SYNTHESIS AND APPLICATIONS OF AND PALLADIUM

NANO

______

A Thesis

presented to

the Faculty of the Graduate School

at the University of Missouri-Columbia

______

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

______

by

SIJIA HU

Dr. Sheila N. Baker, Thesis Supervisor

DEC 2013

The undersigned, appointed by the dean of the Graduate School, have examined the thesis entitled

SYNTHESIS AND APPLICATIONS OF SILVER AND PALLADIUM METAL

NANO FOAMS

Presented by Sijia Hu, a candidate for the degree of master of Science, and hereby certify that, in their opinion, it is worthy of acceptance.

Professor Sheila N. Baker

Professor David Retzloff

Professor John M. Gahl

DEDICATION

This thesis is dedicated to my amazing parents, cousins, and friends that have helped and supported me all the way since the beginning of my research. I could not have done it without you.

Also, this thesis is dedicated to all those who believe in the richness of learning.

ACKNOWLEDGEMENTS

First and foremost, I would like to express my sincere gratitude to Dr. Sheila N.

Baker for the valuable guidance and advice. She inspired me greatly to work in my research. Her willingness to motivate me contributed tremendously to my research project in the past two years.

I would also like to thank Dr. David Retzloff and Dr. John M. Gahl for their willingness to be members of my thesis committee and for offering me with a good environment, facilities and kind help to complete this research project. Also,

I wish to thank Zhifeng Lu who has been supporting me with helpful suggestions and assurance throughout the whole research project. In addition, I wish to thank all the members of Dr. Sheila N. Baker’s research group for their help and encouragement. Finally, an honorable mention goes to my family and friends for their understandings and supports on us in completing this project.

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TABLE OF CONTENTS

ACKNOWLEDGEMENTS ...... ii

TABLE OF CONTENTS ...... iii

TABLE OF FIGURES ...... v

LIST OF ABBREVIATIONS ...... ix

ABSTRACT ...... x

CHAPTER 1: INTRODUCTION ...... 1

1.1 Definition of Nanoporous Metal Foams ...... 1

1.2 Synthetic Methods for NMFs ...... 3

1.3 Applications of NMFs...... 5

1.3.1 Surface-Enhanced Raman (SERS) ...... 5

1.3.2 Viable hydrogen storage ...... 7

1.3.3 Electrochemical actuators ...... 7

1.3.4 ...... 8

1.3.5 High Power Density Batteries ...... 9

1.4 Instruments ...... 10

1.4.1 Microwave ...... 10

1.4.2 Scanning Electron Microscope ...... 12

1.4.3 Brunauer-Emmett-Teller (BET) Analysis ...... 14

1.4.4 Raman Spectroscope ...... 16

1.4.5 Microplate Spectrophotometer ...... 18

1.5 Motivation and Objective for Research ...... 20

CHAPTER 2: EXPERIMENTAL PROCEDURES AND MATERIALS ...... 23

2.1 Polyol Synthesis ...... 23

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2.1.1 Materials ...... 23

2.1.2 Polyol Synthesis Process ...... 23

2.2 Hydrazine Synthesis ...... 25

2.2.1 Materials ...... 25

2.2.2 Hydrazine Synthesis Process ...... 25

2.2.3 Results and Discussions ...... 27

2.2.4 Possible Mechanism...... 34

2.3 Synthesis ...... 36

2.3.1 Materials ...... 36

2.3.2 Sodium borohydride synthesis process ...... 36

2.3.3 Results and Discussions ...... 38

2.2.4 Possible Mechanism...... 44

CHAPTER 3: APPLICATION FOR SILVER ...... 45

3.1 Silver NMFs applied in SERS ...... 45

3.2 Experimental Section ...... 47

CHAPTER 4: APPLICATION FOR PALLADIUM ...... 52

4.1 Palladium NMFs applied in MO decoloration ...... 52

4.2 Experimental Section ...... 53

CHAPTER 5: Future Work and Conclusion ...... 63

5.1 Future work ...... 63

5.2 Conclusion ...... 64

Reference ...... 67

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TABLE OF FIGURES

Figure 1.1 SEM image of Ni nanofaom produced by combustion synthesis. (Scale bar is 200nm). (Luther, Tappan et al. 2009) ...... 2 Figure 1.2 The SEM micrograph of the surface of nanoporous Pt sample.(Weissmuller 2003) ...... 8 Figure 1.3 Image of CEM microwave used for the synthesis of all NMFs, metal including gold, silver, palladium and ...... 11 Figure 1.4 Image of Hitachi S–4700 cold field emission SEM. (UniversityofMissouri-Columbia 1997) ...... 13 Figure 1.5 Principle of SEM electron source ...... 14 Figure 1.6 Image of the B&W TEK i-Raman ...... 18 Figure 1.7 Image of BioTek Powerwave HT Microplate Spectrophotometer, used for the detection of decoloration of MO solution with palladium NMFs...... 19 Figure 2.1 Height adjustment of Hitachi S–4700 SEM specimen ...... 27 Figure 2.2 SEM image of silver NMFs with a magnification is 6k made by hydrazine in the condition of 5 min, 150 W microwave heating...... 29 Figure 2.3 SEM image of silver NMFs with a magnification 13.0k made by hydrazine in the condition of 5 min, 150 W microwave heating...... 30 Figure 2.4 SEM image of silver NMFs with a magnification 45.0k made by hydrazine in the condition of 5 min, 150 W microwave heating...... 30 Figure 2.5 SEM image of Pd NMFs with a magnification 8.03k made by hydrazine in the condition of 5 min, 150 °C microwave heating...... 31 Figure 2.6 SEM image of Pd NMFs with a magnification 13.0k made by hydrazine in the condition of 5 min, 150 °C microwave heating...... 32 Figure 2.7 SEM image of Pd NMFs with magnification of 100k made by hydrazine in the condition of 5 min, 150 °C heating...... 32 Figure 2.8 Nitrogen and desorption isotherm of the silver NMFs using hydrazine synthesis method. The BET surface area is 22.3 m2/g...... 33

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Figure 2.9 Nitrogen adsorption and desorption isotherm of the palladium NMFs using hydrazine synthesis method. The BET surface area is 11.5 m2/g...... 33 Figure 2.10 SEM image of silver NMFs with magnification of 10k made by NaBH4 in the condition of 5 min, 150 W heating...... 39 Figure 2.11 SEM image of silver NMFs with magnification of 18k made by NaBH4 in the condition of 5 min, 150 W heating...... 39 Figure 2.12 SEM image of silver NMFs with magnification of 70.0k made by NaBH4 in the condition of 5 min, 150 W heating...... 40 Figure 2.13 SEM image of Pd NMFs with magnification of 11k made by NaBH4 in the condition of 5 min, 150 °C heating...... 41 Figure 2.14 SEM image of Pd NMFs with magnification of 20k made by NaBH4 in the condition of 5 min, 150 °C heating...... 41 Figure 2.15 SEM image of Pd NMFs with magnification of 30k made by NaBH4 in the condition of 5 min, 150 °C heating...... 42 Figure 2.16 Nitrogen adsorption and desorption isotherm of the palladium NMFs using hydrazine synthesis method. The BET surface area is 83.0 m2/g. .... 43 Figure 3.1 Image of B&W TEK i-Raman microscope ...... 48 Figure 3.2 Raman spectra of R6G molecules deposit on surface of silver NMFs: (a) 6 × 10–5 M; (b) 6 × 10–6 M; (c) 3 × 10–6 M; (d) 2 × 10–6 M. The spectra have been scaled and vertically shifted to enhance the clarity of the presentation...... 49 Figure 4.1 UV-Vis spectra of the MO solution (1×10—3 mol/L) before and after different hour’s immersion at room temperature...... 54 Figure 4.2 UV-Vis spectra of the MO solution (5×10—4 mol/L) before and after different hour’s immersion at room temperature...... 55 Figure 4.3 UV-Vis spectra of the MO solution (1×10—4 mol/L) before and after

different hour’s immersion at room temperature...... 55 Figure 4.4 UV-Vis spectra of the MO solution (5×10—5 mol/L) before and after different hour’s immersion at room temperature...... 55 Figure 4.5 UV-Vis spectra of the MO solution (2×10—5 mol/L) before and after different hour’s immersion at room temperature...... 55

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Figure 4.6 Time variation of MO concentration after immersion of palladium NMFs, different concentration of MO solution were also provided...... 59 Figure 4.7 Time variation of MO concentration after the immersion of palladium NMFs with different concentration of MO solution measured according to a logarithmic scale...... 61

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LIST OF TABLES

Table 1 Comparison of BET surface area of different procedures for generating NMFs ...... 44 Table 2 Kinetics constants for increased MO concentration ...... 61

viii

LIST OF ABBREVIATIONS

nanoporous metal foams NMFs surface – enhanced Raman spectroscopy SERS rhodamine 6G R6G methyl orange MO poly (vinyl alcohol) PVA poly (vinyl pyrrolidone) PVP bovine serum albumin BSA EG

hydrazine N2H4 near-infrared radiation NIR scanning electron microscope SEM

Brunauer-Emmett-Teller BET

sodium borohydride NaBH4

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ABSTRACT

Nanoporous metal foams (NMFs), a relatively new class of materials with low– density, high surface area and conductivity, have been studied recently in nano science. These materials can be engineered to benefit many fields and promise to enable new technologies in areas such as hydrogen storage, high power density battery, surface–enhanced Raman spectroscopy and supercapacitors.

Herein a new developed method is presented to synthesis low–density, nanoporous metal foams of silver and palladium. This study mainly focuses on the synthesis process of NMFs of and it also provides some new applications that are useful in research and daily life.

Silver NMFs were produced by mixing silver nitrate mixed with ethylene glycol, , and a , and heating at 150 W for 5min using a CEM microwave. Higher conversions for this process were obtained using either hydrazine or sodium borohydride as the acceptable reducing agent for this reaction. Palladium NMFs were synthesized in the same way in the condition of

150 °C with either hydrazine or sodium borohydride as the reducing agent.

Silver NMFs were applied in surface–enhanced Raman spectroscopy (SERS) as a material. This material can enhance the detection of the rhodamine

6G (R6G), a model analyte. The limit of detection for rhodamine 6G was found to be 2 × 10–6 M with the help of this silver nanoporous structure. Palladium NMFs was found to degrade methyl orange (MO). An aqueous MO solution will turn nearly colorless after only 10 h of mixing with 0.03g of palladium NMFs at room

x temperature under dark condition. Different concentrations of MO solution were also studied to compare the reaction rates. This application is applicable to the treatment of liquid waste and and is thus conductive to improving the environment.

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CHAPTER 1

INTRODUCTION

1.1 Definition of Nanoporous Metal Foams

Nanoporous metal foams (NMFs), a three dimensional structure with metallic particles which exhibits a porosity of larger than 50% and has a huge amount of sub–micron pores (including micropores, mesopores, and macropores 50–

1000nm in diameter) which contribute to the surface area, have received a tremendous amount of attention due to their excellent properties. NMFs combine properties characteristic of metal such as silver, gold, nickel and palladium, and nano architectures such as . (Tappan, Steiner et al. 2010; Barron 2012)

Figure 1.1 shows a typical define SEM image for Ni nanoporous metal foams presenting general appearance for NMFs. This material was produced via the self–propagating high temperature synthesis method. It has a surface area of approximately 36 m2/g. This compares better than the 20 to 100m2/g typical of

Raney nickel. (Luther, Tappan et al. 2009) Due to the combination of and nano architectures, NMFs represent many useful properties, for instance, low density, large surface area, excellent electrical and thermal conductivity, good mechanical properties, high strength–to–weight ratio and catalytic activity.

(Tappan, Steiner et al. 2010) In addition, they show great promise as components of fuel cells and sensors.(Cheng 2013) For instance, either

1 nanoporous Au or Pd exhibited excellent performance in the electro–oxidation of methanol, ethanol and formic acid when they were engineered as fuel cell anodic catalysts.(Antolini 2009) Moreover, PtIr and PdCd alloys were also reported to be good as sensors and for hydrogen storage, respectively. (Adams, Wu et al. 2009; He, Hong et al. 2013)

Figure 1.1 SEM image of Ni nanofaom produced by combustion synthesis. (Scale bar is 200nm).

(Luther, Tappan et al. 2009)

Great efforts have been devoted to developing reproducible and efficient synthetic methodologies for making noble metal foams due to the growing importance and applications of NMFs. Dealloying and templating growth are the two most–developed methods to generate the NMFs. Nanoporous structures can been grown on alloy surfaces by selective dealloying of the active component of the alloy. For templating growth, porosity and pore diameter are relatively easy to 2 control as the is employed as template. (Tappan, Huynh et al. 2006;

Luther, Tappan et al. 2009; Tappan, Steiner et al. 2010; Wynn 2012; Cheng

2013)

1.2 Synthetic Methods for NMFs

Many different synthetic approaches have been presented for NMFs. For example, templating and dealloying approaches, sol–gel approaches and combustion approaches. Previous work was also reported from other papers. For instance, Brock et al. demonstrate a number of synthetic pathways for producing aerogels of metal chalcogenides by synthesis of surfactant–stabilized followed by controlled oxidative removal of the surfactant groups to invoke gelation. (Mohanan, Arachchige et al. 2005; Arachchige and Brock 2006)

Leventis et al. reported the first synthesis of iron aerogels through nanomelting of hybird /metal oxide aerogels prepared through a sol-gel process.

(Leventis, Chandrasekaran et al. 2009) Besides, Kandarp et al. illustrated a method to generated silver nano particles by sodium borohydride. Poly (vinyl alcohol) (PVA), poly (vinyl pyrrolidone) (PVP) bovine serum albumin (BSA), citrate and cellulose were used as stabilizing agents. (Kandarp Mavani 2013)

Emily M. Hunt and coworkers reported a combustion synthesis method to generate the nickel and aluminum NMFs from reactants. (Hunt,

Pantoya et al. 2006) Jonny J. Blaker and coworkers generated nanocomposite

3 polymer foams from pickering emulsion templates. (Blaker, Lee et al. 2009) In this work, we present an approach using microwave irradiation methods.

Recently an effective approach based on the polyol process of silver and gold nano has been demonstrated with controllable shapes. The reduction of a precursor by ethylene glycol (EG) in the presence of PVP is involved. Sodium chloride and other inorganic salts are added to standard the polyol synthesis.

Network structures of silver can be generated in a range size by controlling the reaction time with microwave irradiation. This is a good method for producing silver nano foams in relatively high yield. (Wiley, Herricks et al. 2004)

Palladium NMFs, however, cannot be generated by this process, due to its different solubility from silver. A facile hydrazine reduction approach using microwave irradiation to produce the nanoporous structure will be reported in this work. Hydrazine (N2H4) is a relatively cheap reducing agent and has been used to produce some other metal nano structures, such as Ni nanowires, Pt nanoparticles and Ag–Ni electrocatalysts. This approach is not only good for palladium, but is also applicable to silver, nickel and bismuth due to the great reducing ability of hydrazine. In our study, EG and ethanol were first mixed with a palladium compound, palladium aceate. Hydrazine, as reducing agent, was then added serving, and the reaction carry out using microwave heating. Palladium

NMFs was generated after microwave irradiation and the same procedure was followed to treat with other metals. (Wang, Shi et al. 2012)

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Another formation of high surface area metal nanoporous structures through an inexpensive method in a green solvent, water, has been reported by Katla Sai

Krishna et al. (Krishna, Sandeep et al. 2010) Following this procedure, we are able to generate a three–dimensional porous structure of silver and palladium by optimizing the concentration of the precursors and reducing agent with the microwave irradiation. It should be pointed out that this procedure works well on both silver and palladium salts, steps were similar to the palladium hydrazine method above. More details on the synthetic procedures are provided in the next chapter. (Krishna, Sandeep et al. 2010; Bin Ahmad, Lim et al. 2011; Sintubin,

Verstraete et al. 2012)

1.3 Applications of NMFs

From the previous discussion, a large number of applications have been found for NMFs due to their excellent characteristics and properties, and new technological possibilities may be discovered. The following provide a summary of main applications of NMFs.

1.3.1 Surface–Enhanced Raman Spectroscopy (SERS)

SERS is a phenomenon in which the Raman scattering intensity from molecules close to the surface of certain finely divided metals is enhanced by a factor of

5

106, and can be as high as 1014 for some systems. The factor is sufficient to allow even single molecule detection using Raman. It provides Raman spectra arising from the vibrational frequencies of molecules. The inherently low sensitivity of conventional Raman scattering limits the applicability in surface chemistry, so conventional Raman scattering is ineffective because the photon of the incident laser light simply propagate through the bulk and the signal from the bulk overwhelms any Raman signal from the surface of analytes. So SERS has resulted in more widespread applications, especially in surface and analytical chemistry. (Campion and Kambhampati 1998; Kosuda, Bingham et al. 2011;

Savaloni and Babaei 2013)

SERS greatly enhanced Raman signal from Raman–active analyte molecules that have been adsorbed onto certain specially–prepared metal surface. The choice of surface metal is also dictated by the resonance frequency.

Visible and near–infrared radiation (NIR) are used to excite Raman modes. Silver and gold are typical metals for SERS due to their plasmon resonance frequencies falling within these wavelength ranges. SERS is sensitive to the surface on which the experiment is taking place. Besides, NMFs have also been demonstrated to be used for substrate enhancement in SERS. (Pelletier 1999;

Hung, Castillo et al. 2003; McCreery 2005; Cheng 2013)

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1.3.2 Viable hydrogen storage

The development of the techniques for using hydrogen energy is one of the most important energy–related projects. Metals with nanoporous structures have been shown to improve reaction kinetics and lower the uptake and release temperatures. Thus, these metals could present useful properties for rapid hydrogen uptake and release. The Palladium – hydrogen system, one of the most widely considered and investigated metal–hydrogen systems so far, has attracted more and more attention, mainly because of the easy procedure and friendly condition to produce a pure sample of palladium and these samples do not require more special surface treatments. (Tappan, Steiner et al. 2010)

1.3.3 Electrochemical actuators

An external stimulus, such as applied voltage, can lead material to reversibly change their dimensions. This type of materials, known as piezoelectric or electrostrictive ceramics, are used as actuators. However, dimension changes were reported for ceramics (T.L. Jordan 2001), (Baughman 1996) and carbon nanotubes (Baughman, Cui et al. 1999), but not for metals so far.

J.Weissmuller and coworkers showed that reversible strain amplitudes can be induced in metals by introducing a continuous network of nanometer sized pores with a high surface area. Figure 1.2 provides the details of this structure, consisting of an interconnected pore space. This structure was generated

7 through our microwave process with or gold. (Weissmuller 2003; Cheng

2013)

Figure 1.2 The SEM micrograph of the surface of nanoporous Pt sample.(Weissmuller 2003)

1.3.4 Green Chemistry

The toxicity caused by azo in the environment has attracted much attention recently. Various catalysts have been researched to remove such environmental pollutants. Nanosized materials, such as nanoparticles and NMFs, are engineered to apply in this field due to their high surface area and leads to a tremendous increase in catalytic reaction rate.

Nanoporous Au and Pd exhibit catalytic degradation of MO solution while bulk Au or Pd without a porous structure does not. Masataka Hakamada et al. show the catalytic degradation of a solution by nanoporous Au, which is distinguished

8 from the conventional photocatalytic effect known in semiconductor oxides.

(Hakamada, Hirashima et al. 2012; Li, Enshuai et al. 2012) In this work, we provided plots of different concentrations of MO solution with the same amount of palladium NMFs generated by sodium borohydride synthesis method. Different reaction kinetic constant were obtained through this work.

1.3.5 High Power Density Batteries

With the development of society, the storage of electrical energy at a high charge and discharge rate is getting more and more important. This technology can enable hybrid electric vehicles and provide back–up for wind and solar energy.

From many previous reports, high charging rates can only be achieved with super–capacitors by surface adsorption reaction, Byoungwoo Kang and coworkers, however, show that batteries which obtain high energy density by storing charge in the bulk of a material can also achieve high discharge rates by using Lithium iron phosphate in nanoscale, known as LiFePO4. (Weissmuller

2003; Kang and Ceder 2009; Cheng 2013)

A new way of making battery electrodes based on Ni and Cu NMFs has been applied to make a lithium– battery that can be 90% charged in two minutes. It could lead to few minutes’ charge for laptop and 30 seconds’ charge for cell phones. This type of battery cathode can be used to hold as much energy as a conventional one, but can recharge a hundred times faster. (Bourzac 2011)

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In addition, Cu and Sn NMFs electrodes have been successfully prepared using electrodeposition process. Three–dimensional porous Cu6Sn5 allow was fabricated as a negative electrode for rechargeable lithium batteries. Fast mass transport and rapid surface reactions were observed.(Shin, Dong et al. 2003;

Choi, Jung et al. 2012)

1.4 Instruments

1.4.1 Microwave

Over the past 25 years, microwave chemistry has moved from a laboratory scale to a well–established synthetic technique used in many academic and industrial laboratories around the world. Although there are still many microwave applications performed in the laboratory, this technology will certainly be applied on a larger scale or in manufacture production in the future due to its huge advantages compared to conventional heating for chemical reactions.

There are two main operational principles for microwave chemistry, the dipolar mechanism and the electrical mechanism. The dipolar mechanism occurs as the polar molecule attempts to follow the electric field in the same alignment under a high frequency electric field. When this happens, the reaction then moves forward by molecules releasing enough heat. In the second mechanism, the irradiated sample is an electrical conductor and the charge carriers are moved through the conducting material under the influence of the electric field. These 10 induced currents will cause heating in the sample due to any electrical resistance. Figure 1.3 shows an external structure of a CEM microwave.

(Mallikarjuna N. Nadagouda 2010; Li, Enshuai et al. 2012; Rastogi, Ganesan et al. 2012; Adhikari, Gyawali et al. 2013)

Figure 1.3 Image of CEM microwave used for the synthesis of all NMFs, metal ions including gold, silver, palladium and nickel.

In nano chemistry, microwave plays an important role as a thermal tool due to considerable advantages over conventional methods, because it provides a fast and uniform heating rate that can be selectively directed towards a targeted area.

A. Vadivel Murugan and coworkers present a microwave irradiation synthesis

11 method of poly(3,4–ethylenedioxythiophene) nanoribbons interleaved between the layers of crystalline V2O5 via the redox polymerization reaction of 3,4– ethylenedioxythiophene monomer and crystalline V2O5. Compared with the conventional 12h for polymerization, the microwave irradiation redox polymerization process proceeds fast within 8 minutes. (Murugan, Kwon et al.

2004; Rhea 2006; Sugihara, Semsarilar et al. 2012)

1.4.2 Scanning Electron Microscope

Invented in 50 years ago, the scanning electron microscope (SEM), a powerful and professional technology in the examination of materials, is now widely used in metallurgy, geology, biology and medicine. It can obtain tremendously higher magnification than normal microscopes, has excellent depth of field, and can also analyze many materials in micro scale providing a large amount of details.

The main components of a SEM instrument are the electron optical column, the scanning system, the detectors, the vacuum system, the electronics controls and the software.

A Hitachi S–4700 (Figure 1.4), a cold field emission SEM, is the instrument used for examining our materials. The term ‘cold’ is used because heat is not used to lower the work potential. It has many useful capabilities. A resolution of 1.5nm can be generated due to an accelerating voltage ranging from 0.5 to 30 kV.

Magnification ranges from 30x to 500,000x and its depth of field is many times greater than that possible with optical microscopy.

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Figure 1.4 Image of Hitachi S–4700 cold field emission SEM. (UniversityofMissouri-Columbia

1997)

The Hitachi S–4700 utilizes a cold cathode field emitter composed of a single crystal of tungsten etched to a fine point. In SEM, electrons escape the source once sufficient heat energy has been applied to exceed the energy potential barrier. In cold field emission electron microscopy, however, an electric field is applied to the tip of the electron source essentially pulling electrons from the emitter. The electrons are accelerated off by two anodes. One of the smallest available beams can be produced by this emitter and then contributes to the high resolution of the instrument due to the microscopic size of the electron source.

Figure 1.6 provides a graphical process for the above description. All the SEM images through entire research for NMFs were generated by this instrument.

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(Vernon-Parry 2000; Argast and Tennis Iii 2004; Beane 2004; Pathan, Bond et al.

2008; Krishna, Sandeep et al. 2010)

Figure 1.5 Principle of SEM electron source

1.4.3 Brunauer–Emmett–Teller (BET) Analysis

In the past few years, research has expanded from the chemistry department into filed of energy, aerospace and medicine. The BET theory aims to primarily to determine the surface area from physical adsorption of a gas on a solid surface. Unlike bulk materials, the surface area to volume ratio is significant for nanoscale particles. Many useful properties, for example,

14 mechanical properties, electrochemical properties, and optical properties, begin to arise especially when their diameter ranges from 1 to 100nm. Hence, determination of their surface area and volume is very important. (Li, Song et al.

2011; Barron 2012)

The BET Theory, an extension of Langmuir theory, was developed by Stephen

Brunauer, Paul Emmett, and Edward Teller in 1938. The first letter of each publisher’s surname was used to name this theory. (Brunauer, Emmett et al.

1938)

BET surface area analysis, providing precise specific surface area evaluation of materials by nitrogen serving for low surface area materials, can be applied to determine the specific surface area of nano scale powders, solids and granules.

Clean solid surfaces adsorb gas molecules and BET provides a model for the process of gas adsorption. This physical adsorption of a gas over the entire exposed surface of a material is called physisorption and can be used in measurement of total surface area and can even analyze pore size of nanopores and micropores. Typical BET analysis may include analysis of specific surface area, pore size, microscope, pore size distribution and porosity.

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.4.4 Raman Spectroscope

Raman Spectroscopy, a spectroscopic technique based on inelastic scattering of monochromatic light, used to observe vibrational, rotational, and other low– frequency modes in a system, and is suitable for the microscopic examination of mineral, materials such as polymers and ceramics, cells, and forensic trace evidence since water doesn’t generally interfere with Raman spectral analysis.

Raman phenomenon is a consequence of sample illumination with a monochromatic photon beam, most of which are absorbed, reflected, or transmitted by the sample, the others interact with the sample. During this interaction, some energy is transmitted to elementary particles of which the materials are constituted. This causes their transition from ground energy levels to ‘virtual’ excited states. These excited states are highly unstable and particles decay instantaneously to the ground state by one of the following three different processes:

1. Rayleigh scattering: The scattering of electromagnetic radiation by

particles with dimensions much smaller than the wavelength of the

radiation, resulting in angular separation of colors and responsible for the

reddish color of sunset and the blue of the sky.

2. Stokes and anti–stokes Raman photons: emission with a photon with an

energy either above or below that of Rayleigh photons, hence generating

a set of frequency shifted Rama photons.

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Raman spectroscopy presents several advantages for microscopic analysis. For instance, specimens don’t need to be fixed or sectioned and the Raman spectra can be generated from a tiny volume, these spectra allow the identification of species present in that volume. Besides, Raman spectra are acquired quickly within seconds; laser light and Raman scattered light can be transmitted by optical fibers over long distance for remote analysis. High resolution is another feature for Raman spectroscopy. Disadvantages to Raman include the need for sensitive, the detection needs sensitive and highly optimized instrumentation due to the extremely weak Raman Effect. Fluorescence is a common background issue since Raman spectroscopy can be swamped by fluorescence emission signal.

A Raman system typically consists of four major components, they are the excitation source (a laser), sample illumination system and light collection optics, wavelength selector, and detector such as photodiode array and CCD.

The Raman microscope applied in our research is i-Raman from B&W Tek Inc.

Figure 1.6 shows a image of the entire instrument. This instrument is unique for its high resolution combined with field portability. It performances comparable to large bench top Raman systems and weighs less than 7 lbs. Besides, the i-

Raman spectrometer system features a CleanLaze® technology with a line width

< 0.3 nm when equipped with 785nm and 830nm laser. This technology results in the correct center wavelength. In addition, the laser output power can be adjusted in the software from 0 - 100%, allowing you to maximize the signal-to- noise ratio and minimize integration time, and the laser lifetime of 10,000 hours

17 ensures quality data for years. The system offers 532 nm, 785 nm and 830 nm excitation wavelength options, it’s ideal for demanding applications involving low concentrations and weak Raman scatters. Besides, this instrument offers wide

Raman shift coverage up to 4000 cm—1, and a TE cooled 2048 pixel CCD array.

Figure 1.6 Image of the B&W TEK i-Raman

1.4.5 Microplate Spectrophotometer

The microplate spectrophotometer, also called plate reader, was used in our research for the application of palladium NMFs, which is a decoloration process of MO solution in the presence of palladium NMFs. In this experiment, we used

BioTek Powerwave HT Microplate Spectrophotometer, it was designed for use in

18 a huge amount of applications, from direct quantitation of nucleic acids using automated pathlength correction to full scale high throughput operations which need speed accuracy and reliability. This instrument is uniquely designed to provide the application flexibility due to its rugged hardware and proven optical performance. Gen5 Data Analysis software comes with this plate reader, it provided clear operated windows and easy operation system.

Figure 1.7 Image of BioTek Powerwave HT Microplate Spectrophotometer, used for the detection of decoloration of MO solution with palladium NMFs.

In addition, this instrument has many useful features, for example, no interference filter is required, and this plate reader can provide continuous wavelength selection from 200nm to 999nm. A 1nm increment for wavelength selection makes this instrument to meet almost all requirements. What’s more, it has fast reading speed, entire 96–well plate can be read in 5 seconds. Besides, the temperature control is another feature. 19

1.5 Motivation and Objective for Research

The first goal for this research was to develop a new method to produce silver and palladium NMFs using microwave irradiation. A polyol process using PVP and EG was first considered based on the paper from Benjamin Wiley et al.

(Wiley, Herricks et al. 2004)However, this process proved only good for silver

NMFs, rather than palladium, so another method needs to be found. Porous structures of silver and palladium were also prepared by the reduction of hydrazine in glycerol–ethanol solution at room temperature by Yue Wang et al.

(Wang, Shi et al. 2012) Hence we applied this method to our system.

Furthermore, sodium borohydride was found to be another reducing agent, capable of replacing hydrazine, becomes a focus development of my research.

Many previous methods in papers use conventional heating methods instead of microwave. Based on the present research and heating mechanism of microwaves, many advantages have been discovered for microwave heating method in nano chemistry, for instance, microwave provides a fast and uniform heating rate that can be selectively directed towards a targeted area. It improves the yield of reactions, enhances physicochemical properties and generally can heat any material containing mobile electric charges. There advantages are significant to lab or industrial application. It is preferred to apply microwave heating for our reactions. But there are also some difficulties to process this reaction, for instance, the amount of reducing agent, hydrazine or sodium borohydride, needs to be well controlled due to its excellent ability to reduce

20 silver acetate or palladium acetate. What’s more, magnetic stirring is needed for the entire process before heating in order to obtain a uniform solution.

The second goal for my research was to apply these NMFs. Although many useful applications were found in different fields, some of them were not environmental friendly and can be only operated in lab research or experiment. A better way to apply palladium was discovered as filter in order to leach the materials that are harmful to the environment. For example, the increased amount of azo dyes leads to toxic pollution, palladium can be applied as catalyst in the reaction to remove this type of pollutant. Besides that, chlorinated compounds were widely used for years, and they are common groundwater contaminants, those pollutants are difficult to treat inexpensively by conventional technology. Palladium catalyst presented good performance for remediating groundwater. What’s more, Tayirjan T. Isimjan et al. present a nanocomposite catalyst comprised of palladium NMFs embedded in polystyrene sulfonic acid, this catalyst was shown to be effective for concersion of 2–phenyl–1,3–dioxolane to toluene or of to cyclohexane. In our work, we found palladium NMFs performed well when bleach methyl orange from dark yellow to almost colorless.

(Centi 2001; Brar, Verma et al. 2010; Zhou, Li et al. 2010; Isimjan, He et al.

2013)

Surface enhanced Raman spectroscopy was studied for years to observe signal from different analytes. (Jeanmaire and Van Duyne 1977) (Kudelski 2005) To enhance the signal of analyte, silver NMFs can be engineered as a substrate to mix with the analyte in this experiment, which means a relatively low limit of

21 detection will be achieved with the presence of silver NMFs. This applications will improve ability of the instrument for observing the signal of analyte.

22

CHAPTER 2

EXPERIMENTAL PROCEDURES AND MATERIALS

2.1 Polyol Synthesis

2.1.1 Materials

All experiments were completed by using Ultrapure Milliopore water (18.2 MΩ).

Silver nitrate (CAS#7761-88-8, AgNO3, ≥ 99%, MW: 169.87), ethylene glycol

(CAS#107-20-1, C2H6O2, ≥ 99%, MW: 62.07) and sodium chloride (CAS#7647-

14-5, NaCl, ≥ 99%, MW: 58.44) purchased from Sigma Aldrich (St. Louis, MO).

All materials were used without any further purification.

2.1.2 Polyol Synthesis Process

In each synthesis, 0.375 mol EG was first added in a test tube by 1000 mL pipette. 0.94 mol silver nitrate and 0.22x10—3 mol sodium chloride was weight by a balance to determine the accurate amount. Silver nitrate was then mixed with the solution. After 20min’s stirring in the 10 mL microwave vial at room temperature, microwave was then applied to heat the solution. Magnetic stirring was applied through the entire process. A microwave vial cap was used. Metal foams were generated in the solution after 5 min of 150°C heating. A spatula was used to extract the solid foams after moving all the solutions into the hazardous

23 unwanted bottles. After extracting the solid foam into a 15 mL plastic centrifuge tube, a centrifuge was then used to centrifuge the solid. Ethanol was used to wash the foams in centrifuge tube. After using a spatula to extract the solid foams from the centrifuge tube, a capped vial was used for keeping them. The solid foam in the capped vial was then dried in the oven for at least 24 hours to remove all liquid. It should be noted that when we dry the foams, no cap was placed on the vial. (Wiley, Herricks et al. 2004; Yao, Xu et al. 2010)

The test tube was washed and labels were removed at first. If a marker was used to mark this tube, ethanol was used to remove the marker writing from the tube.

The tube was then pre–soaked and washed using warm–to–hot tap water, an appropriate brush, and the Alconox wash solution. The tube was washed using warm–to–hot water, a base bath was used in case of the unwanted materials could not be removed. After that, the tube was rinsed with DI water in the carboy and then rinsed using DI water from the quirt bottle. These two steps must be repeated at least 3 times, repectively. Ethanol from another squirt bottle was then used for rinsing the tube. The tube was placed on a mat and was put away for drying.

24

2.2 Hydrazine Synthesis

2.2.1 Materials

Silver nitrate (CAS#7761-88-8, AgNO3, ≥ 99%, MW: 169.87), ethylene glycol

(CAS#107-20-1, C2H6O2, ≥ 99%, MW: 62.07) and ethanol (CAS#64-17-5, C2H6O,

≥ 99.5%, MW: 46.07) were purchased from Sigma Aldrich (St. Louis, MO).

Palladium (II) acetate (CAS#3375-31-3, C12H18O12Pd3, 99.95%, MW: 673.52), was obtained from Sterm Chemicals Inc. Hydrazine monohydrate (CAS#7803-

57-8, H4N2·H2O, ≥ 99%, MW: 50.06) was purchased from Alfa Aesar Chemicals.

Platinum (IV) chloride (CAS# 13454-96-1, PtCl4, 99%, MW: 336.89) was obtained from Acros organics. All materials were used without any further purification.

2.2.2 Hydrazine Synthesis Process

0.043 g (0.25 mmol) silver nitrate was first weighed out, and placed into a 10mL microwave vial containing a stir bar. 1 mL of ethanol and 1 mL of EG were injected in the vial to help prevent aggregation, but they are not necessary for success. Magnetic stirring was then applied for at least 10 min until the solution appeared clear. 0.5 mL hydrazine was added after stirring as a reducing agent,

15 min stirring was necessary in order to ensure all the components mixing well.

A microwave vial cap was then placed on the tube. After 5 min and 150 W heating, NMFs were generated using CEM microwave irradiation. With the same procedure as the first synthesis process, a centrifuge was applied to wash the

25 solid foams. Tubes were washed also following the procedure for the first process. All the foams made by this procedure were preserved in the capped vial. From molecular weight calculations, 0.043g silver nitrate contains 0.027g

Ag, 0.021 g silver NMFs were produced through this process with a yield of 77.8 wt%.

It should be pointed out that palladium foams can’t be generated through the previous polyol synthesis, however, the hydrazine process is suitable for palladium due to the high reducing ability for hydrazine. 0.026 g palladium acetate, which contains 0.012 g palladium particles, was first weighed and then followed the procedure above with the same amount of EG, ethanol and hydrazine. The condition of microwave changed from 150 W to 150 °C with the same time of 5 min. All the other conditions were fixed the same as above.

Palladium NMFs were generated after microwave heating. After extracting from the microwave tube, a centrifuge was also applied and the foam preserved in the vial. 0.01 g Palladium NMFs were generated through this process with a yield of

83.3 wt%. (Pal, Shah et al. 2009; Wang, Shi et al. 2012; Wynn 2012)

Platinum was also studied in our lab through this process. 0.042 g platinum chloride was first weight and also followed the procedure above with the same amount of EG, ethanol and hydrazine. The microwave was set as 150 °C of 5min,

75 W of 5 min and 50 W of 5min, respectively. All the other conditions were set the same. No platinum NMFs generated due to the pressure exceeded the set point of 300 psi. The heating process stopped within 5 min, and at least 45 minutes was needed to wait until the pressure dropped to the released pressure.

26

2.2.3 Results and Discussions

Hitachi S–4700 SEM was used though our entire observing process. Specimen preparation is the first step to use the microscope. Sample NMFs should be placed on the specimen stub and then the stub can be placed on a lock screw, which is an attachment to the specimen exchange rod. Height was adjusted by the specimen height gauge as shown in Figure 2.1. Next, the column vacuum was checked to make sure all the ion pump readings and EAVC power switch were set right. The specimen exchange position was then checked to confirm all the axis control were in right position and stage lock was free. After the preliminary operation, a prepared specimen was introduced into the vacuum chamber using the specimen exchange rod.

Figure 2.1 Height adjustment of Hitachi S–4700 SEM specimen

The HV control window and select accelerating voltage, then click ON button to apply the accelerating voltage. Then operation mode can be setup as ultrahigh resolution. The image was displayed on the observing screen.

27

To obtain a desired image, several steps were applied as following. Low magnification mode was selected first, and the area of interest was located and centered with manual stage controls. High magnification mode was then applied to get more details of the specimen. Manual stage control was used to locate the region of interest. Brightness, contrast and focus were adjusted to obtain a relatively clear image. After these adjustments, alignment was applied. There were four steps for alignment. For beam align, the target was moved into a circle beam. For aperture align, the main target was to eliminate any shift in the specimen image. And then for X and Y align, the same procedure to eliminate any shift was applied. Finishing all the alignments, a clear image was obtained on the screen.

SEM images are provided as follows, figure 2.2 – 2.4 shows the microstructure of the Ag NMFs with different magnifications. The size of silver ligament is as large as several hundred micrometers, which typically consist of aggregated particles.

These particles fused together without any mechanically gathering, then fabricate a porous sub–micro structure. Those NMFs with porous structures are quite robust at elevated temperatures and can endure intense ultrasonic treatment.

The sizes of the constituent fused particles at room temperature are 150nm –

500nm, which is a relatively smaller size compared to the pervious papers, which means an increase of the specific area. (Choi, Jung et al. 2012; Wang, Shi et al.

2012)

Figure 2.2, in relatively low magnification, reveals the good mechanical integrity without any obvious cracks and the homogeneity through the thickness. Figure

28

2.3 illustrates a network structure in higher magnification, a cross–link porous sub–microstructure can be easily observed. Relatively high porosity is also provided. Figure 2.4 shows a detailed porous structure in a magnification of 45 k, an interconnected network structure is obviously observed. The size of the silver aggregates range from 150 nm to 300 nm.

Figure 2.2 SEM image of silver NMFs with a magnification is 6k made by hydrazine in the condition of 5 min, 150 W microwave heating.

29

Figure 2.3 SEM image of silver NMFs with a magnification 13.0k made by hydrazine in the condition of 5 min, 150 W microwave heating.

Figure 2.4 SEM image of silver NMFs with a magnification 45.0k made by hydrazine in the condition of 5 min, 150 W microwave heating.

30

Figure 2.5 – 2.7 reveal a fabricated sub–microstructure for palladium NMFs in different magnifications. Palladium nano particles are highly dispersed and they are not homogeneous placed as silver NMFs. From figure 2.7, palladium aggregates, in high magnification of 100k, present a smaller size of 100 nm –

300 nm than silver NMFs. A poly model pore size was observed, dominated by a small number fraction yet large volume fraction of pores in the 500 nm – 2 µm range, and within the ligaments of foam, pores in 50nm – 200 nm range dominate the number fraction distribution.

Figure 2.5 SEM image of Pd NMFs with a magnification 8.03k made by hydrazine in the condition of 5 min, 150 °C microwave heating.

31

Figure 2.6 SEM image of Pd NMFs with a magnification 13.0k made by hydrazine in the condition of 5 min, 150 °C microwave heating.

Figure 2.7 SEM image of Pd NMFs with magnification of 100k made by hydrazine in the condition of 5 min, 150 °C heating.

32

45 BET surfacearea: 22.3 m2/g

40

35

30

25

adsorbed (cc/g) adsorbed 20 2 2

15

10

Adsorption

Volume ofVolumeN 5 Desorption 0 0 0.2 0.4 0.6 0.8 1 Relative Pressure (P/P0)

Figure 2.8 Nitrogen adsorption and desorption isotherm of the silver NMFs using hydrazine synthesis method. The BET surface area is 22.3 m2/g. 16 BET surface area: 11.5 m2/g

14

12

10

8

adsorbed (cc/g) adsorbed 2 2

6

4

Volume of N ofVolume Adsorption 2 Desorption 0 0 0.2 0.4 0.6 0.8 1 Relative Pressure (P/P0) Figure 2.9 Nitrogen adsorption and desorption isotherm of the palladium NMFs using hydrazine synthesis method. The BET surface area is 11.5 m2/g.

33

The nitrogen adsorption—desorption isotherms obtained at liquid nitrogen temperature show the behaviors for silver and palladium NMFs using hydrazine synthesis method (shown in Figure 2.8 and 2.9). The position of the inflection point depends on the diameter of pores, and the sharpness indicates the narrow pore size distribution. Furthermore, the initial region can be extrapolated back to the origin, confirming the absence of any detectable micropore. The specific area measured using the BET analysis show 23.5 and 11.5 m2/g, respectively. The silver porous structure made at room temperature has a surface area of 6.07 m2/g, where palladium NMFs made at 150 °C of conventional heating has a surface area 4.68 m2/g. (Wang, Shi et al. 2012) Hence with the presence of microwave irradiation, a much higher surface area was obtained through our experiment. Since the porous network structures were formed through the fusion of metal nanoparticles emerged during the nucleation step, they were expected to have surface roughness at the nanoscale which could contribute to their high

BET surface area.

2.2.4 Possible Mechanism

These experimental results indicate that hydrazine is necessary for not only the reduction process but also for the formation of these foam structures in nano scale in our conditions. No metal will be generated without hydrazine in all the reactions. The generated N2 might be important to the formation of these Nano porous structures or framework. 1D or 2D structures can be self–assembled in the present of N2H4 according to the previous work. Hence, in this research, 3D

34 structures were obtained with the presence of ethylene glycol, ethanol and N2H4.

In addition, the relatively large viscosity of ethylene glycol solution might also help to prevent the inter–particle fusion of small particles.

N2H4

n+ M + C2H6O M(C2H6-nO) Ag/Pd Microwave

Scheme 1. Proposed chemical reaction mechanisms. (M = Ag, Pd)

Ethylene glycol, known as a good chelate in many metal complexes, was used in our experiments. Therefore, a possible first step might be the formation of EG complex, noted as M(C2H6-nO). The subsequent addition of N2H4 causes

2+ 2+ the precipitant of Ag or Pd metal in the presence of Ag and Pd , this phenomenon can be detected shortly after the addition. The entire process might be illustrated in Scheme 1. The formation of EG complex, M(C2H6-nO), might be another crucial factor in the generation of nano porous metal structures.

35

2.3 Sodium Borohydride Synthesis

2.3.1 Materials

Silver nitrate (CAS#7761-88-8, AgNO3, ≥ 99%, MW: 169.87), ethylene glycol

(CAS#107-20-1, C2H6O2, ≥ 99%, MW: 62.07) and ethanol (CAS#64-17-5, C2H6O,

≥ 99.5%, MW: 46.07) were purchased from Sigma Aldrich (St. Louis, MO).

Palladium (II) acetate (CAS#3375-31-3, C12H18O12Pd3, 99.95%, MW: 673.52), was obtained from Sterm Chemicals Inc. Sodium borohydride (CAS#16940-66-2,

NaBH4, ≥ 99%, MW: 37.8) was obtained from MP Biochemcials Inc. Platinum (IV) chloride (CAS# 13454-96-1, PtCl4, 99%, MW: 336.89) was purchased from Acros organics. All materials were used without any further purification.

2.3.2 Sodium borohydride synthesis process

Silver NMFs can be synthesized through reduction reaction using sodium borohydride. 0.034 g (0.2 mmol) silver nitrate, which contains 0.022 g silver, was first weighed out by the electric balance, and placed in a 10 mL microwave vial with a stir bar. 1 ml of ethanol and 1 ml of EG were injected in the vial to help prevent aggregation and mixed with silver nitrate at the same time. Magnetic stirring was then applied for at least 15 min until the solution became clear. 0.015 g sodium borohydride, calculated by mass balance, was added as a reducing agent after stirring, 15min additional stirring was necessary in order to ensure all the components were well mixed. Black particles were generated after stirring. A

36 microwave vial cap was added and placed in microwave. After 5 min of 150 W heating, a NMF was generated through this process. With the same procedure as the first synthesis process, a centrifuge was applied to wash foams. All the foams made by this procedure were preserved in a capped vial. The silver nanoparticles were estimated to be 10 to 20 nm in diameter. 0.016 g silver NMFs were produced in our lab with a 72.7 wt% of yield.

With the same procedure, palladium can also be generated in a relatively high yield. 0.062 g palladium acetate, containing 0.029 g palladium, was first mixed with 1mL EG and 1mL ethanol, 0.015 g sodium borohydride was then added after stirring. 150 °C and 5 min were the conditions for microwave heating process. Centrifuge was surely necessary for extracting foams. The foam generated also needed to be kept in a kept vial. 0.024 g palladium NMFs were obtained through this process with a yield of 82.8 %. (Pal, Shah et al. 2009;

Krishna, Sandeep et al. 2010; Bin Ahmad, Lim et al. 2011)

Platinum was tried using sodium borohydride. 0.03 g platinum chloride was first weight and also followed procedure above with the same amount of EG, ethanol and sodium borohydride. After 20 min stirring, the condition of microwave was set at 150 °C for 5min. All the other conditions were set the same. No platinum

NMFs were generated due to the pressure exceeding the set point of 300 psi.

The heating process stopped at 1.5 min, and at least 45 minutes was needed to wait until the pressure dropped to the released pressure.

37

2.3.3 Results and Discussions

Figure 2.10 – 2.12 shows the porous structure of silver NMFs made by sodium borohydride. A porous, interconnected structure can also be observed through the sodium borohydride process used microwave irradiation. Compared to the hydrazine synthesis, more pore ranges from 1 µm to 3 µm appeared in the following images, and the size of the ligaments creating the nanoporous network was smaller, which range from 100 nm to 150 nm. Close observation of these networks show that the ligaments are not of uniform size and often figured with many braches of similar size suggesting thir formation through the fusion of nanoparticles.

38

Figure 2.10 SEM image of silver NMFs with magnification of 10k made by NaBH4 in the condition of 5 min, 150 W heating.

Figure 2.11 SEM image of silver NMFs with magnification of 18k made by NaBH4 in the condition of 5 min, 150 W heating.

39

Figure 2.12 SEM image of silver NMFs with magnification of 70.0k made by NaBH4 in the condition of 5 min, 150 W heating.

Figure 2.13 – 2.15 provide the sponge–like porous network structure for palladium NMFs using sodium borohydride by microwave irradiation. These images indicate the sponge could be larger than 5 µm, which are aggregate irregular particles with size of 50 nm – 100 nm. (Figure 2.15) But the degree of inter particle fusion is not as uniform as that in silver NMFs. There palladium porous structures are more loosely organized in the comparison with the silver sponges, which may be related with the smaller size of the building block, for example, the sizes of the aggregated irregular particles for palladium NMFs obtained are of about 100 nm, which are smaller than the 150 nm – 300 nm of silver NMFs formed by microwave irradiation.

40

Figure 2.13 SEM image of Pd NMFs with magnification of 11k made by NaBH4 in the condition of 5 min, 150 °C heating.

Figure 2.14 SEM image of Pd NMFs with magnification of 20k made by NaBH4 in the condition of 5 min, 150 °C heating.

41

Figure 2.15 SEM image of Pd NMFs with magnification of 30k made by NaBH4 in the condition of 5 min, 150 °C heating.

Figure 2.16 shows the nitrogen adsorption—desorption isotherms obtained at liquid nitrogen temperature for palladium NMFs using sodium borohydride synthesis method. The specific area measured using the BET method show 83.0 m2/g. 81 m2/g and was shown from previous work to generate the porous structure at room temperature.(Krishna, Sandeep et al. 2010) Together with the morphology studies described above, the surface area values are correlated to the overall architecture of their porous structure as well as the size of the constituent particles.

42

800

BET surface area: 83.0 m2/g

700

600

500

400

adsorbed (cc/g) adsorbed 2 2 300

200 adsorption Volume of N ofVolume 100 desorption

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Relative Pressure (P/P ) 0

Figure 2.16 Nitrogen adsorption and desorption isotherm of the palladium NMFs using hydrazine synthesis method. The BET surface area is 83.0 m2/g.

Table 1 shows a comparison of BET surface area, for the silver and palladium

NMFs generated in two different reducing agents and different heating temperatures. From the results obtained in our lab, the NMFs we made have higher surface area reported so far for self-supported silver and palladium nanoporous structures in a simple, template-free method.

43

Table 1 Comparison of BET surface area of different procedures for generating NMFs

BET surface area (m2/g) Silver Palladium

Reducing agent N2H4 N2H4 NaBH4 Microwave 22.3 11.5 83 Room temperature 6.07 4.68 81

2.2.4 Possible Mechanism

The following redox reaction scheme will illustrate the possible mechanism

AgNO3 + NaBH4 Ag + H2 + B2H6 + NaNO3

Scheme 2 Chemical redox reaction for silver nitrate and sodium borohydride

Silver metal will be reduced from the silver nitrate by the presence of sodium borohydride. NaBH4 is not only necessary to the reduction of silver nitrate but also the formation of the porous structures. The borohydride anions were adsorbed onto silver nanoparticles and addition of EG prevented the aggregation of particles. With the help of microwave, the desired nanoparticles can be generated.

44

CHAPTER 3

APPLICATION FOR SILVER

3.1 Silver NMFs applied in SERS

Raman scattering is a phenomenon which was first reported by Fleischmann,

Hendra and McQuillan in 1974, who observed strong and potential–dependent

Raman signals from pyridine adsorbed on a silver electrode that had been electrochemically roughened in potassium chloride aqueous electrolyte.

(Fleischmann, Hendra et al. 1974) The original idea was to generate a high surface area on the roughened metal. After a short period, progress was made by Jeanmaire, Van duyne, Albrecht and Creighton in 1977, they found that a rough silver electrode produces a Raman spectrum that is a million fold more intense than what was expected. (Albrecht and Creighton 1977; Jeanmaire and

Van Duyne 1977)

SERS was applied primarily for observing analytes adsorbed on to Au, Ag and

Cu, and Li, Na and K. (Albrecht and Creighton 1977) Theoretically, any metal would be capable of exhibiting surface enhancement, but these metals satisfy calculable requirements and provide the strongest enhancement. In general, Au and Ag are often used as SERS substrates because they are air stable materials.

Metals such as Pd or Pt also exhibit enhancements of about 102–103 for excitation.

45

Indeed, silver and gold play crucial role in the development of SERS, advancement have been made for Ag and Au nanoparticles with various shapes and with coatings resulting in structures such as Shell–isolated enhanced Raman spectroscopy (Li, Huang et al. 2010), SiO2–encapsulated Au particles (Wustholz, Henry et al. 2010), 2D Au nanomushrrom arrays (Naya, Tani et al. 2008), polyhedral Ag mesocages (Fang, Liu et al. 2011), Si wafers with Ag or Au coating (Dinish, Yaw et al. 2011), and film over nanospheres (Biggs,

Camden et al. 2009).

As mentioned before, silver nanoparticles performed excellent as a substrate.

Many different analytes have been attempted such as rhodamine 6G and 1,2- benzenedithiol. In this work, R6G was used as an analyte. Different limits of detections were reported from many previous works. For instance, the detection limit was successfully reduced by six orders of magnitude from 2 x 10—9 M to 2 x

10—15 M by Yu Chuan Liu and coworkers (Liu, Yu et al. 2006), the rough silver was treated by electrochemical oxidation reduction cycles. (Wang, Ma et al.

2012) What’s more, Dar Nitzan et al. reported that the detection limit of R6G can be down to 10—12 M by the composite structure made of Ag nanoparticles adsorbed on GaN nanowires. (Dar, Wang et al. 2011) Andrzej Kudelski also showed his work to reduce the concentration of R6G to 10—8 M by an electrochemically roughened silver substrates. (Kudelski 2005) Ravula Thirupathi and coworkers showed a detection limit of 1 mM on a gold microrods. (Thirupathi and Prabhakaran 2011) What’s more, Selena Chan et al. reported a detection

46 limit of 10—4 M on small molecules from silver-coated silicon nanopores substrate using 785 nm excitation. (Chan, Kwon et al. 2003)

3.2 Results and Discussion

I-Raman microscope and spectrometer from B&W Tek Inc. was the instrument used through this process. A general procedure was provided for manual operation. To start the system, a key needed to be turned clockwise 90 degrees.

A probe was then positioned close to the sample and was opened by slide switch. Computer software for observation was initialed to find Raman signal.

The probe was repositioned to maximize the focal point of the laser beam on the sample by observing the Raman signal on the computer screen, adjusted until a maximum is reached (Figure 3.1). It should be noted that the ideal focal point is achieved when the regulator is placed against the sample. A dark scan should follow to ensure a clear background and a lower noise of the instrument. After adjusting exposure time and laser power, the sample can be scanned by this instrument, and a Raman spectrum will be shown on the screen.

47

Figure 3.1 Image of B&W TEK i-Raman microscope

In this work, the as-prepared silver NMFs were tested for SERS activity. We used R6G as an analyte to figure out the limit of detection with the presence of silver NMFs. R6G (CAS#989-38-8, C28H31N2O3Cl, 99%, MW: 479.01), was obtained from Sigma–Aldrich. For this purpose, 20 µL of R6G was drop as casted onto a slide containing 10 mg of the silver NMFs sample. Raman spectra were recorded at room temperature using a 785 nm laser as a source.

The characteristic signals for R6G were enhanced multifold when observed over the silver substrates.

48

612

12000

1362

1505

)

1307

564

10000

unit

777

1040

8000 1177 (a)

6000

(b)

4000 Raman Intensity (arbitrary Intensity Raman (c)

2000

(d) 0 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 Raman Shift (cm-1)

Figure 3.2 Raman spectra of R6G molecules deposit on surface of silver NMFs: (a) 6 × 10–5 M; (b) 6 × 10–6 M; (c) 3 × 10–6 M; (d) 2 × 10–6 M. The spectra have been scaled and vertically shifted to enhance the clarity of the presentation.

Figure 3.2 shows plots of Raman spectra of R6G molecules deposit on the surface of silver NMFs. The spectra presented have been recorded using excitation radiation of 785 nm. To deposit on silver NMFs substrate, silver substrate was covered by the drop of R6G aqueous solution, then after about 5 min, water was evaporated. The water evaporation was not completely homogeneous and, at the last stage of the evaporation, we observed on the

49 metal surface a rapid passing of a boundary between wet and dry regions. The size of R6G molecule is around 1.4 nm (Sasai, Fujita et al. 2002). Therefore, we make assumptions that in the densely packed R6G monolayer a single R6G molecule should take no more than 4 nm2 in area. The detection limit of 2 × 10–6

M of R6G is higher compare to the previous work using other metal substrates, or using different morphology treatment of silver Nano particles. From their data,

1mM and 5 × 10–6 M were relatively higher detection limit for R6G, but it should be pointed out there were also some other works that achieved much lower concentration using other gold substrates or different treatment of the surface.

The prominent features shown at 612, 777, 1307, 1362 and 1505 cm–1 were observed, in agreement with previous experimental and theoretical investigations. (Watanabe, Hayazawa et al. 2005; Jensen and Schatz 2006;

Guthmuller and Champagne 2008) The band at 612 cm–1 was obtained due to

Cx–Cx–Cx ring in-plane bending remains unaffected from the dye-host

–1 interactions. We also observed one band at 777 cm because Cx–H out-of-plane bending modes of both condensed rings are degenerate in the solution. In the higher wavelength region, between 1200 cm–1 and 1675 cm–1, most bands were easily observed. The resonance bands at 1307, 1362 and 1505 cm–1 arise due to aromatic Cx–Cx stretching modes. These modes gain intensity due to Franck-

Condon overlap integrals, which depends on the extent of displacement of the excited state potential well along the normal coordinate. (Saini, Kaur et al. 2005)

Almost no Raman signal can be collected from the surface of the wafer, although there are many Ag nanoparticles present. The reason could be that the deposited

50

Ag nanoparticles on the wafer are well separated, and seldom aggregate to create hot spots for strong SERS enhancement. The enhanced electromagnetic field around a single Ag nanoparticle is too weak to enhance the Raman signal of the adsorbed detectable level. The detectable Raman signal is only acquired from the area where the 3D porous structures exist, which indicates the 3D structures serves as nanoparticle holders that have more nanoparticle aggregations than a 2D system. This porous structure could offer stronger couplings between metal nanoparticles and form a better SERS substrate compared to 2D system. From each experiment, each SERS-aggregate gives a somewhat different Raman signature and the absolute intensity and relative ratio of the sharp R6G Raman lines to the underlying continuum vary from one aggregate to the others.

51

CHAPTER 4

APPLICATION FOR PALLADIUM

4.1 Palladium NMFs applied in MO decoloration

As discussed in the Introduction Section, environmental pollution caused by azo dyes has been reported many times recently. Different catalysts and photocatalysts have been discovered and engineered to remove these environmental pollutants. Palladium NMFs, serve as an excellent catalyst aiming at the destruction of contaminants in water and air in non–toxic processes and low operational temperature. Palladium NMFs have been investigated as suitable catalyst for MO decoloration. (Hakamada, Hirashima et al. 2012)

Several methods have been reported to improve this procedure by using bimetals or in other words, binary alloy. To improve the efficiency and rate of decoloration is the most attractive topic for this application, however, from the recent research by Masataka Hakamada et al, smaller ligaments enhanced the reaction not only due to the increase of surface area, but also related to defective and strained surfaces, which are unique characteristics in nanoporous metals.

They have significant effect on the MO decoloration. (Weissmuller 2003;

Hakamada, Hirashima et al. 2012)

From some other current researches, binary alloy, in other words, bimetals will improve the rate of decoloration to 10%–15% and can make MO more colorless than single metal because bimetals can enhance intermolecular effect. For

52 example, palladium can be alloyed with oxide to enhance the reducing ability.

4.2 Results and Discussion

Solid methyl orange (CAS#547-58-0, (CH3)2NC6H4:NC6H4SO3N2, 90%, MW:

327.33), was purchased from Fisher Scientific company. 0.082 g solid MO was mixed with 500 mL water in order to obtain the MO solution with an initial concentration of 1×10—3 mol/L. Lower concentration of MO solution was obtained by diluting the most concentrated solution. 5×10—4, 1×10—4, 5×10—5 and 2×10—5 mol/L of MO were studied through our work. In this experiment, 0.03 g palladium

NMFs, generated by sodium borohydride synthesis process, was first added in 5 mL MO solution. A decoloration reaction then spontaneously occurred at room temperature. A 0.05 mL sample needed to be extracted at different hours for recording to discover the reducing limitation of Palladium catalyst. All the samples were kept in a capped vial. A 96–well UV–Vis plate reader was used for detecting all the samples. Every sample was extracted of 0.05 mL from each capped vial, and then move to each UV–Vis well. The decolorization was monitored by UV-Vis measurement during a time interval under dark condition to be distinguished from photocatalytic degradation conventionally observed TiO2 and ZnO. The decay of the absorbance at around 470 nm, was observed over time. For instance, Figure 4.1 illustrates the UV–Vis spectra of the MO solution at room temperature with different immersion time of 2h, 4h, 6h etc. The obvious decrease of absorbance peak shows that palladium NMFs can serve as an

53 effective photocatalyst. The initial concentration of MO was 1×10—3 mol/L in this experiment. After around 28h’s immersion, the absorbance significantly decreased. Similar plots are provided also for each concentration of different MO solution.

2

1.8

) 1.6 MO

4h

unit

1.4 7h 1.2 18h 20 1 22h 0.8 24h 26h 0.6 28h 44h

Absorbance (arbitraryAbsorbance 0.4 48h 0.2 52h

0 350 400 450 500 550 Time (min) 5

Figure 4.1 UV-Vis spectra of the MO solution (1×10—3 mol/L) before and after different hour’s immersion at room temperature.

54

2

1.8

MO

1.6 2h 1.4 4h 6h 1.2 8h

(arbitary unit) (arbitary 10h 1 12h 0.8 24h 0.6 26h 28h Absorbance 0.4 36h 0.2 48h

0 350 400 450 500 550

Time (min)

Figure 4.2 UV-Vis spectra of the MO solution (5×10—4 mol/L) before and after different hour’s immersion at room temperature. 0.4

0.35

) MO 2h

unit 0.3 4h 0.25 6h 8h 0.2 10h 12h 0.15 24h 26h

0.1 28h Absorbance (arbitraryAbsorbance 36h 0.05 48h

0 350 400 450 500 550

Time (min)

Figure 4.3 UV-Vis spectra of the MO solution (1×10—4 mol/L) before and after different hour’s immersion at room temperature.

55

0.25

) 0.2 MO

unit 2h

4h 0.15 6h 8h 10h 0.1 12h 22h

24h

Absorbance (arbitraryAbsorbance 0.05 26h 28h

0 350 400 450 500 550 Time (min) Figure 4.4 UV-Vis spectra of the MO solution (5×10—5 mol/L) before and after different hour’s immersion at room temperature.

0.14

0.12

) MO

unit 2h 0.1 4h 0.08 6h 8h 10h 0.06 12h 22h 0.04 24h

Absorbance (arbitraryAbsorbance 26h 0.02 28h

0 350 400 450 500 550 Time (min)

Figure 4.5 UV-Vis spectra of the MO solution (2×10—5 mol/L) before and after different hour’s immersion at room temperature.

56

Figure 4.2-4.5 shows the UV-Vis spectra of MO solutions in the concentrations of

5×10—4, 1×10—4, 5×10—5 and 2×10—5 mol/L generated by Microsoft Excel. All data from figure 4.1-4.5 were obtained from the UV–Vis Microplate

Spectrophotometer, each line represented a specific immersion time period. For example, the line of 4 h represents palladium NMFs reacting with MO solution for

4 hours. The initial MO solution was also presented in the plot for comparison.

Obviously, highest intensity appeared at the wavelength of 460 nm – 490 nm, the intensity at the visible light wavelength significantly reduced from 1.826 to 0.452.

The intensity of the azo dye solution in the visible light region is caused by an azo group (–N=N–). (Denney, Sinclair et al. 1987) MO contains two phenyl rings bridged by an azo group in its chemical structure, therefore it is surmised that

MO decomposed to single phenyl rig compounds in the presence of palladium

NMFs.

To discover the time variation of the MO concentration after sample immersion, the ratio between concentration at any time and the initial concentration were necessary to calculate. Beer’s law shows how to calculate those values with different immersion times. C0 is the initial concentration of MO solution, C can be obtained from Beer’s law, the equation 1.

(Equation 1)

57

Where C is the concentration of the compound in solution, expressed in mol L—1.

A is absorbance; b is path length of the sample of each well, which means a path length of the cuvette in which the sample is contained, expressed in centimeters.

ε is the molar absorptivity with unit of L mol—1 cm—1 which varies on different solutions. It is independent to the concentration of solution. Hence, from Beer’s

Law, we can easily find,

(Equation 2)

Time variation of the MO concentration after sample immersion is shown in

Figure 4.6. Different concentration of MO solution immersed with the same 0.03 g of palladium NMFs were presented. Concentration of MO solution was significantly reduced after 12h immersion, but with the increasing of concentration, the reaction rate was also reduced, which means more time was needed until MO degraded to a relatively low concentration and remaining more than 2 hours. For the concentration of 2×10—5 mol/L, more than 55 % of azo dyes was degraded after 12 hours immersion, however, when achieve a higher concentration of 1×10—3 mol/L, the percentage of degradation was only around

30 % at the same immersion time.

58

1

0.9

0 0.8

0.7

0.6

0.5 2.00E-05

MO concentration, C/C concentration,MO 5.00E-05 1.00E-04 0.4 5.00E-04 1.00E-03 0.3 0 100 200 300 400 500 600 700

Time (min)

Figure 4.6 Time variation of MO concentration after immersion of palladium NMFs, different concentration of MO solution were also provided.

The degradation curves of the MO azo dyes by palladium NMFs are well fitted by a mono-exponential curve, suggesting that a pseudo first-order homogeneous reaction model can be taken into consideration for describing the kinetic behavior. To understand the reaction kinetics of MO degradation under different concentrations, the following pseudo first-order model was used to analyze our degradation data:

59

( ) (Equation 3)

where k is pseudo first-order rate constant; C and C0 are defined previously.

Logarithmic plots shown on figure 4.7 are in agreement with the results from previous work. (Li, Li et al. 2006) However, with the increasing of initial concentration of MO solution, the reaction rate constant decreased because k is the apparent pseudo first-order rate constant and is affected by dyestuff concentration. As figure 4.7 presented, the values of k can be obtained directly from the regression analysis of the linear curve in the plot. Values of R2 are also provided on table 1, the linear trendlines fit well with a high value of R2, so we believe the degradation reaction satisfies the pseudo first-order assumption. The decrease of MO concentrations is not simply proportional to the surface area, a higher initial concentration of MO solution can decrease the reaction rate constant.

60

Time (min) 0 0 100 200 300 400 500 600 700 -0.1

-0.2

-0.3

-0.4

) 0

-0.5 ln(C/C -0.6

-0.7 2.00E-05 5.00E-05 -0.8 1.00E-04 -0.9 5.00E-04 1.00E-03 -1

Figure 4.7 Time variation of MO concentration after the immersion of palladium NMFs with different concentration of MO solution measured according to a logarithmic scale.

Table 2 Kinetics constants for increased MO concentration

Concentration (mol L—1) Reaction Constant, k (min—1) R2 2.00E-05 1.1 × 10—3 0.9984 5.00E-05 1× 10—3 0.9968 1.00E-04 0.9 × 10—3 0.9920 5.00E-04 0.8 × 10—3 0.994 1.00E-03 0.6 × 10—3 0.9928

61

Masataka Hakamada et al. reported a similar method by using nanoporouus palladium fabricated by dealloying (Hakamada and Mabuchi 2009; Hakamada and Mabuchi 2009; Hakamada, Tajima et al. 2010) and subjected to the MO degradation also under dark condition. Compare to their work with the same of concentration of 2×10—5 mol/L, palladium NMFs made in our lab has higher degradation rate in the first 12 hours, but relatively lower in the second 12 hours.

Within the same 12 hours immersion time and the same concentration of MO solution, the percentage of degradation deceased from 1 to 0.44 by palladium

NMFs, whereas nanoporous palladium from paper can only reduce the ratio from

1 to 0.56. This is mainly because of the higher surface area 82 m2/g. As a result, the concentration of MO solution has firmly decreased by the simple immersion of the palladium NMFs. The comparative experiments indicated that the atomically defective and strained surfaces, which are unique characteristics in nanoporous palladium, have a significant effect on the MO decoloration. The reaction rate constant agrees with the behavior of pseudo first-order reaction, and this needs no light irradiation unlike phtotcatalysts, and is beneficial in the removal of textile industrial pollution.

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CHAPTER 5

Future Work and Conclusion

5.1 Future work

Besides silver and palladium NMFs, many other metals including gold, copper, nickel, and platinum have also been researched for several years. (Shin, Dong et al. 2003; Hunt, Pantoya et al. 2006; Tappan, Steiner et al. 2010; Li, Enshuai et al.

2012; Cheng 2013) Gold nano porous structure was successfully produced in lab scale at room temperature by Katla and coworkers (Krishna, Sandeep et al.

2010) or microwave heating in our lab. Platinum, however, is still being researched in the lab due to the high reaction pressure when heating by microwave irradiation. The pressure exceeds the 300 psi, which is maximum pressure for microwave, and it takes about 45 minutes for pressure reduction.

Nickel was also discovered for several months in our lab but it has the same pressure problem with platinum. The good news is, nickel NMFs can be generated through this microwave irradiation process although it takes about 1 hour till the pressure dropsdown.

Future work includes attempting the microwave irradiation synthesis of using other metals such as copper and platinum, and studying methods to save time of pressure drop for nickel. Once optimized, these NMFs could be applied in many other fields. For example, nickel NMFs can be applied in heat exchangers, energy absorption and lightweight optics. (Zhou, Wang et al.

63

2012; Wang, Yi et al. 2013) Platinum NMFs is good to use for dust suppression and even still–foam drilling. (Li, Misra et al. 2009; Luther, Tappan et al. 2009) In addition, future work may also include the consideration of different reducing agents, in order to determine the influence on different metal foams.

Research on the lower detection limit of analyte is also included in future work, the morphology treatment of NMFs need to be studied to analyze the reason why

R6G cannot be detected in a lower concentration with the presence of silver

NMFs. Future work may also include ways to improve the degradation rate of palladium NMFs to decolor MO solution.

5.2 Conclusion

Nanoporous metal foam is a three–dimensional structure with high porosity, high surface area and low density. It exhibits many useful properties such as low density, large surface area, excellent electrical and thermal conductivity, good mechanical properties, high strength–to–weight ratio and catalytic activity, due to the combination of different metals, such as Ag, Au and Pd.

In summary we have developed new synthesis method to produce Nano metal foams by microwave heating. Obviously, this type of synthesis is based on principle of redox reaction. Metal compound serves as an oxidant, and hydrazine or sodium borohydride serve as reductant. Microwave heating promotes the reaction moving forward to the reduction of the metal compound. And metal 64 foams can be generated in the presence of EG, NaCl or ethanol. Gold, silver, palladium, nickel have been researched for long time, and the foams made by these metals were already successfully generated in our lab. Microwave is necessary through the entire heating process due to the high efficiency and high coverage area compare to conventional heating. This synthesis method can produce NMFs in relative short time with higher BET surface area.

We also present the applications of these foams especially for silver and palladium NMFs. Silver NMFs can be applied in Surface Enhanced Raman

Spectroscopy where this material can enhance the intensity of signal with relatively low concentration of aqueous R6G. The detection limit of R6G can be achieved as 2 × 10–6 M with presence of silver NMFs, where almost no detectable points existed in the concentration of 6 × 10–5 M without silver NMFs.

The prominent features shown at 612, 777, 1307, 1362 and 1505 cm–1 were observed, in agreement with previous experimental and theoretical investigations.

We also present an application for green chemistry. This material can be used for the decoloration of MO solution because it serves as an excellent catalyst aiming at the destruction of contaminants in water and air in non-toxic processes and low temperature. Different concentrations of MO solution were studied to investigate the ability of palladium NMFs. For each concentration, the absorbance of MO solution decreased significantly after 20h immersion with

0.03g palladium NMFs under dark condition at room temperature. The data of reaction rate constant agrees with the behavior of pseudo first-order reaction,

65 and this catalysis needs no light irradiation unlike phtotcatalysts, and is beneficial in the removal of textile industrial pollution.

66

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