.7 shows the FE - SEM images of AuNP - attached GC

surfaces observed after seed - mediated growth treatment [30] . Here, although the

seeding time was ﬁ xed at 2 h for all samples, the growth period was changed from

5 min to 24 h. This resulted in the dense attachment of AuNPs on the GC surfaces,

and shorter growth times when compared to the cases where the ITO surfaces

were modiﬁ ed. In addition, ﬂ at and smaller AuNPs were clearly formed on the

GC surfaces (see Figure 8.7 a – c). Interestingly, a longer (24 h) treatment in the

growth solution promoted the growth of some microcrystals in local areas, as

shown in Figure 8.7 d. This would be a reﬂ ection of ripening in the growth solu-

tion, to promote crystal growth and to form the bold bodies. The AuNP - attached

8.5 Attachment and Growth of AuNPs on Other Substrates 307

Figure 8.7 FE - SEM images of AuNP - attached GC surfaces.

Samples were prepared using the seed - mediated growth

method, with seeding for 2 h. The growth period was

(a) 5 min, (b) 2 h, (c) 8 h, and (d) 24 h. The observation of

(d) was focused on an area where bold crystals were

dominantly formed. Reproduced with permission from

GC electrodes prepared using the seed - mediated growth method were used for

the electrochemical detection of nitrite [31] .

The method of attachment used for AuNPs on solid surfaces was applied to the

slightly different fabrication of a functional electrode. The technique was used to

attach AuNPs onto mesoporous TiO

ﬁ lms prepared via a liquid - phase deposition

process on GC substrates [32] . Whilst the TiO

ﬁ lm strongly inhibited the electron

transfer process of the [Fe(CN)

]

3 −

/[Fe(CN)

]

4 −

redox couple, electrochemical meas-

urements indicated that the overpotential for the reduction of maleic acid was

signiﬁ cantly decreased when the electrode surface was covered with TiO

ﬁ lm, due

to the electrocatalytic activity. After attaching the AuNPs by the seed - mediated

growth method (Figure 8.8 ), however, the sluggish heterogeneous electron trans-

fer kinetics at the TiO

ﬁ lm was effectively improved while the catalytic activity of

the TiO

ﬁ lm was retained [32] . This example showed the effect of AuNPs on the

nature of less - conducting electrode materials.

308 8 Recent Advances in Metal Nanoparticle-Attached Electrodes

Figure 8.8 FE - SEM images of (a) TiO

ﬁ lm and (b) AuNPs

attached and grown on the surfaces of TiO

ﬁ lm. When the

TiO

ﬁ lm had been prepared on the GC substrate by

liquid - phase deposition, the AuNPs were attached and grown

using the seed - mediated growth method. Reproduced with

8.6

Attachment and Growth of Au Nanoplates on ITO

Although previously, all approaches and trials for preparation have focused on

the seeding process, it is likely that a modiﬁ cation of the growth process should,

potentially, also be capable of altering the nanostructures of AuNPs on the ITO

electrode surfaces.

As a new strategy for attaching Au nanoplates onto the ITO surfaces, a 2 - D

crystal growth of Au was permitted through a liquid - phase reduction from Au

nanoseed particles attached to the ITO surface, using poly(vinylpyrrolidone) ( PVP )

rather than CTAB as a capping reagent in the growth solution [33] . By controlling

the PVP concentration it was possible to form Au nanoplates (see Figure 8.9 ) with

a surface coverage of up to 30%, although variously shaped Au nanocrystals were

formed concurrently on the ITO surface. The Au nanoplates were single crystalline

in nature, with (111) basal planes and an edge - length of up to approximately

∼ 2 μ m, growing parallel to the ITO surface [33] . The concentration of PVP in the

growth solution was a key factor in the Au nanoplate formation, as spherical or

irregularly shaped AuNPs were formed at either higher or lower concentrations

of PVP [33] . The absorption spectra of the Au nanoplate - attached ITO implied

anisotropic and speciﬁ c optical characteristics of the modiﬁ ed ITO glasses.

The Au nanoplate - attached ITO electrode showed some interesting characteris-

tics; for example, an electrochemical response to cytochrome c was observed

without using promoter molecules. However, in order to better examine the

functions of the Au nanoplates, an increase in their coverage is important. Hence,

improved attachment and coverage methods for Au nanoplates are currently

under investigation.

8.7 Attachment and Growth of Silver Nanoparticles (AgNPs) on ITO 309

Figure 8.9 FE - SEM images of Au nanoplates attached

and grown on ITO surfaces. (a) High - magniﬁ cation and

(b) low - magniﬁ cation images. Reproduced with permission

8.7

Attachment and Growth of Silver Nanoparticles ( AgNPs ) on ITO

By applying the same seed - mediated growth method, silver nanosphere and

nanorod particles were successfully attached to the ITO surfaces [34] . As with

AuNPs, the attachment of AgNPs could be performed without using a bridging

reagent (such as MPTMS), by a simple two - step immersion into ﬁ rst the seed

solution, and second into the growth solution containing AgNO

, CTAB, and

varying amounts of ascorbic acid [34] .

The formed nanostructures were found to be very sensitive to the ascorbic acid

content of the growth solution. For example, with an ascorbic acid concentration

of 0.64 m M the AgNPs grew on the ITO surface but retained a moderate dispersion

(Figure 8.10 a). However, when the concentration was raised to 0.86 m M , Ag

nanorod and nanowire formation on the ITO surfaces was much less dispersed

(Figure 8.10 b).

The attachment of AgNPs onto the ITO surfaces was sufﬁ ciently strong for

further use, for example as a working electrode, and consequently AgNP/ITO

electrodes were used in a number of electrochemical measurements. As a result,

it was conﬁ rmed that the outer spheres of the Ag nanoparticles involved in

the redox reaction showed the typical oxidation and reduction waves of Ag metal

[34] . In addition, the redox behavior of [Fe(CN)

]

3 −

/[Fe(CN)

]

4 −

was improved

on the AgNP/ITO electrode, reﬂ ecting the low electron transfer resistance of Ag

(see Figure 8.11 a). This, in turn, indicated that the AgNPs promoted electron

transfer reactions by their presence on the conducting ITO surface. The AgNP/

ITO electrode was also investigated for reduction of the methyl viologen dication

310 8 Recent Advances in Metal Nanoparticle-Attached Electrodes

Figure 8.10 FE - SEM images of AgNP - attached ITO surfaces.

The ITO substrate was ﬁ rst immersed in the seed solution for

2 h and then into the growth solution containing (a) 0.64 m M

or (b) 0.86 m M ascorbic acid for 24 h. Reproduced with

Society.

Figure 8.11 (a) Cyclic voltammograms of

0.5 m M [Fe(CN)

]

3 −

and 0.5 m M [Fe(CN)

]

4 −

0.1 M phosphate - buffered solution (pH 7.0)

recorded using: (curve a) a bare ITO

electrode; (curve b) an Ag seed - attached ITO

electrode; and (curve c) an AgNP/ITO

electrode. Scan rate = 50 mV s

− 1

; (c) Cyclic

voltammograms of 0.5 m M methyl viologen

dication in 0.1 M Na

recorded using:

(curve a) a bare ITO electrode; (curve b) a

conventional Ag electrode; and (curve c) an

AgNP/ITO electrode. Scan rate = 100 mV s

− 1

Reproduced with permission from Ref. [35] ;

8.8 Attachment and Growth of Palladium Nanoparticles on ITO 311

(Figure 8.11 b), in order to determine the native adsorption features of the

fabricated AgNP/ITO electrodes [34] .

8.8

Attachment and Growth of Palladium Nanoparticles PdNPs on ITO

Palladium nanoparticle s ( PdNP s) were also successfully attached and grown on

ITO surfaces using the seed - mediated growth method [35] . The FE - SEM images

recorded after treating the Pd nanoseed particle - attached ITO substrates in a

growth solution for 4 and 12 h, respectively, are shown in Figures 8.12 a and b.

Crystal growth of PdNPs occurred as the immersion time in the growth solution

was increased, with Pd nanocrystals of 60 – 80 nm being identiﬁ ed after 24 h

(Figures 8.12 c and d). The major characteristic of the formed nanostructure of Pd

was that the nanocrystals tended to adhere to each other. Moreover, such aggrega-

tion occurred not only on the surface but was also 3 - D in nature; that is, some

nanocrystals were seen to grow above the basal nanocrystals. The nanostructure

Figure 8.12 FE - SEM images of PdNP - attached ITO surfaces.

The ITO substrates were immersed into the seed solution for

2 h and then into the growth solution for: (a) 4 h; (b) 12 h; and

American Chemical Society.

312 8 Recent Advances in Metal Nanoparticle-Attached Electrodes

of Pd grown on ITO was quite different from that of Au and Ag, mainly because

the AuNPs and AgNPs tended to form in dispersed states (see above). Hence, the

inference was that the identity of the metal changed the aggregation characteristics

during the growth process, despite using CTAB as the same capping reagent in

all growth procedures.

Due to the dense nanoparticle attachment (see Figure 8.12 c), the PdNP/ITO

electrodes had a signiﬁ cantly lowered charge transfer resistance compared to that

of a bare ITO, and the redox reaction of [Fe(CN)

]

3 −

/[Fe(CN)

]

4 −

was observed to be

reversible in 0.1 M phosphate - buffered solution (Figure 8.13 a) [35] . The electro-

catalytic property of PdNPs attached on ITO was conﬁ rmed for the reduction of

oxygen (Figure 8.13 b). In addition, some typical responses were observed in 0.5 M

with the PdNP/ITO electrodes, reﬂ ecting both the characteristics of the

NPs and the thin layer on a nanoscale [35] . The proposed preparation method for

PdNP - attached ITO surfaces should be promising for catalytic applications, as well

as electrochemical uses.

8.9

Attachment of Platinum Nanoparticles PtNPs on ITO and GC

Although attempts were made to prepare platinum nanoparticle s ( PtNP s) on ITO

surfaces using the seed - mediated growth method, some preliminary studies

showed this approach to be difﬁ cult [36] . The problems were caused by the appear-

ance of brown precipitates in the growth solution, despite using the same principle

(i.e., the mixing of K

PtCl

, CTAB and ascorbic acid), before the seed - attached ITO

was immersed.

Figure 8.13 (a) Cyclic voltammograms of

1.0 m M [Fe(CN)

]

4 −

in 0.1 M phosphate -

buffered solution (pH 7.0) with: (curve a) a

bare ITO electrode; (curve b) a bulk Pd

electrode; and (curve c) a PdNP/ITO

electrode prepared via 24 h growth. Scan

rate = 50 mV s

− 1

; (b) Cyclic voltammograms

recorded with: (curves a, b) the PdNP/ITO

electrode prepared via 24 h growth in (curve

a) air - saturated and (curve b) N

- saturated

0.1 M KCl solutions, and with (curve c) a Pd

bulk electrode and (curve d) a bare ITO

electrode in air - saturated 0.1 M KCl solution.

Scan rate = 50 mV s

− 1

. Reproduced with

Chemical Society.

8.9 Attachment of Platinum Nanoparticles on ITO and GC 313

Figure 8.14 FE - SEM images of PtNP - attached ITO surfaces.

The ITO substrate was immersed in the growth solution

containing 0.25 m M K

PtCl

and 5 m M ascorbic acid for 24 h.

(a) Low - magniﬁ cation and (b) high - magniﬁ cation images of

the same surface. Reproduced with permission from Ref. [36] ;

However, with further investigation the attachment of PtNPs on ITO was

achieved by employing a rather simple method, namely a one - step in situ chemical

reduction of PtCl

2−

by ascorbic acid, but without using CTAB [36] . The FE - SEM

images of the PtNP - attached ITO surfaces prepared via this in situ reduction

method are shown in Figure 8.14 . Here, the attached PtNPs were spherical and

showed an agglomerated nanostructure which was composed of small nanoclus-

ters. Based on the morphological changes which were dependent on the growth

time, PtNPs were shown to grow via a progressive nucleation mechanism [36] .

Characteristically, when PtNP/ITO was used as a working electrode, the

charge transfer resistances were found to be signiﬁ cantly lowered due to the PtNP

growth. Hence, for the typical redox system of [Fe(CN)

]

3 −

/[Fe(CN)

]

4 −

, the PtNP/

ITO electrodes exhibited electrochemical responses which were similar to that

of a bulk Pt electrode [36] . It was also apparent that the PtNP/ITO electrodes

had signiﬁ cant electrocatalytic properties for oxygen reduction and methanol oxi-

dation (see Figures 8.15 a and b, respectively) [36] . Those PtNPs which demon-

strated an agglomerated nanostructure should show promise as a new type of

electrode material.

The in situ reduction method used to prepare PtNPs was also applied to the

modiﬁ cation of GC surfaces. This resulted in a thin continuous Pt ﬁ lm which was

composed of small nanoclusters that had a further agglomerated nanostructure

of small grains, and could be attached onto the GC surface [37] . FE - SEM images

of the Pt nanocluster ﬁ lm ( PtNCF ) are shown in Figure 8.16 . The electrochemical

results obtained indicated that the current values for Pt oxidation, Pt oxide reduc-

tion and hydrogen - related redox reactions, when recorded with the PtNCF

electrode, were almost twice those with the Pt nanocluster dispersedly - attached

GC (PtNC/GC) electrode, but this reﬂ ected the higher Pt loading (Figure 8.17 a)

[37] . The electrocatalytic ability of the PtNCF for methanol oxidation was also

314 8 Recent Advances in Metal Nanoparticle-Attached Electrodes

Figure 8.15 (a) Cyclic voltammograms

recorded using (curves a, b) a PtNP/ITO

electrode prepared via 24 h growth in: (curve

a) air - saturated and (curve b) N

- saturated

0.5 M H

solutions, and (curve c) a Pt bulk

electrode for the air - saturated 0.5 M H

solution. Scan rate = 50 mV s

− 1

; (b) Cyclic

voltammograms obtained for 0.1 M methanol

oxidation in the N

- saturated 0.5 M H

solution recorded with: (curve a) the prepared

PtNP/ITO electrode; (curve b) a Pt bulk

electrode; and (curve c) a bare ITO electrode

Scan rate = 50 mV s

− 1

. Reproduced with

Chemical Society.

Figure 8.16 FE - SEM images of PtNCF attached on GC. The

concentration of ascorbic acid for preparation was 5.1 m M .

(a) High - magniﬁ cation and (b) low - magniﬁ cation images. The

left lower corner of (b) was scratched to show the ﬁ lm

thickness. Reproduced with permission from Ref. [37] ;

apparently higher than that of the PtNC/GC or PtNP/ITO electrodes (Figure

8.17 b). In addition, the electrocatalytic performance of PtNCF, when expressed in

term of Pt content, was clearly superior to that of Pt black formed on GC [37] .

Taken together these results indicated that, in spite of the continuous nanos-

tructures, the nanograins of PtNCF functioned effectively for catalytic electrolysis.

At present, PtNCF may be regarded as an interesting thin - ﬁ lm material, which

can be easily prepared via a one - step chemical reduction.