Article pubs.acs.org/JPCC
Influence of Titanium Surface Treatment on Adsorption of Primary Amines ́ Carole Gadois, Jolanta Swiatowska,* Sandrine Zanna, and Philippe Marcus Laboratoire de Physico-Chimie des Surfaces, Chimie ParisTech, 11 rue Pierre et Marie Curie, 75005 Paris, France CNRS (UMR 7045), 11 rue Pierre et Marie Curie, 75005 Paris, France
ABSTRACT: The influence of acid−base properties of titanium (T35) on the adsorption of model amine molecules as a function of surface treatments, necessary for modification of surface morphology and its chemical composition, has been investigated by X-ray photoelectron spectroscopy. A significant increase in the surface energy measured by contact-angle measurements has been observed after surface treatment (thermal, chemical treatment in alkali electrolyte, and electrochemical treatment: anodization) compared with the pristine sample (+75% on average). An important modification of surface morphology was observed by scanning electron microscopy ) after the electrochemical (anodization) and chemical treatment of Ti resulting in formation of TiO2 nanotubes and porous TiO2 surface, respectively. Two amine molecules have been chosen to study the adsorption in a function of Ti surface treatment: propylamine (PPA) and 1,2-diaminoethane (DAE). The increased intensities of N1s and C1s core level peaks indicate a stronger adsorption of DAE than PPA molecule on all type of titanium surfaces. The layer thickness of amines ranged between 0.6 and 0.8 nm and 0.9 and 1.5 nm for PPA and DAE molecule, respectively. It has been shown that higher hydroxyl fraction results in the formation of thicker amine layer. The surface hydroxylation also had an influence on the Brønsted/Lewis distribution. A higher Brønsted interaction was observed with higher surface hydroxylation. For both amines and for all treated surfaces, adsorption occurred in majority via a Lewis-like interaction.
1. INTRODUCTION Since the 1950s, defense and aerospace industries have used titanium alloys for many different applications such as rockets, spacecrafts, gas tanks for satellites, control pressure vessels, and so on.1−3 The high performance of titanium alloys can be explained by their exceptional strength-to-weight ratio, high corrosion resistance, and operation reliability at elevated temperatures. In different assemblies, titanium sheets are bonded together or with composite materials with adhesives rather than by welding or riveting, which allows lighter assemblies and a better stress distribution. This process is less expensive, improves safety, allows time for savings, and is easy to manufacture.4 Strong adhesive bonding between titanium and polymeric composites is thus required for different types of assemblies in industries including the aerospace industry. To enhance the strength of the bonding and to ensure a good polymer/titanium assembly, numerous surface treatments of titanium substrates have been envisaged, such as anodization with chromic acid, abrasion and grit blasting, solvent cleaning, acid etching, and plasma treatment.5,6 © XXXX American Chemical Society
Many of the polymers used as adhesives or anticorrosion coatings are mixtures of epoxy prepolymers such as diglycidilether of bisphenol A (DGEBA) and primary amines used as hardener.7−9 Usually, the application of adhesives on the titanium surface, which is naturally covered by a thin native oxide layer, is followed by curing at elevated temperatures (at around 100−120 °C10,11) necessary for polymerization, which ensures a good adhesion. The liquid epoxy-amine (adhesive)/ metal interphase has a significant influence on the formation of good bonding and the chemical and physical properties of this interphase are different from those of the substrate or adhesive.10,12−18 Despite the crucial importance of the interface, the mechanisms of the interface phenomena are not completely clear. Extensive studies have been carried out on metal/cured epoxy-amine mixtures interphase, and several mechanisms concerning the polymer adhesion on metallic substrate are Received: July 9, 2012 Revised: December 5, 2012
A
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can be used to probe the acid−base nature of oxidized metal surfaces. According to Fowkes,31 the acid−base properties of solid surfaces are of critical importance in adhesion properties of polymer-coated metals. In addition, DAE or PPA can be used to mimic the amine curing agent in epoxy resins.
suggested in literature. For example, hydroxyl groups, which are always present on metal alloy surfaces due to the exposure to ambient atmosphere, can play a significant role as a catalyst during the curing process and increase the degree of cross-linking in the epoxy-amines network19 on the surface. At the interphase, the matrix resin could be overcured with a higher than normal cross-linking density, and the latter might still contain free amine groups.20 The amine−metal chemical bonds are considered to be responsible for the adhesion of polymer to metal surfaces.17 For diamines, molecular adsorption onto the oxidized metal surfaces leading to the formation of a bidentate chelate is often observed, but several mechanisms are suggested in the literature.18,20−22 It has been shown that amines can interact with the acidic surfaces sites of the metals in two different ways: via a Lewis-like interaction (lonely pair of electrons of the nitrogen is given to an empty orbital of titanium)9,23−26 or via a Brønsted-like interaction (protonation of the amine by surface hydroxyl groups).19,25,27,28 Marsh et al.23 showed that 1,2-diaminoethane produced both Lewis and Brønsted acid interactions with acidic surfaces sites of anodized titanium. Debontridder et al.22 proposed a mechanism of interaction between the lone pair of the amine termination and an aluminum cation of the oxide layer (formed on the aluminum surface) leading to the formation of a chelated surface complex. Wielant et al.21 showed that modification of the hydroxyl fraction on iron oxides has an influence on the protonation level of adsorbed nitrogen molecules. A more hydroxylated surface present on iron oxide surfaces promoted bonding of the nitrogen species via a Brønsted-like interaction. Farfan-Arribas et al.26 proposed that amine (ammonia, dimethylamine, and ethylamine) adsorption takes place on stoichiometric and slightly defective TiO2-(110) surfaces by the binding of the nitrogen atom in the amine to a Ti4+ cation. The authors also observed that the coverage by amine layer was smaller on the defective surface than on the stoichiometric surface. The aim of this work was to examine the influence of the surface chemical composition (namely the oxide and hydroxyl fraction) and a surface morphology of titanium substrate on adsorption of probe amine molecules. The chemical and morphological modifications of titanium thin foils of T35 have been obtained by chemical, electrochemical, and thermal treatments. Increased surface roughness, and thus the surface specific area, can enhance the bonding between adhesive and metallic substrate. The effect of increasing surface energy in the adhesive bonding of titanium is not well understood, but in general it is considered that the reactivity and thus the adhesion properties are improved if the surface free energy (SFE) is raised29 (e.g., by surface treatment) and if the wettability is increased. Surface and interfacial free energy determination are widely used to characterize and to predict the adhesive properties of metallic surfaces, especially for polymeric materials.30 Two types of probe amine molecules have been adsorbed on the differently treated titanium surfaces, and the extent of the acid−base interaction type (Lewis or Brønsted) with the surface state (topography) has been investigated. Propylamine (PPA: NH2−CH2−CH2−CH3) is a monoamine, whereas 1,2-diaminoethane (DAE: NH2−CH2−CH2−NH2) contains two amine groups. DAE is considered to be a typical chelating agent in coordination chemistry27 and is also a strong Brønsted (protonic) base and also a strong Lewis (complexing) base.23 Mercier et al.20 showed that on aluminum surfaces, only DAE could lead to a partial dissolution of the oxide. The use of these two similar molecules can give a comparison of the effect of an additional amino group on titanium surfaces. Both molecules
2. EXPERIMENTAL SECTION a. Sample Preparation. Titanium foil (T35, 99.6% purity) with a thickness of 0.3 mm was used for all experiments. Prior to further treatment, each specimen was degreased in ultrasonic bath for 5 min in acetone, ethanol, and ultra pure (UP) Millipore water (resistivity >18 MΩ cm). The thermal oxide film was formed by thermal oxidation of a pristine Ti substrate in air at 500 °C for 5 h. Chemical treatment was performed by immersion of the pristine Ti sample in an alkaline aqueous solution of 5 M NaOH (Analar Normapur reagent grade chemicals VWR BDH Prolabo) at 60 °C for 5 h and then rinsed with ultra pure water. Electrochemical treatment (anodization) was carried out in a two-electrode Teflon cell (with a large platinum grid used as a counter electrode surrounding the titanium sample as a working electrode (T35 thin foil) in a 1 M H3PO4/0.3 wt % HF electrolyte (Analar Normapur reagent grade chemicals VWR BDH Prolabo) at a constant cell voltage of 20 V (provided by a Lambda LLS6060 power supply) for 2 h at room temperature. After the anodization, the samples were thoroughly rinsed in UP water. b. Adsorption of Amines. The adsorption of amine was performed by immersion of titanium sample (pristine Ti sample and Ti after different treatments) during 1 hour in a p-xylene solvent (99.9%) containing 10 wt % of either DAE (99.5%) or PPA (99.5%). All reagents were purchased from Fluka and used as received. After immersion, each sample was rinsed with 50 mL of p-xylene to remove the unadsorbed molecules, dried in air, and introduced directly to the XPS analysis chamber to avoid further contamination. The choice of the p-xylene is justified by a lack of chemical affinity of this solvent to the amine model molecules and the metal/metallic oxide substrate. No visual changes were observed on the sample surface after amine adsorption. c. Surface Characterization. Scanning Electron Microscopy Analysis. For a topographic characterization of the samples before and after treatment, a field-emission scanning electron microscope (FEG-SEM) Gemini LEO 1530 was used. Beforehand samples were coated with a thin layer of graphite to enhance the conductivity of the TiO2 (which is a semiconductor). Contact-Angle Measurements. The contact-angle measurement was used to calculate the SFE using the Owens−Wendt32 method (based on the Fowkes theory), where the surface tension is considered to be a combination of dispersion forces (van der Waals forces) and polar forces (hydrogen bonding): γL(1 + cos θ ) = 2 γLdγSd + 2 γLpγLp
(1)
with γdL and γdS ascribed to the dispersion components of the liquid and the solid (γL = γdL + γpL), respectively, and γpL and γpS ascribed to the polar components of the liquid and the solid (γS = γdS + γpS), respectively.31 Because of the presence of the polar term, the measurements of contact angles on the titanium surfaces with different pretreatments were performed with two different liquids (of known surface tension) with very different values of dispersive and polar components (see Table 1): deionized (DI) water (high polar component) and diiodomethane (high dispersion component). B
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contributions related to contamination (C−O species) from the O1s peak. An assumption on the homogeneous lateral distribution of the contamination and oxide layers was necessary for this calculation. On the basis of basic XPS principles and assuming that the inelastic mean free path (IMFP) of a photoelectron emitted from oxygen in the oxide layer is about the same than Cont the IMFP in the contamination layer (γOx ≈ γO), the O ≈ γO following equations can be written:
Table 1. Surface Free Energy Parameters of the Test: Liquid Free Energy (γL), Dispersive Component (γdL), and Polar Component (γpL) liquid
γL
γdL
γpL
water diiodomethane
72.8 50.8
21.8 50.8
51.0 0
To measure the contact angle (θ) with water or diiodomethane, we deposited an 8 μL droplet from a syringe of either DI water or diiodomethane on the sample surface using an automatic deposition system. The contact angle (θ) was calculated for both sides from the droplet shape using the software program Visiodrop. Five measurements for each probe liquid on each type of samples have been performed, and an average value was taken, with standard deviations within 5% of the mean value. X-ray Photoelectron Spectroscopy Analysis. X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical composition of the sample surfaces (before and after amine adsorption) using an ESCALAB 250 spectrometer with an Al Kα monochromated radiation (hν = 1486.6 eV). The pressure in the analysis chamber was below 10−9 mBar. The survey spectra were recorded with a pass energy of 100 eV; the highresolution spectra (C 1s, O 1s, N 1s, Ti 2p) were recorded with a pass energy of 20 eV and a resolution of 0.1 eV. All spectra were calibrated versus binding energy (BE) of hydrocarbons (C1s at 285.0 eV). Spectra were recorded and analyzed using the ThermoAvantage software. For curve fitting and decomposition, a Shirley-type background subtraction was used and the shape of fitting curves was determined by a 70% Gaussian/ 30% Lorentzian distribution, typical for the spectra fitting for semiconductors.33,34
IC(C − O) IO(C − O) + IO(OH−) =
⎛ −dcont ⎞ ⎟ σCλCcC − O⎜1 − exp λ sin θ ⎠ ⎝ c ⎛⎛ ⎞⎟ −d σOλOcC − O⎜⎜1 − exp λ cont + σOλOcOHexp O sin θ ⎠ ⎝⎝
(
(
)
)
(
= I1
−dcont ⎞ ⎟ λO sin θ ⎠
)
(2)
IC(C − O) 2−
IO(O )
(
(
σCλCcC − O 1 − exp =
(
σOλOcO2−exp
−dcont λc sin θ
−dcont λO sin θ
)
)) = I
2
(3)
where IX(Y) indicates the intensity of the X peak (element X) corresponding to Y species (in brackets); for example, IC(C−O) is the intensity of the C1s peak corresponding to C−O species; σC and σO are the cross sections of the C1s and O1s levels; γC and γO are the IMFPs for C and O in the contamination layer and in the metallic oxide, and CC−O, COH, and CO2− are the atomic concentrations for the indicated species. In eq 2, the denominator corresponds to the intensity of the O1s peak at 531.6 eV, which contains contributions from OH− and C−O, whereas in eq 3, the denominator corresponds to the intensity of titanium oxide contribution at ∼530 eV. The left part of eqs 2 and 3 is the experimental ratio, derived from XPS deconvolution of C1s and O1s peaks. The right part is the calculated value using theoretical values for cross section and IMFPs. In Table 2, the values of cross sections σ and IMFPs γ
3. QUANTITATIVE ANALYSIS OF THE XPS DATA a. Determination of the Hydroxyl Fraction before Amine Deposit. The hydroxyl fraction in an oxide layer was determined from the XPS data using the O 1s and C 1s core level peaks. The surface of each Ti sample is covered by an oxide (with dox thickness) and a contamination layer (with dcont thickness) regardless of sample treatments. The measured O1s signal contains principal contributions: from the oxide matrix (titanium oxide), surface hydroxyl (hydroxides), and organic contaminants (i.e., C−O species). Each O1s photopeak has been fitted with a constant fwhm of 1.4 eV. The oxygen functional groups giving an O1s peak at a BE of around ∼531.6 eV correspond to the presence of contamination (such as C−O species) and to hydroxides (OH− species).35 Hence, for reliable calculations of hydroxyl fractions, it is necessary to subtract the
Figure 1. Schematic representation of the multilayer system: oxideand amine-covered titanium surface.
Table 2. Values of Scofield Cross Sections, Inelastic Mean Free Path (IMFP), Bulk Concentrations, and Transmission Factors of Each Element Used for Thickness Calculation of Titanium Oxide Layer (For Pristine Sample), Contamination Layer (All Samples), and Adsorbed Amine Layer (All Samples)21,46 elements photoionization cross-section IMPF (nm) bulk concentrations (mol/cm3)
C 1s
O 1s
Ti 2p3/2
N1s
σC 0.157 γC 2.30
σO 0.463 γL 1.10
σTi 5.220
σN 1.800
λTi Ti 2.19 DTi Ti 0.094
TiO
λTi 2 2.12 TiO DTi 2 0.049
λDAE Ti 3.22
TTi 3319
transmission factors
C
λPPA Ti 2.91
λDAE N 3.36 DDAE N 0.030
λPPA N 3.04 DPPA N 0.012 TN 3233
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Figure 2. SEM images (top view) of pristine Ti sample (a), thermally grown oxide sample at 500 °C in air, 5 h (b), chemically treated sample in NaOH 5M, 60 °C for 5 h (c), and anodized T35 in H3PO4 1M/0.3% HF, 20 V for 2 h resulting in the formation of nanotubes (d).
used for calculation are given. After combining and rearranging eqs 2 and 3, the following expression for CO2−/COH− can be obtained: c O2 − cOH−
⎛ ⎞⎟ −d σCλC⎜1 − exp λ cont ⎝ I O sin θ ⎠ = 1 ⎛ ⎛ ⎞⎟ I2 −d σCλC⎜1 − exp λ cont − I1σOλO⎜1 − exp ⎝ O sin θ ⎠ ⎝
(
(
Table 3. Contact Angles and Surface Free Energy on Titanium Samples before and after Treatment
)
)
(
−dcont ⎞ ⎟ λO sin θ ⎠
)
(4)
From this equation, the hydroxyl fraction can be calculated easily: 1
OH− fraction (%) = 1+
( ) c O2 −
cOH−
sample
θ (water) (deg)
θ (diiodomethane) (deg)
SFE (mJ/m2)
pristine electrochemical treatment thermal treatment chemical treatment
84 11 16 6
37 24 12 15
41.2 72.6 72.4 74.2
layers (amine and oxide layers) having a constant composition and thickness are homogeneously distributed across the XPS analysis area. These assumptions are necessary for estimation of the thickness of the adsorbed amine layer on sample surface. In a pristine Ti sample (without any treatment), the thicknesses of native oxide layer and amine layer can be calculated by solving the following set of three equations:
× 100 (5)
First, an estimation of the contamination layer thickness dcont was done by minimizing the error between the calculated ratios in eqs 2 and 3 and the experimental data obtained after deconvolution of XPS spectra. The unknown concentrations of C−O, O2−, and OH− were assumed to be between 0 and 10%, 40 and 70%, and 5 and 50%, respectively, for all samples. Atomic percentages obtained after deconvolution gave a good estimation for the iterative calculation. Iteration was stopped when the error was less than 10−4. The contamination layer was assumed to vary between 1 and 2 nm. Then, the hydroxyl fraction was evaluated from eqs 4 and 5 by using the corresponding values for the contamination layer thickness and peak intensities. b. Determination of Amine Thickness after Adsorption. The adsorbed amine layer on the Ti substrate (before and after surface treatments) can be schematically presented as in the Figure 1. To simplify the model, it was assumed that the contamination layer was dissolved by the immersion of the Ti sample in amine-solvent solution. It was also assumed that
⎛ ⎞ ⎛ dox damine ⎞ Ti ⎟·exp⎜− amine = K ·σTi·λ TiTi ·DTi ITi ⎟ Ti · TTi · exp⎜ − TiO2 ⎝ λ Ti ·sin θ ⎠ ⎝ λ Ti ·sin θ ⎠ (6)
⎛ ⎛ ⎞⎞ d TiO2 TiO2 ⎟⎟⎟ · ITi = K ·σTi·λ TiTiO2 ·DTi ·TTi·⎜⎜1 − exp⎜ − TiO ox ⎝ λ Ti 2 ·sin θ ⎠⎠ ⎝ ⎛ damine ⎞ exp⎜ − amine ⎟ ⎝ λ Ti ·sin θ ⎠
(7)
⎛ ⎛ damine ⎞⎞ INNH2 = K ·σN·λNamine ·D Namine ·TN·⎜⎜1 − exp⎜ − amine ⎟⎟⎟ ⎝ λN ·sin θ ⎠⎠ ⎝ (8) D
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Figure 3. XPS survey spectra for the nontreated Ti, the thermally grown oxide (500 °C in air), the chemically treated surface (NaOH 5M), and the anodized surface (nanotubes).
where I is the intensity of a given element, K is an instrumental factor, σX is the cross section for photoionisation of atom X, γYX are the IMPFs for atom X in the matrix Y, θ is the takeoff angle, DYX are the bulk concentrations in terms of number of moles of atoms X/cm3 in the Y matrix, and TX are the transmission factors for element X given by the instrument supplier. As the takeoff angle was 90°, sin θ = 1. Values used for the calculation are given in Table 2. The values for the different γYX were assessed with the TPP2M formula.36 The “amine” term indicates either DAE or PPA. dox and damine are the layer thicknesses of native titanium oxide and amine layers, respectively. The ratio of eqs 7 and 8 allows us to calculate the thickness of the oxide (dox):
(
d
exp − TiO2ox TiO2 λ Ti ·sin θ λ TiTiO2 DTi · Ti · Ti = TiO2 d ITi λ Ti DTi 1 − exp − TiO2ox Ti ITi
(
λ Ti
)
·sin θ
)
(9)
The ratio of eqs 3 and 2 allows us to calculate the thickness of the amine layer (damine) TiO2 INamine σTi λ TiTiO2 DTi T · · · · Ti amine amine TiO2 σN λN TN DN ITi
(
1 − exp − =
damine λNNH2·sin θ
)
⎛ ⎞ dox damine ⎜1 − exp − ⎟exp − TiO2 λ TiNH2·sin θ · λ θ sin ⎝ ⎠ Ti
(
) (
)
(10)
First, estimation for the oxide layer thickness, dox, was made by minimizing the error between the calculated intensity ratio and the experimental data using eq 9. Then, estimation of the amine layer damine was made by using the dox value calculated previously and by minimizing the error between the calculated intensity ratio and the experimental data in eq 10. The iteration was stopped when the error was less than 10−4. For thick oxide layers (thermal oxide, electrochemically and chemically treated samples), the first equation is not required (as the titanium oxide formed on the Ti substrate can be considered to semiinfinitive) and eq 7 is simplified.
Figure 4. Ti 2p spectra for the untreated Ti surface (a) and after thermal treatment (b).
4. RESULTS AND DISCUSSION a. Surface Topography. Figure 2 presents the surface topography of the Ti sample before (a) and after thermal (b), chemical (c), and electrochemical treatments (d) observed by SEM. E
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Table 4. C1s, O1s, Ti2p, and N1s Binding Energies (eV), fwhm (eV), and Atomic Percentage for the Four Studied Samples (Untreated Ti Surface, Thermally Grown Oxide, Chemically Formed Oxide (NaOH), and Electrochemically Formed Oxide, Prior to and after Exposition to DAE and PPA) binding energy (eV) fwhm (eV)
Pristine Sample before adsorption DAE adsorption PPA adsorption
peak area (%)
binding energy (eV) fwhm (eV)
C1sA
C1sB
C1sC
C1sA
C1sB
C1sC
285.0 1.5 285.0 1.5 285.0 1.5
286.6 1.5 286.6 1.5 286.6 1.5
288.4 1.5 288.9 1.5 288.7 1.5
70%
17%
13%
51%
34%
15%
69%
25%
6%
286.6 1.5 286.6 1.5 286.6 1.5
289.0 1.5 288.7 1.5 288.9 1.5
66%
20%
14%
65%
20%
15%
60%
23%
16%
286.4 1.5 286.6 1.5 286.5 1.5
288.8 1.5 289.0 1.5 288.9 1.5
68%
21%
11%
67%
19%
13%
68%
18%
14%
286.6 1.5 286.7 1.5 286.6 1.5
288.5 1.5 288.9 1.5 289.0 1.5
56%
38%
6%
67%
29%
4%
65%
29%
6%
Thermal Treatment before adsorption 285.0 1.5 DAE adsorption 285.0 1.5 PPA adsorption 285.0 1.5 Chemical Treatment before adsorption 285.0 1.5 DAE adsorption 285.0 1.5 PPA adsorption 285.0 1.5 Electrochemical Treatment before adsorption 285.0 1.5 DAE adsorption 285.0 1.5 PPA adsorption 285.0 1.5
binding energy (eV) fwhm (eV)
peak area (%)
intensity ratio
Ti4+ 2p3/2
Ti3+ 2p3/2
Ti2+ 2p3/2
Ti 0 2p3/2
N1s Lewis
N1s Brønsted
N1s Lewis
N1s Brønsted
C1sB/ N1s
458.5 1.3 458.6 1.3 458.5 1.3
457.2 1.3 457.2 1.3 457.2 1.3
455.2 1.3 455.2 1.3 455.2 1.3
454.0 1.3 454.1 1.3 454.1 1.3
400.1 1.7 399.9 1.7
401.9 1.7 401.6 1.7
62%
38%
1.08
69%
31%
1.10
399.8 1.7 400.1 1.7
401.6 1.7 401.8 1.7
81%
19%
1.06
78%
22%
1.04
399.9 1.7 400.0
401.6 1.7 401.7
95%
5%
1.02
87%
13%
1.12
400.2 1.7 399.9 1.7
402.0 1.7 401.7 1.7
64%
36%
1.09
69%
31%
1.10
458.5 1.3 458.6 1.3 458.5 1.3 458.5 1.3 458.6 1.3 458.5 1.3 458.5 1.3 458.6 1.3 458.5 1.3
Values of surface free energies for all samples are reported in Table 3. A significant increase of around 75% of the SFE can be observed after chemical treatment in alkaline solution. The lower contact angles observed on treated samples indicate a better surface wettability,42 which may favor the adhesion of polymer on the surface when adhesive primers are applied. c. Chemical Composition and Hydroxyl Fraction. Figure 3 presents the XPS survey spectra of the four types of surfaces (with and without treatment) before amine deposition. The XPS spectra show principally three peaks: oxygen O 1s, carbon C 1s, and titanium Ti 2p as the main components of all samples. Almost all samples show a very weak N 1s contribution at ∼400.1 eV. The presence of adventitious nitrogen on the pristine Ti sample (without pretreatment) can originate from the organic oils used for lamination process during the fabrication of thin T35 foil. The presence of nitrogen-like contaminations on the Ti samples after pretreatments can be explained by the adsorption of some organic compounds (containing nitrogen) due to sample exposure to air. The amount of nitrogen-like compounds is negligible, and thus this contribution was not taken into account in the quantitative analysis. The oxide layers produced by thermal, chemical, or electrochemical treatments are built up by reaction of titanium with the surrounding oxygen of air, alkaline salts, and/or electrolyte (containing H3P04+HF), respectively, and thus a presence of some contaminants, such as phosphorus, sodium, or fluorine, is evidenced by the XPS data (Figure 3). On the surface of chemically treated sample,
The pristine sample (Figure 2a) shows a relatively smooth surface with no self-organized structure. For the thermal oxide prepared at 500 °C, during 5 h in air (Figure 2b), a modification of the microstructure can be observed, with the formation of small grains (as evidence of the oxide growth) and cracks on the surface. Velten et al.37 reported that thermal oxidation of Ti leads to formation of thick TiO2 layer. For a 5 h oxidation of Ti at 500 °C in air, an estimated value for the oxide thickness is ∼80 nm. The chemical treatment of Ti sample in alkaline bath (5 M NaOH) leads to the formation of a rough, sponge-like structure, with a significant porosity and a high specific area (Figure 2c). A thick, porous TiO2 layer is expected to be formed as a result of this treatment according to the previous study of Nishiguchi et al.38 Anodization in phosphate/fluoride solution results in the formation of a self-organized nanotubes structure (Figure 2d). The similar morphology was already observed after anodization of titanium sample in the phosphate−fluoride electrolyte.39−41 The SEM analysis of the anodized sample shows a perpendicular to the Ti substrate growth of titanium oxide nanotubes with an average diameter of 80 nm (and ∼8 nm thick of a tube side) and an average tube length of ∼350 nm. b. Surface Free Energy. Contact-angle measurements show that the pristine sample has a hydrophobic character (high angle with water and small angle with diiodomethane) and a relatively low SFE due to presence of a contamination layer formed during the fabrication process of the thin Ti foil (principally lamination) with application of organic oils. F
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a non-negligible amount of sodium has been observed (∼6 at %). Fluorine and phosphorus are detected on the sample after electrochemical treatment with an atomic concentration of ∼2% each. For the pristine Ti surface, the Ti 2p3/2 core level peak was decomposed into four contributions (eight, if the Ti 2p1/2 spectrum with a splitting of 5.7 ± 0.2 eV is included) located at the following BEs: 454.0 ± 0.2 eV for metallic titanium (Ti0), 455.2 ± 0.2 eV for Ti2+, 457.2 ± 0.2 eV for Ti3+, and 458.5 ± 0.2 eV for Ti4+ from TiO2 with fwhm = 1.3 eV (see Figure 4a and Table 4). The presence of Ti 2p peaks corresponding to metallic titanium and titanium oxides (Ti2+, Ti3+, and major contribution of Ti4+) confirms the formation of a thin native oxide layer on the Ti surface.43 A similar decomposition of Ti 2p core level spectra for pure titanium surface was reported by Payet et al.44 A calculation of the native oxide layer thickness was performed Ti Ti 2 as described above from the intensity ratio ITiO Ti /ITi, where I corresponds to the Ti 2p3/2 peak intensity of metallic titanium and ITiO2 corresponds to TiO2 neglecting the intensities of the TiO titanium suboxides (
