Interfacial bonding mechanisms of carboxylic coatings on pretreated zinc surfaces

Interfacial bonding mechanisms of carboxylic

coatings on pretreated zinc surfaces

Ph.D. thesis

This research was carried out under the project number MC6.06254 in the framework of the Research Program of the Materials innovation institute (M2i) in the Netherlands (www.m2i.nl).

Interfacial bonding mechanisms of carboxylic

coatings on pretreated zinc surfaces

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op woensdag 14 november 2012 om 10:00 uur

door

Peyman TAHERI

Master of Science in Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran

Copromotor Dr.ir. J.M.C. Mol Samenstelling promotiecommissie: Rector Magnificus, voorzitter

Prof. dr. J.H.W. de Wit, Technische Universiteit Delft, promotor

Prof. dr. H. Terryn, Vrije Universiteit Brussel, België/Technische Universiteit Delft, promotor

Dr. ir. J.M.C. Mol, Technische Universiteit Delft, copromotor Prof. dr. B.J. Thijsse, Technische Universiteit Delft

Prof. S. Lyon, The University of Manchester, Engeland Prof. H.N. McMurray, University of Wales Swansea, Engeland Dr. F. Hannour, Tata Steel Research, Nederland

Prof.dr.ir. J. Sietsma, Technische Universiteit Delft, reservelid

Keywords: Interfacial bonding properties, Surface pretreatment, Surface composition, Oxide semiconductor properties, Polymer functional groups.

ISBN 978-90-77172-85-8 Copyright © 2012 by P. Taheri taheripey@yahoo.com

All rights reserved. No part of the material protected by this copy right notice may be reproduced or utilized in any form or by any means, electronically or mechanically, including photocopying, recording or by any information storage and retrieval system, without written permission from the author.

Chapter 1. Introduction

1.1. Introduction 1

1.2. Scientific question and motivation 1

1.2.1. Delamination, bonding degradation and adhesion 1

1.2.2. Research aim 1 1.2.3. Research approach 1.2.3.1. Research concepts 1.2.3.2. Research assumptions 2 2 4 1.2.4. Research outline 5 1.3. References 8

Chapter 2. Study of the composition, morphology and semiconductor properties of differently treated zinc oxides

2.1. Introduction 10

2.2. Experimental 11

2.3. Results and discussion 12

2.3.1. Composition, morphology and oxide thickness variation of the untreated and

differently treated zinc samples 12

2.3.1.1. Surface composition 12

2.3.1.2. Surface morphology and roughness 14

2.3.1.3. Oxide thickness variation 16

2.3.2. Semiconductor behavior of the untreated and differently treated zinc samples 18

2.3.2.1. Voltammetry study 18

2.3.2.2. EIS study 19

2.4. Conclusions 28

2.5. References 29

Chapter 3. The effects of zinc surface acid-based properties on formation mechanisms and interfacial bonding properties of zirconium-based conversion layers

3.1. Introduction 32

3.2. Experimental 33

3.3. Results and discussion 34

3.3.1. Surface composition and topography of the untreated pure and differently

pretreated zinc samples 34

3.3.2. Surface composition and film formation mechanism of Zr-based conversion layers on the untreated pure and differently pretreated zinc samples 36

3.3.3. Surface topography, composition and acid-base properties of the formed Zr based conversion layers on the untreated pure and differently pretreated zinc samples 41

3.4. Conclusions 48

3.5. References 49

Chapter 4. A comparison of the interfacial bonding properties of carboxylic acid functional groups on zinc and iron substrates

4.1. Introduction 52

4.2. Experimental 53

4.3. Results and discussion 54

4.4. Conclusions 63

4.5. References 65

Chapter 5. Molecular interaction of electro-adsorbed carboxylic acid and succinic anhydride monomers on zinc surfaces

5.1. Introduction 68

5.2. Experimental 69

5.3. Results and discussion 71

5.3.1. Adsorption of succinic acid molecules 71

5.3.2. Adsorption of myristic acid molecules 76

5.3.3. Adsorption of succinic anhydride molecules 80

5.3.4. Configurations of the adsorbed monomers 84

5.4. Conclusions 91

5.5. References 93

Chapter 6. Electrochemical analysis of the adsorption and desorption behaviors of carboxylic acid and anhydride monomers onto zinc surfaces

6.1. Introduction 96

6.2. Experimental 97

6.3. Results and discussion 99

6.3.1. Surface composition of the zinc samples after the molecular adsorption 99 6.3.2. Determination of the adsorption potential of the monomers 101

6.3.3. Electro-adsorption properties of the monomers 102

6.3.4. Desorption properties of the monomers from differently treated samples 106

6.4. Conclusions 111

Chapter 7. The effects of surface treatment and carboxylic acid and anhydride molecular dipole moments on the Volta potential values of zinc surfaces

7.1. Introduction 116

7.2. Experimental 118

7.3. Results and discussion 121

7.3.1. Surface composition of the untreated and differently pretreated zinc samples 121 7.3.2. Volta potential values of the untreated and differently pretreated zinc samples 123 7.3.3. Volta potential shifts of the untreated and differently pretreated zinc samples after adsorption of the carboxylic acid and anhydride molecules 126

7.4. Conclusions 130

7.5. References 131

Chapter 8. Bonding mechanisms at buried interfaces between carboxylic polymers and treated zinc surfaces

8.1. Introduction 134

8.2. Experimental 135

8.3. Results and discussion 140

8.3.1. SEM analysis of the coated and bare Zn substrate 140

8.3.2. Metal-polymer interfacial bonding structure 140

8.3.3. Quantitative metal-polymer interfacial analyses 8.3.3.1. Metal-polymer interfacial mode

8.3.3.2. Polymer interfacial deprotonation density

146 148 149

8.4. Conclusions 152

8.5. References 153

Chapter 9. A novel in-situ study of buried interfacial bonding mechanisms of carboxylic polymers on Zn surfaces

9.1. Introduction 156

9.2. Experimental 157

9.3. Results and discussion 160

9.3.1. Zn oxide surface composition 160

9.3.2. Interfacial bonding mechanism 9.3.3. Interfacial bonding intensity

161 165

9.4. Conclusions 167

9.5. References 168

Chapter 10. In-situ study of Zn-carboxylic polymer interfaces exposed to an aqueous solution by means of an integrated ATR-FTIR and EIS system

10.1. Introduction 170

10.3. Results and discussion 174 10.4. Conclusions 178 10.5. References 180 Chapter 11. Conclusions 11.1. Introduction 181 11.2. Approach 181 11.3. Conclusions 182 11.3.1. Surface pretreatments 182

11.3.1.1. Zn oxide semiconductor properties

11.3.1.2. Zr-based conversion treatment on Zn surfaces

11.3.2. Interfacial bonding properties of the model compounds

182 182 182 11.3.3. Interfacial bonding properties of the real polymer coatings 184 11.4. Remarks

11.4.1. Benefits of the assumed ideal experimental parameters as compared to the real conditions

11.4.2. Deficiencies of the assumed ideal experimental parameters as compared to the real conditions

11.5. Suggestions

Summary Samenvatting List of publications Acknowledgments About the author

185 185 186 186 189 191 193 195 196

1.1. Introduction

Metal surfaces are often coated to increase the system durability. Despite widespread investigations over the past decennia, the adhesion between metal substrate and polymers, e.g. organic coatings and adhesives, and the interfacial bonding degradation in the presence of aqueous environments are not yet well understood. Therefore it is crucial to study the metal-polymer interactions at the interfacial level to optimize the adhesion of organic layers to metal surfaces.

This work consists of a range of different approaches to evaluate the fundamental aspects of the metal-polymer interactions and draw the beneficial treatments. This chapter, introduction, discusses the project motivation, aim and outline. In chapters 2-10 the results of the investigations into several surface treatments, characterization and metal-polymer interfacial properties are presented. Chapter 11 summarizes and correlates the main results of this work followed by remarks and suggestions for future research.

1.2. Scientific question and motivation

1.2.1. Delamination, bonding degradation and adhesion

Polymer coatings should efficiently reduce water, oxygen and ionic diffusion leading to lifelong protection of the metal substrate. However, due to the presence of pores and cracks in polymer structures water, oxygen and ions can access the metal surface. The corrosive medium accumulates at the defect sites of metal-polymer interface resulted in initiation of anodic and cathodic reactions. Oxygen reduction and metal oxidation are respectively the main cathodic and anodic reactions taking place [1]. Diffusion of oxygen through the polymer coating leads to further oxidation of primary corrosion products. Metal oxidation reaction increases the acidity of the interface region leading to instability of the interfacial bonds. Due to the interfacial bonding degradation, the polymer coating at the defect edge detaches from the oxide surface facilitating the diffusion of ions and water along the oxide-polymer interface. This is a frequently observed delamination process of the polymers from metal surfaces called cathodic delamination.

The rate of bonding degradation and cathodic delamination depend on several aspects. Due to various aspects of delamination and bonding degradation, they are evaluated from several disciplinary fields: polymer science, surface physics, chemistry, etc. One important point, among others, is the understanding of the nature and strength of the interfacial bonds between the polymer and the substrate determining the system adhesion. Adhesion is provided by physicochemical interaction between the oxide surface and polymer [2]. As a result, the most important factors of the interfacial bonding degradation process are the polymer density at the interface and the interfacial bonding mechanism determining the adhesion level and system durability.

1.2.2. Research aim

The aim of this work is to study the effects of different surface pretreatments on the interfacial bonding properties of the metal surface-organic coating and characterize their adhesion and interfacial bonding degradation. This work correlates the interfacial bonding

properties to the metal surface composition, acid-base and semiconductor properties and evaluates the important factors in adhesion and disbonding processes.

1.2.3. Research approach 1.2.3.1. Research concepts

As mentioned, the protective properties of organic coatings applied on metal surfaces are determined substantially by the adhesion and polymer-metal interactions. It is reported that the disbonding rate of the coatings is the slowest on the oxide with the highest adhesion strength [3]. On the other hand, polymer coatings can be functionalized to enhance the adhesion of the polymer to the metal surface [4,5]. Carboxylic groups are adhesives functional polymers, which are often used in industrial polymer coatings because of their desirable structural and interfacial performances [6,7]. In addition to the industrial applications, carboxylic groups have attracted much attention because of their importance to understand the fundamental adhesion mechanisms between polymer-metal [8-10].

Several factors depending on the metal and coating properties affect the interfacial bonding properties. Surface pretreatment of metal substrates is usually required before the coating application to promote paint adhesion and improve corrosion performance. In this case, changes in the oxide chemistry and composition have a direct influence on the subsequent bonding behavior with the organic functional groups. Additionally, surface oxide electronic and semiconductor properties are expected to affect the interfacial bonding properties. Consequently, understanding the bonding properties of the polymer coating to the metal surface components, and also comparison of the bonding characteristics of differently pretreated surfaces are crucial to draw up the beneficial treatments.

As soon as the metal oxide interacts with a polymer phase, electrons cross the oxide-polymer interface to achieve an equilibrium state, leading to concomitant changes in the charge distribution at the interface region. The equilibrium state of the oxide-polymer interface is characterized by the flat-band potential, Vfb associated to the extent of band

bending [11-13]. Consequently, interesting interfacial characteristics can be explored by examining the oxide semiconductor properties. Mott-Schottky analysis explores semiconductor properties by correlating the space-charge capacitance and an applied potential in combination with electrochemical impedance spectroscopy (EIS) [14-19]. Conversion pretreatment is an effective method to increase the adhesion of the overlying organic coatings to metal surfaces [20,21]. The existing chromate and phosphate-containing conversion layers are being increasingly replaced with various alternatives because of several health, environmental, energy and process disadvantages [22-24]. Application of zirconium oxides on metal surfaces is a promising treatment technique that came into view as a potential replacement for the traditional treatments [25,26]. The zirconium based pretreatment showed adhesion promotion on a variety of metallic substrates [27,28].

Although studying the interfacial bonding properties is crucial to determine the metal-polymer interactions, the interface region is hard to reach due to the relatively high polymer thickness masking the interface region. Consequently, the interfacial bondings can be modeled by adsorption of the representative monomers. Succinic acid, myristic acid and succinic anhydride molecules can be used as the model compounds to represent the carboxylic functional groups [29,30]. Subsequently, the interfacial bonding properties of the coatings and model compounds on metal surface is studied versus the variations of metal surface properties.

Reductive desorption using cyclic voltammetry (CV) provides the opportunity to study the electrochemical stability of the adsorbents. In this case, the current transient, associated with the removal of the molecules provides valuable information on the mechanistic aspects of the monolayer stability. Additionally, the kinetics of electron transfer reactions is modified by the presence and structural characteristics of the monolayers. CV experiments also lead to a direct measurement of the surface coverage and the desorption kinetics [31-34]. Consequently, the bonding strength between the adsorbents and the effects of interfacial bonding properties on the permeability of the formed monolayers can be evaluated by CV.

It is known that the variation of the interfacial dipole moments and charge distribution shifts the work function of the metal substrate [35]. Kelvin probe has been used successfully in ambient atmosphere to measure the difference in work function between a conducting probe and conducting or semiconducting metals and organic films [36,37]. When the Kelvin probe and a metal surface having different work functions are electrically connected, electrons are distributed to obtain a charge equilibrium state and establish a contact potential. Consequently, the measurement of the contact potential (ΔΨ) provides information about either the electronic properties of the metal surfaces or the polymeric conformation at the metal-organic interfaces.

A destructive method based on analysis of the interfacial region after the polymer removal from the metal surface received intensive attention. In this case, the polymer-metal interface is reached by various processes, such as water intrusion, electrochemical process, pulling-off, and thermal cycle [38]. If a proper method is utilized to remove the bulk polymer, the thin residue film on metal surface presents the metal-polymer interface. For a polymer-metal system exposed to moisture and/or ion containing electrolytes, the mobility of water, hydrated ions and electrons determine the kinetics of the polymer disbonding. In this case, a decrease in adhesion force in presence of moisture occurs due to the strong adsorption of water molecules on oxides and the thereby induced replacement of adsorbed polymeric chains. To evaluate the kinetics of the interfacial bonding degradation, ATR-FTIR Kretschmann geometry can be used in which the thin metal film is subsequently coated with the polymer film [39]. In this case, exposure of the metal-polymer interface in chloride-free solutions provides the opportunity to probe the changes of interfacial bondings due to formation of metal hydr(oxides). This geometry provides the unique opportunity to probe the interfacial bonding formation in-situ as well as the curing steps in the vicinity of the interface region.

Figure 1.1 schematically shows the research approach. In this case, the metal surfaces are subjected to different pretreatments to obtain various surface compositions and semiconductor properties. Additionally, carboxylic model compounds are applied on the differently pretreated metal surfaces and the formed interfacial bonding properties are evaluated. Moreover, the volta-potential shifts due to the adsorbates as well as desorption of the molecular model compounds from metal surfaces is examined. On the other hand, the interfacial bonding mechanisms due to the residue carboxylic polymer coatings are characterized. Furthermore, the interfacial bonding formation and degradation of the carboxylic polymers are assessed.

Figure 1.1. Schematic scheme of the research approach. 1.2.3.2. Research assumptions

This work aims to evaluate the fundamentals of interfacial bonding properties between galvanized steel substrates and polymer coatings. In this case, a thorough control of the experimental parameters of the metal substrate and polymer composition is essential to scrutinize the system principles.

Galvanized steel surfaces compose of various elements such as Zn, Fe, Al, Mg, etc. according to the steel grade, bath composition and the subsequent heat treatments [40,41]. This prohibits a mechanistic interfacial study and increases the system complexity. Consequently, pure Zn is selected in this study as a model substrate to develop the fundamental knowledge and understand the interfacial mechanisms. This provides a solid basis for further researches based on applied approaches.

Various industrial pretreatments and cleaning procedures are applied on metal substrates to improve the polymer adhesion [42]. The solutions used for the treatments composed of different organic and inorganic components changing the surface properties, the extent of which is hardly controllable. Consequently, in this study a set of different aqueous solutions ranging from acid to alkaline were prepared to provide different defined surface compositions.

Polymer coatings are mainly composed of various components such as solvents, adhesives, pigments etc. [43]. Each of these additives may affect the metal-polymer interfacial bonding properties. Adhesives are the functional groups determining the interfacial bonding characterizations [4,5]. Carboxylic functional groups such as polyesters are often used in industrial polymer coatings [6,7]. Additionally, they are used to understand the fundamental adhesion mechanisms between metal surfaces and polymers because of their well-defined reactivity [8-10].

In this work succinic acid, myristic acid and succinic anhydride molecules are used as the interfacial model compounds. Additionally, two types of carboxylic polymers are selected. One polymer system is a 50wt% poly(methyl vinyl ether-alt-maleic acid monobutyl ester) dissolved in ethanol. The polymer is applied on Zn surfaces and dried by ethanol evaporation. The other system is a propoxylated bisphenol A fumarated unsaturated polyester dissolved in styrene and cured with a liquid methyl ethyl ketone peroxide (MEKP). A free-radical chain growth crosslinking reaction is proposed for curing of this system. In this case, the alkenyl function (C=C) present in the chains of

polymer B and styrene dissociate and become activated by MEKP radicals. Subsequently, a crosslinking between polymer B chains is formed by the activated styrene [44].

1.2.4. Research outline

Figure 1.2 schematically shows the different chapters of this thesis. Three different parts form the thesis:

• Part A discusses the effects of different treatments on Zn oxide properties.

• Part B models the interfacial bonding properties between carboxylic model compounds and the differently treated Zn surfaces.

• Part C evaluates the interfacial bonding properties between the real carboxylic polymers and the differently treated Zn surfaces and correlates the results to the studied models.

In the first part of the thesis, i.e. chapters 2 and 3, the effects of different surface pretreatments on electronic properties and composition of the formed oxides are studied. Chapter 2 investigates the composition, morphology and semiconductor properties of differently treated zinc oxides. In this case, electrochemical impedance spectroscopy (EIS) measurements together with Mott–Schottky analysis are used to determine the oxide resistance and semiconductor properties. The electronic properties of the zinc oxide layers are correlated to the surface compositional and structural aspects.

Chapter 3 studies the surface characteristics and deposition procedure of zirconium-based conversion layers on pure zinc substrates. The topography, composition and depth profiles of a set of pure zinc samples treated in alkaline, neutral and acid solutions after deposition of the conversion layers are evaluated. Additionally, the acid-base properties of the obtained conversion layers are evaluated by adsorption of the probe molecules.

Figure 1.2. Schematic structure of the thesis chapters.

Part B of the thesis, presented in chapters 4-7, models the interfacial bonding properties between metal surfaces and carboxylic polymers by adsorption of the representative molecular compounds. Chapter 4 compares the molecular interfacial bondings between succinic acid molecules on zinc and iron substrates as the major compounds forming galvanized steel surface composition. In this chapter the surface compositions, amount of adsorbed molecules and the interaction mechanism between the succinic acid functionalities and the differently pretreated surfaces are evaluated. The results showed a considerable variation of the interfacial bonding properties on Zn and Fe substrates. Consequently, Zn substrate was selected for further detailed studies.

The interfacial bonding properties of succinic acid, myristic acid and succinic anhydride molecules with a set of differently pretreated zinc samples are investigated in chapter 5. Moreover, the interfacial bonding variation due to application of a sufficiently positive potential during the molecular adsorption on the differently pretreated Zn samples is characterized. Chapter 6 studies the (de)adsorption kinetic of the model compounds and evaluates the extent to which the formed carboxylates resisted against the negative potentials and corrosive media. This gives valuable information about the interfacial bonding strength and stability versus the pretreatments and consequently the interface mechanisms.

Chapter 7 evaluates the volta-potential shifts due to the different zinc pretreatments and the interface dipole moments after adsorption of carboxylic acid and anhydride molecules on zinc surfaces by means of scanning Kelvin probe (SKP). Moreover, the variation of the volta-potential is analyzed in lines of hydroxyl fractions and oxide electronic properties.

Part C composed of chapters 8-10 evaluates the interfacial bonding properties of the real carboxylic polymer coatings and differently pretreated Zn samples. In chapter 8, the buried metal-polymer interface is reached by removal of the polymer and evaluation of the residue layers on Zn surfaces. Additionally, the interfacial bondings of the residue film are compared to those formed by adsorption of the model compounds. Moreover, the interfacial bondings are correlated to the semiconductor properties of Zn oxides obtained through the pretreatments.

Chapter 9 investigates the bonding formation mechanism and kinetics at the interfacial region of real carboxylic polymer coatings and Zn samples in a Kretschmann geometry. The interfacial bonding mechanisms are also compared to those obtained by the adsorption of model compounds. Moreover, the interfacial density is quantified based on the Kretschmann geometry. Chapter 10 evaluates the interfacial bonding changes and disbonding process of the real carboxylic polymer coatings on Zn surfaces by means of an integrated ATR-FTIR and EIS system in a Kretschmann geometry. In this case, the interfacial bonding resistivity against the aqueous solution is correlated to the curing process and Zn surface composition.

1.3. References

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Study of the composition, morphology and semiconductor

properties of differently treated zinc oxides

Abstract

This study investigates the composition, morphology and semiconductor properties of differently treated zinc oxides. The characteristics of the oxide layer present underneath the organic coatings play an important role in the delamination rate of the organic coating. The hydr(oxide) fractions of the untreated pure zinc sample and a set of zinc samples treated in alkaline, neutral and acid solutions were calculated using X-ray

photoelectron spectroscopy (XPS). Additionally, scanning electron microscopy (SEM) and atomic force microscopy (AFM) were utilized to evaluate the morphology and topography of the surface layers. In-situ spectroscopic ellipsometry (SE) was used to determine the oxide thickness variation on differently treated zinc samples by ascending applied step potentials, which is a required condition to build up the Mott-Schottky plots. Prior to the electrochemical impedance spectroscopy (EIS) measurements, open circuit potential (OCP) and linear voltammetric measurements were performed to have an insight into the electrochemical behaviors of the untreated and the differently treated zinc samples. EIS measurements together with Mott–Schottky analysis were used to determine the oxide resistance and semiconductor properties. The results clearly showed that the electronic properties of the zinc oxide layers depend strongly on the surface compositional and structural aspects, which in turn are affected by the treatment applied.

Keywords: Hydroxyl fraction; oxide growth rate; semiconductor properties; flatband potential; oxide resistance.

* _{This chapter partially is submitted as a scientific paper:}

Taheri, P.; Hauffman, T.; Flores, J.R.; Hannour, F.; de Wit, J.H.W.; Mol, J.M.C.; Terryn, H. Electrochim. Acta, 2012.

2.1. Introduction

Galvanized steel is often painted to increase the resistance of the system against corrosive media [1]. However, polymer coatings can be subjected to a failure due to the cathodic delamination process [2,3]. In this case, the delamination front is linked to the migration of cations along the polymer/oxide interface. As results, an important factor affecting the cathodic delamination is the electron transfer through the metal oxide [4,5]. On the other hand, depending on steel grade, bath composition, annealing temperature and other parameters, different Zn-based phases are formed in the outer surface of the galvanized steel [6]. Consequently, the zinc oxide resistance is a critical issue determining the delamination rate of the polymer coating from galvanized steels.

As soon as the metal oxide is exposed to an electrolyte, electrons cross the semiconductor/electrolyte interface to achieve an equilibrium state, leading to a band bending and concomitant changes in the charge distribution at the interface region. The equilibrium state of the semiconductor/electrolyte interface is characterized by the flat-band potential, Vfb associated to the extent of band bending. The semiconductors exhibit

various oxide conductivities around the flatband potential [7-9]. Consequently, interesting electrochemical behavior can be explored by monitoring the oxide conductivity at different electrode potentials. By application of an external potential, the charge is distributed at the semiconductor oxide/solution interface resulting in the creation of a depletion zone that can be characterized by a capacitance measurement. The Mott-Schottky equation explains the relation between the space-charge capacitance and the applied potential in combination with electrochemical impedance spectroscopy (EIS) [10-15].

The delamination rate of the coatings from the metal surfaces is closely related to the oxygen reduction at the interface of the oxide surface/polymer coating as well. There are several aspects determining the oxygen reduction at the interface area, e.g. the composition and constitution of the organic coating, the adhesion strength of the organic coating into the metallic substrate and the morphological and electrical properties of the oxide layer [16-21]. One of the possible methods to reduce the oxygen reduction rate at the interface area is chemical modification of the metal surface to retard the oxygen reduction. Consequently, different electronic characteristics of the oxides can be obtained by employing different surface treatments [22]. The oxygen reduction rate can be estimated by measuring the dopant concentration of the oxide film using the Mott-Schottky approach [13,23].

The aim of this study is to evaluate the formation and growth of the zinc oxides and compare the semiconductor properties of the differently treated zinc samples obtained through various treatments. From the X-ray photoelectron spectroscopy (XPS) results, the oxide/hydroxyl fractions of several treated zinc samples were calculated. Moreover, atomic force microscopy (AFM), scanning electron microscopy (SEM) and spectroscopic ellipsometry (SE) were used to evaluate the surface roughness, morphology and oxide thickness variation of the differently prepared oxide layers. Additionally, the electrochemical and semiconductor properties of the formed oxide layers were characterized by analysis of voltammetry measurements and Mott-Schottky plots.

2.2. Experimental

The substrate used in this work was commercially pure zinc sheet (99.95%) supplied by Goodfellow. The samples were mechanically grinded with SiC paper in subsequent steps and then polished to different grade diamond paste (9, 6, 3, 1 and 0.025 µm). Consequently, the samples were cleaned ultrasonically in ethanol and water for two minutes. Afterwards, they were dried under a stream of compressed nitrogen gas. The zinc substrates were treated in different conditions according to the experimental parameters summarized in Table 2.1. Then, they were rinsed with deionized water and dried with compressed nitrogen gas. All of the measurements discussed in this work were repeated 3-5 times to check the reproducibility of the results and/or obtain the mean values.

Table 2.1. The experimental parameters used for the pretreatments of the zinc substrate.

To characterize the surface composition of the samples, XPS analysis was conducted with a PHI 1600/3057 instrument using an incident X-ray radiation (Mg Kα1,2 = 1253.6 eV). The vacuum pressure was approximately 5×10-9 Torr. Narrow multiplex scans were recorded with 29.35 eV pass energy and 0.1 eV step size. The measurements were done at 45° take-off angles with respect to the sample surface. The spectra obtained on the untreated pure oxide layers were shifted to set the C-C/C-H components of the C1s peak at a binding energy of 284.8 eV to correct the sample charging [24].

Tapping mode AFM measurements were performed with a NanoWizardII atomic force microscope (JPK Instruments, Germany) using a silicon cantilever CSC 37 A (Mikromasch, Estonia) with a nominal force/spring constant of 0.65 N/m. The tips are made of silicon nitride, with Al coating on the detector side and CrAu coating on the second side. The tip heights are 12-15 µm and the opening angle of the tips is 30° and the radius of curvature is less than 10 nm. The RMS roughness of the studied samples was analyzed by JPK image processing software v.3. Version 3.3.25. Field emission scanning electron microscopy (FE–SEM) observations were performed using a Jeol JSM–7000F FE–SEM. The acceleration voltage was between 15 and 20 kV and the working distance was 10 mm.

In order to simulate the oxide thickness variation as a function of potential applied in a borate buffer solution (0.075M Na2B4O7·10H2O+0.3M H3BO3, pH=8.4), the oxide

thickness was measured by in situ spectroscopic ellipsometry (SE) within the examined potential range. The in-situ ellipsometry measurements were conducted during the polarization of the samples at each potential for 10 minutes. SE measurements were performed on a M200X, by J.A. Wool- lam Co. The cell was mounted on the goniometer at an angle of 70°. The incident and reflected beam pass the electrolyte and transparent cell windows. The ellipsometric data were analysed using the CompleteEASE software

Sample code Solution pH Temperature (°C) Potential (V) Duration (Min)

Sample 1 0.05 M HCl 1.9 25 - 30

Sample 2 Deionized water 6.6 65 - 30

Sample 3 1 M Na2CO3 11.5 25 - 30

Sample 4 Deionized water 6.6 25 - 30

version 4.41 developed by the J.A. Woollam Co.

The electrochemical measurements were conducted in a conventional three-electrode cell using an EG&G 273 potentiostat. The reference was a saturated calomel electrode (SCE), connected to the main compartment through a salt bridge. A flat platinum plate was utilized as counter electrode (CE), and the zinc sample was used as the working electrode. The electrochemical measurements were performed in the borate buffer solution mentioned and the exposed area was approximately 0.78 cm2. The voltammetric measurements were conducted at the potential range of −1.0 to 0.8 VSCE at the scan rate

of 5 mV s-1. For the electrochemical impedance spectroscopy (EIS) measurements, a Pt wire coupled with a capacitance of 10 nF was employed parallel with the reference electrode to reduce the phase shift induced by the reference electrode in high frequencies [25]. The EIS were performed in the 10 kHz–10 mHz frequency range with 10 frequency points per logarithmic decade and sinusoidal voltage of 10 mV RMS and the potential step size of 100 mV. The experimental impedance data were fitted to an appropriate equivalent circuit using Z-view software. For the Mott-Schottky analysis, the measurements were performed at different DC potentials ranged from -1.0 to 0.8 VSCE.

Mott-Schottky plots were built from parameters obtained from impedance data modeling.

2.3. Results and discussion

2.3.1. Composition, morphology and oxide thickness variation of the untreated and differently treated zinc samples.

2.3.1.1. Surface composition

Figure 2.1.a and Table 2.2 show O1s peak of the zinc samples resolved into three sub-peaks located around 530 eV, 531.5 eV and 533 eV. The sub-sub-peaks can be due to the contribution of O2-, OH- and COx components, respectively [26]. Generally, the

contribution at 533 eV can be ascribed to either molecular water or carboxylate/carbonate-type species [27]. However, our previous investigations showed the absence of water molecules on the zinc surfaces [26]. In situations in which only a thin layer of moisture involved, a small amount of contamination can produce a concentrated electrolyte. For example, a thin layer of moisture in open air is usually saturated with carbon dioxide. Consequently, upon exposure of zinc samples to the air, the water molecules were adsorbed to the surfaces and reacted with carbon dioxide from atmosphere [28,29]. The reactive adsorption of CO2 and H2O resulted in formation of

COx components on zinc surfaces. Table 2.2 shows that sample 3 exhibits the highest

COx fraction among the samples. This can be due to the exposure of this sample in

carbonate including solution (1M Na2CO3) and interaction of zinc surface with

carbonates.

Table 2.2. Peak deconvolution of O1s peaks corresponding to O2-, OH- and COx

components of the untreated and differently pretreated zinc samples. Sample O2- _OH- _CO x BE (eV) FWHM (eV) BE (eV) FWHM (eV) BE (eV) FWHM (eV) Peak area (c/s)

Untreated pure zinc 530.30 1.76 532.10 1.78 533.10 1.53 9.98

Sample 1 529.65 1.53 531.45 1.68 533.16 1.20 7.80

Sample 2 530.37 1.54 531.94 1.48 533.00 1.67 4.02

Sample 3 530.80 1.50 532.00 1.80 533.00 1.72 15.30

Sample 4 529.87 1.50 531.08 1.78 532.78 1.80 5.62

Sample 5 529.77 1.74 531.24 1.80 533.10 1.28 3.65

The surface hydroxyl fraction can be obtained from the O 1s peak fitting shown in Figure 2.1.a. To calculate the actual hydroxyl fraction, the contribution of the surface contamination was subtracted according to the procedure described elsewhere [30]. In the case, the measured O1s signal not only contains contributions from the oxide matrix, but also from the outermost layer of oxygen-containing organic carbon contamination, like C-O and C-O-C=C-O/C-O-C-C-O- _{species. Most oxygen functional groups in polymers give O1s}

binding energies of 532 eV, or approximately at the same location as the OH- peak in the O1s photopeak. In order to obtain a reliable hydroxyl content, it is necessary to subtract the contributions of C-O and O-C=O/O-C-O- _{species from O1s photopeak.}

Figure 2.1.b shows the hydroxyl fractions on the untreated and differently treated zinc samples as determined after the correction of hydroxyl fraction. It can be seen that the hydroxyl fraction increases gradually from sample 1 to 5. The reason of different oxide/hydroxide fractions on zinc samples lies behind their formation and growth mechanisms. Dirkse [31] showed that the oxide/hydroxide formation on zinc substrate proceeds by formation of ZnO/Zn(OH)2 and subsequently dissolving the formed

compounds to Zn(OH)3-/Zn(OH)42- in the electrolyte. Since the dissolving step proceeds

by charge transfer reaction, the process stops when the system reaches a thermodynamic equilibrium state, the level of which is strongly correlated to the electrolyte composition and temperature.

(a) (b)

Figure 2.1. (a) Typical XPS, O1s peak fittings and (b) hydroxyl (OH-) fraction of the untreated and differently pretreated zinc samples.

The low portion of hydroxyl fraction formed on sample 1 can be explained by the low pH (5.4) of 0.05 M HCl solution used for the treatment. In this case, according to Pourbaix diagram [32], the treatment resulted in dissolution of a considerable fraction of surface hydroxyls and formation of metal oxides. On the other hand, the high portion of hydroxyl fraction formed on sample 3 and 5 can be ascribed to the alkaline solutions used for the treatments. The higher hydroxyl fraction formed on sample 5 than that of sample 3 can be due to the application of an external potential as well as the buffering effect of boron components in the solution used.

It can be seen that the hydroxyl fraction detected on the sample treated in water at 25°C is considerably more than that of the sample treated in water at 65 °C. It is noticeable that the temperature plays a substantial role in thermodynamic/kinetic of oxide/hydroxyl formation on zinc surfaces. Formation of hydroxyl on the zinc surface is under a mixed control of diffusion/activation and an increase in the temperature appears to prolong the hydroxyl formation time [33]. Kotnik [34] and Gilbert [35] reported that zinc hydroxyl is usually produced in cold water (0-30°C) whereas in hot water (30-90°C) the surface product is mainly zinc oxide. On the other hand, when the sample treated in hot water is exposed to the ambient air, water molecules are expected to be adsorbed form the air [36]. However, the results show that the oxides formed remain stable even after contact to the air. This can be due to the fact that the hydrated zinc oxide can not take up the adsorbed water to form zinc hydroxide [37]. Consequently, the oxide formed after the treatment is expected to be different from the initial oxide.

2.3.1.2. Surface morphology and roughness

Figure 2.2 presents SEM pictures at high magnification obtained for the untreated and differently treated zinc samples. It can be seen that the oxide morphologies present on the surfaces clearly vary on different samples. Freshly polished zinc surface immediately oxidizes in the air and is covered with a layer of native oxide [38]. It is shown that the resulted film contained inevitability large and densely packed bundles. It can be seen that the oxide present on the untreated pure zinc sample transforms to spherical type oxides on samples 2-4, while sample 1 shows a wurzite type oxide morphology. Aurian-Blajeni et al. [39] showed that the zinc oxide growth is dictated by the diffusion of electrolyte across the formed oxides. In this case, the oxide thickness starts to increase after certain porosity and conductivity reached that is accompanied by an increase in oxide compactness and a transformation of the oxide grains to spherical shapes. An even distribution of the oxide can be observed on sample 3. It is reported that the surface oxide can be homogenized by an increase in carbonate level in the aqueous solution due to the charge transfer controlling the oxide growth [40]. In contrast, a nonhomogeneous oxide morphology is obtained on samples 1 and 2 possibly due to the occurrence of local electrocrystallization events within the layer and/or local mass transfer limitations due to the intensive increase of the oxide layer thickness.

Samples 2 and 4 treated in distilled water present similar oxide morphologies regardless of their different bulk oxide sizes. β-Zn(OH)2 phase is expected to mainly form on the

zinc samples treated in distilled water. It is clear that an increase in the treatment temperature of zinc substrate in water results in an increase in the oxide compactness and

tenacity of the film formed. The same result is observed by Cox et al. [41] studying the morphological aspects of zinc corrosion products. Moreover, the SEM images show that the compactness of the oxides decreases gradually from sample 1 to 5. This can be related to the increase in the hydroxide level from sample 1 to sample 5 proven by the XPS analysis.

(a) (b) (c)

(d) (e) (f)

Figure 2.2. SEM images of the surface oxides formed on the (a) untreated sample and (b) sample 1, (c) sample 2, (d) sample 3, (e) sample 4 and (f) sample 5.

The observed differences in (hydr)oxide level and morphology of the differently treated Zn samples depend on the treatments applied on Zn surfaces. In this case, Lui et al. [42] propose a multistep reaction process for formation and stabilization of the zinc (hydr)oxide. In the first step, an anodic dissolution produces zincate ions, which accumulate near the surface. When a critical concentration is reached, ZnO begins to precipitate. An anodic dissolution process continues through the porous ZnO film till the rate mass transfer of hydroxyl falls below the formation of zincate and ZnO resulted in stabilization of zinc hydroxyl. This mechanism also explains the superior smooth surface obtained on sample 5 treated with the application of an external potential. Another study showed that formation of the passive layer under the potentiostatic condition is coordinated by the second step explained [43]. Consequently, it is probable that the external potential applied together with the buffering effect of the solution stimulate the mass transfer process leading to a higher level of zinc oxide removal and consequently a smoother surface.

To obtain a reliable evaluation of the zinc oxide electronic properties, the effective exposed surface area in contact with the electrolyte has to be known. For this reason, the

oxide roughness is determined by Atomic Force Microscopy (AFM). The higher surface roughness, the higher surface contact area is proposed. Figure 2.3 shows the surface roughness of the untreated pure and differently pretreated zinc samples. It can be seen that the samples are subjected to a nano-scaled topographical rearrangement after the treatments. Samples 1 and 2 treated in acid solution and hot water respectively present rougher surfaces expected to originate from the presence of hexagonal zinc oxides [44]. The rougher surface obtained for sample 2 compared to that of sample 1, regardless of a lower oxide portion on sample 2, can be correlated to the higher pretreatment temperature applied. On the other hand, the alkaline pretreated samples show smoother surfaces. The decrease in the surface roughness from the samples with a low hydroxyl fraction towards those with a high hydroxyl fraction can be due to the dissolution of zinc oxides as proved by the SEM observations.

Figure 2.3. Surface roughness of the untreated and differently treated zinc samples measured by AFM.

2.3.1.3. Oxide thickness variation

To analyze the spectroscopic ellipsometry results, the spectra are fitted to a physical model. The optical model used in the fitting of ellipsometric data consists of two media, i.e. a Lorentz substrate and a porous zinc oxide film layer on top [45,46]. For the porous thin films, multiple unknowns including refractive index, film thickness, porosity and other fit parameters are to be defined. Some researchers showed that Cauchy model is a suitable method to evaluate the ellipsometric characterization of zinc oxide films [47-49]. Consequently, Cauchy dispersion model has been used in this study to determine the film thickness by fitting the ellipsometric data with a three phase [50].

n(λ)=A+B/ λ2+C/ λ4 (2.1) k(λ)=σ exp{β[12400(1/ λ -1/γ)]} (2.2) where n is the refractive index, k the extinction coefficient, λ the wavelength of light and

A, B, C the extinction coefficient amplitudes, σ the exponent factor, and γ the band edge are the fitting parameters. As mentioned, this equation reflects the actual film properties, which highly depend on the fitting parameters. In this case, the best spectral agreement between the experimental and fitted curves is obtained by fitting parameters of A=1.405, B=0.00500 nm2, C=0.00000 nm4 and minimizing the mean square error (MSE<20). The curves of the change in the polarization state, i.e. psi and delta, data and the fitted model obtained for the zinc oxides are schematically shown in Figure 2.4a. Figure 2.4b shows the thickness variations of the untreated and differently treated zinc samples obtained through the SE data fitting as a function of 0.2 VSCE step potential applied

starting from -1.0 VSCE and ending at 0.8 VSCE. In this study, EIS measurements

conducted at each potential consume approximately 10 minutes to sweep the frequency range. Consequently, during the spectroscopic ellipsometry measurements, the samples were polarized at each potential for 10 minutes to meet the condition required for the EIS measurement. It can be seen that the applied potentials, which are cathodic to the potentials around -0.8 VSCE, accelerates the oxide mass transfer process compared to the

oxide deposition step resulting in a partial dissolution of the oxide layers.

(a) (b)

Figure 2.4. (a) Typical SE Psi and delta data/fitted model of the zinc oxide, (b) oxide growth of the untreated and differently treated zinc samples as a function of the applied

potential in borate buffer solution.

On the other hand, the applied potentials anodic to -0.8VSCE lead to an increase in the

oxide thickness due to ion migration induced by the high electric field across the oxide film. It can be seen that the growth rate is high at the initial anodic potentials followed by a gradual decrease in oxide growth rate in more anodic potentials. In this case, the current density thickens the oxide depending logarithmic on the potential gradient in the film [51]. At a higher thickness, diffusion process controls the growth and the ion transport within the film becomes rate determining.

The results show that the degree of the oxide growth depends on the sample type. It can be seen that the rate of oxide growth in the potentials more positive than 0.2 VSCE is lower

Morrison [52] showed that the preferred adsorption site of oxide is into a OLH- group,

where OL is the lattice oxygen ion. Consequently, zinc oxide deposition can be correlated

to the hydr(oxide) fractions present on the surfaces. However, the high oxide growth rate of sample 1 despite the low hydroxyl fraction can be correlated to the low initial oxide thickness as shown in Figure 2.4. This may stimulate the oxide growth to reach a thermodynamic equilibrium status [53].

2.3.2. Semiconductor behavior of the   untreated   and   differently   treated   zinc   samples.

2.3.2.1. Voltammetry study

OCP and polarization studies are conducted on the untreated and differently treated zinc samples. Figure 2.5.a shows the open circuit potentials of the differently treated zinc samples after 10 minutes of immersion in the borate buffer solution. It can be seen that the OCP values obtained for the differently treated zinc samples are negative versus the reference electrode used. In this case, the potential of the zinc electrode in aqueous solutions is expected to get negative versus the potential of zero charge (PZC). Consequently, the electrical double layer is occupied primarily with cations in the inner Helmholtz plane [22]. Additionally, the OCP values obtained for the studied samples (around -0.8 VSCE) verify the observations of Figure 2.4 that an oxide dissolution occurs

in the potentials cathodic to OCPs.

Excluding samples 2 and 3, a gradual increase in the potential levels can be seen from sample 1 to sample 5 showing the role of surface hydr(oxide) fractions in the OCP level. The increase in OCP level by hydroxyl fraction can be related to a decrease in the surface insulation. The extraordinary high OCP levels of samples 2 and 3 can be explained by the high oxide thickness (initial thickness shown in Figure 2.4b) passivating the metal surface in the examined solution. Another factor that may affect the OCP level is PZC depending strongly on the crystalline orientations and surface morphologies obtained through the different treatments [54,55].

Figure 2.5.b presents the polarization curves of the untreated and differently treated zinc samples conducted after 10 minutes exposure in the borate buffer solution. It can be seen that all samples behave the same in the anodic potentials more than 0.2 VSCE despite the

different oxide composition and morphologies. This means that among the reactions taking place in the potentials more than 0.2 VSCE, no ionic transport occurs through the

oxide film. In this case, the reactions mainly occur at the oxide/electrolyte interface instead of the metal/oxide interface [10,56,57].

On the other hand, the anodic currents vary for differently treated samples in the potentials negative than 0.2 VSCE. The anodic potential negative than 0.2 VSCE

presumably is related to oxygen evolution [58]. It can be seen that the anodic current from sample 1 to sample 5, excluding sample 2 decreases gradually demonstrating the importance of (hydr)oxide fraction in the oxygen evolution. This can be correlated to the formation of surface bound peroxides during the oxidation of water by valence-band holes. Thus, oxide fraction plays an important role in the formation of the peroxo species, presumably by providing surface states for the trapping of valence-band holes [59]. The

role of hydroxyl fraction in deficiency of oxygen evolution on titanium has been reported [60]. The low anodic current of sample 2, despite its low hydroxyl fraction, can be correlated to the rough surface increasing the surface area for oxygen evolution. These verify the OCP results regarding the electrochemical activation roles of the hydroxyl fraction and surface roughness.

(a) (b)

Figure 2.5. (a) Open circuit potential and (b) voltammetric curves of the untreated and differently treated zinc samples obtained after 10 minutes immersion in borate buffer

solution.

The observed differences of the electrochemical characteristics of the differently treated samples may originate from a variation of the electronic properties. It is known that in zinc oxide the oxygen atoms are arranged in a hexagonal closed-packed lattice with zinc ions occupying half the tetrahedral sites [61]. Zn2+ _{and O}2- _{are tetrahedrally coordinated}

and consequently positionally equivalent. Due to their noticeable size difference, the ions fill a part of the zinc oxide volume, leaving relatively large open spaces, the extent of which determines the semiconductor properties of zinc oxides.

2.3.2.2. EIS study

In order to construct the Mott-Schottky curves, impedance measurements were performed at potentials between -1.0 and 0.8 VSCE. Figure 2.6 shows the typical Bode plot of the

pure zinc sample performed after 10 minutes of immersion in the borate buffer solution. The plot shows an asymmetric shape of the phase diagram most likely due to the presence of the space charge layer and the Helmholtz layer at the semiconductor/electrolyte interface.

Figure 2.7 shows the equivalent circuit used to fit the experimental data consisting of two parallel capacitance and resistance elements in series with the electrolyte resistance Re. CH/Rct elements represent the double layer in the electrolyte, which includes the charge

transfer resistance (Rct) and Helmholtz double layer capacitance (CH) [62]. Csc/Rox

elements correspond to the space charge capacitance (Csc) and oxide electrical resistance

(Rox) [63]. Considering the non-ideal behavior of the capacitive elements, they were

assumed as constant phase elements (CPE) in the models.

Figure 2.6. Typical Bode plot of the zinc oxide obtained at OCP.

Figure 2.7. Representative equivalent circuit of the zinc oxide/electrolyte system used for the EIS data fitting.

The resistive elements and the CPE-parameters, Q and n, of the untreated pure zinc sample determined through a fitting procedure are presented in Table 2.3. The results show that nsc and nH are in the range of 0.93-1.00 and 0.91-1.00, indicating that the space

charge and Helmholtz double layers are very homogenous. This in turn points out that structurally uniform oxides are prepared as proven by SEM images as well (Figure 2.2). These data summarized in Table 2.3 are used to calculate space charge psuedocapacitance (Csc) proposed by Mansfeld [64] and Helmholtz double layer psuedocapacitance (CH)

according to the procedure explained elsewhere [14,57].

According to the Mott–Schottky concept, the total capacitance (Ctot) of the

semiconductor/electrolyte system is given using the CH and Csc values obtained from the

impedance data fittings according to the following equation [65,66]:

Table 2.3. EIS Fitting data of the untreated pure zinc sample at different potentials. E (VSCE) Re (Ω.cm2_{) (Ω.cm}Rct2_{) (10}-6_.snQ_.ΩH-1_.cm-2₎ nH _(µF.cmCH-2_{) (MΩ.cm}Rox 2_{) (10}-6_.sQn_.Ωsc-1_.cm-2₎ nsc _(µF.cmCsc -2₎ -1.0 142.80 240.0 78.5 0.89 8.24 146.300 203.4 1.00 22.91 -0.8 148.30 91.4 14.6 0.91 14.94 0.016 25.9 0.93 9.61 -0.6 148.80 173.1 10.4 0.92 10.75 0.007 64.0 1.00 26.74 -0.4 149.20 107.2 12.1 0.92 12.13 0.011 30.5 1.00 8.13 -0.2 149.20 128.5 10.3 0.92 10.37 0.009 32.5 1.00 8.38 0.0 148.80 139.7 90.0 0.99 9.00 0.010 27.8 1.00 6.80 0.2 148.60 142.2 72.9 0.93 7.17 0.008 32.1 1.00 6.06 0.4 148.50 154.6 60.4 0.94 5.95 0.007 32.4 1.00 18.27 0.6 148.20 78.5 3.6 1.00 36.01 0.026 4.8 0.94 5.04 0.8 148.20 95.7 1.8 1.00 18.63 0.203 4.5 0.94 4.78 Relative error (%) 0.2-0.8 8-13 0.6-4 0.02-0.1 - 0.001-4 0.2-3 0.02-0.09 -

Morrison [67] showed that most of the semiconductors exhibit a much larger CH value

than that of Csc in depletion mode giving raise to neglecting of the Helmholtz capacitance

in their electrochemical behavior. However, a comparison of the results shown in Table 2.3 shows that the Helmholtz capacitance cannot be neglected in most of the studied potentials. This can be due to the formation of a heavily doped oxide layer and consequently a thin space charge layer compared to most of the semiconductors [68]. Figure 2.8 shows the typical Mott-Schottky curves of the differently treated zinc oxides when (1/Ctot)2 is plotted against the applied potential. It is important to confine the

Mott-Schottky analysis to the potential range in which the oxide film is relatively stable. It can be seen that at the potentials above OCP, the plots have positive slopes indicating n-type semiconductor behavior of the differently treated zinc samples. The decrease of the measured capacity (increase in 1/Ctot) when the applied potential is shifted to cathodic

values is the mark of a n-type semiconductor as well [63]. These results show that donors dominate the electronic behavior of the zinc oxide films [69]. However, the slopes clearly change for the differently treated samples, which can be correlated to the different surface roughnesses and morphologies, varying the effective surface area in contact with the electrolyte. Additionally, a change in the oxide structure is expected to affect the slope of the Mott-schottky plots. In this case a variation in the amount of vacancies and/or defects contribute to the total doping concentration.

In the Mott-Schottky plot a linear region of the graph obtained for samples 1, 2, 4, and 5 indicates the variation of the thickness of the space charge layer with potential [10]. Additionally, the gradual increase of (Ctot-2) in this region indicates that the capacity

varies in a consistent way with the overall oxide thickness, measured by ellipsometry (Figure 2.4). This can be explained by the fact that the oxide growth leads to a change in the valence of the metal ions so that the thickness of space charge increases [22]. The change in the valence of the metal ions (Zn+/Zn2+) in turn can be related to an increase in the electrical conductivity of the surface due to the expansion of the oxide level having a higher defect level than that of metallic zinc. In this case, an effective incorporation of oxygen atoms into the oxide structure leads to the development of the doping concentration (Nsc). Consequently, it can be inferred that the non-linear Mott-schottky

plot behavior of the untreated sample and sample 3 can be due to the rate reduction of the oxide growth as shown by the in-situ ellipsometry measurements.

(a) (b)

(c) (d)

(e) (f)

Figure 2.8. Typical Mott-Schottky plots of (a) the untreated sample, (b) sample 1, (c) sample 2, (d) sample 3, (e) sample 4, (f) sample 5.

Generally, native point defects like vacancies and anti-sites, as well as the presence of hydrogen interstitials as energetically favorable dopants are presented as the most dominant factor in the n-type doping in zinc oxides [63]. Zn+ metal ions, oxygen vacancies and substitutional and interstitial defects are the possible donors in the oxide structure [22]. There is experimental evidence that supports the hypothesis that formation of the zinc ions is strongly correlated to the exposure of the oxide in the environment. It is known that zinc oxide has a large absorptivity for H2, CO and CO2 from the ambient air.

Zinc oxide can partially be reduced by reaction with the adsorbed agents. Each atom of oxygen removed releases an atom of zinc and two electrons. Subsequently, the zinc atom moves to the void space in the form of Zn, Zn+ and Zn2+ [22].

Zinc oxides can be doped with chlorine present in the ambient air as well. This element can theoretically be inserted in the ZnO lattice in substitution to oxygen introducing an extrinsic donor energy level [70]. This intrinsic defect arises from the approach of two oxygen atoms and the formation of a like-dimer of these atoms [71]. When chloride occupies oxygen sites, they act as donor defect, forcing the elongation of the Cl-Zn bond. Additionally, two oxygen atoms near neighbor atoms of Zn are forced to approach forming the VI-VI like-dimer [70]. Figure 2.9 schematically shows the presence of different dopants in the lattice structure of the zinc oxide.

Figure 2.9. Schematic lattice structure of the doped zinc oxide.

The negative slope in the central region observed for sample 3 can be due to a distribution of surface states along the multipod structures that are different from those in the interior of the semiconductor. They act as channels for electron transfer, as long as they lie at the appropriate energy [22]. In such conditions the maximum potential drop occurs within Helmholtz and space charge capacitance layers. When the surface states become fully empty, the capacitance starts to change again with applied potential, the extend of which dependents on the thickness of the oxide layer and the dielectric constant [72]. This indicates a surface-state-mediated mechanism for the charge transfer process at the oxide/electrolyte interface of sample 3 [73]. The surface-state-mediated mechanism defining the semi-conductor properties of sample 3 implies formation of a low-doped oxide structure and a minor role of dopants in oxide electronic characteristics.

Surface states are created within the band gap region due to the selective adsorption of foreign species. Due to the interaction between the zinc surface and the air, some of the electrons near the surface are trapped by physically adsorbed oxygen to form negative ions on the surface. These ions, formed by transfer of electrons from the interior of the zinc oxide to the surface create an upward band bending [73,74].

The Mott-Schottky plots increase at potentials higher than 0.2 VSCE. Consequently, no