University „Alexandru Ioan Cuza” in Iaşi
FacultatyofPhysics
Contributionstothe study of interface fenomena in the multi-layer oxide films
- Abstract -
Scientific Coordinator,
Prof. univ. dr. Dumitru Luca
Ph. D. Candidate,
Dăscăleanu Vasile
INTRODUCTION
The study of oxide materials is a relevant area in the extremely broad field of the science of materials, enjoying a special attention from researchers (physicists, chemists, biologists, mathematicians, engineers, doctors, etc.). This is reflected, including the number of publications in the field of material science in the total annual scientific publications, especially in the pace of discovery and implementation of a large number of innovative materials in everyday life. It is not surprising though that in this context, over the past 50-60 years, a significant number of Nobel prizes have been awarded to winners who worked in the field of material science (including the oxide ones), or for contributions related to progress in this field. Also, the relevance of research in this field is reflected in the allocation of funds for research and the number of projects approved for funding in Europe, Japan and U.S.A.
The society has benefited, especially in these years, to a decisive extent, of the discoveries of science materials, resulting in the appearance of "exotic" materials. We hear almost daily about the discovery of new materials whose properties are controlled by processes at nanoscale, of applications with a bio-mimetic character, or the discovery of so-called meta-materials, to give just a few examples from an impressive list.
Literature is enriched daily with a huge number of publications with fundamental and applied results in improving the properties of oxide materials in applications related to energy production and storage, protection and repair of the environment, hydrogen technology, etc. Many important authors in oxide materials science consider that this has already been, for a long time, an area almost without borders, given the scientific and technological problems that have been or will be resolved, as well as the benefits coming from here. We consider the applications of oxide materials in the production and up taking of energy (fuel cells, solar energy conversion cells, hydrogen technology, water splitting, thermonuclear fusion, etc.), the protection of life and environment (photo-catalytic materials, superhidrofile, bactericidal drugs to destroy pollutants, or as medium/means of protecting life) . There is no need to insist on the progress made in the areas of space technology, high temperature super-conductivity, physics and the technology of extreme temperatures and pressures, all based on the use of innovative materials.
There is research on improving the applying performance of some classes of oxide materials with nano- and micro-structure by increasing thin films, nano-fibers or nano-tubes, aerogels and nano-particle systems. We mention only as an example significant results in literature regarding the use of oxide materials or compounds with other organic or inorganic materials, the number of those increasing quite rapidly.
Restricting our discussion to photocatalytic materials, the research field has expanded almost exponentially after the popularization of high applicative potential discoveries in the early 70s. Now they have become known as photocatalytic properties of titanium dioxide for photocatalytic water dissociation, and for converting solar energy into chemical energy. Subsequent research has focused on investigating ways of structural changes, chemical and morphological characteristics that could lead to increased efficiency of photocatalytic titanium dioxide and other photocatalytic materials.
TiO2 has become in the meanwhile standard material for photo-catalytic process; in pure state, it can be photo- activated only by ultraviolet light (due to the large width of the forbidden band). The improvement of its photo-catalytic efficiency can be made by reducing the actual width of the forbidden band, provided by doping with different kinds of anions and/or cations as well as by increasing the lifetime of photo-generated carriers, involving spatial separation, in order to compensate their recombination on material defects.
It should be noted that the beneficial effects brought by the two above-mentioned approaches can be found in the case of titanium dioxide in the increase of the photocatalytic efficiency as well as of its surface hydrofilicity (although these two distinct properties are related to various causes). The quantitative evaluation of the effects of doping is easier to carry out through experiments for measuring the specific surface energy, or the contact angle, rather than the measurement of the photocatalytic efficiency. This was the very reason for our choice to make contact angle measurements, to assess the effects of material changes investigated in the present thesis.
In the studies conducted in the present doctoral study, we chose to investigate the macroscopic properties as full effects of some physical processes at nano- and microscopic scale in the interface region between two oxide materials with different electronic structure levels, both showing photocatalytic activity: carbon dioxide titanium and tungsten trioxide. We chose these materials, based on information at the initiation time of our research, which seemed to be of interest for the construction of hetero-junction semiconductors which should ensure a higher life time of photo-generated charge carriers by irradiation with light in the superior limited spectral range to higher values in the wavelength 390-450 nm. The results obtained on thin layer samples of the WO3/TiO2, may be considered as test-examples for other photocatalytic materials of interest as well. We have successfully tested the possibility of bi-layer preparation of the two above mentioned materials via plasma-assisted methods. We characterized these configurations from the structural, elemental and chemical viewpoint via methods and cutting edge techniques. The results were interpreted taking into account the physical phenomena in the interface region, i.e. local atomic ordering, investigated by EXAFS technique through measurements using synchrotron radiation. These were made in the measurement sessions HASILAB synchrotron facility DESY in Hamburg, Germany. The results were correlated with the measurements of crystallographic structure (XRD) and the composition of the item (XPS), as well as macroscopic effects of changes induced at the interface by measuring the contact angle of the surface of the samples with distilled water. The results were also discussed with reference to the effects of doping with a titanium dioxide and iron oxide. Characterization in this regard was made via contact angle measurements, experiments for the photocatalytic performance evaluation going to be conducted in the near future.
The thesis is divided into six chapters, followed by a section of conclusions and a list of references. It ends with the list of original contributions of the author expressed in publications in journals and papers at scientific conferences in the field. The first 3 chapters have got a monographic character and the following 3 present the author's own contributions.
In Chapter 1, entitled "Oxide materials based on titanium and tungsten" reviews the current data present in the literature about the physical properties of the two kinds of oxide materials in solid state or in the form of thin layers, together with their most important applications. In the second part of this chapter there are presented preliminary data on the characteristics of the bi-layer TiO2/WO3 structures.
Chapter 2, entitled "Getting oxide thin films," contains monographic information about obtaining oxide films, presenting the main physical and chemical methods for their preparation. Since the method of preparation of the materials investigated in this thesis is based on the spray deposition in a plane magnetron geometry type there are discussed here the main theoretical and practical aspects related to this choice, and the physical characteristics of magnetron discharge on the plasma-cathode interface.
Chapter 3, entitled "Methods for characterizing the physical surface" is dedicated to presenting information on the characterization of surfaces and interfaces via modern techniques of photoelectron spectroscopy, structural analysis through x-ray diffraction, x-ray absorption and specific energy measurements surface. It should be mentioned that, even in this case, we selected only those methods that are relevant to the purpose and at the same time available in the laboratory in which we worked or through access to third parties. It is known that the number of these methods is very high, for example only the number of spectroscopic techniques for area analysis being over 20.
Chapter 4, entitled "Plants for the preparation and characterization of the studied oxide films" presents the experimental arrangement for preparing investigated materials, together with the instrumentation used to characterize the elemental, structural and atomic ordering on the local scale. This includes technical specifications of the facilities used, together with the technical solutions chosen for the synthesis and characterization of the researched materials.
Chapter 5, entitled "Characterization of TiO2 thin oxide films doped oxide" presents results of research conducted for the synthesis and characterization of homogeneous oxide structures as thin layers doped with nitrogen and iron. These results were used as reference for the comparison with the TiO2/WO3 two-layer structures type.
Chapter 6, entitled "Bi-layer TiO2/WO3 and WO3/TiO2 Systems" is dedicated to the analysis results on the role of atomic ordering processes in the interface region of the two oxide materials. We used the XPS analysis techniques to determine the elemental and chemical concentration profile in the interface region, as well as XANES, EXAFS technique which has allowed the study of the effects of local arrangement at the interface between the two thin films. The data obtained are useful for understanding the functioning of the two-layer structures as TiO2/WO3 heterojunctions.
Investigations contained in the thesis have led to the accumulation of a large number of data, which were only partially recovered up to now. For example, we introduced here, only partially, the results of the effects of heat treatment on structure and spatial extent of the region of the interface, since their analysis requires laborious analysis and the development of a suitable model. We are going to carry out in the near future tests of photocatalytic activity of the two-layer structures investigated, as well as an analysis of the concentration gradient of the characteristics of the heterojunction interface region.
4. PREPARATION AND CHARACTERIZATION OFOXIDE FILMS
The research carried out in this paper relates to the study of the properties of the films of oxide materials, of doped titanium dioxide and some of the tungsten trioxide film in structures of mono- and bi-layer type. The preparation of these materials in the form of thin films made by a series of experiments aimed at finding the optimum parameters of the deposition (which should provide samples with the properties in the range of reproducible and controllable properties of interest for practical applications). Since the main purpose of the research conducted was to find the appropriate means to increase the photocatalytic efficiency, we chose as oxide material prototype the titanium dioxide - a highly intensively researched material over the last decades, as presented in detail in Chap. 1.
As shown above, in order to improve the efficiency of the photocatalytic oxide of these classes of materials, particularly of titanium dioxide, it is possible through an extension of the photo-activation, as well as increasing the life time of the photo-generated charge carriers in the region of surface/interface. Therefore, in a first series of experiments we followed the effect of Fe doping atoms, N atoms, respectively, the latter being a topic addressed in other projects in Plasma Physics Laboratory and Laboratory of Surface and Interface Physics.
The results were interpreted in terms compared to those obtained in a second series of experiments for the synthesis and characterization of the two-layer-type structures of TiO2/WO3 and WO3/TiO2 type, with hetero-junction semiconductor function. The properties of such structures have been studied by methods which allowed the structural, elemental, chemical, spectral characterization, as well as the study of the local atomic ordering in the interface region. Finally, the properties of the studied structures were compared in order to find, in addition to the explanation of physical phenomena involved, as many arguments as possible for extending the field of applicability. Although it has been a subject of this thesis, we assume that our results could be used also in the development of hetero-junction of the type WO3/D:TiO2, with the idea of using the potential benefits of the combination of the two approaches.
5. CHARACTERIZATION OF THIN OXIDE LAYERS OF DOPED TiO2
5.1. Thin layers TiO2: N
In order to assess comparatively the macroscopic effects of the processes of bilayer interface type structures of titanium oxide/tungsten oxide, regarding hidrofilicity properties and photocatalytic efficiency, we prepared 'standard' samples of doped TiO2. Their properties were compared to those of the bilayers in the arrangement WO3/TiO2/glass, respectively TiO2/WO3/glass. As mentioned above, this allowed the highlighting role of interface processes in the compensation of photogenerated charge losses by separating them and extending their life time.
In a first instance, we have prepared webs of a dissociation of an atom of titanium with a nitrogen atom, being much lower than that of a TiN atom bond (the free enthalpy of 464 kJ/mol compared to 640 kJ/mol in the case of Ti-O bond) [87].
We chose to use for the preparation of TiO2 thin films: A method described in reference [88]. It consists in using a ceramic spray TiN put to a target bombardment of Ar+ ions in a mixture of Ar discharge reaction with O2. Compared with the classical method which consists in nitriding TiO2 surface during film growth (by introducing nitrogen in gas discharge), we had a way to adjust the nitrogen content in a much broader scope. [89]
To obtain thin films there have been used the following settings: RF generator power (75W), the constant flow of argon (5.6 sccm) and O2 flow rate varies between 0.0 and 2.0 sccm (in steps of 0.5 sccm). In these circumstances, we could adjust the partial pressure of O2 unloading in the limit of 2.3×10-5 mbar (in the only presence of residual oxygen in the atmosphere) and a maximum value of 1.3×10-3 mbar.
The total pressure of the discharge of the gas mixture was kept constant by adjusting the flow rate of the exhaust gas from the deposition chamber. This is done by adjusting the gas flow with a gate valve between the discharge chamber and the turbo-pump. As substrate deposition there were used borosilicate glass plates and wafers of Si (100) (25 mm×40 mm), which were previously cleaned in an ultrasonic bath with acetone and alcohol and dried in air, and during deposit they were automatically maintained at a temperature of 200°C using a temperature controller.
We determined the crystal structure of those and watched their evolution according to the flow of oxygen introduced during preparation. Thin films of varying thickness around 150 nm were analyzed using XRD and XPS techniques [76].
By increasing the oxygen flow to 0.0 sccm to 2.0 sccm, the samples were noted in Table 4.1 (DV41 = 0.0 sccm, DV44 = 0.5 sccm, DV47 = 1.0 sccm, DV50 = 1, 5 sccm, DV53=2.0 sccm) and the diffractograms show:
- an increase in the peak intensity of a specific crystalline phase of TiO2;
- a decrease in the specific peak TiN;
- fluctuating values of the peaks due to the small thickness of the films; however we identify A(101), A(004), A(200), TiN(111), TiN(200) and TiN(220). All the analyzed XRD samples were deposited on a Si support (we identify specific peaks (111) Si at 27.80°, (220)Si at 46.80° and (311)Si at 55.80°). These diffraction patterns were obtained with the help of the XRD system of DRON II with radiation of Xa Cu Kα.
Table 5.1. Values of the peak areas A(004) and TiN(200)
Test / AreaA(004) (u.a.) / 2θ / Crystallite size / Area
TiN (200) (u.a.) / 2θ / Crystallite size
DV41 / 3.57 / 37.69 / 22 nm / 2.82 / 41.42 / 24 nm
DV53 / 6.28 / 37.63 / 24 nm / 8.67 / 41.46 / 25 nm
DV50 / 9.25 / 37.83 / 24 nm / 8.77 / 41.55 / 29 nm
As a general conclusion, we find that the concentration of dopant (nitrogen) in films, fluctuating between 1.2 and 1.8 %, even if the partial pressure In our opinion, this may be due, most likely, to the formation of a titanium oxide film at the target area, which is maintained evenunder zero oxygen flow introduced into the cooking chamber of films of oxygen has a minimum value (depending on the residual traces of oxygen from the atmosphere). The result was shown in earlier experiments in which it has been demonstrated using the technique of elastic scattering of low energy ions (LEIS) [90] that, even in the case of threshold pressure of 10-7mbar, the formation of oxide is present.
The general aspect of the sample area is generally the same, with differences in the maximum values of the maximum roughness of the order differences of the order of a few nm. Systematic XPS measurements allowed determination of the ratio of the concentrations of dopant atoms, which - as we shall see below - range between 0.031 and 0.040 (see Table 5.2).
Table 5.2. The concentration values of chemical elements in the analyzed samples
N(%) / O/Ti / N/Ti
DV41 / 53.5 / 1.8 / 1.197 / 0.040
DV53 / 56.7 / 1.6 / 1.360 / 0.038
DV50 / 57.2 / 1.2 / 1.196 / 0.031
High resolution XPS spectra of N 1s signal of the samples studied are shown in Fig. 5.4, along with the peaks obtained after the deconvolution. We notice two net peaks, denoted by N1 and N2, at values of the binding energies BEN1 = 396.3 eV, and respectively BEN2 = 399.6 eV respectively. The N1 component is associated with the Ti-N bond, and the second component (N2) is linked to the rest of the nitrogen present in the material [89]. It is obvious that the peak area varies in a complementary manner with the decreasing nitrogen content of thematerial, the role of the component due to the Ti-N bond decreasing gradually.
The analysis of XPS spectra of high resolution Ti 2p there are found relatively small variations in their appearance after the deconvolution with 3 components corresponding to the states of ionization of Ti (Ti2+, Ti3+ and Ti4+. The results of the quantitative analysis are shown in Fig. 5.4. They show that the effect of nitrogen substitution for oxygen doping is reflected only in a small degree in the proportion of chemical states of Ti.
Table 5.3. Peak area values after deconvolution N 1s
Sample / Peak area N1 / Peak concentration N1(arbitrary units) / Peak area N2 / Peak concentration N2 (arbitrary units)
DV41 / 684 / 42.97 / 908 / 57.03
DV44 / 261 / 43.38 / 341 / 56.62
DV47 / 497 / 63.04 / 291 / 36.96
DV53 / 83 / 25.91 / 238 / 74.09
DV50 / 363 / 53.28 / 319 / 46.72
The result can be interpreted with a view to the nature of the material sprayed from the spray target (titanium nitride), it seeming to suggest that the doping of the material linked to the ON substitution occurs mainly during the growth of the film and not at the spraying target surface, or during the transit of the material through the net between the cathode and the substrate.