Thesis for the degree of Doctor of Philosophy

Colloidal synthesis of metal nanoparticles

Mechanistic studies and development of flow synthesis routes

A NNA P EKKARI

Department of Chemistry and Chemical Engineering

C HALMERS U NIVERSITY OF T ECHNOLOGY
Gothenburg, Sweden 2020
Colloidal synthesis of metal nanoparticles
Mechanistic studies and development of flow synthesis routes
Anna Pekkari

© Anna Pekkari, 2020.

ISBN 978-91-7905-392-5

Doktorsavhandlingar vid Chalmers tekniska högskola,

Ny serie nr. 4859
ISSN 0346-718X

Department of Chemistry and Chemical Engineering

Chalmers University of Technology
SE-412 96 Gothenburg
Sweden
Telephone +46 (0)31 772 1000

Cover:
Illustration of the synthesis of Au nanoparticles in a continuous segmented flow reactor

Typeset in LATEX
Printed by Chalmers Reproservice
Gothenburg, Sweden 2020
Colloidal synthesis of metal nanoparticles
Mechanistic studies and development of flow synthesis routes

Anna Pekkari
Department of Chemistry and Chemical Engineering
Chalmers University of Technology

Abstract
Metal nanoparticles (NPs) are central in a wide range of industrial areas including cat-
alytic and biomedical applications. Due to their interesting physical properties, the
demand of these precious materials is steadily increasing, which has created a need
for the development of effective and high-performing NPs. Because the properties are
closely correlated to NP size, shape, and crystal structure, the development of con-
trolled synthesis of metal NPs has emerged. However, the challenge of understanding
how reaction parameters influence the outcome of the synthesis currently limits the pro-
duction of precisely designed metal NPs. Additionally, low reproducibility and control
during scale-up has often restricted the production to small scale which limits the use
in industrial applications.
The studies presented in this thesis focus on the controlled synthesis of uniform metal
NPs using solution-based colloidal methods. Firstly, the multiple roles of the NP stabiliz-
ers were investigated. In the synthesis of Cu NPs, alkanethiol stabilizers only provided
temporary stabilization of the Cu NPs and decomposed under heating in inert atmo-
sphere, forming Cu2 S NPs. In another study, uniform Pd NPs stabilized with a binary
surfactant combination were synthesized without using traditional reductants. The fatty
acid stabilizer contributed to the reduction of Pd-precursors, and the reduction kinet-
ics follow a pseudo-first order kinetics. The specific stabilizers investigated influence
reduction kinetics, NP sizes and shapes. Secondly, to address scale-up challenges in
NP synthesis, the development of flow synthesis routes were explored. Uniform Pd
nanocubes (NCs) and PdPt core-shell NPs were produced in a single-phase flow reactor,
and a segmented flow reactor was developed to produce uniform Au NPs. The flow
production was scaled-up, and the uniformity of Au NPs was confirmed by inline optical
spectroscopy quality control. Catalytic evaluation of the function of PdPt core-shell NPs
in a model NO2 reduction reaction showed improved catalytic activity, selectivity and
temperature stability compared to Pd NCs.

Keywords: copper, palladium, platinum, gold, nanoparticles, colloidal synthesis, stabi-

lizers, reduction kinetics, flow chemistry, core-shell, nanocube, segmented flow, inline
quality control, NO2 reduction

i
List of Publications

This thesis is based on the following appended papers:

I. Synthesis of Cu nanoparticles: stability and conversion into Cu2 S nanoparticles

by decomposition of alkanethiolate
Christian Rohner, Anna Pekkari, Hanna Härelind and Kasper Moth-Poulsen
Published, Langmuir, 2017, 33, 13272-13276

II. Synthesis of highly monodisperse Pd nanoparticles using a binary surfactant

combination with sodium oleate as reductant
Anna Pekkari, Xin Wen, Jessica Orrego-Hernández, Robson Rosa da Silva, Eva Olsson,
Hanna Härelind, and Kasper Moth-Poulsen
Submitted

III. Continuous microfluidic synthesis of Pd nanocubes and PdPt core-shell

nanoparticles and their catalysis of NO2 reduction
Anna Pekkari, Zafer Say, Arturo Susarrey-Arce, Christoph Langhammer,
Hanna Härelind, Victor Sebastian and Kasper Moth-Poulsen
Published, ACS Applied Materials and interfaces, 2019, 11, 36196-36204

IV. Continuous hydrothermal flow synthesis of monodisperse citrate-capped Au

nanoparticles: a non-fouling efficient route by using paraffin-water segmented
system and inline liquid-liquid phase separation
Anna Pekkari, Orlane Nicolardot, Robson Rosa da Silva, Xin Wen, Jessica Orrego-Hernández,
Hanna Härelind, and Kasper Moth-Poulsen
Submitted

iii
Additional publications not included in this thesis:
Guided selective deposition of nanoparticles by tuning of the surface potential
J. Eklöf, A. Stolaś, M. Herzberg, A. Pekkari, B. Tebikachew, T. Gschneidtner,
S. Lara-Avila, T. Hassenkam and K. Moth-Poulsen
Published, Europhysics Letters, 2017, 1, 18004

Microwave-heated γ-Alumina Applied to the Chemoselective Reduction of

Aldehydes to Alcohols
B. Dhokale, A. Susarrey-Arce, A. Pekkari, A. Runemark, K. Moth-Poulsen,
C. Langhammer, H. Härelind, M. Busch, M. Vandichel, and H. Sundén
Accepted ChemCatChem, (2020)

iv
My Contributions to the Publications

Paper I
Second author. I conducted synthesis experiments, TEM and UV-Vis analysis. XRD and
electron diffraction was performed by Christian Rohner. Part of the writing and proof
reading.

Paper II
Main author. I conducted all the synthesis experiments, sample preparation, UV-Vis and
FTIR analysis. HRTEM and electron diffraction was performed by Xin Wen. NMR mea-
surements was performed by Jessica Orrego-Hernández. I wrote the first draft and was
responsible for writing the manuscript.

Paper III
Main author. I conducted all the synthesis experiments, SEM analysis and performed
data analysis and interpreted the results with my co-authors. TEM, HRTEM and EDX
analysis was performed by Victor Sebastian. The catalytic evaluation was performed by
Zafer Say. I wrote the first draft and was responsible for writing the manuscript.

Paper IV
Main author, shared equally with Orlane Nicolardot. I conducted synthesis experiments,
UV-Vis, Zeta potential measurements and TEM analysis. HRTEM was performed by Xin
Wen. I wrote the first draft and was responsible for writing the manuscript.

v
vi
Contents

1 Introduction 1
1.1 Scope of thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Shaped metal nanoparticles 3

2.1 Colloidal synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.1 Shape-control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.2 Bimetallic nanoparticles . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Flow synthesis of nanoparticles . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 Pd and Pt nanoparticles in catalysis . . . . . . . . . . . . . . . . . . . . . 9

3 Synthesis and characterization methods 11

3.1 Colloidal synthesis and kinetic evaluation . . . . . . . . . . . . . . . . . 11
3.1.1 Synthesis of Cu and Cu2 S nanoparticles . . . . . . . . . . . . . . 11
3.1.2 Synthesis and reduction kinetics of Pd nanoparticles . . . . . . . 12
3.2 Flow synthesis of nanoparticles . . . . . . . . . . . . . . . . . . . . . . . 13
3.2.1 Single-phase flow synthesis of Pd nanocubes and
PdPt nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2.2 Continuous segmented flow synthesis of Au nanoparticles . . . . 14
3.3 Characterization of nanoparticles . . . . . . . . . . . . . . . . . . . . . . 15
3.3.1 Transmission electron microscopy . . . . . . . . . . . . . . . . . . 15
3.3.2 Scanning electron microscopy . . . . . . . . . . . . . . . . . . . . 16
3.3.3 Ultraviolet-visible spectroscopy . . . . . . . . . . . . . . . . . . . 16
3.3.4 Powder x-ray diffraction . . . . . . . . . . . . . . . . . . . . . . . 16
3.3.5 Microwave plasma atomic emission spectroscopy . . . . . . . . . 17
3.3.6 Fourier transform infrared spectroscopy . . . . . . . . . . . . . . 17
3.3.7 Nuclear magnetic resonance . . . . . . . . . . . . . . . . . . . . . 17
3.3.8 Zeta potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

vii
 3.3.9 Catalytic evaluation . . . . . . . . . . . . . . . . . . . . . . . . . 18

4 Results and discussion 19

4.1 Stabilizer effects and reduction mechanisms for colloidal nanoparticles . 19
4.1.1 Multiple roles of stabilizers . . . . . . . . . . . . . . . . . . . . . 19
4.1.2 Evaluation of reduction kinetics . . . . . . . . . . . . . . . . . . . 24
4.2 Flow synthesis of colloidal nanoparticles . . . . . . . . . . . . . . . . . . 27
4.2.1 Development of a single-phase flow synthesis . . . . . . . . . . . 27
4.2.2 Development of an automated segmented flow synthesis . . . . . 29
4.2.3 Flow synthesis optimization - effect of temperature . . . . . . . . 31
4.2.4 Scale-up of flow production . . . . . . . . . . . . . . . . . . . . . 33
4.2.5 Catalytic evaluation of Pd and PdPt nanoparticles . . . . . . . . . 37

5 Conclusions and Reflections 39

5.1 Contribution to the field of nanoparticle synthesis . . . . . . . . . . . . . 40
5.2 Reflections on future development of
nanoparticle synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Acknowledgements 43

Bibliography 45

viii
Chapter 1
Introduction

The development of metal nanoparticles (NPs) (including Cu, Au, Pd and Pt) has gained
immense interest in the recent years due to their interesting and unique physical prop-
erties, and has led to their applications in a range of areas including catalysis[1, 2], fuel
cells[3], and biomedical applications[4,5]. The steadily increasing demand of these pre-
cious materials has created a need for the development of effective and high-performing
NPs. Since the properties of the metal NPs are closely related to the size, shape and com-
position, the development of synthesis methods with a high degree of control over these
properties have emerged[6, 7]. By replacing NPs with heterogeneous morphology with
precisely designed NPs with higher performance, the material loading in the defined
application can be reduced and the cost lowered, and thus a more sustainable use of
these rare metals can be achieved[8]. Among the wide range of existing NP synthesis
methods, solution-based colloidal methods have created a range of defined shaped NPs
and benefit from its simplicity, reproducibility and high precision in controlling NP prop-
erties[9–11]. However, the lack of understanding of how different reaction parameters
influence the outcome of the synthesis is currently a challenge in the development of
precisely structured metal NPs[12]. Moreover, for controlled metal NPs to target indus-
trial applications the production has to be scaled-up to the kilogram-scale[8, 13]. The
batch reactors traditionally used for the synthesis of controlled metal NPs are often lim-
ited to small scale due to inefficient mixing and heat transfer during production scale-up
which has led to poor reproducibility between batches[8, 14]. An approach to address
these challenges is to synthesize NPs in flow reactors. Reagents are continuously infused
in micro- or millimeter sized channels, which provide fast mass transfer and heating
with high control over reaction parameters[15–18]. The synthesis can be scaled up by
flow reactor parallelization[19], increase of flow channel dimensions[13, 20], and ex-
tended operation times and NP quality can be continuously monitored by inline quality
control[21].

1
1.1 Scope of thesis
The primary objective of the work presented in this thesis was to develop an increased
understanding of what factors control the synthesis of uniform metal NPs using solution-
based colloidal methods. The first part of this thesis focuses on understanding synthesis
mechanisms and the influence on NP properties. In Paper I, thiol-stabilized Cu NPs
were synthesized and the decomposition route of the thiol stabilizer and the forma-
tion of Cu2 S NPs was studied. In Paper II, Pd NPs stabilized with a binary surfactant
combination were synthesized without traditional reducing agents, and the reduction
mechanisms and kinetics were studied. The second part of the thesis involves the de-
velopment of continuous flow synthesis routes to address scale-up challenges in NP
synthesis. In Paper III a flow synthesis is designed for the synthesis of Pd nanocubes
(NCs) and PdPt core-shell NPs, and process scalability explored. Furthermore, the cat-
alytic performance of the Pd and PdPt NPs was evaluated in a model NO2 reduction
reaction. Paper IV focuses on exploring the development of an automated segmented
flow synthesis to produce uniform citrate-capped Au NPs in large scale. The Au NP reac-
tion phase is separated by liquid-liquid phase separation, and product quality monitored
inline by optical spectroscopy.

2
Chapter 2
Shaped metal nanoparticles

2.1 Colloidal synthesis

Precisely shape and size-controlled metal NPs have mainly been synthesized using solution-
based colloidal methods, which have successfully explored a wide range of metals and
structures[9–11]. In the general procedure, a metal salt precursor is reduced in solu-
tion by a reductant, which supplies the metal with electrons. A variety of reductants
have been employed in the synthesis of NPs, such as ascorbic acid, sodium borohydride,
trisodium citrate and alcohols. Apart from donating electrons, the reductant can some-
times have several roles such as capping agent, colloidal stabilizer or solvent. Careful
choice of reductant is essential since its physicochemical properties influence the re-
duction kinetics and subsequent outcome of the NP synthesis[22]. Other components
normally present in the colloidal method are capping agents or stabilizers that pro-
vide colloidal stabilization and prevent aggregation by adsorption onto the NP surface.
Commonly employed stabilizers in NP synthesis include surfactants, thiols, polymers,
and fatty acids[23].
LaMer theory [24] is the predominant theory that explains NP formation in so-
lution, which is divided into two distinct stages; nucleation and growth (Figure 2.1).
Initially (I), the metal precursor is rapidly reduced, and when the metal atom con-
centration reaches a critical concentration (II), the burst nucleation point, plenty of
metal nuclei form. Particles continue to grow by coalescence of the nuclei followed by
slow growth by diffusion (III). A rapid nucleation that creates multiple particle seeds
is considered crucial for the production of uniform NPs[25]. Uniform or monodisperse
NPs are defined as having a narrow size distribution with a size deviation below 10 %
[26]. A precise control of size and shape of NPs can be achieved by fine tuning the
nucleation and growth steps. This can be attained either by thermodynamic (reaction
temperature, reduction potentials) or kinetic control (reagent concentration, reaction

3
rate, solubility)[25, 27]. Nonetheless, to understand the effect of each parameter and
how it influences the outcome of the synthesis is challenging. Several models have tried
to explain NP growth and kinetics[28], but additional knowledge is needed to fully
understand reduction mechanisms involved in NP synthesis[12, 29].

Figure 2.1: The principle of NP nucleation and growth according to LaMer theory. The
curve shows the metal precursor concentration as a function of time. [29] - Published
by The Royal Society of Chemistry.

2.1.1 Shape-control
In the synthesis of metal NPs, the shape can be directed by thermodynamics and kinetics
[25, 27, 30, 31], presented in the energy diagram in Figure 2.2. The lowest free energy
of the NP system is the product formed under thermodynamic control. A kinetic product
which has a higher energy can form at a local energy minimum. The driving force for
any kinetic product is to convert to the thermodynamic state, where a certain amount
of energy is required to cross the activation energy barrier. Depending on the size of
the energy barrier and thermal energy present in the system, the kinetic product can be
stable for different periods of time [27, 30].
For a crystalline material, surface area is not the only factor determining the sur-
face free energy. The type of crystal structure possesses different surface free energies
depending on their atom arrangements. The metals applied in this thesis work (Cu,
Au, Pd, Pt) are face-centered cubic (fcc) metals for which three of the low-index crystal
planes are (111), (100) and (110) (Figure 2.3a). The more dangling bonds a surface
contains, the more unstable it is and the higher surface free energy it has. The number
of dangling bonds increases for (111), (100) and (110) crystal planes from 3,4 to 6,
and consequently the surface free energies follow the same pattern [27]. The Wulff
polyhedron is the thermodynamically most favorable shape, having the lowest surface
energy [31]. It consists of (111) and (100) facets and is referred to as polycrystalline.

4
Figure 2.2: Schematic illustration of kinetic and thermodynamic NP products with
relation to the total free energy. The arrow marks the activation energy required to
transform a kinetic product to a thermodynamic product. Copyright (2020) Wiley. Used
with permission from Ref. [30].

Capping agents can influence both the thermodynamic and kinetics by selective
adsorption to crystal facets, and thus anisotropically change the surface free energies.
By lowering the activation energy for a specific facet through selective adsorption, the
capping agent can direct the growth into a specific shape [23,25]. Figure 2.3b visualizes
an example of the role of the capping agent in the growth of a metal NP into different
shapes. When a capping agent selectively passivate the (100) facets, the NP grow into a
cube, whereas a different capping agent that passivates (111) facets directs the growth
into an octahedral shape. Furthermore, by selective adsorption the capping agent can
physically block a certain crystal facet and direct the NP shape through kinetic control.
In NP synthesis, kinetics and thermodynamics are not separate events and are often
closely connected. Among the different reaction parameters, temperature has the most
power in determining if thermodynamic or kinetic control is dominating in NP synthe-
sis. Hence, a significant rise or decrease in temperature can switch the reaction from
thermodynamic to kinetic control [25, 27].

5
 a)
a)

b)

Figure 2.3: a) Models of the (111), (100), and (110) planes of a fcc-metal and the
corresponding numbers of dangling bonds per surface unit cell (NB ). b) Schematic
illustration of the role of a capping agent in directing the growth of a single-crystal seed
into NPs with different shapes. Reprinted with permission from Ref. [27]. Copyright
(2015) American Chemical Society.

2.1.2 Bimetallic nanoparticles

Due to their interesting properties, bimetallic metal NPs have been intensively studied
in recent years which has led to the development of a plethora of different structures
[32–41].To simplify, bimetallic NPs can be divided into two main structures, core-shell
or alloy NPs. In the synthesis of bimetallic NPs, elemental composition and shape-
control is more complex than for single metal NPs. Generally, the reduction can be
performed simultaneously, or by a two-step seed-growth [32, 33]. The co-reduction
approach can create either alloys or core-shell NPs, whereas the two-step seeded-growth
normally provides core-shell structures unless significant kinetic energy is provided. I
limit this section to introducing the co-reduction approach, since it is more relevant for
the experimental part of this thesis.
In colloidal synthesis of bimetallic NPs, apart from the parameters mentioned previ-
ously, several factors are important in determining NP shape and composition including
redox potentials, interfacial energy, and metal precursor reduction rates [32, 33, 35].
The standard reduction potential is a quantitative measure on how easy a metal precur-

6
sor can be reduced. The reduction potentials of metals can be lowered by selecting a
capping agent that strongly coordinates to the metal, which slows down the reduction
rate. The interfacial energy is determined by two factors, lattice mismatch between the
two metals and the bond between the substrate metal and the surface atoms. A large
lattice mismatch will lead to large lattice strain and high interfacial energy. Hence, a
small lattice mismatch such as for Pd and Pt (0.77 %) facilitates the combination pro-
cess [32]. Finally, the reduction rate is determined by several factors such as reduction
potential, reductant and reaction temperature. By manipulating the reduction kinetics,
the shape and the elemental composition of a NP can be altered from a core-shell struc-
ture to an alloy [32, 35, 37]. In a one-pot synthesis at slow reduction rate conditions,
the metal with the highest reduction potential will be reduced prior to the other metal,
thus creating a core-shell structure. In contrast, high reduction rates normally favors
the formation of alloy NPs [32, 35].

2.2 Flow synthesis of nanoparticles

The production of colloidal metal NPs using traditional batch reactors is often limited to
small scale due to challenges with reproducibility in large reactors. A possible solution is
the use of flow reactors, where reagents are pumped through micro- or millimeter sized
channels for a certain reaction time and temperature. The high surface area-to-reaction
volume provides fast heat transfer and the small reaction volumes gives excellent mass
transfer and rapid mixing, and metal NPs can be formed in a short amount of time[15–
17, 42–44].
One type of flow reactor is the single-phase reactor which was designed in Paper III.
In this single-phase reactor (Figure 2.4a) the flow is in the laminar regime and reactants
are mixed mainly by diffusion. This creates a parabolic velocity profile that gives rise
to residence time distributions inside the reactor and may result in wide particle size
distributions[45–47]. A frequent challenge in single-phase flow reactors is fouling, i.e.
the deposition of NPs on the reactor walls. Fouling is caused by heterogeneous nucle-
ation and uncontrolled NP growth at the reactor walls, which could be explained by a
high affinity between the reagents and the reactor. Reactor fouling causes wide particle
size distributions and product losses, and will eventually result in channel clogging and
reactor failure[45–47].
An approach to remedy fouling is to synthesize NPs in biphasic reactors, so called
segmented flow reactors[8, 14, 18, 46, 47]. In the segmented flow reactor (Figure 2.4b),
the reaction phase is infused into an immiscible carrier phase which creates sub-microliter
sized droplets or segments of reaction phase. Fouling can be significantly reduced in the
segmented flow reactor since contact between the reaction phase and the reactor wall

7
is minimized. Additionally, the efficient turbulent mixing inside the segments created
by the slip velocity between the reaction phase and the carrier phase minimize axial
dispersion effects normally encountered in single-phase reactors that cause wide par-
ticle size distributions[46, 47]. While fouling is reduced in segmented flow reactors,
the carrier phase generates large amount of solvent waste. A recycling strategy can be
implemented by separating the immiscible phases by polarity using liquid-liquid phase
separation[46,48]. This approach is applied in Paper IV, where the organic carrier phase
can be separated and reused in the synthesis.
Contrary to batch reactors that are limited to small production scales due to poor
reproducibility, flow reactors can successfully be scaled up[15, 49]. Scale-up can be
achieved by operation of the reactor for extended operation times, by reactor paral-
lelization [15, 18, 19], or increased flow channel dimensions [13, 20, 44]. The possibil-
ity of full automation of the process enables an effective, safe and sustainable produc-
tion[14, 15, 18]. NP production scale-up is studied in a single-phase reactor (Paper III)
and in an automated segmented flow system (Paper IV). Furthermore, the integration
of inline quality control can provide process control by the monitoring of NP growth
to provide valuable data of reaction kinetics and synthesis outcome[16, 21, 44, 46, 49].
There exist several examples of inline quality measurements applied during metal NP
flow synthesis including dynamic light scattering [50], UV-Vis spectroscopy [13, 50–53]
and small angle x-ray scattering (SAXS) [51], that can provide structural and/or el-
emental information [21, 46]. However, inline quality control is currently limited for
fully automated segmented flow production of Au NPs and is studied in Paper IV.

(a)

(b)

Figure 2.4: Mixing characteristics in a) single-phase laminar flow, b) segmented flow

with turbulent mixing in segments.

8
2.3 Pd and Pt nanoparticles in catalysis
Noble metals including Pd and Pt NPs are important industrial catalysts in a range
of reactions[11]. These rare noble metals are limited resources with extremely low
abundance in the earths crust, notably at parts per billion concentrations [54]. The
increasing demand of these scarce metals has sparked the development of more high-
performing catalysts to optimize the use of these metals. The catalytic performance,
i.e. reactivity and selectivity is closely related to NP size and shape, crystal facets and
elemental composition. By applying shaped NPs with enhanced catalytic properties,
the catalytic loading can be reduced which leads to lower costs and provide a more
sustainable use of these rare metals[8]. However, in order to enhance NP catalyst per-
formance for a certain reaction, understanding of the correlation between NP properties
and catalytic activity and selectivity needs to be improved.
Shaped Pd and Pt-based NPs have been studied as catalysts in several reactions
including reduction and oxidation reactions in fuel cells, carbon-carbon bond formation
and hydrogenation reactions[11]. Especially interesting is the combination of Pd and
Pt into bimetallic NPs, which may not only combine the individual properties but could
enhance the catalytic performance and temperature stability due to synergy effects be-
tween the metals[33–35, 55]. In Paper III catalytic evaluation of Pd NCs and PdPt
NPs with a core-shell structure is performed, where the effect of shape and elemental
composition on catalytic activity and selectivity in a model NO2 reduction reaction is
evaluated.

9
10
Chapter 3
Synthesis and characterization methods

3.1 Colloidal synthesis and kinetic evaluation

To evaluate the effect of different reagents including stabilizers and reductants, and
their influence on NP synthesis outcomes, different solution-based colloidal synthesis
methods were developed and evaluated. This section describes the synthesis methods
that were developed to produce Cu NPs (Paper I) and Pd NPs (Paper II), and the evalu-
ation of reduction kinetics of Pd NPs (Paper II).

3.1.1 Synthesis of Cu and Cu2 S nanoparticles

The synthesis of Cu NPs was performed under N2 atmosphere using a Schlenk line (Pa-
per I). A scheme for the reaction can be found in Figure 3.1. First, the copper precursor
(CuCl2 *2H2 O) was dried into brown copper chloride (CuCl2 ) powder by heating to 60
°C for 30 minutes. Subsequently, the stabilizer dodecane thiol and solvent, dodecane,
were added to the flask, and the mixture was heated to 145 °C forming a yellow dis-
persion. The reducing agent, tert-butylamineborane complex (TBAB) was added and
the reaction was left to proceed for 5-90 min. The black Cu NP dispersion was then
left to cool in room temperature. For the synthesis of Cu2 S NPs the Cu NP dispersion
was reheated to 175 °C for 2 hours or left overnight under N2 , giving an orange Cu2 S
NP dispersion. NP dispersions were transferred to N2 -filled centrifuge tubes filled with
degassed ethanol, were centrifuged and the supernatant was discarded. The procedure
was repeated twice, followed by drying in N2 followed by dispersion in dry toluene. A
detailed description of the synthesis steps and the purification of the NPs is explained
in Paper I.

11
Figure 3.1: (Top) Reaction Scheme of the reaction of CuCl2 and dodecane thiol yield-
ing Cu(0)-dodecane thiol and didodecyl disulfide. (Bottom) Formation of metastable
thiolate-capped Cu NPs and their decomposition paths forming Cu2 S and Cu2 O, in
the presence of excess thiol under ambient conditions or N2 atmosphere, respectively.
Reprinted with permission from Ref. [56]. Copyright (2017) American Chemical Soci-
ety.

3.1.2 Synthesis and reduction kinetics of Pd nanoparticles

Synthesis of Pd NPs stabilized with sodium oleate (NaOL) and hexadecyltrimethylam-
monium chloride (CTAC) was performed in batch (Paper II), and in the absence of
traditional reducing agent. Briefly, in a vial solutions of CTAC, NaOL and H2 PdCl4 were
mixed, which created a turbid orange suspension. The vial was capped and placed in a
heated convection oven for 4h, which gave a black Pd NP suspension.
UV-Vis spectroscopy was used to evaluate the reduction kinetics of Pd NPs. Samples
of the reaction solution were taken at specific time points during the reaction (5-300
min), and were added to room temperature saturated KCl-solution and mixed. Aliquots
of this stock solution was added to 3 Eppendorf tubes containing KCl-solution. Dilution
with KCl solution was performed to avoid hydrolysis of the PdCl4 2- complex and to
quench the reaction. The final solutions were centrifuged to separate Pd NPs from
the PdCl4 2- complex, and the supernatant was analyzed with UV-Vis spectroscopy. The
percentage of PdCl4 2- remaining in the reaction solutions was calculated from a standard
plot. A detailed description of the synthesis and kinetic evaluation can be found in in
Paper II.

12
3.2 Flow synthesis of nanoparticles
This section describes the flow syntheses which were developed to synthesize Pd NCs
and PdPt core-shell NPs using a single-phase flow reactor (Paper III), and Au NPs using
a segmented flow reactor (Paper IV).

3.2.1 Single-phase flow synthesis of Pd nanocubes and

PdPt nanoparticles

Synthesis of Pd NCs was first established by adapting a batch protocol from Niu et
al[57], to a micro-sized single-phase flow synthesis (Figure 3.2). Two streams of so-
lutions were infused in micro-sized polytetrafluoroethylene (PTFE) tubing (inner di-
ameter 800 μm) at a constant flow rate by the use of syringe pumps, interfaced in a
t-junction followed by heating in a temperature controlled water bath for a certain res-
idence time. The NP dispersion was then collected and purified by centrifugation. In
the synthesis of Pd NCs, the first stream contained an aqueous solution of Pd-precursor
(H2 PdCl4 ) and stabilizer, hexadecyltrimethylammonium bromide (CTAB), which was
reduced by a second stream consisting of an aqueous solution of reducing agent (L-
Ascorbic acid). To synthesise PdPt NPs the first stream was composed of an aqueous so-
lution of CTAB and a combination of Pd-precursor(H2 PdCl4 ) and Pt-precursor(H2 Cl6 Pt)
with varying molar ratios of the metals. A detailed description of the flow synthesis
method and the purification steps can be found in Paper II.

a) b)

Figure 3.2: The single-phase flow system used to synthesize Pd NCs and PdPt NPs. a)
Photograph of the equipment, b)Schematic setup.

13
3.2.2 Continuous segmented flow synthesis of Au nanoparticles
A continuous segmented hydrothermal flow synthesis was developed to produce citrate-
capped Au NPs, adapted from a modified Turkevich method in batch by Kettemann et. al
[58]. The segmented flow system (Figure 3.3) featured two peristaltic pumps, an ultra-
smooth flow chemistry syringe pump, automated back-pressure control and full automa-
tion control using the connected computer with integrated software. The reagents were
pumped in high purity grade perfluoroalkoxy (PFA) tubes. Aqueous citrate solution
was interfaced with Au-precursor solution in water in an Ethylene tetrafluoroethylene
(EFTE) T-junction. The outlet was connected to a glass-capillary connected to a sec-
ond t-junction (EFTE). Microliter sized segments were produced when carrier phase,
IsoparTM L was infused to the second T-junction. The outlet of the T-junction was con-
nected to a coiled tube microreactor (PFA), heated by hot air to a constant temperature.
The outlet of the flow reactor was connected to an automated active back-pressure regu-
lator to maintain constant pressure in the reactor and avoid gas formation. The aqueous
reaction phase was separated by a liquid-liquid separator, and the organic carrier phase
IsoparTM L was reused in the synthesis. An inline UV-Vis spectrometer flow cell was used
to monitor the quality of Au NPs in the reaction phase. The carrier phase IsoparTM L was
kindly sponsored by ExxonMobil. A detailed description of the segmented flow setup
and experimental parameters can be found in Paper IV.

a) b)

Au NP
suspension

Figure 3.3: Setup of the automated segmented flow reactor for the synthesise of citrate-
capped Au NPs with liquid-liquid phase separation and inline optical spectroscopy qual-
ity control, a) Photograph of the equipment, b) Schematic setup.

14
3.3 Characterization of nanoparticles
The synthesized NPs were analyzed with respect to morphology, size, crystal struc-
ture, elemental composition and catalytic activity using the characterization methods
described in this section.

3.3.1 Transmission electron microscopy

Characterization of the morphology, size, crystal structure and elemental distribution of
the synthesized NPs in Paper I-IV were studied using transmission electron microscopy
(TEM). The electrons in the TEM are accelerated by a high voltage, 200-300 kV, which
results in a very small wavelength. Compared to an optical microscope where the spa-
tial resolution is limited to the wavelength of the visible light, the TEM provides sig-
nificantly higher spatial resolution that can be as low as 0.1 nm dependent on various
effects including instabilities and lens abberations[59]. In the TEM, an electron beam is
transmitted through a very thin sample by the focusing of a series of magnetic lenses. An
area of the sample is illuminated by the electron beam, and the objective lens provides
an image of the area which is magnified by the projector lenses and a highly magnified
image of the sample can be observed on a viewing screen or by a camera. With the
use of an aperture, the selected area electron diffraction (SAED) pattern of the NPs can
be recorded in the back focal plane of the objective lens[59]. By comparison to known
crystal structures determination of crystal facets of the NPs can be detected. SAED was
used to determine the crystal structure of Cu NPs (Paper I), Pd NPs (Paper II) and Au
NPs (Paper IV).
In Scanning TEM (STEM) the electron beam is focused to a small spot and scanned
over the specimen, and the intensity of the scattered electrons is recorded by a high
angle annual dark field (HAADF) detector, which is atomic number sensitive and thus
provides qualitative information of the sample[59]. In Paper III STEM-HAADF was used
in the imaging of Pd NCs and PdPt core-shell NPs.
Interactions between the electron beam in the TEM and the sample produces a
range of secondary signals providing elemental information from the NP sample. En-
ergy dispersive x-ray spectroscopy (EDX) records the emission of characteristic x-rays
resulting from the inelastic scattering of electrons in the atoms of the sample. A spec-
trum can be constructed with characteristic peaks that corresponds to individual ele-
ments in a selected area of the sample, which can be converted to quantitative data of
the elemental distribution in the sample[59]. In Paper I, EDX was used to study the
elemental composition in Cu NPs and in Paper III to determine the relative elemental
distribution in PdPt core-shell NPs.

15
3.3.2 Scanning electron microscopy

In the Scanning Electron Microscope (SEM), an electron beam focused by magnetic

lenses is scanned over the sample surface in a raster pattern. When the electron beam
interacts with the electrons in the sample excited secondary electrons are created which
can be detected and a topological image of the sample constructed[60]. SEM imaging
of NP samples can provide information about particle shape and morphology. In Paper
III, SEM imaging was performed on Pd NCs and PdPt core-shell NPs deposited onto
Si-substrates to study the effect of heat treatment on NP morphology.

3.3.3 Ultraviolet-visible spectroscopy

Ultraviolet-visible (UV-Vis) spectroscopy is a technique used to measure absorbance of

molecules that undergo electronic transitions in the ultraviolet and visible light range.
Quantitative measurements can be used by following the Lambert-Beer law stating that
the absorbance is related to the concentration of the analyte and the path length of
the light. The wavelength of the absorbance peak can be used to analyze metal NPs,
and metal precursor complexes present in solution[61]. When metal NPs interact with
incoming light, excitation of the surface plasmons gives rise to absorption and excitation
events. This in turn creates different perceived colors of the NP suspension, determined
by the chemical environment and composition. The position and width of the absorption
peak in the spectrum can provide information of shape, size and elemental composition
of the NPs[62]. In Paper II, a quantitative evaluation of the reduction kinetics in the
synthesis of Pd NPs was studied with UV-Vis spectroscopy. Cu NPs were analyzed with
UV-Vis spectroscopy under N2 atmosphere to study the stability over time (Paper I), and
inline quality control using UV-Vis spectroscopy was applied to continuously monitor
the consistency of flow-synthesized Au NPs (Paper IV).

3.3.4 Powder x-ray diffraction

In powder x-ray diffraction (XRD), a powder sample is irradiated with an X-ray beam
scanned at different incident angles which interacts with the electrons in the sample
and result in different scattering intensities at various angles. The resulting diffraction
pattern can be identified by comparison to a known standard or a database[63]. XRD
was applied to study the changes in the crystal structure of Cu NPs that over time formed
Cu2 S NPs (Paper I).

16
3.3.5 Microwave plasma atomic emission spectroscopy

Microwave plasma atomic emission spectroscopy (MP-AES) is an atomic emission tech-

nique that was used to quantify the synthesis yield for batch and flow synthesized Pd
NCs (Paper III). Microwave plasma excites the elements in the sample and an emission
spectrum with characteristic wavelengths for a specific element can be created. The in-
tensity of the emitted light is proportional to the number of atoms, and qualitative and
quantitative information about the elements in the sample is provided [64]. To quantify
the Pd content in Pd NC suspensions (Paper III), the emission spectrum of completely
dissolved Pd NP solutions were compared to standards with known Pd concentration.

3.3.6 Fourier transform infrared spectroscopy

In fourier transform infrared (FTIR) spectroscopy an infrared source is interacting with

a sample which cause molecular vibrations and absorbance. The infrared source is
scanned over a wide spectral range to produce an absorption spectra. In a molecule,
the atoms absorb frequencies that are characteristic of their structure, depending on
their mass and arrangement. Since every molecule has slightly different vibration mode,
the spectrum produced is unique and can be used to identify and study chemical com-
pounds[65]. In Paper II, FTIR spectroscopy with an attenuated total reflection (ATR)
diamond crystal was used to study the chemical composition of reagents and Pd NPs.

3.3.7 Nuclear magnetic resonance

Nuclear magnetic resonance (NMR) is a technique to study local interaction between

magnetic moment of atomic nuclei and an external magnetic field. A radiofrequency
transmitter excites the nuclei, creating magnetic resonance of the nuclei, and the signals
are detected by a receiver and recorded as spectral lines. A spectrum can be generated
for compounds containing atoms with non zero magnetic movement, such as the proton
1
H. In a molecule, the magnetic resonance of an atom is dependent on the magnetic
field of the surrounding atoms. Thus, NMR can provide detailed information about the
electronic structure and individual functional groups in a molecule. Since the magnetic
field is characteristic for a certain compound, NMR can be used for identification and
can provide structure determination of unknown samples, study dynamics, reaction
mechanisms, and chemical environment[66]. In Paper II, 1 H NMR was used to study
the structure and chemical environment of the reagents NaOL, CTAC and Pd NPs.

17
3.3.8 Zeta potential
Zeta potential can be applied to evaluate the surface charges of NPs. When a charged
particle is dispersed in a solvent, an adsorbed electrical double layer forms at its surface.
The layer closest to the particle is of opposite charge to the particle, referred to as the
Stern layer. Due to the electrostatic field of the NPs a diffuse layer consisting of opposite
and same charge as the particle form on top of the Stern layer. Together with the
Stern layer this forms the electrical double layer. Application of an electric field results
in charges in the diffuse layer moving towards the opposite electrode. The slipping
plane is a hypothetical plane acting as an interface between the moving charges and
the dispersant around them, and the zeta potential is the potential at this interface[67].
Zeta potential was used to evaluate the colloidal stability of Au NPs (Paper IV).

3.3.9 Catalytic evaluation

In Paper II, Pd NCs and PdPt core-shell NPs were evaluated as catalysts in the NO2 re-
duction reaction. The flat samples were assembled in a tube “pocket reactor”, previously
described by Bu et al[68]. Catalyst samples were prepared by drop casting concentrated
NP suspension onto plasma-treated fused silica substrates. The NP samples were first
pre-treated under a gas flow and heated (150 °C), and cooled down to 48°C. The sam-
ples were then exposed to the reaction mixture (2200 ppm NO2 and 2.2 % H2 in argon)
with the temperature gradually ramped up to 136 °C. The outlet gas composition was
detected by mass spectroscopy, and the conversion and product selectivity quantified. A
detailed description of the experimental setup can be found in Paper III.

18
Chapter 4
Results and discussion

4.1 Stabilizer effects and reduction mechanisms for

colloidal nanoparticles
This section focuses on the multiple roles of the stabilizers in NP synthesis and how they
can influence NP properties. I investigated the role of the stabilizers in the synthesis of
Cu NPs and the subsequent formation of Cu2 S NPs (Paper I), and in the synthesis of Pd
NPs stabilized with a binary surfactant combination (Paper II). Furthermore, quantita-
tive evaluation of reduction kinetics in the synthesis of Pd NPs (Paper II) was evaluated
to investigate the mechanisms involved in the synthesis.

4.1.1 Multiple roles of stabilizers

In Paper I, I developed a synthesis of spherical Cu NPs (3-10 nm) and investigated their
stability and elemental composition over time. A range of reaction parameters and the
effect on the particle size distributions were evaluated. The uniform Cu NPs (Figure
4.1a) consist mainly of Cu(0) and CuO species that formed after oxidization of the NPs
on the TEM-grid, which is confirmed by SAED (Figure 4.1b). When exposed to ambient
atmosphere, the Cu NPs oxidized and the color of the NP suspension changed from black
to brown along with the disappearance of the plasmon resonance maximum at 597 nm.
The Cu NP suspensions were not stable for more than 12h even under protected N2
atmosphere and showed aggregation and precipitation. Interestingly, under N2 atmo-
sphere the Cu NP suspension changed color from blue to orange, despite the absence
of oxygen, and showed poor colloidal stability. To evaluate the effect of temperature,
the Cu NP suspension aged at room temperature was heated to 175 °C which yielded
uniform Cu2 S NPs (Figure 4.1c). SAED of the NPs shows the presence of hexagonal
Cu2 S and Cu (0) species, but no Cu(I)O (Figure 4.1d).

19
 a) b)

Cu (O)

Cu (I) Oxide

c) d)

Cu (O)

Cu (I) Sulfide

Figure 4.1: TEM images of a) Cu NPs, c) Cu2 S NPs. b) and d) shows the corresponding
SAED of the NPs from a) and b), respectively. Reference patterns of Cu(0), JCPDS-Nr.
4-0836 (white semicircles), Cu2 O, JCPDS-Nr. 5-0667 (red semicircles), and hexagonal
Cu2 S, space group P63/mmc, JCPDS-Nr. 46-1195, (yellow semicircles). Adapted with
permission from Ref.[56]. Copyright (2017) American Chemical Society.

The stability of freshly synthesized thiolate-protected Cu NPs in N2 atmosphere

was evaluated by UV-Vis spectroscopy (Figure 4.2). With time (0-275 min), a grad-
ual disappearance of the plasmon resonance band occurred and the peak maximum
slowly decreased and shifted to longer wavelengths, however at a much slower rate
than previously reported for oxidation of Cu[69]. It can therefore be excluded that the
diminishing absorbance maximum was caused by Cu NP oxidation, but may instead be
a result of NP aggregation. The presence of sulfur in the Cu NP samples was confirmed
by EDX analysis. However, EDX and XRD analysis on the samples aged at room tem-
perature could not detect crystalline Cu2 S species. XRD analysis of the heated Cu NP
suspension on the other hand shows the presence of hexagonal Cu2 S phases. Therefore,
it can be concluded that Cu2 S were not formed in room temperature, and heating of
the NP suspension was necessary to yield crystalline Cu2 S NPs, or Cu NPs coated with a
Cu2 S shell.
These findings point to the general conclusion that the thiolate stabilizers can not
provide sufficient colloidal stabilization of Cu NPs under longer time in room temper-
ature, even under protected atmosphere. Furthermore, heating of the suspension was
necessary to form crystalline Cu2 S NPs. As the only sulfur source in the Cu NP synthesis,

20
it can be concluded that during heating of the Cu NP suspension, the alkane thiolate
stabilizer decomposed through cleavage of the C-S bond at the surface of the Cu NPs,
and only provided temporary stabilization of the Cu NP surface. This agrees well with
previous findings by Vollmer et. al [70].

a) b)

a )

Figure 4.2: (a) Temporal evolution of UV-Vis absorption spectra of freshly synthesized
Cu NP suspension. Photograph of the Cu NP suspension after b) 0 min, and c) after 275
min. Adapted with permission from Ref. [56]. Copyright (2017) American Chemical
Society.

Apart from decomposing on the surface to alter the chemical composition of NPs,
the stabilizer can have other roles in the synthesis of metal NPs. In Paper II I investigated
the influence of the stabilizers in the development of a synthesis of Pd NPs stabilized
with a binary surfactant mixture of NaOL and CTAC. The synthesis was performed in
the absence of traditional reducing agents, hence the Pd-precursors were reduced spon-
taneously in the reaction mixture at 100 °C. Based on earlier findings [71], our initial
hypothesis was that the electron dense alkyl double bond in the stabilizer NaOL was the
main contributor to the reduction of Pd-precursors. After 4 hours of reaction uniform
Pd NPs formed (Figure 4.3) with an average size of 29.7 nm ± 5.7 % (Inset in Figure
4.3a). The Pd NPs are polycrystalline, seen by multiple diffraction rings (inset (Figure
4.3b), and can be visualized in the HRTEM image of a single multiple-twinned Pd NP
(Figure 4.3c,d).
A range of reaction parameters were investigated to study the influence on the Pd
NP properties, thoroughly described in Paper II. In this thesis I focus the discussion on
the effects of altering the stabilizers on Pd NP properties. To evaluate the hypothe-
sis that the double bond is responsible for the reduction of Pd-precursors, NaOL was
replaced with the structurally similar saturated fatty acid, sodium stearate (NaST). Sur-
prisingly, this stabilizer combination could also produce Pd NPs, which are smaller than

21
 a) b)

20 25 30 35 40
)

) )

Figure 4.3: Structural characterization of Pd NPs stabilized with NaOL and CTAC. a),b)
TEM images of monodisperse Pd NPs at relatively low magnification, where inset in a)
shows histogram of particle size distribution with an average size of 29.7 nm ± 5.7 %,
inset in b) shows the SAED pattern of Pd NPs. c) HRTEM image of an individual Pd
NP shows a multiple-twinned structure, where twin boundaries are marked with red
arrows, d) HRTEM image of the selected area of the Pd NP marked by a red square in
c). The inserted image in d) shows a corresponding FFT pattern.

with NaOL, with an average size of 13 nm ± 19 % (Figure 4.4a). The reaction was
slower than with NaOL, observable by a later color change of the reaction solution. The
reduction speed was further investigated with UV-Vis spectroscopy, as explored in the
following section. Reduction occurred despite the NaST lacking double bonds which in-
dicates that the reduction mechanisms are more complex than the previously proposed
hypothesis.
When CTAC was replaced with CTAB, the equivalent ammonium salt with bromide
as counter ion in the surfactant mixture, Pd NPs with different shapes formed (Figure
4.4b). This includes bars, cubes and “arrow” shaped Pd NPs. After 4 hours reaction,
excess particle seeds are present and Pd NPs are smaller than than those stabilized with

22
 a) b)

5 10 15 20 25 30 35
)

Figure 4.4: a) TEM image of Pd NPs stabilized with NaST and CTAC, where Inset in a)
shows a correspoding histogram of size distributions with an average size of 13 nm ±
19 %. b) TEM image of Pd NPs stabilized with NaOL and CTAB.

NaOL and CTAC (Figure 4.4b). Despite using the same reductant (NaOL), a slower
reduction was evidenced by a slower color change of the reaction solution. The differ-
ence in shapes obtained when CTAC was replaced with CTAB could be due to bromide
ions (Br- ) present in CTAB, well-known to selectively adsorb onto (100) crystal facets
and have been applied extensively in the controlled synthesis of cubic [57, 72] and rod-
shaped Pd NPs [73]. In the synthesis of CTAB-stabilized Au nanorods, Meena et. al [74]
showed that Br- adsorption is not the only contributor to the selective surface passiva-
tion of (100) facets, but is a driving force for CTAB micelle adsorption and stabilization
of the Au nanorod surface. CTAB was shown to form dense surfactant micellar layers
on gold surfaces. In contrast, more isotropic shaped particles were obtained with CTAC,
due to less facet selectivity and the low presence of Cl- and micellar structures protect-
ing the Au surface. These findings may explain the difference in shapes observed when
CTAC was replaced with CTAB in the synthesis of Pd NPs. Furthermore, the slower re-
duction of Pd-precursors observed in the presence of CTAB could partly be explained by
the strong complexation between bromide ions in CTAB and Pd-precursors (PdBr4 2- ).
Additionally, the dense and thick surfactant layer of CTAB may lower the accessibility of
NaOL to reduce Pd-precursors which could contribute to a slower reduction rate.
In order to produce colloidally stable Pd NPs it was necessary to use a binary sta-
bilizer combination with the anionic surfactant NaOL/NaST and the cationic surfactant
CTAC. When synthesis was performed in the absence of CTAC, reduction occurred and
Pd NPs formed but the stabilizers (NaOL/NaST) provided poor colloidal stability and
extensive visual particle aggregation in solution could be observed. On the other hand,
synthesis with only CTAC does not result in any reduction of Pd-precursors. These

23
findings lead to the conclusion that CTAC was necessary to provide sufficient colloidal
stability of Pd NPs, but did not contribute to the reduction of Pd-precursors. To study
the interactions between the stabilizers NaOL and CTAC with Pd NPs, qualitative anal-
ysis was performed using FTIR and NMR, which showed that the stabilizers provided
colloidal stabilization by adsorption onto the NP surface. Details of the experiments
and qualitative analysis can be found in Paper II. It is clear that NP stabilizers can have
multiple roles in the NP synthesis and their influence of the outcomes can be complex.

4.1.2 Evaluation of reduction kinetics

To investigate the kinetics involved in the Pd NP synthesis (Paper II), a quantitative
evaluation of reduction kinetics was performed by UV-Vis spectroscopy. Reduction ki-
netics were monitored by following the diminishing absorbance peak of PdCl4 2- (280
nm) over time (5-300 min). In coherence with previous observations, it is clear that the
reduction of Pd precursors stabilized with NaOL and CTAC (Figure 4.5a) is faster than
the combination with NaST and CTAC (Figure 4.5b). A quantitative analysis of the re-
duction (Figure 4.5c), reveals that after 1 hour of reaction 50.8 % of PdCl4 2- is reduced
in the CTAC and NaOL system, which is increased to 87.2 %, and 89.4 % after 4 and 6
hours of reaction, respectively. In comparison, for the system with NaST and CTAC only
7.4 %, 15.4 % and 16.3 % PdCl4 2- is reduced after 1, 4, and 6 hours, respectively.
In the evaluation of reduction kinetics, several kinetic models have been developed
to describe metal NP formation [28,75–77]. Since the reduction reaction in the solution-
based colloidal synthesis is governed by electron transfer between a reductant and metal
precursor, the reaction is normally considered bimolecular, and the rate of reaction
depend on both reactants [12, 78]. As the reductant normally is supplied in excess the
reaction can be simplified to a pseudo first-order reaction rate law, where the reaction
rate is dependent on the concentration of the metal precursor. When the first-order
reaction law is applied to the data for Pd NPs (Figure 4.5d), the two systems show
adequate correlation as the plot of ln [PdCl4 2- ] decreases linearly over time. The rate
constants were calculated to k=1.27·10-4 s-1 for NaOL and CTAC, and k=1.17·10-5 s-1
for NaST and CTAC. The time points at 250 and 300 min slightly deviates from the
model. It could therefore be discussed if the reduction mechanisms at this time in the
reaction involve other mechanisms such as particle aggregation. For the pseudo-first
order rate law, the concentration of the reductant, in our case NaOL, is assumed to
remain constant throughout the reaction. For our system, the concentration of NaOL
was not supplied in excess in relation to Pd-precursors. Despite this fact, our data
correlate fairly well with the pseudo first-order model. It could be argued that NaOL
might have the capability of reducing several Pd-ions into metallic Pd, and thereby

24
 a) 1 b) 1
a 5 min a 5 min
30 min 30 min
0.8 60 min 0.8 60 min
Absorbance (a.u.)

Absorbance (a.u.)
120 min 120 min
180 min 180 min
0.6 240 min 0.6 240 min
300 min 300 min

0.4 0.4

0.2 0.2

0 0
250 275 300 325 350 250 275 300 325 350
Wavelength (nm) Wavelength (nm)

) 120
)
5

100 4.5

4
80
2-

ln [PdCl 2- ]
4

4
3.5
% PdCl

NaST NaST
60
NaOL NaOL
3
40
2.5
20
2

0 1.5
0 50 100 150 200 250 300 0 50 100 150 200 250 300
Time (min) Time (min)

Figure 4.5: Analysis of the reduction kinetics in the synthesis of Pd NPs. UV-Vis spectra
of PdCl4 2- in the reaction solution after 5-300 min for Pd NPs stabilized with a) CTAC
and NaOL, b) CTAC and NaST. c) A plot showing the percentage of PdCl4 2- remaining
in the reaction solutions for Pd NPs synthesized with NaOL and CTAC (blue), and with
NaST and CTAC (red) measured from the absorbance peak at 280 nm as a function of
time. d) Plots of ln [PdCl4 2- ] over time, which shows the pseudo-first order reaction
kinetics involved in the synthesis of Pd NPs stabilized with NaOL and CTAC, and with
NaST and CTAC, respectively.

retain a constant concentration through the reaction. Since the exact mechanisms of the
reduction are still not fully clear, further studies into this aspect should be performed to
provide a better understanding. To evaluate the influence of the stabilizer on reduction
kinetics, the effect of varying the concentration of reductant, i.e. NaOL/NaST, on the
reduction kinetics could be performed. Moreover, application of the Finke-Watzky (F-
W) kinetic model [75] to our data was performed since it could provide a model for
relatively slow reduction reactions [12]. The F-W model describes the reduction through
several steps including nucleation, homogeneous aggregation and autocatalytic surface
growth. Application of the model to our data shows poor agreement, where our system
presents a slow continuous reduction as opposed to the F-W model which includes a
slow reduction followed by fast autocatalytic growth.

25
 The type of metal precursor and the metal precursor and ligand coordination is
known to substantially influence reduction kinetics [12, 77]. Therefore, in the synthesis
of Pd NPs it would be interesting to evaluate the effect on reduction kinetics of changing
type of Pd-precursor, i.e. the type of metal precursor complex. It is clear from this study
that CTAC plays an important role in providing shape and colloidal stability of the Pd
NPs. The effect on shape and reduction kinetics when CTAC was replaced with CTAB,
caused by the stronger complexation between Pd-precursors and Br– ions serves as a
first example and it would be interesting to study other cationic surfactants and their
effect on reduction kinetics and NP properties.
Another possible direction to extend investigations of reduction kinetics could be
the study of other metals such as Au. Syntheses of various shaped Au NPs have been
performed with traditional reducing agents and stabilization of NaOL and CTAB [71]
and oleic acid and CTAB [79]. During the synthesis a noticeable color change of the Au-
precursor solution have been claimed to indicate that NaOL may act as partial reducing
agent for Au(III) precursors. Yet, further studies would be needed to understand the
reduction kinetics and mechanisms for Au NPs in the absence of these reducing agents.
From the quantitative evaluation of the reduction kinetics involved in the synthesis
of Pd NPs, it can be concluded that the alkyl double bond in NaOL is not necessary
to reduce Pd-precursors. Nonetheless, it may influence the reduction rate since reduc-
tion with NaOL is faster and more effective than Pd NP synthesis with the saturated
fatty acid NaST. However, the exact mechanisms governing the reduction needs to be
further studied to elucidate what electron donating groups that contribute to reducing
Pd-precursors. Elemental analysis of byproducts formed during NP synthesis using e.g.
NMR and Mass spectroscopy may improve understanding of the mechanisms during
reduction [80], which could elucidate the mechanisms involved in the reduction of Pd-
precursors (Paper II) but also to understand the decomposition of alkane thiolates on
the surface of Cu NPs (Paper I). The evaluation of byproduct chemistry during metal NP
synthesis may be challenging since byproducts can be present in small quantities, and
many reagents are involved in the synthesis (metal precursors, stabilizers, reductants
etc.) which creates a multitude of possible compounds and combinations to evaluate. In
this study, we applied NMR and Liquid chromatography and mass spectroscopy (LCMS)
to evaluate the byproducts formed during the Pd NP synthesis. Nonetheless, these
analyses led to inconclusive results and further analyses are needed to draw significant
conclusions on the reduction mechanisms involved in the synthesis. Despite these re-
sults, byproduct evaluation could be a valuable tool in evaluation of NP properties and
contribute in the development of metal NP synthesis procedures.

26
4.2 Flow synthesis of colloidal nanoparticles
The second focus of this thesis is the development of flow synthesis methods, due to
their potential to provide excellent control over NP properties. First, I developed a
synthesis of Pd NCs and PdPt NPs using a single-phase flow reactor (Paper III), and
an automated segmented flow synthesis was developed for the production of Au NPs
(Paper IV). A range of reaction parameters were evaluated and the effects of synthesis
temperature discussed. Furthermore, the scale-up of NP flow production was studied,
with the future aim of targeting real applications. Finally, evaluation of catalytic per-
formance in a model catalytic reaction of Pd NCs and PdPt NPs was performed (Paper
III).

4.2.1 Development of a single-phase flow synthesis

In Paper III, I developed a method to synthesize Pd NCs using a single-phase flow re-
actor. First, Pd NCs were synthesized in batch (Figure 4.6a) according to the protocol
by Niu et al[57], and then the synthesis was translated to a single-phase flow reactor
(Figure 4.6b). A range of reaction parameters were investigated to find the optimum
flow synthesis conditions, including temperature, stabilizer concentration and reaction
time. The STEM-HAADF image in the right inset in (Figure 4.6b) shows the [100] crys-
tal planes of the Pd NCs. The more efficient mixing, heat and mass transfer in the flow
reactor enabled a 10-time shortening of the reaction time, which resulted in smaller Pd
NCs. The batch synthesized Pd NCs have a size of 27.3 nm ± 11 % (inset in Figure 4.6a),

a) b)
2 nm

0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40
) )

Figure 4.6: TEM images of Pd NCs synthesized in a) batch, b) in single-phase flow.The

inset in b) shows a high-resolution TEM-image of a single Pd NC. The corresponding
histogram of particle size distribution in inset a) shows a size of 27.3 nm ± 11 % for
the batch reactor, and inset in b) 14.4 nm ± 11 % for the flow reactor.

27
and Pd NCs synthesized in flow 14.4 nm ± 11 % (inset in Figure 4.6b). Comparison
of NP uniformity shows no significant difference between the two methods. Nonethe-
less, when the particle yield was examined using MP-AES, observing the amount of Pd
precursor that has been reduced into metallic Pd NCs, a significant improvement in re-
action yield can be seen for flow synthesized Pd NCs (94 % in flow, 63 % in batch). This
improvement in synthesis yield could be explained by the more efficient heat and mass
transfer in the flow reactor[43].
Motivated by the enhanced catalytic properties of bimetallic NPs, I modified the
synthesis to incorporate a second metal to produce bimetallic PdPt NPs. By varying the
relative molar ratios of Pd:Pt in the precursor stream, the morphology of the formed
PdPt NPs can be controlled, which gradually changes from rough cubic to a spherical
dendritic shape with increasing Pt concentration (Figure 4.7).

a) b) )

Figure 4.7: TEM-images of flow-synthesized PdPt NPs. The molar ratio of Pd:Pt in the
particles are a) 6:1, b) 3:1, c) 1:1. Scale bars are 50 nm, in inset images scale bars are
10 nm. Reprinted with permission from Ref.[81]. Copyright (2019) American Chemical
Society.

Characterization using STEM-HAADF of the PdPt NPs with the highest Pt concen-
tration clearly visualizes the dendritic surface topography (Figure 4.8a,c,d). The ele-
mental distribution, analyzed by a line scan on the NP using STEM-EDX (Figure 4.8b)
reveals a core-shell structure with Pd dominated in the core and a Pt-rich surface. The
formation of bimetallic NPs is directed by the kinetics and reduction potential of the
two metals. Despite its lower reduction potential, Pd is reduced first to form the parti-
cle core. The slower reduction rate of Pt precursors could be explained by the stronger
complexation between Pt precursors and CTAB [82], which then lead to the formation
of PdPt core-shell NPs.

28
 a) b)

) )

Figure 4.8: a) HAADF-STEM image of flow synthesized PdPt NPs with the highest Pt
ratio. b) EDX line scan of a single PdPt NP which shows the relative distribution of Pd
and Pt, a core-shell structure. c) and d) shows high-resolution HAADF-STEM images
of PdPt NPs. Reprinted with permission from Ref.[81]. Copyright (2019) American
Chemical Society.

4.2.2 Development of an automated segmented flow synthesis

In Paper IV, I developed a hydrothermal segmented flow synthesis of citrate-capped

Au NPs. The synthesis was fully automated, and the carrier phase was separated from
the reaction phase and reused in the synthesis. After 2 minutes of reaction, uniform
Au NPs (Figure 4.9a,b), with a narrow size distribution of 9.5 nm ± 0.8 nm (8.4 %)
(inset Figure 4.9a) were produced. The majority of Au NPs are quasi-spherical and
polycrystalline seen by multiple diffraction rings (inset in Figure 4.9b) and the HRTEM
image (Figure 4.9c). The size and size-distribution of Au NPs was confirmed by UV-Vis
spectroscopy where the NPs show an absorbance maximum at 518 nm (Figure 4.9d).
During the development of the segmented flow synthesis of Au NPs, a range of reaction
parameters were evaluated including, temperature, reaction time and volume ratio of
reaction phase and carrier phase.

29
 a) b)

7 8 9 10 11 12
)

) )
d(111)=0.225 nm
)
a
ba
b

d(200)=0.225 nm d(200)=0.225 nm
a )

Figure 4.9: a), b) TEM images of uniform citrate-capped Au NPs at relatively low
magnification. The histogram of size distribution in inset in a) shows that the Au NPs
have a size of 9.5 nm ± 0.8 nm (8.4 %). The inset in b) shows the SAED pattern of
Au NPs. c) HRTEM image of a single Au NP shows a polycrystalline structure, where
boundaries between different crystalline domains are marked with red arrows. d) UV-
Vis absorbance spectrum of Au NPs with an absorbance maximum at 518 nm.

The narrow size distribution of the Au NPs synthesized in the segmented flow reac-
tor could be attributed to the internal back-mixing related to the slip velocity between
the segments of reaction phase and carrier phase that cause turbulent mixing. This
chaotic mixing eliminates axial dispersion effects normally experienced in single-phase
reactor that can cause wide particle size distributions [47, 52, 83, 84]. Furthermore, our
segmented flow reactor was integrated with automated back-pressure control which
enables the potential to conduct the synthesis at higher pressure and temperature, so
called hydrothermal conditions. A higher reaction temperature may shorten of reaction
time due to a faster nucleation and growth. The segmented flow reactor enables pre-
cise controlled synthesis under hydrothermal conditions, which is not easily achieved in
conventional batch reactors.

30
4.2.3 Flow synthesis optimization - effect of temperature

In the development of the flow synthesis of Pd NCs and PdPt NPs (Paper III), and Au NPs
(Paper IV), a systematic evaluation of different reaction parameters were performed to
evaluate the influence on NP properties. Details of all the tested parameters can be
found in Paper III and Paper IV. Since the temperature has a big influence on reaction
kinetics in the synthesis of metal NPs, directing the outcome of the synthesis [12, 27],
the effect on nucleation and growth of NPs by variation of the synthesis temperature
was evaluated for both flow systems.
When the temperature was lower than the optimal conditions (60 °C for Pd NCs
and PdPt NPs, 100 °C for Au NPs), larger Pd NPs (Figure 4.10a), PdPt NPs (Figure
4.10c) and Au NPs (Figure 4.10e) form. When the synthesis temperature is lowered,
the nucleation rate is slower which results in fewer particle seeds that subsequently
grow into larger NPs. Ftouni et. al observed an increase in Au NP size when the flow
synthesis temperature was reduced from 100 °C to 60 °C [85]. At the lower synthesis
temperature, PdPt NPs (Figure 4.10c) formed highly dendritic and porous NPs. Pd NPs
synthesized at the same temperature consisted of different shapes including nanorods,
triangles, multiple twinned NPs and NCs (Figure 4.10a). The slower nucleation rate
experienced at lower synthesis temperature may have increased the prevalence of Pd
NP seeds with stacking faults or multiple twinned structures that subsequently direct
the growth into different structures such as multiple twinned particles and nanorods.
However, further structural characterization is needed to confirm the exact structures
of the Pd NPs. Both thermodynamics and reaction kinetics, largely influenced by tem-
perature, play important roles in determining the structure of NP seeds [27]. Since
the shape of the NPs strongly depends on the initial structure of the seeds (presence of
stacking faults, twinned structure or single-crystal), control of temperature can direct
these properties [12]. Wang et. al [86] correlated the reduction rate with the struc-
ture of the seeds and found that relatively slow reduction of Pd-precursors generated
seeds with stacking faults and/or twin planes, whereas higher reduction rates yielded
single-crystal Pd NPs.
When the synthesis temperature was increased (130 °C for Pd NCs and PdPt NPs,
140 °C for Au NPs) various effects on sizes and shapes can be seen. Pd NCs (Figure
4.10b) have a wide distribution of sizes and shapes including rods, cubes, and twinned
NPs. Plenty of the Pd NPs are smaller than the Pd NCs synthesized at the optimal con-
ditions (96 °C). Similar size distribution effects are observed for the PdPt NPs (Figure
4.10d). The wide size and shape distributions observed for Pd NPs and PdPt NPs synthe-
sized at 130 °C could be explained by the difference in nucleation and growth rates. The
nucleation and growth are competing processes that occur simultaneously [12], and at

31
this temperature, the optimal parameters of nucleation and growth differ, which may
cause shape and size variations. In contrast, Au NPs synthesized at 140 °C experienced
extensive aggregation and formed large structures (Figure 4.10f).

a) b)

) )

) )

Figure 4.10: TEM images of a) Pd NCs and c) PdPt NPs synthesized in single-phase flow
at 60 °C. b) Pd NCs and d) PdPt NPs synthesized in flow at 130 °C. Au NPs synthesized
in segmented flow at e) 100 °C and f) 140 °C. Scale bars are 50 nm.

When the synthesis temperature was raised, the acceleration of nucleation of growth
created many small NP seeds. Without proper colloidal stabilization, the citrate-capped
Au NPs aggregated in an effort to minimize the high surface energy of the individual
small NPs. This correlates well with previous findings [87], where nuclei aggregation

32
and polydisperse Au NPs can be seen when synthesis temperature was increased. The
aggregation may be minimized by a shorter reaction time, and the colloidal stabiliza-
tion improved by increasing the concentration of stabilizer [87,88]. The fact that citrate
also acts as reductant and pH-mediator in the synthesis makes is difficult to predict the
effects of increased citrate concentration on Au NP morphology [88]. Furthermore, the
insufficient colloidal stabilization of Au NPs at 140 °C may have originated from partial
degradation of citrate. It has been shown that degradation of citrate occurs at higher
temperature [89, 90], but its thermal stability may be affected when applied as capping
agent [90, 91].

4.2.4 Scale-up of flow production

A common aim for Paper III and Paper IV was to investigate flow synthesis scalability,
with the aim of targeting future applications. Flow reactors benefit from easy scalabil-
ity, safe operation and sustainability. However, several scalability challenges exist which
include maintaining NP uniformity and minimize fouling, and integration of inline mon-
itoring of NP consistency. Scale-up of microfluidic flow reactors is achieved by operation
of the reactor for longer time periods, by several reactors run in parallel, or the dimen-
sions of the flow channels can be increased. The production of Pd NCs (Paper III) was
first scaled up by setting up a micro-sized nucleation zone (inner diameter 800 μm) fol-
lowed by connection to a milli-sized growth zone (inner diameter 1.4 mm). The larger
volume of the reactor led to an 8-fold increase of the production rate. Nonetheless, the
synthesis yield in this flow reactor decreased drastically compared to the micro-sized
flow reactor (33 %) which could be due to the extensive fouling on the reactor walls.
Interestingly, the Pd NCs synthesized in the milli-sized flow reactor have equivalent size
and shape uniformity (14 nm ± 11 %) compared to the micro flow reactor. It should be
noted that the particle size distribution was calculated from TEM-images which may not
be representative of the quality of the whole batch. Furthermore, the second scale-up
approach of Pd NCs and PdPt NP synthesis was by extension of the operation time of
the micro-sized flow reactor (120 min). During the extended synthesis times, fouling
occurred in the polymer tubing which led to more heterogeneous sized and shaped Pd
NCs (Figure 4.11a) and PdPt NPs (Figure 4.11d).
The motivation to scale-up the synthesis of Pd NCs and PdPt NPs was the cat-
alytic evaluation in a model NO2 reduction reaction. During catalytic evaluation, the
NP catalysts were exposed to reagents at elevated temperatures and the effect on the
morphology is therefore studied (Figure 4.11).

33
 a) b) )

) ) )

Figure 4.11: Structural characterization by SEM of Pd NCs (a,b and c) and PdPt NPs
(d,e and f) synthesized in large-scale in the single-phase microsized flow reactor. NP
samples were deposited on Si-substrates. a) Pd NCs and d) PdPt NPs without treatment.
b) Pd NCs and e) PdPt NPs after treatment in 48-136 °C temperature interval. c) Pd
NCs and f) PdPt NPs after treatment in 48-220°C temperature interval. Prior to heat
treatments, pre-treatment was performed at 158 °C. Heat treatments were conducted
in 2200 ppm NO2 and 2.2 % H2 in Ar(g). Scale bars are 100 nm.

After treatment in reaction conditions (48-136 °C), Pd NCs have slightly rounded
edges (Figure 4.11b), and upon further temperature increase (220 °C), particles under-
went sintering and formed large agglomerates(Figure 4.11c). The PdPt core-shell NPs
on the other hand showed good thermal stability and retained the shapes at reaction
conditions (Figure 4.11e), and the shape remained preserved at elevated temperatures
(Figure 4.11f). The improved thermal stability experienced by the PdPt core-shell NPs
may be attributed to the dendritic crystal structure, and the combination of Pd with Pt,
which has a higher temperature stability [92]. The severe aggregation of the Pd NCs ex-
perienced at elevated temperatures (>220 °C), where the NPs lost their size and shape
can be explained by sintering. Sintering can occur by two different mechanisms; Ost-
wald ripening, where small particles dissolve and redeposit onto large particles, or par-
ticle migration and coalescence[92, 93]. It is not fully understood by which mechanism
sintering occurred for the Pd NPs, but a correlation between high NP concentration and
sintering could be observed, where shorter inter-particle distance seemed to enhance
the sintering process. It should also be pointed out that for this model catalytic reaction
the NP catalysts were simply deposited on Si-substrates. Heterogeneous catalysis is nor-
mally performed with metal NP catalysts immobilized onto a porous support material.
Several strategies have been employed to improve the sintering stability of metal NP
catalysts to minimize diffusion, including enhancing metal-support interactions [94].

34
It would hence be interesting to re-evaluate the thermal stability of the NPs on meso-
porous support materials. Moreover, it has been shown that highly monodisperse NPs
experience less sintering due to Ostwald ripening effects [95]. Therefore, it would be
very interesting to evaluate and compare the temperature stability of more shape- and
size-uniform Pd NCs and PdPt NPs.
Fouling, which was experienced during scale-up in the flow reactors in Paper III,
is a commonly encountered problem in single-phase flow reactors. Any interfaces can
act as nucleation zones during NP growth, and since the reactants in single-phase flow
reactors are in constant contact with the reactor wall there is high risk for heteroge-
neous nucleation and accelerated growth to occur on the reactor wall, which can lead
to reactor fouling [45–47]. A possible solution is to use segmented flow reactors where
the contact between the reactor wall and the reaction phase can be minimized by the
use of an organic carrier phase that effectively wets the reactor walls. In the segmented
flow synthesis of citrate-capped Au NPs no fouling was observed in the flow reactor,
even when the synthesis was scaled-up for an extended time period (120 min). This
indicates that the segmented flow effectively minimizes contact between the reaction
phase and the reactor wall. It would be interesting to develop a fouling-free segmented
flow synthesis of the Pd NCs and PdPt NPs that would enable an effective scale-up of
the synthesis and that likely would produce more uniform NPs.
The segmented flow created large volumes of organic solvent waste (carrier phase).
In Paper IV, a recycling strategy was implemented to separate Au NPs from the organic
phase by using LLS, where the two phases efficiently can be separated based on po-
larity and the carrier phase recovered and reused in the synthesis. Au NPs after LLS
show a zeta potential of -60 mV and maintain colloidal stabilization after separation
of the reaction phase. Slight agglomeration could be observed when the Au NPs were
placed onto TEM-grids, which could be an effect from drying the NP suspension or may
originate from after the LLS.
An important part in continuous flow production scale-up is the monitoring of
growth and NP product consistency using inline quality control [21, 46]. During the
scale-up of the hydrothermal segmented flow synthesis of Au NPs, the product con-
sistency was monitored inline using an UV-Vis spectroscopy flow cell during 54 min
of synthesis (Figure 4.12). As can be seen in the contour plot (left Figure 4.12), the
absorbance spectra of the Au NPs stay fairly consistent, as well as the absorbance max-
imum for each spectrum (marked with red diamonds) that presented an average peak
maximum of 520.3 nm with a standard deviation of 2.8 nm (0.5 %). The representative
inline UV-Vis spectrum of Au NPs (right Figure 4.12) correlates well with the ex-situ
UV-Vis spectrum (Figure 4.9d). Some points in the contour plot show slightly lower

35
 b ba
)

)
a

a
) b ba

Figure 4.12: Left panel shows a contour plot which visualizes the inline UV-Vis ab-
sorption spectra of citrate-capped Au NPs recorded between 60-3120 seconds during
continuous production. The absorbance maxima of each spectrum are marked with red
diamonds. Right panel shows a representative inline UV-Vis spectrum of Au NPs.

absorbances and red-shift of the absorbance maxima, which may be due to slight ag-
glomeration of Au NPs. It should be noted that the spectra were recorded during short
integration time (1 ms) and the flow cell may be sensitive to external disturbances such
as vibrations and air bubbles that may contribute to deviations in absorbance. Since
the plasmon peaks are not very defined for Pd NCs and PdPt NPs, quality monitoring by
inline UV-Vis spectroscopy is more challenging for these flow synthesized NPs.
When comparing the two systems developed to synthesize NPs in flow in larger
scale there exist several important differences. The type of pumps used is important
when it comes to scaling up the flow synthesis. In Paper III, a syringe pump was used
to infuse the reagents. This type of pump is consequently limited by the volume of the
syringes and can only produce a certain volume of NP suspension. In Paper IV, auto-
mated peristaltic pumps were used and the production volume was not as limited since
bottled reagents were used for the precursor solutions. The advantage of the fouling-
free segmented flow synthesis of citrate-capped Au NPs is that it was fully automated
and integrated with inline quality control. The full automation provides lower mainte-
nance and safer operation and this system demonstrates the capability of NP synthesis
scale-up for future real applications like the catalysis of NO2 reduction, as explored for
a test reaction in the following section.

36
4.2.5 Catalytic evaluation of Pd and PdPt nanoparticles
In Paper III, a coauthor performed a temperature programmed NO2 reduction reaction
(48-136 °C with a gas feed of 2200 ppm NO2 and 2.2 % H2 in argon) using a flow
“pocket reactor” to study the catalytic activity of Pd NCs and PdPt core-shell NPs. The
corresponding NO2 conversion efficiencies (Figure 4.13a) show that PdPt core-shell NPs
exhibited slightly higher catalytic activity at lower temperatures. The lower activity of
Pd NCs could partly be related to the cubic structure, which is composed of (100) crystal
facets, known to exhibit lower activity compared to high-index facet counterparts[96–
99] that are highly abundant in the dendritic PdPt NPs. Additionally, synergistic effects
between Pd and Pt may have contributed to the improved catalytic activity of PdPt NPs.
Product selectivity is another aspect studied. During NO2 reduction by H2 three prod-
ucts form; NO, N2 O, and N2 , where N2 is the most desired one due to its non-toxicity
compared to the others, which are greenhouse gases and/or toxic. There is a clear
difference in selectivity between the two types of NPs (Figure 4.13b), where the PdPt
core-shell NPs show higher selectivity towards N2 , as well as lower selectivity for NO
formation compared to the Pd NCs who show nearly 98 % NO selectivity. It should be
noted that due to restructuring behavior that was observed for the NPs at elevated tem-
peratures, the operation temperature was limited to 150 °C. An improved temperature
stability would enable the temperature window to be extended, likely resulting in lower
NO selectivity. Moreover, since the synthesis scale-up produced heterogeneous NPs it
would be interesting for future studies to evaluate the catalytic activity and selectiv-
ity of uniform shaped NPs, which would deepen the understanding of the relationship
between catalytic performance and shape.

37
a) 100
PdPt NPs
b)
Pd NPs
80 Blank

NO N 2O N2
NO2 Conversion

60
PdPt NPs 68 20 10

40
Pd NPs 98
1.3 0.7
20
0 20 40 60 80 100
Product selectivity (%)
0
0 20 40 60 80 100 120 140
Temperature (° C)

Figure 4.13: Catalytic evaluation of Pd NCs and PdPt NPs. a) Relative NO2 conversion
efficiencies of Pd NCs, PdPt NPs and blank sample. b) Calculated relative percent selec-
tivity of NO, N2 O and N2 in 48 – 136 °C temperature interval for Pd NCs and PdPt NPs.
Adapted with permission from Ref. [81]. Copyright (2019) American Chemical Society.

38
Chapter 5
Conclusions and Reflections

The aim of the work presented in this thesis was to improve the understanding of the
factors that control the synthesis of shape and size-controlled metal NPs using solution-
based colloidal methods. From the studies presented in the first part of this thesis, it can
be concluded that the stabilizers strongly influence the NP properties which include sta-
bility, shape and reduction kinetics. I found that the selected stabilizers can have more
roles than providing colloidal stabilization. Thiolate stabilizers decompose at the sur-
face of Cu NPs and only provide temporary stabilization of the NP surface, even under
inert atmosphere. In the synthesis of Pd NPs in the absence of traditional reductants,
it can be concluded that the fatty acid stabilizers NaOL and NaST contribute to the re-
duction of Pd-precursors. Quantitative evaluation of the reduction kinetics show that
these stabilizers exhibit pseudo first-order reduction kinetics in the synthesis of Pd NPs,
where the unsaturated fatty acid NaOL provides faster and more effective reduction.
The different stabilizers’ influence on reduction kinetics direct the sizes and shapes of
the Pd NPs.
The second part presented in this thesis focused on the development of flow syn-
thesis routes for the design of shaped and uniform metal NPs, and provided synthesis
scalability and catalytic evaluation. First, I developed a single-phase flow reactor that
produced shape- and size-controlled Pd NCs and PdPt NPs. Precise morphological con-
trol of core-shell PdPt NPs was achieved by varying the elemental composition. Sec-
ondly, a fouling-free segmented flow synthesis was developed to produce uniform Au
NPs. In both syntheses, among the evaluated reaction parameters temperature strongly
influenced NP properties. Synthesis scale-up in the single-phase reactor led to extensive
fouling and heterogeneous NPs. Exposure of these NPs to elevated temperatures showed
that the bimetallic PdPt NPs exhibit better stability compared to the monometallic Pd
NCs. Furthermore, from a proof-of-concept catalytic evaluation in the NO2 reduction
reaction it can be concluded that PdPt NPs also exhibit higher catalytic activity and

39
improved product selectivity. The scale-up of the segmented flow system provided uni-
form Au NPs and the product consistency was confirmed by inline optical spectroscopy.
The fully automated and segmented flow system thus presents a promising system for
scalable synthesis of precisely controlled NPs.

5.1 Contribution to the field of nanoparticle synthesis

I gained several insights from the studies presented in this thesis. I hope these findings
will contribute to the further development within the research field of NP synthesis.
Firstly, it is well-known that stabilizers can have multiple roles in the synthesis of metal
NPs. I have gained an improved understanding of the roles of some stabilizers applied
in the synthesis of metal NPs, how the stabilizers can direct the NP stability, shape and
reduction kinetics. I am convinced that these studies can contribute to better knowledge
on how these specific stabilizers interact and influence NP properties. Secondly, the
different flow synthesis routes presented in this thesis will possibly serve as examples
and inspiration for further refinements and development of new flow techniques for the
production of controlled metal NPs. The process scalability of the two flow methods was
demonstrated for the production of NPs. Nonetheless, the production scales evaluated
in these studies are still in the milligram to gram scale. Additional work is needed to
further refine and develop the methods to target kilogram scale production capacity
required for real industrial applications. Several approaches can be explored to reach
this goal such as operating several reactors in parallel, incorporating in-situ quality
control and automated feedback loops. These are some of the aspects that I will discuss
in the next section.

5.2 Reflections on future development of

nanoparticle synthesis
The future of metal NP synthesis holds exciting opportunities, where automation and
intelligent synthesis are interesting areas for exploration. In the coming years, NP syn-
thesis development will likely expand from traditional trial-and-error approaches and
include intelligent synthesis optimization, using artificial intelligence (AI) and machine
learning [21, 100–102]. Fully automated segmented flow systems are promising plat-
forms for this development. Instead of replacing experimentalists, I believe these new
tools will strengthen and speed up the NP development processes. From the studies
presented in this thesis I found that some stabilizing molecules can act as reductants in
the synthesis, and further research into these capabilities might expand the flow synthe-
sis but only if we understand how these reductants are functioning. This may require

40
a combination of automated flow synthesis and machine learning to change reaction
conditions quickly, and experimentalists to choose the most likely factors to test.
Furthermore, NP synthesis should take inspiration from flow synthesis of organic
compounds, which in recent years have seen developments in automated synthesis us-
ing AI in planning and screening of reaction conditions to predict optimal reaction con-
ditions [103]. This can also be applied to NP synthesis, where machine learning can
serve as a powerful tool for synthesis optimization by the use of automated feedback
loops. By bringing knowledge from existing NP synthesis data, reaction parameter space
optimization can be performed to estimate and optimize NP outcomes [49, 104, 105].
Fully automated segmented flow systems with in-situ monitoring coupled to machine
learning and optimization algorithms may enable effective reaction monitoring and NP
synthesis optimization. Altogether, I believe these future technological developments
could facilitate production scale-up for real industrial applications and may shorten the
time for new high-performing controlled NPs to reach industrial applications.

41
42
Acknowledgements
These PhD studies have been a journey for me, both professionally and personally, and
has involved deep valleys and important life lessons learned over these four years. I
want to thank all the people who have been involved in this process to support me to
finally reach this big goal.

My second supervisor Prof. Kasper Moth-Poulsen for giving me the opportunity to start
this PhD project. For all your scientific inputs and guidance, and I am glad to have
experienced how our relationship has developed over the years.

My main supervisor Prof. Hanna Härelind for your strong support and supervision and
guidance. You helped me believe in myself, all the way to the PhD. Thanks for all the
amazing times together during research projects, and outside work at the division par-
ties.

My examiner Prof. Martin Andersson. You sparked my interest in research when you
took me in as a master thesis student in 2014, which eventually led me to start this
journey. Thank you for your scientific support and guidance during this time.

Prof. Victor Sebastian for hosting me during my research stay in University of Zaragoza
in Spain. I am grateful for your supervision in developing flow chemistry methods and
the personal developments I experienced during this research stay.

The Flow team: Jessica and Robson. Thanks for great times together in the lab, with
lots of laughter, interesting scientific discussions and achievements.

My friends and colleagues at the division of applied chemistry. Thank you for amazing
times together with fikas, parties, trips, and good times in the lab. No one mentioned,
no one forgotten.

My dear friends, who mean so much to me. You have been by my side all this time. You
know who you are. Thank you all for your support.

43
Ett speciellt tack till min familj. Ni har stöttat mig och alltid funnits där för mig i de bra
och de svåra stunderna. Jag är väldigt tacksam för det. Jag älskar er.

Älskade Carl-Robert, min klippa. De snart två år som du har varit en del av mitt liv har
varit fantastiska. Du har funnits där och har stöttat mig varje dag i att kämpa på och
fortsätta. Jag ser nu fram emot att fortsätta vår resa tillsammans mot en spännande
framtid. Jag älskar dig.

44
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