Characterization of thin films prepared by co-sputtering iron and titanium precursors and thermal oxidation under air atmosphere

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under air atmosphere

A. Djarri, A. Achour, M.A. Soussou, N. Sobti, S. Achour

To cite this version:

A. Djarri, A. Achour, M.A. Soussou, N. Sobti, S. Achour. Characterization of thin films prepared by

co-sputtering iron and titanium precursors and thermal oxidation under air atmosphere. Materials

Characterization, Elsevier, 2018, 135, pp.139 - 145. �10.1016/j.matchar.2017.10.008�. �hal-01705411�

Characterization of thin films prepared by co-sputtering iron and titanium precursors and thermal oxidation under air atmosphere

A. Djarri

, A. Achourb

, M. A. Soussou

, N. Sobti

, S. Achour

*

a

Mentouri Universty, Constantine, Algeria, Research Unit in Material Science and Application Mentouri University, Constantine, Algeria.

b

Institut des Materiaux Jean Rouxel (IMN), Universite de Nantes, CNRS, 2 rue de la Houssiniere, BP32229, 44322 Nantes Cedex 3, France.

c

Namur university, Belgium.

d

Laboratory of Thermal Processes, Research and Technology Center of Energy, Borj Cedria Science and Technology Park, 2050 Hammam-Lif, Tunisia.

e

University of Batna, Algeria.

f

National Polytechnic School of Constantine, Research Unit in Material Science and Application Mentouri University, Constantine, Algeria.

ABSTRACT

Nanocomposites of metal oxides are useful materials for operation in many energy

conversion systems. In this study, such nanocomposites were prepared by oxidation of mixtures of iron and titanium precursor metallic thin films at 520 °C under air atmosphere. The metallic films with different iron percentages were obtained by radio frequency (RF) magnetron

sputtering on glass and silicon substrates. The films were characterized by means of X-Ray Diffraction (XRD), Raman spectroscopy, UV-vis spectroscopy, High Resolution Scanning Transmission Electron Microscopy (HRSTEM), Ellipsometry and X-ray Photo-electron Spectroscopy (XPS). The results show that these films present mainly two nanometric phases, namely Fe2O3 and TiO2. A phase separation was observed; the film surface was found to be iron rich oxide or clusters of iron rich oxide (Fe

O

, x > y), whereas titanium accumulated deeply in the bulk, forming TiO

far below the film surface. A red shift of optical absorption as well as a relatively stable high refractive index (varying between 3.1 and 3.5) over a broad band of optical frequencies was observed for the films containing higher iron initial concentrations.

Keywords : Fe

O

-TiO

, Thin films, XPS, HR-STEM, phase separation, segregation.

1. Introduction

Nanocomposites of metal oxides are useful materials in energy conversion systems.

Extensive earlier experimental findings indicate that inert oxides such as titanium oxide can greatly improve the reactivity of iron oxide, for example, over multiple redox cycles [1] . The promotion or inhibition of solid state reaction in nanocomposite films, such as phase formation, phase separation, segregation, crystallization, etc., plays an important role in ultimate

performance of the films. The extraordinary interest in nanocomposite materials is mainly due to the vast range of properties that can arise from the combination of the peculiar characteristics of each component [2] . TiO

has been studied as photo catalyst as well as in many other

applications. However, its large band gap which approaches 3.2 eV, depending on the phase structure (rutile: ∼3.0 eV [3] , anatase: ∼3.4 eV [4] and brookite: ∼3.3 eV [5] ), confines its uses to only the UV region of the electromagnetic spectrum. Hence, lowering the band gap of titania below 3 eV to shift the optical response to the visible light range is a highly active area of research with attention focused on the improvement of the catalytic activities of TiO

through doping, metal loading and semiconductor mixing.

Depending on preparation methods, iron oxide can be found in several phases such as magnetite (Fe3O4), wustite (FeO) and trioxide (α-Fe

O

, β-Fe

O

, γ-Fe

O

, and ε-Fe

O

) among others. The three most common forms of iron oxides are magnetite (Fe

O

), maghemite (γ-Fe

O

), and hematite (α-Fe

O

). Formation and reactivity of iron oxide at the nanoscale are rather complicated. For instance, Both Fe

O

and FeO can be detected in 20% iron oxide– silica aerogel calcined at 500 °C [6] . Hematite, which is the main phase detected in the present

investigation, is an interesting n-type semiconductor material with a band gap of about 2.2 eV

[7] . It has been investigated for its application in environmental purification [8] as well as for

water splitting [9] despite the unfavorable position of the conduction band. For instance,

photoelectrochemical (PEC) water splitting at Au-modified nanocrystalline TiO

films have

been studied [10] as alternative materials to TiO

and the inverse opal photonic crystals (PC)

design has been applied to photocatalytic iron oxides [11] and binary oxides [12] . It has been

found that coupling a SOFC (Solid Oxide Fuel Cell) with an external reducer, using Fe-Ti-O as

an oxygen carrier is a good candidate for electricity generation [13] . Fe

O

can be considered as

a sensitizer for TiO

, since it has a flat band potential of 0.32 V versus NHE (Normal Hydrogen

Electrode). Some authors [14] have published a correlation between the enhanced photo

reactivity and higher surface activity resulting from the addition of metal oxides to TiO

. This suggests that hetero elements incorporated to TiO

may create novel binary metal oxide photo catalysts with better performance. It has been pointed out that an efficient way to use the majority of sunlight may be found in the composites of nano semiconductors such as TiO

- Fe

O

[15, 16] . Therefore, these kinds of nanocomposites should gain more attention. So far, most of the studies focus on developing Fe

O

-TiO

powder photocatalyst by various methods [17, 18, 19] such as hydrothermal and sol-gel. However, there are some drawbacks of the practical use of the powder photocatalyst [20] ; (1) separation of powder from water is difficult;

(2) the suspended powder tends to aggregate, especially, at high concentrations. Therefore, developing supported photocatalysts is of obvious practical significance. For instance, Zhang and Lei, prepared Fe

O

-TiO

coatings supported on activated carbon fiber by metal organic chemical vapor deposition [21] and Jin Kawakita et al [22] fabricated coatings of nano-sized composite of TiO

and Fe

O

by the warm spray process for photo-electric conversion function by TiO

and electron charge/discharge function by Fe

O

. In order to improve the conductivity of hematite, Ti-doped (up to 19.7% by atomic ratio) micro-nanostructured hematite films have been prepared by an in situ solid-state reaction method [23] . Mechanical milling has been used to prepare nanometric iron-titanium oxides for anodic materials in lithium cells [24].

On the other hand, preparation and nanostructuring of mixed oxide thin films where two single oxides of different optical properties are mixed together [25, 26] can be considered as one of the common approaches for tailoring the photonic properties of the materials, such as refractive index and frequency-selective light trapping and absorption. Knowledge of phase and element distribution as well as evolution of the nanoscale morphology of the Fe–Ti thin films during oxidation is thus of considerable importance. The present work is devoted to the

structural and optical characterization of thin films prepared by RF magnetron co-sputtering Fe and Ti precursor metallic films on glass and Si substrates and subsequent oxidation at relatively low temperature and under air atmosphere. The surface of these films was found to be

composed of iron rich nanospheres with 10 nm diameter or less.

2. Material and Methods

Films that are mixtures of metallic iron and titanium were deposited on glass substrates by

radio frequency magnetron sputtering of pure Ti (99.8 %) target with Fe (99.5%) in the form of

circular sheets superimposed on the Ti target (Fig. 1). The ratio of the Fe sputtered area to the

total sputtered area of Ti was changed, in order to obtain various composites. It was possible to change the concentration of Fe in Ti-Fe thin films by changing the relative area covered with Fe sheets. The base pressure of the system was 3.10-3 Pa and the sputtering Ar gas (99.99) pressure was at 5.10-1 Pa. The RF power was fixed at 10 W/cm2 and the substrate temperature in this case did not exceed 100 °C. After sputtering, the metallic (mixture of Ti and Fe) films were oxidized in air under atmospheric pressure at 520 °C during 8 hours.

Ti

Fe

Fig. 1. Schematic of the Ti target (yellow) on which Fe discs (red) are superimposed for the deposition of Ti-Fe mixture by magnetron sputtering

The preparation conditions and sample designations are summarized in Table 1.

Table1 . Sample preparation conditions Sample

designation Iron percentage in term of sputtered area ratio

(S

/S

) (%)

Deposition time (min) Oxidation time (h) at 520 °C

S

S

S

10 17 22

3 3 3

8 8 8

The oxidized films were characterized by XRD (Siemens D8000) using Cu Kα radiation at

normal incidence in the 2θ range of 20-80° with 40 keV, 35 mA. Raman spectra were recorded

with a Bruker Senterra Raman spectrometer equipped with a laser source radiating at

wavelength = 532 nm and power = 10 mW. X- ray photoelectron spectroscopy (XPS) depth profiling was carried out on a Kratos Nova system using Al Kα (1486.6 eV) radiation. Both elemental and topographical scanning transmission electron microscopy images were taken in a FEI STEM of type Talos F200A. Focused Ion Beam (FIB) was used to prepare thin cross section of the film for TEM analysis and UV-vis spectrometry (Shumadzu UV 3101 PC spectrometer) was used to record the optical absorbance spectra. Ellipsomertry was performed using a Jobin Yvon UVISEL NIR Spectroscopic Phase Modulated Ellipsometer with an

automatic goniometer. The ellipsometric measurements were collected at an angle of incidence of 70° across the spectral range 190 - 830 nm. It should be noted that the analysis results of samples S2 and S3 are not very different. So the exposed results will be focused on S1 and S3 except where is useful.

3. Results and Discussion

3.1. XRD and Raman spectroscopy

Fig. 2 shows the XRD patterns of the films that were deposited on glass substrates. Both

anatase (a) which is characterized by the most intense diffraction peak at 25.5° corresponding

to the (101) planes and rutile (r) which is characterized by the (110) reflection at 27.6° are

visible in the explored region of XRD. It can be seen that no inter metallic FeTi is detected in

these films. The other reflections can be assigned to α-Fe

O

and the intensity of these

reflections increased with the increase of the Fe content.

20 40 60 80 0

86 172 258

Inte nsity (Arb. Unit)

2 ^ (degree)

S3 S2 S1

r h

h h

h ah

h h h h

Fig. 2 . XRD patterns of the prepared films, h: hematite, a: anatase, r: rutile

The formation of these phases (anatase, rutile and hematite) is confirmed by Raman spectra displayed in Fig. 3 . Many peaks of the Raman spectra, as shown for samples S

and S

, matches that of typical hematite (α-Fe

O

), rutile and anatase. Factor group analysis suggests 7 Raman-active vibrational modes (2A1g + 5Eg) for hematite corundum structure, situated at 226, 245, 292, 301, 411, 497, 613 cm

[27] . Also, the Raman lines appearing at 225, 245, 291, 410, 495, and 611 cm

are characteristic of α-Fe

O

[28] . All of them are clearly observed in the spectra of samples S

, S

(not shown) and S

, revealing the presence of α-Fe

O

. Raman modes bands for rutile are reported around 144 - 147, 238 - 240, 447- 448, and 610 - 612 cm

[29, 30] or 143, 450, 612, 826 [31] while for anatase are found around 144, 394, 514, and 636 cm

[32] or at 146, 197, 400, 516, 520, and 641 cm

[33] . The peaks at 267 and 520 cm

(for sample S

) originate from the glass substrate since these peaks disappear when the film

thickness increases (S

). The increased intensity of the hematite Raman peaks is related to the

increase of iron concentration in the precursor films. The intensities of Raman bands related to

TiO

are very weak compared to that of iron oxide because the titanium oxide is deeply buried

in the film bulk, contrary to the iron oxide which exists near the surface as will be revealed in

sections 3.2 and 3.3 below.

0 200 400 600 800 1000

Int ens ity (Arb. Unit)

Raman shift (cm-1)

S

S

148229 243

267

292 411

442

609

495 609 441 291 408

243 224

144 116

r r,a

h,r

h h

h r

h h 520

Fig.3. Raman spectra of the films S1 and S3; h, a, r are for hematite, anatase and rutile, respectively.

3.2. XPS analysis

The XPS depth profiling of the prepared films is shown in Fig. 4 . It can be seen that iron

diffuses during annealing upward to the film surfaces where it accumulates either as metallic or

metal rich oxide, while titanium and oxygen together follow the same profile and accumulate in

the bulk, indicating a phase separation. Apparently, the segregation of pure metallic iron or

metal rich oxide beneath the film surface is accompanied by titanium inward diffusion to the

bulk. Iron outward diffusion seems to be faster than the oxygen inward diffusion as can be seen

from the XPS profile shapes. This may be due to the significant size difference between either

Fe(II) or Fe(III) which are 0.74 Å and 0.64 Å, respectively and O anion (1.4 Å) [34] . The

outward Fe diffusion and migration proceed, probably along the grain boundaries between

nanoparticles. The Formation of titanium oxide in the bulk far below the surface can be

evidenced by the oxygen and titanium profile behaviors which at certain depths adopt atomic

concentration corresponding almost to TiO

( Fig. 4 ). It should be noted that the binding energy

of Ti 2p

agrees well with the 2p binding energy of Ti

ions (459-458.5 eV) [23] for the

sample S

. However, a shift of about 0.8 eV towards lower energies for the sample S

( Fig. 5a ) is observed, which may be attributed to the existence of reduced titanium oxide or metallic Ti traces in the vicinity of the surface (more probably at about 10 nm, since the measurements were taken on the non eroded freshly prepared surface). Fe 2p, however, remains at the same position and increases only slightly in intensity ( Fig. 5b ) when the iron concentration increases.

The binding energy of Fe 2p3/2 is located at 709.9 eV, which can be compared to those of metallic Fe (706.8 eV), FeO (709.5 eV), Fe

O

(710.4 eV), and Fe

O

(710.6 eV). [35] .The presence of a satellite peak at about 719 eV indicates that the iron species are not associated with Fe

O

[36] , excluding its formation under the present conditions of preparation.

0 5000 10000 15000 20000 25000

0 10 20 30 40 50 60 70 80

O 1s C 1s Ti 2p Fe 2p Si 2p

Atomic percentage [ %]

etch time [s]

S

0 5000 10000 15000 20000 25000

0 10 20 30 40 50 60 70 80

O 1s C 1s Ti 2p Fe 2p Si 2p

Atomic percentage [ %]

etch time [s]

S

0 5000 10000 15000 20000 25000 0

10 20 30 40 50 60 70 80

O 1s C 1s Ti 2p Fe 2p Si 2p

Atomic percentage [ %]

etch time [s]

S

Fig. 4 . XPS depth profiles of O, C, Ti, Fe and Si elements for the films S

, S

and S

From this comparison, one can suggest the formation of iron oxide just at the top of the surface where the oxygen concentration is relatively high as is shown, for sample S

, in Fig. 4 , whereas iron rich clusters precipitate at the subsurface in the form of spherical nanoparticles as can be seen from TEM analysis below. In fact, examining the atomic concentrations (Fig. 4 ), one can see that the oxygen atomic concentration at a certain depth near the surface can approach almost half of iron, indicating the formation of iron rich oxide which, according to this observation, should be in the form of Fe

O or Fe

O where x > 1. The formation and properties of such unusual oxide which results from the interaction of atomic oxygen with iron clusters has been, theoretically and experimentally, studied for small iron clusters [37] .

It should be mentioned that the results of Raman and XPS analysis do not give the same phases for iron oxide phases because hematite is mainly located in the bulk while the iron rich phase is located at the surface as a very thin layer. This can be understood since, contrary to Raman spectroscopy and XRD, the XPS is a very sensitive tool only for surface analysis and the iron rich surface layer is too small to be detected by XRD and Raman analysis. These later two techniques indicate the presence of only the hematite phase which excludes any

transformation under Raman laser heating during analysis.

450 455 460 465 470 0

1 2 3

Intensi ty (k count)

Binding energy (eV)

S

on Si S

on Si Ti 2p

458.7

457.9 (a)

705 710 715 720 725 730 735 0

2 4 6 8 10 12

Inten sity (k coun t)

Binding energy (eV)

S

on Si S

on Si Fe 2p

(b)

709.9

Fig. 5. XPS spectra related to Ti 2p (a) and Fe 2p (b) core level binding energies in the samples S1 and S3, measured on the as prepared surfaces.

3.3. TEM analysis

Fig. 6 shows the HAADF STEM cross section images of the sample S

. The images show a

contrast between the volume and the surface of the film where spherical nanoparticles, with

diameters mainly ranging between 3 and10 nm, can be distinguished and where the bulk seems to be highly porous compared to the surface region. From these images, the thickness of this film can be estimated to be in the order of 140 nm in accordance with ellipsometry

measurement (see section 3.5 below). The accumulated spherical nanoparticles at the film surface are identified to be iron rich as can be seen from EDX elemental mapping shown in Fig. 7 (in agreement with the XPS depth profiling results). It should be noted that the EDX oxygen mapping shows clearly a distribution with oxygen poor zones corresponding to the spherical nanoparticle locations. Therefore, these particles should represent an iron riche phase.

What may prohibit the rapid oxidation of iron in the present mixture is, probably, the presence of titanium which would pump oxygen to the detriment of iron, because of the much higher affinity of titanium to oxygen.

Titanium oxide accumulates far below the surface where iron oxide becomes rare. The digital line profiles, shown in Fig. 8 , confirm the iron and titanium compound separation in the surface region of the films in accordance with the XPS analysis. The phase separation extends along 20 nm below the surface. A possible explanation of this phenomenon may be as follows:

At the beginning, the oxygen should diffuse from the surface to the bulk and react

preferentially with titanium because the affinity of titanium to oxygen is much higher than that

of iron. Consequently, iron could easily diffuse to the surface where it can segregate or slowly

react with oxygen that is continuously pumped by titanium. Moreover, the accumulation of iron

ions beneath the surface may produce an internal electric field which could sweep titanium ions

inward. Some of the iron rich spherical nanoparticles can be found at depths greater than 20

nm. It has been shown that not only the diffusion of Fe ions was found to be faster than Ti ions,

during Fe-Ti oxidation at 700 °C, but the outward diffusion of Fe ions is more favorable than Ti

ions [1] , which is in accord with the present results.

Fig. 6 . HAADF STEM cross section image of the S3 film

HAADF Fe K O K Ti K

40nm

40nm 40nm 40nm

Fig. 7. HAADF image and EDX mapping showing Fe, O and Ti distribution across the S3 film thickness. Iron, in a spherical form, is segregated at the surface while titanium is segregated deeply in the film volume.

Fig. 8 . EDX analysis of the S3 film, showing quantified (atomic %) digital line profile which

starts from the bulk to the surface of the film along the shown arrow on the inset image.

The observed iron rich nanoparticles were further analyzed using HRTEM which reveals that the biggest particles (about 10 nm diameter) (Fig. 9a), actually, are formed of smaller ones with different orientations as can be seen in Fig. 9b and c . It should be noted that the measured inter reticular distances (Fig. 9b) do not correspond to any of the known iron oxides (FeO, Fe

O

, Fe

O

) which confirms that the observed iron rich nanoparticles could be in the form of Fe

O

where x > y.

From the above analysis, an illustration of the formation of these nanoparticles and the phase separation can be sketched as shown in Fig. 10 . Because the iron and titanium profiles,

determined by XPS and EDX, show considerable gradients in phase distribution and separation beside the formation of monodisperse iron rich nanoparticles, the observed process, although complicated, may be mainly diffusion controlled

Fig. 9. HR TEM image of the film surface and selected area diffraction of an iron rich

nanoparticle composed of three clusters (Fe

O

) that are located in the subsurface of the S3

film.

ACCEPTED MANUSCRIPT

Fig. 10 . Illustration of Fe-Ti composite oxidation and phase separation under heating at 520 °C for 8h.

3.4. Optical properties

The UV-Vis absorbance spectra are presented in Fig. 11 . As expected, pure TiO

almost has no absorption above its absorption edge rising at about 400 nm. In contrast, the Fe

O

-TiO

films show a strong absorption in the visible range which increases with iron oxide increase.

The enhanced absorption in the visible region can be attributed not only to the increase in the film thicknesses but also to the photosensitization of Fe

O

[21]. The red shift of the absorption peak as well as the appearance of shoulders at the peaks with increasing iron content may result from the contribution of Fe

O

to the total absorption as well as from interference effects. The high visibility interference fringes for S

, S

and S

indicate that the films are smooth and relatively dense.

300 400 500 600 700 800

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Absorb an ce (Arb . Un it)

Wavelenght (nm)

TiO₂ S₁ S₂

S₃

Fig. 11 . Absorbance spectra of S

, S

and S

with pure TiO

film.

It is apparent in these curves that as the amount of Fe in the films increases the amplitude of the oscillations observed in the absorbance curves increases. This behavior seems to be

congruent with an increase in the refractive index (n) of the films. Simultaneously, the position

of the absorption edge (Eg) shifts to lower energy values, changing from the value of TiO

(about 3.2 eV) to that of Fe

O

(about 2.2 eV). A simple estimate for the optical gap can be given by assuming that its value is near the position of the absorption peak maximum.

3.5. Ellipsometry

The nanocomposites feature electromagnetic properties different from those of the constituting materials. They can display high refractive index over a wide range of

wavelengths. Higher refractive index (n) is often achieved by increasing polarizability and/or increasing density. In Fig. 12 , the dependence of n and k (extinction coefficient) on wavelength for S

and S

films is presented. The refractive index of all films is fairly high but the refractive index of the S

film is the highest. The value of the refractive index n increases up to about 3.5 at 550 nm as the amount of Fe

O

in the films increases. Thereafter, it decreases, slightly, to about 3 and remains constant at higher wavelengths. Above 550 nm, both the refractive index and the coefficient of extinction become constant indicating very low dispersion. Here, the interesting result is that the extinction coefficient rapidly drops to a very low value just above 550 nm, and then remains constant over a large interval of wavelength, while the refractive index also remains constant but at exceptional high value. It should be noted that the average n values of rutile and anatase single crystals are 2.75 and 2.52, respectively [38] . Simultaneously, the optical gap defined by the edge of the extinction coefficient k shifts towards longer

wavelengths as the amount of Fe

O

in the films increases. The sharp drop in extinction

coefficient indicates the onset of optical absorption in these films which can be estimated to be

about 3.2 eV for the sample S1 and 2.1 eV for S

and also for S

(not shown). Finally, the

thickness and roughness, deduced from ellipsometry measurement are given in table 2. In

particular, the thickness of S

film is comparable with the value deduced from TEM imaging

(about 140 nm). From these observations one could, effectively, control the optical properties

by controlling the film composition, as can be seen from both ellipsometry and UV-vis

spectroscopy, where these properties, considerably, depend on composition.

200 300 400 500 600 700 800 1.8

2.0 2.2 2.4 2.6 2.8 3.0

Wavelength (nm)

n

0.0 0.2 0.4 0.6 0.8 1.0

k

S

200 300 400 500 600 700 800

1.0 1.5 2.0 2.5 3.0 3.5 4.0

Wavelength (nm)

n

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

k

S

Fig. 12 . Refractive indexes and extinction coefficients as function of wavelength for S1 and S3 films.

Table 2: Thickness and roughness of the studied films

Sample Thickness (nm) Roughness (nm)

S

100.5 12.6

S

121.8 negligible

S

134.2 negligible

Conclusions

Thin films of iron oxide and titanium oxide prepared by sputtered metallic iron and titanium mixture and subsequent oxidation were thoroughly characterized using state of the art analysis instruments. Beside TiO

/Fe

O

formation, segregation which leads to phase separation in Fe- Ti composite thin films was observed after annealing under air at 520 °C for 8h. Unusual partially oxidized iron or iron oxide Fe

O

(x > y), in the form of spherical nanoparticles, was found to segregate at the film subsurface while titanium segregates deeply in the bulk and forms TiO

. The refractive index of the prepared films can be exceptionally high and the optical absorption could be shifted towards the visible light region. It is foreseen that these results and the preparation method will open new avenues to engineer the structure, optical and electrical properties of metal oxide mixtures, by surface composition modification.

Acknowledgements

The authors are grateful to Dr. Christoph Mitterbauer from FEI Company BV, Eindhoven, The Netherlands, for his valuable help in STEM analysis and M. Kloul from Horiba Jobin Yvon (France) for his help in the ellipsometry measurement. The XPS analysis was done at IMN, Université de Nantes, CNRS 44322 Nantes Cedex 3, France.

Conflict of interest: The authors declare that this research is not for profit sector and there is

no conflict of interest.

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