Self-assembled monolayers (SAM) are ordered molecular assemblies and are commonly used to modify aluminum surfaces. They are spontaneously formed by the adsorption of molecules with head groups that show affinity to a specific substrate [13
]. Phosphonic acids form SAMs on aluminum surfaces by condensation reactions of the acid functional group with basic surface-bound alumino-hydroxyl species. Thereby, the phosphonic acid head group shows a strong affinity to the aluminum substrate, creating an alumino-phosphonate linkage via a strong Al–O–P chemical bond:
R–PO(OH)2 + Al–OH→R–(OH)OP–O–Al + H2O [5–7].
To investigate a successful formation of the organophosphorus layers, including information about the properties of the films (surface morphology, structural ordering, density and uniformity, binding mode to the aluminum substrate, as well as stability), different analytical methods have been applied in the literature. A detailed characterization of the layers is necessary to understand their intended function (for instance corrosion protection, adhesion promotion, (non)-wettability, surface passivation). Table 1
summarizes the various analytical techniques that have been used in the last 20 years, or so, to study the organization and chemistry of organophosphorus compounds on an aluminum substrate. Various characterization techniques have been categorized by us into three groups: Surface morphology; presence, composition, and stability of layers; orientation of molecules and binding mode to aluminum, and are all briefly explained in the remainder of this section.
2.2. Presence, Composition, and Stability of Layer
A variety of surface sensitive analytical techniques has been used to investigate the presence of the layer and its composition. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) are both methods which determine the elemental composition in the first couple of nanometers of a surface and, hence, were often applied to verify the presence of phosphorus, indicating a successful organophosphorus coating chemically-bound to aluminum [6
]. As a general preparative step, the samples were usually rinsed/washed after the coating process to remove the residual chemicals and physically adsorbed organophosphorus compounds. In addition to the determination of the presence of the organophosphorus coating, XPS was also used to study the concentration of the organic molecules on the aluminum surface [18
]. The experimental phosphorus to aluminum concentration ratio allowed the quantification of the organophosphorus molecule surface concentration and modelling of the XPS data helped to determine the thickness of the monolayer [18
]. Thereby, the ratio of signals from the coating and from the underlying aluminum substrate was recorded, from which the coating thickness can be derived. Depth profiling via argon ion sputtering in combination with XPS was also used to determine the coating thickness (especially for thicker layers). The comparison between experimental and theoretical values for the carbon-to-phosphorus ratio was used as proof that chemically-uniform organophosphorus films formed on aluminum [7
]. The recording of characteristic binding energies allowed to follow the individual steps in the decomposition of the phosphonates on the aluminum surface [20
]. Table 2
summarizes XPS binding energies found in the literature for organophosphorus layers on metal surfaces.
Another analytical technique often used to study the presence and composition of organophosphorus layers on aluminum is reflectance Fourier-transform infrared spectroscopy (FTIR), often measured at a grazing angle to increase the surface sensitivity. FTIR was applied to investigate the adsorption of the coating by detecting specific vibrational modes involving phosphorus bonds [8
]. Table 3
contains a summary of frequencies of vibrational modes involving the phosphorus atom found in the literature for self-assembled organophosphorus monolayers on aluminum.
Time-of-flight secondary ion mass spectrometry (ToF-SIMS), as a complementary technique to XPS, has also been used to demonstrate the presence of the organophosphorus layer, thereby investigating selective functionalization on aluminum and glass regions of a surface by mapping analysis [6
]. In addition, surface plasmon resonance spectroscopy (SPR) was applied to examine the kinetics of the adsorption process [27
]. Thereby, it was found that the adsorption process of phosphonic acids on aluminum starts very quickly and reaches a plateau after some minutes.
Water contact angle (WCA) measurements were used to investigate the hydrophilicity and hydrophobicity of the surface. A change compared to a reference untreated aluminum surface indicated the successful formation of an organophosphorus layer. In addition, WCA measurements were also applied to study the orientation of the molecules (see also below) because in well-ordered monolayers, the access of the water drop to the aluminum surface is reduced [7
]. From the investigation of the temporal behavior of WCA, information about surface coverage and chain ordering was obtained [5
]. Finally, the stability of the adsorbed layers was determined by recording advancing and receding contact angles by repeated cycling [10
2.3. Orientation of Molecules and Binding Mode to Aluminum
In order to characterize the binding (orientation and mode) of the organophosphorus molecules to an aluminum surface, mostly three methods have been used in the literature: Infrared reflection absorption spectroscopy (IRRAS) at grazing angle (often applying polarized infrared radiation), X-ray photoelectron spectroscopy (XPS) at a fixed electron take-off angle as well as angle-resolved (ARXPS) and solid-state 31P nuclear magnetic resonance (NMR) spectroscopy.
The most expected orientation with the phosphonic acid group reacting with the surface hydroxyl groups of the aluminum substrate and the hydrocarbon tail and/or terminal functional groups on top was proven by angle-resolved XPS. The carbon (from the tail) to phosphorus (from the head group) intensity ratio was determined at low and high electron take-off angles, representing two different information depths [7
]. In addition, using infrared reflection absorption spectroscopy (IRRAS) and comparing the intensities of vibrations polarized perpendicular or parallel to the hydrocarbon backbone, information about the orientation of the organophosphorus molecule was also obtained [5
The phosphonic group can bind to the aluminum atom via a direct P–O–Al bond in different modes, i.e., mono-, bi-, or tridentate, depending on whether one, two or all three oxygen atoms from the phosphonic group are involved, respectively. It has been found that both the deposition mode (for instance stirring, sonication), as well as the structure of the phosphonates, play a role [12
]. Additionally, the evolution from tridentate to lower binding modes as film formation proceeds (the increase of coverage can change the chain ordering) was observed in the literature [5
]. These behaviors might explain the variety of binding modes reported in the literature for organophosphorus layers on aluminum.
summarizes the frequencies of infrared vibrational modes found in the literature for self-assembled organophosphorus layers on aluminum. By observing which vibrational bands are present and which are not, certain binding modes have been assigned (mostly bidentate [16
] and tridentate [12
], and some monodentate [30
]). For instance, a missing P–OH stretching mode together with the presence of PO32−
stretching modes (deprotonation of the phosphonic acid group) in the IR spectrum indicates the formation of a bidentate binding. Mixtures of binding modes have also been found [5
As a second technique to investigate the binding mode of organophosphorus molecules to aluminum, X-ray photoelectron spectroscopy (XPS) was used. Table 2
summarizes the XPS binding energies found in the literature for self-assembled organophosphorus layers on aluminum. As can be seen from Table 2
, due to overlapping bands, neither the aluminum nor the phosphorus signal allows to clearly differentiate between various binding modes of phosphorus to aluminum. It has to be noted that a trend in the experimental binding energy to increase for the structure [POn
is observed as the ratio of n
of “free” O ligands (n
) to covalently bound OR ligands (m
) is stepwise changed from 4:0 to 3:1 to 2:2 and to 1:3. Therefore, also the XPS phosphorus binding energy has been used to assign the binding mode (i.e., mono-, bi-, or tridentate) [34
]. However, a final conclusion cannot be drawn just from the P 2p signal and further experimental evidence is needed. The O 1s signal is of particular interest regarding the assessment of the type of bond between organophosphorus and the aluminum substrate. As the binding energy for P=O and P–O–Metal is different from the one for P–OH and P–OR (see Table 2
), the ratio of the intensities at these two binding energies was used to determine the binding mode. In agreement with other studies using IR (see above), a mixed monodentate and bidentate mode was found [34
]. As a third method, the linewidth and shifts of peaks in 31
P NMR spectra have been used to study the bonds between the aluminum and the oxygen atoms of the organophosphorus molecule and to assign a specific binding mode by comparison with literature data [12