Synthesis of the metal nanoparticles from the microemulsion was first reported by Boutonnet et al. in 1982 [6], who synthesized the metal nanoparticles of Pt, Pd, Rh and Ir by the reduction of the salt dissolved in the aqueous phase of W/O microemulsion using hydrazine or hydrogen. Nanoparticles were used for the preparation of the supported catalysts. Kishida et al. [7, 8] described a new method of preparation of Rh catalysts with the very narrow metal dispersion. Isolated metal crystallites were first synthesized by hydrazine reduction of a metal precursor in the microemulsion. Subsequently the catalyst support was precipitated by the hydrolysis of metal alkoxide (tetraethyl orthosilicate, aluminium triisopropoxide, zirconium or titanium tetrabutoxide) in the microemulsion and the metal nanoparticles were immobilized on the catalyst support. However, under such conditions, a part of the active metal particles is closed in pores of the support and therefore is catalytically inactive. By this phenomenon, the authors explained the differences between the values obtained from chemisorption measurements and TEM (transmission electron microscopy). Some of the metal particles buried in the support were not exposed on the surface for CO chemisorption [8]. Nevertheless, the catalysts prepared in this way showed higher activity than similar ones prepared by conventional incipient wetness impregnation method. These authors studied the effect of the surfactant selection and its molar ratio to water in the preparation of catalysts using the microemulsion. The dependence of the effect of the molar ratio of water and surfactant on the resulting particle size went through a minimum, but for each microemulsion system was specific. Kosydar et al. [9] used two microemulsion systems with anionic bis(2-ethylhexyl)sulfosuccinate (AOT) and non-ionic polyoxyethylene(7–8)octylphenyl ether (Triton-X 114) surfactants. Pd nanoparticles with sizes in a narrow range but various average sizes (8–9 and 4 nm) were obtained using hydrazine hydrate or NaH2 PO2 as a reducing agent. Semagina et al. [10], Heshmatpour et al. [11] and Noh et al. [12] also reported on the application of water/AOT/isooctane microemulsion system for the preparation of Pd nanoparticles. Different nanoparticle size and size distributions were obtained depending on the reducing agent or precursor salt. Using N2 H4 as a reducing agent, larger nanoparticles and a broader size ranges (6–13 nm) were obtained compared to NaBH4 (3.9–5 nm). Although reducing agent plays an important role in the size of the metal nanoparticles formed, Chen et al. [13] reported on the formation of similar Pd nanoparticle size when N2 H4 or NaBH4 was used as a reducing agent, thus indicating the contribution of other factors. Thus Perez-Coronado et al. [14] synthetized Pd nanoparticles by a water-in-oil (w/o) microemulsion method using the water/ AOT/isooctane system. N2 H4 and NaBH4 were used as reducing agents and water-to-surfactant ratios between 3 and 7 were considered. Results [14] showed that Pd nanoparticles with higher size were obtained when the water-to-surfactant molar ratio was increased. Lower reaction time and higher evaporation temperature led to the formation of smaller Pd nanoparticles. Higher values of mean size Pd nanoparticles were obtained with N2 H4 compared to NaBH4 . Wojcieszak et al. [15] investigated the size of palladium nanoparticles (1.6 wt.%) synthesized by the modified water-in-oil microemulsion method using hydrazine as the reducing agent by varying the nature of the organic surfactant and solvent. The results confirmed the dependence of the particle size on the nature of organic surfactants. Smaller particles were obtained with organic solvents and anionic surfactants. Kim et al. [16] compared the catalytic performance of Pd/ZrO2 catalysts prepared from microemulsion with the ones prepared by the conventional impregnation method. The catalysts prepared by the microemulsion method exhibited a much higher activity for the hydrogenation of CO, the size distribution of Pd particles of the catalysts was remarkably narrow, and the average particle size was much smaller than that of the catalysts prepared by the impregnation method. It was found that the Pd particle size of the catalyst prepared by the microemulsion method could be controlled by the concentration of palladium salts in water pools. Kim et al. [16] also found that some of Pd particles of the catalyst were partly or wholly buried in zirconia support. Significantly higher activity of Pt catalysts for the combustion of toluene prepared from the microemulsion compared to those prepared by the impregnation with the aqueous solution of H2 PtCl6 was observed by Rymeš et al. [1] as well. Pt particle size was up to ten times smaller. Pocoroba

the corresponding olefins in C3 fraction or selective hydrogenation of phenols to ketones, e.g. cyclohexanone from phenol. Palladium is also very active in the hydrogenation of nitro com-

The activity of supported catalysts containing noble metals is influenced by the type of catalyst support, the type of noble metal, the amount of an active component in the catalyst, the way of catalyst preparation, etc. The preparation of the supported catalysts in most cases requires the highest metal dispersion and thus the largest surface area of an active component. For these reasons, the particle size of Pd in supported catalysts can have a significant effect on catalytic activity and selectivity of these catalysts. For some types of reactions, it is necessary to optimize the size of metal crystallites at a certain critical value. If the average particle size of Pt is higher or lower than its optimal value, the activity of such a catalyst for toluene combustion in

The distribution of the active component in the catalyst pellet along with the diameter and shape of the active component particles is closely connected with the method of the preparation [2]. That is why it is important to choose a suitable method for the preparation of the catalyst to provide an optimal dispersion of the active component in the catalyst and also its optimal location in the catalyst. There are various methods for catalyst preparation. The most commonly practiced techniques are incipient wetness impregnation with a metal salt solution, ion exchange with a metal-containing cation or anion and the use of organometallic complexes as precursors [3]. Using incipient wetness impregnation, the nanoparticles in solution can be stabilized by capping agents such as sugars (glucose, starch, carboxymethyl cellulose) to avoid aggregation and widening of the particle size distribution [4]. The use of microemulsion could be another preparation method promising the formation of small metal nanoparticles of a narrow size distribution, located on the external surface of the support.

The term microemulsion was first defined by Schulman and Friend in the 1940s [5]. Microemulsion (a reverse micellar system) is defined as a thermodynamically stable dispersion of two immiscible liquids stabilized by a surface film of emulsifier. It is a three-component system consisting of organic phases: oil, an aqueous phase and a surfactant. The surfactant is a substance which stabilizes the emulsion by reducing the interfacial tension between the dispersed particles of water in oil to a minimum, thus preventing the formation of large aggregates. For the preparation of heterogeneous catalysts, microemulsions of type W/O (water-in-oil) are important since

Synthesis of the metal nanoparticles from the microemulsion was first reported by Boutonnet et al. in 1982 [6], who synthesized the metal nanoparticles of Pt, Pd, Rh and Ir by the reduction of the salt dissolved in the aqueous phase of W/O microemulsion using hydrazine or hydrogen. Nanoparticles were used for the preparation of the supported catalysts. Kishida et al. [7, 8] described a new method of preparation of Rh catalysts with the very narrow metal dispersion. Isolated metal crystallites were first synthesized by hydrazine reduction of a metal precursor in the microemulsion. Subsequently the catalyst support was precipitated by the hydrolysis of metal alkoxide (tetraethyl orthosilicate, aluminium triisopropoxide, zirconium or titanium tetrabutoxide) in the microemulsion and the metal nanoparticles were

highly dispersed particles of the metal salts or the metal particles can be prepared.

pounds and it is used in the production of aniline by DuPont and the Bayer process.

252 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

the gas phase decreases [1].

This method is described in this chapter.

et al. [17] prepared 2.5 wt.% Pd/Al2 O3 exhaust gas catalysts by incipient wetness impregnation technique and from microemulsion as well. Contrary to other authors, the catalysts prepared by incipient wetness impregnation technique consisted of smaller crystallite size than 2 nm and Pd was found to be well dispersed in comparison to microemulsion when the particle size was in the 20–40 nm range and the distribution of noble metal was irregular.

acid in such amount to obtain the catalyst (J [IMP]) with 3 wt.% of Pd. 20.0 g H2

393 K for 2 h, then calcined in air at 623 K for 2 h.

**2.2. Catalyst reduction**

*2.3.1. Surface area*

suspension on Cu grid.

USA.

at the same temperature for 1 h.

**2.3. Characterization of the catalysts**

*2.3.2. Size and distribution of palladium particles*

atmosphere. The treatment in the reducing H2

ods was subsequently reduced to metallic palladium.

mixture was stirred for 24 h at room temperature. After filtration, the catalyst was first dried at

Preparation of Highly Dispersed Pd-Supported Catalysts for Hydrogenation Processes…

The active component of the catalyst in the form of palladium oxide prepared by both meth-

Reduction of Pd precursors was carried out either in gas phase using hydrogen or in liquid phase using hydrazine monohydrate. The catalysts reduced in the gas phase by a mixture of hydrogen and helium (10:90) were placed in a quartz tube. Reduction was performed at 723 K with a heating rate 15 K/min for 2 h and was followed by desorption of hydrogen in inert gas

Reduction in the liquid phase was done using hydrazine monohydrate in the stoichiometric excess of 3:1 at 363 K for 2 h. After reduction, the sample had to be washed with ethanol to

BET surface areas of the catalysts were determined using Pulse Chemisorb 2700, Micromeritics,

Morphology, particle size and particle size distribution of metal were determined by transmission electron microscopy (HRTEM) on JEOL JEM 3010 operating at 300 kV and equipped with an energy-dispersive spectroscopy (EDS) detector (INCA/Oxford) and CCD Gatan (Digital Micrograph software). The transmission electron analysis was carried out on samples that were subsequently dispersed in propan-2-ol, followed by the application of a drop of a diluted

Pd particle size was determined using CO chemisorption as well. For the measurements, a custom-made apparatus was used [19]. The catalyst was dried prior to measurement in the air at 393 K for 2 h. The sample treatment was performed first in the inert and then in the reducing

of PdO contained in the catalyst. Since hydrogen is not only very strongly absorbed on the surface of Pd, but also dissolved in the subsurface layers of the metal it is necessary to include hydrogen desorption step at 523 K after reduction of the catalyst. Hydrogen desorption provided decomposition of the resulting β-hydride of Pd, which is very easily formed even at room temperature and, according to literature, is unstable at high temperatures and low pressure [20]. CO chemisorption measurements were carried out with CO (5 vol.% in He) at 298 K using pulse technique. To calculate average Pd particle size (dAV) and specific surface area of Pd (SPd) from chemisorption data, the following parameters were used: constant depending

atmosphere was carried out for the reduction

remove excess reducing agent and was dried at 393 K in an inert atmosphere.

O was used. The

255

http://dx.doi.org/10.5772/intechopen.72986

Eriksson et al. [18] published an extensive review focused on the preparation of catalysts from microemulsion and their applications in heterogeneous catalysis. This review covered most of the applications concerning heterogeneous catalysts, not only Pd supported but other noble metals were mentioned as well. It was concluded that the majority of the catalysts obtained from microemulsion showed high catalytic activity and the main advantages of using microemulsion technique were related to the possibility of controlling properties such as particle size, morphology and size distribution.

The aim of the study described here was to prepare highly dispersed Pd hydrogenation catalyst by the microemulsion method since the microemulsion method is more suitable for metal catalysts preparation [7, 8, 16] in comparison with conventional incipient wetness impregnation method. In this research, the focus was on optimizing the microemulsion preparation conditions, investigating the influence of reduction conditions, evaluating the physical properties and comparing them with the properties of the catalysts prepared by incipient wetness impregnation method. Details of these procedures and results are presented in the following pages. It is noted that the results presented in the figures and tables are original and not published anywhere before.

#### **2. Experimental**

#### **2.1. Catalyst preparation**

The reverse micellar system W/O was prepared from a commercial surfactant IGEPAL CO-520 dissolved in cyclohexane. The reason for choosing non-ionogenic surfactant was to reduce the influence of impurities such as Na+ , K+ and SO4 2− (which are contained in ionogenic surfactants) on the properties of the resulting catalyst. The concentration of the surfactant in cyclohexane was 0.5 mol/dm3 . The aqueous solution of palladium chloride (0.85 g of PdCl2 solution of concentration 1.0 mol/dm3 ) was added to the microemulsion prepared in such amounts that the catalyst (B [ME]) with demanded amount 3 wt.% of Pd was obtained. The exact amount of commercial support (2.49 g of γ-Al2 O3 , SBET = 313 m2 /g) calcined at 623 K was inserted in the microemulsion. The commercial surfactant IGEPAL CO-520 in the amount of 4.23 and 15.0 g of cyclohexane was used. This suspension was stirred under reflux and tempered at 298 K for 24 h. Precipitates were decanted, filtered, washed with ethanol and then dried at 393 K overnight. The samples were calcined in air at 623 K for 2 h. The catalyst prepared by this method will be denoted by [ME].

Catalysts I [IMP] and J [IMP] were prepared by incipient wetness impregnation. Impregnation of the commercial support (2.25 g of γ-Al2 O3 , SBET = 313 m2 /g) with aqueous solution of palladium chloride (0.99 g of PdCl2 solution of concentration 1.5 mol/dm3 ) stabilized with hydrochloric acid in such amount to obtain the catalyst (J [IMP]) with 3 wt.% of Pd. 20.0 g H2 O was used. The mixture was stirred for 24 h at room temperature. After filtration, the catalyst was first dried at 393 K for 2 h, then calcined in air at 623 K for 2 h.

The active component of the catalyst in the form of palladium oxide prepared by both methods was subsequently reduced to metallic palladium.

#### **2.2. Catalyst reduction**

et al. [17] prepared 2.5 wt.% Pd/Al2

size, morphology and size distribution.

**2. Experimental**

**2.1. Catalyst preparation**

cyclohexane was 0.5 mol/dm3

solution of concentration 1.0 mol/dm3

of the commercial support (2.25 g of γ-Al2

chloride (0.99 g of PdCl2

reduce the influence of impurities such as Na+

exact amount of commercial support (2.49 g of γ-Al2

prepared by this method will be denoted by [ME].

O3

254 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

was in the 20–40 nm range and the distribution of noble metal was irregular.

technique and from microemulsion as well. Contrary to other authors, the catalysts prepared by incipient wetness impregnation technique consisted of smaller crystallite size than 2 nm and Pd was found to be well dispersed in comparison to microemulsion when the particle size

Eriksson et al. [18] published an extensive review focused on the preparation of catalysts from microemulsion and their applications in heterogeneous catalysis. This review covered most of the applications concerning heterogeneous catalysts, not only Pd supported but other noble metals were mentioned as well. It was concluded that the majority of the catalysts obtained from microemulsion showed high catalytic activity and the main advantages of using microemulsion technique were related to the possibility of controlling properties such as particle

The aim of the study described here was to prepare highly dispersed Pd hydrogenation catalyst by the microemulsion method since the microemulsion method is more suitable for metal catalysts preparation [7, 8, 16] in comparison with conventional incipient wetness impregnation method. In this research, the focus was on optimizing the microemulsion preparation conditions, investigating the influence of reduction conditions, evaluating the physical properties and comparing them with the properties of the catalysts prepared by incipient wetness impregnation method. Details of these procedures and results are presented in the following pages. It is noted that the results presented in the figures and tables are original and not published anywhere before.

The reverse micellar system W/O was prepared from a commercial surfactant IGEPAL CO-520 dissolved in cyclohexane. The reason for choosing non-ionogenic surfactant was to

, K+

surfactants) on the properties of the resulting catalyst. The concentration of the surfactant in

amounts that the catalyst (B [ME]) with demanded amount 3 wt.% of Pd was obtained. The

inserted in the microemulsion. The commercial surfactant IGEPAL CO-520 in the amount of 4.23 and 15.0 g of cyclohexane was used. This suspension was stirred under reflux and tempered at 298 K for 24 h. Precipitates were decanted, filtered, washed with ethanol and then dried at 393 K overnight. The samples were calcined in air at 623 K for 2 h. The catalyst

Catalysts I [IMP] and J [IMP] were prepared by incipient wetness impregnation. Impregnation

, SBET = 313 m2

O3

solution of concentration 1.5 mol/dm3

and SO4

O3

. The aqueous solution of palladium chloride (0.85 g of PdCl2

, SBET = 313 m2

) was added to the microemulsion prepared in such

2− (which are contained in ionogenic

/g) with aqueous solution of palladium

) stabilized with hydrochloric

/g) calcined at 623 K was

exhaust gas catalysts by incipient wetness impregnation

Reduction of Pd precursors was carried out either in gas phase using hydrogen or in liquid phase using hydrazine monohydrate. The catalysts reduced in the gas phase by a mixture of hydrogen and helium (10:90) were placed in a quartz tube. Reduction was performed at 723 K with a heating rate 15 K/min for 2 h and was followed by desorption of hydrogen in inert gas at the same temperature for 1 h.

Reduction in the liquid phase was done using hydrazine monohydrate in the stoichiometric excess of 3:1 at 363 K for 2 h. After reduction, the sample had to be washed with ethanol to remove excess reducing agent and was dried at 393 K in an inert atmosphere.

#### **2.3. Characterization of the catalysts**

#### *2.3.1. Surface area*

BET surface areas of the catalysts were determined using Pulse Chemisorb 2700, Micromeritics, USA.

#### *2.3.2. Size and distribution of palladium particles*

Morphology, particle size and particle size distribution of metal were determined by transmission electron microscopy (HRTEM) on JEOL JEM 3010 operating at 300 kV and equipped with an energy-dispersive spectroscopy (EDS) detector (INCA/Oxford) and CCD Gatan (Digital Micrograph software). The transmission electron analysis was carried out on samples that were subsequently dispersed in propan-2-ol, followed by the application of a drop of a diluted suspension on Cu grid.

Pd particle size was determined using CO chemisorption as well. For the measurements, a custom-made apparatus was used [19]. The catalyst was dried prior to measurement in the air at 393 K for 2 h. The sample treatment was performed first in the inert and then in the reducing atmosphere. The treatment in the reducing H2 atmosphere was carried out for the reduction of PdO contained in the catalyst. Since hydrogen is not only very strongly absorbed on the surface of Pd, but also dissolved in the subsurface layers of the metal it is necessary to include hydrogen desorption step at 523 K after reduction of the catalyst. Hydrogen desorption provided decomposition of the resulting β-hydride of Pd, which is very easily formed even at room temperature and, according to literature, is unstable at high temperatures and low pressure [20]. CO chemisorption measurements were carried out with CO (5 vol.% in He) at 298 K using pulse technique. To calculate average Pd particle size (dAV) and specific surface area of Pd (SPd) from chemisorption data, the following parameters were used: constant depending on the shape of metal particles and contact with the support (k = 6); surface density of metal atoms (CPd = 1.26 × 1019 atom/m2 ) and stoichiometry of adsorption Pd:CO (SAV = 2) [21].

#### *2.3.3. Reducibility of active centres of catalyst*

Temperature-programmed reduction (TPR) was used to describe the reducibility of different active centres and their concentrations. The samples were first dried in air at 393 K for 2 h to constant weight. TPR analysis of the samples was performed using a commercial device Autochem II 2920 Micromeritics, USA, with H2 /Ar mixture 5/95. During the analysis, the temperature was increased linearly with time to a temperature of 923 K with a heating rate of 5 K/min.

#### *2.3.4. Metal content determination*

The metal content was determined directly by X-ray fluorescence (XRF) analysis and indirectly by atomic absorption spectrometry (AAS). The indirect determination of Pd content in the catalyst consisted of a liquid sample analysis, which was prepared by multiple leaching of a defined amount of Pd catalyst using aqua regia. Quantitative transfer of Pd in the solution was indicated by the support discoloration and the last extract from the catalyst was colourless as well. Quantitative dissolution of Pd gave PdCl4 2− solution which was analysed by atomic absorption spectroscopy.

microemulsion, the resulting average Pd particle size determined by CO chemisorption was

**] wPd [%] dAV (Pd)a**

Preparation of Highly Dispersed Pd-Supported Catalysts for Hydrogenation Processes…

catalyst was evaluated, an efficiency of Pd loading on the support was studied as well. When using both 1 and 0.5 M solutions, highly dispersed catalysts with Pd particle size of approxi-

catalyst with desired Pd content of 3 wt.% was obtained, while at the concentration of 0.5 M,

Another parameter influencing the final metal particles is the molar ratio of water and surfactants, the so-called R factor. The amount of surfactant in the microemulsion affects the size and shape of water drops dispersed in an oil phase. Too high a value of R factor can lead to the formation of unstable microemulsion and its breaking. To evaluate the influ-

concentration of 1 mol/dm3

The values reported in **Table 2** show that the molar ratio of water and the surfactant does not affect the final Pd content loading on the support since the Pd content was close to the desired

When increasing the surfactant content in microemulsion, Pd dispersion increased. When R factor was equal to 8, the final Pd particle size determined from chemisorption data was 2.3 nm.

concentration on the final particle size of Pd in the prepared

solution of 1 M concentration, the

 **[nm] SPd [m2**

http://dx.doi.org/10.5772/intechopen.72986

**/gPd]**

257

was used based on the results

**/gPd]**

 **[nm] SPd [m2**

2.2 nm. When the effect of PdCl2

Determined from chemisorption measurements.

**Catalyst c(PdCl2**

**Table 1.** Effect of PdCl2

a

the Pd content was only 2.5 wt.%.

ence of the R factor values, PdCl2

value of 3 wt.% for all catalysts.

Determined from chemisorption measurements.

shown in **Table 1**.

a

*3.1.2. Effect of molar ratio of water to surfactant*

mately 2.3 nm were obtained. However, when using PdCl2

**Catalyst R factor wPd [%] dAV (Pd)a**

E [ME] 4 2.9 2.2 180.9 F [ME] 8 2.9 2.3 172.4 G [ME] 12 2.9 2.8 142.7 H [ME] 16 2.8 4.1 96.9

**Table 2.** Effect of molar ratio of water to surfactant on Pd crystallite size in the resulting catalyst.

**) [mol/dm3**

A [ME] 1.50 2.9 3.0 131.4 B [ME] 1.00 2.9 2.2 180.9 C [ME] 0.50 2.5 2.3 172.8 D [ME] 0.25 2.1 2.6 152.2

concentration on Pd crystallite size in the resulting catalyst.

#### **3. Results and discussion**

#### **3.1. Effect of preparation conditions of microemulsions on the physical properties of catalysts**

When the effect of preparation conditions of microemulsions was monitored, attention was paid to two parameters namely the concentration of metal salts in the aqueous solution and the molar ratio of water and surfactant.

#### *3.1.1. Effect of PdCl2 concentration*

According to literature [7, 8, 16], the concentration of metal component precursor in the form of soluble salts in the aqueous solution has a significant impact on the final size of metal particles. Therefore, the effect of PdCl2 concentration in the aqueous solution was studied. Palladium catalysts were prepared from PdCl2 solutions of different concentrations in the range from 0.25 to 1.5 mol/dm3 . Other preparation conditions of microemulsions such as surfactant type, its concentration in the solvent and the molar ratio of water to surfactant remained constant. The molar ratio of water to surfactant was 4 to evaluate the influence of PdCl2 concentration on Pd crystallite size in the final catalyst.

The observed Pd crystallite size dependence on the concentration of PdCl2 goes through a minimum as is shown in **Table 1**. When using 1 M solution of PdCl2 for the preparation of the

**Table 1.** Effect of PdCl2 concentration on Pd crystallite size in the resulting catalyst.

microemulsion, the resulting average Pd particle size determined by CO chemisorption was 2.2 nm. When the effect of PdCl2 concentration on the final particle size of Pd in the prepared catalyst was evaluated, an efficiency of Pd loading on the support was studied as well. When using both 1 and 0.5 M solutions, highly dispersed catalysts with Pd particle size of approximately 2.3 nm were obtained. However, when using PdCl2 solution of 1 M concentration, the catalyst with desired Pd content of 3 wt.% was obtained, while at the concentration of 0.5 M, the Pd content was only 2.5 wt.%.

#### *3.1.2. Effect of molar ratio of water to surfactant*

on the shape of metal particles and contact with the support (k = 6); surface density of metal

Temperature-programmed reduction (TPR) was used to describe the reducibility of different active centres and their concentrations. The samples were first dried in air at 393 K for 2 h to constant weight. TPR analysis of the samples was performed using a commercial device Autochem

The metal content was determined directly by X-ray fluorescence (XRF) analysis and indirectly by atomic absorption spectrometry (AAS). The indirect determination of Pd content in the catalyst consisted of a liquid sample analysis, which was prepared by multiple leaching of a defined amount of Pd catalyst using aqua regia. Quantitative transfer of Pd in the solution was indicated by the support discoloration and the last extract from the catalyst was

**3.1. Effect of preparation conditions of microemulsions on the physical properties of**

When the effect of preparation conditions of microemulsions was monitored, attention was paid to two parameters namely the concentration of metal salts in the aqueous solution and

According to literature [7, 8, 16], the concentration of metal component precursor in the form of soluble salts in the aqueous solution has a significant impact on the final size of metal particles.

concentration in the solvent and the molar ratio of water to surfactant remained constant. The

molar ratio of water to surfactant was 4 to evaluate the influence of PdCl2

minimum as is shown in **Table 1**. When using 1 M solution of PdCl2

The observed Pd crystallite size dependence on the concentration of PdCl2

concentration in the aqueous solution was studied. Palladium

. Other preparation conditions of microemulsions such as surfactant type, its

solutions of different concentrations in the range from 0.25

increased linearly with time to a temperature of 923 K with a heating rate of 5 K/min.

colourless as well. Quantitative dissolution of Pd gave PdCl4

) and stoichiometry of adsorption Pd:CO (SAV = 2) [21].

/Ar mixture 5/95. During the analysis, the temperature was

2− solution which was analysed

concentration on Pd

for the preparation of the

goes through a

atoms (CPd = 1.26 × 1019 atom/m2

II 2920 Micromeritics, USA, with H2

*2.3.4. Metal content determination*

by atomic absorption spectroscopy.

**3. Results and discussion**

the molar ratio of water and surfactant.

 *concentration*

**catalysts**

*3.1.1. Effect of PdCl2*

to 1.5 mol/dm3

Therefore, the effect of PdCl2

catalysts were prepared from PdCl2

crystallite size in the final catalyst.

*2.3.3. Reducibility of active centres of catalyst*

256 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

Another parameter influencing the final metal particles is the molar ratio of water and surfactants, the so-called R factor. The amount of surfactant in the microemulsion affects the size and shape of water drops dispersed in an oil phase. Too high a value of R factor can lead to the formation of unstable microemulsion and its breaking. To evaluate the influence of the R factor values, PdCl2 concentration of 1 mol/dm3 was used based on the results shown in **Table 1**.

The values reported in **Table 2** show that the molar ratio of water and the surfactant does not affect the final Pd content loading on the support since the Pd content was close to the desired value of 3 wt.% for all catalysts.

When increasing the surfactant content in microemulsion, Pd dispersion increased. When R factor was equal to 8, the final Pd particle size determined from chemisorption data was 2.3 nm.

**Table 2.** Effect of molar ratio of water to surfactant on Pd crystallite size in the resulting catalyst.

On further lowering the R factor, the particle size remained almost unchanged. Therefore, for the further preparation of catalyst, the value of R = 4 was used. This result is in compliance with the conclusions mentioned by Perez-Coronado [14] and Kim et al. [16], who minimized the Pd particle size in a similar system (Pd/ZrO2 ) using R factor equal to 4.4.

#### **3.2. Controlled sintering of metal component**

The process of calcination is necessary for removing the surfactant after catalyst preparation using the microemulsion technique. Calcination is also responsible for the decomposition of metal precursors to the formation of oxides; therefore, calcination is carried out in air atmosphere at high temperatures. Calcination at various high temperatures can affect the resulting metal particle size in the prepared catalyst because of the sintering of the particles. Recently it has been reported by Forman et al. [22] that silica-encapsulated Pd nanoparticles prepared from microemulsion showed more sintering-resistant behaviour compared to the particles prepared by the impregnation method.

Pd catalyst prepared from microemulsion and subsequently calcined at 723 K showed high dispersion with a narrow size distribution of Pd crystallites since almost 80% of Pd particles were in the range 1–3 nm based on HRTEM measurements (**Figures 1** and **2**).

P0 [ME] catalyst was divided into five parts (**Table 3**), which were treated in the range from 773 to 1223 K in an inert atmosphere for 2 h. In this temperature range, no irreversible changes of the catalyst support were observed. BET surface areas of 3% Pd/Al2 O3 catalysts calcined at 723 and 1223 K were 274 and 264 m2 /g, respectively.

Results obtained from both chemisorption measurements (**Table 3**) and HRTEM analyses (**Figure 2**) for very small metal particles (in units of nm) showed high degree of sintering at a temperature of 923 K. However, when using temperatures higher than 1023 K, further significant decrease of Pd specific surface area (SPd) did not occur. SPd values of the catalysts treated

 **[nm] SPd [m2**

Preparation of Highly Dispersed Pd-Supported Catalysts for Hydrogenation Processes…

http://dx.doi.org/10.5772/intechopen.72986

259

**/gPd]**

**Figure 2.** Histograms of Pd particle sizes determined by HRTEM in catalysts (P0 [ME] and P5 [ME]) prepared by the

at temperatures 1023, 1123 and 1223 K were almost constant (**Table 3**).

P0 [ME] 723 2.2 169.6 P1 [ME] 773 5.7 70.1 P2 [ME] 923 16.3 25.6 P3 [ME] 1023 27.6 14.4 P4 [ME] 1123 31.3 12.7 P5 [ME] 1223 32.0 12.4

**Table 3.** Effect of calcination temperature on Pd crystallite size in the resulting catalyst.

**Catalyst T calcination [K] dAV (Pd)a**

Determined from chemisorption measurements.

microemulsion technique.

a

**Figure 1.** HRTEM micrographs of highly dispersed catalysts P0 [ME] and P5 [ME] (bar length = 100 nm).

Preparation of Highly Dispersed Pd-Supported Catalysts for Hydrogenation Processes… http://dx.doi.org/10.5772/intechopen.72986 259

On further lowering the R factor, the particle size remained almost unchanged. Therefore, for the further preparation of catalyst, the value of R = 4 was used. This result is in compliance with the conclusions mentioned by Perez-Coronado [14] and Kim et al. [16], who minimized

The process of calcination is necessary for removing the surfactant after catalyst preparation using the microemulsion technique. Calcination is also responsible for the decomposition of metal precursors to the formation of oxides; therefore, calcination is carried out in air atmosphere at high temperatures. Calcination at various high temperatures can affect the resulting metal particle size in the prepared catalyst because of the sintering of the particles. Recently it has been reported by Forman et al. [22] that silica-encapsulated Pd nanoparticles prepared from microemulsion showed more sintering-resistant behaviour compared to the particles

Pd catalyst prepared from microemulsion and subsequently calcined at 723 K showed high dispersion with a narrow size distribution of Pd crystallites since almost 80% of Pd particles

P0 [ME] catalyst was divided into five parts (**Table 3**), which were treated in the range from 773 to 1223 K in an inert atmosphere for 2 h. In this temperature range, no irreversible changes

/g, respectively.

**Figure 1.** HRTEM micrographs of highly dispersed catalysts P0 [ME] and P5 [ME] (bar length = 100 nm).

were in the range 1–3 nm based on HRTEM measurements (**Figures 1** and **2**).

of the catalyst support were observed. BET surface areas of 3% Pd/Al2

) using R factor equal to 4.4.

O3

catalysts calcined at

the Pd particle size in a similar system (Pd/ZrO2

258 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

**3.2. Controlled sintering of metal component**

prepared by the impregnation method.

723 and 1223 K were 274 and 264 m2

**Figure 2.** Histograms of Pd particle sizes determined by HRTEM in catalysts (P0 [ME] and P5 [ME]) prepared by the microemulsion technique.

Results obtained from both chemisorption measurements (**Table 3**) and HRTEM analyses (**Figure 2**) for very small metal particles (in units of nm) showed high degree of sintering at a temperature of 923 K. However, when using temperatures higher than 1023 K, further significant decrease of Pd specific surface area (SPd) did not occur. SPd values of the catalysts treated at temperatures 1023, 1123 and 1223 K were almost constant (**Table 3**).

**Table 3.** Effect of calcination temperature on Pd crystallite size in the resulting catalyst.

Pd catalyst prepared from microemulsion and subsequently calcined at 723 K showed high dispersion with a narrow size distribution of Pd crystallites since almost 80% of Pd particles were in the range 1–3 nm based on HRTEM measurements (**Figures 1** and **2**).

The results obtained from HRTEM analyses confirmed that Pd particle size after reaching a certain size did not increase. The largest Pd crystallites found in the HRTEM micrographs had the size of approximately 80 nm. At the Pd content of 3 wt.%, these crystallites were large enough to be separated from each other on the support. The further sintering of Pd particles could occur only with difficulty.

Catalysts treated at higher temperatures compared to P0 [ME] catalyst showed a wider size distribution (**Figure 2**). For example, the catalyst sintered at 1223 K contained 70% of Pd particles in the range 15–40 nm. The average particle size determined by CO chemisorption was 32 nm. Controlled sintering at high temperatures does not lead to a catalyst with a narrow particle size distribution. Sintering is very fast probably due to the high metal content in the catalyst. To prepare a series of the catalysts with different Pd dispersion, but with the narrow particle size distribution, it is necessary to choose a different method than controlled sintering, e.g. to control the particle size with microemulsion preparation conditions. It would be necessary to optimize several parameters so that the catalysts prepared under different conditions showed, besides the Pd dispersion, the same parameters such as content of metal and impurities.

explained by recrystallization of Pd particles, their diffusion on the support and a formation of larger clusters. The formation of larger Pd crystallites during reduction in the liquid phase

**/gPd] dAV (Pd)c**

Preparation of Highly Dispersed Pd-Supported Catalysts for Hydrogenation Processes…

 **[nm]**

http://dx.doi.org/10.5772/intechopen.72986

261

This observation is inconsistent with the commonly known theory, according to which the reduction performed in the liquid phase should be the more efficient way to keep high disper-

Method of incipient wetness impregnation of the support with aqueous solutions of salts and acids is a basic procedure for the preparation of heterogeneous catalysts. This procedure was used to compare such prepared catalyst with that prepared from the microemulsion. The amount of impregnation solution was chosen to keep the same Pd content and concentration in the resulting suspension with the support as in the preparation of A [ME] catalyst using the microemulsion. The other preparation conditions namely impregnation time, calcination temperature and reduction in the gas phase have been retained as well. The Pd particle size in

The determined Pd content in such prepared catalyst (I [IMP]) was only 1.5 wt.% (**Table 5**). Therefore, for further comparison, the catalyst with two-times higher Pd content (J [IMP]) in the impregnation solution was prepared. Increasing Pd content on the support led to the increase of Pd crystallite size and the decrease in the specific surface area of Pd in the catalyst. J [IMP] catalyst of comparable Pd content, prepared by incipient wetness impregnation, contained Pd particles of average size 5.6 nm. Obtained results therefore indicate that the microemulsion preparation technique leads to higher dispersion of Pd than wet impregnation method.

**/gPd] dAV (Pd)a**

 **[nm]**

was confirmed by dAV (Pd) determination from HRTEM measurements (**Table 4**).

**3.4. Preparation of catalysts by incipient wetness impregnation**

(g) 169.6 2.2

(l)a 65.8 —

(l)b 70.6 5.8

**Table 4.** Effect of reducing agent on Pd specific surface area and on Pd crystallite size.

Drying of the catalyst sample at 393 K after reduction in the liquid phase.

Drying of the catalyst sample at 623 K after reduction in the liquid phase.

the resulting catalyst was determined by CO chemisorption.

**Catalyst wPd [%] SPd [m2**

I [IMP] 1.5 79.2 4.7 J [IMP] 2.7 66.8 5.6

**Table 5.** Properties of catalysts prepared by incipient wetness impregnation of support.

sion of the metal on the support.

**Reducing agent SPd [m2**

H2

N2 H4

N2 H4

a

b

c

a

Determined by HRTEM.

Determined by HRTEM.

#### **3.3. Effect of catalysts' reduction on Pd particle size**

Reduction of metal precursors of the catalysts can be performed in several ways. The most common is reduction in the gas phase using hydrogen or its mixture with an inert gas as a reducing agent. Another way is reduction in the liquid phase using hydrazine or formaldehyde.

To determine the temperature needed to reduce PdO, produced during calcination of the metal precursors in air atmosphere at high temperatures, TPR was used. On the TPR record of the catalyst, two reduction maxima were observed in the temperature range from 578 to 663 K. Therefore for the reduction of Pd/Al2 O3 catalyst in the gas phase, the temperature of 723 K was chosen. At the temperature of approximately 363 K, a negative peak was clearly visible. It represented a decomposition of β-hydride of Pd described in literature [20] which is formed due to contact of Pd with hydrogen.

The dispersion of Pd in the catalysts reduced in the gas and the liquid phase was determined from the CO chemisorption measurements of Pd specific surface area. It was found that the catalyst reduced in the liquid phase showed lower SPd value determined by CO chemisorption. It was assumed that this difference can be caused by the blocking of active sites for CO adsorption by unreacted hydrazine. This was denied by experiment (*b* in **Table 4**) using the higher drying temperature for desorption of impurities in an inert atmosphere (623 K). SPd value of that catalyst was close to the previous experiment (*a* in **Table 4**). Since hydrazine reduction was performed at 363 K, no sintering of Pd crystallites can occur. Also, desorption at 623 K should not cause the sintering of Pd crystallites because reduction in the gas phase was carried out at 723 K and this phenomenon was not observed. The lower values of SPd can be

a Drying of the catalyst sample at 393 K after reduction in the liquid phase. b Drying of the catalyst sample at 623 K after reduction in the liquid phase. c Determined by HRTEM.

Pd catalyst prepared from microemulsion and subsequently calcined at 723 K showed high dispersion with a narrow size distribution of Pd crystallites since almost 80% of Pd particles

The results obtained from HRTEM analyses confirmed that Pd particle size after reaching a certain size did not increase. The largest Pd crystallites found in the HRTEM micrographs had the size of approximately 80 nm. At the Pd content of 3 wt.%, these crystallites were large enough to be separated from each other on the support. The further sintering of Pd particles

Catalysts treated at higher temperatures compared to P0 [ME] catalyst showed a wider size distribution (**Figure 2**). For example, the catalyst sintered at 1223 K contained 70% of Pd particles in the range 15–40 nm. The average particle size determined by CO chemisorption was 32 nm. Controlled sintering at high temperatures does not lead to a catalyst with a narrow particle size distribution. Sintering is very fast probably due to the high metal content in the catalyst. To prepare a series of the catalysts with different Pd dispersion, but with the narrow particle size distribution, it is necessary to choose a different method than controlled sintering, e.g. to control the particle size with microemulsion preparation conditions. It would be necessary to optimize several parameters so that the catalysts prepared under different conditions showed, besides the Pd dispersion, the same parameters such as content of metal

Reduction of metal precursors of the catalysts can be performed in several ways. The most common is reduction in the gas phase using hydrogen or its mixture with an inert gas as a reducing agent. Another way is reduction in the liquid phase using hydrazine or formaldehyde.

To determine the temperature needed to reduce PdO, produced during calcination of the metal precursors in air atmosphere at high temperatures, TPR was used. On the TPR record of the catalyst, two reduction maxima were observed in the temperature range from 578 to

O3

723 K was chosen. At the temperature of approximately 363 K, a negative peak was clearly visible. It represented a decomposition of β-hydride of Pd described in literature [20] which is

The dispersion of Pd in the catalysts reduced in the gas and the liquid phase was determined from the CO chemisorption measurements of Pd specific surface area. It was found that the catalyst reduced in the liquid phase showed lower SPd value determined by CO chemisorption. It was assumed that this difference can be caused by the blocking of active sites for CO adsorption by unreacted hydrazine. This was denied by experiment (*b* in **Table 4**) using the higher drying temperature for desorption of impurities in an inert atmosphere (623 K). SPd value of that catalyst was close to the previous experiment (*a* in **Table 4**). Since hydrazine reduction was performed at 363 K, no sintering of Pd crystallites can occur. Also, desorption at 623 K should not cause the sintering of Pd crystallites because reduction in the gas phase was carried out at 723 K and this phenomenon was not observed. The lower values of SPd can be

catalyst in the gas phase, the temperature of

were in the range 1–3 nm based on HRTEM measurements (**Figures 1** and **2**).

260 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

could occur only with difficulty.

**3.3. Effect of catalysts' reduction on Pd particle size**

663 K. Therefore for the reduction of Pd/Al2

formed due to contact of Pd with hydrogen.

and impurities.

**Table 4.** Effect of reducing agent on Pd specific surface area and on Pd crystallite size.

explained by recrystallization of Pd particles, their diffusion on the support and a formation of larger clusters. The formation of larger Pd crystallites during reduction in the liquid phase was confirmed by dAV (Pd) determination from HRTEM measurements (**Table 4**).

This observation is inconsistent with the commonly known theory, according to which the reduction performed in the liquid phase should be the more efficient way to keep high dispersion of the metal on the support.

#### **3.4. Preparation of catalysts by incipient wetness impregnation**

Method of incipient wetness impregnation of the support with aqueous solutions of salts and acids is a basic procedure for the preparation of heterogeneous catalysts. This procedure was used to compare such prepared catalyst with that prepared from the microemulsion. The amount of impregnation solution was chosen to keep the same Pd content and concentration in the resulting suspension with the support as in the preparation of A [ME] catalyst using the microemulsion. The other preparation conditions namely impregnation time, calcination temperature and reduction in the gas phase have been retained as well. The Pd particle size in the resulting catalyst was determined by CO chemisorption.

The determined Pd content in such prepared catalyst (I [IMP]) was only 1.5 wt.% (**Table 5**). Therefore, for further comparison, the catalyst with two-times higher Pd content (J [IMP]) in the impregnation solution was prepared. Increasing Pd content on the support led to the increase of Pd crystallite size and the decrease in the specific surface area of Pd in the catalyst. J [IMP] catalyst of comparable Pd content, prepared by incipient wetness impregnation, contained Pd particles of average size 5.6 nm. Obtained results therefore indicate that the microemulsion preparation technique leads to higher dispersion of Pd than wet impregnation method.

**Table 5.** Properties of catalysts prepared by incipient wetness impregnation of support.

above 773 K. Catalysts calcined at higher temperatures in the range from 1023 to 1223 K showed almost the constant value of the dAV (Pd) ~ 30 nm. Pd particle size after reaching a

Preparation of Highly Dispersed Pd-Supported Catalysts for Hydrogenation Processes…

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263

Furthermore, it was found that the method of reduction has a significant effect on the resulting Pd dispersion. The average particle size of palladium in catalysts reduced in the gas phase using hydrogen was lower than the one reduced in the liquid phase using hydrazine mono-

This work was realized within the Operational Programme Prague—Competitiveness (CZ.2.16/3.1.00/24501) and "National Program of Sustainability" (NPU I LO1613) MSMT-43

and Květa Jirátová2

2 Institute of Chemical Process Fundamentals of the CAS, v. v. i., Prague, Czech Republic

[1] Rymeš J, Ehret G, Hilaire L, Boutonnet M, Jirátová K. Microemulsions in the preparation of highly active combustion catalysts. Catalysis Today. 2002;**75**:297-303. DOI: 10.1016/

[2] Radic N, Grbic B, Jovanovič Ž, Terlecki-Baričevič A, Arsinjevič Z. Microemulsions in the preparation of highly active combustion catalysts. In: Proceedings of the Seventh Congress

[3] Hegedus LS, editor. Catalyst Design: Progress and Perspectives. New York: Wiley; 1987.

[4] Bacik DB, Zhang M, Zhao D, Roberts CB, Seehra MS, Singh V, Shah N. Synthesis and characterization of supported polysugar-stabilized Pd nanoparticle catalysts for enhanced hydro-dechlorination of trichloroethylene. Nanotechnology. 2012;**23**:294004.

[5] Schulman JH, Friend JA. Light scattering investigation of the structure of transparent oil-water disperse systems. II. Journal of Colloid Science. 1949;**4**:497-509. DOI:

certain value did not further increase. The largest particles were up to 80 nm.

hydrate. These results were confirmed by HRTEM measurements.

\*, Jiří Krupka1

1 Department of Organic Technology, UCT Prague, Prague, Czech Republic

\*Address all correspondence to<EMAIL_ADDRESS>
**Acknowledgements**

760/2015.

Jan Patera1

**References**

288 p

**Author details**

, Iva Paterová1

S0920-5861(02)00082-2

on Catalysis; Varna; 1996. p. 377

DOI: 10.1088/0957-4484/23/29/294004

10.1016/0095-8522(49)90046-X

**Figure 3.** Histogram of Pd particle sizes determined by HRTEM in I [IMP] catalyst.

Based on HRTEM measurements it was found that the Pd particle size distribution of I [IMP] catalyst was broader in comparison with P0 [ME] catalyst (**Figures 2** and **3**). The highest frequency of particles was in the range 2–6 nm. However, I [IMP] catalyst contained particles larger than 10 nm unlike P0 [ME] catalyst.

#### **4. Conclusions**

Experimental results presented here show that the Pd/Al2 O3 catalysts prepared from the microemulsions contained much smaller Pd particles and had narrower distribution of Pd particle size than the catalysts prepared by classical incipient wetness impregnation of the supports with water solutions of palladium chloride.

Study of the effect of the preparation conditions of the microemulsions was focused on the concentration of PdCl2 in an aqueous solution and the molar ratio of water and surfactant. It was found that the optimal PdCl2 concentration was equal to 1 mol/dm3 . Using this concentration, catalysts with the highest Pd content and high Pd dispersion were obtained. The average particle size in such catalysts was 2.2 nm. Another condition of the microemulsion preparation technique influencing the final size of metal particles was the molar ratio of water and surfactant. It was found that with decreasing ratio of water and surfactant, Pd dispersion on the support increased. The optimal value of R factor equal to 4 for the microemulsion system with used surfactants was found. Thus, the prepared catalyst showed a narrow particle size distribution, containing 80% Pd crystallites in the range 1–3 nm.

Using controlled sintering at high temperatures the catalyst with the narrow particle size distribution could not be obtained. Pd particle sintering proceeded very fast at temperatures above 773 K. Catalysts calcined at higher temperatures in the range from 1023 to 1223 K showed almost the constant value of the dAV (Pd) ~ 30 nm. Pd particle size after reaching a certain value did not further increase. The largest particles were up to 80 nm.

Furthermore, it was found that the method of reduction has a significant effect on the resulting Pd dispersion. The average particle size of palladium in catalysts reduced in the gas phase using hydrogen was lower than the one reduced in the liquid phase using hydrazine monohydrate. These results were confirmed by HRTEM measurements.

#### **Acknowledgements**

This work was realized within the Operational Programme Prague—Competitiveness (CZ.2.16/3.1.00/24501) and "National Program of Sustainability" (NPU I LO1613) MSMT-43 760/2015.

#### **Author details**

**Figure 3.** Histogram of Pd particle sizes determined by HRTEM in I [IMP] catalyst.

262 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

Experimental results presented here show that the Pd/Al2

distribution, containing 80% Pd crystallites in the range 1–3 nm.

supports with water solutions of palladium chloride.

larger than 10 nm unlike P0 [ME] catalyst.

**4. Conclusions**

concentration of PdCl2

was found that the optimal PdCl2

Based on HRTEM measurements it was found that the Pd particle size distribution of I [IMP] catalyst was broader in comparison with P0 [ME] catalyst (**Figures 2** and **3**). The highest frequency of particles was in the range 2–6 nm. However, I [IMP] catalyst contained particles

microemulsions contained much smaller Pd particles and had narrower distribution of Pd particle size than the catalysts prepared by classical incipient wetness impregnation of the

Study of the effect of the preparation conditions of the microemulsions was focused on the

tion, catalysts with the highest Pd content and high Pd dispersion were obtained. The average particle size in such catalysts was 2.2 nm. Another condition of the microemulsion preparation technique influencing the final size of metal particles was the molar ratio of water and surfactant. It was found that with decreasing ratio of water and surfactant, Pd dispersion on the support increased. The optimal value of R factor equal to 4 for the microemulsion system with used surfactants was found. Thus, the prepared catalyst showed a narrow particle size

Using controlled sintering at high temperatures the catalyst with the narrow particle size distribution could not be obtained. Pd particle sintering proceeded very fast at temperatures

concentration was equal to 1 mol/dm3

O3

in an aqueous solution and the molar ratio of water and surfactant. It

catalysts prepared from the

. Using this concentra-

Jan Patera1 , Iva Paterová1 \*, Jiří Krupka1 and Květa Jirátová2

\*Address all correspondence to<EMAIL_ADDRESS>
1 Department of Organic Technology, UCT Prague, Prague, Czech Republic

2 Institute of Chemical Process Fundamentals of the CAS, v. v. i., Prague, Czech Republic

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**Chapter 12**

**Provisional chapter**

**Symbiosis in Plasmonic Nanoparticles**

**Symbiosis in Plasmonic Nanoparticles**

DOI: 10.5772/intechopen.71730

The focus of this chapter is on how the symbiotic relationship existing in nature can easily be translated to the nanoscale systems, particularly in plasmonic nanoparticles. Here, we discuss the synthesis and properties of bimetallic nanoparticles, consisting of plasmonic silver (Ag) with ferromagnetic cobalt (Co). The symbiotic properties in the Co-Ag bimetallic plasmonic nanoparticles are discussed in the chapter where Ag and Co are the beneficiary elements due to the presence of each other. These bimetallic plasmonic nanomaterials demonstrate multi-functionalities which are not just limited to well-known bio-sensing or magneto-optical effects but also expand to highly unexpected and exotic properties such as extreme oxidation resistance, ferroplasmons, improved quality factor,

**Keywords:** plasmonics, ferromagnetic, bimetallic nanoparticles, symbiosis,

environment in which the ecosystem and its evolution are symbiotic [2].

ferroplasmon, quantum efficiency, quality factor, EELS, galvanic coupling, nanosphere

Symbiosis is a natural phenomenon where the co-existence of two entities mutually benefits their survival. Symbiosis is the key ingredient for the existence of life on this planet whether on the surface or under the water. Some of the most common and sought out symbiotic pairs on land are honey bee and flower, birds/mammals and fruit of plants, and in the oceans are corals and algae or clown fish and sea anemones [1]. Even the plants coexist with the natural

This behavior, when observed at the nanoscale, is expected to lead to many interesting phenomena in the field of nanomaterials. For symbiosis to occur at the nanoscale, multi-metallic nanoparticle systems are required where individual components can mutually benefit

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Abhinav Malasi and Ritesh Sachan

Abhinav Malasi and Ritesh Sachan

http://dx.doi.org/10.5772/intechopen.71730

**Abstract**

Additional information is available at the end of the chapter

and tunable radiative quantum efficiency.

lithography, dewetting

**1. Introduction**

Additional information is available at the end of the chapter

**Provisional chapter**

## **Symbiosis in Plasmonic Nanoparticles**

**Symbiosis in Plasmonic Nanoparticles**

#### Abhinav Malasi and Ritesh Sachan Abhinav Malasi and Ritesh Sachan Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.71730

#### **Abstract**

The focus of this chapter is on how the symbiotic relationship existing in nature can easily be translated to the nanoscale systems, particularly in plasmonic nanoparticles. Here, we discuss the synthesis and properties of bimetallic nanoparticles, consisting of plasmonic silver (Ag) with ferromagnetic cobalt (Co). The symbiotic properties in the Co-Ag bimetallic plasmonic nanoparticles are discussed in the chapter where Ag and Co are the beneficiary elements due to the presence of each other. These bimetallic plasmonic nanomaterials demonstrate multi-functionalities which are not just limited to well-known bio-sensing or magneto-optical effects but also expand to highly unexpected and exotic properties such as extreme oxidation resistance, ferroplasmons, improved quality factor, and tunable radiative quantum efficiency.

DOI: 10.5772/intechopen.71730

**Keywords:** plasmonics, ferromagnetic, bimetallic nanoparticles, symbiosis, ferroplasmon, quantum efficiency, quality factor, EELS, galvanic coupling, nanosphere lithography, dewetting

#### **1. Introduction**

Symbiosis is a natural phenomenon where the co-existence of two entities mutually benefits their survival. Symbiosis is the key ingredient for the existence of life on this planet whether on the surface or under the water. Some of the most common and sought out symbiotic pairs on land are honey bee and flower, birds/mammals and fruit of plants, and in the oceans are corals and algae or clown fish and sea anemones [1]. Even the plants coexist with the natural environment in which the ecosystem and its evolution are symbiotic [2].

This behavior, when observed at the nanoscale, is expected to lead to many interesting phenomena in the field of nanomaterials. For symbiosis to occur at the nanoscale, multi-metallic nanoparticle systems are required where individual components can mutually benefit

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

and introduce multi-functionalities. In the field of nanomaterials, there is a deliberate drive towards synthesizing materials with multiple functionalities in a single nanostructure and the most simplistic system is bimetallic nanoparticles. The studies on bimetallic nanoparticle systems have shown improved magneto-plasmonic response, enhanced catalytic activities, efficient energy conversion, tunable plasmonic properties, leading to phenomenon like plasmon hybridization and Fano resonances (the interference between narrow bandwidth plasmons and the broader plasmons/continuum) to name a few [3–7]. These properties exhibited by multi-metallic nanomaterial system and has led to the seminal research on the symbiosis in bimetallic particles at the nanoscale. The focus of this book chapter is on the symbiotic properties of bimetallic nanoparticles, particularly those of silver (Ag), an excellent plasmonic material, and cobalt (Co), a ferromagnetic material. These material systems find applications in sensing of biological and chemical agents, catalysis, energy harvesting, data storage, and nanoelectronics.

to the breakage of the film to more stable state in the form of droplets. Some of the most common examples of spinodal dewetting are the hydrophobic leaf surfaces, oil droplet formation on the water surface, the windshield of vehicles after rain etc. This technique has been used to

The SEM in **Figure 1(a)** shows the typical Co-Ag nanoparticle arrays of dewetted metal film. The inset of the figure depicts the mono-modal size distribution. The average nanoparticle size and spacing of the dewetted nanoparticles depends on the initial film thickness. This gives the control to precisely synthesize the desired size of the nanoparticles. The nanoparticle spacing and size cannot be varied independently and are dependent on each other because of material volume conservation [11, 12]. This suggests that by independently controlling the film thickness of individual metals in a bilayer system, a wide size range can be achieved. Using the theoretical relation between the film thickness and the nanoparticle size, a parameter space of nanoparticle size as a function of Ag volume percentage is plotted for total thickness varying

and Ag/Co/SiO2

interesting to note here that a wide array of nanoparticles of different sizes and composition can be synthesized using this technique. This technique also gives the flexibility to vary the nanoparticle size by keeping the Ag volume fixed or varying the Ag volume amount by keep-

The drawback of dewetting is the lack of independent control of either of the nanoparticle size or spacing which restricts its use for synthesizing nanoparticles within the interacting regime (interparticle spacing ≤ nanoparticle size). If this barrier could be broken, this technique would be highly useful for making large 2D nanoparticles arrays beneficial for plasmonic

**Figure 1.** (a) SEM image of Co-Ag nanoparticle with a monomodal size distribution of 113 ± 30 nm. The inset shows the size distribution of the nanoparticles. (b) Theoretically generated nanoparticle size parameter space as a function

and Ag/Co/SiO2

between 2 and 20 nm. (a) from [13] is used in accordance with the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/). (b) is reproduced from Ref. [12]. © IOP Publishing. Reproduced with

regions as shown in **Figure 1(b)**. It is

Symbiosis in Plasmonic Nanoparticles http://dx.doi.org/10.5772/intechopen.71730 269

with the total film thickness of the system varied

synthesize a wide variety of nanoparticles of different sizes, shapes, and materials.

in the range 2–20 nm for Co/Ag/SiO2

of Ag amount for the configurations Co/Ag/SiO2

permission. All rights reserved.

ing the size fixed.

The contents of the book chapter are organized as follows: Section 2 covers the synthesis techniques involved in the fabrication of bimetallic Co-Ag nanoparticle arrays; Section 3 focuses on the basic characterization techniques that were involved in various studies discussed in Section 4. In Section 4, a detailed discussion on the various symbiotic properties is presented followed by a discussion on other existing symbiotic systems in Section 5 and concluding remarks in Section 6.

#### **2. Synthesis of bimetallic nanoparticles**

The synthesis of bimetallic nanoparticles has been reported by both physical and chemical routes using the top-down as well as bottom-up approaches. In the view of this focused chapter, the techniques using physical vapor deposition of metal thin films are specifically emphasized. The two discussed techniques viz. (i) pulsed laser induced dewetting (PLiD) and (ii) colloidal lithography, are adopted due to their merits of being robust, time efficient and comparatively cost efficient as compared to other methods incorporating vapor deposition systems.

#### **2.1. Pulsed laser induced dewetting (PLiD)**

The nanoparticles synthesized by PLiD fall in the category of self-organization. Selforganization is a phenomenon in which the intrinsic force plays a prominent role in arranging the patterns with definite length scale and shape, thus stabilizing the system. Some of the common examples of self-organization are skin pigmentation in animals and solar system. At the nanoscale, examples of self-organization can be seen in protein folding self-assembly, the formation of micelles and droplet formation from the breakage of thin polymer and metal films. The phenomenon of breaking of thin metal or polymer films into droplets is called spinodal dewetting which leads to characteristic size and spacing [8–10]. Spinodal dewetting works on the principle of thermodynamic instabilities occurring due to the mismatch in the surface energy of a thin liquid metal film on the non-wetting substrate surface, which leads to the breakage of the film to more stable state in the form of droplets. Some of the most common examples of spinodal dewetting are the hydrophobic leaf surfaces, oil droplet formation on the water surface, the windshield of vehicles after rain etc. This technique has been used to synthesize a wide variety of nanoparticles of different sizes, shapes, and materials.

and introduce multi-functionalities. In the field of nanomaterials, there is a deliberate drive towards synthesizing materials with multiple functionalities in a single nanostructure and the most simplistic system is bimetallic nanoparticles. The studies on bimetallic nanoparticle systems have shown improved magneto-plasmonic response, enhanced catalytic activities, efficient energy conversion, tunable plasmonic properties, leading to phenomenon like plasmon hybridization and Fano resonances (the interference between narrow bandwidth plasmons and the broader plasmons/continuum) to name a few [3–7]. These properties exhibited by multi-metallic nanomaterial system and has led to the seminal research on the symbiosis in bimetallic particles at the nanoscale. The focus of this book chapter is on the symbiotic properties of bimetallic nanoparticles, particularly those of silver (Ag), an excellent plasmonic material, and cobalt (Co), a ferromagnetic material. These material systems find applications in sensing of biological and chemical agents, catalysis, energy harvesting, data storage, and

The contents of the book chapter are organized as follows: Section 2 covers the synthesis techniques involved in the fabrication of bimetallic Co-Ag nanoparticle arrays; Section 3 focuses on the basic characterization techniques that were involved in various studies discussed in Section 4. In Section 4, a detailed discussion on the various symbiotic properties is presented followed by a discussion on other existing symbiotic systems in Section 5 and concluding

The synthesis of bimetallic nanoparticles has been reported by both physical and chemical routes using the top-down as well as bottom-up approaches. In the view of this focused chapter, the techniques using physical vapor deposition of metal thin films are specifically emphasized. The two discussed techniques viz. (i) pulsed laser induced dewetting (PLiD) and (ii) colloidal lithography, are adopted due to their merits of being robust, time efficient and comparatively cost efficient as compared to other methods incorporating vapor deposition

The nanoparticles synthesized by PLiD fall in the category of self-organization. Selforganization is a phenomenon in which the intrinsic force plays a prominent role in arranging the patterns with definite length scale and shape, thus stabilizing the system. Some of the common examples of self-organization are skin pigmentation in animals and solar system. At the nanoscale, examples of self-organization can be seen in protein folding self-assembly, the formation of micelles and droplet formation from the breakage of thin polymer and metal films. The phenomenon of breaking of thin metal or polymer films into droplets is called spinodal dewetting which leads to characteristic size and spacing [8–10]. Spinodal dewetting works on the principle of thermodynamic instabilities occurring due to the mismatch in the surface energy of a thin liquid metal film on the non-wetting substrate surface, which leads

nanoelectronics.

remarks in Section 6.

systems.

**2. Synthesis of bimetallic nanoparticles**

268 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

**2.1. Pulsed laser induced dewetting (PLiD)**

The SEM in **Figure 1(a)** shows the typical Co-Ag nanoparticle arrays of dewetted metal film. The inset of the figure depicts the mono-modal size distribution. The average nanoparticle size and spacing of the dewetted nanoparticles depends on the initial film thickness. This gives the control to precisely synthesize the desired size of the nanoparticles. The nanoparticle spacing and size cannot be varied independently and are dependent on each other because of material volume conservation [11, 12]. This suggests that by independently controlling the film thickness of individual metals in a bilayer system, a wide size range can be achieved. Using the theoretical relation between the film thickness and the nanoparticle size, a parameter space of nanoparticle size as a function of Ag volume percentage is plotted for total thickness varying in the range 2–20 nm for Co/Ag/SiO2 and Ag/Co/SiO2 regions as shown in **Figure 1(b)**. It is interesting to note here that a wide array of nanoparticles of different sizes and composition can be synthesized using this technique. This technique also gives the flexibility to vary the nanoparticle size by keeping the Ag volume fixed or varying the Ag volume amount by keeping the size fixed.

The drawback of dewetting is the lack of independent control of either of the nanoparticle size or spacing which restricts its use for synthesizing nanoparticles within the interacting regime (interparticle spacing ≤ nanoparticle size). If this barrier could be broken, this technique would be highly useful for making large 2D nanoparticles arrays beneficial for plasmonic

**Figure 1.** (a) SEM image of Co-Ag nanoparticle with a monomodal size distribution of 113 ± 30 nm. The inset shows the size distribution of the nanoparticles. (b) Theoretically generated nanoparticle size parameter space as a function of Ag amount for the configurations Co/Ag/SiO2 and Ag/Co/SiO2 with the total film thickness of the system varied between 2 and 20 nm. (a) from [13] is used in accordance with the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/). (b) is reproduced from Ref. [12]. © IOP Publishing. Reproduced with permission. All rights reserved.

related research requiring energy hotspots. Recently, Yadavali and Kalyanaraman reported that the particle spacing of the nanoparticles can be independent of the initial film thickness which is not the case in classical spinodal dewetting by creating Rayleigh-Taylor instabilities at the metal film surface in contact with liquid creating large pressure gradients due to evaporation [14].

#### **2.2. Colloidal lithography**

Lithographic techniques are well suited towards making large area 2D periodic ordered structures. The formation of these periodic structures involves the three basic steps, i.e. mask formation, material deposition, and mask etching. It gives the flexibility to print complex geometries of ordered 2D arrays of nanostructures on large areas. The drawback to most of the lithographic techniques are the cumbersome steps to achieve the final product which is time and resource consuming, and also requiring the use of expensive instruments. Van Duyne and co-workers came out with a fast and inexpensive technique to pattern large area 2D periodic nanostructures by forming masks of self-assembled colloidal nanoparticles called the nanospheres lithography (NSL) [15]. The self-assembly of a monolayer of hexagonally packed colloidal nanoparticles can be either done on hydrophilic substrates or at the air-water interface [15, 16]. Once the mask is ready, the metal is deposited through the gaps between the colloidal beads arrangement. The template is then etched, leaving the hexagonally arranged patterns of metal nano-triangles on the substrate. The in-plane dimensions of the features formed by NSL depend solely on the size of the colloidal nanoparticles [15].

volume ratio of the two metals remained intact after the nanoparticle synthesis. EDX analysis revealed the average Co:Ag X-ray count of the synthesized nanoparticles followed a linear

**Figure 2.** (a) Cross-sectional HAADF image of Ag-Co hemispherical nanoparticle synthesized by PLiD. Core-loss EELS mapping of (b) Ag and (c) Co for the hemispherical nanoparticle. (d) The EELS spectra measured for the nanoparticle measured at three different regions marked in (a). Reproduced from Ref. [12]. © IOP Publishing. Reproduced with

Symbiosis in Plasmonic Nanoparticles http://dx.doi.org/10.5772/intechopen.71730 271

In the present case of characterizing symbiotic bimetallic nanoscale systems, core-loss EELS (electron energy loss spectroscopy) is employed to understand the elemental distribution, inter-mixing or alloying, chemical reactivity and electronic structure in a single nanoparticle. An example is shown in **Figure 2** where the elemental distribution of Ag and Co is mapped using Ag-M45 and Co-L23 edges in bimetallic nanoparticles. A high-angle annular dark field (HAADF) image of Ag-Co bimetallic nanoparticle is shown in **Figure 2(a)**, along with the Ag (**Figure 2(b)**) and Co (**Figure 2(c)**) elemental maps. The representative core-loss EELS edges of Ag and Co obtained from the region (i), (ii) and (iii) regions are shown in **Figure 2(d)** [12]. This method is also used to quantify the surface oxidation in the Ag-Co nanoparticles and to study the oxidation state of formed metal oxides [29]. The information about the optical behavior is

In this section, we discuss the tuning and improvement or enhancement in four different properties caused by the mutual sharing or transfer of free electrons between Ag and Co,

The materials displaying optical properties at the nanoscale are termed as plasmonic materials. Plasmonics can be defined as when the metal nanoparticles interact with light of sizes less than or equivalent to the wavelength, the free electrons start to resonate at a certain frequency called the plasmon resonance frequency. The plasmon resonance is a material property and depends on the shape, size, composition, and environment of the nanoparticle. The other viable option to vary the plasmon resonance is by synthesizing multi-metallic nanoparticle composites. Multi-metallic nanoparticles open the door to incorporate multiple functionalities in a single nanoparticle. Depending on the synthesis approach and the reaction conditions,

relationship with the Co:Ag film thickness ratio [12].

**4. Symbiotic properties**

permission. All rights reserved.

leading to symbiosis at the nanoscale.

**4.1. Tuning the plasmon resonance and its sensitivity**

also obtained using low-loss EELS and will be discussed in Section 4.3.

The biggest challenge using this technique is to avoid the formation of a large number of vacancy and dislocation defects [17]. To tackle the control of defect formation is to control the evaporation rate of the fluid containing the beads [18, 19]. The evaporating fluid forces the beads to move towards the crystallized area due to the internal flow of the fluid. Other ways are to confine the solution and apply mechanical or physical force by gas flow to form ordered arrangement [16, 20, 21].

NSL requires the direction of metal flux being deposited to be perpendicular to the substrate plane. This restricts the control to vary the nanoparticle geometry and is limited to nanotriangles, which are truncated pyramids and can be further tuned to semi-spherical shape [22, 23]. Many variations of colloidal lithography in the form of angle-resolved NSL, shadowsphere lithography, hole mask colloidal lithography, and shrinkable hole mask lithography exists which can create complex geometries like crescents, dimers, pillars, chiral structures etc. [24–27].

#### **3. Characterization**

The as-prepared samples were characterized for their morphology by SEM and optical properties by UV-vis spectroscopy. The statistical information on the average nanoparticle size and distribution were calculated by analyzing the SEM images using image analysis software (ImageJ by NIH) [28]. EDX mapping study on bimetallic Co-Ag nanoparticles revealed the

**Figure 2.** (a) Cross-sectional HAADF image of Ag-Co hemispherical nanoparticle synthesized by PLiD. Core-loss EELS mapping of (b) Ag and (c) Co for the hemispherical nanoparticle. (d) The EELS spectra measured for the nanoparticle measured at three different regions marked in (a). Reproduced from Ref. [12]. © IOP Publishing. Reproduced with permission. All rights reserved.

volume ratio of the two metals remained intact after the nanoparticle synthesis. EDX analysis revealed the average Co:Ag X-ray count of the synthesized nanoparticles followed a linear relationship with the Co:Ag film thickness ratio [12].

In the present case of characterizing symbiotic bimetallic nanoscale systems, core-loss EELS (electron energy loss spectroscopy) is employed to understand the elemental distribution, inter-mixing or alloying, chemical reactivity and electronic structure in a single nanoparticle. An example is shown in **Figure 2** where the elemental distribution of Ag and Co is mapped using Ag-M45 and Co-L23 edges in bimetallic nanoparticles. A high-angle annular dark field (HAADF) image of Ag-Co bimetallic nanoparticle is shown in **Figure 2(a)**, along with the Ag (**Figure 2(b)**) and Co (**Figure 2(c)**) elemental maps. The representative core-loss EELS edges of Ag and Co obtained from the region (i), (ii) and (iii) regions are shown in **Figure 2(d)** [12]. This method is also used to quantify the surface oxidation in the Ag-Co nanoparticles and to study the oxidation state of formed metal oxides [29]. The information about the optical behavior is also obtained using low-loss EELS and will be discussed in Section 4.3.

#### **4. Symbiotic properties**

related research requiring energy hotspots. Recently, Yadavali and Kalyanaraman reported that the particle spacing of the nanoparticles can be independent of the initial film thickness which is not the case in classical spinodal dewetting by creating Rayleigh-Taylor instabilities at the metal film surface in contact with liquid creating large pressure gradients due to

Lithographic techniques are well suited towards making large area 2D periodic ordered structures. The formation of these periodic structures involves the three basic steps, i.e. mask formation, material deposition, and mask etching. It gives the flexibility to print complex geometries of ordered 2D arrays of nanostructures on large areas. The drawback to most of the lithographic techniques are the cumbersome steps to achieve the final product which is time and resource consuming, and also requiring the use of expensive instruments. Van Duyne and co-workers came out with a fast and inexpensive technique to pattern large area 2D periodic nanostructures by forming masks of self-assembled colloidal nanoparticles called the nanospheres lithography (NSL) [15]. The self-assembly of a monolayer of hexagonally packed colloidal nanoparticles can be either done on hydrophilic substrates or at the air-water interface [15, 16]. Once the mask is ready, the metal is deposited through the gaps between the colloidal beads arrangement. The template is then etched, leaving the hexagonally arranged patterns of metal nano-triangles on the substrate. The in-plane dimensions of the features

The biggest challenge using this technique is to avoid the formation of a large number of vacancy and dislocation defects [17]. To tackle the control of defect formation is to control the evaporation rate of the fluid containing the beads [18, 19]. The evaporating fluid forces the beads to move towards the crystallized area due to the internal flow of the fluid. Other ways are to confine the solution and apply mechanical or physical force by gas flow to form ordered

NSL requires the direction of metal flux being deposited to be perpendicular to the substrate plane. This restricts the control to vary the nanoparticle geometry and is limited to nanotriangles, which are truncated pyramids and can be further tuned to semi-spherical shape [22, 23]. Many variations of colloidal lithography in the form of angle-resolved NSL, shadowsphere lithography, hole mask colloidal lithography, and shrinkable hole mask lithography exists which can create complex geometries like crescents, dimers, pillars, chiral structures

The as-prepared samples were characterized for their morphology by SEM and optical properties by UV-vis spectroscopy. The statistical information on the average nanoparticle size and distribution were calculated by analyzing the SEM images using image analysis software (ImageJ by NIH) [28]. EDX mapping study on bimetallic Co-Ag nanoparticles revealed the

formed by NSL depend solely on the size of the colloidal nanoparticles [15].

evaporation [14].

**2.2. Colloidal lithography**

270 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

arrangement [16, 20, 21].

**3. Characterization**

etc. [24–27].

In this section, we discuss the tuning and improvement or enhancement in four different properties caused by the mutual sharing or transfer of free electrons between Ag and Co, leading to symbiosis at the nanoscale.

#### **4.1. Tuning the plasmon resonance and its sensitivity**

The materials displaying optical properties at the nanoscale are termed as plasmonic materials. Plasmonics can be defined as when the metal nanoparticles interact with light of sizes less than or equivalent to the wavelength, the free electrons start to resonate at a certain frequency called the plasmon resonance frequency. The plasmon resonance is a material property and depends on the shape, size, composition, and environment of the nanoparticle. The other viable option to vary the plasmon resonance is by synthesizing multi-metallic nanoparticle composites. Multi-metallic nanoparticles open the door to incorporate multiple functionalities in a single nanoparticle. Depending on the synthesis approach and the reaction conditions, amount of mixing of metals within a nanoparticle can vary from being immiscible like coreshell, to partially miscible, or to completely miscible and also gives advantage on tuning the nanoparticle morphology from spheres to rods and from discs to crescents [30, 31].

second study, nanoparticles were synthesized by dewetting of stacked Co/Ag/SiO2

and the nanoparticle bandwidth increases.

plasmon bandwidth?

**4.2. Oxidation stability**

The optical properties of different sized nanoparticles were studied by varying the thickness of Co film from 1 to 5 nm while keeping the thickness of Ag film fixed at 5 nm. Similarly, pure Ag nanoparticles were synthesized by varying the initial film thickness from 1 to 10 nm. The plasmon resonance energy and plasmon bandwidth for Ag and Co-Ag nanoparticles as a function of nanoparticle volume are plotted in **Figure 3(b)** and **(c)**, respectively. Similarly, on increasing the nanoparticle volume, the plasmon resonance energy decreases or red-shifts

After tuning the plasmon resonance energy, the sensitivity and stability of these nanoparticles are tested. On conducting the refractive index (RI) sensing of the bimetallic nanoparticles, it is observed that for some composition and sizes of Co-Ag nanoparticles, they showed improved RI sensitivity over pure Ag nanoparticles, shown in **Figure 3(d)**. In another study, bimetals of Co-Ag nanoparticles (83.3% Ag) heated to 150° C, showed stable RI sensitivity while Ag nanoparticles for the same starting film thickness are highly unstable as evident from **Figure 3(e)**. The reason for this ultra-stability is discussed in Section 4.2. A general idea about the sensor's performance can also be acquired by calculating its quality factor. The quality factor is a direct measure of the sensitivity of a system to detect, higher the quality factor means more sensitive the system. For a plasmonic sensor, the experimental quality factor can be calculated as *Q* = *Eres*/*Г*, *Eres* being the plasmon resonance energy and *Г* the bandwidth of the plasmon resonance. The quality factor comparison of the samples discussed in second study is plotted as a function of resonance energy in **Figure 3(f)**. The crossover of the quality factor in **Figure 3(f)** is explained by comparing the bandwidths (**Figure 3(b)** and **(c)**) of the two systems for plasmon resonance energies greater than 2.5 eV. At the same plasmon resonance energy, the nanoparticle with higher quality factor means that it has a narrow bandwidth. This leads to the question: what are the different factors contributing to the broadening of

To explain the broadening of the bandwidth, we use the two-level model to explain the results, which is analogous to molecular spectroscopy in which the plasmon decays by dephasing of coherent oscillations (remember the definition of plasmon, which is a coherent oscillation of electrons) [37]. The dephasing occurs as a result of many interactions occurring due to electron-electron, electron-phonon, electron-defect and electron-surface interactions [37, 38]. These events follow Matthiessen's rule as these scattering events are statistically independent channels and thus can be added up [37, 38]. So, the total bandwidth of Co-Ag bimetallic nanoparticles should be the sum of the contributions from individual metals, which clearly is not the case as seen in **Figure 3(f)**. This leads to the possibility of some sort of electronic interaction taking place at the nanoscale between Ag and Co. It also suggests that the sensitivity is

highly dependent on nanoparticle size and composition, as evident from **Figure 3(d)**.

Metal structures at the nanoscale have high reactivity and so are susceptible to oxidation. Because of this reason, Ag which has the best optical properties is not favored over Au [39]. The inertness of Au makes it a more plausible material than Ag which scums to its chemical

system.

273

Symbiosis in Plasmonic Nanoparticles http://dx.doi.org/10.5772/intechopen.71730

Kalyanaraman and co-workers extensively studied the Ag-Co bimetallic system synthesized by PLiD [11, 12, 29, 32–35]. Two separate studies were conducted on these bimetallic nanoparticles, one in which the bimetal composition was fixed while the nanoparticle size was varied. In the other case, the Ag amount was fixed and the Co amount was varied. In the first study, three different compositions of Ag (71.5%, 83.3%, and 100%) in the nanoparticles were studied which were achieved by keeping the starting volume ratio of the metals fixed. It was observed that for fixed composition and increasing nanoparticle size, the plasmon resonance red shifted as shown in **Figure 3(a)**. This observation is explained by using Maxwell-Garnett (MG) approach. The MG approach calculates the effective complex dielectric of a system in which small grains of metal 1 are embedded in metal 2, and is only dependent on the volume fraction and complex dielectric functions of the metals, suggesting for fixed composition, the effective complex dielectric should be the same [36]. Hence an increase in size shifts the plasmon to higher wavelength, consistent with the plasmonic theory. For the

**Figure 3.** (a) Comparison of the localized surface plasmon resonance (LSPR) wavelength for different composition of Ag in a single nanoparticle as a function of three different nanoparticle sizes. (b) and (c) Comparison of the plasmon resonance energy and bandwidth of pure Ag and Co-Ag nanoparticles as a function of total nanoparticle volume, respectively. (d) LSPR positions of three different compositions of Ag measured at three different refractive index liquids. The slopes of the straight-line fits give the refractive index sensitivity of the nanoparticles denoted by *S*. (e) Refractive index sensitivity comparison of Co-Ag and Ag nanoparticles synthesized from 1 nm Co on 5 nm Ag and 5 nm Ag as a function of annealing temperature (modified color scheme). (f) Experimentally measured quality factor comparison of different compositions of Co-Ag nanoparticles and different sizes of Ag nanoparticles as a function of their plasmon resonance energy. (a) and (d) from Ref. [12]. © IOP Publishing. Reproduced with permission. All rights reserved. (b), (c) and (f) from Ref. [13], used in accordance with the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/). (e) reproduced with permission from Ref. [29].

second study, nanoparticles were synthesized by dewetting of stacked Co/Ag/SiO2 system. The optical properties of different sized nanoparticles were studied by varying the thickness of Co film from 1 to 5 nm while keeping the thickness of Ag film fixed at 5 nm. Similarly, pure Ag nanoparticles were synthesized by varying the initial film thickness from 1 to 10 nm. The plasmon resonance energy and plasmon bandwidth for Ag and Co-Ag nanoparticles as a function of nanoparticle volume are plotted in **Figure 3(b)** and **(c)**, respectively. Similarly, on increasing the nanoparticle volume, the plasmon resonance energy decreases or red-shifts and the nanoparticle bandwidth increases.

After tuning the plasmon resonance energy, the sensitivity and stability of these nanoparticles are tested. On conducting the refractive index (RI) sensing of the bimetallic nanoparticles, it is observed that for some composition and sizes of Co-Ag nanoparticles, they showed improved RI sensitivity over pure Ag nanoparticles, shown in **Figure 3(d)**. In another study, bimetals of Co-Ag nanoparticles (83.3% Ag) heated to 150° C, showed stable RI sensitivity while Ag nanoparticles for the same starting film thickness are highly unstable as evident from **Figure 3(e)**. The reason for this ultra-stability is discussed in Section 4.2. A general idea about the sensor's performance can also be acquired by calculating its quality factor. The quality factor is a direct measure of the sensitivity of a system to detect, higher the quality factor means more sensitive the system. For a plasmonic sensor, the experimental quality factor can be calculated as *Q* = *Eres*/*Г*, *Eres* being the plasmon resonance energy and *Г* the bandwidth of the plasmon resonance. The quality factor comparison of the samples discussed in second study is plotted as a function of resonance energy in **Figure 3(f)**. The crossover of the quality factor in **Figure 3(f)** is explained by comparing the bandwidths (**Figure 3(b)** and **(c)**) of the two systems for plasmon resonance energies greater than 2.5 eV. At the same plasmon resonance energy, the nanoparticle with higher quality factor means that it has a narrow bandwidth. This leads to the question: what are the different factors contributing to the broadening of plasmon bandwidth?

To explain the broadening of the bandwidth, we use the two-level model to explain the results, which is analogous to molecular spectroscopy in which the plasmon decays by dephasing of coherent oscillations (remember the definition of plasmon, which is a coherent oscillation of electrons) [37]. The dephasing occurs as a result of many interactions occurring due to electron-electron, electron-phonon, electron-defect and electron-surface interactions [37, 38]. These events follow Matthiessen's rule as these scattering events are statistically independent channels and thus can be added up [37, 38]. So, the total bandwidth of Co-Ag bimetallic nanoparticles should be the sum of the contributions from individual metals, which clearly is not the case as seen in **Figure 3(f)**. This leads to the possibility of some sort of electronic interaction taking place at the nanoscale between Ag and Co. It also suggests that the sensitivity is highly dependent on nanoparticle size and composition, as evident from **Figure 3(d)**.

#### **4.2. Oxidation stability**

amount of mixing of metals within a nanoparticle can vary from being immiscible like coreshell, to partially miscible, or to completely miscible and also gives advantage on tuning the

Kalyanaraman and co-workers extensively studied the Ag-Co bimetallic system synthesized by PLiD [11, 12, 29, 32–35]. Two separate studies were conducted on these bimetallic nanoparticles, one in which the bimetal composition was fixed while the nanoparticle size was varied. In the other case, the Ag amount was fixed and the Co amount was varied. In the first study, three different compositions of Ag (71.5%, 83.3%, and 100%) in the nanoparticles were studied which were achieved by keeping the starting volume ratio of the metals fixed. It was observed that for fixed composition and increasing nanoparticle size, the plasmon resonance red shifted as shown in **Figure 3(a)**. This observation is explained by using Maxwell-Garnett (MG) approach. The MG approach calculates the effective complex dielectric of a system in which small grains of metal 1 are embedded in metal 2, and is only dependent on the volume fraction and complex dielectric functions of the metals, suggesting for fixed composition, the effective complex dielectric should be the same [36]. Hence an increase in size shifts the plasmon to higher wavelength, consistent with the plasmonic theory. For the

**Figure 3.** (a) Comparison of the localized surface plasmon resonance (LSPR) wavelength for different composition of Ag in a single nanoparticle as a function of three different nanoparticle sizes. (b) and (c) Comparison of the plasmon resonance energy and bandwidth of pure Ag and Co-Ag nanoparticles as a function of total nanoparticle volume, respectively. (d) LSPR positions of three different compositions of Ag measured at three different refractive index liquids. The slopes of the straight-line fits give the refractive index sensitivity of the nanoparticles denoted by *S*. (e) Refractive index sensitivity comparison of Co-Ag and Ag nanoparticles synthesized from 1 nm Co on 5 nm Ag and 5 nm Ag as a function of annealing temperature (modified color scheme). (f) Experimentally measured quality factor comparison of different compositions of Co-Ag nanoparticles and different sizes of Ag nanoparticles as a function of their plasmon resonance energy. (a) and (d) from Ref. [12]. © IOP Publishing. Reproduced with permission. All rights reserved. (b), (c) and (f) from Ref. [13], used in accordance with the Creative Commons Attribution (CC BY) license

(https://creativecommons.org/licenses/by/4.0/). (e) reproduced with permission from Ref. [29].

nanoparticle morphology from spheres to rods and from discs to crescents [30, 31].

272 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

Metal structures at the nanoscale have high reactivity and so are susceptible to oxidation. Because of this reason, Ag which has the best optical properties is not favored over Au [39]. The inertness of Au makes it a more plausible material than Ag which scums to its chemical and structural instabilities [40, 41]. Shortage of stable plasmonic materials creates the demand for research on alternate or new stable plasmonic materials required for various applications at the nanoscale [42, 43]. A lot of different solutions or alternative routes have been presented to stabilize metal nanoparticles but they either are short-term solutions or undermine the pure metal properties. The brighter side of these solutions has resulted in finding new and improved properties in some cases. By synthesizing core-shell of Au-Ag, the Ag could be stabilized via electron transfer from Au core, but the nanoparticle properties degrade over time due to the alloying between Au and Ag at the interface [44]. These synthesized core-shell structures at a critical thickness of Ag shell are resilient to harsh chloride ion environment without any visible degradation under transmission electron microscope.

In another study, the Ag nanostructures were stabilized by processing them with low-temperature single hydrogen atoms. The idea was to stabilize nanostructures by filling the interstitial sites with a hydrogen atom to prevent Ag atom diffusion [41]. Covering Ag nanostructures with a monolayer of graphene could protect them from oxidation for almost a period of a month [45]. The other possibility is by surface passivation of Ag nanostructures with organic molecules. The choice of an organic molecule can preferentially control the optical or the electrical properties [46].

Sachan et al. stabilized the Ag nanoparticles by synthesizing bimetals of Ag and Co having segregated regions within a single nanoparticle using PLiD. The oxidation behavior was studied using optical spectroscopy and core-loss EELS mapping and showed prolong optical stability of Ag nanoparticles [12, 29, 34]. This ultra-stability was due to the galvanic coupling between Co and Ag at the nanoscale where Co was acting as a sacrificial anode, thus protecting Ag surface by providing electrons as shown in **Figure 4(a)**. The study was further extended by Malasi et al. to understand the role of Co composition in these bimetals and how the Co-Ag nanoparticle life could be extended [34]. On studying the extended optical bandwidth decay of these nanoparticles, two observations were made. Firstly, the decay behavior of Co and Ag of the bimetals followed two different trends displaying a sharp boundary demarcating the change in trends from inverse logarithmic to exponential decay shown in **Figure 4(b)**. Second observation suggested that the decay trends observed in pure metal oxidation were consistent with the decay of the individual metals of the bimetallic nanoparticle and overlapped perfectly. The change in decay behavior suggested that the two metals have segregated regions, the results of which are consistent with the EELS observation. The only question that remained here to answer was whether the Co oxidized completely or not before the change in the optical bandwidth decay trends. To answer this, a theoretical model is used to quantify the oxidation of Co in the bimetallic nanoparticle system, details of which are available in Ref. [34]. Co-Ag bimetals showed almost 12 times better shelf life than pure Ag nanoparticles for 50% degradation in the optical bandwidth and depending on Co amount, the Ag could be oxide free for almost a year. The lifetimes were established using effective medium theory, which is a versatile way to study nanoparticles in embedded media [47, 48].

on top of Co films showed much improved EOT stability in ambient air as can be visualized from the SEM images shown in **Figure 4(c)–(f)**. The study was carried out for 100 days which showed 1.9 times better plasmon peak stability and 1.7 times better bandwidth stability over pure Ag EOT. The contour maps in **Figure 4(g)** and **(h)** shows the plasmon resonance decay as a function of days. The map shows the change in the behavior of Ag EOT, initially the plasmon significantly red-shifts leading up to day 40 after that the plasmon resonance intensity starts decaying very rapidly in comparison to Ag/Co EOT showing slower red-shift and stable plasmon resonance intensity. The stabilization of bimetal EOT is attributed to the underlying Co layer, which prevents Ag to dewet on the glass substrate. This is evident from the SEM images where material build-up is present for pure Ag EOTs due to dewetting on glass sub-

**Figure 4.** (a) Schematic showing the galvanic coupling in a single bimetallic Co-Ag nanoparticle. (b) Optical bandwidth decay of two different concentrations of Co (solid triangles and hollow squares) in the bimetallic Co-Ag nanoparticles plotted as a function of days. Each curve follows two different trends, inverse logarithmic and exponential decay corresponding to that of pure Co and pure Ag, respectively. The inset shows the overlap of Co oxidation in Co-Ag nanoparticles, where the dashed line is an inverse logarithmic function as a guide to the eye. SEM images depicting the morphology of triangular shaped EOTs of pure Ag captured on (c) 0 days and (d) after 100 days. Similarly, SEM images for the morphology of Ag/Co EOT captured on (e) day 0 and (f) after 100 days. (g) and (h) are the contour maps for the resonance decay of Ag and Ag/Co EOT devices, respectively. (a) reproduced with permission from Ref. [29]. (b) reproduced from Ref. [34]. © IOP Publishing. Reproduced with permission. All rights reserved. (c)–(h) from Ref. [49], used in accordance with the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

Symbiosis in Plasmonic Nanoparticles http://dx.doi.org/10.5772/intechopen.71730 275

As discussed in the preceding sections, due to the scarcity of plasmonic materials or rather stable plasmonic materials at room temperature, a lot of attention has been diverted on the study of bimetallic nanoparticle systems. The interaction between the bimetallic nanoparticles

strate but absent in Ag/Co EOTs.

**4.3. Ferro-plasmons**

To explore the other possibilities of stability studies in Co-Ag bimetal system, Kalyanaraman and co-workers studied the extraordinary optical transmission (EOT) of perforated holes of triangular shapes in Ag and Ag/Co films [49]. The findings suggested that Ag films deposited

**Figure 4.** (a) Schematic showing the galvanic coupling in a single bimetallic Co-Ag nanoparticle. (b) Optical bandwidth decay of two different concentrations of Co (solid triangles and hollow squares) in the bimetallic Co-Ag nanoparticles plotted as a function of days. Each curve follows two different trends, inverse logarithmic and exponential decay corresponding to that of pure Co and pure Ag, respectively. The inset shows the overlap of Co oxidation in Co-Ag nanoparticles, where the dashed line is an inverse logarithmic function as a guide to the eye. SEM images depicting the morphology of triangular shaped EOTs of pure Ag captured on (c) 0 days and (d) after 100 days. Similarly, SEM images for the morphology of Ag/Co EOT captured on (e) day 0 and (f) after 100 days. (g) and (h) are the contour maps for the resonance decay of Ag and Ag/Co EOT devices, respectively. (a) reproduced with permission from Ref. [29]. (b) reproduced from Ref. [34]. © IOP Publishing. Reproduced with permission. All rights reserved. (c)–(h) from Ref. [49], used in accordance with the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

on top of Co films showed much improved EOT stability in ambient air as can be visualized from the SEM images shown in **Figure 4(c)–(f)**. The study was carried out for 100 days which showed 1.9 times better plasmon peak stability and 1.7 times better bandwidth stability over pure Ag EOT. The contour maps in **Figure 4(g)** and **(h)** shows the plasmon resonance decay as a function of days. The map shows the change in the behavior of Ag EOT, initially the plasmon significantly red-shifts leading up to day 40 after that the plasmon resonance intensity starts decaying very rapidly in comparison to Ag/Co EOT showing slower red-shift and stable plasmon resonance intensity. The stabilization of bimetal EOT is attributed to the underlying Co layer, which prevents Ag to dewet on the glass substrate. This is evident from the SEM images where material build-up is present for pure Ag EOTs due to dewetting on glass substrate but absent in Ag/Co EOTs.

#### **4.3. Ferro-plasmons**

and structural instabilities [40, 41]. Shortage of stable plasmonic materials creates the demand for research on alternate or new stable plasmonic materials required for various applications at the nanoscale [42, 43]. A lot of different solutions or alternative routes have been presented to stabilize metal nanoparticles but they either are short-term solutions or undermine the pure metal properties. The brighter side of these solutions has resulted in finding new and improved properties in some cases. By synthesizing core-shell of Au-Ag, the Ag could be stabilized via electron transfer from Au core, but the nanoparticle properties degrade over time due to the alloying between Au and Ag at the interface [44]. These synthesized core-shell structures at a critical thickness of Ag shell are resilient to harsh chloride ion environment

In another study, the Ag nanostructures were stabilized by processing them with low-temperature single hydrogen atoms. The idea was to stabilize nanostructures by filling the interstitial sites with a hydrogen atom to prevent Ag atom diffusion [41]. Covering Ag nanostructures with a monolayer of graphene could protect them from oxidation for almost a period of a month [45]. The other possibility is by surface passivation of Ag nanostructures with organic molecules. The choice of an organic molecule can preferentially control the optical or the

Sachan et al. stabilized the Ag nanoparticles by synthesizing bimetals of Ag and Co having segregated regions within a single nanoparticle using PLiD. The oxidation behavior was studied using optical spectroscopy and core-loss EELS mapping and showed prolong optical stability of Ag nanoparticles [12, 29, 34]. This ultra-stability was due to the galvanic coupling between Co and Ag at the nanoscale where Co was acting as a sacrificial anode, thus protecting Ag surface by providing electrons as shown in **Figure 4(a)**. The study was further extended by Malasi et al. to understand the role of Co composition in these bimetals and how the Co-Ag nanoparticle life could be extended [34]. On studying the extended optical bandwidth decay of these nanoparticles, two observations were made. Firstly, the decay behavior of Co and Ag of the bimetals followed two different trends displaying a sharp boundary demarcating the change in trends from inverse logarithmic to exponential decay shown in **Figure 4(b)**. Second observation suggested that the decay trends observed in pure metal oxidation were consistent with the decay of the individual metals of the bimetallic nanoparticle and overlapped perfectly. The change in decay behavior suggested that the two metals have segregated regions, the results of which are consistent with the EELS observation. The only question that remained here to answer was whether the Co oxidized completely or not before the change in the optical bandwidth decay trends. To answer this, a theoretical model is used to quantify the oxidation of Co in the bimetallic nanoparticle system, details of which are available in Ref. [34]. Co-Ag bimetals showed almost 12 times better shelf life than pure Ag nanoparticles for 50% degradation in the optical bandwidth and depending on Co amount, the Ag could be oxide free for almost a year. The lifetimes were established using effective medium theory, which is a versatile way to study nanoparticles in embedded media [47, 48]. To explore the other possibilities of stability studies in Co-Ag bimetal system, Kalyanaraman and co-workers studied the extraordinary optical transmission (EOT) of perforated holes of triangular shapes in Ag and Ag/Co films [49]. The findings suggested that Ag films deposited

without any visible degradation under transmission electron microscope.

274 Noble and Precious Metals - Properties, Nanoscale Effects and Applications

electrical properties [46].

As discussed in the preceding sections, due to the scarcity of plasmonic materials or rather stable plasmonic materials at room temperature, a lot of attention has been diverted on the study of bimetallic nanoparticle systems. The interaction between the bimetallic nanoparticles has led to interesting optical phenomena such as plasmon hybridization and Fano interference resonance. These optical phenomena occur due to the interactions between various plasmon modes, dark and bright modes, or sub-radiant and super-radiant modes. Hybridization of two plasmonic resonances leads to a lower energy resonance, i.e. the sub-radiant mode with narrow bandwidth and the higher energy resonance acting as a continuum, i.e. the super-radiant mode, in asymmetric nanoparticle systems [50, 51]. The above mentioned optical phenomena are observed in a wide variety of plasmonic nanostructures such as multishells, cavities, and heterodimers [52–54].

One such optical phenomenon observed in the bimetallic Ag-CoFe and Ag-Co nanoparticles is the ferroplasmon (FP), intense LSPR (localized surface plasmon resonance) excited in the visible spectrum of light on the surface of ferromagnetic CoFe and Co when in contact with plasmonic Ag [32]. FP is an intense LSPR with long plasmon decay length scales in the visible spectrum of light. As mentioned earlier, the exhibition of a strong LSPR in the visible spectrum is the characteristic property of the noble metals (Ag, Au, and Cu). Other metals, including the ferromagnetic metals (e.g. Co, Ni, Fe) have non-existent or highly damped visible plasmons. Recent studies of Ni nanostructures have demonstrated the sustenance of surface plasmon in the visible spectrum of light [55]. The discovery of FP is possible due to the advances in transmission electron microscopy equipped with a monochromated electron probe in STEM with energy resolutions of ~150 meV, enabling the quantitative study of well-resolved plasmon peaks at the nanoscale using low-loss EELS with high energy and spatial resolution. **Figure 5** shows the comparative study of plasmon excitations on the surface of bimetal Ag-CoFe, pure Ag and pure CoFe nanoparticles, resolved by low-loss EELS. The EELS spectra are shown in **Figure 5(a)** from Ag region of Ag-CoFe nanoparticle and the isolated Ag nanoparticle, taken from the square area marked in the HAADF images in **Figure 5(b)** and **(c)**. Both the spectra exhibit peaks in the visible range corresponding to the well-known surface plasmon in Ag. In contrast, the EELS spectrum shown in **Figure 5(e)** obtained from CoFe in Ag-CoFe show a strong and distinct plasmon peak, referred to as FP, at 2.7 eV, unlike the featureless spectrum for the isolated CoFe nanoparticle. The EELS spectra for CoFe corresponds to the red square marked on the CoFe side of the bimetal nanoparticle and the isolated nanoparticle marked on the HAADF images in **Figure 5(b)** and **(d)**, respectively. The evolution of FP in the Ag-CoFe bimetallic system is thoroughly studied as a function of nanoparticle size and found to be consistently present [32]. The presence of FP is an intricate play between the nanoparticle morphology and the interface shared between them, which was demonstrated by the absence of FP in dimers of Ag-Co nano-triangles placed in close proximity [56].

additional high-order peak, which is not observed experimentally in EELS. In the hybridization model, the coupling can be assumed as an instantaneous Coulomb interaction between the surface charge density of various components. In another approach, electrodynamic simulations were performed showing the interaction between spherical Ag and Co nanoparticles with their surfaces separated by a nanometer gap [58]. This gives rise to new plasmon mode at higher energy due to the interaction between Ag and Co nanospheres, as shown in **Figure 5(f)** where the optical spectrum of pure Co and pure Ag nanospheres are plotted in blue and orange, respectively. The green color corresponds to the spectrum obtained from the interaction between the Ag-Co nanospheres shown in the schematic. This analytical approach depends on the interaction of two spherical nanoparticles and the use of non-contacting geometry, assuming the contact between Ag and Co will not influence the interaction process. FP nanoparticle system still lacks a detailed theoretical study which is only possible by finite difference time domain or discrete dipole approximation simulations by taking account the

**Figure 5.** (a) EELS spectra from the surface of Ag side of Ag-CoFe (solid line) and isolated Ag nanoparticle. (b), (c) and (d) are the HAADF images of bimetallic Ag-CoFe, isolated Ag, and isolated CoFe nanoparticles, respectively. (e) EELS spectra from the surface of CoFe side of Ag-CoFe (solid line) and isolated CoFe nanoparticle. The EELS spectra are measured from the small red boxes marked on the HAADF images. (f) Comparison of extinction spectra simulated for pure Ag (orange line), pure Co (blue line) and dimer of Ag-Co (green line) nano-spheres. The schematic represents the Ag-Co dimer system separated by 1 nm. Reprinted (adapted) with permission from [32, 58]. Copyright (2014) American

Symbiosis in Plasmonic Nanoparticles http://dx.doi.org/10.5772/intechopen.71730 277

Plasmonic nanoparticles are an efficient medium for transferring the coupled electromagnetic waves in and out of the system. How well the nanoparticles will couple to the electromagnetic waves depends on their shape, size, material, and ambient dielectric environment and is measured by the radiative quantum efficiency. The dependence of radiative quantum efficiency on the Au nanoparticle shape and size was first demonstrated by Sonnichsen et al. [59]. As the volume of nanostructure increased, the radiative contributions in the bandwidth increased, leading to increased quantum efficiencies. The other interesting observation regarding Co-Ag bimetal system is the tuning of radiative quantum efficiency in a much wider energy range by controlling the Co amount and keeping the Ag volume fixed [13]. This is achieved by

hemispherical shaped nanoparticle.

Chemical Society.

**4.4. Radiative quantum efficiency**

The physical origin of FP is explained by qualitative theoretical models such as dipole-dipole interaction, hybridization model, and Fano interference as these all arise due to the coupling between different plasmon modes [50, 51, 57]. A simple dipole-dipole interaction approach considers the coupling between same (dipole-dipole) or different (dipole-quadrupole) modes and successfully describes the electromagnetic energy transfer between two nanoparticles. However, it lacks the complete explanation due to inherently neglecting the interaction of other modes of plasmon oscillation. Alternatively, another model based on the plasmon hybridization effectively explain the existence of FP peak evolved at 2.7 eV, but also predict

**Figure 5.** (a) EELS spectra from the surface of Ag side of Ag-CoFe (solid line) and isolated Ag nanoparticle. (b), (c) and (d) are the HAADF images of bimetallic Ag-CoFe, isolated Ag, and isolated CoFe nanoparticles, respectively. (e) EELS spectra from the surface of CoFe side of Ag-CoFe (solid line) and isolated CoFe nanoparticle. The EELS spectra are measured from the small red boxes marked on the HAADF images. (f) Comparison of extinction spectra simulated for pure Ag (orange line), pure Co (blue line) and dimer of Ag-Co (green line) nano-spheres. The schematic represents the Ag-Co dimer system separated by 1 nm. Reprinted (adapted) with permission from [32, 58]. Copyright (2014) American Chemical Society.

additional high-order peak, which is not observed experimentally in EELS. In the hybridization model, the coupling can be assumed as an instantaneous Coulomb interaction between the surface charge density of various components. In another approach, electrodynamic simulations were performed showing the interaction between spherical Ag and Co nanoparticles with their surfaces separated by a nanometer gap [58]. This gives rise to new plasmon mode at higher energy due to the interaction between Ag and Co nanospheres, as shown in **Figure 5(f)** where the optical spectrum of pure Co and pure Ag nanospheres are plotted in blue and orange, respectively. The green color corresponds to the spectrum obtained from the interaction between the Ag-Co nanospheres shown in the schematic. This analytical approach depends on the interaction of two spherical nanoparticles and the use of non-contacting geometry, assuming the contact between Ag and Co will not influence the interaction process. FP nanoparticle system still lacks a detailed theoretical study which is only possible by finite difference time domain or discrete dipole approximation simulations by taking account the hemispherical shaped nanoparticle.

#### **4.4. Radiative quantum efficiency**

has led to interesting optical phenomena such as plasmon hybridization and Fano interference resonance. These optical phenomena occur due to the interactions between various plasmon modes, dark and bright modes, or sub-radiant and super-radiant modes. Hybridization of two plasmonic resonances leads to a lower energy resonance, i.e. the sub-radiant mode with narrow bandwidth and the higher energy resonance acting as a continuum, i.e. the super-radiant mode, in asymmetric nanoparticle systems [50, 51]. The above mentioned optical phenomena are observed in a wide variety of plasmonic nanostructures such as multi-

One such optical phenomenon observed in the bimetallic Ag-CoFe and Ag-Co nanoparticles is the ferroplasmon (FP), intense LSPR (localized surface plasmon resonance) excited in the visible spectrum of light on the surface of ferromagnetic CoFe and Co when in contact with plasmonic Ag [32]. FP is an intense LSPR with long plasmon decay length scales in the visible spectrum of light. As mentioned earlier, the exhibition of a strong LSPR in the visible spectrum is the characteristic property of the noble metals (Ag, Au, and Cu). Other metals, including the ferromagnetic metals (e.g. Co, Ni, Fe) have non-existent or highly damped visible plasmons. Recent studies of Ni nanostructures have demonstrated the sustenance of surface plasmon in the visible spectrum of light [55]. The discovery of FP is possible due to the advances in transmission electron microscopy equipped with a monochromated electron probe in STEM with energy resolutions of ~150 meV, enabling the quantitative study of well-resolved plasmon peaks at the nanoscale using low-loss EELS with high energy and spatial resolution. **Figure 5** shows the comparative study of plasmon excitations on the surface of bimetal Ag-CoFe, pure Ag and pure CoFe nanoparticles, resolved by low-loss EELS. The EELS spectra are shown in **Figure 5(a)** from Ag region of Ag-CoFe nanoparticle and the isolated Ag nanoparticle, taken from the square area marked in the HAADF images in **Figure 5(b)** and **(c)**. Both the spectra exhibit peaks in the visible range corresponding to the well-known surface plasmon in Ag. In contrast, the EELS spectrum shown in **Figure 5(e)** obtained from CoFe in Ag-CoFe show a strong and distinct plasmon peak, referred to as FP, at 2.7 eV, unlike the featureless spectrum for the isolated CoFe nanoparticle. The EELS spectra for CoFe corresponds to the red square marked on the CoFe side of the bimetal nanoparticle and the isolated nanoparticle marked on the HAADF images in **Figure 5(b)** and **(d)**, respectively. The evolution of FP in the Ag-CoFe bimetallic system is thoroughly studied as a function of nanoparticle size and found to be consistently present [32]. The presence of FP is an intricate play between the nanoparticle morphology and the interface shared between them, which was demonstrated by the absence of FP in dimers of Ag-Co

The physical origin of FP is explained by qualitative theoretical models such as dipole-dipole interaction, hybridization model, and Fano interference as these all arise due to the coupling between different plasmon modes [50, 51, 57]. A simple dipole-dipole interaction approach considers the coupling between same (dipole-dipole) or different (dipole-quadrupole) modes and successfully describes the electromagnetic energy transfer between two nanoparticles. However, it lacks the complete explanation due to inherently neglecting the interaction of other modes of plasmon oscillation. Alternatively, another model based on the plasmon hybridization effectively explain the existence of FP peak evolved at 2.7 eV, but also predict