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Dynamic, in situ study of self-assembling organic phosphonic acid monolayers from ethanolic solutions on aluminium oxides by means of odd random phase multisine electrochemical impedance spectroscopy
Electrochimica Acta 106 (2013) 342–350
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Dynamic, in situ study of self-assembling organic phosphonic acid monolayers from ethanolic solutions on aluminium oxides by means of odd random phase multisine electrochemical impedance spectroscopy Tom Hauffman a,∗ , Yves van Ingelgem a , Tom Breugelmans a,b , Els Tourwé a , Herman Terryn a , Annick Hubin a a b
Vrije Universiteit Brussel, Department of Electrochemical and Surface Engineering, Pleinlaan 2, 1050 Brussels, Belgium Artesis University College of Antwerp, Applied Engineering and Technology-Chemistry, Paardenmarkt 92, 2000 Antwerp, Belgium
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Article history: Received 8 January 2013 Received in revised form 2 April 2013 Accepted 3 April 2013 Available online 3 June 2013 Keywords: SAM Ethanol In situ ORP EIS
a b s t r a c t The study of self-assembling monolayers on various oxide substrates is a scientific field still in full development. One of the challenges in this type of work is to probe the interactions in situ and dynamically. In this study a novel approach to investigate the adsorption of n-octylphosphonic acid on aluminium oxide from an ethanolic solution through odd random phase electrochemical impedance spectroscopy is presented. A model is proposed to describe the system and its validity is statistically established. It is observed that molecules adsorb on the surface. It is proven that the acid–base condensation reaction expels water which stays nearby the hydrophilic surface. Furthermore, it is shown that the phosphonic molecules bind ionically with the oxide surface. The work in this manuscript clearly shows that ethanol as a solvent is not suited to form stable organic acid layers on the surface. Due to the fact that water diffuses slowly in the bulk solvent, hazardous local environments are created at the oxide surface. During adsorption, the oxide is at the same time attacked. In this work, it is shown that odd random phase multisine electrochemical impedance spectroscopy is the ideal technique to not only investigate in situ dynamically the adsorbing behaviour of very thin films, but also to comprehend what happens with the buried substrate. Moreover, complex models can be used to fit the datasets obtained as it is possible with this analysis technique to prove statistically that they are correct. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction The adsorption of a wide range of self-assembling organic monolayers (SAMs) on a variety of substrates is being studied through mainly ex situ surface analysis methods. The best known system is the adsorption of thiol functionalities on ideal Au(1 1 1) substrates [1–3]. However, due to the lack of organic functionalities which are able to assemble on the noble substrates, scientific interest is shifting towards other substrates like oxide films. For example, the adsorption of phosphonic acids self-assembling monolayers on aluminium oxides is of substantial interest due to potential application in corrosion protection, biosensing, miniaturisation, adhesion promoting molecules, etc. [4,5]. The work described in literature on the adsorption of phosphonic acids on aluminium oxides is not fully consistent. It has
∗ Corresponding author. Tel.: +32 26293538. E-mail address: [email protected] (T. Hauffman). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.04.025
been tried to assemble the molecules from aqueous solutions where it has been shown by several ex situ techniques that short molecules (e.g. n-octylphosphonic acid) have difficulties to form self-assembling layers on aluminium oxides [6,7]. Other authors claim to form SAM’s with phosphonic acids, having less than 10 carbons [8–11]. SAM’s on various oxides are formed with carbon chain lengths above 12 carbon atoms [12–15]. The main issue here is the Vanderwaals forces involved in the assembly of SAM’s. Longer chains exhibit larger forces with respect to each other, causing a more stable monolayer. The discrepancies in literature can have a number of reasons: lack of knowledge of the blank substrate, lack of knowledge of the influence of the solvent interactions with the oxide surface and ex situ analysis approaches, where the coated substrate is taken out of its original deposition environment. We recently analysed the deposition of n-octylphosphonic acid molecules from water on pretreated and very well characterised aluminium oxides. It was observed that in none of the cases (ex situ neither in situ) a monolayer was formed [6,16]. This could be due to the fact that the organic molecules’ aliphatic tails are repulsed by
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the polar solvent and create multilayers on the oxide surface. Therefore, a next step is using a solvent that is significantly less polar than water, in an in situ approach. Such solvents, like ethanol, are also necessary when using longer molecules that cannot be dissolved in water. In a preliminary study, the influence of ethanol as a solvent, using the same supporting electrolyte as used in this work, on the aluminium oxide was analysed [17]. Odd random phase multisine electrochemical impedance spectroscopy showed that ethanol adsorbs on the oxide surface through an acid–base condensation reaction. Furthermore, the influence of the ethanol adsorption on the oxide chemistry was characterised. Prior to the phosphonic acid layer formation, the samples were stabilised in ethanol for at least 24 h. This way, the influence of ethanol adsorption itself on the experimental data obtained in the continuation study of the deposition of n-octylphosphonic acid on the oxide is outruled. The study of the layer formation itself asks for a dynamic technique that is not destructive towards the substrate. It is known that the self-assembly process of monolayers in general, and more in particular the assembly process of phosphonic acids on aluminium oxides, can take from a couple of minutes to almost 24 h [18,13,19]. As such, different time scales of immersion should be experimentally covered and measured in a fast way. The technique put forward here is odd random phase multisine electrochemical impedance spectroscopy (ORP EIS) [20,21]. It enables a dynamic study, thanks to the shortened measurement time and the information on the non-stationary behaviour [22–24]. Furthermore, the ability to compare noise data with residuals of fits, provides a toolbox to evaluate the fitting quality. This is, to the authors knowledge, the first time that the adsorption of ultrathin films and organic monolayers is being probed in situ and dynamically using electrochemical impedance spectroscopy over a large frequency range.
2. Experimental N-octylphosphonic acid is purchased from Alfa Aesar (purity 98%) and used as received. The aluminium was obtained from Hydro Aluminium and has a 99.99% purity. The aluminium samples are pretreated as described in a previous paper [6]. First, the sample was immersed during 1 min in a 25 g/l aqueous solution of NaOH at 70 ◦ C. Next, the sample was rinsed during 15 s in water, whereafter it was ultrasonically cleaned with water for 2 min. After drying, the degreased substrate was electropolished during 6 min, in an 80 vol% ethanol – 20 vol% perchloric acid solution with a current density of 70 mA/cm. The sample was then rinsed again for 15 s with water. After drying, the sample was galvanostatically anodised with a current density of 5 mA/cm, with the voltage going up to 20 V, instead of up to 150 V. The electrolyte used is a 0.1 M di-ammoniumtartrate solution. This way, thinner oxide layers are obtained which are more suitable for electrochemical impedance spectroscopy. After rinsing it for 15 s with water, the sample was dried with compressed nitrogen. The oxide layer is 25 nm thick (as measured by visual ellipsometry and non-porous). The impedance spectra are acquired using a two-electrode set-up. The counter electrode is a platinum grid. The electrolyte consists of a 0.1 M NaClO4 (·H2 O) ethanolic solution. The concentration of n-octylphosphonic acid is 10−3 M. At this concentration, the molecules behave as free molecules in the solution [17]. The perturbation signal applied is a 10 mV RMS variation around the open circuit potential. The impedance spectrum is acquired between 0.1 and 104 Hz. The measurement time is less than 1 min. Before measuring the adsorption of the n-octylphosphonic acid molecules, the oxide samples are let to stabilise in ethanol. The influence of
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ethanol on the aluminium oxide and the necessity of this stabilisation period are outlined in a previous article [17].
3. Results and discussion In the Bode plots, presented in Figs. 1 and 2(a) and (b), an evolution throughout immersion time can be observed. As it is known from previous work that this evolution is not due to the influence of ethanol or the supporting electrolyte on the aluminium oxide, this effect should come from the interaction between the oxide and the phosphonic acids [17]. Fig. 1 represents the first 18 h of interaction. A continuous increase in the impedance amplitude is observed throughout immersion time in the frequency region between 10−1 and 101.5 Hz, together with a small shift of the phase to lower frequencies. After 19 h, the amplitude of the impedance in the same frequency region decreases drastically from 50,000 to 20,000 , as can be seen in Fig. 2, where the impedance curves are shown between 18 h and 53 h of submersion. This is accompanied by a phase shift towards the higher frequency regions. While the phase shift at high frequencies lowers, the one at low frequencies increases. In order to visualise possible mass transfer rate influencing behaviour, the Nyquist diagrams for both time frames can be found in Figs. 1 and 2(c). A clear influence can be noticed in the frequency region lower than 0.5 Hz. Additional information about the occurring phenomena is obtained from complimentary surface analysis techniques. Ex situ as well in situ, organophosphonic acid layers on the substrates have been detected using X-ray photoelectron spectroscopy and visual ellipsometry [6,16], in line with literature [9,10]. Secondly, secondary electron spectroscopy images were taken before the aluminium oxide was brought in contact with the phosphonic acid environment and after 52 h (see Fig. 3). It can be clearly seen, that, where the left image in Fig. 3 exhibits a smooth oxide surface, the right one shows a surface that is highly deformed by corrosion processes. Several cracks are observed at the surface and the oxide layer is flaking off. The Bode plots clearly show that more than one time constant can be resolved. With complementary techniques we evidenced at least two phenomena, i.e. adsorption of the organic layer and deterioration of the oxide layer. Therefore, we propose the model, presented in Fig. 4. Its ability to describe the system under investigation is discussed later on in the manuscript. As the double layer capacitance, that is in series with Ctotal , will be large compared to Ctotal and thus will exhibit a small impedance value, it will be not visualised in the measurement and as such it is left out of the model. From the fitting of model Fig. 4(a) it became clear that the resistance coupled with the Warburg element Wadsorption resulted in very small values (around 2 ) with large relative errors (nearly 100%). It is decided to omit this parameter since it hardly influences the total impedance. The high uncertainties for this element hinder the determination of the other elements. Therefore the model in Fig. 4(b) is put forward. In this model, Relec represents the resistance of the electrolyte, Ctotal the capacitive behaviour of the combination adsorbed organic layer – aluminium oxide, Wadsorption the Warburg element representing mass transfer behaviour related to the adsorption of the phosphonic acid molecules, CPEorganic the constant phase element describing the growing organic layer and the underlaying oxide, Rorganic the resistance of the solution in the defects of the organic layer. Coxide represents the capacitive behaviour of the oxide layer where no organic layer is adsorbed, Roxide the resistance of the defects in the oxide layer and Wcorrosion the Warburg element which describes mass transport due to corrosion reactions taking place at the oxide surface. Fig. 5 represents the magnitude Bode plot, modelled curve, residual curve and the different noise levels (noise on the excited
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Fig. 1. Bode plots (a and b) and Nyquist diagram (c) for n-octylphosphonic acid–aluminium oxide interactions in water for the first 18 h of immersion (0.16, 0.5, 1, 3 and 18 h of immersion).
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Fig. 2. Bode plots (a and b) and Nyquist diagram (c) for n-octylphosphonic acid–aluminium oxide interactions in water after 18 h of immersion (18, 19, 25, 29, 33, 45 and 53 h of immersion).
Fig. 3. SEM images of a blank (a) and corroded (b) aluminium oxide sample.
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relative error is always less than 1.2% (except for the two measurement points at the two lowest frequencies, where the residual relative error is less than 5.5%). We decided that the relative error is low enough to accept the model. 3.1. Fitting results In the following subsections, the fitted parameters are shown and discussed. In the figures, the parameter values with their standard deviations are given. The relative errors mentioned in the text itself are calculated by dividing these standard deviations by the corresponding parameter values. 3.1.1. Electrolyte resistance In Fig. 6, one can observe that the electrolyte resistance can be considered constant throughout immersion time (all fitted values are between 170 and 180 cm2 ). Up to 1718 min of immersion time the relative error is below 6%. It increases to around 15% after 1950 min. 3.1.2. Total capacitance Ctotal (Fig. 7) exhibits a stable value up to 1213 min. At 1478 min of immersion, a sudden increase in capacitance value is observed. After 2438 min, the value of the capacitance drops again and starts to evolve to its initial value. The increase in capacitance value at 1478 min is accompanied by an increase in standard deviation.
Fig. 4. Electrical equivalent circuit to model the phosphonic acid–aluminium oxide interactions happening in an ethanol environment. Relec = electrolyte resistance, Ctotal = total capacitance of the adsorbed organic layer and the underlying oxide, Wadsorption = Warburg element due to the reaction between the phosphonic acid molecules and the oxide and Radsorption the corresponding electron transfer resistance, CPEorganic and Rorganic = adsorbing organic layer, Coxide and Roxide = non covered oxide layer, Wcorrosion = Warburg element due to corrosion reaction.
frequencies and noise on the non excited frequencies) of the measurements at 10 and 3158 min of immersion. It should be remarked that the curves of the noise on the non excited and the noise on the excited frequencies make an almost perfect overlay. This means that the system is behaving stationary during the measurement. Using the technique described in [25] it can also be demonstrated that the systems response is linear. The residual is the difference between measured values and modelled ones. A good fit implies that the residual is situated within the noise level of the measurement. This is the case for the measurement at 10 min. At 3158 min, the difference between the noise and the residual becomes higher. Firstly, this is due to a decrease in signal to noise ratio by a factor of two at high frequencies. Secondly, it is observed that the residual
3.1.3. Organic layer parameters The constant phase element representing the adsorbing organic layer on the oxide surface, is recalculated to a pseudo capacitance, using the formula of Brug [26] (Fig. 8(a)). This pseudo capacitance can be considered stable up to 1213 min of immersion. Hereafter, the capacitance decreases with an order of magnitude, whereafter it becomes unstable and exhibits large standard deviations. The resistance, representing the defects in the organic layer, exhibits the same trend (Fig. 8(b)). Only in the time frame where a stable pseudo capacitance is observed, the resistance decreases continuously. At 1478 min, a sudden drop is observed. After 2250 min it increases again drastically to 5.5 k, decreasing afterwards to values as observed in the beginning of the immersion time. The alpha value of the constant phase element varies between 0.85 and 0.89 throughout immersion time. The electron transfer resistance coupled with the Warburg element Wadsorption was too small to be determined. Therefore is it not possible to calculate any physical meaningful parameters out of the Warburg elements fitted. 3.1.4. Oxide layer parameters The oxide layer exhibits a stable capacitive behaviour in the first 1213 min of immersion (Fig. 9(a)). The resistance of the electrolyte in the defects of the oxide increases in the same time frame (Fig. 9(b)). Then, the fitted capacitance becomes very unstable and exhibits large standard deviations. This behaviour is combined with a steep decrease of the resistance in the defects of the oxide layer. For the corrosion reaction, there is a resistance coupled with the Warburg element. In Fig. 10, the influence of this electron transfer resistance with respect to the mass transfer resistance is compared for three adsorption times: 10 min of immersion, 1958 min of immersion and 4118. A discussion of the parameter trends in this figure will be given in the following section. 3.2. Discussion of the physical meaning of the fitted parameters It is clear that three major time frames can be determined from 0 to 1213 min (hereafter referred to as frame A), from 1478 to
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Fig. 5. Magnitude Bode plot, modelled curve, residual curve, noise on the excited frequencies and noise on the non excited frequencies of the measurements at (a) 10 and (b) 3158 min of immersion.
2198 min (frame B) and from 2438 min to the end of the measurement series (frame C). 3.2.1. Time frame A In frame A, the change in the value in the parameters is small but meaningful as it is higher than the standard deviations calculated for every parameter value on every measurement step. An impedance magnitude increase is observed. This behaviour can mainly be attributed to the reactions between the phosphonic acid molecules and the oxide hydroxyls, as is also confirmed by XPS analysis in another scientific publication [27]. As the layers grow and Ctotal is constant, most possibly the width and height growth compensate each other in this capacitive behaviour (Fig. 7). The pseudocapacitance, Corganic (Fig. 8), which represents the interaction layer of the organophosphonics with the oxide surface, increases because more and more surface will be taken by adsorbing molecules. An increasing surface leads to an increasing capacitor value. In this time frame, the resistance correlated with the organic layer pores Rorganic (Fig. 8) exhibits a small global decreasing tendency. It is not possible to correlate the resistance of these pores univocally with a physical phenomenon
Fig. 6. Electrolyte resistance and its standard deviation as a function of immersion time.
going on, as this behaviour will be e.g. influenced by the changing concentrations of supporting electrolyte and phosphonic acids, the water content which is expelled due to the acid–base reaction, the ethanol content, etc. In time frame A, the oxide surface is not changing. The increasing resistance of the electrolyte in the oxide defects can be attributed to the adsorption of the organophosphonics (Fig. 9(b)), filling up the pores. Due to the chemical reactions between the surface hydroxyls and the phosphonic acid functional groups, water is expelled [28]. This water is diffusing to the bulk solvent due to concentration differences. However, aluminium oxide is hydrophilic and attracts the water. Therefore, mass transfer is influencing the system, which is shown by the presence of the Warburg element Wadsorption . It is known from literature that phosphonic acids bind with the surface through the formation of a surface salt [29,30]. This implies an ionic bound. As such, if we vary the potential of the surface, these ionic bounds will respond to that and this will occur in the impedance data. The binding of the phosphonic acids to the surface is strongly influenced by the presence of water (chemical equilibria). When water does not diffuse immediately to the bulk of the solvent, this will have an effect on the adsorption (and thus on the ionic bonds) and this will be visible in the impedance spectra as a Warburg element.
Fig. 7. Ctotal and its standard deviation as a function of immersion time.
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(b)
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Fig. 8. Organic layer parameters and their standard deviation as a function of immersion time.
(a)
(b) Fig. 9. Oxide parameters and their standard deviation as a function of immersion time.
Normally, a resistance is coupled with the Warburg element in the electrical equivalent circuit. However, due to the fact that this resistance is coupled with the presence of ionic bonds, it is very small and difficult to determine. Because it does influence the overall impedance only by a very small amount, the resistance can be left out of the equivalent circuit.
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4
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2
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0
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Fig. 10. Comparison of the electron transfer resistance Roxide with the mass transfer resistance (Wcorrosion ) for different immersion times.
Concerning the Warburg element connected to the corrosion at the oxide surface (Fig. 10), it can be seen that the electron transfer resistance is dominant in time frame A. Here, almost no corrosion is going on, thus the diffusion of corrosion products is negligible. 3.2.2. Time frame B In time frame B, a sudden event occurs, which in a later phase restabilises. The only parameter that immediately exhibits a continuous trend is the resistance Roxide (Fig. 9(b)) coupled with the oxide matrix. In this time frame a corrosion reaction is dominating the global mechanism. This corrosion reaction is mainly initiated by the expelled water from the phosphonic acid adsorption reaction. As can be seen in frame A, water diffuses slowly to the bulk solvent. Phosphonic acids can dissolve in water, creating an environment with a pH sufficiently low to initiate aluminium oxide corrosion. This is confirmed by the sudden decrease of Roxide . Such a corrosion breakthrough measured is only possible when in time frame A also local corrosion systems exist. This is not seen in the global impedance character of time frame A, because the adsorption of the phosphonic acids is dominating. The organic layer sudden delaminates because the oxide structure is removed by the corrosion. This explains the drastic decrease of the organic layer resistance, Rorganic (Fig. 8). In this time frame, it can be seen that the standard deviations of the fitted capacitances become higher. This can be attributed to the sudden changes at the interface which firmly destabilise the building-up of the organic layer.
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fit increases in this time frame, it is acknowledged that the model proposed does not fully exhibit the possibility to describe the system under study. More complex models ought to be applied, but as physical knowledge is limited at this stage it is too speculative to use more complex models than the presented open to describe the process.
3.3. Final adsorption mechanism of n-octylphosphonic acid on aluminium oxide from an ethanolic solution It can be seen in the discussion above that all parameters follow trends which can be correlated with complementary surface analysis techniques. Furthermore, all residual relative errors of the modelled curves versus the experimental ones situate themselves within acceptable ranges. The standard deviations are acceptable in the situation where there is mainly adsorption of the noctylphosphonic acid molecules. When corrosion breaks through, standard deviations become higher due to the very unstable of the interface. The fact that complementary analysis techniques [27], parameter trends, residual errors on the modelled curves and behaviour of the standard deviations all can be correlated in an unambiguous way, established the validity of the model proposed. The model, used here, suits every experiment performed in these conditions. Ten different sets of measurements were performed and all of them can be fitted using the model and all of them establish the mechanism proposed. The total scheme of what is going on during the three defined time frames, based on ex situ experimental results, ORP EIS data and statistical evaluation of EIS fits, can be found in Fig. 11.
4. Conclusions Fig. 11. Physical representation of the interactions between n-octylphosphonic acid and aluminium oxide when using ethanol as a solvent.
The corrosion breakthrough is also visualised in the Warburg element connected to the corrosion reaction at the oxide surface (Fig. 10): in zone B, the Warburg modulus is the highest of all three time frames. 3.2.3. Time frame C In time frame C, the oxide layer parameters exhibit an unstable trend, caused by the continuously ongoing corrosion reaction (Fig. 9). This is especially shown by the decreasing Roxide . Due to the ongoing corrosion reaction the capacitance behaviour is unstable, what translates itself in higher standard deviations on the Coxide parameter. Ctotal increases to a value higher than the value measured after 10 min of immersion (Fig. 7). A local stable surface seems to exist when no corrosion breaks through, although the capacitance is decreasing again in time frame C. This can be attributed to again local corrosion spots, where organic layer and oxide material is being removed. Due to the ongoing corrosion reaction, the interaction layer is ceaselessly disturbed. In time frame C, the Warburg modulus of the corrosion reaction (Fig. 10) is comparable to zone B, but the electron transfer resistance still continues to decrease. This confirms the ongoing corrosion of the oxide, creating defects in the matrix. In both time frames B and C, mass transfer plays a dominant role in the lower frequency region (starting from 100.3 Hz for B and 100.5 Hz for C). In view of the fact that the standard deviations on the fitted parameters increase together with the fact that the residual of the
In this work, an in situ and dynamic characterisation approach was established for the adsorption of n-octylphosphonic acid molecules on aluminium oxides using odd random phase multisine electrochemical impedance spectroscopy. This novel technique enables us to measure large frequency band impedance spectra in a short time. This way, dynamic systems can be measured in a stationary way. A model was proposed to represent the interactions between the phosphonic acids and the aluminium oxides. The model was statistically proven to be correct. It was shown that the adsorption behaviour of phosphonic acids on the oxide can be classified in three time frames. At first, the molecules adsorb on the surface, filling oxide cracks and forming stable layers. It could be shown that the binding between aluminium hydroxyls and the n-octylphosphonic acid functional group is, at least partly, an ionic one. Secondly, the water expelled from the adsorption interaction forms, in combination with the acids, hazardous environments for the oxide. A corrosion breakthrough is observed in the second time frame, removing oxide and organic layer. This corrosion is a consequence of the reaction between the phosphonic acid molecules and the surface and as such is inherently part of the layer formation process. In a third time zone, the organic layer tries to restabilise on the surface, but the combination of the ongoing attack at the oxide leads to very unstable layers. Therefore, in this work, it could be established that ethanol as a solvent for the deposition of organic self-assembling monolayers is not always suitable. If the molecules interact with the oxides through an acid–base elimination condensation reaction and if they are soluble in water, then adsorption always goes hand in hand with the deterioration of the oxide film itself. As such, it is impossible to form monolayers of these molecules from ethanolic solutions.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Dynamic, in situ study of self-assembling organic phosphonic acid monolayers from ethanolic solutions on aluminium oxides by means of odd random phase multisine electrochemical impedance spectroscopy Tom Hauffman a,∗ , Yves van Ingelgem a , Tom Breugelmans a,b , Els Tourwé a , Herman Terryn a , Annick Hubin a a b
Vrije Universiteit Brussel, Department of Electrochemical and Surface Engineering, Pleinlaan 2, 1050 Brussels, Belgium Artesis University College of Antwerp, Applied Engineering and Technology-Chemistry, Paardenmarkt 92, 2000 Antwerp, Belgium
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Article history: Received 8 January 2013 Received in revised form 2 April 2013 Accepted 3 April 2013 Available online 3 June 2013 Keywords: SAM Ethanol In situ ORP EIS
a b s t r a c t The study of self-assembling monolayers on various oxide substrates is a scientific field still in full development. One of the challenges in this type of work is to probe the interactions in situ and dynamically. In this study a novel approach to investigate the adsorption of n-octylphosphonic acid on aluminium oxide from an ethanolic solution through odd random phase electrochemical impedance spectroscopy is presented. A model is proposed to describe the system and its validity is statistically established. It is observed that molecules adsorb on the surface. It is proven that the acid–base condensation reaction expels water which stays nearby the hydrophilic surface. Furthermore, it is shown that the phosphonic molecules bind ionically with the oxide surface. The work in this manuscript clearly shows that ethanol as a solvent is not suited to form stable organic acid layers on the surface. Due to the fact that water diffuses slowly in the bulk solvent, hazardous local environments are created at the oxide surface. During adsorption, the oxide is at the same time attacked. In this work, it is shown that odd random phase multisine electrochemical impedance spectroscopy is the ideal technique to not only investigate in situ dynamically the adsorbing behaviour of very thin films, but also to comprehend what happens with the buried substrate. Moreover, complex models can be used to fit the datasets obtained as it is possible with this analysis technique to prove statistically that they are correct. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction The adsorption of a wide range of self-assembling organic monolayers (SAMs) on a variety of substrates is being studied through mainly ex situ surface analysis methods. The best known system is the adsorption of thiol functionalities on ideal Au(1 1 1) substrates [1–3]. However, due to the lack of organic functionalities which are able to assemble on the noble substrates, scientific interest is shifting towards other substrates like oxide films. For example, the adsorption of phosphonic acids self-assembling monolayers on aluminium oxides is of substantial interest due to potential application in corrosion protection, biosensing, miniaturisation, adhesion promoting molecules, etc. [4,5]. The work described in literature on the adsorption of phosphonic acids on aluminium oxides is not fully consistent. It has
∗ Corresponding author. Tel.: +32 26293538. E-mail address: [email protected] (T. Hauffman). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.04.025
been tried to assemble the molecules from aqueous solutions where it has been shown by several ex situ techniques that short molecules (e.g. n-octylphosphonic acid) have difficulties to form self-assembling layers on aluminium oxides [6,7]. Other authors claim to form SAM’s with phosphonic acids, having less than 10 carbons [8–11]. SAM’s on various oxides are formed with carbon chain lengths above 12 carbon atoms [12–15]. The main issue here is the Vanderwaals forces involved in the assembly of SAM’s. Longer chains exhibit larger forces with respect to each other, causing a more stable monolayer. The discrepancies in literature can have a number of reasons: lack of knowledge of the blank substrate, lack of knowledge of the influence of the solvent interactions with the oxide surface and ex situ analysis approaches, where the coated substrate is taken out of its original deposition environment. We recently analysed the deposition of n-octylphosphonic acid molecules from water on pretreated and very well characterised aluminium oxides. It was observed that in none of the cases (ex situ neither in situ) a monolayer was formed [6,16]. This could be due to the fact that the organic molecules’ aliphatic tails are repulsed by
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the polar solvent and create multilayers on the oxide surface. Therefore, a next step is using a solvent that is significantly less polar than water, in an in situ approach. Such solvents, like ethanol, are also necessary when using longer molecules that cannot be dissolved in water. In a preliminary study, the influence of ethanol as a solvent, using the same supporting electrolyte as used in this work, on the aluminium oxide was analysed [17]. Odd random phase multisine electrochemical impedance spectroscopy showed that ethanol adsorbs on the oxide surface through an acid–base condensation reaction. Furthermore, the influence of the ethanol adsorption on the oxide chemistry was characterised. Prior to the phosphonic acid layer formation, the samples were stabilised in ethanol for at least 24 h. This way, the influence of ethanol adsorption itself on the experimental data obtained in the continuation study of the deposition of n-octylphosphonic acid on the oxide is outruled. The study of the layer formation itself asks for a dynamic technique that is not destructive towards the substrate. It is known that the self-assembly process of monolayers in general, and more in particular the assembly process of phosphonic acids on aluminium oxides, can take from a couple of minutes to almost 24 h [18,13,19]. As such, different time scales of immersion should be experimentally covered and measured in a fast way. The technique put forward here is odd random phase multisine electrochemical impedance spectroscopy (ORP EIS) [20,21]. It enables a dynamic study, thanks to the shortened measurement time and the information on the non-stationary behaviour [22–24]. Furthermore, the ability to compare noise data with residuals of fits, provides a toolbox to evaluate the fitting quality. This is, to the authors knowledge, the first time that the adsorption of ultrathin films and organic monolayers is being probed in situ and dynamically using electrochemical impedance spectroscopy over a large frequency range.
2. Experimental N-octylphosphonic acid is purchased from Alfa Aesar (purity 98%) and used as received. The aluminium was obtained from Hydro Aluminium and has a 99.99% purity. The aluminium samples are pretreated as described in a previous paper [6]. First, the sample was immersed during 1 min in a 25 g/l aqueous solution of NaOH at 70 ◦ C. Next, the sample was rinsed during 15 s in water, whereafter it was ultrasonically cleaned with water for 2 min. After drying, the degreased substrate was electropolished during 6 min, in an 80 vol% ethanol – 20 vol% perchloric acid solution with a current density of 70 mA/cm. The sample was then rinsed again for 15 s with water. After drying, the sample was galvanostatically anodised with a current density of 5 mA/cm, with the voltage going up to 20 V, instead of up to 150 V. The electrolyte used is a 0.1 M di-ammoniumtartrate solution. This way, thinner oxide layers are obtained which are more suitable for electrochemical impedance spectroscopy. After rinsing it for 15 s with water, the sample was dried with compressed nitrogen. The oxide layer is 25 nm thick (as measured by visual ellipsometry and non-porous). The impedance spectra are acquired using a two-electrode set-up. The counter electrode is a platinum grid. The electrolyte consists of a 0.1 M NaClO4 (·H2 O) ethanolic solution. The concentration of n-octylphosphonic acid is 10−3 M. At this concentration, the molecules behave as free molecules in the solution [17]. The perturbation signal applied is a 10 mV RMS variation around the open circuit potential. The impedance spectrum is acquired between 0.1 and 104 Hz. The measurement time is less than 1 min. Before measuring the adsorption of the n-octylphosphonic acid molecules, the oxide samples are let to stabilise in ethanol. The influence of
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ethanol on the aluminium oxide and the necessity of this stabilisation period are outlined in a previous article [17].
3. Results and discussion In the Bode plots, presented in Figs. 1 and 2(a) and (b), an evolution throughout immersion time can be observed. As it is known from previous work that this evolution is not due to the influence of ethanol or the supporting electrolyte on the aluminium oxide, this effect should come from the interaction between the oxide and the phosphonic acids [17]. Fig. 1 represents the first 18 h of interaction. A continuous increase in the impedance amplitude is observed throughout immersion time in the frequency region between 10−1 and 101.5 Hz, together with a small shift of the phase to lower frequencies. After 19 h, the amplitude of the impedance in the same frequency region decreases drastically from 50,000 to 20,000 , as can be seen in Fig. 2, where the impedance curves are shown between 18 h and 53 h of submersion. This is accompanied by a phase shift towards the higher frequency regions. While the phase shift at high frequencies lowers, the one at low frequencies increases. In order to visualise possible mass transfer rate influencing behaviour, the Nyquist diagrams for both time frames can be found in Figs. 1 and 2(c). A clear influence can be noticed in the frequency region lower than 0.5 Hz. Additional information about the occurring phenomena is obtained from complimentary surface analysis techniques. Ex situ as well in situ, organophosphonic acid layers on the substrates have been detected using X-ray photoelectron spectroscopy and visual ellipsometry [6,16], in line with literature [9,10]. Secondly, secondary electron spectroscopy images were taken before the aluminium oxide was brought in contact with the phosphonic acid environment and after 52 h (see Fig. 3). It can be clearly seen, that, where the left image in Fig. 3 exhibits a smooth oxide surface, the right one shows a surface that is highly deformed by corrosion processes. Several cracks are observed at the surface and the oxide layer is flaking off. The Bode plots clearly show that more than one time constant can be resolved. With complementary techniques we evidenced at least two phenomena, i.e. adsorption of the organic layer and deterioration of the oxide layer. Therefore, we propose the model, presented in Fig. 4. Its ability to describe the system under investigation is discussed later on in the manuscript. As the double layer capacitance, that is in series with Ctotal , will be large compared to Ctotal and thus will exhibit a small impedance value, it will be not visualised in the measurement and as such it is left out of the model. From the fitting of model Fig. 4(a) it became clear that the resistance coupled with the Warburg element Wadsorption resulted in very small values (around 2 ) with large relative errors (nearly 100%). It is decided to omit this parameter since it hardly influences the total impedance. The high uncertainties for this element hinder the determination of the other elements. Therefore the model in Fig. 4(b) is put forward. In this model, Relec represents the resistance of the electrolyte, Ctotal the capacitive behaviour of the combination adsorbed organic layer – aluminium oxide, Wadsorption the Warburg element representing mass transfer behaviour related to the adsorption of the phosphonic acid molecules, CPEorganic the constant phase element describing the growing organic layer and the underlaying oxide, Rorganic the resistance of the solution in the defects of the organic layer. Coxide represents the capacitive behaviour of the oxide layer where no organic layer is adsorbed, Roxide the resistance of the defects in the oxide layer and Wcorrosion the Warburg element which describes mass transport due to corrosion reactions taking place at the oxide surface. Fig. 5 represents the magnitude Bode plot, modelled curve, residual curve and the different noise levels (noise on the excited
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Fig. 1. Bode plots (a and b) and Nyquist diagram (c) for n-octylphosphonic acid–aluminium oxide interactions in water for the first 18 h of immersion (0.16, 0.5, 1, 3 and 18 h of immersion).
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Fig. 2. Bode plots (a and b) and Nyquist diagram (c) for n-octylphosphonic acid–aluminium oxide interactions in water after 18 h of immersion (18, 19, 25, 29, 33, 45 and 53 h of immersion).
Fig. 3. SEM images of a blank (a) and corroded (b) aluminium oxide sample.
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relative error is always less than 1.2% (except for the two measurement points at the two lowest frequencies, where the residual relative error is less than 5.5%). We decided that the relative error is low enough to accept the model. 3.1. Fitting results In the following subsections, the fitted parameters are shown and discussed. In the figures, the parameter values with their standard deviations are given. The relative errors mentioned in the text itself are calculated by dividing these standard deviations by the corresponding parameter values. 3.1.1. Electrolyte resistance In Fig. 6, one can observe that the electrolyte resistance can be considered constant throughout immersion time (all fitted values are between 170 and 180 cm2 ). Up to 1718 min of immersion time the relative error is below 6%. It increases to around 15% after 1950 min. 3.1.2. Total capacitance Ctotal (Fig. 7) exhibits a stable value up to 1213 min. At 1478 min of immersion, a sudden increase in capacitance value is observed. After 2438 min, the value of the capacitance drops again and starts to evolve to its initial value. The increase in capacitance value at 1478 min is accompanied by an increase in standard deviation.
Fig. 4. Electrical equivalent circuit to model the phosphonic acid–aluminium oxide interactions happening in an ethanol environment. Relec = electrolyte resistance, Ctotal = total capacitance of the adsorbed organic layer and the underlying oxide, Wadsorption = Warburg element due to the reaction between the phosphonic acid molecules and the oxide and Radsorption the corresponding electron transfer resistance, CPEorganic and Rorganic = adsorbing organic layer, Coxide and Roxide = non covered oxide layer, Wcorrosion = Warburg element due to corrosion reaction.
frequencies and noise on the non excited frequencies) of the measurements at 10 and 3158 min of immersion. It should be remarked that the curves of the noise on the non excited and the noise on the excited frequencies make an almost perfect overlay. This means that the system is behaving stationary during the measurement. Using the technique described in [25] it can also be demonstrated that the systems response is linear. The residual is the difference between measured values and modelled ones. A good fit implies that the residual is situated within the noise level of the measurement. This is the case for the measurement at 10 min. At 3158 min, the difference between the noise and the residual becomes higher. Firstly, this is due to a decrease in signal to noise ratio by a factor of two at high frequencies. Secondly, it is observed that the residual
3.1.3. Organic layer parameters The constant phase element representing the adsorbing organic layer on the oxide surface, is recalculated to a pseudo capacitance, using the formula of Brug [26] (Fig. 8(a)). This pseudo capacitance can be considered stable up to 1213 min of immersion. Hereafter, the capacitance decreases with an order of magnitude, whereafter it becomes unstable and exhibits large standard deviations. The resistance, representing the defects in the organic layer, exhibits the same trend (Fig. 8(b)). Only in the time frame where a stable pseudo capacitance is observed, the resistance decreases continuously. At 1478 min, a sudden drop is observed. After 2250 min it increases again drastically to 5.5 k, decreasing afterwards to values as observed in the beginning of the immersion time. The alpha value of the constant phase element varies between 0.85 and 0.89 throughout immersion time. The electron transfer resistance coupled with the Warburg element Wadsorption was too small to be determined. Therefore is it not possible to calculate any physical meaningful parameters out of the Warburg elements fitted. 3.1.4. Oxide layer parameters The oxide layer exhibits a stable capacitive behaviour in the first 1213 min of immersion (Fig. 9(a)). The resistance of the electrolyte in the defects of the oxide increases in the same time frame (Fig. 9(b)). Then, the fitted capacitance becomes very unstable and exhibits large standard deviations. This behaviour is combined with a steep decrease of the resistance in the defects of the oxide layer. For the corrosion reaction, there is a resistance coupled with the Warburg element. In Fig. 10, the influence of this electron transfer resistance with respect to the mass transfer resistance is compared for three adsorption times: 10 min of immersion, 1958 min of immersion and 4118. A discussion of the parameter trends in this figure will be given in the following section. 3.2. Discussion of the physical meaning of the fitted parameters It is clear that three major time frames can be determined from 0 to 1213 min (hereafter referred to as frame A), from 1478 to
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Fig. 5. Magnitude Bode plot, modelled curve, residual curve, noise on the excited frequencies and noise on the non excited frequencies of the measurements at (a) 10 and (b) 3158 min of immersion.
2198 min (frame B) and from 2438 min to the end of the measurement series (frame C). 3.2.1. Time frame A In frame A, the change in the value in the parameters is small but meaningful as it is higher than the standard deviations calculated for every parameter value on every measurement step. An impedance magnitude increase is observed. This behaviour can mainly be attributed to the reactions between the phosphonic acid molecules and the oxide hydroxyls, as is also confirmed by XPS analysis in another scientific publication [27]. As the layers grow and Ctotal is constant, most possibly the width and height growth compensate each other in this capacitive behaviour (Fig. 7). The pseudocapacitance, Corganic (Fig. 8), which represents the interaction layer of the organophosphonics with the oxide surface, increases because more and more surface will be taken by adsorbing molecules. An increasing surface leads to an increasing capacitor value. In this time frame, the resistance correlated with the organic layer pores Rorganic (Fig. 8) exhibits a small global decreasing tendency. It is not possible to correlate the resistance of these pores univocally with a physical phenomenon
Fig. 6. Electrolyte resistance and its standard deviation as a function of immersion time.
going on, as this behaviour will be e.g. influenced by the changing concentrations of supporting electrolyte and phosphonic acids, the water content which is expelled due to the acid–base reaction, the ethanol content, etc. In time frame A, the oxide surface is not changing. The increasing resistance of the electrolyte in the oxide defects can be attributed to the adsorption of the organophosphonics (Fig. 9(b)), filling up the pores. Due to the chemical reactions between the surface hydroxyls and the phosphonic acid functional groups, water is expelled [28]. This water is diffusing to the bulk solvent due to concentration differences. However, aluminium oxide is hydrophilic and attracts the water. Therefore, mass transfer is influencing the system, which is shown by the presence of the Warburg element Wadsorption . It is known from literature that phosphonic acids bind with the surface through the formation of a surface salt [29,30]. This implies an ionic bound. As such, if we vary the potential of the surface, these ionic bounds will respond to that and this will occur in the impedance data. The binding of the phosphonic acids to the surface is strongly influenced by the presence of water (chemical equilibria). When water does not diffuse immediately to the bulk of the solvent, this will have an effect on the adsorption (and thus on the ionic bonds) and this will be visible in the impedance spectra as a Warburg element.
Fig. 7. Ctotal and its standard deviation as a function of immersion time.
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(b)
(a)
Fig. 8. Organic layer parameters and their standard deviation as a function of immersion time.
(a)
(b) Fig. 9. Oxide parameters and their standard deviation as a function of immersion time.
Normally, a resistance is coupled with the Warburg element in the electrical equivalent circuit. However, due to the fact that this resistance is coupled with the presence of ionic bonds, it is very small and difficult to determine. Because it does influence the overall impedance only by a very small amount, the resistance can be left out of the equivalent circuit.
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5
4
3
2
-2
-1
0
1
2
3
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Fig. 10. Comparison of the electron transfer resistance Roxide with the mass transfer resistance (Wcorrosion ) for different immersion times.
Concerning the Warburg element connected to the corrosion at the oxide surface (Fig. 10), it can be seen that the electron transfer resistance is dominant in time frame A. Here, almost no corrosion is going on, thus the diffusion of corrosion products is negligible. 3.2.2. Time frame B In time frame B, a sudden event occurs, which in a later phase restabilises. The only parameter that immediately exhibits a continuous trend is the resistance Roxide (Fig. 9(b)) coupled with the oxide matrix. In this time frame a corrosion reaction is dominating the global mechanism. This corrosion reaction is mainly initiated by the expelled water from the phosphonic acid adsorption reaction. As can be seen in frame A, water diffuses slowly to the bulk solvent. Phosphonic acids can dissolve in water, creating an environment with a pH sufficiently low to initiate aluminium oxide corrosion. This is confirmed by the sudden decrease of Roxide . Such a corrosion breakthrough measured is only possible when in time frame A also local corrosion systems exist. This is not seen in the global impedance character of time frame A, because the adsorption of the phosphonic acids is dominating. The organic layer sudden delaminates because the oxide structure is removed by the corrosion. This explains the drastic decrease of the organic layer resistance, Rorganic (Fig. 8). In this time frame, it can be seen that the standard deviations of the fitted capacitances become higher. This can be attributed to the sudden changes at the interface which firmly destabilise the building-up of the organic layer.
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fit increases in this time frame, it is acknowledged that the model proposed does not fully exhibit the possibility to describe the system under study. More complex models ought to be applied, but as physical knowledge is limited at this stage it is too speculative to use more complex models than the presented open to describe the process.
3.3. Final adsorption mechanism of n-octylphosphonic acid on aluminium oxide from an ethanolic solution It can be seen in the discussion above that all parameters follow trends which can be correlated with complementary surface analysis techniques. Furthermore, all residual relative errors of the modelled curves versus the experimental ones situate themselves within acceptable ranges. The standard deviations are acceptable in the situation where there is mainly adsorption of the noctylphosphonic acid molecules. When corrosion breaks through, standard deviations become higher due to the very unstable of the interface. The fact that complementary analysis techniques [27], parameter trends, residual errors on the modelled curves and behaviour of the standard deviations all can be correlated in an unambiguous way, established the validity of the model proposed. The model, used here, suits every experiment performed in these conditions. Ten different sets of measurements were performed and all of them can be fitted using the model and all of them establish the mechanism proposed. The total scheme of what is going on during the three defined time frames, based on ex situ experimental results, ORP EIS data and statistical evaluation of EIS fits, can be found in Fig. 11.
4. Conclusions Fig. 11. Physical representation of the interactions between n-octylphosphonic acid and aluminium oxide when using ethanol as a solvent.
The corrosion breakthrough is also visualised in the Warburg element connected to the corrosion reaction at the oxide surface (Fig. 10): in zone B, the Warburg modulus is the highest of all three time frames. 3.2.3. Time frame C In time frame C, the oxide layer parameters exhibit an unstable trend, caused by the continuously ongoing corrosion reaction (Fig. 9). This is especially shown by the decreasing Roxide . Due to the ongoing corrosion reaction the capacitance behaviour is unstable, what translates itself in higher standard deviations on the Coxide parameter. Ctotal increases to a value higher than the value measured after 10 min of immersion (Fig. 7). A local stable surface seems to exist when no corrosion breaks through, although the capacitance is decreasing again in time frame C. This can be attributed to again local corrosion spots, where organic layer and oxide material is being removed. Due to the ongoing corrosion reaction, the interaction layer is ceaselessly disturbed. In time frame C, the Warburg modulus of the corrosion reaction (Fig. 10) is comparable to zone B, but the electron transfer resistance still continues to decrease. This confirms the ongoing corrosion of the oxide, creating defects in the matrix. In both time frames B and C, mass transfer plays a dominant role in the lower frequency region (starting from 100.3 Hz for B and 100.5 Hz for C). In view of the fact that the standard deviations on the fitted parameters increase together with the fact that the residual of the
In this work, an in situ and dynamic characterisation approach was established for the adsorption of n-octylphosphonic acid molecules on aluminium oxides using odd random phase multisine electrochemical impedance spectroscopy. This novel technique enables us to measure large frequency band impedance spectra in a short time. This way, dynamic systems can be measured in a stationary way. A model was proposed to represent the interactions between the phosphonic acids and the aluminium oxides. The model was statistically proven to be correct. It was shown that the adsorption behaviour of phosphonic acids on the oxide can be classified in three time frames. At first, the molecules adsorb on the surface, filling oxide cracks and forming stable layers. It could be shown that the binding between aluminium hydroxyls and the n-octylphosphonic acid functional group is, at least partly, an ionic one. Secondly, the water expelled from the adsorption interaction forms, in combination with the acids, hazardous environments for the oxide. A corrosion breakthrough is observed in the second time frame, removing oxide and organic layer. This corrosion is a consequence of the reaction between the phosphonic acid molecules and the surface and as such is inherently part of the layer formation process. In a third time zone, the organic layer tries to restabilise on the surface, but the combination of the ongoing attack at the oxide leads to very unstable layers. Therefore, in this work, it could be established that ethanol as a solvent for the deposition of organic self-assembling monolayers is not always suitable. If the molecules interact with the oxides through an acid–base elimination condensation reaction and if they are soluble in water, then adsorption always goes hand in hand with the deterioration of the oxide film itself. As such, it is impossible to form monolayers of these molecules from ethanolic solutions.
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