VYTAUTAS MAGNUS UNIVERSITY

Karolis GEDVILAS

RESEARCH OF PHOTOCATALYTIC WATER SPLITTING ON

THE SURFACE OF TITANIUM ACTIVATED BY WATER

VAPOUR PLASMA

Summary of Doctoral Dissertation Physical Sciences, Physics (02P)

Kaunas, 2015

2

Dissertation was prepared at Vytautas Magnus University and Lithuanian Energy Institute in 2010 – 2015. Scientific supervisor:

Prof. dr. habil. Liudvikas Pranevi�ius (Vytautas Magnus University, Physical Sciences, Physics – 02P)

Scientific consultant: Prof. dr. Darius Mil�ius (Lithuanian Energy Institute, Technological Sciences, Material Engineering – 08T) The dissertation defence session is to be held at the Board of Physics Science Branch of Vytautas Magnus University: Chairman:

Prof. dr. habil. Julius Dudonis (Kaunas University of Technology, Physical Sciences, Physics – 02P)

Members: Prof. dr. Giedrius Laukaitis (Kaunas University of Technology, Physical Sciences, Physics – 02P) doc. dr. Saulius Mickevi�ius (Vytautas Magnus University, Physical Sciences, Physics – 02P) dr. Vitas Valin�ius (Lithuanian Energy Institute, Technological Sciences, Energy and Thermal Engineering – 06T) dr. Janis Kleperis (University of Latvia, Technological Sciences, Physical Sciences, Physics – 02P)

The official defence of the dissertation will be held at 11.00 a.m. on 30 June 2015, at the Vytautas Magnus University, Vileikos Str. 8, 220 a., LT-44404, Kaunas, Lithuania. The summary of the doctoral dissertation was mailed on 29 May 2015. The dissertation is available at the National M. Mažvydas Library and the Library of Vytautas Magnus University.

3

VYTAUTO DIDŽIOJO UNIVERSITETAS

Karolis GEDVILAS

FOTOKATALITINIO VANDENS SKAIDYMO ANT VANDENS

GAR� PLAZMA AKTYVUOTO TITANO PAVIRŠIAUS

TYRIMAS

Daktaro disertacijos santrauka Fiziniai mokslai, Fizika (02 P)

Kaunas, 2015

4

Disertacija rengta 2010 – 2015 metais Vytauto Didžiojo universitete ir Lietuvos energetikos institute. Mokslinis vadovas:

Prof. habil. dr. Liudvikas Pranevi�ius (Vytauto Didžiojo universitetas, fiziniai mokslai, fizika – 02P) Mokslinis konsuntaltas: Prof. dr. Darius Mil�ius (Lietuvos energetikos institutas, technologiniai mokslai, medžiag� inžinerija – 08T)

Disertacija ginama Vytauto Didžiojo universiteto Fizikos mokslo krypties taryboje. Pirmininkas:

Prof. habil. dr. Julius Dudonis (Kauno Technologijos universitetas, fiziniai mokslai, fizika – 02P) Nariai:

Prof. dr. Giedrius Laukaitis (Kauno Technologijos universitetas, fiziniai mokslai, fizika – 02P)

doc. dr. Saulius Mickevi�ius (Vytauto Didžiojo universitetas, fiziniai mokslai, fizika – 02P)

dr. Vitas Valin�ius (Lietuvos energetikos institutas, technologiniai mokslai, energetika ir termoinžinerija – 06T)

dr. Janis Kleperis (Latvijos universitetas, technologiniai mokslai, fiziniai mokslai, fizika – 02P)

Disertacija bus ginama viešame Fizikos mokslo krypties tarybos pos�dyje 2015 m. birželio 30d. Gamtos moksl� fakulteto 220 auditorijoje. Adresas: Vileikos g. 8, LT-44404, Kaunas, Lietuva. Disertacijos santrauka išsi�sta 2015 m. geguž�s 29 d. Disertacij� galima perži�r�ti nacionalin�je M. Mažvydo ir Vytauto Didžiojo universiteto bibliotekose.

5

INTRODUCTION

Relevance of the Problem:

Titanium dioxide (TiO2) is considered one of the most suitable photocatalyst for decomposition of many organic pollutants in water and air. For many applications, titanium dioxide is used in form of thin layers that are deposited by atmospheric pressure CVD, evaporation, magnetron sputtering or sol gel methods [1, 3–5]. However, the reaction mechanism on TiO2 photocatalyst is still not very clear; in addition, the reaction intermediates and product selectivity are not well understood. The band gap of TiO2 is 3.2 eV, and it only absorbs the light of the high-energy ultraviolet region. A wide-ranging search in order to improve properties of photocatalyst has been undertaken, but the hurdles to practical use remain high [1, 2].

One of the most important properties of titanium is that it can be easily oxidized and reduced. The oxidation of titanium occurs in different oxidation atmospheres, such as air, oxygen and water vapour even at room temperature. At room temperature, the amorphous very thin (5-10 nm thick) passive film is formed, which is composed of three layers: the first layer adjacent to metallic titanium is TiO2, the intermediary layer is Ti2O3, and the third layer, in contact with the environment, is TiO2. The latter layer is the most important layer responsible for the interaction with the adsorbate molecules and chemical compounds [6]. Reduction of titania leads to the formation of oxygen vacancies on the surface. The controllability of reducibility of titanium oxides is essential for many applications in heterogeneous catalysis. In many cases, the reduced TiO2 surfaces exhibit a much higher catalytic activity for water split reactions. In the presence of oxygen vacancies, water molecules readily adsorb onto the hydrophilic surface by forming chemical bonds with the nonstoichiometric titanium oxide and stay as individual molecules or tend to form small water islands [7]. Heterogeneous reactions are also kinetically affected by surface properties such as crystallinity, porosity, particle size, and surface topography.

There are different ways to reduce the surface of oxidized titanium. The ultraviolet light removes some oxygen atoms from the surface of the titania. In vacuum experiments, O vacancies are easily created either by electron bombardment, ion sputtering, or simply by thermal annealing. Plasma reduced TiO2 surfaces possess chemisorptive and catalytic properties different from stoichiometric surfaces. The reason for the difference between stoichiometrically oxidized and plasma reduced surfaces is probably a different electronic configuration of the ions, and a lower degree of coordinative unsaturation in the fully oxidized oxides. The situation becomes more complicated in the environment of water vapour plasma when oxidation and reduction reactions proceed simultaneously. Coatings immersed in plasma are exposed to the fluxes of incident neutral water molecules, wide range of electromagnetic radiation, electron and ion bombardment [8]. Water vapour plasma can supply many radicals and atoms as well as some ions into the surface, and then layers with non-stoichiometric ratio would form on the stoichiometric TiO2 layer. Large numbers of significant variables, the surface properties of samples of partially reduced oxides may be quite different and dependent on the details of the reduction process even for the same degree of reduction.

6

The potential of using water-vapour plasmas excited by microwaves as an ultraviolet (UV) light source has been investigated by using various pressures and input powers [9]. Although there are excited states that emit light in this wavelength range there is experimental evidence [10] that photocatalysis induced by plasma-photon radiation cannot explain plasma-driven photoactivation of TiO2. Complementary, it has been registered that TiO2 shows plasma-induced catalytic activity under conditions where there is no UV or very little emitted by the plasma. It follows that other processes may contribute to one or more of these catalyst activation mechanisms in plasma.

Plasma surface modification involves changes of microstructure and composition. For example, implantation of hydroxyl groups into a TiO2 surface using water vapour plasma. Plasma irradiation by X-ray and ionic component modifies kinetics of oxidation reactions and steady state stoichiometry [9, 11]. Under ion bombardment, surface is sputtered preferentially removing oxygen atoms and then upper layers with non-stoichiometric ratio form on the stoichiometric TiO2 layer [12, 13]. Ion erosion of stoichiometric TiO2 leads to preferential loss of O from the near-surface region. Among all the defects identified in TiO2, oxygen vacancy is one of the most important and is supposed to be the prevalent defect determining photocatalytic property. The relatively high defect (oxygen vacancy) mobility in TiO2 makes this system dynamic and flexible [14]. Under ion bombardment, the interface between the TiO2 film and layer of adsorbed water is composed of reduced titanium oxides.

The effects of plasma treatment are related to the enhancement of chemical reactions by irradiation with photons, electrons, ions, or activated species produced in the plasma, as this leads to the formation of functional groups on the surface thus increasing the wettability. Moreover, plasma treatment causes near-surface defects that modify the surface roughness, stoichiometry, and structure of titanium dioxide and form traps for atomic hydrogen in the near-surface region. A team of researchers led by Samuel S. Mao [15] found out that by introducing a specific kind of disorder, mid-gap electronic states accompanied by a reduced band gap are created. In this study, there is indirect evidence that the band gap of the near-surface region of TiO2 under direct plasma treatment (in situ) might be significantly reduced. The photocatalytic properties of TiO2 under H2O-plasma are modified and affected by the creation of reactive sites by ion impact under the present conditions.

Many controllable plasma-processing parameters can be optimized to favour the desired processes. In order to access the complicated processes at the TiO2 surface, methodologically, we begin with the metal surface and study the evolution of titanium oxide phase and surface morphology in dependence on the thickness of an adsorbed water layer, controllably changing power of plasma generator, exposure time and water vapor pressure. The objective is to photocatalytically split water into hydrogen and oxygen on the surface of plasma activated titanium dioxide and to electrochemically separate hydrogen.

7

The Principle Aim and Tasks of the Dissertation: The aim of the dissertation is to get a deeper understanding of water molecules

dissociation on the surface of titanium thin films activated by water vapour plasma. In order to achieve the aim of the work, the following tasks are to be fulfilled:

1. To form nanostructured 300-700 nm thick titanium films on the silicon and silicon dioxide substrates using magnetron sputtering technique.

2. To perform experiments exposing thin titanium films in the low temperature water vapour plasma by varying different parameters such as: i) power of plasma power supply ii) time of exposition iii) working gas pressure iv) the intensity of plasma radiation.

3. With the help of experimental results and theoretical models check the assumption that chemical activity of the surface of Ti films oxidized in plasma significantly increases due to formation of non-stoichiometric TiOx compounds, which turns the surface in H2O plasma into hydrophilic, the adsorbed water spreads on the surface and forms the islet structure.

4. To check the assumption that TiO2 immersed in water vapour plasma forms the electrochemical element, in which the oxidation reaction occurs on non-stoichiometric TiO2 surface and the reduction reaction on the interface of TiO2/Si. The nanocrystalline TiO2 is a solid electrolyte conductible to H ions.

5. To perform the analysis of properties of Ti films oxidized in H2O plasma: i) nano-relief, ii) surface composition and structure, and iii) structural and phase transformations clarifying the mechanism of dissociation of photocatalytic water molecules.

6. To explain the photocatalytic occurrence that takes place on the surface of modified titanium films during exposition of water vapour plasma.

Novelty of the Work: In this work, novel approach presents the ability to increase the efficiency of

water splitting. Methodologically, a thin titanium film is formed at the beginning, which is later oxidized in water vapour plasma. Studies have shown that surface of oxidized Ti film acts like photoelectrochemical cell solid electrolyte, splits on film surface adsorbed water molecules, and positive hydrogen ions are reduced on the other side of electrolyte. Possibly high splitting efficiency is explained by removal of plasma initiated oxygen atoms from the surface and activated photonic emission in the presence of cascade collisions. Plasma surface modification influences processes of film microstructure and composition changes. In water photocatalysis, synergetic effects may be attributed predominantly to the TiO2 catalyst, which is activated by plasma radiation.

8

Overview of the Dissertation: The dissertation is structured as follows: introduction, literature analysis,

experimental technique, research results, results modeling, conclusions and a list of references.

Introduction provides an overview of the relevance of the topic of the work, its scientific novelty, and approval of the work, structure and the contents of the presented work. The first literature analysis chapter gives an overview of the different water splitting methods. In addition, significant works of foreign scientist who have been working in the field of photocatalytic water splitting using TiO2 are reviewed. In the experimental technique chapter, equipment used for thin titanium films formation and thin film exposing in water vapour plasma is described. In the research results chapter, the main obtained results are presented. In addition, explanation of acting processes involved in water splitting during film processing in plasma is presented. In the results modeling chapter, the mechanism of film transformation and photocatalytic splitting are proposed. Conclusions and the list of references are the final parts of the dissertation. Propositions Presented for the Defense:

1. Non-stoichiometric titanium oxides form on thin (300-700 nm) titanium films oxidized in water vapour plasma. Formed oxides activate H2O chemical adsorption process. Adsorbed H2O molecules are forming islets structure.

2. During exposition in plasma, photoelectrochemical element is formed: plasma and film surface interaction region corresponds to photo anode, TiO2 layer – solid electrolyte, TiO2 and silicon substrate interface – photocathode.

3. After electrochemical element formation, split hydrogen is transferred from the surface to the TiO2/Si interface. The principle of this action is: water oxidation reaction occurs on the non-stoichiometric TiO surface, leading to the formation of H ions. Then, H ions are transferred through the solid electrolyte (nanocrystal TiO2) towards TiO2/Si interface. At this interface the reduction reaction occurs, where H ions react with electrons, which are generated on the surface of TiO2 and arrive to reaction zone by external contour.

4. In low-pressure (~ 10 Pa), water vapour plasma modified titanium films are chemically active and splits adsorbed water molecules into O and H atoms.

5. It was determined that the main mechanism, which forms oxygen vacancies on the TiO2 surface, is TiO2 erosion with positive ions, which comes from plasma.

6. In water vapour plasma at simultaneous H2O adsorption and ion bombardment, TiO2 surface layer with saturated defects forms. Optical and electronic properties of this layer significantly differ from crystalline TiO2. New energetic levels form in the electron energy structure, which reduces the width of band-gap and enables conditions for electron-hole pair formation.

9

EXPERIMENTAL TECHNIQUE

The water vapour plasma-surface interactions have been studied using 300-700 nm thick Ti films, which were deposited on Si and silicon dioxide substrates with the dimensions of 10x20 mm using magnetron-sputtering method in vacuum chamber [16, 17]. The water vapour plasma was produced in a cylindrical stainless steel vacuum chamber (40 cm in diameter, and 47 cm high) using industrial magnetron device. One cap of experimental chamber was connected to a pumping system, which allowed evacuation of reactor down to a residual pressure lower than 10-5 Pa. The water vapour was introduced through a mass flow controller (MFC) at designated flow rates. Chamber pressure was regulated during the experiments with a MFC. A diffuse plasma region was usually produced in the chamber surrounding the magnetron and extending longitudinally for about 5 cm. Samples exposed in the reactor were treated under different plasmas and gas-phase conditions, depending on the location, the pressure and the applied power.

The depth distribution profiles of Ti, O and H atoms for untreated and plasma treated samples were measured by the Auger electron spectroscopy (AES, PHI 700XI) and the glow-discharge optical emission spectroscopy (GDOES, Spectrum Analytic GMBH). The film thickness was measured using the nanoprofilometer (Ambios XP 200). The microstructure of film was characterized by X-ray diffraction method using Bruker diffractometer (Bruker D8). The measurements were performed with �-� geometry goniometer in the range 20º–70º using CuK� radiation in steps of 0.01o. The identification of peaks had been done using Search – Match function of EVA software. The surface topography of untreated and plasma treated samples were studied using a SEM (JEOL JSM – 5600) equipped with an Energy Dispersive Spectrometer (EDS, Bruker Quad 5040), and optical (Nikon Eclipse Lv150) microscopes. Optical transmission spectra measurements were carried out at room temperature using a scanning double-beam spectrophotometer with a photomultiplier tube detector (Jasco V–650).

RESULTS In this chapter, the results obtained by performing analysis of films formed during

experiments are presented. Given results correspond to the defensive propositions and are presented in explanatory nature, which means that results obtained after exposition in plasma (ex-situ) are related to processes, which occur during exposition in plasma (in-situ). In this chapter following processes are interpreted: i) film surface reduction; ii) formation of hydrophilic surface; iii) formation of electrochemical cell; iv) hydrogen transfer and processes in electrochemical cell; v) structural and phase transformations of titanium films; vi) changes of surface roughness, morphology and elemental composition; vii) influence of water layer for film reactivity; viii) processes involved in water molecules splitting mechanism.

Film surface reduction At the initial moment, titanium film is coated with thin (up to 10 nanometres)

titanium oxide layer, which is naturally formed in the environment. During titanium film affection by water vapour plasma, due to the selective oxygen sputtering, partly reduced TiO2 layers are formed on the sample surface. The resulting amount of reduced

10

titanium oxide is growing, and its external limits presumably overlap. Consequently, in case of removing oxygen atoms, oxygen vacancies are formed simultaneously and their amount is growing. These processes change optical and electronic properties, and their presence is demonstrated in Fig. 1 where optical microscope images of titanium films exposed in water vapour plasma are provided. Fig. 1 shows clearly visible formation of interference colours. Kinetics of reduction of titanium oxide and vacancies formation processes depend on energy and density of ions coming from plasma. Whereas samples were located on the magnetron cathode, sample surfaces were affected by inhomogeneous magnetron plasma, and the density of ion current was different in different areas (approximately from 1 mA/cm2 to 10 mA/cm2), which resulted in formation of different layers of titanium oxide and different colours seen on the surfaces of samples. During reduction of TiO2 surface, the large concentration of surface defects is created, part of oxygen atoms are sputtered, and film becomes light or dark blue [23]. The concentration of hydrogen in the oxide also has influence on surface colour. Insertion of hydrogen into titanium oxide changes films colour to blue or purple, and at higher concentrations to black. Optical properties of modified films also depend on the formed structure, crystallite size (or presence of amorphous phase) and the concentration of impurity atoms [24]. Whereas during plasma processing, reduction of oxide layer depends on incident radiation intensity, the colour is different for different irradiation areas.

Fig. 1. Optical microscope images of titanium films exposed in water vapour plasma.

The surface of TiO2 can be reduced in different ways. UV radiation coming from plasma removes part of oxygen atoms from film surface. During electronic erosion, ion bombardment and sputtering, or due to thermal effects, oxygen vacancies are created. Due to oxide bombardment with energetic particles, oxygen atoms are removed from stationary positions. Thus, oxygen vacancies are generated and various sub-oxides are formed on the surface. However, due to the simultaneous oxidation process, it is not possible to remove all oxygen atoms and the number of formed vacancies is only in the few percent range and changes depending on radiation intensity coming on the surface of sample. At the time of radiation, photo generated electron and hole pairs move towards titanium dioxide surface and in such way cause reduction and oxidation reactions of absorbed materials (reactions 1-4).

������ � � � �� ��� � � � (1) ������ � �� � �� ��� � (2) ������ � � � �� � �

�� (3) ������ � � � �� � �� ��� � (4)

11

When e-/h+ pair comes into contact with absorbed water, it gets oxidized by positive holes, which in the process form hydroxyl radicals (•OH) (1) with strong oxidative decomposing power. Under certain conditions, oxygen reduction takes place instead of hydrogen generation, and superoxide anions (O2

•-) (3) are formed, which attach to the intermediate products in the oxidative reaction, forming peroxide or changing to hydrogen peroxide and later to water. When hydrogen peroxide is reduced by negative electron, hydroxides and hydroxide radicals are formed.

Surface wetting effect In the initial state, the titanium film has hydrophobic properties, and in the water

vapour environment, water molecules are dissociatively adsorbed on the surface to form surface hydroxyls, on which further H2O molecules are physisorbed through hydrogen bonding. It is known that during exposure to UV radiation, wetting angle of water drops on the titanium dioxide surface decreases, and under certain conditions, full wetting (super wettability) can be observed. In water vapour plasma, in case of surface reduction and vacancies formation, surface becomes chemically active. Thus, on the activated film surface, dissociative adsorption of water molecules occurs – adsorbed water molecules dissociate into OH- and H+, which creates surface coated by hydroxides. Due to the increased hydroxyl group content on the surface, Van der Waals forces and hydrogen bonds interaction between the water molecules and hydroxyls is also increasing. For this reason, the film surface becomes hydrophilic (Fig. 2.). Film surface changes from hydrophobic to hydrophilic are related to photo induced electron jumps over the band gap. In the titanium oxide crystal, generated free electrons reduce oxidation level of titanium from Ti+4 to Ti+3. At the same time, generated holes oxidize O-2 ions. These processes create oxygen vacancies and, as it has been already mentioned above, the surface becomes chemically active. On the vacancy sites, hydroxides, which come from water vapour plasma environment or from dissociated water molecules, are adsorbed. Concentration of adsorbed hydroxides depends on surface reduction rate. Due to hydroxide coated surface and increased hydrophilicity, other water molecules can be easily adsorbed on the non-stoichiometric titanium oxide surface, thus, forming water islets, drops or coated film surface with thin water monolayers. The stable molecule layer adsorbed on the surface may consist of 2-5 molecular monolayers. Hydrophilicity properties of titanium films exposed in water vapour plasma also depend on the changes of surface roughness. The changes of the film surface roughness can improve wettability properties [14].

Fig. 2. The formation of the hydrophilic surface in the water vapour plasma.

12

Experimental studies have shown that along with the water vapour, water molecules of different size clusters and microdroplets, whose size can vary from a few nanometers to a few micrometers, can be adsorbed on the surface of oxidized titanium films. For example, water microdroplets of large diameter spread upon the surface and form a thin layer of water (right-side in Fig. 2.). Wetted surface of titanium dioxide film under UV irradiation and ion bombardment from the plasma becomes super hydrophilic. Such surface is very favourable for photocatalytic processes that involve oxidation and reduction reactions. Splitting of water molecules on the hydrophilic surface leads to simultaneous oxidation and hydrogenation.

Fig. 3. presents optical microscope images, in which the resulting image of titanium film surface after exposure in water vapour plasma can be seen. It is seen that the areas which were coated by water microdroplets became white. Despite interference, white colour is characteristic of nanocrystalline titanium dioxide. Blue, purple or black colours, as it was mentioned above, are characteristic of hydrogenated areas, i.e., TiO2 areas, the surface layer of which is saturated with hydrogen atoms. This can be explained by the fact that the surface of Ti layer in water vapour plasma is saturated with oxygen and hydrogen atoms, which means that there are conditions for doped titanium dioxide formation. As S. Mao and other scientists say [15], formation of such materials improves the catalytic properties of water decomposition in daylight.

Fig. 3. Optical microscope images of titanium films processed in water vapour plasma. Formation of photoelectrochemical cell Titanium films, which are immersed in water vapour plasma environment, are

simultaneously oxidized and hydrogenated and can be interpreted as plasma activated photoelectrochemical cell. During irradiation, when absorbed particles energy is greater than width of band gap, electrons and hole pairs (excitons) are created (5). Charge separation takes place by migrating and diffusing to the surface. In such way, they are trapped by titanium lattice (6) and in reactive oxygen areas (7). According to electronic activity, Ti3+ ions are formed (8). However, some of the charge carriers recombine and do not participate in further reactions. Trapped holes react with TiO2, breaking the connections between titanium and oxygen ions and thus forming reactive oxygen vacancies. Water molecules dissociatively adsorb on the vacancy areas and hydroxyl groups are whilst in chemisorption on the surface, where Ti3+ ions are re-oxidized to Ti+4 ions (9). Some of the trapped holes can decompose to water molecules, to hydrogen and hydroxyl radicals (10). So formed double charge areas (TiO-OH) provide high

13

hydrophilicity characteristics for surface. A multilayer film (TiO-OH/TiO2/TixOy/Si) formed during such processes acts as a photoelectrochemical cell, where super hydrophilic Ti-OH layer meets photo anode, and TixOy/Si interface meets photo cathode. TiO2 formed during plasma exposure acts as a solid electrolyte with high proton and low electron conductivity.

TiO2 + h� � e- (TiO2) + h+ (TiO2) (5) e-(TiO2) + Ti4+ � Ti3+ (trapped electrons) (6) 4h+(TiO2) + 2O2- � O2� (trapped holes) (7) e- (TiO2) + 4H+ + Ti4+ + 2O2- � Ti3+ + 2H2O (8) Ti3+ + 2H2O � e- (TiO2) + 4H+ + TiO2 (9) h+(TiO2) + H2O � •OH + H+ (10) H2O + h� � 2H+ + ½ O2 + 2e- (TiO2) (11) 2H+ + 2e- � H2 (12) 2H2O + 2h� � O2 + 2H2, (13)

On the photo-activated TiO-OH layer, the oxidation reaction occurs (11), by

which hydrogen ions and electrons are generated. Part of hydrogen ions combines with electrons and forms neutral atoms at the surface. The other part is cached in surface radiation defect areas. Next, hydrogen ions diffuse into the volume between the crystalline boundaries through the crystalline TiO2 electrolyte. Part of ions is absorbed into metallic titanium layer due to high titanium affinity for hydrogen. During the hydrogen diffusion and capture at trapping centres in the film, the outer layer of TiO2 doped with hydrogen atoms is formed. The remaining part of ions continues to diffuse to the depth until it reaches silicon substrate and stops, because silicon is nonconductive to hydrogen ions. At the interface between silicon and film of oxidized titanium, hydrogen ions recombine by attaching electrons, which come from plasma through silicon substrate and thus form neutral hydrogen molecules (12), which accumulate at this interface. The overall water splitting reaction, when radiation energy is converted to chemical energy, is given in (13).

Fig. 4. presents the scheme of electrochemical cell, which is formed during titanium film exposure in water vapour plasma.

Fig. 4. Scheme of electrochemical cell, which is formed during titanium film exposure in water

vapour plasma.

14

Hydrogen transfer and effects in electrochemical cell During oxidation, hydrogen ions formed by reactions (1, 10) diffuse through

oxide layer into bulk. Due to pure hydrogen adsorption on the oxide surface and slow hydrogen transfer kinetics, its speed of diffusion through oxidized titanium is lower than in metallic titanium. However, energetic incident ions coming from plasma create many defects on the surface; thus, hydrogen can move through defects and trapping centres. In addition, during exposure in plasma, the proceeding simultaneous oxidation and hydrogenation of titanium film causes stress, which forms cracks. Those cracks can act like hydrogen transfer channels. Moreover, hydrogen transfer into bulk may be due to phase transition of titanium, in which the formation of titanium dioxide crystals occurs; intergranular boundaries may also serve as transfer channels. Hydrogen ion can spread along oxide surface while it is captured at the trapping centres or on the spreading way, meets other hydrogen ion and recombines to molecular hydrogen (H2) and diffuses from oxide surface. Hydrogenated titanium oxide forms new electronic states, and thus the width of band gap might be lowered. However, hydrogen ions can also diffuse through intergranular titanium lattice boundaries and dislocations. Defects of material and dislocations can act as hydrogen trapping centres. Hydrogen ions, which do not form hydrides, move toward silicon substrate, which is impermeable for them. As it was mentioned in the previous paragraph, at the interface between silicon and film of oxidized titanium, hydrogen ions recombine by attaching electrons, which come from plasma through silicon substrate, and so form neutral hydrogen molecules, which accumulate at this interface. Accumulation of hydrogen molecules leads to formation of large surface defects and deformities.

Fig. 5. presents a schematic view of deformations, which occur during water plasma exposure for titanium films of different thickness (up to 300 nm and up to 700 nm). It is seen that for thin up to 300 nm thick films, during hydrogen accumulation at the film and substrate interface, bubbles are formed on the film surface. For the thicker films (up to 700 nm), due to lower elasticity or the poorer film adhesion with Si substrate, continuous blisters and curves form; the film becomes wrinkled or cracked, or even raises off the substrate.

a) b) Fig. 5. The hydrogen accumulation and influence during electrochemical process in H2O plasma

of films with different thicknesses, when film thickness: a) 300 nm and b) 700 nm. Artifacts of the above-described processes are observed by optical microscopy

images of film surface topography (Fig. 6. and Fig. 7.). Fig. 6 shows that after 5 minutes of treatment of the thin (300 nm) titanium film in the water vapour plasma, when

15

plasma generator power equals 150 W, a large amount of bubbles of different sizes is formed. Their diameter depends on formed hydrophilic areas and on the size of surface adsorbed water microdroplets. The height of bubbles achieves up to few hundreds nanometres. The dark blue colour also proves the formation of titanium oxide hydride on the film surface.

Fig. 6. Surface height profile and optical view of plasma treated 300 nm thick Ti film for 5 min at 150 W [25].

Fig. 7. Surface height profile and optical view of plasma treated 600 nm thick Ti film for 5 min at 150 W [25].

From the profilogramm in Fig. 7., it is seen that after plasma treatment, the height

of thicker films (600 nm) increases up to 40 times. At optical image, this is seen as bulging and wrinkled areas. Such deformations are formed by large stresses during oxidation, hydrogenation and hydrogen accumulation. From the optical image (Fig. 7.), it is also seen that the oxidized Ti film is quite homogeneous in coloration, and dark blue and purple colours prove formation of hydrogenated titanium oxide layer. After EDS analysis, it was registered that O/Ti ratio is equal to 0.3 and 0.4, respectively, Fig. 6. and Fig. 7.

The analysis of XRD shows simultaneous titanium film oxidation and hydrogenation processes occurring during the water vapour plasma exposure. XRD curves presented in Fig. 8 show structural and phase changes in dependence of exposure in plasma time and power of plasma generator. It is seen that after 5 minutes of exposure, when plasma generator power is 200W (curve 1), Ti2O and TiO2 compounds are formed. Under the same plasma excitation power, after 20 and 60 minutes exposure (curves 2 and 3) it is noticeable that the peak at 38.8°, 51.9° and 68.8° angles decreases, and the peak at 22.9° and 37.4° angles increases, which means that the increasing exposure time intensifies the oxidation process and increases the amount of implanted oxygen into the film. Additionally, after 20 minutes exposure at 34.8° and 40.8° angles, HTi compound is observed, which after 60 minutes exposure can be transformed into H2Ti. Increasing the plasma generator power up to 300W, just after 5 minutes exposure

16

(curve 4), stable TiO2 compound was registered. In addition, in this case formation of titanium silicide is observed (TiSi2).

Fig. 8. Water vapour plasma treated titanium film diffraction patterns when the exposure time: 1, 4 – 5 minutes, 2 – 20 min, 3 – 60 min, and plasma excitation power 1, 2, 3 – 200 W; 4 – 300

W. Structural and phase transformations of Ti films, activated by water vapour

plasma Images presented below show how metallic Ti film consistently transforms into

TiO2 during water vapour plasma treatment depending on plasma excitation power and exposure time.

Fig. 9. Diffraction pattern of Ti film deposited on Si (111) substrate after exposure at the water

vapour plasma (5 min, 200 W).

17

In diffractogram presented in Fig. 9, it is seen that after 5 min. exposure at water vapour plasma using 200 W plasma power generator, formation of TiO and TiO2 compounds in the film is registered. Crystalline metallic Ti is not registered.

Fig. 10. Diffraction pattern of Ti film deposited on Si (111) substrate after exposure at the water

vapour plasma (20 min, 200W). In diffractogram presented in Fig. 10, it is seen that after 20 min. exposure, TiO

and TiO2 compounds are registered. Peak at 37.5o typical to TiO increases, and peaks characteristic of TiO2 decrease. Apparently, amorphous Ti oxidizes and turns into a crystalline TiO, and TiO2 phase is reduced.

Fig. 11. Diffraction pattern of Ti film as deposited on Si (111) substrate after exposure of the

water vapour plasma (5 min, 300 W). In diffractogram presented in Fig. 11, it is seen that after increasing plasma

generator power up to 300W, even after 5 min exposure, stable TiO2 compound was registered. In addition, formation of titanium silicide is observed.

After increasing plasma exposure time up to 60 min, when plasma generator power is 300W, from diffractogram presented in Fig. 12, it is seen that in the film stable TiO2 and TiSi2 compounds are dominant.

18

Fig. 12. Ti film as deposited on Si (111) substrate after exposure of the water vapour plasma (60 min, 300 W) diffraction pattern.

Fig. 13. Diffractograms of Ti films: As-deposited – curve 1, and after plasma treatment, when

exposure time 2 – 5 min, 3 – 20 min and 4 – 60, when plasma excitation power is 200 W. In Fig. 13, juxtaposed as-deposited metallic titanium film and film processed

during experiment under 200 W plasma generator power are illustrated. It is seen that as-deposited Ti film had amorphous structure (Fig. 12. curve 1). After 5 minutes exposure, a small peak corresponding to TiO2 (rutile) and the peak corresponding to TiO are observed. It is interesting that after 20 and 60 minutes exposure, intensity of TiO2 peaks does not increase, and amplitude of TiO peak is rapidly increasing. This can be explained in this way: surface layer is rapidly oxidized during the first minutes of the reactions of the neutral molecules of water coming from the surrounding environment, and the coating consists of TiO2 and TiO compounds, which are registered (curve 2). Along with the oxidation process, selective oxygen erosion with corpuscular particles coming from the plasma reducing TiO2 to TiO occurs. The experiment shows that in the presence of short exposure time, oxidation dominates until the stationary element composition of the surface layer stabilizes, where TiO2 phase dominates (curve 4). The experimental results correlate well with each other.

19

In summary done by XRD analysis, it can be concluded: In Ti film, which is placed in water vapour plasma, TiOx compounds are formed. Stoichiometric oxide phase and number depend on the Ti film exposure time in the water vapour plasma and plasma generator power.

Study of surface roughness, initiated by interaction with water vapour

plasma In this paragraph, the study of the changes of surface topography of Ti films

initiated by plasma treatment under low and high-flux irradiation by ions extracted from water vapour plasma is presented.

Figs. 14-17 illustrate typical profiles of surface topography and SEM surface views for different samples: Fig. 14 – as-deposited titanium film, Figs. 15 and 16, respectively, for thin (up to 300 nm) and thick (up to 700nm) Ti films after plasma treatment for 20 min at 300W (a – low, ~ 1-2 mA/cm2 and b – high, ~10 mA/cm2 flux radiation), and Fig. 17 – thick Ti film after plasma treatment for 60 min at 50W (high ion flux radiation), respectively.

Fig. 14. Surface topography and SEM surface view of as-deposited titanium film.

a)

20

b)

Fig. 15. Surface topography and SEM surface view of plasma treated thin Ti film for 20 min at 300 W under a) high and b) low-flux ion irradiation.

a)

b) Fig. 16. Surface topography and SEM surface view of plasma treated thick Ti film for 20 min at

300 W affected by a) low and b) high flux ion-radiation.

21

Fig. 17. Surface topography and SEM surface view of plasma treated thick Ti film for 60 min at 50 W under high-flux ion irradiation.

Tables 1 and 2 summarize surface roughness measurement results for thin (up to

300 nm) and thick (up to 700 nm) Ti films, respectively.

Table 1. Results of surface roughness of thin Ti films. Thin Titanium films Roughness difference before and after

treatment (R_after – R_before), nm Plasma power,

W

Time, min

Sample No. H, Rz L, Rz H, Rq L, Rq

200 5 1 4.4 2.7 5.7 -0.3

20 2 24.3 -0.1 57.2 2.3

300 5 3 180.9 47.3 147.3 49.9

20 4 74.8 38.8 136.5 57.9

Table 2. Results of surface roughness of thick Ti films. Thick Titanium films Roughness difference before and after

treatment (R_after – R_before), nm Plasma Power,

W

Time, min

Sample No. H, Rz L, Rz H, Rq L, Rq

20 60 5 3.1 3.6 2.8 2.6 50 60 6 -0.3 5.8 -3.8 -0.4

300 20 7 564.7 885.7 223 464 Plasma treatment parameters, such as duration and power dissipated in plasma,

are included in the tables. Letters L and H indicate low and high-flux treatment modes, respectively. For all as-deposited Ti films, the average surface roughness is equal to about 4.5 nm for Rz, and about 4 nm for Rq. It is seen that almost for all cases, the surface roughness increases after plasma treatment. It was registered that the mean surface roughness increases significantly after exposition in high-density water vapour plasma at 300 W: to 181 nm (Rz) for thin, and to 886 nm (Rz) for thick Ti film. However, it was observed that changes of surface roughness are different for samples treated by low and high-flux ion irradiation: for thin films, roughness is higher for Ti film areas treated by high-flux than for areas treated by low-flux irradiation (Table 1, samples 3 and 4), and for thick films – conversely, roughness is higer for areas treated

22

by low-flux than for areas treated by high-flux irradiation (Table 2, sample 7). The same trend is registered also for other plasma processing power parameters (Tables 1 and 2, samples 1, 2, 5 and 6). This fact is considered as the manifestation of the formation of new oxide phases in the near-surface region of Ti film. It was observed that after the exposition in low density water vapour plasma at 50 W for 60 min, the Ti film surface becomes periodically bumpy (Fig. 17.) with height amplitude equal to 8 nm, and period equal to around 15-18 μm. After the exposition of Ti fim in highly ionized water vapour at 300 W for 20 min, thin film becomes holey (Fig. 15.). The mean diameter of holes corresponds to the size of water island layers, and their height is equal to the film thickness of 300 nm. When the film is thick, the observed buckle-and-crack network (Fig. 16.) can be attributed to TiO2 phase precipitation within the Ti film. Presumably, the atomic oxygen interacts at the sites formed during film bulk cracking process and produces M-O links which, in turn, activates formation of new cracks. Therefore, spontaneous film cracking may be maintained by the oxidation process. The generation rate of new sites for oxygen rather than the strain may be the major factor for oxidation kinetics.

Surface views show that oxidation is favored on surfaces containing defects. There is a suggestion that water adsorption on the surface of growing oxide islands acts to form a more permeable scale and thus allows gas access and thereby enhances the H and O permeation rate. An important observation is that whisker formation is encouraged. It is possible that the tip of the whisker is catalytically active in promoting dissociation of the oxidant. The protrusions evolve even under very mild plasma conditions. The surface protrusions dictate the nucleation and subsequent oxidation of a Ti film by promoting the development of a three-dimensional granular morphology in the film.

For incident ions extracted from water vapor plasma, the altered chemical composition of the surface layers leads to changes in surface topography. At high fluences, a saturation concentration of trapped ions is reached, which is determined by a balance among the incident ion flux, the diffusion of the implanted ions into the bulk, the flux of backscattered and removed atoms due to sputtering. It is necessary to divide the surface topography changes on the microscopic (atomic) and macroscopic level. The development of surface structures is closely related to the range and trapping mechanism of implanted gas ions and to the properties of the gas-solid system. It means that the development of surface geometry depends on the type and energy of incident ions. In the range of low ion energies and heavy ions, penetration process is negligible and sputtering process prevails in the formation of surface geometry. It will produce the rearrangement of the first atomic layer and the appearance of micro non-homogeneities.

Water vapor plasma treated titanium has been shown to cleave. Cutting in the form of “popping off” discrete blisters was observed. The location of the cut correlates well with the wetted areas and can be explained by damage induced in-plane stress and the corresponding elastic out-of-plane strain. One of the main issues concerning their mechanical performance is the type and magnitude of residual stresses around the crystalline precipitates. These stresses arise due to the thermal and elastic mismatch between the crystalline precipitates and the matrix. Residual stresses may or may not generate microcracks around the precipitates depending on their magnitude and crystal size.

23

Images obtained by optical and scanning electron microscope analysis Surface views obtained by scanning electron and optical microscopes give

information about mechanism of films degradation in water vapour plasma. During studies, a systematic analysis of Ti films was conducted with the aim to understand the physicochemical processes, which mechanically violate Ti film. It is presumed that main effects are: i) water vapor plasma performs Ti oxidation, what leads to increase of film volume about 20-30 % from its initial volume. According to the latter reason, large internal mechanical stresses breaking the film are produced; ii) H atoms accumulate in the film, and when their concentration exceeds solubility limit, hydrogen bubbles are created, which leads to mechanical stresses, initiating changes in topography and mechanical fragmentation on the surface; iii) at the interface of Ti-Si substrate hydrogen atoms accumulate, because crystalline silicon is impermeable to H atoms, and forms stresses which in the presence of the critical values rip off the film from the substrate. Titanium films, as previously, are also divided into two groups: i) “thick” films, with the thickness of 500-700 nm and ii) “thin” films, with the thickness of 200-300 nm. The optical and scanning electron microscopy surface images are grouped according to the thickness groups and water vapor plasma exposure settings.

SEM images analysis Thick films additionally can be divided into three groups: (i) the images typical to

those presented in Fig. 18 a) and b), in which homogeneous films with wavy surface are observed; ii) the images typical to those presented in Fig. 19 a) and b), in which submicron structures are observed, which can be a consequence of micro discharges, and iii) the images typical to those presented in Fig. 20 a) and b) in which formed indentation and crack network are observed, which can be explained by forming of TiO2 phase. Some of the samples had images typical to those presented in Fig. 21 a) and b), where films lifted off from the substrate surface can be seen.

a) 5 min, 200 W, H b) 60 min, 200 W, H Fig. 18. Ti film remaining structurally homogeneous after exposure in water vapor plasma.

24

a) 5 min, 300W, L b) 60 min, 300W, L Fig. 19. Submicron derivatives formed during exposure in the water vapour plasma.

a) 20 min, 300 W, L b) 20 min, 300 W, H Fig. 20. Indentation and crack network formed during exposure in the water vapour plasma.

a) 5 min, 300 W, H b) 60 min, 300 W, H Fig. 21. During exposure in the water vapour plasma lifted off from substrate and broken films.

By processing thin titanium films, it was observed that they become holey (Fig.

22. b) along the entire film layer. The density of holes in the film depends on plasma generator power, radiation intensity and exposure time. This can be explained by hydrogen accumulation at the film-substrate interface. When hydrogen accumulates, bubbles (Fig. 22. a) are formed. When accumulated hydrogen is in the excess pressure state, i.e., the internal pressure reaches the limit of material resistance to fracture, film removal from the substrate through the entire thickness of the film layer occurs.

Fig

whersubstimpe

oxidecondits coabouthe nanosomeO/Tisampof ab

films

a) Fig. 22. B

g. 23. The cro Fig. 23. pr

re the formtrate interfaermeable for

Optical m After inter

e film, struditions. Thisolour due to

ut structural surface, sh

ometres. Ane colouratio ratio of 1.3

ples of ratiobout 1.8-1.9

In images s differ dep

20 min, 100 Bubbles and

oss-sectional

resents crosmed columnace can be sr hydrogen,

icroscope im

raction withucture, coms results in to interferendeformatio

howing thatalysis of the

on indicates3. The rose o 1.6-1.7. Th9, and stoich presented iending on p

W holes forme

l SEM image

ss-sectionalar structureeen. In addi, mechanica

image analy

h water vapomposition anthat the filmnce effects. ons of filmst maximumese sampless reduced olayer has a

he light beihiometricallin Fig. 24.,plasma para

d during exp

es of Ti film

l view of tites and possition, it is s

ally degrade

ysis

our plasma,nd thicknes

m after its diThe colour

s. A wide ram oxide ths by SEM-Eoxygen contan O/Ti ratioige with a gy oxidized Tit is seen th

ameters and

b)posure in the

s after expos

tanium filmsible formaeen that Ti

es.

, surface of ss of whichifferent prepr of sampleange of inte

hickness is EDS and optent. The suo of 1.5. Th

grey tinge uTi films hav

hat after plad exposure t

) 20 min, 150water vapou

sure in water

ms treated intion of bubfilm, depos

f Ti film is ch depends paration con

e gives primerference co

of the orptical microsunset-yellowhe dull grey

underneath hve the white

asma treatmtime; chem

0 W ur plasma.

r vapour plas

n vapour plabble at the sited on sub

covered withon experimnditions cha

mary informolours apperder of hunscope showw colour hy colouratiohas an O/Tie colouratioent colours ical compo

25

ma.

asma, film-

bstrate

h thin mental anges

mation ear on ndred

ws that has an on has i ratio on.

of Ti sition

26

and oafter whicthe fi

Fig.

sampwith circumicroinducatomdepewatethe fto hy

obtai(quansemiobtai

optical propwater vapo

ch can be exfilm is mech

. 24. Optical The form

ple after intmicro form

ular form (thons (Fig. 2ced by radia

ms occurs, dunds on conr condensat

film thickneydrogen, the

During einment, whntum efficiconductor ined during

perties of trour plasma xplained as hanically da

a)

c) microscope

mation of citeraction wmations of he minimum

24. b and cation cominuring whichnditions at tte react inteess, temperae possibilityxperimentshereas, as ency) is abphotocatalythese exper

eated films treatment bthe emerge

amaged due

images of th

ircular shapith water vwater mole

m energy lac). During ing from plash titanium othe interfac

ensively. Doature, and suy of film lift, considerashown by

bout a hundyst. Fig. 25riments.

are differenbecomes roent hydrogeto appearin

he film surfac

pe structurevapour plasmecules (droaw determininteraction sma occur;

oxide and hyce of titaniuominant proubstrate mating from suable attentiy scientistsdred times 5 presents

nt. It is seenough, wavy en bubbles. ng large stre

ce after expo

es can be ma before pplets), whicnes this). Twith plasmwater moleydrogen forum film anocesses that aterial. If suubstrate wouion was g [15], TiOhigher thanoptical mi

n that the reand with c

In Fig. 24 desses.

b)

d) osure to wate

interpreted plasma formch is close

Their dimensma, photocat

cules reactirm. The kinend plasma. A

damage theubstrate wouuld decreasegiven to bO2 photocan that achieicroscope i

elief of the circular hilld), it is seen

er vapour pla

as followsmation is c

to the plasions are tetalytic procion with titaetics of reacAreas coatee film depenuld be perme. lack TiO2

atalytic proeved using images of

films locks, n that

asma.

s: the coated ne of

ens of cesses anium ctions ed by nd on

meable

film operty other films

27

a) b) Fig. 25. Optical microscope images of the surface of Ti films after exposure in water vapour

plasma, when a) plasma generation power – 100 W, exposure time – 20 min, radiation intensity – low, and b) respectively, 120 W, 10 min, radiation intensity – low.

In Fig. 25, disorderly spaced dark and light (film colour) islets on the surface of

titanium oxide are seen. This can be explained by the fact that the surface in water vapour plasma is unevenly covered with hydroxyl groups (islet with a diameter equal to about 10 microns). Meanwhile, plasma activation of the surface of a small surface area (up to few mm2) is even. Therefore, it can be stated that islet form water vapour adsorption from plasma causes that apart from black islets (a diameter of 10-20 microns), white islets as well exist (their diameter is also 10-20 microns). It means that at the first stage, titanium oxide islets form, which are hydrogenated during the growing time, and when TiO2 with 0.3 % of H is reached, its colour becomes black. As with the previous results, it can be seen that due to increase of plasma generator power, oxidation and hydrogenation processes rapidly take place.

Under certain experimental conditions, when samples are placed above the magnetron cathode, the surface is coated evenly by water vapour (with hydroxyl groups). When it is seen that in the tracking area of the film, the colour does not change, islets are not observed either (Fig. 26.). This means that the surface is coated with water vapour plasma film evenly, and restructuring takes place evenly throughout the entire area.

28

Fig. 26. Optical microscope images of the surface of Ti films after exposure in water vapour

plasma, when samples are located above the magnetron cathode. Water layer influence on reactivity of film During experiments, it was obtained that in dependence on the operating pressure

in the chamber, the surface of films possibly absorbs different water layers, which results in the difference in water splitting efficiency on the film surface. Further analysis shows how the properties of titanium film change, when films are processed at different operating pressures.

Fig. 27. XRD patterns of Ti films: as deposited – 1, and plasma treated at 10 Pa – 2, and at 300

Pa – 3 [18]. X-ray diffraction was used to assess the effect of water vapour pressure on the

plasma treated Ti film phase composition and crystallite size. Fig. 27 includes XRD patterns of Ti films: as-deposited – curve 1, and plasma treated at water vapor pressure of 10 Pa and 300 Pa – curves 2 and 3, respectively. A very small amount of anatase TiO2 embedded in the matrix of amorphous titanium was registered for an as-deposited Ti film (Fig. 27, curve 1). It is difficult to accurately locate the anatase peaks at low levels of crystallinity. The amount of oxidized Ti was observed to increase drastically with the introduction of water vapor at 10 Pa (Fig. 27, curve 2). The registered

29

dominant phases were rutile TiO2 and TiSi2. It indicates the intensive structural and compositional transformations occurring in Ti film and at the interface titanium film/Si substrate immersed in water vapor under plasma radiation. The XRD pattern of Ti film plasma treated at water vapor pressure equal to 300 Pa (Fig. 27, curve 3) shows the presence of small amount of anatase TiO2, and XRD peak corresponding to Ti3O5 becomes dominant [18].

The optical surface views of films treated under different plasma conditions demonstrated general features of water adsorption. The colour of as-deposited titanium film was metallic silver independent of its thickness and homogeneousness. After plasma exposure at 10 Pa pressure, the Ti film changed its color and the contours of micrometric adsorbed water islands, which were connected by stripes of micrometric length and submicrometric width, were observed on the surface (Fig. 28 a). The arriving micrometer scale water droplets spread on the surface of plasma activated hydrophilic TiO2 and drastically changed surface morphology. It was observed that the local surface areas coated by water droplets were completely oxidized and lifted in the form presumably due to accommodation of hydrogen at TiO2/Si interface (Fig. 28 b). However, as water vapor pressure increased up to 100-300 Pa, when TiO2 surface was coated by water multilayer, the metallic film color changed to bright blue (Fig. 28 c), and water droplets arriving to the surface of TiO2 did not initiate any changes of surface topography and optical properties (Fig. 28 d) [18].

a) b)

c) d)

Fig. 28. Optical surface views of plasma treated Ti films coated by adsorbed water island structure (a), water droplets on the surface of hydrophilic TiO2 (b), water multilayer without (c)

and with water droplets (d) on the surface [18].

30

Fig. 29 includes SEM surface views of Ti films as-deposited (Fig. 29 a), and plasma treated at water vapor pressure of 10 Pa without (Fig. 29 b) and with water droplets on the surface of hydrophilic TiO2 (Fig. 29 c). The images obtained revealed that the initially flat surface of the as-deposited Ti film (Fig. 29 a) became bumpy (Fig. 29 b) with nanometric height amplitude and periodicity equal to 7-9 μm after plasma treatment of Ti film covered by water island structure. The deposited water droplets spread on the hydroxyl-rich hydrophilic surface during plasma treatment and resulted in formation of blisters and cover liftings. The SEM surface views did not register any noticeable modifications in surface morphology of Ti films treated at pressure of 300 Pa (Fig. 29 d) [18].

a) b)

c) d)

Fig. 29. SEM surface views of Ti films: as-deposited (a) and plasma treated covered by adsorbed water island structure (b), water droplets on the surface on the surface of

hydrophilic TiO2 (c), and water multilayer (d) [18]. Fig. 30 includes surface topography height profiles of Ti films: as-deposited

(curve 1) and plasma treated at water vapour pressure of 10 Pa (curve 2) and 300 Pa (curve 3). The measured mean square surface roughness of the as-deposited Ti film on silicon substrate equaled to 3-4 nm (curve 1). The roughness of Ti films treated at 10 Pa pressure increased up to 17 nm (curve 2), and no noticeable changes of roughness were registered after treatment at 300 Pa (curve 3). The mean distance between height peaks in surface topography profile measured by stylus profilometer is in agreement with the mean size of water islands observed by optical microscopy (Fig. 27 a) and the mean distance between topographic amplitudes registered by SEM (Fig. 28 b) [18].

31

Fig. 30. Surface topography height profiles of Ti films: as-deposited – 1, and plasma treated at 10 Pa – 2 and 300 Pa – 3 [18].

Summarizing results, presented in this paragraph, it can be stated that chemical

reactivity of titanium films exposed in water vapor plasma depends on thickness of water layer adsorbed on the surface. When water vapour pressure is from 10 to 100 Pa, hydrophilic TiO2 surface is coated by the layer of adsorbed water islets, and surface hydroxylation occurs, which is one of the main factors leading to splitting of adsorbed water molecules. Coming water molecules wet the surface rapidly, and so initiate increase of the density of hydroxyl groups and water splitting to atomic components. Micrometer-sized water droplets spread on hydrophilic surface as monolayers coating and decompose into atomic components – hydrogen and oxygen. However, when water vapour pressure increases up to 300 Pa, the surface of TiO2 is coated by water multilayer. The latter layer prevents redox kinetics and electrochemical water splitting.

Photocatalytic water splitting in plasma Treatment film surface with plasma is forming partly reduced TiOx layers on the

stoichiometric titanium dioxide. Properties of those layers strongly depend on the degree of reduction. Vacancies are formed on the surface. When their concentration reaches critical value, they annihilate by restructuration of lattice. Formed rudiments of reduced oxide grow, and their external boundaries overlap. Due to sub-oxides and vacancies, the surface becomes reactive.

In comparison, plasma reduced surfaces of titanium dioxide have different chemisorption and catalytic properties than stoichiometric surfaces. The reason of those differences can be different electronic configuration of ions and lower coordinated unsaturation degree for fully oxidized oxides. In participation of oxygen vacancies, water molecules easily adsorb on the surface and form chemical bonds with non-stoichiometric titanium dioxide and stay as separate molecules or form small water islets. It was observed that adsorbed water layer is quite non-homogeneous and depends on surface roughness.

In such conditions, adsorbed water clusters wet the surface rapidly by increasing concentration of hydroxyl groups on the oxide surface. Hydroxyl radicals are a neutral hydroxide ion form, half-life of which is 10-9 s [19]. It is one of most powerful oxidizing agents (Table 3). Concentration of OH• radicals depends of the degree of reduction.

32

hydrAffecoxyg

proceprocelayerpassaclose

be inelectWheto eqadjacrelocquan

Table3. ThOxidizer

OH•

O3

H2O2

O2

Due to inc

ophilic. Adcting such

gen and hydSo an elec

ess, generatess by attacr connects bage of posited circuit (F

F Energy di

nterpreted introlyte (TiOreas atom gquilibrium cent to eleccation of atonta are emi

he comparisor O

O

H

crease of Odsorbed watfilms with

drogen. ctrochemicating electronching electrboth procestive H+ ions

Fig. 31).

Fig. 31. Energ

iagram of pn the follow

O2), duringgets excess oposition. Fctrolyte. Inoms takes pitted, energ

on of hydroxyOxidation re

OH•+H++e

O3+2H++2e=H

H2O2+2H++2e

O2+4H++2e=

OH• concentter moleculeradiation, O

al cell is fons on the acrons. For reses, acting s). Electrica

gy diagram o

plasma-assiswing way:

g interactioof energy, aor this rea

n this way, lace. For ongy of whic

yl radical oxeaction

e=H2O

H2O+O2

e=2H2O

=2H2O

trations in tes form thinOH• radical

orming, in wctivated TiOtention of eas proton c

ally conduct

of plasma ass

sted electrocwhen energn, it relea

atom immedson, the exthe chain

ne volume ach can be

xidizing poweThe ox

the surface n water layls decompos

which half O2 surface, oelectrical neconducting stive plasma

sisted electro

chemical cegetic ion ar

ases its enediately “triexcited atom

reaction statom interac

equal to

er [20, 21]. xidation pote

2.80

2.07

1.77

1.23

oxide, the yer on the ase into atom

cycle conson the othereutrality in solid electro

a allows elec

ochemical ce

ell presenterrives from ergy for e

es” to give itm bumps in

tars inside cting with a3.3 eV. A

ential (eV)

surface becactivated surmic compon

ists of oxidr side – reduboth sides, olyte (allowctrons to flo

ell.

ed in Fig. 3plasma to

electrolyte at back and rto another electrolyte

another one,As it is kn

comes rface. nents:

dation uction

TiO2 ws the ow in

1 can solid

atom. return atom , and , light nown,

33

photocatalytic effect is based on generation of electron-hole pair. Thus, when energy of photon exceeds energy of band gap (3.0-3.2 eV), electron (e-) from valence band jumps to conduction band, and thereby the hole (h+) is formed at valence band. Photo-generated electrons and holes can recombine in the semiconductor volume or on its surface during the time interval (recombination time) by releasing absorbed photon energy in phonon form. However, some of these electron-hole pairs diffuse to the surface and participate in chemical reduction-oxidation reactions with adsorbed donor or acceptor molecules. Thus, during these reactions, water splits to O2 and H2. Whereas potential of band bottom conductivity is more negative than oxidation – reduction potential (EH+/H2 = 0V), conductivity band electron reduces hydrogen. At the same time, oxygen oxidation takes place, i.e., oxygen gives electron for semiconductor, because valence band top potential is more positive than oxidation – reduction potential (EO2/H2O = +1.23V). According to the above-described process, on-going water decomposition reactions are given below.

Generation of electron and hole

��� ���

��������� �������

� (14) Oxidation reaction

�����

���� � �� � ���� �� � ���� (15) Reduction reaction

�� � �����

�� � �� � !� (16)

The overall reaction equation ��

��

�� � � �� "�� � #���� (17)

UV radiation comes from plasma to the surface of the sample. Fig. 32 a) includes

emission spectra of water vapour plasma [22]. It is seen that UV area has two peaks of OH groups (~290 nm and ~320 nm). Fig. 32 b) presents illustration of vapour plasma radiation.

Fig. 32. a) emission spectra of H2O plasma [22] b) H2O plasma radiation.

34

As it was mentioned in the introduction, the potential of using water-vapour plasmas excited by microwaves as an ultraviolet (UV) light source has been investigated by using various pressures and input powers [9]. Although there are excited states that emit light in this wavelength range, there is experimental evidence [10] that photocatalysis induced by plasma-photon radiation cannot explain plasma-driven photoactivation of TiO2. Complementary, it has been registered that TiO2 shows plasma-induced catalytic activity under conditions where there is no UV or very little emitted by plasma. It follows that other processes may contribute to one or more of these catalyst activation mechanisms in plasma.

A team of researchers led by Samuel S. Mao [15] found out that by introducing a specific kind of disorder, mid-gap electronic states are created accompanied by a reduced band gap. Our recent investigations have shown that the optical band gap of titanium dioxide measured ex situ decreases from 3.27 eV for the pure TiO2 to 2.7-2.6 eV after treatment in H2O-plasma. Moreover, there is indirect evidence that the band gap of the near-surface region of TiO2 under direct plasma treatment (in situ) may be even smaller. The photocatalytic properties of TiO2 under H2O-plasma are modified and affected by the creation of reactive sites by ion impact under the present conditions.

The effects of plasma treatment are related to the enhancement of chemical reactions by irradiation with photons, electrons, ions, or activated species produced in the plasma as this leads to the formation of functional groups on the surface thus increasing the wettability. Under negative bias voltage (100-200 V), the outermost layer of titanium dioxide, which is affected by simultaneous ion bombardment and water vapor adsorption, becomes highly dynamic. Structural and phase transformations and free energy or chemical potential, which is the thermodynamic function that determines the equilibrium state of interface, modify chemisorptive ant catalytic properties.

Surface of TiO2 titanium film in water vapor environment is partially or entirely covered by adsorbed water layer. The arriving water molecules and microdroplets, driven by superwettability effects on hydrophilic surface of plasma reduced titanium dioxide, converge to water island layers. The competing oxidation and reduction reactions occur at the very top surface of oxide. In these conditions, the kinetics of redox processes and the concentration of hydroxyl groups on the surface depend on the oxidation state of titanium oxide, which depends on the flux of incident energetic ions and the flux of water molecules arriving from plasma. In the presence of generated electrons and holes, the water molecules are decomposed. Photons needed for water decomposition can arise from ion interaction with matter. Energetic particles cause the inner layer of TiO2 surface ionization. The outer layer electrons change the vacancy and the photons of the inner layer, which generates electron-hole pairs interacting with water molecules. In the presence of negative bias voltage, energetic ions arising from the plasma generate photons, which initiate electron and hole pair generation on the plasma modified titanium film surface. Water splitting reactions take place according to (1-17) described reactions. Simultaneously, the hydrogen ions move through the solid electrolyte TiO2, when electrons move through plasma and neutralize ions at the titanium dioxide coating and silicon connection area.

35

CONCLUSIONS

1. Ionic bombardment and inextricably linked with it selective surface erosion turn hydrophobic surface of TiO2 into hydrophilic, and physical adsorption into chemical. Under ion bombardment, the interface between TiO2 film and layer of adsorbed water consists of non-stoichiometric titanium oxides.

2. During oxidation processes, a part of oxygen atoms migrates on the surface and desorbs from it when oxygen molecules are created. Another part of oxygen atoms diffuses into bulk of titanium film. In the water vapour plasma environment, reduced surface of TiO2 becomes favorable for effective splitting of water and extraction of hydrogen.

3. Water molecules and microdroplets affected by wettability effect of hydrophilic surface of titanium dioxide reduced in plasma converge to water islet layers. Oxidation and reduction reactions occur in the upper layer of oxide.

4. Studies have shown that surface layer of oxidized Ti film acts as a solid electrolyte of photoelectrochemical cell and splits on the film surface adsorbed water molecules while positive ions of hydrogen are reduced on the other side of electrolyte. Probably high efficiency of splitting can be explained by preferential removal of oxygen atoms from the surface and photonic emission of atoms activated during cascade collisions.

5. Due to the formed vacancies of oxygen, surface of TiO2 in water vapour plasma environment is covered with hydroxyl radicals, which are involved in H2O dissociation reactions on the activated catalyst. During these reactions, molecular oxygen, positive ions of H and electrons are produced.

6. Splitting of water molecules on the surface of TiO2 takes place according to reactions described in (14-17). Then H ions are transferred through the solid electrolyte, while electrons are transferred through the plasma and neutralize ions at the titanium dioxide film and silicon substrate interface. Formed hydrogen (H2) accumulates at TiO2/Si interface.

7. TiO2 film oxidized in water vapour plasma is characterized by increased chemical activity, which is manifested through observation of chemical adsorption of H2O molecules coming from the ambient. Adsorbed H2O molecules form an islet structure, and the microdroplets evenly spread on the surface by covering surface of TiO2 with a thin (nanometric) layer. For effective electrochemical water splitting, a thin film (up to 300 nm) and relatively low plasma excitation power are sufficient. Processes should take place at a low pressure of about 10 Pa.

8. Ion erosion of stoichiometric TiO2 leads to preferential loss of oxygen from the surface. Among all the defects identified in TiO2 film, formation of oxygen vacancy is one of the most important processes determining photocatalytic properties.

9. At simultaneous H2O adsorption and ion bombardment, the stochastic mixing of adsorbed and adsorbent atoms takes place resulting in formation of TiO2 surface layer with saturated defects.

10.Optical and electronic properties of modified layer significantly differ from crystalline TiO2. Studies of elemental composition using GDOES method have shown that near-surface layer formed in plasma is saturated with hydrogen atoms. As it was shown by US researchers [15], such layers form TiO2 with a reduced band gap.

36

REFERENCES 1. Henderson, M. A., A surface science perspective on TiO2 photocatalysis. Surface

Science Reports, vol. 66 (6-7), (2011), 185-297. 2. Gan, W. Y., Chiang, K., Brungs, M., Amal, R., Dense TiO2 thin film:

photoelectrochemical and photocatalytic properties. Int. J. Nanotechnol., vol. 4 (5), (2007), 574-587.

3. Matthews R. W., Photooxidation of organic impurities in water using thin films of titanium dioxide. J. Phys. Chem. vol. 91 (1987) 3328-3333.

4. Nakata K., Fujishima A., TiO2 photocatalysis: Design and applications, Journal of Photochemistry and Photobiology C: Photochemistry Reviews, vol. 13, (2012), 169-189.

5. Mathews N. R., Jacome M. A. C., Morales E. R., Antoniom J. A. T., Structural and spectroscopic study of the Fe doped TiO2 thin films for applications in photocatalysis, Physica Status Solidi, vol. 6, no.S1, (2009), 219-223.

6. Pouilleau J., Devilliers D., Garrido F., Durand-Vidal S., Mahe E., Structure and composition of passive titanium oxide films, Mater. Sci. Eng., vol. B47, (1997), 235-243.

7. Zheng J. M., Chin W. C., Khijniak E., Pollack GH, Surfaces and Interfacial Water: Evidence that Hydrophilic Surfaces have Long-Range Impact. Adv Colloid Interface Sci. vol. 127, (2006), 19-27.

8. Shuaibov A. K., General A. A., Kel'man V. A., Shevera I. V., The emission characteristics of low-pressure water vapor discharge UV emitters, Tech. Phys. Lett. vol. 34 (7), (2008), 588.

9. Oh, J.-S., Kawamura, K., Pramanik, B. K., Hatta, A., Investigation of Water-Vapor Plasma Excited by Microwaves as Ultraviolet Light Source. IEEE Transactions on Plasma Science, vol. 37, (2009), 107-112.

10. Sano T., Negishi N., Sakai E., Matsuzawa S., Contributions of photocatalytic/catalytic activities of TiO2 and -Al2O3 in nonthermal plasma on oxidation of acetaldehyde and CO. Journal of Molecular Catalysis A: Chemical, vol. 245, (2006), 235-241.

11. Nguyen V. T. S., Foster J., Gallimore A., Operating a radio-frequency plasma source on water vapour. Review of scientific instruments, vol. 80, (2009).

12. Kubart T., Nyberg T. and Berg S., Modelling of low energy ion sputtering from oxide surfaces. J. Phys. D: Appl. Phys. vol. 43, No. 20, (2010), 205204.

13. Christie A. B., Sutherland I., Walls J. M., Studies of the composition, ion-induced reduction and preferential sputtering of anodic oxide films on Hg0.8Cd0.2Te by XPS. Surface Science, vol. 135, Issues 1-3, (1983), 225-242.

14. Pan X., Yang M-Q., Fu X., Zhang N., Xu Y-J., Defective TiO2 with oxygen vacancies: synthesis, properties and photocatalytic applications. Nanoscale, vol. 5, (2013), 3601-3614.

15. Mao S., Chen X., Liu L., Yu P. Y., Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science, vol. 331, (2011), 746.

16. Pranevicius L. L., Milcius D., Tuckute S., Gedvilas K., Preparation of hydrogenated-TiO2/Ti double layered thin films by water vapor plasma treatment. Appl. Surf. Sci. vol. 258, (2012), 8619-8622.

37

17. Pranevicius L. L., Tuckute S., Gedvilas K., Milcius D., Oxidation of thin Ti films and its simultaneous hydrogenation by water vapor plasma. Thin Solid Films, vol. 524, (2012), 133-136.

18. Pranevicius L., Pranevicius L. L., Vilkinis P., Baltaragis S., Gedvilas K., Water surface coverage effects on reactivity of plasma oxidized Ti films. Appl. Surf. Sci. vol. 295, (2014), 220-244.

19. Helmut S., Strategies of antioxidant defense. European Journal of Biochemistry vol. 215 (2), (1993), 213-219.

20. Strengths of Oxidizing and Reducing Agents. Table of Standard Electrode Potentials. Electronic source: http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/electrode2.html

21. Background on advanced oxidation processes. Electronic source: http://catarina.udlap.mx/u_dl_a/tales/documentos/lamb/rivas_c_f/capitulo5.pdf

22. Nguyen V. T. S., Hydrogen Production in a Radio-Frequency Plasma Source Operating on Water vapor. The University of Michigan, dissertation, (2009).

23. X. Pan, M-Q. Yang, X. Fu, N. Zhang, Y- J. Xu, Defective TiO2 with oxygen vacancies: synthesis, properties and photocatalytic applications, Nanoscale, 5, 2013, p. 3601.

24. J. M. McKay, V. E. Henrich, Preparation and properties of the Ti2O3(047) surface, Applied Physics, 137, (2–3), 1984, p. 463–472

25. Jansonait� S., Gedvilas K., Pranevicius L., Studies of surface topography kinetics and volume processes of thin Ti film at water vapor plasma, Radiation interaction with material and its use in technologies, 2012, p. 242-245.

Publications on the Subject of the Dissertation and Approbation Articles

1. L. Pranevicius, L. L. Pranevicius, K. Gedvilas, P. Vilkinis, S. Baltaragis, “Water surface coverage effects on reactivity of plasma oxidized Ti films”. Applied Surface Science, vol. 295, p. 240-244. (2014) [ IF 2.53 ]

2. L. Pranevicius, S. Tuckut�, L. L. Pranevicius, K. Gedvilas, “Water vapor-plasma-enhanced oxidation of thin titanium films”. Acta Physica Polonica A, vol. 123, No. 5, (2013). [ IF 0.60 ]

3. L. Pranevicius, M. Urbonavicius, L. L. Pranevicius, S. Tuckut�, K. Gedvilas, T. Rajackas, D. Girdzevicius, “Study of oxidation of thin film by reduction processes in water vapor plasma”. e-Journal of Surface Science and Nanotechnology, vol. 10, p. 613-618, (2012). [ SJR 0.29 ]

4. L. Pranevicius, M. Urbonavicius, S. Tuckut�, K. Gedvilas, T. Rajackas, L. L. Pranevicius, D. Milcius, “Structural and phase transformations in water vapour plasma treated hydrophilic TiO2 films”. Advances in Materials Science and Engineering, article ID 592485, doi:10.1155/2012/592485. (2012). [ IF 0.89 ]

5. S. Jansonait�, K. Gedvilas, L. Pranevicius, “Studies of surface topography kinetics and volume processes of thin Ti film at water vapor plasma”. Radiation interaction with material and its use in technologies, p. 242-245, (2012)

6. L. L. Pranevi�ius, S. Tuckut�, K. Gedvilas, D. Milcius, “Oxidation of thin Ti films and its simultaneous hydrogenation by water vapor plasma”. Thin solid films, vol. 524, p. 133-136. (2012). [ IF 1.86 ]

38

7. L. L. Pranevicius, D. Milcius, S. Tuckut�, K. Gedvilas, “Preparation of double-layered TiO2 thin films by water vapour plasma treatment”. Applied surface science, vol. 258. Iss.22, p. 8619-8622. (2012). [ IF 2.53 ]

8. K. Gedvilas, L. L Pranevicius, D. Milcius, “Surface topography analysis of water vapour plasma irradiation induced effects in Ti films”. Nanomaterials: Applications and Properties, vol. 1, Part II, (2011).

Approbation of the Results

1. International congress “Materials and Renewable energy”, Hong Kong, China. 2014. “Surface water coverage effects on titanium oxidation and water splitting efficiency in water vapour plasma”, (Poster presentation).

2. 11th International conference “Chemija 2013”, Vilnius, Lithuania, 2013. “Surface reactivity of titanium films immersed in water vapour plasma”, (Poster presentation).

3. SPSSM 2012: 4th international symposium on structure-property relationships in solid-state materials, Bordeaux, France, 2012. “Structural and phase transformations in water vapour-plasma-treated titanium films”, (Poster presentation).

4. The vital nature sign: 6th international scientific conference, Kaunas, Lithuania, 2012. “Studies of surface height topography and volume processes of thin titanium film at water vapour plasma”, (Poster presentation).

5. 3d International Conference on Radiation interaction with materials: fundamentals and applications, Kaunas, Lithuania, 2012. “Studies of surface topography kinetics and volume processes of thin Ti film at water vapour plasma”, (Poster presentation).

6. The vital nature sign, 5th international scientific conference, Kaunas, Lithuania, 2011. “Development of surface topography of titanium films under water plasma irradiation”, (Oral presentation).

39

CURRICULUM VITAE Name Surname: Karolis Gedvilas Date of birth: 06.07.1984 Citizenship: Lithuanian E-mail: k.gedvilas@gmf.vdu.lt Education: 2010 – 2015 Vytautas Magnus University, Faculty of Natural Sciences, PhD of

Physics. Theme: Research of photocatalytic water splitting on the surface of titanium activated by water vapour plasma.

2008 – 2010 Kaunas University of Technology, Faculty of Telecommunications and Electronics, Master of Biophysics (Biomedical engineering).

2007 – 2009 Vytautas Magnus University, Faculty of Informatics, Bachelor of Informatics.

Work experience: 2013 – Present JSC “Nepriklausomi Tyrimai”; Raman, NIR and FTNIR spectroscopy

consultant. Work with “Thermo Scientific” portable analytical instruments for identification of different kind of materials (pharmaceutical, plastics, metals, feeds).

2011 – Present Vytautas Magnus University, Faculty of Natural Sciences, Department of Physics, junior lecturer.

2008 – 2011 JSC “Senuku prekybos centras”, Database administrator of storehouse management systems. Responsible for performance and development of logistics systems.

2005 – 2007 Vytautas Magnus University, Faculty of Social Science and Vocational Education and Research centre. IT administrator.

40

Karolis Gedvilas

FOTOKATALITINIO VANDENS SKAIDYMO ANT VANDENS

GAR� PLAZMA AKTYVUOTO TITANO PAVIRŠIAUS TYRIMAS

Rezium�

Tyrim� sritis ir problemos aktualumas

Šiame darbe yra pateikiamas naujas b�das �galinantis padindinti vandens

skaidymo efektyvum�. Metodologiškai, pirmiausiai yra suformuojama plona titano danga, kuri v�liau yra oksiduojama vandens gar� plazmoje. Atlikti tyrimai parod�, kad paviršinis oksiduotos Ti dangos sluoksnis veikia kaip fotoelektrochemin�s cel�s kietas elektrolitas ir skaido ant dangos paviršiaus adsorbuotas vandens molekules, o teigiami vandenilio jonai yra redukuojami kitoje kieto elektrolito pus�je. Galimai aukštas skaidymo efektyvumas yra paaiškinamas plazmos inicijuojamu preferenciniu deguonies atom� pašalinimu nuo paviršiaus ir kaskadini� susid�rim� metu aktyvuot� atom� fotonin�s emisijos. Plazmin� paviršiaus modifikacija �takoja dangos mikrostrukt�ros ir sud�ties pasikeitimo procesus. Pavyzdžiui, panaudojant vandens gar� plazm�, gali b�ti vykdoma hidroksilo jon� implantacija � titano dioksido pavirši�. Taikant plazmini apdorojim�, visi pamin�ti procesai �vyksta vienu metu, ta�iau j� �taka gali b�ti modifikuojama. D�l šios priežasties, siekiant išgauti norimus procesus vykstan�ius s�veikoje su dangos paviršiumi, gali b�ti optimizuojami �vair�s kontroliuojami plazminio apdorojimo parametrai.

Esant tiesioginiam plazmos poveikiui (in situ) titano dioksido dangos paviršiaus draustin�s juostos plotis gali b�ti kur kas mažesnis (nei 3.2 eV). Vandens gar� plazmoje, esant atitinkamoms s�lygoms, titano dioksido fotokatalitin�s savyb�s yra modifikuojamos ir �takojamos reaktyvi� sri�i� k�rimosi d�l joninio poveikio.

Plazminio poveikio efektai yra susiejami su chemini� reakcij� padid�jimu skatinam� fotonin�s, elektronin�s ar jonin�s apšvitos arba su aktyvuot� element� plazmoje susidarymu, kas nulemia funkcini� grupi� ant paviršiaus formavim�si taip padidinant dr�kinimo efekt�. Prid�jus neigiam� prieš�tamp� (100-200 V) išorinis titano dioksido sluoksnis, kuris yra veikiamas vienalaikio joninio bombardavimo ir vandens gar� adsorbcijos, tampa labai dinamiškas. Vykstan�ios strukt�rin�s ir fazin�s transformacijos modifikuoja chemisorbcines ir katalitines titano dioksido dangos paviršiaus savybes.

Darbo tikslai ir uždaviniai Disertacijos tikslas – Gilinti mokslin� supratim� apie vandens molekuli�

disociacij� ant vandens gar� plazmoje oksiduojam� titano dang� paviršiaus, aktyvinam� plazmos spinduliuote.

41

Disertacijos uždaviniai: 1. Magnetroninio nusodinimo metodu suformuoti nanostrukt�rines 300-700 nm

storio titano dangas ant silicio ir silicio dioksido pad�kl�; 2. Atlikti eksperimentus apdorojant plonas titano dangas žematemperat�rin�je

vandens gar� plazmoje parenkant skirtingus parametrus: i) plazmos generatoriaus galia, ii) ekspozicijos laikas iii) darbini� duj� sl�gis, iv) plazmin�s apšvitos intensyvumas;

3. Eksperimentiniais rezultatais ir teoriniais modeliais patikrinti darom� prielaid�, kad plazmoje oksiduot� Ti dang� paviršiaus cheminis aktyvumas ženkliai did�ja d�l nestechiometrini� TiOx jungini� susidarymo, ko rezultate paviršius H2O plazmoje tampa hidrofilinis, o adsorbuotas vanduo pasklinda po pavirši� ir formuoja salelin� strukt�r�;

4. Patikrinti prielaid�, kad TiO2 danga vandens gar� plazmoje formuoja fotoelektrochemin� element�, kuriame oksidacijos reakcija vyksta ant nestechiometrinio TiO2 paviršiaus, o redukcijos reakcija skiriamoje riboje TiO2/Si. Nanokristalinis TiO2 – kietas elektrolitas laidus H jonams.

5. Atlikti Ti dang�, oksiduot� H2O plazmoje, savybi�: i) paviršiaus nanoreljefo, ii) paviršiaus sud�ties bei strukt�ros ir iii) strukt�rini� ir fazini� virsm�, aiškinan�i� fotokatalitin� vandens molekuli� skilimo mechanizm�, analiz�.

6. Paaiškinti fotokatalitinius reiškinius, vykstan�ius ant modifikuojam� titano dang� paviršiaus ekspozicijos H2O plazmoje metu.

Ginamieji teiginiai

1. Ant vandens gar� plazmoje oksiduot� plon� (300-700 nm) Ti dang� paviršiaus formuojasi nestechiometriniai titano oksidai, kurie aktyvina H2O chemin�s adsorbcijos proces�. Paviršius tampa hidrofiliniu. Adsorbuotos H2O molekul�s formuoja salelin� strukt�r�.

2. Ekspozicijos plazmoje metu formuojasi fotoelektrocheminis elementas, kuriame fotoanod� atitinka plazmos ir dangos paviršiaus s�veikos sritis, elektrolit� – TiO2 sluoksnis ir fotokatod� TiO2 sluoksnio ir silicio pad�klo riba.

3. Susidarius fotoelektrocheminiam elementui atskil�s vandenilis pernešamas nuo paviršiaus link TiO2/Si skiriamosios ribos. Jo veikimo principas: ant nestechiometrinio TiO paviršiaus vyksta vandens oksidacijos reakcija, kurios metu susidaro H jonai, kurie pernešami per kiet� elektrolit� (nanokristalin� TiO2) link TiO2/Si skiriamosios ribos, kur vyksta redukcijos reakcija, kurios metu H jonai reaguoja su elektronais, susidariusiais ant TiO2 paviršiaus, ir patekusiais � reakcijos zon� išoriniu kont�ru.

4. Žemo sl�gio (~10 Pa) vandens gar� plazmoje modifikuojamos titano dangos yra chemiškai aktyvios, disociatyviai skaldan�ios adsorbuotas vandens molekules � O ir H atomus.

5. Nustatyta, kad pagrindinis mechanizmas, formuojantis deguonies vakansijas ant TiO2 paviršiaus, yra selektyvi TiO2 erozija teigiamais jonais, ateinan�iais iš plazmos

6. Vandens gar� plazmoje esant vienalaikei H2O adsorbcijai ir joniniam bombardavimui formuojasi defektais prisotintas TiO2 paviršinis sluoksnis, kurio optin�s ir elektronin�s savyb�s ženkliai skiriasi nuo kristalinio TiO2. Susidaro nauji energetiniai lygmenys energetin�je zonoje, kurie mažina uždraustos zonos plot� ir sudaro s�lygas elektronin�m – skylin�m poroms susidaryti.

Karolis GEDVILAS

RESEARCH OF PHOTOCATALYTIC WATER SPLITTING

ON THE SURFACE OF TITANIUM ACTIVATED

BY WATER VAPOUR PLASMA

Summary of Doctoral Dissertation

Išleido ir spausdino – Vytauto Didžiojo universiteto bibliotekos Leidybos skyrius (S. Daukanto g. 27, LT-44249 Kaunas)

Užsakymo Nr. K15-071. Tiražas 40 egz. 2015 05 26. Nemokamai.