Electrosynthesis of Hydrogen Peroxide in an Acidic Environment with
RuO2 as a Water Oxidation Catalyst
&
Silver Nanoparticles in Zeolite Y: Surface Enhanced Raman Spectroscopic
(SERS) Studies
THESIS
Presented in Partial Fulfillment of the Requirements for the Degree Master of
Science in the Graduate School of The Ohio State University
By
Kevin D. Cassidy, B.S.
Graduate Program in Chemistry
The Ohio State University
2011
Committee:
Professor Prabir K. Dutta, Advisor
Professor Susan V. Olesik
Abstract i
Hydrogen peroxide (H2O2) was electrosynthesized by reduction of
dissolved oxygen in a H2SO4 solution.
In the electrosynthesis of H2O2 the
evolution of oxygen on the anode is the rate limiting step. To increase the rate of
O2 evolution at the reticulated vitreous carbon (RVC) anode, rutile RuO2 was
electrodeposited onto the anode. The RVC anode electrodeposited with RuO2
showed a 200 % increase in current efficiency over the plain RVC anode of 16.77
% to 48.08 %, and demonstrated a comparable efficiency to a Pt wire anode.
Operational parameters such as H2SO4 concentration, oxygen flow rate, effect of
O2 vs. air being pumped into solution, and cathodic potential were systematically
studied to improve the current efficiency of the electrochemical cell. Results
indicated that the optimal conditions for the generation of H2O2 are a cathodic
potential of -0.55 V vs. Ag/AgCl reference electrode, oxygen flow rate of 100
units, 0.1 M H2SO4 concentration, and O2 gas pumped into solution.
ii
Abstract ii
Previous studies have shown that when silver nanoparticles are in close
proximity (1-2 nm) and exposed to visible light the resulting electric fields overlap
and produce a strong surface-enhanced Raman spectroscopy (SERS) signal
greater than a single particle’s response. The difficulty lies in producing silver
particles that are both of the proper size (no greater than 20 nm) and proximity.
Zeolite Y is a mesoporous and microporous material whose structure’s pore size
and distance between pores would be ideal to use as a platform for silver
nanoparticle synthesis. The zeolite framework contains cations that are readily
exchanged with silver ions and the pore size itself would prevent the silver from
aggregating into an unsuitable size. In this work silver was ion-exchanged into
Na zeolite Y from a 0.1 M AgNO3 solution. The Ag+ zeolite Y was then reduced
with Hydrazine (N2H4) to form silver nanoparticles inside the zeolite cages. By
varying the concentration of the hydrazine and injecting the hydrazine in portions
over time a SERS capable platform was produced.
iii
Dedicated to my family and friends for keeping me sane
iv
Acknowledgements
I would like to thank my advisor Dr. Prabir K. Dutta for his patience and
guidance throughout my research projects. I would also like to thank for their
suggestions and support past and present group members: John Spirig, William
Schumaker, Toni Ruda, Haoyu Zhang, Joe Obirai, Cheruvallil Rajesh, Supriya
Sabbani, Xiaogan Li, Weizhen Xiong, Brian Peebles, Jeremy White, Adedunni
Adeyemo, Julia Rabe, Michael Severance, Betsy Heck, Andrew Zane, Suvra
Mondal, Joselyn Del Pilar, and Prasenjit Kar.
v
Vita
April 3rd, 1982.....................................................................Born – Portsmouth, OH
2005.......................................................................B.S. Chemistry, Shawnee State
University
2006-present...........................................................Graduate Teaching Associate,
The Ohio State University
Fields of Study
Major Field: Chemistry
Minor Field: Analytical Chemistry
vi
Table of Contents
Page
Abstract..................................................................................................................ii
Dedication.............................................................................................................iv
Acknowledgements................................................................................................v
Vita........................................................................................................................vi
List of Tables..........................................................................................................x
List of Figures.......................................................................................................xii
Chapters:
1 Electrosynthesis of Hydrogen Peroxide in an Acidic Environment with RuO2 as
a Water Oxidation Catalyst
1.1 Introduction.........................................................................................1
1.2 Experimental.......................................................................................10
1.2.1 Voltage Selection……………………………………………..…10
1.2.2 Cell…………………………………………………...........……..10
1.2.3 Electrode Holders……………………………………………….17
1.2.4 Membrane……………………………………………..…………22
1.2.5 Electrodes………………………………………………..………25
1.2.6 Electrosynthesis…………………………………………………30
1.2.7 Electroparamagnetic Resonance (EPR)……………………...31
1.3 Results and Discussion......................................................................32
vii
1.4 Conclusion.........................................................................................43
1.5 References........................................................................................44
2 Silver Nanoparticles in Zeolite Y: Surface Enhanced Raman Spectroscopic
(SERS) Studies
2.1 Introduction........................................................................................48
2.1.1 Motivation……………………….............................................48
2.1.2 Previous Studies on SERS Substrates.................................52
2.2 Experimental......................................................................................56
2.2.1 Silver Loading and Reduction……………………………........56
2.2.2 X-ray Diffraction (XRD)……………………….........................57
2.2.3 Raman Spectroscopy…………………………………………..58
2.3 Results and Discussion......................................................................58
2.3.1 Formation of Highly Reduced Silver Zeolite.........................58
2.3.2 Raman Studies on Highly Reduced Silver Zeolite with
Pyridine as Analyte........................................................................64
2.3.3 Raman Studies with Partially Reduced Silver Zeolites with
Pyridine as Analyte........................................................................81
2.3.4 Raman Studies with Partially Reduced Silver Zeolites with
Benzenethiol as Analyte................................................................87
2.3.5 SERS Sample Preparation by Initial Hydrazine Reduction
Followed by Laser Reduction........................................................87
2.3.6 SERS Sample Preparation by Simultaneous Hydrazine and
Laser Reduction............................................................................88
viii
2.3.7 SERS sample preparation by initial hydrazine reduction
followed by photolysis of solid surface..........................................89
2.3.8 SERS Sample Preparation by Initial Hydrazine Reduction
Followed by Photolysis of the Entire Sample................................90
2.4 Conclusion.......................................................................................109
2.5 References......................................................................................110
Bibliography………………………………………..…………………………………115
ix
List of Tables
Table 1.1: Common Industrially Available Oxidants..............................................3
Table 1.2: Plain RVC Working Electrode/Pt Wire Auxiliary Electrode.................33
Table 1.3: Plain RVC Working Electrode/Ruthenium (II) Oxide-Modified RVC...34
Table 1.4: Plain RVC Working Electrode/Plain RVC Auxiliary Electrode............35
Table 2.1: Pyridine Peak Assignment..................................................................68
Table 2.2: SER Spectral Data (cm-1) and Vibrational Assignments of
Benzenethiol Adsorbed on Silver Powder............................................................94
x
List of Figures
Figure 1.1: Reactions of the AO process...............................................................6
Figure 1.2: Simplified Riedl-Pfleiderer Reaction....................................................7
Figure 1.3: Reduction Peak Position....................................................................11
Figure 1.4: Effect of De-Aerating the Solution.....................................................12
Figure 1.5: Initial Cell Design...............................................................................13
Figure 1.6: Tubular U-Shaped Cell Design..........................................................14
Figure 1.7: Current Cell Design...........................................................................16
Figure 1.8: Initial Teflon Electrode Holders (Images a and b are the lids for the
anodic compartment, images c and d are for the cathodic compartment)...........18
Figure 1.9: Platinum Wire Electrode Holder........................................................19
Figure 1.10: Teflon Electrode Holder with Sleeve................................................21
Figure 1.11: NAFION® Chemical Formula...........................................................23
Figure 1.12: Electrodeposition of RuCl3 on RVC.................................................26
Figure 1.13: SEM Images of the RVC Foam Before and After Depositing
RuO2
............................................................................................................................28
xi
Figure 1.14: RVC Anode After Decomposition in Cathodic Solution...................36
Figure 1.15: RVC Anode After Decomposition Dried...........................................37
Figure 1.16: RVC Anode Before Run in 1 M Sulfuric Acid...................................38
Figure 1.17: Plain RVC Dry..................................................................................39
Figure 2.1: Raman Scattering..............................................................................55
Figure 2.2: Comparison of Synthesized Nano-Scale Zeolite Y (Upper) to that of
the Previously Characterized Nano-Scale Zeolite Y Standard (Lower)...............61
Figure 2.3: TEM of Excess Hydrazine Treated Ag+ Zeolite Y..............................62
Figure 2.4: XRD Pattern of Nano-Scale Zeolite Y Ion-Exchanged with 0.1M
AgNO3 and Reduced with Hydrazine. Pattern Matches for Silver (Upper) and
Zeolite Y (Lower) are Included for Comparison...................................................63
Figure 2.5: Liquid Pyridine (b), Aqueous Pyridine (c) Raman Spectra................66
Figure 2.6: SERS Spectra of Pyridine on Silver..................................................67
Figure 2.7: Raman Spectra of 0.1M Pyridine Solution at 3.2 mW Power with a
633 nm Excitation Line........................................................................................69
Figure 2.8: 3.2 mW Power Scan with a 633 nm Excitation Line of a 0.1 M
Pyridine Solution on Nanoscale Ag Zeolite Y......................................................70
Figure 2.9: Nanoscale Ag Zeolite with 0.1 M Pyridine 0.4 mW Power 633 nm
Excitation Line.....................................................................................................71
xii
Figure 2.10: Micronscale Ag Zeolite with 0.1 M Pyridine 3.2 mW Power with a
633 nm Excitation Line.........................................................................................72
Figure 2.11: BaSO4 Raman Spectrum at 3.2 mW Laser Power with a 633 nm
Excitation Line......................................................................................................75
Figure 2.12: BaSO4 Pellet with Ag Zeolite Y with 0.1 M Pyridine Cycling Focus
Scan at 3.2 mW Laser Power with a 633 nm Excitation Line..............................76
Figure 2.13: BaSO4 pellet with Micron Ag Zeolite Y with 0.1 M pyridine at 3.2 mW
Laser Power with a 633 nm Excitation Line.........................................................77
Figure 2.14: pH dependence of Pyridine/Pyridinium Conversion........................78
Figure 2.15: Calcined Ag Zeolite Y Pellet with 0.1 M Pyridine at 3.2 mW Laser
Power with a 633 nm Excitation Line...................................................................79
Figure 2.16: Calcined Ag Zeolite Y Pellet with 0.1 M Pyridine Evaporated at 3.2
mW Laser Power with a 633 nm Excitation Line..................................................80
Figure 2.17: Controlled Addition of Hydrazine with 0.1 M Pyridine at 3.2 mW
Laser Power with a 633 nm Excitation Line.........................................................83
Figure 2.18: 0.1 M Pyridine on Ag Zeolite Y after Laser Illumination 3.2 mW
Power off Spot with a 633 nm Excitation Line......................................................84
Figure 2.19: Single Drop 0.1 M Pyridine on Ag Zeolite Pellet at 3.2 mW Laser
Power with a 633 nm Excitation Line...................................................................85
xiii
Figure 2.20: Single Drop 0.1 M Pyridine on Na Zeolite Pellet at 3.2 mW Laser
Power with a 633 nm Excitation Line...................................................................86
Figure 2.21: Raman (top) and SERS spectra (bottom) of benzenethiol..............93
Figure 2.22: Raman Sample Holder 1 cm Wide x 1 cm Long x 1/2 cm Deep......94
Figure 2.23: Raman Spectra of Teflon Wafer at 3.2 mW Laser Power with a 633
nm Excitation Line................................................................................................96
Figure 2.24: 0.1 M Benzenethiol on Ag Zeolite Pressed in Wells at 3.2 mW Laser
Power with a 633 nm Excitation Line...................................................................97
Figure 2.25: Signal Intensity of Benzenethiol Concentration 0.1 M Benzenethiol
Red and 0.001 M Benzenethiol Black (Spectra Collected at 3.2 mW Laser Power
with a 633 nm Excitation Line).............................................................................98
Figure 2.26: 0.001 M Benzenthiol on Ag Zeolite Reduced with 1.90*10-3 M
Hydrazine at 3.2 mW Laser Power with a 633 nm Excitation Line......................99
Figure 2.27: 0.001 M Benzenthiol on Ag Zeolite with 9.51*10-3 M Hydrazine
Reduction at 3.2 mW Laser Power with a 633 nm Excitation Line....................100
Figure 2.28: 0.001 M Benzenethiol on Ag Zeolite Pressed in Wells at 3.2 mW
Laser Power with a 633 nm Excitation Line......................................................101
Figure 2.29: Spectral Shifts for Benzenethiol Binding to Different Cluster
Sizes
..........................................................................................................................102
xiv
Figure 2.30: Ethanol Raman Spectra at 3.2 mW Laser Power with a 633 nm
Excitation Line....................................................................................................103
Figure 2.31: Effect of Hydrazine Concentration with Surface Photolysis Setup
(Collected at 3.2 mW Laser Power with a 633 nm Excitation Line)...................104
Figure 2.32: Effect of Hydrazine Concentration with Suspension Photolysis Setup
(Collected at 3.2 mW Laser Power with a 633 nm Excitation Line)...................105
Figure 2.33: 2.57 mM Hydrazine Reduction TEM (Top Left is at 50 nm
Resolution, Top Right is at 20 nm Resolution, and Bottom is at 100 nm
Resolution).........................................................................................................106
Figure 2.34: XRD of Photolyzed Zeolite Y Suspension with 640 C NIST
Standard............................................................................................................107
Figure 2.35: XRD of 2.57 mM Hydrazine Reduced Ag Zeolite..........................108
xv
Chapter 1
Introduction
1.1.1 Hydrogen Peroxide
Wastewater historically was either treated with formaldehyde, chlorine or
iron salt, but increasingly there has been a demand for safer and more
environmentally friendly methods such as hydrogen peroxide treatment. The
concentration of hydrogen peroxide required for wastewater treatment has
expensive transportation and storage costs; this led to a demand for a cheaper
on-site production method. Traditional techniques of hydrogen peroxide
generation involve nonaqueous solvents and dangerous byproducts, whereas
electrochemical production minimizes the byproduct formation [1]. By careful
selection of electrodes and catalysts hydrogen peroxide can be electrochemically
generated from water and oxygen.
Hydrogen peroxide is used in a wide range of industries, such as; pulp and
paper bleaching, textiles, detergents, electronics, metallurgy, wastewater
treatment, and as a chemical oxidizer. Given its decomposition products of water
and oxygen, H2O2 is by far a safer alternative than most chemicals used in these
industries. There are a wide range of reactions that H2O2 can undergo, seen in
equations 1-5. Hydrogen Peroxide can be added to urea to create urea
hydrogen peroxide [Eq. (1)], a teeth whitener.
Addition:
H2O2 + (NH2)2CO  (NH2)2CO H2O2
1
(1)
H2O2 is considered to be an environmentally safe substance, because when
H2O2 decomposes it breaks down into water and oxygen [Eq. (2)].
Decomposition:
2H2O2  2H2O + O2
(2)
H2O2 is a strong oxidizing agent [Eq. (3)]. H2O2 is a more efficient oxidizing
agent than most other oxidants as can be seen in Table 1.1, H2O2 has a higher
active oxygen content than most commercially used oxidants and has a more
benign environmental response since it reduces to water during the reaction.
Oxidation:
H2O2 + M  MO + H2O
(3)
As a reducing agent H2O2 is capable of reducing oxidizing agents stronger than
itself [Eq. (4)], such as KMnO4, which is used in this thesis for the determination
of H2O2 produced, and Ce(SO4)2, which can also be used to determine the
concentration of H2O2.
Reduction:
H2O2 + Rn+2  Rn +2H+ + O2
(4)
H2O2 is also used in organic syntheses for substitution reactions [Eq. (5)].
Substitution:
H2O2 + RX  ROOH + HX
2
(5)
Oxidant
Active Oxygen (% w/w)
By-Product
H2O2
47.1
H2O
tBuOOH
17.8
tBuOH
HNO3
25
Nox, N2O, N2
N2O
36.4
N2
NaClO
21.6
NaCl
NaClO2
35.6
NaCl
NaBrO
13.4
NaBr
KHSO5
10.5
KHSO4
NaIO4
29.9
NaI
PhIO
7.3
PhI
Table 1.1: Common Industrially Available Oxidants [2]
3
Hydrogen peroxide (H2O2) was first identified by Louis Jacques Thenard in
1818 as a product of the substitution reaction of barium peroxide and nitric acid,
seen in equation 6.
BaO2 + 2HNO3 → H2O2 + Ba(NO3)2
(6)
Thenard then changed the reaction mixture from nitric acid to sulfuric acid [Eq.
(7)].
BaO2 + H2SO4  H2O2 + BaSO4
(7)
Prior to the 1940’s most commercial production of H2O2 involved inorganic
processes with the electrolysis of an aqueous sulfuric acid solution or acidic
ammonium bisulfate (NH4HSO4), followed by hydrolysis of the peroxodisulfate
((SO4)2)2− which is formed, this resulted in a higher concentration of H2O2
produced than the barium salts method.
The most widely used current production method of H2O2 began in the
1940’s. This is the anthracene autoxidation process (AO), the main reactions
follow the Riedl-Pfleiderer process [3]. A 2-alkylanthraquinone (AQ, typically 2ethylantraquinone) in an appropriate solvent or mixture of solvents is
hydrogenated in the presence of a catalyst to a anthraquinol or
anthrahydroquinone (AHQ). A side reaction involving the hydrogenation of the
unsubstituted aromatic ring in AHQ results in the formation of 5,6,7,8tetrahydroanthrahydroquinone (THAHQ). This side reaction results in the need
to replenish the AQ source eventually as the efficiency drops. The AHQ and
THAHQ are separated from the catalyst and oxidized in the presence of air to
4
regenerate the AQ and form tetrahydroanthraquinone (THAQ) and an equal
amount of H2O2. The series of reactions can be seen in Figure 1.1. A simplified
mechanism for the whole reaction can be seen in Figure 1.2, which leads to the
overall process governed by the reaction in Eq. 8.
H2 + O2  H2O2
(8)
This reaction doesn’t take into account the aforementioned, and other, side
reactions that lead to a consumption of AQ. Since the process has been used
there haven’t been any other methods efficient enough to convince most
manufacturers to replace it. There has however been a good deal of research
done to enhance the process.
Most refinements in the process have had to deal with the catalyst used in
the hydrogenation step. This step has the most side reactions occurring out of all
of the steps. Ni and Pd catalysts supported on SiO2, Al2O3 or a mixture of
Al2O3/SiO2 are the most common catalysts used in industry today [4]. Newer
catalysts for this reaction have focused on amorphous Ni/B [5,6] and Ni/Cr/B [7]
alloys. If La is added to the Ni/B alloy at an atomic ratio of La/Ni = 0.34 it leads to
an increase in the hydrogenation activity. La causes a better dispersion of Ni
which results in a greater number of hydrogenation sites [8,9]. The incorporation
of the Cr into the Ni/B alloy leads to a greater stability in the reaction system,
which results to an increase in yield of H2O2 and a minimization of side reactions
[4].
From the 1950’s to 1980 Shell ChemicalTM produced H2O2 from the
oxidation of primary and secondary alcohols, producing an aldehyde from
5
Figure 1.1: Reactions of the AO process
6
Figure 1.2: Simplified Riedl-Pfleiderer Reaction
7
primary alcohols and a ketone from secondary alcohols [10, 11]. These
reactions can be seen in Equations 9 and 10.
CH3-CH2-CH-OH + H2  H2O2 + CH3-CH2-CH=O
(9)
(10)
Due to the solubility of alcohol in the peroxide phase the purity is lower with this
method than the AO process.
H2O2 was first electrochemically synthesized by Meidinger in 1853.
Meidinger oxidized sulfate to persulfate at the anode and then produced H2O2 via
the hydrolysis of the persulfate [12] at the cathode. This process was used as
the primary synthesis method of H2O2 until the anthraquinone method displaced
it.
H2O2 can also be synthesized electrochemically by electrolysis of a dilute
NaOH solution in an electrolytic cell, this is referred to as the Dow Process. The
process involves the following reactions [Eq. 11, 12, 13].
Anode:
2OH-  H2O + ½O2 + 2e-
(11)
Cathode:
H2O + O2 + 2e-  HO2- + OH-
(12)
Overall:
NaOH + ½O2  HO2Na
(13)
A cell being supplied with 2.3 V and 62 mA cm-2 yields a NaOH/HO2- weight ratio
ranging from 1.6–1.8:1 with a current efficiency of 90%. Pairing the trickle-bed
cathode, where a co-current reactant gas and liquid flow through fixed beds of
8
catalyst particles, with an ion-exchange membrane affords a 2.1% w/w HO2solution in 5.0% w/w NaOH and increases current efficiency to 95% [13].
Lobyntseva et al. used a modification of the trickle-bed process above,
where the trickle-bed electrode was replaced with a rotating disk cathode
consisting of a high-surface area glassy carbon electrode modified with either an
athraquinone derivative or gold nanoparticles paired with a platinum foil counter
electrode and a saturated calomel reference electrode in a 0.1 M KOH solution
[14].
The work done for this thesis is not the first time RVC has been utilized as
a cathode material for H2O2 electrosynthesis, nor is it the first time H2O2 has
been synthesized in an acidic environment.
Alvarez-Gallegos and Pletcher
utilized the high-surface area nature of RVC to fabricate a cathode for the
electrochemical production of H2O2 for the removal of organics [15]. The solution
was kept at an acidic pH due to the presence of Fe(II) as a catalyst for the
oxidation of the organic compounds via H2O2. However, this is the first report of
using ruthenium oxide on the anode to facilitate the oxygen reduction reaction.
1.1.2 RuO2 as a Water Oxidation Catalyst
There has been a great deal of interest of finding catalysts for the splitting
of water into hydrogen and oxygen, mostly from those interested in mimicking the
process of photosynthesis for energy production. Ruthenium (IV) oxide (RuO2) is
a well established water splitting catalyst [16]. RuO2 is an insoluble compound,
an important feature for an aqueous system, which can form a rutile crystalline
structure when heated to over 200 °C [17]. The mechanism for the oxidation of
9
water on RuO2 is not fully understood. Titanium dioxide (TiO2) is another
important water splitting catalyst, which shares a similar structural unit as rutile
RuO2 [18]. Along the c-axis of the rutile structure of RuO2, there are ruthenium
atoms bridged by two oxygen atoms [17]. If these sites are important then the
morphology also plays a role.
Experimental
1.2.1 Voltage Selection
Oxygen is known to undergo 2 e- and 4 e- reduction on glassy carbon
electrodes at around -0.5 V and -0.8 V potential regions [19]. Cyclic
voltammetry (CV) was used to probe the reduction potential of oxygen-saturated
solution of 1M H2SO4 on the Reticulated Vitreous Carbon (RVC) electrode
utilizing a Ag/AgCl reference electrode. The cyclic voltammogram (CV) of the
air-saturated solution of H2SO4 is shown below in Figure 1.3. We also
interrogated the effects of de-aerating the solution with N2 to see if the reduction
waves were actually related to the oxygen. The CVs shown in Figure 1.4
suggested that de-aerating the solution resulted in the loss of the reduction wave
related to O2 2e- process.
1.2.2 Cell
The first cell design was a glass cube, with the top removed, with a glass
divider in the middle to make two compartments. Each compartment was
capable of holding 20 mL of solution. A hole was drilled into the divider and a
spherical quartz frit was welded into that position, the frit had 5 micron pores.
The frit was then coated with a NAFION® membrane, which will be discussed in
10
greater detail in a later section. A lid was placed over top of the cell with a slot to
allow the RVC electrode to enter solution, and two small circular holes to allow
the insertion of a Ag/AgCl reference electrode in the cathodic compartment and
the Pt auxiliary electrode in the anodic compartment. The pore sizes for this frit
were too small, which resulted in low concentrations of hydrogen peroxide
production. Images of the first cell can be seen in Figure 1.5.
The next cell designed revolved around a tubular U shape. A 25-50
micrometer pore size frit was placed in the middle to separate the two
compartments. The frit was then coated with a NAFION® membrane. The cell
was made of Pyrex of ⅛ inch thickness, the base measured 2 ⅝ inches in length
and 1 ¼ inches in width, each cylinder was 3 ⅜ inches tall with an outer diameter
of 1 inch. Each compartment was capable of holding a maximum of 28 mL. An
additional cell was made identical to this cell; however the frit on the second cell
had pore sizes ranging from 50-75 micrometers. This larger pore size however
did not prevent hydrogen peroxide from travelling to the anodic compartment.
Images for this design can be seen in Figure 1.6.
11
8
7
6
5
Current (mA)
4
3
2
1
0
-1200
-1000
-800
-600
Voltage (mV)
Figure 1.3: Reduction Peak Position
12
-400
-200
0
-1
25
8
6
Deaerated
Aerated
15
4
10
2
5
0
0
0
-200
-400
-600
Potential (mV)
Figure 1.4: Effect of De-Aerating the Solution
13
-800
-1000
Aerated Current (mA)
Deaerated Current (mA)
20
Cell cover
20 ml
Ag/AgCl ref.
electrode
20 ml
RVC working
electrode
Figure 1.5: Initial Cell Design
14
Nafion membrane
on quartz filter
Platinum aux. electrode
O2 Pumped
in via
Micropipette
Teflon® Lids
Ag/AgCl
Reference
Electrode
RuO2/RVC
Auxiliary
Electrode
RVC
Working
Electrode
Magnetic
Stir Bar
Figure 1.6: Tubular U-Shaped Cell Design
15
NAFION®
Membrane
The current cell design is simply a modification of the previous cell. An Lshaped Pyrex® tube was joined to the cathodic compartment with the center of
the tube positioned 2 cm above the base. The tube had an inner diameter of 1
cm. The tube was added to allow the insertion of a gas diffusion tube to
minimize formation of oxygen bubbles. In addition, it also prevented O2 bubbles
from sticking to the surface of the electrodes and to allow better dispersion of the
bubbles into solution. This addition increased the maximum volume of the
cathodic compartment to 35 mL. This cell included the frit with pore sizes
ranging from 25-50 micrometers. This cell design can be seen in Figure 1.7.
16
Teflon®
Electrode
Sheaths
Ag/AgCl
Reference
Electrode
Gas
Diffusion
Tube
RVC
Working
Electrode
®
Magnetic NAFION
Stir Bar Membrane
Figure 1.7: Current Cell Design
17
RuO2/RV
C Auxiliary
Electrode
1.2.3 Electrode Holders
Initially the electrodes were suspended in the solution using the tension
provided by the connecting wires to keep them from dropping too far into
solution, where the wire would touch the solution. The working and auxiliary
electrodes would never be the same distance from each other between runs,
also the reference electrode would at times touch the working electrode. This
resulted in irreproducible runs, and it was decided that holders would have to be
fabricated for the electrodes in order to minimize variations between runs.
The first cell holders that were designed for the RVC electrodes were
Teflon® lids sized to fit snugly on the tubes of the electrochemical cell. The
Teflon® lids had slots cut into them for the RVC electrodes to be inserted. A
small hole, 3/16 inch diameter, was also drilled into the Teflon® lid that was to go
over the working electrode compartment to allow for the Ag/AgCl reference
electrode to be inserted. A larger hole, ¼ inch diameter, was drilled behind the
smaller hole to allow the insertion of a tube for pumping in gas. These lids can
be seen in Figure 1.8.
The cell holder designed for the platinum wire electrode was a Pyrex lid
with a ⅛ inch diameter tube of 2 ¾ inches in length fused to the lid. The tubing
had several small, ⅛ inch by ¼ inch, slots cut into to allow the solution easier
access to the platinum wire and yet still hold the wire in position. The lid was
sized to fit snugly on top of the auxiliary compartment tube. The Pyrex® lid can
be seen in Figure 1.9.
18
Figure 1.8: Initial Teflon® Electrode Holders (Images a and b are the lids for the
anodic compartment, images c and d are for the cathodic compartment)
19
Figure 1.9: Platinum Wire Electrode Holder
20
The Teflon® lids did help stabilize the electrodes, however they still
allowed the electrodes to move around in the solution. The lids also added extra
mechanical stress onto the electrodes. Because the alligator clip connects the
electrodes to the voltage source was much heavier than the electrodes it caused
the electrodes to rest on the slots to stabilize their positions. This combined with
the fragility of the electrodes themselves resulted in many broken electrodes.
Also gas being pumped in caused the electrodes on the working compartment,
(which was the only compartment that had gas being pumped into solution) to
jump up and down which added to the mechanical stress being applied to the
electrodes.
The current holder was designed to help alleviate this mechanical stress
placed on the electrodes. In order to minimize the stress placed on the
electrodes it was decided the wires attached to the electrode would first be
wrapped around a screw and then the voltage supplier would be connected to
the screw via an alligator clip. The electrodes themselves would be inserted into
an open sleeve attached to the lid, the sleeve had windows cut into it to allow the
solution greater access to the electrode. The sleeve and the lid were made from
Teflon®. The sleeve was attached to the lid with nylon cotter pins. The lid also
had a 3/16 inch diameter hole drilled into it behind the slot where the sleeve was
attached, for the insertion of the Ag/AgCl reference electrode. The larger hole
was removed from this design as the cell itself was redesigned for the pumping
of gas. The most current holder can be seen in Figure 1.10.
21
Figure 1.10: Teflon® Electrode Holder with Sleeve
22
1.2.4 Membrane
The electrochemical separation of the two compartments was achieved
using a NAFION® 117 membrane deposited on the filter. The membrane
formation used a NAFION® solution using ethanol as solvent. The NAFION®
solution is deposited onto a glass frit in the electrochemical cell on both sides and
allowed to dry, excess residue on the glass is removed with ethanol. NAFION®
was chosen for its high degree of OH- ion rejection which is needed for hydrogen
peroxide electrosynthesis. The chemical formula for NAFION® can be seen in
Figure 1.11 [20]. NAFION® has excellent permeselectivity to protons, the
transference number of H+ ions remains constant at unity until an external H2SO4
concentration of 2 M is reached [21].
It is necessary to regenerate the NAFION® membrane after a few
hundred hours of use as polymer degradation can become a problem [22-24].
The old membrane is removed with ethanol and a new one is deposited onto the
glass frit. Some suspect that membrane degradation is mainly caused by
chemical attack of the polymer [21, 25]. Previous work has suggested that
hydroxyl free radicals, generated from homolytic cleavage of H2O2 catalyzed by
metal impurities, can attack polymer end groups having H-containing terminal
bonds (such as –CF2COOH) that are formed during membrane processing [5].
Carbon radicals in the electrode and unknown radical centers in the membrane
have been identified in a PEM fuel cell using electron spin resonance (ESR) [26,
27].
23
Figure 1.11: NAFION® Chemical Formula
24
1.2.5 Electrodes
Two sizes of reticulated vitreous carbon (RVC) foams were obtained from
ERG Materials and Aerospace Corporation: ~2x125x125 mm and 4x125x125
mm, of 100 pores per inch (ppi) and 60 ppi, respectively. For a preliminary
investigation, the 2x125x125 mm RVC was used to determine the reduction
potential of oxygen in 1.0 M H2SO4. To achieve this, 2x5x40 mm RVC was cut
from the as-received RVC and used as the working electrode in a three electrode
electrochemical cell. The reference electrode was a Ag/AgCl (saturated with 3 M
KCl) electrode and a coiled platinum wire was used as the counter electrode.
The electrochemical synthesis of H2O2 in this project requires a specially
prepared anode made of RuO2 deposited on a conducting material, carbon was
chosen as the conducting material for this experiment. Two different methods of
depositing RuO2 are investigated. The first method was a cathodic
electrochemical deposition of a 5 mM solution of RuCl3 followed by thermal
oxidation of the RuCl3 to RuO2 in a furnace, following the procedure used by Hu
et al [29]. The second approach involved soaking the RVC in a saturated
solution of RuCl3 and allowing the material to dry followed by thermal oxidation of
the RuCl3 to RuO2 in a furnace.
For the electrochemical deposition, 60 voltammetric cycles, at a rate of 50
mV/s, were performed and the features of the voltammograms were used to
monitor the progress of the electrodeposition. The deposition bath consisted of 5
mM RuCl3:xH2O in 0.1M NH4Cl + 0.01M HCl in the cathodic compartment and
0.1M NH4Cl + 0.01M HCl in the anodic compartment. A Ag/AgCl reference
25
electrode and a Pt wire counter electrode were used. Though the
voltammograms [Figure 1.12] were almost featureless, the growth of the
deposited film could be ascertained from the oxidation potential around 0.6 V
which shifted to 0.4 V as the deposition continued. The deposition was stopped
after 60 voltammetric cycles and the electrode was rinsed with distilled water
before transferring into a furnace and heat treated at 200 °C for 10 hrs.
The second approach of depositing the ruthenium oxide on the RVC was
to soak the RVC in a saturated solution of RuCl3. Several drops of a saturated
RuCl3 solution in ethanol were placed on the RVC and allowed to dry each time.
The dropping was continued until 2 ml content of the ethanol solution had been
used. This was to make sure that enough quantity of the RuCl3 had been soaked
onto the RVC. Following this, the RuCl3+RVC material was transferred into a
furnace and heat treated at 250 °C for 10 hrs. The resultant material was washed
with water and baked in the oven for about 5 hrs before analysis. It was
observed that the washed material still had a lot of RuCl3 based on the brown
residue that formed from the washing.
26
10
8
6
Current (mA)
4
-400
2
0
-200
-2
-4
0
200
400
60th 
-6
-8
Voltage (mV)
Figure 1.12: Electrodeposition of RuCl3 on RVC
27
600
800
1000
1st
1200
The SEM images of the materials suggest that the electrochemical route
gave a better surface modification than the solution-based process (Fig 1.13).
The experiment therefore adopted the electrochemical deposition method for the
anode preparation. It seems that while the SEM image shows that the ruthenium
oxide had been deposited on the RVC, there might be some layer of the RuCl3
in-between the ruthenium oxide and the RVC surface thus making the electron
transfer process from the oxide inaccessible to the RVC.
28
Plain as-received
RVC
Electrodeposited
RuO2 on RVC
Deposited RuO2 on
RVC via ethanolic
RuCl3:xH2O
Figure 1.13: SEM Images of the RVC Foam Before and After Depositing RuO2
29
1.2.6 Electrosynthesis
All electrosynthesis runs were performed with an EG&G Princeton Applied
Research Model 273 Potentiostat/Galvanostat. Electrosynthesis runs were
performed in a three electrode cell as described previously. Electrosynthesis
runs were conducted under chronoamperometric mode with the following
settings; purge time was passed, equilibriation time was set to 120 s, initial
voltage of -0.550 V, final voltage of -0.551 V, run time of 3600 s, current
maximum of 1 A, time/pt 5.000s, number of points 730, calibrated for a Ag/AgCl
reference electrode. Chronoamperometry was chosen after voltage fluctuations
were observed during a controlled potential run; by choosing two voltages close
together in value one can simulate a controlled potential run using
chronoamperometry.
In order for the solution to be fully saturated with oxygen, oxygen is
pumped into the solution for 15 minutes prior to a run. Initially oxygen was
pumped into solution via a micropipette connected to an oxygen tank; the
micropipette was inserted into the cathodic compartment behind the working
electrode. A flow meter was used to control and monitor flow rate. This set up,
particularly with the initial electrode holders, caused the electrodes to jostle
violently causing structural damage to the electrodes. In this setup the bubbles
from the introduced oxygen would often cling to the surface of the RVC
electrodes thereby reducing usable surface area for the reaction.
A stir bar was introduced into the cathodic compartment to try to remove
30
any clinging oxygen bubbles from the surface of the electrodes. The stir bar did
help remove some of the clinging bubbles, but did not remove all of them.
The cell was then redesigned, which can be seen above, by adding a Lshaped tube to the cathode compartment where oxygen could be bubbled into
solution safely away from the electrode. The micropipette was exchanged with a
gas diffusion tube as the method of introduction for the oxygen gas. This setup
allowed the bubbles to then disperse into solution with no large clinging bubbles
on the surface of the electrodes being observed.
1.2.7 Electron Paramagnetic Resonance EPR
Possible radical presence was determined using Electron Paramagnetic
Resonance (EPR) spectroscopy. Four samples of RVC electrodes were
scanned using EPR; all samples were suspended in 1 mL of solution and placed
into a capillary tube. The first sample consisted of RVC electrode that
decomposed after ten reduction cycles suspended in 1 mL of the cathodic
compartment solution. The second sample consisted of RVC electrode that
decomposed after ten reduction cycles suspended in 1 mL of nanopure water.
The third sample consisted of unused RVC suspended in 1 mL of 1 M H2SO4.
The fourth sample consisted of unused RVC suspended in 1 mL of nanopure
water.
All EPR spectra were collected on a Bruker ESP300 spectrometer. Each
spectrum consisted of 5 scans performed at 20 mW, with a modulating frequency
of 100 kHZ, an amplitude of 0.525 Gauss, a microwave frequency of 9.77 GHz,
and a center field of 3450 Gauss with a sweep width of 100 Gauss.
31
1.3 Results and Discussion
Chronoamperometry measures the current as a function of time. By
integrating the graph of current versus time, one can determine the total amount
of charge (Q) passed during the trial. This Q value can then be used to
determine the theoretical number of moles of oxygen reduced to hydrogen
peroxide. The following equation was used to determine the theoretical number
of moles of oxygen reduced from the Q values
Moles = (Q/nF)
(EQ 1)
where Q is the total amount of charge passed during a trial; n is the number of
electrons; and F is the Faraday constant.
Potassium permanganate titrations, using 2 mM KMnO4, were performed
on samples removed from the cathode compartment to determine experimentally
the concentration of hydrogen peroxide generated during a trial. By comparing
the theoretical number of moles of oxygen reduced to the experimentally
determined number of moles reduced, the efficiency of the cell can be
determined using the following equation.
Efficiency =
Experimental Number of Moles Reduced
*100
Theoretical Number of Moles Reduced
(EQ 2)
Trial hydrogen peroxide generations were performed using the previously
described experimental setup and a plain as-received RVC working electrode,
Ag/AgCl reference electrode, and coiled Pt wire auxiliary electrode. The results
can be seen in the table 1.2. The efficiencies for the trials were averaged to be
33.3 ± 5.4.
Trial hydrogen peroxide generations were then performed using a plain
32
as-received RVC working electrode, Ag/AgCl reference electrode, and ruthenium
(II) oxide-modified RVC auxiliary electrode. The results can be seen in the table
1.3. The efficiencies for the trials were averaged to be 33.56±6.47.
Trial hydrogen peroxide generations were then performed using a plain
as-received RVC working electrode, Ag/AgCl reference electrode, and a plain asreceived RVC auxiliary electrode. The results can be seen in the table 1.4.
The efficiencies for the trials were averaged to be 16.77±3.89.
33
Table 1.2: Plain RVC Working Electrode/Pt Wire Auxiliary Electrode
34
Table 1.3: Plain RVC Working Electrode/Ruthenium (II) Oxide-Modified RVC
Auxiliary Electrode
35
Table 1.4: Plain RVC Working Electrode/Plain RVC Auxiliary Electrode
36
The efficiency averages for the first two sets of experiments show
comparable results using the Pt wire auxiliary electrode and the cheaper
alternative of the ruthenium (II) oxide-modified RVC auxiliary electrode. While
using plain RVC on both sides of the electrochemical cell produced resulted in
over a 50% drop in efficiency compared to both the trials using RuO2 modified
RVC as the anode and the trials using Pt wire as the anode.
Qiang et. al. reported obtaining current efficiencies of 81% [28]. In this
study pH was controlled using a pH-stat (Model pH-40, New Brunswick Scientific
Co., Edison, NJ), HClO4 (1 M) and NaOH (1 M) solutions. Temperature was
controlled using a thermostat (Model EX-200, Brookfield Engineering
Laboratories, Inc., Stoughton, MA) and a water bath. This study included the use
of sodium perchlorate (NaClO4) as an inert supporting electrolyte. Hydrogen
peroxide generations had run times of 7200 s, which is twice what was
performed in these experiments. They observed significant H2O2 degradation at
elevated temperatures (greater than 23 ºC). The work involved in this present
study did not optimize pH, temperature, or vary the run times of experiments.
These optimizations could lead to possible increases in current efficiency.
Gallegos reported obtaining current efficiencies of 20% at -1.5 V (vs.
stainless steel), but drops to 10% at -2.0 V [29]. Utilizing voltages in this range
involved competing 2 electron and 4 electron oxygen reductions. Voltages in this
range can also lead to hydrogen gas evolution. Gallegos’ research involved the
use of a stainless steel anode with the possibility of Fe3+ and Fe2+ ions dissolving
into solution. Fe(II) is known to react with H2O2 to form the Fenton reagent, thus
37
lowering current efficiency [30].
Anodic decomposition occurs in the RVC electrodes, resulting in
electrode failure after approximately 10 trial runs. EPR was performed in order to
detect the presence of free radicals thought to responsible for the degradation of
the anodes. Four samples were prepared; the first sample was taken from an
anode after decomposition occurred with the anodic solution and can be seen in
Figure 1.14, the second sample consists of a portion taken from a decomposed
anode dried to see if the free radical was in the solution or on the electrode and
can be seen in Figure 1.15, the third sample was taken from a piece of RVC
anode before any runs and placed in 1 M sulfuric acid to see if the RVC anode
itself contained anything that would be detected by EPR shown in Figure 1.16,
the fourth sample was a piece of plan RVC anode material shown in Figure 1.17.
Large peaks shown in Figures 1.14 and 1.15 clearly indicated the presence of
unpaired electrons. Figures 1.16 and 1.17 showed no visible peaks indicating an
absence of unpaired electrons. All EPR spectra were taken one week after
decomposition occurred, this rules out the detection of any short lived free radical
species, such as hydroxyl radicals [31]. It is possible that during the reaction a
reactive oxygen species reacted with the anode causing its degradation. The
observation of carbon-centered radicals by EPR suggests that these
intermediates which exhibit extremely long lifetimes, detectable for months after
being created [32] may be involved in the degradation.
38
Intensity
Gauss
Figure 1.14: RVC Anode After Decomposition in Cathodic Solution
39
Intensity
Gauss
Figure 1.15: RVC Anode After Decomposition Dried
40
Intensity
Gauss
Figure 1.16: RVC Anode Before Run in 1 M Sulfuric Acid
41
Intensity
Gauss
Figure 1.17: Plain RVC Dry
42
1.4 Conclusions
The RuO2-Modified RVC electrode shows comparable efficiencies to the
Pt wire electrodes. The RuO2-Modified RVC electrode does show an advantage
over utilizing a plain RVC electrode, which results in a 50 % reduction in
hydrogen peroxide production efficiency compared to the RuO2-Modified RVC
electrode. Although the RuO2-Modified RVC electrode does show comparable
efficiency to the Pt wire electrode the anodic decomposition of the RVC electrode
renders it useless for multiple trials. The electrosynthesis of H2O2 on the RVC
electrode is resulting in the generation of free radicals, which in turn is eroding
the anode. With a more inert supporting material RuO2 should be a good
candidate for a water splitting catalyst in the anodic compartment.
43
References
1. Schumb WC, Satterfield CN, Wentworth RL. Hydrogen peroxide Reinhold
Publishing Co.: New York, NY, 1955. p. 392, 515, 535-546.
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Hydrogen peroxide synthesis : an outlook beyond the anthraquinone
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Angewandte Chemie, International Edition (2006), 45(42),
6962-6984.
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13. K. Henricson (Ahlstroem Oy), WO9321106, 1993 [Chem. Abstr. 1993,
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Kaido; Kontturi, Kyoesti. Electrochemical synthesis of hydrogen peroxide:
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oxide-zeolite Y catalyst for photochemical oxidation of water to dioxygen,
Microporous and Mesoporous Materials, 22, (1998), 475-483.
17. J. M. Fletcher, W.E. Gardner, B.F. Greenfield, M. J. Holdoway, M. H.
Rand, J. Chem. Soc. (A) (1968) 653.
18. W. Rhittinger, G. C. Dismukes, Chem. Rev. 97 (1997) 1.
19. Xue, T. Mass transfer in Nafion membrane systems: effects of ionic size
and charge on selectivity Journal of Membrane Science, v. 58 issue 2,
1991, p. 175.
20. Pourcelly, Gerald; Lindheimer, Arlette; Gavach, Claude; Hurwitz, Henry D.
Electrical transport of sulfuric acid in Nafion perfluorosulphonic
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Electrochemistry (1991), 305(1), 97-113.
45
21. D.E. Curtin, R.D. Lousenberg, T.J. Henry, P.C. Tangeman, M.E. Tisack, J.
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Lett. 7 (2004) A209.
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24. M. Pianca, E. Barchiesi, G. Esposto, S. Radice, J. Fluorine Chem. 95
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25. A. Panchenko, H. Dilger, E. M¨oller, T. Sixt, E. Roduner, J. Power Sources
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26. C.-C. Hu, K.H. Chang, Electrochim. Acta 45 (2000) 2685.
27. G. Hubner, E. Roduner, J. Mater. Chem. 9 (1999) 409.
28. Qiang, Zhimin. Electrochemical generation of hydrogen peroxide from
dissolved oxygen in acidic solutions Water Research, v. 36 issue 1, 2001,
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29. Gallegos, Alberto Alvarez; Garcia, Yary Vergara; Zamudio, Alvaro. Solar
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46
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197-213.
47
Chapter 2
2.1 Introduction
2.1.1 Motivation
Traditional Raman scattering is a weak phenomenon, as the scattering
observed is the result of weak inelastic scattering as compared to a more intense
elastic Rayleigh scattering.
Traditional Raman spectroscopy typically has a
detection limit no greater than 0.1 M [1, 2].
The ultimate goal in chemical
detection is the ability to detect the presence of single molecules/atoms, which
cannot
be
accomplished
with
normal
Raman
spectroscopy.
Sample
fluorescence in the scanning range can also obscure important peak information
in addition to the already weak signal.
Raman scattering is the result of scattering with a change in frequency,
corresponding to a change in the vibrational energy of a molecule. The Raman
effect can be visualized as a transition from the ground state to a virtual excited
state, followed by subsequent scattering to a vibrationally excited state. There
are mainly three possible scattering mechanisms which can occur. The most
intense seen is the elastic scattering or Rayleigh scattering that results in no
energy change, the incident and emitted photon has the same energy.
The
inelastic scattering, Stokes and Anti-Stokes scattering, which occurs is where the
Raman Effect is observed. In Stokes scattering a molecule is excited to a virtual
state and the final vibrational state of the molecule is more energetic than its
48
initial state, this causes the scattered photon to be shifted to a lower frequency in
order for the total energy of the system to remain balanced. However, if the final
vibrational state is lower in energy than the initial state, then in order to balance
the total energy the scattered photon will be shifted to a higher frequency, and
this is known as Anti-Stokes scattering.
Figure 2.1 shows the differing
scatterings that can be observed.
Surface-enhanced Raman spectroscopy (SERS) was first noticed in 1974
by Fleischmann with the molecule pyridine adsorbed onto a roughened silver
electrode [3]. Fleischmann noticed that the electrochemically roughened coinage
metal surface produced a Raman signal six orders of magnitude larger than was
expected from the normal Raman cross-section.
After this discovery many
researchs tried to discover the mechanisms behind the 106 fold enhancement.
The enhancement factor is considered to be the product of an electromagnetic
(EM) enhancement mechanism and a poorly understood chemical enhancement
mechanism. The mechanisms arise because the intensity of Raman scattering is
proportional to the induced dipole moment, μind, which is itself a product of
polarizability, α, and the incident electromagnetic field, E [4].
μind = α * E
(1)
The electromagnetic enhancement mechanism is often stated to correspond to
104 factor enhancement while the chemical enhancement theory accounts for the
leftover 102 enhancement. The chemical enhancement effect is considered to
arise from the electronic intracluster excitation via a type of Förster excitation
transfer [5] or metal/adsorbate charge transfer resonance [6].
49
The enhancement factor, based on the electromagnetic enhancement
mechanism, E at each molecule is (approximately) given by
E = |E(ω)|2|E(ω’)|2,
(2)
where E(ω) is the local electric-field enhancement factor at the incident
frequency ω and E(ω’) is the corresponding factor at the Stokes-shifted
frequency ω’ [7].
E is often approximated such that E(ω) and E(ω’) are
considered equal, and hence E = |E(ω)|4.
When an EM wave interacts with a roughened metal surface, or small
nanoparticles, the wave may excite localized surface plasmons on the metal,
which amplifies the EM fields near the surface. A localized surface plasmon,
LSPR, is produced when the collective oscillation of valence electrons in a metal
nanoparticle is in resonance with the frequency of incident light. For a spherical
particle, (this shape is assumed for this thesis based on observed TEM and the
size/shape of the zeolite supercages), with a radius a, and irradiated by z
polarized light of λ wavelength, we can assume the electric field around the
nanoparticle is uniform. This allows Maxwell’s equations to be replaced by the
Laplace equation of electrostatics [7] and allows one to determine the EM field
outside the particle, Eout, is given by
,
(3)
where x, y, and z are the Cartesian coordinates, r is the radial distance,
are the Cartesian unit vectors, and α is the polarizability which
can be further broken down into the following
50
α = ga3,
(4)
g is a function of the dielectric constant of the nanoparticle, εin, and the dielectric
constant of the medium , εout, expressed as,
g = (εin-εout)/(εin+2εout).
(5)
Equation 3 illustrates the distance dependence of the EM enhancement factor.
The field enhancement decays with r-3, indicating that adsorbates will have a
stronger enhancement than molecules that are farther removed from the surface
[8].
The EM field enhancement is taken to the fourth power, the distance
dependence scales with this, increasing r-3 to r-12. One can imagine however that
due to the distance enabling enhancement that shells of molecules can surround
the nanoparticle increasing the surface area by a factor of r2. Taking this into
account to one can calculate the SERS Raman intensity, ISERS, at a distance as
follows:
ISERS = [(a+r)/a]-10
(6)
where a is the particle size and r is the distance between the analyte and the
substrate.
As the dielectric functions are wavelength dependent, the magnitude of
the enhancement is also wavelength dependent. The maximum enhancement is
seen in Equation 5, where εin=-2εout. This condition is met in the visible region of
the electromagnetic spectrum for coinage metals such as gold and silver [9]. An
enhancement factor, EF, can be determined experimentally by using the
following equation:
51
EF = [(ISERS/INRS)*(NSURF/NSOL)]
(7)
ISERS is the intensity of the SERS active Raman scattering, INRS is the normal
Raman intensity for a solution of the same concentration as used in the SERS
experiment without the SERS active substrate, NSURF is the number of molecules
bonded to the SERS active substrate, and NSOL is the number of molecules in the
focal volume. Note that this equation does not take into account any analytes not
directly bonded to the SERS active substrate but still within range of the
enhancing fields.
2.1.2 Previous Studies on SERS Substrates
An ideal SERS substrate will provide a substantial enhancement factor, be
in uniform in composition, reproducible, and stable. Previously studied SERSactive substrates have included; Ag and Au aggregated colloids [10-15], metal
electrodes [16-19], metals generated by reduction of silver-exchanged zeolites
[20-21]. Coinage metal nanoparticles have shown that it is possible to detect
single molecules of rhodamine 6G with Raman [22], unfortunately reproducibility
of particle size and long-term stability of the colloidal solution plagues this
method [23-25].
Metal electrodes are yet another commonly studied SERS
substrates, in fact they were the initial SER substrates.
These electrodes
typically undergo a roughening process via electrochemistry or chemical etching
to produce more prolate structures suitable for SERS work [26].
The metal
electrodes do not have issues with particles falling out of solution due to
aggregation, thus are much more stable than colloidal solutions. The roughened
52
surfaces on the electrodes do not provide as great an enhancement factor as
seen in the colloidal solution.
The work done in this thesis utilizes the unique properties of the zeolite to
facilitate the formation of a SERS substrate. Zeolites are photochemically and
thermally stable.
Many types of zeolites, including zeolite Y, contain
exchangeable cations present in the framework that are needed to balance out
the negative charge of the framework itself. Zeolites have been used before as a
host for the formation of metal and semiconductor clusters [27]. A number of
metallic and organometallic compounds have been formed in zeolites, of which
silver is one. Ag is the only noble metal whose ion can be completely exchanged
into a zeolite from an aqueous solution because it is the only noble monopositive
charged cation that forms a stable mononuclear species in water [28-30]. At
lower Ag+ ion exchange levels, all the Ag+ in hydrated zeolites are easily reduced
to form clusters, the water has been suggested to facilitate the diffusion of silver
[31]. There is also the added benefit that the redox of Ag zeolite is reversible,
and that the oxygen treatment of dehydrated Ag-Y causes a migration of Ag+ ions
into the sodalite units from the supercages [32].
There have only been two studies published using zeolites as a SERS
platform. The first study was performed by Dutta and Robins, where Robins
produced a silver-coated zeolite Y substrate. Ion-exchanged Ag zeolite Y was
reduced with hydrazine, and Robins observed that the silver had migrated to the
surface of the zeolite forming a silver-coating around the surface [20]. Yan
investigated silver-coated zeolite A and zeolite NaX crystals deposited by
53
vacuum deposition as surface-enhanced Raman scattering (SERS) substrates.
Yan found that the substrates were active for the enhancement of Raman signal.
scattering from uranyl ions with a detection limit of 10-5 M for uranyl was obtained
using silver-coated zeolite A [21].
54
Energy
Virtual Energy
Level
Rayleigh
Scattering
Stokes
Shift
First Excited State
Ground State
Figure 2.1: Raman Scattering
55
Anti-Stokes
Shift
2.2 Experimental
2.2.1 Silver Loading and Reduction
A 500 mg sample of Na+ Zeolite Y was calcined at 550°C for 24 hours
under a flow of oxygen to remove any trace organics. The calcined zeolite was
then treated with humidified air during the cool down cycle once the temperature
reached 80°C, to prevent the reintroduction of organics inside the zeolite
framework. Then 100 mg of the hydrated calcined zeolite was removed and
placed into a 20 mL screw top vial, the remaining zeolite was placed into a
dessicator in another screw top vial for storage. The 100 mg sample was added
to 5 mL of a 0.1 M AgNO3 solution and stirred overnight for Ag+ exchange with
the Na+ in the zeolite framework. The solution was then washed repeatedly to
remove any unreacted AgNO3 remaining in solution, and the resulting
supernatant was treated with NaCl in the dark for detection of Ag+ ions.
The Ag+ Zeolite Y was placed into 5 mL of nanopure water and 1 mL of
hydrazine at different concentrations. The first method involved adding 1 mL of
99.7% hydrated hydrazine to the zeolite suspension; this resulted in the
previously white suspension turning to a dark gray almost black color. This color
shift indicates that the silver aggregated into large clusters during reduction. The
next method involved adding several lower concentrations of hydrazine to the
zeolite suspension. The third method utilized was a controlled addition of the
concentrations from method 2.
In the controlled addition method, while the
zeolite suspension was being stirred 10 μL aliquots of the hydrazine solution
56
were added to the zeolite suspension every 6 minutes for an hour, for a total of
100 μL of the hydrazine solution.
During one of the Raman sequential scans, the peaks changed noticeably
as the scans progressed for the first few scans and then stabilized. This seemed
to indicate that the silver clusters may be undergoing further reduction from the
laser light. Following this observation several Raman scans were performed on
the hydrazine reduced Ag zeolites following sample illumination by the laser at
varying power levels. These studies only provided illumination onto a tiny part of
the sample, and in order to maximize the sample exposure to light the hydrazine
treated Ag zeolite was suspended in 1 mL of nanopure water and illuminated in a
photolysis chamber while being stirred with a microstir bar.
Bulk photolysis
experiments were also performed where 100 g of hydrazine treated Ag zeolite
was suspended in 50 mL of water with a stir bar and stirred while exposed to light
from the photolysis set-up.
2.2.2 X-ray diffraction (XRD)
In order to determine whether the zeolite supercage structure was left
intact from Ag+ reduction, X-ray diffraction (XRD) studies were performed first on
a Na Y zeolite sample and on reduced Ag Y zeolite. XRD studies were carried
out on a Rigaku Geigerflex diffractometer with a nickel filtered CuKa source with a
wavelength of 1.5405 Å. The Cu source irradiated X-rays at 40 kV and 25 mA.
Diffraction measurements were taken at a scan rate of 0.02 degree/step, with a
dwell time of 0.5 s/step, and a scan range of 3-50 degrees. The x-rays were
57
collected by a scintillation counter, with a divergence slit of 0.6 degrees, and
scattering and receiving slits of 0.5 degrees.
2.2.3 Raman Studies
All Raman spectra were collected on a Renishaw - Smiths combined
Raman - IR Microprobe utilizing either a 514.5 nm laser line of an Ar ion laser or
a 632.8 nm laser line of a He – Ne laser. The laser was edge-focused for all
spectra, and both dynamic and static scans were performed.
2.2.4 Transmission Electron Microscopy (TEM)
Reduced silver zeolite samples were imaged by a transmission electron
microscope (TEM) to characterize zeolite and silver particle size, and silver
particle dispersion and uniformity. Silver zeolite samples were taken from the
same batches as used for Raman and XRD experiments.
All images were
obtained using a Tecnai F20 TEM. All samples were dispersed into distilled
water to create a 10 % w/w solution, a 2 μL aliquot was then placed on a 300 nm
copper TEM grid and allowed to evaporate.
2.3 Results and Discussion
2.3.1 Formation of Highly Reduced Silver Zeolite
Nano-scale zeolite Y was synthesized with a modification of the Holmberg
method [1]. The Holmberg method mixes a solution of aluminum isopropoxide
(Al(OC3H7)3), H2O, and tetramethylammonium hydroxide (TMAOH) with a Ludox
SiO2 solution. The aluminum source and silica solutions are then stirred for one
hour until the solution turns clear at which point tetramethylammonium bromide
(TMABr), as a templating agent, is added to the solution. The solution is then
58
stirred again for one hour and then transferred to a bomb and placed in a 100 °C
oven for 72 hours. Instead of adding base directly to the aluminum source, as is
called for by the Holmberg method, it was split (80/20) between the aluminum
source and the silica source. This was done to help dissolve the silica source
before mixing. Also, the Al(OC3H7)3 is slightly insoluble so it was heated in a
water bath between 60-70 °C until the solution was clear before adding the
TMABr. Only after these two steps was the aluminum and silica source mixed.
XRD was performed on the newly made samples, prior to silver-ion exchange, in
order to determine if proper crystalline structure was observed. Figure 2.2 clearly
shows that the newly synthesized sample matches up with the previous nanoscale zeolite Y synthesis, and consistent with reported patterns. Smaller scale
crystallites show broader peaks in their patterns relative to larger scale
crystallites. In order to determine the size, uniformity, and position of the silver
particles inside the zeolite, TEM was performed. As can be seen in the two TEM
images in Figure 2.3 the silver particles greatly vary in size. Most particles are
clearly over 10 nm in diameter making them much too large to be located in the
interior of the zeolite and therefore must be located on the surface.
Once the synthesis method was verified with XRD, a silver-ion exchange
was performed on the nano-scale zeolite. A 100 mg sample of the nano-scale
zeolite was placed into 5 mL solution of 0.1 M AgNO3 and stirred overnight. The
ion-exchanged nano-scale zeolite then underwent washing several times until no
excess Ag was observed. The freshly washed sample was then treated with 1
mL of 19.26 M hydrated hydrazine, the sample immediately shifted from a milky
59
white suspension to a dark grey/black suspension. In order to determine if the
crystallinity of the zeolite Y was still intact the sample was once again subjected
to XRD analysis. Figure 2.4 shows that the sample still matches up with zeolite
Y, indicating that the supercage structure is mostly still intact however there is a
noticeable decrease in peak intensity showing some degradation of the zeolite
has taken place. The 2 new peaks at 38.1 2Θ and 44.3 2Θ are due to silver.
60
Figure 2.2: Comparison of Synthesized Nano-Scale Zeolite Y (Upper) to that of
the Previously Characterized Nano-Scale Zeolite Y Standard (Lower)
61
Figure 2.3: TEM of Excess Hydrazine Treated Ag+ Zeolite Y
62
38.177
44.301
Figure 2.4: XRD Pattern of Nano-Scale Zeolite Y Ion-Exchanged with 0.1M
AgNO3 and Reduced with Hydrazine. Pattern Matches for Silver (Upper) and
Zeolite Y (Lower) are Included for Comparison
63
2.3.2 Raman Studies on Highly Reduced Silver Zeolite with Pyridine as
Analyte
Pyridine is an extensively studied SERS substance, and seemed well
suited to determine if the silver zeolite Y produced in this thesis was a good
candidate for SERS activity.
Figures 2.5 and 2.6 show the normal Raman
response for neat liquid pyridine, aqueous pyridine, and SERS response for
pyridine on silver, as determined by Pagliai et. al.. The peak assignments and
relative intensities determined by Muniz-Miranda are shown in Table 2.1. These
reported bands shifts are used as comparisons to determine if the Raman
spectra observed in this thesis are illustrating a SERS response.
The main
bands of interest are the very strong peaks associated with the ring-breathing
modes in the non-SERS Raman position of 1000 cm-1 and 1032 cm-1 which blue
shift in the SERS response to 1008 cm-1 and 1036 cm-1 (as seen by comparison
of Figure 2.5 and 2.6). These shifts are used to verify a surface-enhancement
effect.
A 1 mL sample of a 0.1 M pyridine solution was placed in a capillary tube
with the ends capped. The pyridine sample was then placed in the Renishaw
Raman instrument and scanned using a 633 nm He-Ne laser at 3.2 mW. The
resulting spectrum can be seen in Figure 2.7, which matches the expected
spectrum seen in Figure 2.4c.
Another 1 mL sample from the same 0.1 M
pyridine solution was added to 10 mg of nano silver zeolite Y and placed in a
capillary tube and then into the Raman instrument. The 3.2 mW power scan and
the 0.4 mW power scans can be seen in Figures 2.8 and 2.9, respectively. The
64
3.2 mW power spectrum exhibited a great degree of fluorescence masking any
possible peaks in the spectrum.
Dropping the power to 0.4 mW helped to
alleviate the fluorescence masking the peaks, but only resulting in very weak
peaks.
The peaks observed matched more closely to the expected peak
positions associated with the SERS effect at 1008 cm-1 and 1036 cm-1
The synthesis process for the nano-scale zeolite Y has templated organics
which lead to the fluorescence problems. However the micron-scale zeolites
have no organics involved in their synthesis, for this reason it was decided to
switch from the nano-scale to the micron-scale synthesis.
The micron-scale
synthesis was developed by the Dutta group, it requires that two solutions be
combined slowly and then stirred for 4 hours. Solution A consists of 85 mL of DI
water, added to 7.2955 g or 0.1838 moles of NaOH, combined with Solution B
which consists of 13.8574 g of Ludox SM-30 for the silica source added to 2.208
g or 0.01081 moles of Al(OH)3.
Once the solution has stirred for 4 hours it is
then placed in an autoclave at 90 °C for 24 hours, it is then removed from the
autoclave and allowed to cool and then undergoes a washing process to remove
any unreacted reagents. A 10 mg sample of micron-scale zeolite Y was added to
1 mL of a 0.1 M pyridine solution and placed into a capillary tube and then
scanned with the Raman instrument. The resulting spectrum can be seen in
Figure 2.10, the 1002 cm-1 and 1034 cm-1 peaks from this spectrum more closely
match up with the aqueous pyridine Raman spectrum of 1000 cm-1 and 1032 cm1
than the SERS peak positions of 1008 cm-1 and 1036 cm-1.
Without silver
particles, the zeolite Y itself is incapable of facilitating a SERS response.
65
Raman Shift cm-1
Figure 2.5: Liquid Pyridine (b), Aqueous Pyridine (c) Raman Spectra [33]
66
Intensity
Raman Shift cm-1
Figure 2.6: SERS Spectra of Pyridine on Silver [33]
67
Table 2.1: Pyridine Peak Assignments [34]
68
Intensity
Raman Shift cm-1
Figure 2.7: Raman Spectra of 0.1M Pyridine Solution at 3.2 mW Power with a
633 nm Excitation Line
69
1800000
1600000
1400000
Intensity
1200000
1000000
800000
600000
400000
200000
0
0
200
400
600
800
1000
1200
1400
1600
-1
Wavenumber
Raman
Shift cm
Figure 2.8: 3.2 mW Power Scan with a 633 nm Excitation Line of a 0.1 M Pyridine
Solution on Nanoscale Ag Zeolite Y
70
1800
Intensity
Raman Shift cm-1
Figure 2.9: Nanoscale Ag Zeolite with 0.1 M Pyridine 0.4 mW Power 633 nm
Excitation Line
71
Intensity
Raman Shift cm-1
Figure 2.10: Micronscale Ag Zeolite with 0.1 M Pyridine 3.2 mW Power with a
633 nm Excitation Line
72
The capillary setup was fraught with instability issues leading to loss of
laser focus. Though the Raman instrument was situated on a stabilizing bench,
because the capillary was situated unsecured on a glass slide, any vibrations
experienced would result in a physical shift of the capillary. In order to alleviate
these stability issues, a small vessel was made that had a base with a screw-top
cap with a glass window. The base had a well bored into that had a 2 cm
diameter and was 2 cm deep.
It was then decided to switch from a suspension to having a BaSO4 pellet
composed of 10% w/w silver zeolite Y. The barium sulfate was to act as a
Raman reference, to ensure that laser was optimally focused. Figure 2.11 shows
a Raman spectrum of the BaSO4 used for the pellet formation. Figure 2.12
shows the Raman spectrum of a BaSO4/Ag zeolite Y pellet with a 1 mL drop of
0.1 M pyridine placed on top, the spectrum shows both the SERS pyridine peaks
and the BaSO4 peaks. Figure 2.13 is the result of 30 accumulated Raman scans
of a 90/10% w/w BaSO4/Ag zeolite Y pellet with a 1 mL drop of 0.1 M pyridine
(laser focused on the zeolite), this resultant spectrum is missing the peak of 987
cm-1 from BaSO4 but shows the 1008 and 1035 cm-1 peaks corresponding to the
pyridine SERS response. A closer inspection of the area between the two 1000
cm-1 peaks shows a third peak at 1029 cm-1. Our explanation for the 1029 cm-1
peak is that it is coming from pyridine converting to pyridinium by reacting with H+
contained in the zeolite. This assignment is supported by literature as seen in
Figure 2.14, pyridine predominately makes up the solution at a basic pH of 8.3
and as the solution becomes more acidic pyridine starts reacting to form
73
pyridinium, both pyridine peaks (1035 cm-1 and 1002 cm-1) and prydinium peaks
(1029 cm-1 and 1011 cm-1) can be seen at pH 5.3 and once the solution reaches
an acidic pH of 2.8 pyridine peaks disappear showing only pyridinium peaks.
The first step in lowering the fluorescence background was to calcine the
zeolite to remove any organics, while the micron scale synthesis does not itself
introduce any trace organics some may have been absorbed into the zeolite
while it was dried.
Figure 2.15 shows a slight reduction in the heightened
baseline from the calcination process. Pyridine was evaporated from a sample
and then subjected to Raman spectroscopy in order to determine whether
pyridine was reacting with the silver or is just physisorbed onto the zeolite, Figure
2.16 shows though that no peaks indicative of pyridine were present, which
indicates that pyridine is only physisorbed onto the silver zeolite.
74
2500000
Intensity
Intensity
2000000
1500000
1000000
500000
0
150
315
480
645
810
975
Raman
Shift cm
Wavenumber
1140
1305
1470
1635
-1
Figure 2.11: BaSO4 Raman Spectrum at 3.2 mW Laser Power with a 633 nm
Excitation Line
75
Intensity
Raman Shift cm-1
Figure 2.12: BaSO4 Pellet with Ag Zeolite Y with 0.1 M Pyridine Cycling Focus
Scan at 3.2 mW Laser Power with a 633 nm Excitation Line
76
Intensity
Raman Shift cm-1
Figure 2.13: BaSO4 pellet with Micron Ag Zeolite Y with 0.1 M pyridine at 3.2
mW Laser Power with a 633 nm Excitation Line
77
Figure 2.14: pH dependence of Pyridine/Pyridinium Conversion [35]
78
Intensity
Raman Shift cm-1
Figure 2.15: Calcined Ag Zeolite Y Pellet with 0.1 M Pyridine at 3.2 mW Laser
Power with a 633 nm Excitation Line
79
Figure 2.16: Calcined Ag Zeolite Y Pellet with 0.1 M Pyridine Evaporated at 3.2
mW Laser Power with a 633 nm Excitation Line
80
2.3.3 Raman Studies with Partially Reduced Silver Zeolites with Pyridine as
Analyte
The previous studies involving an “overkill” dose of 19.26 M hydrazine
resulted in silver particles that were not only too large and of varying size, but
also located outside of the zeolite supercage structure. It was decided to then
shift from a one dose treatment in hopes of reducing as much silver as possible
to a controlled addition of hydrazine in the hopes that the silver would be smaller
sized and placed within the zeolite. Varying concentrations of hydrazine were
made, in each case 100 mg of silver ion-exchanged zeolite Y was placed in 10
mL of DI water. The zeolite suspension was then stirred in the dark while 10 µL
of a hydrazine solution, concentrations given in Figure 2.17, was added every 6
minutes over an hour for a total of 10 additions. The solution was then washed
to remove any excess hydrazine and the zeolite was allowed to dry before it was
pressed into a pellet, BaSO4 was no longer used in the pellet formation as it
proved difficult to press reliably. The pellet was then placed on a Teflon spacer
inside the sample holder and a 2 µL drop of 0.1 M pyridine was added to the
each sample and studied with Raman spectroscopy. Figure 2.17 shows that the
6.42*10-3 M or 6.42 mM solution resulted in the best spectrum, with greatest
intensities at the 1012 cm-1 and 1039 cm-1 bands.
While watching as spectra were accumulated during a 30 scan sequence
it was noticed that peak positions and intensities were shifting as time elapsed.
One hypothesis is that this peak shifting is due to further reduction of the silver
due to the laser light. In order to study if this was in fact true, the hydrazine
81
treated zeolite was illuminated for 10 minutes prior to a run with 3.2 mW laser
power at a focused point. Spectra were then collected at the laser focus point
and at a position removed from the laser focus point. Figure 2.18 shows the
resultant spectrum from an area outside of the laser focal site, the pyridine peaks
do correspond to those associated with the SERS effect however enhancement
appears to be low because the much weaker zeolite peak at 512 cm-1 is in
greater intensity than the two characteristic pyridine peaks of 1010 cm-1 and
1040 cm-1. The on laser site can be seen in Figure 2.19; the pyridine peaks
clearly match with the expected SERS response peaks, and are of a greater
intensity than either of the characteristic peaks of either Teflon at 733 cm-1 or
zeolite Y at 505 cm-1. A cross check sample was performed on micron-scale
zeolite Y pellet without any silver present; the sample was illuminated for 10 min
at 3.2 mW laser power with a 633 nm laser just like the previous experiment.
The results can be seen in Figure 2.20, which shows no characteristic SERS
peaks, only those expected from a normal Raman response of an aqueous
pyridine sample along with a small peak at 505 cm-1 from the zeolite Y.
82
Figure 2.17: Controlled Addition of Hydrazine with 0.1 M Pyridine at 3.2 mW
Laser Power with a 633 nm Excitation Line
83
Raman Shift cm-1
Figure 2.18: 0.1 M Pyridine on Ag Zeolite Y after Laser Illumination 3.2 mW
Power off Spot with a 633 nm Excitation Line
84
Raman Shift cm-1
Figure 2.19: Single Drop 0.1 M Pyridine on Ag Zeolite Pellet at 3.2 mW Laser
Power with a 633 nm Excitation Line
85
Raman Shift cm-1
Figure 2.20: Single Drop 0.1 M Pyridine on Na Zeolite Pellet at 3.2 mW Laser
Power with a 633 nm Excitation Line
86
2.3.4
Raman
Studies
with
Partially
Reduced
Silver
Zeolites
with
Benzenethiol as Analyte
Other SERS molecules include benzenethiol. Benzenethiol has a Raman
cross-section even larger than pyridine [36], has a similar structure, and has
been extensively studied. Figure 2.21 shows the normal Raman spectrum for
neat liquid benzenethiol and the SERS response for benzenethiol on silver, as
determined by Aggarwal et al., these responses are used to determine if the
Raman spectra observed in this thesis are eliciting a SERS response, specifically
the red shifts of the 1002 cm-1, 1026 cm-1, and 1093 cm-1 bands to 999 cm-1,
1022 cm-1, and 1073 cm-1, respectively.
The peak assignments and relative
intensities determined by Tai Ha Joo can be seen in Table 2.2.
A new sampling technique was devised; the Teflon spacers that had been
used were modified to contain 5 sample wells in an X formation. All wells were
cut to be 2 mm wide x 2 mm long x 1/2 mm deep, seen in Figure 2.22. Each well
would contain a separate sample to allow for faster analysis. Because the wells
were so much smaller than the previous setup, a scan of the Teflon surface was
needed to ensure if any of the peaks in the Raman were arising from the Teflon.
By focusing on the Teflon well, the spectrum obtained can be seen in Figure
2.23.
2.3.5 SERS Sample Preparation by Initial Hydrazine Reduction Followed by
Laser Reduction
The silver zeolite Y was pressed into the well after an initial hydrazine
reduction, and then illuminated for 10 minutes at 3.2 mW laser power, after which
87
a 2 μL aliquot of a benzenethiol solution was placed on the surface and the
Raman spectrum obtained.
Four sites were chosen on the sample and
compared to determine homogeneity. As seen in Figure 2.24 all four sites show
similar results and the shifting of the 999 cm-1, 1026 cm-1 and 1093 cm-1 peaks to
999 cm-1, 1026 cm-1, and 1073 cm-1 confirms that a SERS effect is taking place.
In order to determine the limit of detection the benzenethiol was then diluted by a
factor of 100 to 1 mM, and the resulting spectrum was compared to a 0.1 M
sample. The results seen in Figure 2.25 show comparable intensity, this led to
the thought that even lower concentrations should be readily detectable.
However when a solution of 1*10-5 M benzenethiol was added to the silver
zeolite, none of the characteristic peaks of SERS or normal response was seen.
2.3.6 SERS Sample Preparation by Simultaneous Hydrazine and Laser
Reduction
The next set of experiments involved simultaneous reduction via dilute
hydrazine and laser light.
Two samples were prepared in which silver-ion
exchanged zeolite Y was pressed into two separate wells, one well had a 2 μL
aliquot of 1.90*10-3 M benzenethiol added to it and the other had a 2 μL aliquot of
9.51*10-3 M benzenethiol added, and then both samples were illuminated for 10
minutes with 3.2 mW laser power before being scanned. The resulting spectrum
can be seen in Figures 2.26 and 2.27, respectively. Both spectrum show peaks
characteristic of a SERS response, however the sample reduced with 9.51*103
M benzenethiol also appears to exhibit peaks characteristic of a normal Raman
response at 1093 cm-1 and large unknown peaks at 883 cm-1 and 1048 cm-1.
88
The benzenethiol was dissolved in an ethanol solution, and neither peak
corresponds to an ethanol spectrum. A second trial using the same conditions
was carried out, the scan was set between the 800 and 1300 cm-1 region to
monitor the three suspect bands and the three SERS response bands. The new
trial matched up with the previous experiment and can be seen in Figure 2.28.
After searching the literature it was thought that the new peaks may be due to an
interaction with a Ag7 cluster as seen in Figure 2.29.
A fresh sample was
prepared following the same sample preparation method utilized in Figures 2.27
and 2.28, but with no benzenethiol present in the ethanol solvent, the peaks
shown in Figure 2.30 clearly match with the three suspected peaks seen in 2.27
and 2.28. This removes any notion that the three suspect peaks were likely due
to benzenethiol bound to a Ag7 cluster, and are purely caused by a denaturing
agent in the ethanol solvent. Switching to absolute ethanol should alleviate this
problem.
2.3.7 SERS sample preparation by initial hydrazine reduction followed by
photolysis of solid surface
The simultaneous reduction of the silver-ion exchanged zeolite resulted in
only a small portion being properly reduced because of the small size of the laser
focus, leaving most of the sample containing only partially reduced silver
particles. In order to create a more uniform reduction, unreduced samples were
pressed into the Teflon wells, had a 2 μL aliquot of varying hydrazine
concentrations added, and placed into a photolysis setup where UV light was
focused on the samples and illuminated for 30 minutes. The samples were then
89
exposed to 1mM benzenethiol and spectra were collected. The 1.90*10^-3 M
solution of hydrazine showed the sharpest peaks and the least amount of noise
as can be seen in Figure 2.31. When the samples were removed it was noticed
that only the surface layer exposed to the UV light was properly reduced, the rest
of the sample remained white.
2.3.8 SERS Sample Preparation by Initial Hydrazine Reduction Followed by
Photolysis of the Entire Sample
Further modification of the reduction method involved the following. The
20 mg of the unreduced zeolite was placed into 1 mL of water, a 10 μL aliquot of
varying hydrazine concentrations added, and placed into a photolysis setup
where UV light was focused on the samples and illuminated for 30 minutes while
being stirred via a magnetic stirbar. The samples were then exposed to 1mM
benzenethiol and Raman spectra were collected.
The 0.193 mM solution of
hydrazine was the only reduced sample that showed characteristic SERS peaks
for benzenethiol, which can be seen in Figure 2.32.
The samples reduced via photolysis of the suspension were subjected to
TEM imaging. The TEM images can be seen in Figure 2.33; the silver particles
now are far more uniform in size than in the previous TEM of the reduction via
excess hydrazine. The size of the particles based on the TEM scale seems to be
approximately 1 nm, which indicates that the silver particles are small enough to
be contained within the zeolite itself and not only present on the surface.
The Scherrer Equation seen in equation 7 can be used to calculate the
silver particle size, τ, as long as the particles are below 100 nm. Where Κ is the
90
shape factor, equal to 0.9 typically, λ is the X-ray wavelength, β is the FWHM or
breadth in radians, and Θ is the Bragg angle. A powder diffraction standard with
known particle size is used as a reference to calculate the silver particle size.
Because the standard has minimal strain, any broadening of the peaks is
considered to be contributed solely by the instrument itself. The reduced silver
zeolite was mixed with NIST 640 C powder diffraction standard in a 50/50 ratio
with a calibrated particle size of 4.9 μm and characterized with XRD.
The
resulting XRD pattern can be seen in Figure 2.34. The standard has two peaks
that fall within the scanned range, one at 28.4409 2Θ and one at 47.3003 2Θ.
Due to the intensity of the peaks observed from the standard all silver peaks
were lost in the pattern. XRD was repeated on the same Ag Zeolite source
without the NIST 640 C standard to measure the β of the silver peaks and can be
seen in Figure 2.35 The peak at 47.3003 2Θ being the closest to a silver peak, at
44.301 2Θ, was used to determine the breadth, β. The breadth, β, refers to the
broadening due to the particle-size effect alone, equation 8 is used to determine
β from the measured breadth of the sample in question βm and βs the measured
breadth of the standard. The Jade program was used to deconvolute the peaks
where silver and zeolite Y pattern peaks overlap. The silver particle size was
calculated to be 40.7 nm after adjusting for the inherent broadening from the
instrument. The difference calculated by the Scherrer Equation than that seen in
the TEM images is attributed to the heterogeneity of the particles, as some large
(greater than 50 nm) silver particles were observed.
(7)
91
β2 = β2m-β2s
(8)
92
Figure 2.21: Raman (top) and SERS spectra (bottom) of benzenethiol [36]
93
Table 2.2: SER Spectral Data (cm-1) and Vibrational Assignments of
Benzenethiol Adsorbed on Silver Powder [37]
94
Figure 2.22: Raman Sample Holder 2 mm Wide x 2 mm Long x 1/2 mm Deep
95
10000
733
9000
8000
7000
Counts
6000
5000
4000
1381
3000
2000
1297
1210
1000
0
400
600
800
1000
1200
1400
1600
Raman Shifts
Figure 2.23: Raman Spectra of Teflon Wafer at 3.2 mW Laser Power with a 633
nm Excitation Line
96
1800
999
1073
1022
Raman Shift cm-1
Figure 2.24: 0.1 M Benzenethiol on Ag Zeolite Pressed in Wells at 3.2 mW Laser
Power with a 633 nm Excitation Line (Multiple Focal Sites)
97
Raman Shift cm-1
Figure 2.25: Signal Intensity of Benzenethiol Concentration 0.1 M Benzenethiol
Red and 0.001 M Benzenethiol Black (Spectra Collected at 3.2 mW Laser Power
with a 633 nm Excitation Line)
98
Raman Shift cm-1
Figure 2.26: 0.001 M Benzenthiol on Ag Zeolite Reduced with 1.90*10-3 M
Hydrazine at 3.2 mW Laser Power with a 633 nm Excitation Line
99
Raman Shift cm-1
Figure 2.27: 0.001 M Benzenthiol on Ag Zeolite with 9.51*10-3 M Hydrazine
Reduction at 3.2 mW Laser Power with a 633 nm Excitation Line
100
Raman Shift cm-1
Figure 2.28: 0.001 M Benzenethiol on Ag Zeolite Pressed in Wells at 3.2 mW
Laser Power with a 633 nm Excitation Line
101
Figure 2.29: Spectral Shifts for Benzenethiol Binding to Different Cluster Sizes
[38]
102
70000
60000
50000
Intensity
40000
30000
882.605
20000
10000
1047.86
1092.36
0
800
850
900
950
1000
1050
Raman Shifts (cm-1)
1100
1150
1200
Figure 2.30: Ethanol Raman Spectra at 3.2 mW Laser Power with a 633 nm
Excitation Line
103
1.93 mM Hydrazine Controlled Dilution
9.63 mM Hydrazine Controlled Dilution
3.85 mM Hydrazine Controlled Dilution
2.57 mM Hydrazine Controlled Dilution
0.193 mM Hydrazine Controlled Dilution
Raman Shift cm-1
Figure 2.31: Effect of Hydrazine Concentration with Surface Photolysis Setup
(Collected at 3.2 mW Laser Power with a 633 nm Excitation Line)
104
0.193 mM Hydrazine
1.93 mM Hydrazine
3.85 mM Hydrazine
Raman Shift cm-1
Figure 2.32: Effect of Hydrazine Concentration with Suspension Photolysis
Setup (Collected at 3.2 mW Laser Power with a 633 nm Excitation Line)
105
Figure 2.33: 2.57 mM Hydrazine Reduction TEM (Top Left is at 50 nm
Resolution, Top Right is at 20 nm Resolution, and Bottom is at 100 nm
Resolution)
106
7000
28.440
6000
5000
Counts
4000
3000
47.30
2000
1000
0
0
-1000
5
10
15
20
25
30
35
40
45
50
2-Theta
Figure 2.34: XRD of Photolyzed Zeolite Y Suspension with 640 C NIST Standard
107
2500
2000
Counts
1500
1000
500
44.30
0
0
5
10
15
20
-500
25
30
35
2 Theta
Figure 2.35: XRD of 2.57 mM Hydrazine Reduced Ag Zeolite Y
108
40
45
50
2.4 Conclusion
SERS response can be verified by observing the red shift of the 1093 cm-1
peak to 1073 cm-1 peak in the benzenethiol, as well as the blue shifting of the
1000 cm-1 and 1032 cm-1 to 1008 cm-1 and 1036 cm-1 in pyridine. TEM images
illustrate that the silver particle size within the zeolites are of proper size to
produce a SERS response as well. The increase in band intensity also verifies
that the experiments have shown that it is possible to create a SERS capable
platform, by reducing silver inside a zeolite structure. Initial reductions of silver
with excess hydrazine resulted in silver particles too large to be contained within
the zeolite supercage structure and were of a nonuniform size. By controlling the
hydrazine concentration and simultaneously photoreducing results in silver
particles that are smaller, more uniform in size, and more equally dispersed in
the zeolite structure.
109
References
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2. Strommen, D.P.; Nakamoto, K. J. Chem. Educ., 1977, 54, 474.
3. Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Raman spectra of
pyridine adsorbed at a silver electrode.
Chemical Physics Letters
(1974), 26(2), 163-6.
4. Stiles, Paul L.; Dieringer, Jon A.; Shah, Nilam C.; Van Duyne, Richard P.
Surface - enhanced Raman spectroscopy.
Annual Review of
Analytical Chemistry (2008), 1 601-626.
5. Sun, Mengtao; Wan, Songbo; Liu, Yajun; Jia, Yu; Xu, Hongxing.
Chemical mechanism of surface- enhanced resonance Raman
scattering via charge transfer in pyridine-Ag2 complex.
Journal of
Raman Spectroscopy (2008), 39(3), 402-408.
6. Creighton, J. A. Surf. Sci. 1986, 173, 665.
7. Schatz, George C.; Young, Matthew A.; Van Duyne, Richard P..
Electromagnetic mechanism of SERS.
Topics in Applied Physics
(2006), 103(Surface-Enhanced Raman Scattering), 19-4.
8. Liu, Fanxin; Cao, Zhishen; Tang, Chaojun; Chen, Ling; Wang, Zhenlin.
Ultrathin Diamond-like Carbon Film Coated Silver Nanoparticles Based Substrates for Surface-Enhanced Raman Spectroscopy.
ACS
Nano (2010), 4(5), 2643-2648.
9. Moskovits M. 2005. Surface-enhanced Raman spectroscopy: a brief
retrospective. J. Raman Spectrosc. 36:485–96.
110
10. Zhang, Jian-Bing; Fang, Yan. Surface-enhanced Raman scattering of phydroxybenzoic acid in pure Ag colloids produced by laser ablation.
Colloids and Surfaces, A: Physicochemical and Engineering Aspects
(2005), 266(1-3), 38-43.
11. Horimoto, Noriko; Ishikawa, Nobuaki; Nakajima, Atsushi. Preparation of a
SERS substrate using vacuum-synthesized silver nanoparticles.
Chemical Physics Letters (2005), 413(1-3), 78-83.
12. M. Zhu, G. Qian, G. Ding, Z. Wang, M. Wang Materials Chem. & Phys 96
(2006) 489.
13. Loo, B. H.; Tse, Y.; Parsons, K.; Adelman, C.; El-Hage, A.; Lee, Y. G.
Surface-enhanced Raman spectroscopy of imidazole adsorbed on
electrode and colloidal surfaces of Cu, Ag , and Au.
Journal of Raman
Spectroscopy (2006), 37(1-3), 299-304.
14. L. Rivas, S. Sanchez-Cortes, J.V. Carcía-Ramos, G. Morcillo Langmuir 17
(2001) 574.
15. U. Nickel, A. Castell, K. Pöppl, S. Schneider, Langmuir 16 (2000) 9087.
16. Y. Chen, H. Kim Materials Lett. 61 (2007) 5040.
17. H. Lin, J. Mock, D. Smith, T. Gao, M. Sailor J. Phys. Chem. B 108 (2004)
11654.
18. Whitcomb, D. R.; Stwerka, B. J.; Chen, S.; Cowdery-Corvan, P. J. SERS
characterization of metallic silver nanoparticle self - assembly within
thin films.
Journal of Raman Spectroscopy (2008), 39(3), 421-426.
111
19. Gutes, Albert; Carraro, Carlo; Maboudian, Roya. Silver Dendrites from
Galvanic Displacement on Commercial Aluminum Foil As an
Effective SERS Substrate.
Journal of the American Chemical Society
(2010), 132(5), 1476-1477.
20. Dutta, Prabir K.; Robbins, Daniel. Silver-coated faujasitic zeolite crystals
as surface-enhanced Raman spectroscopic substrates.
Langmuir
(1991), 7(10), 2004-6.
21. W. Yan, L. Bao, S. Mahurin, S. Dai Applied Spectroscopy 58 (2004) 18.
22. Nie, Shuming; Emory, Steven R.. Probing single molecules and single
nanoparticles by surface-enhanced Raman scattering.
Science
(Washington, D. C.) (1997), 275(5303), 1102-1106.
23. Li Qing-yu; Li Shi-rong; Si Min-zhen; Zhang Peng-xiang Comparative
study on SERS and stability of negative silver colloids.
Guang pu xue
yu guang pu fen xi = Guang pu (2007), 27(2), 299-301.
24. Guingab, J. D.; Lauly, B.; Smith, B. W.; Omenetto, N.; Winefordner, J. D.
Stability of silver colloids as substrate for surface enhanced Raman
spectroscopy detection of dipicolinic acid.
Talanta (2007), 74(2), 271-
274.
25. Prochazka, Marek; Mojzes, Peter; Stepanek, Josef; Vlckova, Blanka;
Turpin, Pierre-Yves. Probing Applications of Laser-Ablated Ag Colloids
in SERS Spectroscopy: Improvement of Ablation Procedure and SERS
Spectral Testing.
Analytical Chemistry (1997), 69(24), 5103-5108.
112
26. Kruszewski, Stefan. Dependence of SERS signal on surface
roughness.
Surface and Interface Analysis (1994), 21(12), 830-8.
27. Ozin, G. A.; Gil, C. J. Chem. Rev. 1989,89, 1749.
28. Sun, Tao; Seff, Karl. Silver Clusters and Chemistry in Zeolites.
Chemical Reviews (Washington, DC, United States) (1994), 94(4), 85770.
29. Baes, C. F.; Mesmer, R. E. The Hydrolysis of Cations; Wiley: New York,
1976.
30. Brown, D. R.; Kevan, L. J. Phys. Chem. 1986,90, 1129.
31. Kevan, L.; Narayana, N. ACS Symp. Ser. 1983,218,283.
32. Holmberg, Brett A.; Wang, Huanting; Norbeck, Joseph M.; Yan, Yushan.
Controlling size and yield of zeolite Y nanocrystals using
.tetramethylammonium bromide. Microporous and Mesoporous Materials
(2003), 59(1), 13-28.
33. Pagliai M, Bellucci L, Cardini G, Muniz-Miranda M, Schettino V (2006)
Phys Chem Chem Phys 8:171.
34. Muniz-Miranda, Maurizio; Cardini, Gianni; Schettino, Vincenzo. Surfaceenhanced Raman spectra of pyridine and pyrazolide on silver colloids:
Chemical and electromagnetic effects. Theoretical Chemistry Accounts
(2004), 111(2-6), 264-269.
35. D.J. Rogers, et al. Surface Enhanced Raman Spectroscopy of Pyridine,
Pyridinium Ions and Chloride Ions adsorbed on the Silver Electrode. J.
Electroanal. Chem., 167 (1984) 237-249.
113
36. Aggarwal, R. L.; Farrar, L. W.; Diebold, E. D.; Polla, D. L. Measurement
of the absolute Raman scattering cross section of the 1584-cm-1 band
of benzenethiol and the surface-enhanced Raman scattering cross
section enhancement factor for femtosecond laser-nanostructured
substrates.
Journal of Raman Spectroscopy (2009), 40(9), 1331-1333.
37. Tai Ha Joo, Myung Soo Kim, Kwan Kim Journal of Raman Spectroscopy
1987, v. 18, 57-60.
38. Saikin. “On the chemical bonding effects in the Raman response:
Benzenethiol adsorbed on silver clusters.” Physical Chemistry Chemical
Physics, v. 11 issue 41, 2009, p. 9401.
114
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CHAPTER 2
1. Morris, M; Wallan, D.J. Anal. Chem., 1979, 51, 182A.
2. Strommen, D.P.; Nakamoto, K. J. Chem. Educ., 1977, 54, 474.
3. Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Raman spectra of
pyridine adsorbed at a silver electrode.
Chemical Physics Letters
(1974), 26(2), 163-6.
4. Stiles, Paul L.; Dieringer, Jon A.; Shah, Nilam C.; Van Duyne, Richard P.
Surface - enhanced Raman spectroscopy.
Annual Review of
Analytical Chemistry (2008), 1 601-626.
5. Sun, Mengtao; Wan, Songbo; Liu, Yajun; Jia, Yu; Xu, Hongxing.
Chemical mechanism of surface- enhanced resonance Raman
118
scattering via charge transfer in pyridine-Ag2 complex.
Journal of
Raman Spectroscopy (2008), 39(3), 402-408.
6. Creighton, J. A. Surf. Sci. 1986, 173, 665.
7. Schatz, George C.; Young, Matthew A.; Van Duyne, Richard P..
Electromagnetic mechanism of SERS.
Topics in Applied Physics
(2006), 103(Surface-Enhanced Raman Scattering), 19-4.
8. Liu, Fanxin; Cao, Zhishen; Tang, Chaojun; Chen, Ling; Wang, Zhenlin.
Ultrathin Diamond-like Carbon Film Coated Silver Nanoparticles Based Substrates for Surface-Enhanced Raman Spectroscopy.
ACS
Nano (2010), 4(5), 2643-2648.
9. Moskovits M. 2005. Surface-enhanced Raman spectroscopy: a brief
retrospective. J. Raman Spectrosc. 36:485–96.
10. Zhang, Jian-Bing; Fang, Yan. Surface-enhanced Raman scattering of phydroxybenzoic acid in pure Ag colloids produced by laser ablation.
Colloids and Surfaces, A: Physicochemical and Engineering Aspects
(2005), 266(1-3), 38-43.
11. Horimoto, Noriko; Ishikawa, Nobuaki; Nakajima, Atsushi. Preparation of a
SERS substrate using vacuum-synthesized silver nanoparticles.
Chemical Physics Letters (2005), 413(1-3), 78-83.
12. M. Zhu, G. Qian, G. Ding, Z. Wang, M. Wang Materials Chem. & Phys 96
(2006) 489.
13. Loo, B. H.; Tse, Y.; Parsons, K.; Adelman, C.; El-Hage, A.; Lee, Y. G.
Surface-enhanced Raman spectroscopy of imidazole adsorbed on
119
electrode and colloidal surfaces of Cu, Ag , and Au.
Journal of Raman
Spectroscopy (2006), 37(1-3), 299-304.
14. L. Rivas, S. Sanchez-Cortes, J.V. Carcía-Ramos, G. Morcillo Langmuir 17
(2001) 574.
15. U. Nickel, A. Castell, K. Pöppl, S. Schneider, Langmuir 16 (2000) 9087
16. Y. Chen, H. Kim Materials Lett. 61 (2007) 5040.
17. H. Lin, J. Mock, D. Smith, T. Gao, M. Sailor J. Phys. Chem. B 108 (2004)
11654.
18. Whitcomb, D. R.; Stwerka, B. J.; Chen, S.; Cowdery-Corvan, P. J. SERS
characterization of metallic silver nanoparticle self - assembly within
thin films.
Journal of Raman Spectroscopy (2008), 39(3), 421-426.
19. Gutes, Albert; Carraro, Carlo; Maboudian, Roya. Silver Dendrites from
Galvanic Displacement on Commercial Aluminum Foil As an
Effective SERS Substrate.
Journal of the American Chemical Society
(2010), 132(5), 1476-1477.
20. Dutta, Prabir K.; Robins, Daniel. Silver-coated faujasitic zeolite crystals
as surface-enhanced Raman spectroscopic substrates.
Langmuir
(1991), 7(10), 2004-6.
21. W. Yan, L. Bao, S. Mahurin, S. Dai Applied Spectroscopy 58 (2004) 18.
22. Nie, Shuming; Emory, Steven R.. Probing single molecules and single
nanoparticles by surface-enhanced Raman scattering.
(Washington, D. C.) (1997), 275(5303), 1102-1106.
120
Science
23. Li Qing-yu; Li Shi-rong; Si Min-zhen; Zhang Peng-xiang Comparative
study on SERS and stability of negative silver colloids.
Guang pu xue
yu guang pu fen xi = Guang pu (2007), 27(2), 299-301.
24. Guingab, J. D.; Lauly, B.; Smith, B. W.; Omenetto, N.; Winefordner, J. D.
Stability of silver colloids as substrate for surface enhanced Raman
spectroscopy detection of dipicolinic acid.
Talanta (2007), 74(2), 271-
274.
25. Prochazka, Marek; Mojzes, Peter; Stepanek, Josef; Vlckova, Blanka;
Turpin, Pierre-Yves. Probing Applications of Laser-Ablated Ag Colloids
in SERS Spectroscopy: Improvement of Ablation Procedure and SERS
Spectral Testing.
Analytical Chemistry (1997), 69(24), 5103-5108.
26. Kruszewski, Stefan. Dependence of SERS signal on surface
roughness.
Surface and Interface Analysis (1994), 21(12), 830-8.
27. Ozin, G. A.; Gil, C. J. Chem. Rev. 1989,89, 1749.
28. Sun, Tao; Seff, Karl. Silver Clusters and Chemistry in Zeolites.
Chemical Reviews (Washington, DC, United States) (1994), 94(4), 85770.
29. Baes, C. F.; Mesmer, R. E. The Hydrolysis of Cations; Wiley: New York,
1976.
30. Brown, D. R.; Kevan, L. J. Phys. Chem. 1986,90, 1129.
31. Kevan, L.; Narayana, N. ACS Symp. Ser. 1983, 218, 283.
32. Holmberg, Brett A.; Wang, Huanting; Norbeck, Joseph M.; Yan, Yushan.
Controlling size and yield of zeolite Y nanocrystals using
121
tetramethylammonium bromide. Microporous and Mesoporous Materials
(2003), 59(1), 13-28.
33. Pagliai M, Bellucci L, Cardini G, Muniz-Miranda M, Schettino V (2006)
Phys Chem Chem Phys 8:171.
34. Muniz-Miranda, Maurizio; Cardini, Gianni; Schettino, Vincenzo. Surfaceenhanced Raman spectra of pyridine and pyrazolide on silver colloids:
Chemical and electromagnetic effects. Theoretical Chemistry Accounts
(2004), 111(2-6), 264-269.
35. D.J. Rogers, et al. Surface Enhanced Raman Spectroscopy of Pyridine,
Pyridinium Ions and Chloride Ions adsorbed on the Silver Electrode. J.
Electroanal. Chem., 167 (1984) 237-249.
36. Aggarwal, R. L.; Farrar, L. W.; Diebold, E. D.; Polla, D. L. Measurement
of the absolute Raman scattering cross section of the 1584-cm-1 band
of benzenethiol and the surface-enhanced Raman scattering cross
section enhancement factor for femtosecond laser-nanostructured
substrates.
Journal of Raman Spectroscopy (2009), 40(9), 1331-1333.
37. Tai Ha Joo, Myung Soo Kim, Kwan Kim Journal of Raman Spectroscopy
1987, v. 18, 57-60.
38. Saikin. “On the chemical bonding effects in the Raman response:
Benzenethiol adsorbed on silver clusters.” Physical Chemistry Chemical
Physics, v. 11 issue 41, 2009, p. 9401.
122