Study on the transfer and interaction of charged particles using ac glow discharge on liquids toward noble nanoparticle synthesis

Study on the transfer and interaction of charged particles using AC glow discharge on liquids toward noble nanoparticle synthesisA thesis presented by THAI VAN PHUOC Department of Energy

Study on the transfer and interaction of charged particles using AC glow discharge on liquids toward noble nanoparticle synthesis

A thesis presented

by

THAI VAN PHUOC

Department of Energy and Environment Science

Nagaoka University of Technology

January 2020

I certify that the work presented in this thesis with my best efforts is original Ihereby certify that I have not submitted this study for a degree or diploma at anyuniversity or other institutes I confirm that all sources on which is based have beenacknowledged in the bibliography.

i

In this study, the author experimentally investigated the transfer and interaction

of charged particles from an alternating current glow discharge on the liquids andthe ability of this process to synthesize noble metal nanoparticles The mechanism ofthe interaction and transfer of species from the plasma on liquids has been studied.These works are based on the investigation only on each species or charged particle

at the interface of a plasma-liquid couple To understand the synthesize noble metalnanoparticles, the effect of the oppositely charged particles should be confirmedalternate impact and synthesize it on a liquid

To achieve these goals, we have investigated using an experimental apparatusthat generate AC glow discharge on liquids The change of the chemical compositions

in the liquids was observed to study the effect of AC glow discharge on the liquid.The process was monitored via the measurement of pH and electrical conductivity.The results indicated that AC glow discharge acidifies the solutions of neutral orbase, and alkalizes the solutions of nitrate salt The results indicated that AC glowdischarge alternately generates both positive ions and free electrons The interaction

of positive ions from AC glow discharge with water molecules leads to the generation

of OH radicals and hydronium cation H3O+ Free electrons from AC glow dischargetransfer and absorb into the liquid to form solvated electrons

The ability in the nano-synthesis of AC glow discharge was also investigated.The results showed that gold and silver nanoparticles were successfully synthesized

by AC glow discharge on the precursor solution The process of the nano-synthesis

by AC glow discharge is based on the reduction reactions between solvated electrons

iii

and metallic ions The results also indicated that the pH value in the precursorsolutions affects the morphology of gold nanoparticles At a low pH level, solvatedelectrons are captured by the available of H3O+ instead of reacting with metallicions, and hence results in a small the size of gold nanoparticles At a high pH level,there is the change in the form of gold ions and therefore leads to a decrease in theredox potential of the reaction eaq and gold ions

The synthesis of platinum and copper nanoparticles by AC glow dischargewas also monitored The results indicated that it is possible to generate platinumnanoparticles in small yield Meanwhile, there were not copper nanoparticles synthe-sized during the discharge Solvated electrons play the main role in reducing species

to reduce these ions to neutral atoms In contrast, OH radicals oxidize copper ionsand platinum ions back to a higher oxidized value The role of redox potential E0 inthe synthesis of noble nanoparticles was also clarified At low redox potential, thereaction of eaq and metal ions occurs at a low equilibrium constant The concentra-tion of neutral atoms generated hence is not enough to reach a supersaturation forthe nucleation process In contrast, the neutral atoms generated in the reaction athigh equilibrium constant are sufficient for the existence of the process of nucleationand subsequent growth

Our findings broaden new understandings of the plasma-liquid interactions Theresults indicate that it is possible to use AC glow discharge to synthesize silver,gold nanoparticles This also suggests a new way to control the morphology ofnoble nanoparticles via the control of the frequency of the AC power supply It

is an advantage in the nanofabrication because no substance needs to add to theprecursor

The study has been done with the contributions of numerous people who havegiven me enormous support in academic activity as well as life assistance I wouldlike to express my grateful acknowledge to everyone who accompanied me to finalthis study.

First of all, I would like to express my sincere appreciation to Associate ProfessorToru Sasaki, who is my supervisor in the Department of Electrical, Electronics andInformation Engineering at Nagaoka University of Technology I wish to expressthanks to him for his valuable support, accurate advice, and helpful commentsduring the doctoral course He is always willing and patient to give me convincingexplanations from his vast knowledge Without his persistent support, all resultsand goals of this study would not have been realized

I would like to pay my special thanks to President Nob Harada, who is nowPresident National Institute of Technology, Kitakyushu College He connected me

to study at Nagaoka University of Technology and gave me the support to get thescholarship for my doctoral course

I would like to thank Associate Professor Takashi Kikuchi, who is in theDepartment of Electrical, Electronics and Information Engineering at NagaokaUniversity of Technology His detailed comments during my doctoral course not onlyhelped me achieve my research results but also helped me find new ways to complete

I would like to thank Assistant Professor Kazumasa Takahashi, who is in the

v

Department of Electrical, Electronics and Information Engineering at NagaokaUniversity of Technology, for his valuable advice to overcome any issues on myresearch activity He is always willing to share his knowledge to help me betterunderstand and be integrated into Japan life

I wish to show my gratitude to Associate Professor Nobuo Saito, who is now

in the Department of Materials Science and Technology, Nagaoka University ofTechnology, for his thoughtful instructions on materials analysis techniques Hiscomments are extremely precious to improve my research

I am indebted to Dr Kenichiro Kosugi, Mr Hideto Furuno, Mr KatsukiWatanabe, who are in the Department of Materials Science and Technology atNagaoka University of Technology for their assistance and valuable discussions onTEM, DLS, and XRD operations

I would also like to thank Professor Hisayuki Suematsu and Mr NguyenDuy Hieu, who are in the Department of Nuclear System Safety Engineering atNagaoka University of Technology, for his valuable discussions on transmissionelectron microscopy analysis and assistance with TEM operations

I would like to thank Professor Weihua Jiang, who is in the Department ofNuclear System Safety Engineering at the Nagaoka University of Technology, for hisvaluable comments and reviews of this thesis

I wish to show my gratitude to Dr Tran Ngoc Dam, who is in the Faculty

of Mechanical engineering at the HCMC University of Technology and Education

He was the first person who brings me to plasma science and also gave me manysupports to study in Japan

I wish to thank all members of the MHD laboratory, who not only gave memuch support in my study but also shared with me many interesting experiences in

a lifetime Their support helped me be integrated into Japan life

I am indebted to Mr Thanet Kladphet for his precious discussion and experience

He was always close to me and helped me overcome the surprises in my first dayswhen I arrived in Japan.

I would like to recognize the invaluable assistance from staff in the Division

of International affairs at Nagaoka University They are always willing to help meovercome obstacles in life in Japan

I would like to thank the Department of International Cooperation, the Ministry

of Education and Training, Vietnam for precious financial support during my doctoralcourse

I would like to say an enormous thanks to the support and great love of myfamily They are always near me, keep me going, and give me their warm support.This study would have been impossible to obtain without their support

• Journal Publications

1 Thai, P V., Abe, S., Kosugi, K., Saito, N., Takahashi, K., Sasaki, T andKikuchi, T., 2019 Size/shape control of gold nanoparticles synthesized byalternating current glow discharge over liquid: the role of pH MaterialsResearch Express, 6(9), p.095074

2 Thai, P V., Abe, S., Kosugi, K., Saito, N., Takahashi, K., Sasaki, T andKikuchi, T., 2019 Interaction and transfer of charged particles from

an alternating current glow discharge in liquids: Application to silvernanoparticle synthesis Journal of Applied Physics, 125(6), p.063303

1 Thai, P V., Abe, S., Kosugi, K., Saito, N., Takahashi, K., Sasaki, T andKikuchi, T., 2019, December Nano-synthesis by AC Glow Discharge onLiquids: The Role of Redox Potential The 11th Asia-Pacific Interna-tional Symposium on the Basics and Applications of Plasma Technology(APSPT-11)

2 Thai, P V., Abe, S., Kosugi, K., Saito, N., Takahashi, K., Sasaki, T andKikuchi, T., 2019, November Gold nanoparticle synthesis by AC glowdischarge on liquid and the effect of pH value in the size of synthesizednanoparticle The 7th International Workshop on Nanotechnology andApplication (IWNA 2019)

ix

3 Thai, P V., Abe, S., Kosugi, K., Saito, N., Takahashi, K., Sasaki, T andKikuchi, T., 2019, March AC glow discharge in liquids: its influenceand ability to silver nanoparticles The 11th International Symposium onAdvanced Plasma Science and 12th International Conference on Plasma-Nano Technology and Science (ISPlasma2019/IC-PLANTS2019)

4 Thai, P V., Abe, S., Kosugi, K., Saito, N., Takahashi, K., Sasaki, T.and Kikuchi, T., 2018, November Synthesis Silver Nanoparticles viaReduction Reaction by AC Glow Discharge at Atmospheric Pressure withLiquid In 2018 4th International Conference on Green Technology andSustainable Development (GTSD) (pp 60-62) IEEE

1 Introduction 1

1.1 Breakdown mechanism in Gaseous Phase 2

1.1.1 Townsend discharge 2

1.1.2 Paschen’s law 4

1.2 Transfer and interaction at the gas-liquid interface 5

1.2.1 Electrons 5

1.2.2 Ions 7

1.2.3 Neutral species 9

1.3 UV interaction 11

1.4 Mass and heat transfer 12

1.5 PLIs for nanomaterial synthesis 14

1.6 Scope and outline of the thesis 15

2 AC glow discharge on liquid 17 2.1 Experimental setup 17

2.2 Spectral measurement 19

2.3 Excitation temperature 20

2.4 Heat transport and evaporation of water liquid 22

2.5 Conclusion 26

3 Transfer and interaction of charged particles on liquid 27 3.1 Introduction 27

3.2 Methodology 28

3.3 Results 29

xi

CONTENTS xii

3.3.1 Acidification of liquid 29

3.3.2 Alkalization by NO−3 ions 30

3.3.3 Generation of OH radical 33

3.4 Discussion 37

3.4.1 Interfacial reactions and the transfer of free electrons 37

3.4.2 Interfacial reactions and the transfer of positive ions 38

3.5 Conclusions 42

4 Noble metal nanoparticles synthesized by AC glow discharge 43 4.1 Introduction 43

4.2 Methodology 44

4.3 Results 45

4.3.1 Silver nanoparticles 45

4.3.2 Gold nanoparticles 46

4.4 Discussion 57

4.4.1 Silver nanoparticles 57

4.4.2 Gold nanoparticles 59

4.5 Conclusion 62

5 Copper and Platinum synthesis: The role of redox potential 65 5.1 Introduction 65

5.2 Methodology 66

5.3 Results 67

5.3.1 Platinum 67

5.3.2 Copper 68

5.4 Nano-synthesis in the mix solution of CuSO4 and HAuCl4 72

5.5 Nano-synthesis in the mix solution of CuSO4 and AgNO3 74

5.6 Discussion 75

5.7 Conclusion 82

1.1 Schematic diagram of transfer and interaction of some important

species at plasma-liquid interface [8] 2

1.2 Schematic of Townsend avalanche 3

1.3 Voltage-current characteristics of an electrical discharge [9] 3

1.4 Paschen curves for helium, argon, nitrogen, and neon [11], [12] 5

1.5 Schematic illustration of some processes and fluxes from gas into a particle bulk: kinetic flux of gas for the surface collisions (Jcoll), adsorption onto the surface/interface (Jads) and absorption into the particle bulk (Jabs), desorption out of the interface (Jdes), and net uptake by the condensed phase (Jnet) The orange arrows illustrate production and loss of chemical species by reactions at the interface (Ps, Ls) and in the particle bulk (Pb, Lb) [X]g and [X]s describe the gas and surface concentrations of species X [37] 10

2.1 Schematic of the experimental apparatus 18

2.2 Waveform of applied voltage and current for AC glow discharge 19

2.3 Optical emission spectra of AC glow discharge on solutions: a) H2O; b) 1 mM NaOH; c) 1 mM HNO3; d) 1 mM NaNO3; and e) 5 mMAgNO3 for 5 minutes of discharge 21

2.4 Optical emission spectra of AC glow discharge on solutions after 5 minutes, covering the spectral region 320–350 nm; solid line - AgNO3 solutions, dotted line - other solutions 22

xiii

LIST OF FIGURES xiv

2.5 Boltzmann plot used for calculating the excitation temperature for

HeI on solutions under AC glow discharge 232.6 The temperature increase during the discharge (a) and volume lossafter 10 minutes of discharge (b) for the solutions of H2O, 5 mMAgNO3, 25 mM AgNO3, and 50 mM AgNO3 242.7 The change in root mean square current IRMS (a) and the resistance(b) during the discharge on the solution of H2O, 5 mM AgNO3, 25

mM AgNO3, and 50 mM AgNO3 242.8 The simplified model for the experimental setup: Rplasma, Rliquid, and

Cglass are the resistance of plasma, the resistance of liquid, and thecapacitor of the glass beaker, respectively 26

3.1 The color change in solutions during the discharge: (a) 0.1 mMNaOH + 0.5 mL Bromothymol Blue, (b) 5 mM NaNO3 + 0.5 mLBromothymol Blue 293.2 Changes in pH values of NaOH, NaNO3, and HNO3 solutions during

AC glow discharge Error bars represent standard error 303.3 Changes in electrical conductivity (EC) of NaOH, H2O, and HNO3solutions during AC glow discharge Error bars represent standarderror 313.4 Changes in pH values of NaNO3, AgNO3 solutions during AC glowdischarge Error bars represent standard error 313.5 Changes in EC values of NaNO3, AgNO3 solutions during AC glowdischarge Error bars represent standard error 323.6 The reaction between TA (a) and NaTA (b) with OH radical to formHTA, Na-HTA [66] 333.7 Schematic for the calibration of 2-hydroxyterephthalic acid 343.8 Schematic for the measurement of the concentration OH radical 343.9 Time evolution of the fluorescence of HTA in the solution at pH 10.4

by Helium gas (exposure time 0.5 s) 35

3.10 Time evolution in (a) UV-vis spectra of the solution at pH 10.5 and(b) the concentration of OH radical in the solutions during the discharge 363.11 Time evolution in the concentration of OH radical in the solution(pH 10.5) during the discharge at 12kV: (a) using Helium gas and (b)using Argon gas 363.12 Schematic of the impact of free electrons and positive ions from ACglow discharge on the liquid due to the change of the voltage polarity 393.13 Schematic of the impact of free electrons and positive ions from ACglow discharge on the solution of AgNO3 and NaNO3 403.14 Changes in pH values of AgNO3 solutions after the discharge Errorbars represent standard error 41

4.1 Time evolution of AgNPs formed by AC glow discharge; a) Solution

of 5mM AgNO3; b) Solution of 25 mM AgNO3; c) Solution of 50 mMAgNO3 [73] 464.2 TEM image of AgNPs synthesized by AC glow discharge in solution5mM AgNO3 474.3 EDS mapping of silver nanoparticles synthesized by AC glow discharge

in solution 5mM AgNO3 The elemental maps in (a)–(c) correspond

to silver, nitrogen, and oxygen, respectively 474.4 EDS spectrum of silver nanoparticles synthesized by AC glow discharge

in solution 5mM AgNO3 484.5 The color change in solutions with pH in the range of 2 to 13 during

10 minutes of AC glow discharge The spots in samples are air bubbles 494.6 Changes in pH value of solutions during AC glow discharge [74] 504.7 TEM images of AuNPs synthesized in the solutions of pH = 13 – 8(top-down) after 1 (left) and 10 minutes (right) of AC glow discharge.All scale bars are 100 nm 514.8 Box plot of size distribution of AuNPs synthesized in the solutions of

pH 9 – 13 after 10 minutes of discharge [74] 52

LIST OF FIGURES xvi

4.9 Size distribution from DLS measurement of AuNPs synthesized in thesolutions of pH 9 – 13 after 10 minutes of discharge [74] 534.10 The color change in the solution pH12 at 10 min of AC glow discharge(a) and (b) at 20 min after stopping the discharge (c) and (d) arethe TEM images of AuNPs in the solutions (a) and (b), respectively 544.11 The change in the morphology of AuNPs in the solution at pH 9during AC glow discharge The number in the left-top shows the time

of the discharge All scale bars are 50 nm 544.12 TEM image of AuNPs synthesized in pH 10 solution after 1 min ofdischarge The average size of 123.8 ± 35.6 nm The other absorptionpeaks were emitted from the TEM grid and specimen holder [74] 554.13 The change in root mean square current IRM S due to pH value duringthe discharge 554.14 The evolution of UV-VIS absorption spectra of AuNPs in solutionsduring 10 minutes of AC glow discharge [74] 564.15 Schematic illustrates the process in the synthesis of AgNPs under ACglow discharge 584.16 Schematic illustrates the process in the synthesis of AuNPs under ACglow discharge 615.1 TEM image of PtNPs synthesized in the solution of 5.1 × 10−2 mM

K2PtCl6 at pH 9 after 10 minutes of AC glow discharge 675.2 EDS mapping of platinum nanoparticles synthesized by AC glowdischarge in the solution of 5.1 × 10−2mM K2PtCl6 at pH 9 after

10 minutes of AC glow discharge The elemental maps in (a)-(c)correspond to platinum, sodium, and chlorine, respectively 685.3 EDS profile of platinum nanoparticles synthesized by AC glow dis-charge in the solution of 5.1 × 10−2 mM K2PtCl6 at pH 9 after 10minutes of AC glow discharge 695.4 The evolution of UV-VIS absorption spectra of PtNPs in the solution

of 5.1 × 10−2 mM K2PtCl6 at pH 9 during the discharge 69

5.5 The change in UV-vis absorption spectra of CuSO4 solutions: (a) before the discharge and (b) - after 10 minutes of discharge 705.6 The change in UV-vis absorption spectra of mix solution CuSO4 +PEI during AC glow discharge: (a) - before the discharge and (b) -after 10 minutes of discharge 715.7 XRD pattern for the solution of 10mM CuSO4 before and after 10min of AC glow discharge 715.8 TEM image of nanoparticles synthesized in the mix solution of 10

-mM CuSO4 and 5.1 × 10−2 mM HAuCl4 after 10 minutes of ACglow discharge The elemental maps in (a)-(b) are gold and copper,respectively 725.9 EDS profile of nanoparticles synthesized in the mix solution of 10

mM CuSO4 and 5.1 × 10−2mM HAuCl4 after 10 minutes of AC glowdischarge 735.10 XRD pattern for the mix solution of 10 mM CuSO4 and 5.1 ×

10−2mM HAuCl4 after 10 minutes of AC glow discharge 735.11 TEM image of nanoparticles synthesized in the mix solution of 10 mMCuSO4 and 10 mM AgNO3 after 10 minutes of AC glow discharge.The elemental maps in (a)-(b) are silver and copper, respectively 745.12 EDS profile of nanoparticles synthesized in the mix solution of 10 mMCuSO4 and 10 mM AgNO3 after 10 minutes of AC glow discharge 755.13 Schematic representation of LaMer’s nucleation and growth of nanopar-ticles 765.14 Schematic representation of the total free energy related to free surfaceenergy and bulk crystal energy 775.15 Schematic representation of Equilibrium constant of a reaction 795.16 Schematic representation of pathway reaction between noble ions with

eaq and OH radical Red and blue numbers are indexed for redoxpotential (E0) and equilibrium constant (Keq) calculated followingEquations 5.4-5.6, respectively 80

LIST OF FIGURES xviii

5.17 Box plot of the size distribution of AuNPs synthesized in the solutionsdue to redox potentials of precursor gold ions 81

1.1 Some reactions of solvated electrons and other species [25] 7

1.2 Summary of the properties of solvated electron in an aqueous solution [17] 7

1.3 Kinetic values for uptake on water surface and Henry’s constant kH for some neutral species [38] [39] 11

1.4 Some important reaction of VUV in liquid [48], [49] 12

1.5 Henry’s law constant and mass diffusivity for some species 14

2.1 Experimental parameters used in the study 19

2.2 The minimum energies for formation of excited fragments by the dissociative excitation of water [59] 20

2.3 Excitation energy and transition strength at the wavelength peaks of HeI [64] 22

3.1 Physical properties of OH in water [69] 33

3.2 Some reactions of OH radical and other species 36

5.1 Some reactions between solvated electrons with some noble metal ions [25] 78

xix

The interaction of plasma and liquid has been increasingly studied in the field

of plasma science and other technologies Plasma-liquid interactions (PLIs) are animportant tool in a variety of applications of nanotechnology, medicine, and watertreatment [1]–[3] The success of PLIs in expanding practical applications is due

to their unique properties The combination between the fourth state matter andliquid results in generate in the bulk liquid a variety of reactive species, hydratedelectrons, and UV radiation The processes in plasma contacting liquid water aresignificantly complex and can be summed up as shown in Fig 1.1 These processesinclude transfer and interfacial interactions between neutral, charged particles fromplasma and liquid, mass and heat transfer, and UV radiation [4]

The classification of plasma in contact with liquids could be based on themethod of plasma-generation It includes ”direct liquid phase plasmas”, ”gas-phaseplasmas”, and ”multiphase plasmas” In the term of direct liquid phase plasma, theplasma is generated within a bulk liquid [5] For gas-phase plasmas, the discharge atatmospheric-pressure is operated over a liquid surface [6] In the case of multiphaseplasmas, the plasmas are generated in the environment of dispersed liquid phase-aerosol, bubbles, and foams [7] The difference in the media density leads to thedistinction in the plasma properties such as electron density, ionization degree, powerdensity, and more

1

1.1 Breakdown mechanism in Gaseous Phase 2

H, O, OH HNO 3 , HNO 2

break-of the study will also be pointed out And a short description will present thestructure of this thesis included the topics and main points for each of its parts

Townsend discharge is the earliest theory to explain the principle of the gasdischarge The process starts when free electrons move in an electric field, collide withsurrounding gas molecules, consequently, generate more additionally free electron.The process continually repeats and creates the avalanche of free electrons in themultiplication scale These free electrons, finally, can go through the gas

Figure 1.2 shows the schematic of electron avalanche in a Tonwsend’s experiment.The current traveling between two planar parallel plates as shown in Fig 1.2 canexpress as:

Cathode Anode

Ionising electron path Liberated electron path

Figure 1.2: Schematic of Townsend avalanche

Current (A)

Figure 1.3: Voltage-current characteristics of an electrical discharge [9]

with I presenting the average current in the gap, Io the initial current at the cathode,

αn is the first Townsend ionisation coefficient, and d is the distance between theplates of the device

During the Townsend discharge, the change of applied voltage leads to thechange in the current Figure 1.3 shows voltage-current characteristics of electricaldischarge In the region A-D, the current strongly increases to a few µA when appliedvoltage increases close to a breakdown voltage The process of ionization starts at alow intensity as called the dark charge In the region of D-F called corona discharge,

1.1 Breakdown mechanism in Gaseous Phase 4

the voltage is suddenly reduced while the current continually rise The plasma emits

a faint glow in the region F-I called glow discharge when voltage increases back And

in the region of arc discharge I-K, a large amount of radiation is produced when thecurrent intensively rises

There have limitations to the Townsend theory Townsend theory can explainthe discharge at low gas pressure that the ionization results in an increase of current.However, the effect of gas pressure or the geometry of the gap on the breakdownvoltage could be not explained by the Townsend mechanism

The breakdown voltage is a fundamental parameter in the field of plasma science.Paschen and Friedrich experimentally investigated Paschen’s law to calculate thebreakdown voltage as the function of pressure and gap length [10] Paschen’s lawovercomes the gaps of Townsend’s theory limited at the low-pressure condition Thevoltage breakdown at the pressure condition in a large range could determine usingPaschen’s law The law can be described in the following equation [11]:

Figure 1.4: Paschen curves for helium, argon, nitrogen, and neon [11], [12]

in-terface

The properties and products of the interaction of free electrons at the liquid interface depend on the energy of these electrons At high energy levels,electrons could vibrationally excite, dissociatively attach, dissociate into neutral, andionize the water molecules The study in the interaction of high energetic electronsand liquid is based on the cross-section for electron collision with water molecules[13] Electrons at low energetic energy adsorb and absorb into the bulk liquid tobecome hydrated electrons or solvated electrons eaq Atmospheric-pressure plasmacommonly generates free electrons with electron temperature of a few eV [14] Thisstudy will, therefore, mainly focus on low energetic electrons

plasma-The transfer of free electrons into the gas-liquid interface is not clearly stood Meesungnoen et al investigated a Monte Carlo simulation to calculatethe penetration range of electrons in liquid water [15] The results indicated thepenetration depth of electrons with initial energy from 0.2 eV to 10 eV is the range of

under-1.2 Transfer and interaction at the gas-liquid interface 6

0.4 − 10 nm Electrons with energy from 10 eV up to 200 eV could penetrate up to10.2 nm In the opposite direction, solvated electrons can move back from the bulkliquid to the gas phase Hart et al reported there is the leave of solvated electronsout of the liquid surface with the energy of 1.56 eV [16] Free electrons with thehydration energy (∆G) of −156 kJ.mol−1 feature hydrophobic property [17] Thus,the structure of the solvated electron in the liquid is a topic of ongoing debate Thesolvation of a solvated electron has been proposed as the polarization of surroundingwater molecules due to the electron-dipole interactions Also, the electric field ofextra electrons inducing a potential well which then traps and stabilizes the electron[18] Other experimental studies [19] [20] proposed a model that one O–H from each

of several H2O molecules is directly hydrogen-bonded to eaq as glassy matrices.Other properties of solvated electrons have been investigated Some studiesconfirmed solvated electron featured optical absorption at 720 nm [21] [22] Thisspectroscopic feature is important for experimental studies to determine the presence

of eaq in the liquid Solvated electrons are species owing to high mobility Schmidt

et al used a model of transient conductivity measurements to calculate the diffusion

of solvated electrons It is estimated to approximate 4.7 × 10−5cm2 s−1 [23] Table1.2 summarizes some important properties of solvated electrons

The solvated electron eaq is a strongly reduced agent with the redox potential

of -2.77 V [24] Solvated electrons could react almost radicals or species in theirpenetration The rate constant of these reactions is in the range from 106M−1s−1 to

1010M−1s−1 [25] Consequently, the life-time of solvated electrons in the bulk liquid

is strongly contingent on the kind of scavengers presenting in the liquid Solvatedelectrons can even react with water molecules in the rate constant of 6.0 × 109M−1s−1.This results in the generation of ions OH− in the bulk liquid Many studies reportedthe irradiation of electrons from atmospheric pressure plasma over liquid leads toalkaline the bulk liquid [26][27] The solvated electrons are also important for theapplication of nanomaterial synthesis It will be late discussed in other sections.Some important reactions of solvated electrons and other species are summarized inTable 1.1

Table 1.1: Some reactions of solvated electrons and other species [25].

R1 eaq + H2O → H + OH− 103 s−1R2 2eaq + 2H2O → H2 + 2OH− 6.0 × 109M−1s−1R3 eaq + H+→ H 2.4 × 1010M−1s−1R4 eaq + H → OH− + H2 2.4 × 1010M−1s−1R5 eaq + 2OH → 2OH− 2.8 × 1010M−1s−1R6 eaq + H2O2 → OH + OH− 1.2 × 1010M−1s−1R7 eaq + HO2− → OH + 2OH− 3.5 × 109 M−1s−1R8 eaq + O2 → O2− 1.8 × 1010M−1s−1R9 eaq + O3 → O3− 3.6 × 1010M−1s−1R10 eaq + O− → OH− 2.2 × 1010M−1s−1

Table 1.2: Summary of the properties of solvated electron in an aqueous solution[17]

eaqAbsorption maximum (nm) 720Extinction coefficient,  (L mol−1 cm−1) 19000Diffusion coefficient (cm2 s−1× 105) 4.75Equivalent conductivity (S cm−2) 190Mobility (cm2 V−1 s−1× 103) 1.98Redox potential (V) - 2.77

The interaction of ions with water molecules at the plasma-liquid interface couldresult in the ejection, transfer of water molecules into the gas phase, excitation ofwater molecules, secondary electron emission, and more These are closely dependent

on the initial energy of ions The interaction of ions and the water surface at theatomic scale have been investigated using computer simulations [28] [29] The results

of these simulations provide a lot of information in the term of the sputtering ofwater molecules, the penetration depth of ions, increase in temperature of the liquid

It was found that ions O2+ with the energy of 50-500 eV can eject from 2 to 450water molecules out of the water surface [28] The results in Minagawa’s studyshowed ions O+ in the range of 10-100 eV can penetrate the surface water from 1.5

nm to 3 nm [29].The impact of these ions in the range from 10 eV to 100 eV leads

to the sputtering of water molecules with the average number of 0.5 to 7 The ion

1.2 Transfer and interaction at the gas-liquid interface 8

bombardment on the water surface also leads to an increase in the liquid temperature.The temperature increase in the region of 3 nm depth by the impact of a single ionwas estimated at 100 K

The impact of ions at high energy also leads to emit secondary electrons andchange the composition in the liquid Cserfalvi and Mezei investigated a model ofelectrolyte-cathode discharge to observe the impact of positive ions at 10-100 eV [30].The results showed the occurrence of secondary electron emission and the generation

of H+ ions in the bulk liquid Cserfalvi and Mezei proposed a chemical processevolving in two steps as described by Eq (1.3) to explain their results

continuously reacts with a surrounding water molecule H2O to generate a radical

OH and one hydronium cation H3O+

The transport of low energetic ions through the gas-liquid interface is lessclear However, the interaction of ions (0.1 - 10 eV) and water molecules have beeninvestigated in many studies [31]–[36] These works are based on the cross-section of

a guided-ion beam and water molecules H2O The collisions have been studied asthe charge-transfer from the low energetic ion to the water molecule as follows:

X+ + H2O → X + H2O+, (1.4)where X denotes Ar [31], N2 [31], N [32], Kr [34], and He [35], [36]

The results showed that the cross-section is a linear function of ion energy with a

negative slope The cross section of the charge-transfer is approximately 10−15cm−2

at 1.0 eV and reduces due to an energy increase of the ion The rate coefficient for thereaction of He+ and H2O at atmospheric pressure is up to 5.6 × 10−10cm3 mol−1 s−1[35], [36]

The degree of ionization in the non-equilibrium plasma is low because of ing at the condition of ambient gas A large number of neutral atoms are contained inthe normal pressure plasma Therefore, the transfer of neutral species into the bulkliquid has been much attracted The interaction and transfer of these species withbulk liquid involve many kinetic processes and transport [37] Kolb et al describedthe processes of the net gas uptake by atmospheric particles as shown in Fig 1.5.These include the collisions at the gas-liquid interface, adsorption onto the liquidsurface and then absorption into the bulk of particles, and desorption out to thegas phase Some parameters of ω-the mean free path of X in the gas phase; αa and

operat-αb-the surface and bulk accommodation coefficients; τd-the desorption lifetime, γ-thenet uptake coefficient are available in literature [38] as shown in Table 1.3

The quantity in the solvation of neutral species into the bulk liquid is possible

to estimate Henry’s law indicates that the concentration of gas dissolved into theliquid is proportional to its pressure It can be calculated as the following equation:

where Sg is the concentration of solvated of species into the bulk liquid (mol/L), kH

is the Henry’s constant available in literature [39] (mol/(L × atm)), and Pg is thepartial pressure (atm)

Another approaches to explain the process of transfer of neutral species atatomic scale is via computational simulation These involve the processes of thediffusion of species in the bulk liquid [40], energy of activation [41], and the formation

in solvation structures around OH radicals [41], [42] Base on the difference of free

1.2 Transfer and interaction at the gas-liquid interface 10

of the interface (Jdes), and net uptake by the condensed phase (Jnet) The orangearrows illustrate production and loss of chemical species by reactions at the interface(Ps, Ls) and in the particle bulk (Pb, Lb) [X]g and [X]s describe the gas and surfaceconcentrations of species X [37]

energy, these works could predict the concentration according to the penetration ofneutral species from the gas phase into the gas-liquid interface and the bulk liquid.The concentration ratio of two position is defined as [43]:

c1

c2 = exp(−

∆G

where c1 and c2 are the concentration of species at two positions ”1” and ”2” and

∆G is the free energy difference between these points

Robert Vacha et al used Eq (1.6) to calculate the free energy minimum of bothhydrophobic gases (N2, O2, and O3) and hydrophilic species (OH, HO2, and H2O2)

at the gas-liquid interface The calculation indicated the difference in ∆G betweenliquid and gas for the hydrophobic gases is positive The values of ∆G for H2O2,

OH2, and OH are − 10 kcal mol−1, − 7 kcal mol−1, and − 4.5 kcal mol−1, respectively.The results reasonably explain the hydrophobic and hydrophilic properties for thetwo groups

The lifetime and diffusion coefficient of species are also important to determinetheir concentration at the gas-liquid interface and in the bulk liquid The lifetime

of OH radicals is estimated at around 149 ps in the calculation of Wick et al [44]

Table 1.3: Kinetic values for uptake on water surface and Henry’s constant kH forsome neutral species [38] [39].

Species αs αb kb

( M−1s−1)

T(K)

kH(mol/(L × atm))

This result is reasonable to explain the result of Yusupov’s simulation [45], in which

OH radicals are not found at the gas-liquid interface Yusupov’s results showedthat the diffusion coefficient for H2O2, OH2, and OH are approximate 0.13 ˚A2 ps−1,0.07 ˚A2 ps−1, and 0.84 ˚A2 ps−1, respectively

Ultraviolet (UV) light is an important production of plasma irradiation Theyield of UV light emission is strongly related to the energy of the discharge, thecharacteristic of plasma-liquid interactions, and the type of gases fed to the plasma

UV photons directly pass to the liquid with the change in the refraction index atthe gas-liquid interface The irradiation of UV with the water molecules is clearunderstood via photochemical reactions

In the term of photochemistry, VUV has been attracted much attention due toits quantum yield VUV is the ultraviolet light with the wavelength in the rangefrom 75 nm to 185 nm The penetration depths of VUV at 172 nm and 185 nmare 0.036 cm and 0.1 cm, respectively [46] The absorption coefficient of VUV inliquid at < 185 nm is 101− 105cm−1 [47] This shows 90% of the VUV irradiation

is absorbed by the liquid at the depth of 10−1− 10−5cm below the liquid interface.The irradiation between VUV light and water molecules results in generate

a variety of radicals O, H, OH and eaq The type of produced species is closely

1.4 Mass and heat transfer 12

Table 1.4: Some important reaction of VUV in liquid [48], [49]

H2Oaq + hv → [H2O+, e−]

[H2O+, e−] + H2O → H3O+ + eaq−+ OH

124147

0.10.07

H2Oaq + hv → OH + H

124147185

0.930.650.3

dependent on the wavelength of VUV Table 1.4 summarizes some important reactions

of VUV in the liquid

The photochemical reactions of UV light at larger wavelengths most occur withother compounds instead of water molecules These reactions most dissociate thecompounds into smaller groups The absorption of UV light at 200-280 nm for ozoneleads to generate oxygen atoms or hydrogen peroxide as Eq (1.7) [50] Absorption

of UVC (200-280 nm) for hydrogen peroxide results in the dissociation into hydroxylradicals as Eq (1.8) Nitrate and nitrite ions under the irradiation of UVC alsorelease to NO2 and NO These productions are very high reactive, consequently,usually lead to other secondary reactions

The process of mass and heat transfer between plasma and liquid is also tant to consider The process is based on the mass and heat transfer between the

impor-gas and liquid phases, and modelled as the Navier-Stokes equations [51]:

And the water evaporation at the surface for a couple of gas-liquid phases isexpress in the following [51]:

Qb = Jz,H2O· Hvap= −DH2O,g·∂CH2O(g)

∂z

to gas phase with a coefficient lifetime τd These species then diffuse and convectinto the bulk liquid and react with other available chemical compositions Table 1.5shows the Henry’s law constant and diffusion coefficient for some species

The liquid convection can play the main role to transport these species in theliquid The diffusion coefficient (D) for most species is in the range of 10−5cm2 s−1.This leads to the diffusion speed (D/L) determined by diffusion coefficient dividedthe length (L) is about 10−5 cm s−1 This speed is too low to compare with theliquid convection caused by plasma, gas flow, and heat increase in liquid In theopposite direction, the evaporation at the plasma-liquid could bring out an amount

1.5 PLIs for nanomaterial synthesis 14

Table 1.5: Henry’s law constant and mass diffusivity for some species

Species Henry’s law constant [52]

[mol m−3 Pa−1]

Diffusion coefficient [51], [53]

[m2 s−1]Gas phase Liquid phase

In general, the temperature in the plasma is mostly higher than the temperature

in the liquid The contact of plasma and liquid leads to the heat transfer from thegas-plasma to the liquid surface The heat increase at the liquid surface leads to theevaporation of water molecules in the liquid into the gas phase The temperature

of the bulk liquid is also increased due to the heat transfer and mass convection.The process has been well investigated in the studies of simultaneous heat and masstransfer [51]

The interaction of plasma and liquid provides a variety of reactive radicalsand solvated electrons in the bulk liquid These species are precious materials forthe process of nanomaterial synthesis Applying plasma-liquid interaction in thenanofabrication, therefore, has been increasingly studied

The mechanism of the processes is mainly based on the neutralization of solvatedelectrons (eaq) with metal ions M+n into the metal neutral M before growing tothe nanoscale The radical H also has the ability to neutralize metal ions via thecharge-exchange reaction The chemistry of the processes can be simply expressedreactions as shown in Eq (1.12)

The effect of other parameters of the bulk liquid on the synthesis process has alsobeen studied This is related to temperature, stirring, which changes the residencetime, reaction rate of the reactions listed Eq (1.12) [57] The effect of pH level onthe nanoparticle morphology has been reported, in which the change in pH levelleads to the change in the form of metals ions [58].

The processes of transfer and interaction of species from plasma into the bulkliquid have been well studied The mechanism of these processes has been studiedvia the investigation on each specie or charged particle at the plasma-liquid interface.Note that the impact of charged particles on the liquid results in generating theproducts which can mutually eliminate For instance, the transfer of free electronsinto the liquid form solvated electron eaq, while the interaction of positive ions leads

to produce hydronium cation H3O+ These new generated particles eaq, H3O+quicklyreact together Here, we have not understood clearly the effect of the oppositelycharged particles when alternately impact the bulk liquid The possibility to use theoppositely charged particles together for synthesizing or even control the morphology

1.6 Scope and outline of the thesis 16

of nanomaterial is still unknown

The goals of this work is to experimentally investigate the impact of oppositelycharged particles generated from AC glow discharge in liquids The change ofchemical compositions in liquids was monitored during the discharge process Also,the ability to synthesize noble metallic nanoparticles by AC glow discharge was alsoexamined

The thesis includes six chapters of which start with an introduction of a literaturereview of the current studies in plasma-liquid interaction This chapter provides

an overview of the transfer and interaction of species from plasma into the liquid.Chapter 2 presents the experimental apparatus and typical parameters for thegeneration of AC discharge on the liquid

Chapter 3 shows the results in the change of the chemical composition in thebulk liquid during the discharge The generation and the concentration of OH radical

in the liquid are also illustrated Based on these results, the author discusses aboutthe the impact of oppositely charged particles from AC glow discharge in liquids

In chapter 4, the results of the synthesis for gold, silver nanoparticles by using

AC glow discharge on liquid are illustrated The role of charged particles and otherspecies from plasma for the process synthesis is clarified The impact of the liquid’s

pH level on the morphology of gold nanoparticles is also analyzed

Chapter 5 shows the results of the synthesis for copper and platinum nanoparticle.From these results, we explain the role of redox potential in the nanoparticle synthesisand the limitation of PLIs for nanomaterial The author also presents a new solution

to control the size of gold, silver nanoparticles via alternating glow discharge using aflexible frequency In finally, the author summarizes the results with newly obtainedunderstandings on AC glow discharge on liquids and, finally, concludes this study inchapter 6

AC glow discharge on liquid

In this chapter, we present an experimental apparatus to generate an alternatingcurrent (AC) glow discharge on the liquid The plasma properties of the dischargehave been investigated using a spectrometer The effect of heat transferring fromplasma to liquid has been monitored We also observed the change in the currentduring the discharge

AC glow discharge was generated in a glass beaker (Ø53 × 70 mm) containing asolution The anode electrode (copper tube) with outer and inner diameters of 4

mm and 2 mm, respectively, was fixed at 15 mm above the liquid’s surface Gas ofhelium was fed to the anode electrode at a rate of 5 L min−1 during the irradiation

17

2.1 Experimental setup 18

High voltage

probe

Figure 2.1: Schematic of the experimental apparatus

period An aluminium plate connected to the ground was put below the beaker

The solutions of silver nitrate (AgNO3) 1 M, nitric acid (HNO3) 1 M, sodiumnitrate (NaNO3) 1 M (Hayashi Pure Chemical Ind., Ltd), sodium hydroxide (NaOH)

1 M (Nacalai Tesque Inc.) were used without further purification These solutionswere used to observe the change in the chemical composition during the discharge Fornoble nanoparticle synthesis, the solutions of silver nitrate (AgNO3) 1 M, chloroauricacid (HAuCl4) 1 M in HCl 1 M including 1 mg Au/mL (1000 ppm), potassiumhexachloroplatinate (K2PtCl6) 1 M in HCl 1 M including 1 mg Pt/mL (1000 ppm)(Hayashi Pure Chemical Ind., Ltd), copper (II) sulfate (CuSO4) 1 M (Nacalai TesqueInc.) were used without further purification Deionized water (DI H2O) was usedduring the preparation of the samples The volume of each prepared sample was 100mL

The high-power supply for generating AC glow plasma consisted of a 2 × 4invertor system using an insulated-gate bipolar transistor The voltage and main

−5

0 5 10

−0.50

−0.25 0.

0.25 0.50

Figure 2.2: Waveform of applied voltage and current for AC glow discharge

Table 2.1: Experimental parameters used in the study

Experimental parametersApplied voltage (peak-peak) 18 kV

Flow rate of helium gas 5 L min−1

Liquid sample of(100 mL)

Deionized H2ONaOH

HNO3AgNO3NaNO3

frequency supplied were fixed at 18 kV and 38 kHz, respectively Voltage and currentvalues were measured using a high voltage probe (Iwatsu HV-P30) and currenttransformer with a recording oscilloscope (Keysight, DSOX2024A) The waveform

of applied voltage and current is shown in Figure 2.2 Spectroscopic measurementswere performed using a multichannel visible spectrometer (Hamamatsu, PMA-12)

We also performed optical emission spectroscopy to estimate the species present

in large quantities in AC glow discharge during discharge Figure 2.3 shows theoptical emission spectrum of the AC glow discharge for a variety of solutions The

AC glow discharge on liquid are illustrated The role of charged particles and otherspecies from plasma for the process synthesis is clarified The impact of the. .. from AC glow discharge in liquids The change ofchemical compositions in liquids was monitored during the discharge process Also ,the ability to synthesize noble metallic nanoparticles by AC glow discharge. .. illustrated Based on these results, the author discusses aboutthe the impact of oppositely charged particles from AC glow discharge in liquids

In chapter 4, the results of the synthesis for gold,