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Self-assembled monolayers of functional group-terminated molecules on Au [Elektronische Ressource] / by Jinxuan Liu

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154 pages
Self-assembled Monolayers of FunctionalGroup-terminated Molecules on AuDissertationbyJinxuan LiuFakultät für Chemie und BiochemieRuhr-Universität BochumDecember 2010Referees: Prof. Dr. Ch. WöllProf. Dr. R. A. FischerDate of Defense: 2010.12.10iDedicationiiAbstractThe purpose of this thesis was to investigate the structures of self-assembled mono-layers (SAMs) with functionalized end groups, e.g. pyridine, triptycene on Au sur-face. Another aim was to indentify the vibrational band of mercapto-group on Ausurface. Finally, adsorption properties of small molecules on metal-organic frame-work (MOF) thin films were examined.Various surface-analytical techniques, e.g. ultra-high vacuum-Infrared reflectionadsorption spectroscopy, x-ray photoelectron spectroscopy, near edge X-ray ab-sorption fine structure spectroscopy, scanning tunneling microscopy and thermaldesorption spectroscopy were applied.The main results of the thesis were that pyridine SAMs can be formed on Au sur-face with two dierent structures, which are almost perpendicular to Au surfacewith non-coplanar aromatic rings; MOF thin films are established as model systemto study adsorption properties of small molecules which can determine the chemi-cal or physical states of metals in MOFs.KeywordsSAMs, MOF thin films, pyridine, HKUST-1Das Ziel dieser Arbeit war die strukturelle Untersuchung von selbstorganisierendenMonoschichten (SAMs) mit funktionalisierten Endgruppen, z.B.
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Self-assembled Monolayers of Functional
Group-terminated Molecules on Au
Dissertation
by
Jinxuan Liu
Fakultät für Chemie und Biochemie
Ruhr-Universität Bochum
December 2010Referees: Prof. Dr. Ch. Wöll
Prof. Dr. R. A. Fischer
Date of Defense: 2010.12.10
iDedication
iiAbstract
The purpose of this thesis was to investigate the structures of self-assembled mono-
layers (SAMs) with functionalized end groups, e.g. pyridine, triptycene on Au sur-
face. Another aim was to indentify the vibrational band of mercapto-group on Au
surface. Finally, adsorption properties of small molecules on metal-organic frame-
work (MOF) thin films were examined.
Various surface-analytical techniques, e.g. ultra-high vacuum-Infrared reflection
adsorption spectroscopy, x-ray photoelectron spectroscopy, near edge X-ray ab-
sorption fine structure spectroscopy, scanning tunneling microscopy and thermal
desorption spectroscopy were applied.
The main results of the thesis were that pyridine SAMs can be formed on Au sur-
face with two dierent structures, which are almost perpendicular to Au surface
with non-coplanar aromatic rings; MOF thin films are established as model system
to study adsorption properties of small molecules which can determine the chemi-
cal or physical states of metals in MOFs.
Keywords
SAMs, MOF thin films, pyridine, HKUST-1
Das Ziel dieser Arbeit war die strukturelle Untersuchung von selbstorganisierenden
Monoschichten (SAMs) mit funktionalisierten Endgruppen, z.B. Pyridin oder Trip-
tycen auf Au-Oberflächen. Ein weiteres Ziel war die Identifizierung der Schwingungs-
Bande von SH-Gruppen auf Au-Oberflächen. Schlielich wurde das Adsorptionsver-
halten von kleinen Molekülen auf dünnen MOF-Filmen untersucht.
Hierzu wurden verschiedene Oberfl?chentechniken wie UHV-IRRAS, XPS, NEX-
AFS, STM und TDS angewendet.
Die wichtigsten Ergebnisse der Arbeit waren zu einen, dass sich Pyridin SAMs
auf Au-Oberflächen in zwei senkrecht zur Oberfläche orientierten Strukturen mit
´nicht-koplanaren aromatischen Ringen ausbilden. Zum anderen wurden dlznne´
MOF-Filme als Modellsystem etabliert, um damit die Adsorptionseigenschaften
von kleinen Molekülen, die die chemischen oder physikalischen Zustände von Met-
allen in MOFs bestimmen, zu untersuchen.
Stichwort
´SAMs, dlznnen´ MOF-Filmen, Pyridin, HKUST-1
iiiContents
1 Introduction 1
1.1 Self-assembled monolayers (SAMs) . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Outline of the thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2 Experimental Techniques 9
2.1 Surface Analytical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2 Quantum Chemical Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3 Chemicals and Preparation of SAMs 43
3.1 Chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.2 Preparation of SAMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4 Pyridine-terminated SAMs 50
4.1 SAMs Made from Pyridine Disulfides . . . . . . . . . . . . . . . . . . . . . . 50
4.2 SAMs Made from Monothiols . . . . . . . . . . . . . . . . . . . . . 60
4.3 Small molecules adsorbed on Pyridine Terminated SAMs . . . . . . . . . . . . 80
5 Triptycene thiol and selenol SAMs 84
5.1 Preparation of SAMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
5.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
5.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
6 Vibrational Spectroscopic Investigation of SH-Terminated SAMs 98
6.1 Preparation of SAMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
6.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
6.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
7 Loading Gas Phase Species in HKUST-1 Metal Organic Framework Grown onto a
COOH-Terminated SAM 105
7.1 Preparation of HKUST-1/MHDA/Au . . . . . . . . . . . . . . . . . . . . . . . 105
7.2 Bands Assignments of HKUST-1/MHDA/Au . . . . . . . . . . . . . . . . . . 106
7.3 Thermal Stability of HKUST-1/MHDA/Au . . . . . . . . . . . . . . . . . . . 107
7.4 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
ivCONTENTS
7.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
8 Summary and Outlook 119
8.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
8.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
Appendix1 121
Appendix2 122
Appendix3 124
References 144
Publications 145
Acknowledgement 146
Curriculum Vitae 147
v1 Introduction
The surfaces of solids play a significant role in chemical processes, which are very important
for basic chemical research and the application of knowledge to the solution of prac-
[1, 2]tical problems. Heterogeneous catalysis , which can take place at solid surfaces, has been a
[3]central process in the chemical industry for a century with regard to ammonia synthesis using
an iron-based catalyst or the exhaust gases of cars using catalyst system that converts carbon
[4]monoxide and hydrocarbons to carbon dioxide. Corrosion , which is caused by chemical re-
actions at surfaces, happens in our daily life such as at surfaces of airplanes and ships. These
[5]damages can be reduced by modifying the properties of the surface . The Nobel Prize in chem-
istry was awarded to Gerhard Ertl in 2007 for his thorough studies of fundamental molecular
processes at the gas-solid interface. The physical and chemical properties of metal surfaces and
oxide surfaces have been well understood since the last decade. Nowadays organic surfaces are
[6–12]increasingly attracting attention .
1.1 Self-assembled monolayers (SAMs)
The history of organic thin films development can be traced back more than 200 years ago
[13]from observation of the calming influence of oil on water surfaces by Franklin . After that
[14]time Zisman performed systematic research on systems related to self-assembled monolayers
[7–12, 15–21](SAMs). Hitherto self-assembled monolayers (SAMs) attract tremendous attentions.
The main reason why are so attractive is the tunability of the surface
properties by selectively modifying functional groups. In addition, from the application point of
[22–27]view these materials often exhibit interesting diverse properties in molecular electronics ,
[28–32] [33–38]electrochemistry , or biochemistry as shown in Fig.1.1.
Nowadays, research on SAMs mainly focuses on the potential technological applications, nev-
ertheless the fundamental understanding of the formation of SAMs and construction of new
structures of SAMs require persistent research in this field. Very recently, it had been found that
gold adatoms are involved at all stages of alkane thiol self-assembly, including the dissociation
of the disulfide (S-S) and hydrogen-sulfide (S-H) bonds and subsequent formation of the self-
[39–42]assembled structure .
Various types of ligands have been synthesized to construct SAMs on diverse substrates. These
[7]systems have been throughly investigated including thiols (RSH)/selenols (RSeH) on metal
11.1 Self-assembled monolayers (SAMs)
Figure 1.1: Applications of self-assembled monolayers.
(Au, Ag, Cu, Pt, Pd, Zn, Hg etc) or semiconductor surface (CdSe, CdS, Ge, Si etc), carboxylic
acids on Al O and Ti/TiO , alkyl siloxane monolayers on SiO surfaces, organo silanes on2 3 2 2
oxidized surfaces (SiO , Al O , ITO glass etc), alcohols and amines on Pt and Si.2 2 3
1.1.1 Constitution of SAMs
Self-assembled monolayers (SAMs) are formed spontaneously on a substrate by chemisorption
of a molecule with a specific anity of its head group. It is one of the methods to prepare
organic thin films on a substrate.
The constituents of an aromatic SAM-molecule and the angles defining the orientation of the
[7]molecule are schematically shown in Fig.1.2 . The structure and stability of SAMs are signifi-
[43, 44]cantly determined by head group, molecular backbone and end group of a SAM-molecule .
The sulfur atom is widely used as a head group on metallic surfaces like Au, Ag, Pt, etc. due
to the formation of very strong chemical covalent bonds between sulfur and metals. Recently,
another element selenium, which belongs to the same group of the periodic system of the ele-
ments as sulfur is chose as the head group as well. It has been found that very high quality of
[45]SAMs structures can be obtained using selenium as head group in certain cases .
Flexible alkanes and rigid aromatic compounds can be selected for the backbone of a SAM-
molecule. In earlier work on SAMs the focus was on n-alkanethiolates on gold to unravel
[7, 9]fundamental aspects of film formation, structure and properties . In later years aromatic thi-
olates have attracted an increasing amount of attention. This interest results from the higher
rigidity of the molecular backbones which in many cases have allowed for a better control of
[46–53]the monolayer structure .
21.1 Self-assembled monolayers (SAMs)
Today, SAMs gain an increasing importance with regard to the generation of organic surface
[21, 54]exposing predefined functionalities . Attaching an appropriate functional end group such
[55–57] [58–61] [62–64] [65–67] [68, 69] [68, 70–72] [73–79]as fluorocarbon , SH , CN , NH , OH , COOH or pyridine2
allow to tailor the wettability and the reactivity of the organic surfaces exposed by the SAMs,
[22–25] [28–32]which have numerous potential applications in molecular electronics , electrochemistry ,
[33–38]or biochemistry .
Figure 1.2: Schematic drawing of an ideal self-assembled alkanethiolate SAM adsorbed on
Au(111) surface showing the end group, molecular backbone and head group. The constituents
and characteristics of the SAM are highlighted.
1.1.2 Formation of thiolates SAMs on Au surfaces
Self-assembled monolayers can be prepared on various substrates, metal or semiconductor sub-
strates, for example, Au, Ag, Cu, Pt and Pd for metal SiO , Al O , TiO , ZnO for2 2 3 2
[7]semiconductor surface (see Ref. and references therein), however the most widely used sub-
strate for studying SAMs is gold because (1) it is an inert metal and does not be oxidized at
room temperature in air; (2) it can be used for many other surface analytical techniques.
[9]Organic thin films supported by a substrate can be prepared using several routes (see Ref.
and references therein) such as Langmuir-Blodgett (LB) films, organic molecular beam epitaxy,
self-assembled monolayers prepared from vapor under vacuum conditions and from solutions.
Comparing with the other preparation methods, SAMs in particular grown from solutions are
attractive for the following main reasons: (1) the ease of preparation; (2) low cost.
A schematic process of SAMs preparation and a model of fully assembled SAM are shown in
Fig.1.3. A clean gold substrate is immersed into a solution containing thiols, these molecules
will mainly overcome the energy of van der Waal’s interactions and gauche defect (deviation
from fully extended backbone) during immersion and finally adsorb on Au surface forming
[9, 18]SAMs .
3