How Organophosphonic Acid Promotes Silane ... - ACS Publications

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How Organophosphonic Acid Promotes Silane Deposition onto Aluminum Surface: A Detailed Investigation on Adsorption Mechanism Juan Torras,*,† Denise S. Azambuja,‡ Johanna M. Wolf,§ Carlos Alemán,∥,⊥ and Elaine Armelin*,∥,⊥ †

Department of Chemical Engineering, EEI, Universitat Politècnica de Catalunya, Plaça del Rei, 15, 08700, Igualada, Spain Instituto de Química, Universidade Federal do Rio Grande do Sul (UFRGS), Av. Bento Gonçalves 950, CEP 91501-970, Porto Alegre, RS, Brazil § Technische Universitat Darmstadt, Karolinenplatz, 5, 64289, Darmstadt, Germany ∥ Department of Chemical Engineering, ETSEIB, Universitat Politècnica de Catalunya, Av. Diagonal 647, 08028, Barcelona, Spain ⊥ Centre for Research in Nano-Engineering, Universitat Politècnica de Catalunya, Campus Sud, Edifici C’, C/Pasqual i Vila s/n, E-08028, Barcelona, Spain ‡

S Supporting Information *

ABSTRACT: Many research works have evidenced the importance in using inorganic−organic hybrid materials to protect metal surface or to serve as intermediate adhesion promoter layer for further coating deposition. The main aim of the present work was to elucidate the mechanism of silane deposition onto aluminum surface, in the presence of organophosphonic acid as adhesion promoter and using experimental and theoretical techniques. Transparent thin films of layered inorganic−organic composites were prepared by the sol−gel synthesis of tetraethylorthosilicate and vinyltrimethoxysilane in the presence of 1,2-diaminoethanetetrakis-methylenephosphonic acid. Aluminum surface was characterized by X-ray diffraction, SEM and XPS techniques. On the other hand, inorganic− organic coating was characterized by FTIR and XPS spectroscopies. Density functional theory was employed to evaluate three simplified molecules, orthosilicic acid (Si(OH)4), methylphosphonic acid (MePA), and Si(OH)3OMePA, by using different coordination modes, in order to approach the most stable chemical bonding between silane-phosphonic groups with modified aluminum surface (i.e., with boehmite as oxidized layer).

1. INTRODUCTION

mechanical properties, although it has the highest rate of corrosion due its heterogeneous microstructure.2 In fact, a survey analysis of recently published surface treatment3−6 and protection studies allows us to realize the importance of prolonging the life of the materials based on certain types of aluminum alloys.7−9 Recently, Boeing researchers developed “green” sol−gel coatings as an environmentally friendly alternative to traditional aerospace finishing materials and processes.9−12 Sol−gel hybrid networks were based on a reactive mixture of an organofunctionalized silane with a stabilized zirconium complex. They checked that the hybrid coatings provided durable adhesion for paints, adhesives, and sealants, and even reducing the occurrence of “rivet rash” adhesion failures. Modifications of the basic inorganic/organic hybrid networks have yielded

Many decades of experience with aluminum alloys use in buildings, public works, ship-building, automotive area, and so on, have confirmed the observations of the 19th century chemists. Aluminum and its alloys have an excellent resistance to atmospheric corrosion in marine, urban and industrial environments.1 This good corrosion resistance, as much as lightness, the suitability for surface treatments, and the ease of aluminum recycling, explain the development of increasing numerous aluminum applications such as lightweight materials for automotive, equipment, and components for transport sector, household appliances, decoration, building, and others. However, the addition of alloying elements to the aluminum matrix and metallurgical treatments has relevant influence on several properties, especially mechanical, thermal, electrical conductivity and corrosion resistance. Among these alloys, those classified as aluminum−copper alloy has copper as the primary alloying element. This kind of aluminum alloy is commonly known by the trade name of AA2024 and is one of the most employed aluminum alloy, due to its excellent © 2014 American Chemical Society

Received: May 12, 2014 Revised: July 14, 2014 Published: July 17, 2014 17724

dx.doi.org/10.1021/jp5046707 | J. Phys. Chem. C 2014, 118, 17724−17736

The Journal of Physical Chemistry C

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multifunctional coatings with potential applications in fields such as corrosion control and oxidation protection.13,14 Sol−gel technologies using silane depositions have been reported to be one of the promising methods to protect the aluminum surface after the appropriate pretreatment.15−17 The most employed silanes for sol−gel depositions are TEOS (tetraethylorthosilicate), VTMS (vinyltrimethoxysilane), VTES (vinyltriethoxysilane), 3-aminopropyltriethoxysilane (APTES), and GPTMS (3-glycidoxypropyltrimethoxysilane), the first being the less expensive raw material.18−21 However, it was found that the adherence, homogeneity, and film thicknesses are usually poor, if only silicate is used as a reagent. Dalmoro et al.22 were the pioneering in prove that coatings prepared by combining silane derivatives with phosphonic acid compounds like 1,2-diaminoethanetetrakis-methylenephosphonic acid (EDTPO) or aminotrimethylenephosphonic acid (ATMP) enhance the protection against corrosion of aluminum alloys, specifically the AA1100 and AA2024 alloys.22−24 Novelty of their work was to obtain stable self-assembling monolayers (SAM) films using high water content in silane bath, whereas coatings obtained from organosilane sol−gel processes are usually promoted by high alcohol concentration. Furthermore, sol−gel matrices resulted in a particular synergistic behavior among metal alkoxide precursor and trialkoxysilane coupling molecules. Khramov et al.25 have demonstrated that stable hybrid coatings with phosphonate functionalities can chemically react with the surface of magnesium substrates increasing both adhesive and corrosion resistance properties of the coatings for long time. An interesting review published by Mutin et al.26 have summarized the synergistic interaction occurring in hybrid materials from organophosphorus coupling molecules. Nevertheless, despite several works have reported the covalent bonding between organosilane derivatives and aluminum oxide and with phosphorus derivatives,27,28 the chemical mechanism related to the effect of the phosphonic groups in the deposition and adhesion of a homogeneous layer of silane network has not been completely elucidated. In order to approach the role of phosphonic molecules in the formation of covalent linkages between the silane layer and the aluminum surface, herein we present a comprehensive study involving both the experimental techniques and theoretical calculations. Finally, a detailed mechanism for the stable organosilane coating deposition has been presented.

with distilled water and dried under a hot air stream before sol−gel deposition. Functionalization of Aluminum Surface. Two silanizing baths were prepared. A solution containing only silane derivatives was prepared by mixing 50 mL of ethanol, 46 mL of deionized water, 3 mL of VTMS, and 1 mL of TEOS. The solution was stirred mechanically for 1 h at room temperature and then stored for 3 days prior to use. The coating obtained from this solution was denoted TEOS-VTMS. Another solution was prepared with the same reagents, solvents and proportions (v/v) that the first mixture and incorporating 2.3 mg of phosphonic acid (EDTPO), which corresponds to a very low concentration of 3.75 × 10−5 mol·L−1. The mixture was stirred mechanically for 1 h at room temperature and then stored for 3 days prior to use. The coating obtained from the latter solution was denoted TEOS-VTMS-EDTPO. Immediately after the pretreatment, several AA2024 panels were coated by a dipcoating procedure using a Model 50 dip-coater (Chemat Technology, Inc.) with a withdrawal speed of 0.2 cm/min. The coated samples were dried under ambient conditions for at least 24 h and cured in an oven at 110 °C for 1 h prior to testing. The thicknesses of the as-prepared TEOS-VTMS and TEOSVTMS-EDTPO films were measured by a profilometer Dektack Veeco 150. Measurements. Fourier transform reflection−absorption infrared (FTIR-RA) spectra of the pretreated aluminum surface and the silane film deposited were obtained using a Nicolet 6700 FT-IR spectrometer equipped with a Smart SAGA (Specular Aperture Grazing Angle) accessory and Omnic software. Spectra were collected with an incidence angle of 80° from the normal surface at a resolution of 8 cm−1 (total of 120 scans), using a polished dish of aluminum alloy as background. Scanning electron microscopy (SEM) studies were carried out using a Focused Ion Beam Zeiss Neon 40 scanning electron microscope equipped with energy dispersive X-ray spectroscopy (EDS) analysis and operating at 30 kV. X-ray diffraction patterns were registered with a PANalytical X’Pert PRO MRD diffractometer, using CuKα radiation (λ = 0.15406 nm). Analyzed samples were aluminum dishes with 19 mm of diameter and 4 mm of height, before and after the applied pretreatment. X-ray photoelectron spectroscopy (XPS) analyses were performed in a SPECS system equipped with a highintensity twin-anode X-ray source XR50 of Mg/Al (1253 and 1487 eV, respectively) operating at 150 W, placed perpendicular to the analyzer axis, and using a Phoibos 150 MCD-9 XP detector. The X-ray spot size was 650 μm. The pass energy was set to 25 and 0.1 eV for the survey and the narrow scans, respectively. For the flood gun, the energy and the emission current were 0 eV and 0.1 mA, respectively. Spectra were recorded with a pass energy of 25 eV in 0.1 eV steps at a pressure below 6 × 10−9 mbar. The C 1s peak was used as an internal reference with a binding energy of 284.8 eV. Highresolution XPS spectra were acquired by Gaussian/Lorentzian curve fitting after S-shape background subtraction. Theoretical Methods. Simulation of the real surface in theoretical studies is a difficult and time-consuming task. In the present study, the substrate for the adsorption of the SAM of alcoxisilanes and methylphosphonic acids, as precursors of a protective layer against corrosion of AA2024 aluminum alloys, was modeled using the crystal structure of boehmite (hereafter, γ-AlOOH). In fact, surface pretreatment was expected to produce a protective oxidized layer. Although natural oxide on aluminum is predominantly amorphous, Alexander et al.29

2. EXPERIMENTAL AND THEORETICAL METHODS Materials. The chemical names and chemical formulas associated with the reagents used are vinyltrimethoxysilane (VTMS; C5H12O3Si), tetraethyl orthosilicate (TEOS; Si(OC2H5)4), and 1,2-diaminoethanetetrakis-methylenephosphonic acid solution (EDTPO; C6H20N2O12P4). All reagents were purchased from Sigma-Aldrich, Spain, and were used without further purification. The nominal composition of the aluminum alloy used in the present study is (units: mass %): Cu = 3.8−4.9; Mg = 1.2−1.8; Mn = 0.3−0.9; Fe and Si = 0.5; Cr = 0.1; Zn = 0.15; Ti = 0.15, other elements = 0.15 and Al balance to 100%. Pretreatment of Aluminum Alloy. The surfaces of the aluminum alloy samples (area = 2 × 3 cm2, thickness = 1 mm) were prepared by grinding with silicon carbide paper up to grade #1200, followed by thorough washing with distilled water and, finally, immersed in a 0.05 mol L−1 acetic acid solution (pH 3) for 5 min. After this, samples were carefully washed 17725



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showed that a thin pseudoboehmite layer (mainly made of γAl2O3 and γ-AlOOH phase) is present at the hydroxylate surface of aluminum oxide under atmospheric conditions. In order to avoid difficulties with having to simulate an amorphous surface with a possible mixed-oxide structure, the chemisorption study was performed considering the crystalline γ-AlOOH (010) surface. The geometry of this simple surface model is terminated by hydroxyl groups, which easily allow reacting with the orthosilicic (hereafter, Si(OH)4) and methylphosphonic acids (hereafter MePA) via an acid−base condensation mechanism.30,31 All performed calculations were based on the densityfunctional theory (DFT) in the standard Kohn−Sham formalism, as implemented in the SIESTA package32,33 with periodic boundary conditions. The generalized gradient approximation (GGA) was used on the calculation of exchange-correlation energy employing the Perdew−Burke− Ernzerhof (PBE) functional.34 All atoms were represented by the Troullier-Martins norm-conserving pseudopotentials,35 and a numerical double-ζ basis set with polarization function. Initial structural configurations of bulk crystal and γ-AlOOH (010) surface were derived from experimental characterization of γAlOOH crystal structure.36,37 Initial structures were allowed to relax under periodic boundary conditions by means of conjugate gradient minimization. Thus, the atom coordinates were optimized until the forces acting on each atom were smaller than 0.04 eV/Å, using a mesh cutoff of 350 Ry. Sampling of the irreducible Brillouin zone was performed according to the scheme proposed by Monkhorst and Pack38 with a k-points mesh made of 16 × 20 × 16 for crystal. The search for the equilibrium structure at the surface was done in the slab geometry. Indeed, two different sizes of supercell surface were considered in order to fit either individual molecules (2 × 1 × 2, with 64 atoms) or the condensed Si(OH)3OMePA molecule made of Si(OH)4 and MePA fragments (4 × 1 × 4, with 256 atoms). Moreover, the surface structure on the b-direction of the bulk unit cell presents the smallest unit cell possible but the necessary number of atomic levels to hold the surface reconstruction. Luschtinetz et al.31 reported that relaxation of α-Al2O3 (0001) surface extends for seven atomic layers in depth. Thus, the (010) surface model of γ-AlOOH was created by repeating the bulk unit cell taken from the optimized bulk and adding a vacuum region of 25 Å along the b-direction. The coordinates of all atoms except those that belong to the last four atomic layers (from 13 up to 16) were free to relax. Sampling of the irreducible Brillouin zone on the γ-AlOOH (010) surfaces are performed with a k-points mesh made of 16 × 1 × 16 and 8 × 1 × 8 for the 2 × 1 × 2 and 4 × 1 × 4 supercells, respectively. Finally, we built adsorption complexes between the two sizes of relaxed surfaces and the three studied molecules, Si(OH)4, MePA, and Si(OH)3OMePA, by using different coordination modes. In order to compare the stability between different adsorption complexes, the binding energy (BE) of reaction 1 was calculated.

mode. Binding energies were corrected with the basis set superposition error (BSSE) by mean of the standard counterpoise (CP) method but incorporating relaxation energy into correction.39 The binding energy of the complex is defined as usual by the following equation: BEBSSE = Ecomplex + Ewat − (ESurf + EAdsor) + E BSSE

where Ecomplex is the CGA/DFT energy of the optimized complex, and Ewat, ESurf, and EAdsor are the CGA/DFT energies after optimization of the water molecules associated with the condensation reaction, the isolated γ-AlOOH surface, and the isolated adsorbate, respectively. Moreover, the stability of several structures involving a sixmembered hydrogen bond ring (Si−OH···OP−O), when are adsorbed on amorphous alumina, were calculated by means of the Gaussian0340 program at B3LYP/6-31+G* level on the gas phase.

3. RESULTS AND DISCUSSION 3.1. Experimental Results. Surface Characterization. In order to check if the aluminum surface was affected by pitting or aluminum desmutting, the AA2024 surface was characterized after pretreatment with acetic acid. The effect of the acid solution pretreatment on AA2024-T3 grinded dishes was studied by XRD and SEM-EDX. A summary of the XRD results, in which the reference spectrum (AA2024) is compared to that of the dishes freshly treated with diluted acetic acid, is shown in Figure 1a. As it can be seen, the pretreatment did not create a new phase, the two spectra being very similar. The interplanar spacing evidence the appearance of Al2CuMg (Sphase, orthorhombic), Cu3Mn2Al, and Al6(Cu, Fe, Mn) intermetallic phases, indicating that the acid pretreatment does not promote the dealloying of the metal surface or the appearance of corrosion products composed by metal oxides. SEM results are fully consistent with the XRD results. Detection of the intermetallic phases was possible by using electron backscatter diffraction detector. As it can be seen in Figure 1b, these phases (white spots on SEM micrograph) are distributed randomly in the aluminum surface and correspond mainly to the S-phase. EDS analysis revealed the presence of Al, Cu, Fe, and Mg elements (Figure 1c). Combination of the results from XRD and SEM/EDS analyses revealed that the intermetallic particles were mainly roughly spherical Al2CuMg (S-phase), of approximate size 5 μm. Additionally, some irregularly shaped Al(Cu,Fe,Mn) phase, of approximate size 10 μm, was also observed in zones different from that displayed in Figure 1b (not shown). Silane coating depositions are usually favored by the presence of a passivating nanometric layer constituted by Al2O3 or Al(OOH) phases, which actuates as anchoring element for the formation of Si−O−Al covalent bonds at the interface. Unfortunately, the presence of this very thin layer was not detected by XRD and SEM analyses. Coating Characterization. Silane film formation on the metal surface was evaluated to prove the catalytic effect promoted by phosphonic acid in the covalent adhesion of the silane to the substrate. For this purpose, silane coatings prepared with and without incorporation of phosphonic acid to the sol−gel bath have been characterized and compared. Although the incorporation of phosphonic onto silica network−metal surface has been evidenced by Kannan et al.41 and by Dalmoro et al.,24 studies involving organophosphonic

Surf − (OH)n + Adsor − (OH)n → Surf − (O)n − Adsor + nH 2O

(2)

(1)

The BE was defined as the energy involved in a condensation reaction between an hydroxylate surface (Surf-(OH)n) and the adsorbate (Adsor-(OH) n ), where n is referring to a monodentate (n = 1) or bidentate (n = 2) coordination 17726



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sition as a function of depth. Figure 2 shows the atomic percentage evolution of aluminum, carbon, oxygen, silicon, and nitrogen and phosphorus elements during the depth profiling study. Moreover, survey spectrum is presented in Figure S2 (see SI). Accurate analyses of comparative atomic concentration evolution with time drive to elucidate how silane derivatives and phosphorus are covalently bonded to the aluminum surface. The concentration of Al 2p is negligible for the initial etching time either for aluminum coated with TEOSVTMS or TEOS-VTMS-EDTPO films, proving that the surface is well covered. However, the silane thickness is slightly lower for substrates covered with TEOS-VTMS-EDTPO films (637 nm) since the aluminum concentration is similar to that of bare metal after 1h of attack, whereas TEOS-VTMS film exhibits a very low atomic percentage of aluminum at the same sputtering time (Figure 2a) according to its high coating thickness (1412 nm). Similar behaviors were found for the atomic evolution of C 1s and O 1s in TEOS-VTMS-EDTPO films, which increased with time until 40 min of depth profile and then decreased to concentrations similar to that of bare AA2024 after 1 h (Figure 2b,c). In the case of the sample without phosphonic acid (TEOS-VTMS), the decay is considerably less marked than the films with EDTPO. After 4 h the percentages of aluminum, carbon, oxygen, silicon, and nitrogen in either TEOS-VTMS or TEOS-VTMSEDTPO coatings are similar to those of AA2024 bare alloy after 1 h of XPS sputtering. Another interesting observation is that silicon (Figure 2d) is not detected in a great percentage at first stage, whereas oxygen is present in a high concentration. This is consistent with FTIR results, which evidenced the presence of −OH groups from high hydrophilic coating surface. The atomic percentage of Si shows large variations for samples coated by silane films, particularly in the case of silane without EDTPO (e.g., after 1 h of etching time). Some aspects of experimental work have to be taken into account when evaluating these results: (i) the pretreatment applied does not eliminate the Si particles present in the AA2024 alloy surface, (ii) the coating thickness obtained for samples without EDTPO molecules is higher than TEOS-VTMS-EDTPO, and (iii) there is the possibility to find also Si content from some impurities from silicon carbide paper used for grinding the surface in the pretreatment step in the XPS results. Other elements, like nitrogen, were always detected either in the AA2024 bare metal or in the metal coated with silane derivatives (Figure 2e). Moreover, the high concentration of N 1s in bare AA2024 at initial depth profiling, which is absent in samples coated with silane, indicates contamination from pretreatment. In any case, the overall nitrogen concentration is too low to be considered as relevant for the present study. Moreover, TEOS-VTMSEDTPO XPS results indicate that its chemical composition is practically the same that the one of AA2024 bare metal in only 1 h of depth profiling (Figures 2a−e). On the other hand, some interesting conclusions can be taken from P 2p evolution (Figure 2f). At initial etching time, phosphorus element does not appear, being detected for first time after 1 h of attack. This feature proves the hypothesis that phosphorus is located in a region near of the AA2024−silane interfacial zone, establishing a stable Al−O−P and P−O−Si linkages. The concentration of phosphorus increases with the depth profile evidencing a mechanism of phosphonic deposition. EDTPO is able to establish monodentate and bidentate bridges with the aluminum substratum and hydrogen bonding interactions with the silanol groups (Si−OH) from

Figure 1. Comparison of the XRD spectra between the AA2024 mechanically polished and the AA2024 surface after pretreatment with acetic acid. (a) XRD spectrum, (b) SEM micrograph, (c) EDS analyse.

molecules have been mainly focused on their interactions, either by adsorption or covalent bonding with ceramic substrates (e.g., ITO, Al2O3, and GaAs).42−45 On the other hand, despite FTIR studies showing evidence of the presence of Si−O−Al absorption groups in the silane-cured film,24,41 identification of P−O−Al or P−O−Si linkages by this technique is problematic because of both the low concentration of phosphonic molecules in the sol−gel composition and the similarity of the absorbance regions for P−O−metal and Si− O−metal stretchings and vibration modes. FTIR results are discussed in the Supporting Information (SI) as complementary data (Figure S1). XPS has been proven to be an excellent technique to support the covalent bonding of chemical elements. In this study we performed a depth profile attack (from 0 to 4 h) on the surfaces of AA2024, AA2024 coated with TEOS-VTMS, and AA2024 coated with TEOS-VTMS-EDTPO to determine the compo17727



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Figure 2. Comparison of the XPS atomic composition of (a) Al 2p, (b) C 1s, (c) O 1s, (d) Si 2p (e) N 1s, and (f) P 2p for AA2024-T3 without coating and coated with TEOS-VTMS and TEOS-VTMS-EDTPO silane films.

1s, and O 1s were obtained and shown in Figure 3. Deconvolution of experimental XPS spectra was performed choosing component peaks to reflect possible chemical composition of the investigated materials that were correlated with literature data.46−51 Deconvoluted curves of each individual sample are supplied in the SI. High-resolution XPS spectra have been used to investigate the chemical environment of a particular element. The naturally formed Al oxide on the as-ground sample surface cleaned with acetic acid, which is a few nanometers thick, becomes undetectable after 10 min of depth profiling. The narrow and unique Al 2p peak at the sputtered surface of cleaned AA2024 substrate is observed at 72.3 eV (Figure 3a) and has been attributed to the Al matrix. After 1 h of sputtering, the Al 2p for AA2024 substrate shifts toward a slightly higher binding energy (72.6 eV) and a second broad peak appears at ∼74.8 eV (Figure 3b), the latter being attributed to Al3+ oxidized state.47 According to some authors,

TEOS, evidenced by FTIR, resulting in the formation of a very stable six membered (O−PO···H−O−Si−O) hydrogenbonded rings between PO from phosphonic molecules and Si−OH from TEOS. Phosphorus element is not detected in the TEOS-VTMS-EDTPO coating surface with few nanometers of onset, instead it is found close to the metal surface in a low concentration after more than 60 min of depth profile (Figure 2f). Another interesting observation is that the reduction of Si 2p content occurs at the same time that the P 2p content increases, corroborating with the assumption that silane film is mainly located as an outer layer, whereas phosphonic molecules forms only an inner adhesion layer to metal substrate (see Figure S3). Therefore, phosphonic molecules promote the regular deposition of a silane coating at the aluminum surface, acting as an adhesion promoter for the silane attachment. To further investigate the chemical linkages of the coated AA2024, high resolution spectra of Al 2p3/2, Si 2p3/2, P 2p3/2, C 17728



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Figure 3. Normalized XPS high resolution spectra recorded at depth profiling of 10 min and 1 h of sputtering for AA2024 bare (served as a control), and AA2024 coated with TEOS-VTMS and TEOS-VTMS-EDTPO films. (a, b) Al 2p, (c, d) C 1s, (e, f) O 1s, (g, h) Si 2p, (i) P 2p.

Theoretical Methods). High-resolution spectra of Al 2p in coated AA2024 surfaces are similar for the two silane films at initial depth profiling, the height of the peaks being much lower than for the uncoated surface. This feature indicates that the surface is well covered. The Al 2p spectrum recorded for the bare substrate after 1 h was deconvoluted in two components

Al 2p binding energies ranging from 74 to 76 eV are probably related to Al2O3 or AlOOH clusters isolated or chelated with other metals from alloy elements.46,47 Indeed, in recent work,52 the Al 2p peak at 74.3 eV was associated with pseudoboehmite structures in pure minerals, justifying the approach used in this work for theoretical calculations (see Experimental and 17729



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Si), while the component associated with highest binding energy has been associated with the silicon bonded to aluminum oxide (Si−O−Al).47,48 Nevertheless, films modified with phosphonic groups changed from one (10 min) to two components at 102.8 and 98.8 eV (1 h; Figure S7c). The first peak corresponds to the silicon bonded to aluminum oxide (Si−O−Al), as mentioned before.55 Finally, the P 2p peak only appeared after 1 h of attack for the sample with EDTPO component. This peak is very broad and deconvoluted into four peaks at 132.6 (P 2p3/2), 133.4 (P 2p1/2), 134.9 (P 2p3/2), and 135.8 eV (P 2p1/2), which correspond to P−O−metal and PO bonds (Figure S8).55,56 The overall XPS results obtained for TEOS-VTMS samples show the preferential formation of Si−O−Si and Si−O−Al linkages at the outer and inner films, respectively, whereas the spectra recorded for TEOS-VTMS-EDTPO samples reveal Si− O−Si linkages in the upper polymeric network and both P−O− Al and Si−O−Al linkages at the interfacial region between the coating and the metallic substrate. 3.2. Theoretical Results. Bulk and Surface Structures. Previous studies regarding the use of SAMs with organosilane molecules and modeling studies are scarce. Dkhissi et al. have reported several works with SAMs deposition of silane onto silica surfaces (SiO2) and using modeling calculations.57−59 However, to the best of our knowledge, physical chemistry studies related to silane SAM formation onto aluminum hydroxide surface was not found. The suitability of the computational methodology used in this work has been proved by calculating the bulk lattice constants of γ-AlOOH. Comparison of the results after relaxation of both coordinates and cell vectors with experimental data and previous theoretical estimations reveals very small deviations. The verification of the computational methods accuracy for one of the phases of boehmite structure is briefly discussed in the SI (Table S1). The more frequent exposed faces on the γ-AlOOH crystals are the (100), (010), (001), and (101) (see Figure S9). Among them, the most stable in gas-phase is the basal (010) surface, which presents a higher coordination number of Al and bridging OH groups.60 We chose the (010) surface to investigate the chemisorption of both Si(OH)4 and MePA, due to the accessible hydroxyl groups that facilitate a condensation reaction on the chemisorption process (Figure 4).

centered at 71.5 and 74.1 eV for TEOS-VTMS coating, whereas Al 2p spectrum from TEOS-VTMS-EDTPO coating presented four components at 72.2, 72.9, 74.7, and 77.7 eV (Figure S4). According to Hoque et al.,53 Al 2p peaks at low binding energy correspond to aluminum oxides, while those at high binding energies are related to Si−O−Al and P−O−Al linkages. The broad C 1s peak is deconvoluted into several components (Figure S5). The component centered at 284.8 eV, which is present in both coated and uncoated AA2024 (Figure 3c), correspond to the carbon bounded to another carbon or hydrogen (C−C, C−H). On the other hand, the peak at 282.2 eV corresponds to the carbon bonded to silicon atom (C−Si), whereas the peak at 287 eV is attributed to the carbon linked to oxygen through a single bond (C−O; Figure 3c,d). Another interesting difference between uncoated and coated AA2024 substrate appears in the high-resolution spectra of O 1s (Figure 3e,f). This element presents a broad peak that can be deconvoluted in three main peaks at 529.7, 531.8, and 533.6 eV for uncoated AA2024 surface, either in 10 min or 1 h. The deconvoluted curves for AA2024 (Figure S6a) present a high concentration of O 1s localized at 531.9 eV, which is mainly associated with aluminum oxides or hydroxides,46,47 whereas the component at 533.6 eV should be related to Al2O3−COO− from surface pretreatment with acetic acid.47 However, the O 1s position varied slightly for the coated systems due to the presence of Si−O and P−O linkages. A new peak at 532.9 eV, which corresponds to the Si−O−Si bonds on silica network, was observed at initial depth profiling (Figure 3e) for both silane coatings.34,46,51 The peak at 529−531.9 eV, attributed to the Al−O linkages from Al oxides/hydroxides in the AA2024 surface, does not appear at first stages of depth profiling in the coated samples, corroborating that the surface is well covered. Interestingly, O 1s deconvolution peaks changed greatly after 1 h of sputtering for the film of silane modified with EDTPO. In this case, as the attack reached the aluminum surface, the main contributions have been attributed to Al−O interactions at 531.5 eV and also to a high concentration of Al−O−P functional groups at 533.1 eV (Figure S6c). XPS analyses reported by Tsud and Yoshitake54 revealed the presence of two components at 532.2 and 533.4 eV in the O 1s peak of phenyl phosphonic acid adsorbed on amorphous alumina, which were attributed to Al2O3 and the Al−O−P linkages, respectively. On the other hand, as TEOS-VTMS coating presents high thickness compared to TEOS-VTMS-EDTPO, the main O 1s component at 532.8 eV comes from the Si−O−Si silica network (Figures 3f and S6b). Alkoxysilane-organophosphorus films have usually high thickness compared to other hybrid materials, as evidenced by SEM or perfilometer measurements.23,25 The bonding of phosphorus to the aluminum surface (P− O−Al) and to silicon (P−O−Si) forming the organic− inorganic network is supported by the high resolution spectra of O 1s, Si 2p, and P 2p. Deconvolution of the Si 2p spectra registered for the initial attack of TEOS-VTMS and TEOSVTMS-EDTPO films (Figure 3g,h) resulted in an unique component centered at 103.0 eV, which have been assigned to Si−O−Si linkages in the silica network.28,51 With time profiling increases (Figure 3h), the silicon neighboring for TEOS-VTMS samples does not change until the depth reaches the Al surface (after 4h). Therefore, new components appear at 98.4, 103.0, and 107.5 eV (Figure S7b). The former one has the lowest height and has been attributed to the silicon−metal bonds (Al−

Figure 4. Bidentate adsorption sites on γ-AlOOH (010) surface model. Al, O, and H atoms are represented by cream, red, and white spheres, respectively. 17730



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Surface relaxation pattern derived from calculations performed on both 2 × 1 × 2 and 4 × 1 × 4 supercells remains almost unchanged. Attending to the interlayer distance (di), which is measured as the distance between Oa atom and the Al atom that is just below on the adjacent layer (see Figure S9a), a low distortion of Δdi = 0.024 Å is observed with respect to the bulk unit cell in both supercells (Δdi = disurf − dibulk). Similarly, comparison of the y-coordinate between the relaxed surface and the bulk crystal for a given i-th atomic layer (Δ⊥i) allows us to estimate the atomic displacements. The most affected atomic layer is the first, which is made of hydrogen atoms from hydroxyl groups. This is displaced inward by −0.13 Å, while the rest of the atomic layers displace outward from bulk coordinates by +0.02 Å. The relaxation goes deep inside the crystal up to the eighth layer with the following pattern of Δ⊥i: −0.13, 0.02, 0.02, 0.03, 0.02, 0.02, 0.03, and 0.03 Å. No appreciable differences were obtained on the relaxation pattern of the 4 × 1 × 4 supercell. Chemisorbed Species. The synergistic combination of tetraethylorthosilicate and multiphosphonic acid has been previously shown in a previous work24 as an excellent precursor of corrosion protection layer to aluminum alloy. Specifically, the work suggest that alkylphosphonic acid may interact with the surface through a monodentate coordination mode and, in addition, form a strong and stable linkage with silicon through a nonhydrolyzable bond. In order to improve the understanding of this synergistic process, structural and energetic studies have been carried out with three chemisorbed spices, Si(OH)4, MePA, and a condensed molecule complex of Si(OH)3OMePA, on γ-AlOOH. The surface model has different adsorption sites where both MePA and Si(OH)4 spices may adsorb in a mono- and bidentate chemisorption sites. In the following these sites will be labeled ms and bs, respectively. Zenobi et al.61 recently studied the chemisorption of alkylphosphonic acids onto γAlOOH, suggesting that the monodentate coordination is the most favorable chemisorbed site. However, the own authors did not discard the presence of bidentate chemisorbed spices. Similarly, alkylsilanes have been shown forming a multilayer attachment to the surface of hydroxylated alumina, being the alkylsilane adsorbate on the lowest layer linked to the surface by a single Al−O−Si covalent bond.62 The γ-AlOOH (010) surface presents only one monodentate adsorption site (ms1; Figure 4) because of the symmetry of the hydroxyl groups, which are equally accessible to bind either MePA or Si(OH)4 molecules. However, there are three different bidentate coordination sites (bs1, bs2, and bs3). Initially, we build adsorption complexes of MePA and Si(OH)4 in mono- and bidentate coordination sites, considering and (2 × 1 × 2) supercell symmetries. For each adsorbate several dihedral angles (θ, Figure 5a) between chemisorbed molecule and surface were considered as starting structure before geometry optimization. Specifically, dihedral angles for monodentate coordination sites ranged from 0 up to 270°, whereas only two angles (±120° with respect to O−O−P plane linked to the surface) were considered on bidentate adsorption complexes of MePA. For each chemisorbed specie, the calculated dihedral angles θ, relative energies (ΔEr), and binding energy (BEBSSE, eq 2) obtained for the different adsorption sites of the γ-AlOOH (010) surface after geometry optimization are shown in Table 1.

Figure 5. Monodentate chemisorption of methylphosphonic and orthosilicic acid. Details on the (a, c) ac crystallographic plane and (b, d) bc crystallographic plane of γ-AlOOH (010) surface. Al, O, and H atoms are represented by the same colors indicated in Figure 4, whereas Si, P, and C atoms are represented by brown, green, and gray spheres, respectively.

The most stable adsorbed complex obtained for the monodentate chemisorption site of MePA is the ms_c minimum (θ = 73.4°, Figure 5a,b), which is stabilized through the formation of two hydrogen bonds between surface and MePA, that is, PO and OH moieties. Steric repulsions between the methyl group of MePA and the neighboring hydroxyl groups of the surface are responsible of the destabilization of complexes with theta values of −178.3° (ms_e) and 12.3° (ms_b). On the other hand, result displayed in Table 1 do not allow us to define specific and selective sites for the monodentate chemisorption of Si(OH)4 onto the γ-AlOOH (010) surface, all relative energies of the optimized complexes being lower than 2 kcal/mol. The ms_f (Figure 5c,d), which is the absolute minimum of energy, presents a positive BEBSSE value of 0.9 kcal/mol. These results suggest that the monodentate coordination in a high surface coverage, which is represented through the 2 × 1 × 2 supercell, is slightly more stable for MePA than for Si(OH)4. Regarding bidentate complex energies, as shown in Table 1, both species are destabilized by about 25 kcal/mol with monodentate complexes. Specifically, for the MePA chemisorption the bidentate site bs2 is the most stable, the bs1 being 17731



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Table 1. Dihedral Angles with the Surface (θ, degrees), Relative Energies (ΔEr), and Binding Energies (BEBSSE) of the Adsorption Complexes of Methylphosphonic Acid (MePA) and Orthosilicic Acid (Si(OH)4) on the Various Adsorption Sites in a Supercell 2 × 1 × 2 of γ-AlOOH (010) Surfacea MePA site

θ

b

−24.3 12.3 73.4

ms_a ms_b ms_c ms_d ms_e ms_f ms_g

−178.3 −92.0 −87.9

bs1_a bs1_b bs2_a bs2_b bs3_a bs3_b

118.3 −117.2 118.3 −114.9 120.9 −126.9

ΔEr

Table 2. Relative Energies (ΔEr) and Binding Energies (BEBSSE) for Complexes of Methylphosphonic Acid (MePA), Orthosilicic Acid (Si(OH)4) and the Condensed Molecule of Si(OH)3OMePA adsorbed onto Various Selected Sites of a Supercell 4 × 1 × 4 of γ-AlOOH (010) Surfacea MePA

Si(OH)4 BE

BSSE

θ

c

Monodentate 0.8 −0.5 −19.8 6.4 5.1 13.4 0.0 −1.2 61.1 106.1 18.1 16.0 141.2 3.9 2.2 −143.8 3.5 1.6 −124.7 Bidentate 5.3 29.6 117.2 8.5 32.6 1.1 24.3 119.1 0.0 22.9 63.8 86.8 128.4 73.6 96.4

ΔEr

coord.

ΔEr

ms_c ms_f bs2_a bs2_b

1 1 2 2

0.0

−13.4

0.0 1.5

Sims-Pms Sibs1-Pms Sibs2-Pms

2 3 3

0.0 0.0 20.4

21.3 22.6 Si(OH)3OMePA −24.9 6.3 24.9

site BE

BSSE

1.3 0.3 2.0 1.3 1.4 0.0 1.5

2.1 1.4 3.4 2.2 2.4 0.9 1.9

4.0

26.4

0.0

27.0

68.9

94.1

a

BEBSSE

Si(OH)4 ΔEr

BEBSSE

0.0 0.0

−9.2 16.8

All energies are in kcal/mol.

species, as observed from the BEBSSE (Table 2), which makes it difficult to populate the γ-AlOOH (010) surface with this chemisorption model. Moreover, taking into account the hypothesis of synergistic combination between alkylorthosilicate and alkylphosphonic acid, the interest of this work is to provide understanding about the possible mechanism. Accordingly, three different chemisorbed geometries, involving two tridentate (Figure 6) and one

a All energies are in kcal/mol. bDihedral angles defined between atoms Al1−O2−P−O in monodentate coordination sites and between O− O−P plane linked to the surface and carboxylic oxygen in bidentate coordination sites, (O−O−P−O). cDihedral angles defined between atoms Al1−O2−Si−O in monodentate coordination sites and between O−O−Si plane linked to the surface and the oxygen from one of the hydroxyl groups of Si(OH)4 in bidentate coordination sites, (O−O− Si−O).

ΔEr = 5.3 kcal/mol unfavored. Regarding the chemisorption of Si(OH)4, bs2 is the most stable, while bs1 is disfavored by only ΔEr = 4.0 kcal/mol. As was observed in monodentate sites, MePA retains some stability compared to Si(OH)4 despite of the high obtained values of BE. For both compounds the most disadvantaged coordination mode is the bs3, which presents a much higher instability due to the tension between the oxygen atoms of the surface and chemisorbed molecule. However, this short (2 × 1 × 2) surface model is affected by closer neighboring images of adsorbates that might stress the BEBSSE of the system. Despite this restriction, these calculations allow us to take a picture of the possible minima and their relative stability in a populated surface. In order to check how relative stability of these systems could be affected in a less covered surface, new models with low coverage surface (4 × 1 × 4) were optimized and compared with previous minima of crowded surface. The starting geometries were constructed using the lowest energy minimum obtained for each surface using the (2 × 1 × 2) surface model. Table 2 lists the relative and binding energies of selected minima as well as of the condensed molecule of Si(OH)3OMePA on the various adsorptions sites of γ-AlOOH (010) surface. As can be seen, lowering surface population results in a stabilization of about 10 kcal/mol, in all cases, with exception of the bidentate sites for MePA, which are less affected by the movement of the adsorbate away from the boundaries and neighboring images. We want to notice that the formation of monodentate Si(OH)4 complex is now energetically favorable but still less favorable than that of monodentate MePA. However, the bidentate site is still disfavored for the two

Figure 6. Tridentate binding coordination (Sibs1-Pms) of condensed Si(OH)3OMePA molecule chemisorbed on the 4 × 1 × 4 supercell of γ-AlOOH (010) surface. Details on the (a) ac crystallographic plane and (b) bc crystallographic planes. Al, O, Si, P, C, and H atoms are represented by cream, red, brown, green, gray, and white spheres, respectively.

bidentate (Figure 7) coordination sites of the condensed Si(OH)3OMePA molecule, have been examined. More specifically, the formers are made of a bidentate Si(OH)4 and a monodentate MePA moieties. The main difference between the two tridentate complexes consists on the adsorption model used to link the Si(OH)3 moiety to the surface, which is based on the bidentate sites bs1 or bs2 (hereafter, Sibs1-Pms or Sibs2Pms, respectively). On the other hand, bidentate coordination of condensed Si(OH)3OMePA molecule is made of two monodentate sites, one for each moiety (hereafter Sims-Pms). The most stable geometry corresponds to Sims-Pms complex, which is shown in Figure 7, with a BEBSSE of −24.9 kcal/mol. We want to notice that this chemisorbed complex is 2.2 kcal/ mol more stable than the two monodentate complexes of 17732



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The overall results indicate that a synergic condensation between MePA and Si(OH)4 is expected close to the surface when protective layers of silica over oxidized aluminum surfaces are formed. More specifically, adsorption occurs through two monodentate coordination sites, one for each fragment of the condensed molecule. Stability Induced by the Six Membered Hydrogen Bond Ring. Experimental evidence in a previous work evidenced the existence of Si−OH···OP hydrogen bonding interactions between the two fragments forming the condensation molecule, giving place to the formation of a six-membered ring.24 As discussed above, the γ-AlOOH (010) surface presents a stable complex of bidentate Si(OH)3OMePA (BEBSSE = −24.9 kcal/ mol). From a structural point of view, this model is with the formation of six membered hydrogen bonded rings in the first layer, which is in contact with the γ-AlOOH (010) surface. The energy stability (ΔE) induced by this six-membered hydrogen bond ring was obtained by disrupting the hydrogen bond cycle, which was estimated in 10.7 kcal/mol. Alternatively, other hydrogen bonding schemes would be formed considering the adsorption onto a hydroxylated amorphous alumina surface. Unstructured surfaces may hold several coordination sites. Thus, besides the studied chemisorbed complexes on the γAlOOH surface, we also considered additional structures holding a six membered hydrogen bonded ring that may or may not attach to amorphous alumina. Accordingly, a simple model consisting of a distorted AlO6H6 octahedron has been used to represent amorphous boehmite, four possible scenarios further than the minimum energy structure found on the γAlOOH (010) surface being considered to describe the interaction with the adsorbate (Figure 8). In the first of these

Figure 7. Bidentate binding coordination of condensed Si(OH)3OMePA molecule chemisorbed on the 4 × 1 × 4 supercell of γ-AlOOH (010) surface, showing the stable six-membered ring between phosphonic and silanol groups. Atoms are represented with the same colors indicated in Figures 4 and 6

MePA and Si(OH)4 individually chemisorbed on the γ-AlOOH (010) surface. This indicates a favorable synergic combination between alkylorthosilicate and alkylphosphonic acid, the stability of the protective layer on the alumina surface increasing upon the presence of MePA. The key of this overstabilization originates not only from several hydrogen bonding interactions with the surface, but also from an intramolecular hydrogen bond between the two moieties of the condensation molecule. Specifically, two six-membered hydrogen bonds involving the carboxylic group of MePA moiety are detected. One involves the surface hydroxyls (Al− OH···OP−O) and the other one is made within the chemisorbed molecule (Si−OH···OP−O), with hydrogen bonding distances (dOH···O) of 1.83 and 2.07 Å, respectively. The intramolecular hydrogen bond was previously suggested in the experimental work pointing out to the synergetic behavior between both moieties on the condensed molecule of Si(OH)3OMePA.24 Additionally, a weak hydrogen bond with dOH···O = 2.11 Å involving an oxygen atom from hydroxyls of Si(OH)3 fragment and the surface hydroxyls is appreciated (not shown). On the other hand, the tridentate complexes present higher instability in terms of BEBSSE than the bidentate SimsPms models (Table 2). In fact, the synergy induced by the monodentate chemisorption of MePA moiety is not enough to counter the penalty of energy associated with the bidentate coordination of the Si(OH)3 fragment. Thus, the most favorable tridentate complex involves a bs2 bidentate site on the Si(OH)3 fragment with a BEBSSE of 6.3 kcal/mol.

Figure 8. Different structures of stable six membered hydrogen bond ring made on the condensed molecule of Si(OH)3OMePA, depending on its coordination site to the γ-AlOOH surface; (A) nonchemisorbed, (B) bidentate Si(OH)4 moiety, (C) monodentate MePA moiety, and (D) tridentate coordination.

structures (A), no group of the condensation molecule is attached to the surface. The other three structures, denoted B− D, reflect the adsorption of the bidentate orthosilicic acid fragment (B), the monodentate adsorption of the methylphosphonic fragment (C), and the simultaneous adsorption of orthosilicic and methylphosphonic acid moieties (D). For each structure the energy stability (ΔE) associated with the six membered hydrogen bonded ring has been estimated by 17733



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calculating the energy penalty through the disruption of the hydrogen bond cycle (Table 3).

Scheme 1. Schematic View of Building up TEOS-VTMSEDTPO Films on AA2024 Surface

Table 3. Energy Difference between the Models Calculated Considering the Presence and Absence of Six Membered Hydrogen Bond Ring Structures (ΔE in kcal/mol), Hydrogen Bond Distance (dOHO in Å), and Hydrogen Bond Angle (αOHO in Degrees) structures (see Figure 8)

ΔE

dOHO

αOHO

A B C D

12.0 9.1 4.3 6.9

2.01 1.84 2.48 1.87

141.6 148.0 125.7 146.3

Comparison of the different minimum energy conformations indicates that the molecular strain provoked by the formation of O−H···O intramolecular hydrogen bonds is responsible for the differences among stabilities. Thus, ΔE values displayed in Table 3 could be influenced by additional hydrogen bonds, biasing the stabilization associated with the six-membered ring The dOH···O values (Table 3) indicate significant differences in the strength of the intramolecular hydrogen bond. In fact, in structure C the carboxylic oxygen of the MePA moiety and the hydroxyl groups from the Si(OH)3 moiety are involved in several stabilizing interactions with a surface hydroxyl group (Figure 8), which leads to a weakening of the six membered hydrogen bond ring (dOH···O = 2.48 Å). However, when the orthosilicic acid is attached to the surface in a bidentate coordination, the six membered hydrogen bond ring is reinforced (dOH···O = 1.84 and 1.87 Å for structures B and D, respectively), because no additional hydrogen bond is formed. However, the most stable six-membered hydrogen bond ring is found for the minimum of energy for the structure made of chemisorbed spices on γ-AlOOH (010) surface, the stabilization energy being 10.7 kcal/mol. The latter value is very close to that obtained for the free condensation molecule, 12.0 kcal/ mol (structure A in Table 3), indicating that the strain is very small. Therefore, theoretical calculations corroborate with the XPS results regarding the Si−O−P and P−O−Al covalent bonding stability between silane coating and oxidized aluminum surface. The synergic combination study of both species through the condensed Si(OH)3OMePA molecule, chemisorbed onto the γ-AlOOH (010) surface through two monodentate coordination sites, shows an overstabilization with respect to the independently chemisorbed spices of 2.2 kcal/mol. Also, the stability of the bidentate complex is reinforced by an intramolecular six-membered hydrogen bond cycle. Thus, the presence of MePA on the surface of boehmite enhances the stability of the Si(OH)3 deposition by creating a stronger interaction with the γ-AlOOH surface. Based on the experimental and theoretical investigations, we can propose the following explanation for silane deposition and phosphorus interaction. In the first step, hydrolyzed TEOS and VTMS molecules are attracted by the hydrophilic surface of boehmite. Then, EDTPO molecules catalyze the covalent adhesion of TEOS-VTMS molecules to the modified metal surface by hydrogen bond interactions between hydroxyl and PO linkages. When the surface is stabilized with the solution and slightly evaporated (24 h in air), the cured process take place and the network is finally formed with organophosphonic groups and silane as inner layer and Si−OH as outer layer (Scheme 1).

As a summary, in this work both experimental measures and theoretical calculations have been used to examine the phosphonic-silane synergistic interaction for covalent bonding with aluminum surface. Detailed analyses of FTIR and XPS results have enabled us to propose a mechanism for the stable deposition of this hybrid coating, which has been subsequently corroborated by first principle theoretical calculations. Understanding of the proposed mechanism is important for the development of new improved coatings for aluminum protection. Furthermore, the excellent agreement between theoretical calculations and experimentally determined parameters indicate that combined experimental-computational approaches can be successfully used to rationally understand the mechanism of the adsorption process and optimize improved coatings.

4. CONCLUSIONS Analysis of XPS spectra with depth profiling was of crucial importance for understanding the mechanism of silane network formation when it is in contact with organophosphonic molecules. Additionally, theoretical calculations performed, using relatively simple models systems, allowed us to establish a preferred linkage by the formation of a bidentate adsorption model and six-membered hydrogen bonded ring bond above both bohemite (010) and amorphous aluminum surfaces. Therefore, the present study has confirmed and demonstrated that phosphonic groups act as adhesion promoters for TEOSVTMS deposition onto aluminum surface, creating a homogeneous and thin coating, compared to the irregular and thick layer of samples obtained in absence of phosphonic molecules. Moreover, experimental results proved that silanol groups are mainly located outside the hybrid organic−inorganic film, whereas phosphorus and siloxane rings are found close to the metal surface. Nevertheless, the presence of very thin layers of γ-AlOOH or other crystalline structures over the metal surface was not possible to confirm by XRD or XPS analysis. The synergistic combination of MePA and Si(OH)4 chemisorbed on a γ-AlOOH (010) surface contributes to the successful formation of both stable linkages to the Al surface through monodentate and bidentate coordination sites and highly favored hydrogen bonds between the PO and HO−Si-O groups. Thus, the phosphonic group presence on the surface of boehmite enhances the stability of silane deposition by creating 17734



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(11) Larson-Smith, K. L.; Blohowiak, K. Y.; Seebergh, J. E.; Sirkis, M. R.; Sundaram, V. S.; Hybrid Coatings and Associated Methods of Application. U.S. Patent 2010/196621-A1, August 5, 2010. (12) Liu, J.; Chaudhury, M. K.; Berry, D. H.; Seebergh, J. E.; Osborne, J. H.; Blohowiak, K. Y. Effect of Surface Morphology on Crack Growth at a Sol-Gel Reinforced Epoxy/Aluminum Interface. J. Adhes. 2006, 82, 487−516. (13) Warren, S. C.; Perkins, M. R.; Adams, A. M.; Kamperman, M.; Burns, A. A.; Arora, H.; Herz, E.; Suteewong, T.; Sai, H.; Li, Z.; et al. A Silica Sol−Gel Design Strategy for Nanostructured Metallic Materials. Nat. Mater. 2012, 11, 460−467. (14) Vallé, K.; Belleville, P.; Pereira, F.; Sanchez, C. Hierarchically Structured Transparent Hybrid Membranes by In Situ Growth of Mesostructured Organosilica in Host Polymer. Nat. Mater. 2006, 5, 107−111. (15) Khramov, A. N.; Balbyshev, V. N.; Voevodin, N. N.; Donley, M. S. Nanostructured Sol−Gel Derived Conversion Coatings Based on Epoxy- and Amino-Silanes. Prog. Org. Coat. 2003, 47, 207−213. (16) Zheludkevich, M. L.; Miranda Salvado, I.; Ferreira, M. G. S. Sol−Gel Coatings for Corrosion Protection of Metals. J. Mater. Chem. 2005, 15, 5099−5111. (17) Naderi, R.; Fedel, M.; Urios, T.; Poelman, M.; Olivier, M.-G.; Deflorian, F. Optimization of Silane Sol-Gel Coatings for the Protection of Aluminium Components of Heat Exchangers. Surf. Interface Anal. 2013, 45, 1457−1466. (18) Conde, A.; Durán, A.; de Damborenea, J. J. Polymeric Sol-Gel Coatings as Protective Layers of Aluminium Alloys. Prog. Org. Coat. 2003, 46, 288−296. (19) Naderi Zand, B.; Mahdavian, M. Evaluation of the Effect of Vinyltrimethoxysilane on Corrosion Resistance and Adhesion Strength of Epoxy Coated AA1050. Electrochem. Acta 2007, 52, 6438−6442. (20) Yim, H.; Kent, M. S.; Tallant, D. R.; Garcia, M. J.; Majewski, J. Hygrothermal Degradation of (3-Glycidoxypropyl)trimethoxysilane Films Studied by Neutron and X-ray Reflectivity and Attenuated Total Reflection Infrared Spectroscopy. Langmuir 2005, 21, 4382−4392. (21) Feng, Z.; Liu, Y.; Thompson, G. E.; Skeldon, P. Crack-Free Solgel Coatings for Protection of AA1050 Aluminium Alloy. Surf. Interface Anal. 2010, 42, 306−310. (22) Dalmoro, V.; dos Santos, J. H. Z.; Azambuja, D. S. Corrosion Behavior of AA2024-T3 Alloy Treated with Phosphonate-Containing TEOS. J. Solid State Electrochem. 2012, 16, 403−414. (23) Dalmoro, V.; dos Santos, J. H. Z.; Armelin, E.; Alemán, C.; Azambuja, D. S. Phosphonic Acid/Silica-Based Films: A Potential Treatment for Corrosion Protection. Corros. Sci. 2012, 60, 173−180. (24) Dalmoro, V.; dos Santos, J. H. Z.; Armelin, E.; Alemán, C.; Azambuja, D. S. A Synergistic Combination of Tetraethylorthosilicate and Multiphosphonic Acid Offers Excellent Corrosion Protection to AA1100 Aluminum Alloy. Appl. Surf. Sci. 2013, 273, 758−768. (25) Khramov, A. N.; Balbyshev, V. N.; Kasten, L. S.; Mantz, R. A. Sol−Gel Coatings with Phosphonate Functionalities for Surface Modification of Magnesium Alloys. Thin Solid Films 2006, 514, 174−181. (26) Mutin, P. H.; Guerrero, G.; Vioux, A. Hybrid Materials from Organophosphorus Coupling Molecules. J. Mater. Chem. 2005, 15, 3761−3768. (27) Templin, M.; Franck, A.; Du Chesne, A.; Leist, H.; Zhang, Y.; Ulrich, R.; Schadler, V.; Wiesner, U. Organically Modified Aluminosilicate Mesostructures from Block Copolymer Phases. Science 1997, 278, 1795−1798. (28) Lukes, I.; Borbaruah, M.; Quin, L. D. Direct Reaction of Phosphorus Acids with Hydrosy of Silanol and on the Silica Gel Surface. J. Am. Chem. Soc. 1994, 116, 1737−1741. (29) Alexander, M. R.; Thompson, G. E.; Beamson, G. Characterization of the Oxide/Hydroxide Surface of Aluminum Using X-Ray Photoelectron Spectroscopy: A Procedure for Curve Fitting the O 1s Core Level. Surf. Interface Anal. 2000, 29, 468−477. (30) Hector, L. G.; Opalka, S. M.; Nitowski, G. A.; Wieserman, L.; Siegel, D. J.; Yu, H.; Adams, J. B. Investigation of Vinyl Phosphonic

stronger interactions with the aluminum surface, improving coating adhesion and allowing the formation of a more compact and homogeneous protective layer. According to previous results with other aluminum alloys, this mechanism can be extrapolated to other phosphonic and silane derivatives. The key for silane adhesion is a good pretreatment (chemical cleaning, mechanical polishing, or abrasion) combined with a catalytic amount of phosphonic molecules, before any further coating deposition (organic paints) for aluminum protection.

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ASSOCIATED CONTENT

S Supporting Information *

Tables S1−S3 and Figures S1−S9. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by MICINN and FEDER funds (Project No. MAT2012-34498) and by the DIUE of the Generalitat de Catalunya (Contract Nos. 2009SGR925 and XRQTC). Support for the research of C.A. was received through the prize “ICREA Academia” for excellence in research funded by the Generalitat de Catalunya.

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REFERENCES

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