Multilayer Deposition of Silica SolâGel Films by Electrochemical

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Multilayer Deposition of Silica Sol-Gel Films by Electrochemical Assisted Techniques Gianmarco Giordano, Christian Durante, Armando Gennaro, and Massimo Guglielmi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10040 • Publication Date (Web): 06 Dec 2016 Downloaded from http://pubs.acs.org on December 7, 2016

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The Journal of Physical Chemistry

Multilayer Deposition of Silica Sol-Gel Films by Electrochemical Assisted Techniques Gianmarco Giordano,1 Christian Durante,2 Armando Gennaro,2 Massimo Guglielmi1* 1 Dipartimento di Ingegneria Industriale, Università di Padova, Via Marzolo 9, 35135 Padova, Italy 2 Dipartimento di Scienze Chimiche, Università di Padova, Via Marzolo 1, 35131 Padova, Italy

*Corresponding Author Massimo Guglielmi, Università degli Studi di Padova, Via Marzolo 9, Padova (Italy) e-mail: [email protected] phone/fax: +39 049 8275509/+39 049 8275505

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ABSTRACT: In the last decade, electro-assisted sol-gel deposition was employed by different groups in the world to deposit thin coatings on a variety of substrates. By applying a negative potential, the reduction reactions of water and oxygen occur at the working electrode and the concurring generation of hydroxide makes the pH basic, thus enabling a faster condensation of sol-gel precursors. In this work we present two new methods that allow one to modulate the thickness of the deposited layer. The first one consists in an electrochemically assisted deposition of multilayers, with a linear growth of the film up to ten layers. A final thickness of 1 ± 0.1 m is achievable, without cracks and with no heat treatment required. The second method uses a pulsed potential in order to control the diffusion of hydroxide ions and, as a consequence, the growth of the silica layer on the substrate, determining a good homogeneity of the film. The starting solution contains a mixture of tetraethyl orthosilicate (TEOS), triethoxymethylsilane (MTES), water, ethanol, hydrochloric acid and potassium nitrate as electrolyte. The samples are characterized by ellipsometery, using Cauchy model.


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INTRODUCTION The deposition of thin layers by the sol-gel method is widely employed and many products on the market are obtained by different deposition methods and with different compositions, ranging from simple oxides to nanocomposite formulations. Although the method is, in principle, simple and a vast literature is available on both the science behind the process and its technological aspects, especially in relation to the various coating strategies, there are still limitations to the extensive use of the sol-gel technology. A particularly important point is the difficulty to obtain thick layers and, somehow related to it, the difficulty to coat objects with complex or hidden surfaces. The thickness is limited by the shrinkage of the gel and the resulting mechanical stresses arising when the deposited gel film dries under the constraint of the substrate. Above a certain critical thickness, that depends on the chemical composition but mainly on the structure of the gel and on its initial density, the layer starts to crack and above a certain thickness it can even delaminate completely from the substrate. This is a well-known phenomenon, described at the beginning of the nineties by Brinker1 and Scherer2 who showed that the maximum thickness obtainable by a single step dip-coating deposition cannot exceed 1 m. Different strategies have been studied in the past to overcome these limitations: increasing the compliance of the gel and its initial density by using organically modified tetraethoxysilane-methyltriethoxysilane (TEOSMTES) sols3, adding denser silica nanoparticles (colloids) to a TEOS-MTES sol4, incorporation of a polymer as stress-relaxing agent5-7, multilayer deposition, with each deposition step followed by a thermal treatment so as to relax the mechanical stresses8. The last strategy is of course, time consuming, even if in specific cases automatic deposition-rapid thermal annealing systems can be employed.


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In 1999 Shacham et al.9 developed a new deposition method based on the electrochemical generation of catalysts (hydroxide ions or protons) on a conductive surface to locally increase the rate of sol-gel reactions. Since then the method has been studied by several researchers and applied to different systems and for different purposes: zirconia and titania precursors were tested as alternative materials;10-11 nanoparticles were incorporated into nanocomposites;12-14 a gold coral-like nanostructure was co-deposited and templated by the silica network before its removal;15 silica films were applied for corrosion protection;16-18 ordered mesoporous silica was successfully obtained on ITO and other substrates.19-21 This method presents clear limitations, the most important being the need for an electrically conductive surface, but also introduces several advantages. Among these, the possibility of depositing a layer under local basic conditions using a stable acidic sol allows, in principle, to obtain denser layers. Although there are no direct demonstrations of this, indirect proofs can be easily obtained, as it will be discussed later in this paper. Recently the authors started to study the electrochemically assisted sol-gel deposition method (EASG) to try and clarify the roles of the many parameters involved in the process.22 It was observed that, while increasing the deposition time with the intent of increasing the thickness, the homogeneity of the layer progressively got worse and above a critical thickness of few tenths of micrometer the layer was heavily damaged. This was, in some way, unexpected as it was believed that an increased density of the layer was the premise for a thicker critical thickness. These findings showed the necessity for further experimental work aimed at clarifying the correlation between the experimental conditions, the resulting film thickness and its maximum achievable value.


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Here we report the results of this study, where two different approaches were used to try and solve the problem and, at the same time, to better understand the basic aspects of the deposition process. In particular, two set of experiments are described: the deposition of multiple layers by repeated extraction and re-immersion of the sample from the batch after electrochemical assisted deposition, and the deposition of a single layer, constantly immersed in the sol, by applying a pulsed potential.

EXPERIMENTAL Tetraethyl orthosilicate (≥99.0%) (TEOS), Methyl Triethoxysilane (>90%) (MTES) and Ethanol (≥ 99.8%) were purchased from Sigma Aldrich and used as received. Potassium nitrate (KNO3) was purchased from Carlo Erba. The coating solution was prepared by mixing ethanol (~31.5 mL), bi-distilled water (10 ml), hydrochloric acid (0.1 M, 0.5 mL), TEOS (6.4 mL) and MTES (1.63 mL, TEOS:MTES=70:30 molar ratio). The solution was pre-hydrolyzed overnight at room temperature under stirring. Before the deposition, KNO3 (0.03 M) was added under magnetic stirring until complete dissolution. The electrodeposition was performed by using a potentiostat/galvanostat Bio-Logic SP300 in a three-electrode electrochemical cylindrical cell (diameter: ~4 cm). The reference electrode was a saturated calomel electrode (SCE) and the counter electrode was a glassy carbon plate (16 mm x 20 mm). The working electrodes were obtained from a stainless steel sheet (AISI 304), covered on one side by a protective removable polymer film, which was cut in plates with a size of 30 mm x 30 mm x 1 mm. The deposition surface was polished to a mirror-like finishing. Teflon tape was used to mask the edges of the plates during the depositions leaving an exposed


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circular area of about 200 mm2. No stirring was applied. After deposition the samples were withdrawn at a low constant speed (~50 m s-1) to minimize the thickness of the layer deposited by dip-coating. For the multilayer deposition, the samples were extracted and reinserted in the cell after 5 minutes, enabling the solution to locally recover the initial pH conditions (neutralization of HOby means of protons). In order to measure the thickness for each layer, different samples were produced for each deposition step, assuming that the deposited layers were not affected by further immersion in the solution. For the pulsed potential method, “intermittent duty-cycles” were used (Figure 1) between a maximum potential of -1.4 V (“ON”) and a minimum of 0 V (“OFF”) vs. the reference electrode, with a “ON” time of 4, 5 and 6 s, and a “OFF” time ranging between 2.5 and 7 s with steps of 0.5 s. In all the experiments the total “ON” time was 300 s, i.e. each cycle was repeated from 50 (6 s x 50 = 300 s) to 75 (4 s x 75 = 300 s) times. During deposition no stirring was applied. The samples were dried at room temperature for at least 24 h before the characterization. The thickness was measured by a variable-angle spectroscopic Ellipsometer (J. A. Wollam Co., Inc.) at incident angles of 70° and 80°, with a wavelength range of 300-1200 nm. The thickness was fitted using a Cauchy model.


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Figure 1. Schematic representation of the “intermitted duty-cycles” used for the pulsed potential experiments.

RESULTS AND DISCUSSION The premise to this work was the difficulty of increasing the thickness of the deposited layer by simply increasing the deposition time. The homogeneity of the layer was observed to worsen with increasing deposition time, with the layer starting to crack above a certain critical thickness. This effect is commonly observed when coatings are deposited by traditional methods (dipping, spinning, etc.), as sol-gel layers obtained from acidic solutions are subjected to a large structural contraction upon drying, which in a constrained system (substrate-film) determines a tensile stress, which increases with the layer thickness. With the electrochemical deposition the local basic environment is expected to determine the formation of a denser and stiffer gel, thus increasing the critical thickness at which the film starts to crack. This raises some fundamental questions: why doesn’t it work? Why does the homogeneity, which is very high at the beginning, progressively get worse? A possible answer is that, with increasing time, the initially acidic sol becomes more and more basic at increasing distances from the surface of the working electrode (due to the diffusion


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of hydroxyl ions), and hydrolysis and condensation start being promoted not only at the surface, but also in the sol, producing complex species which are added to the surface layer in a nonhomogeneous way.20 Although the control of the pH close to the surface, and its variation with distance from it, is experimentally quite difficult, and could not be afforded, samples obtained by keeping the potential for 15 minutes showed the complete delamination of the obtained thick layer. Two approaches were adopted to avoid this unwanted effect: (i) the multilayer deposition method, where the diffusion of hydroxide ions and protons is allowed to regenerate the pH homogeneity in the sol during the “extraction” period, and the substrate is subsequently reimmersed in a sol which is compositionally much closer to the initial situation; (ii) the pulsed potential method, where the time for sol homogenization is given by stopping the production of HO- with the substrate remaining inside the sol. Silica Multilayer Deposition. The electrodeposition of a sol-gel layer can be controlled by two main variables: potential and time. The multiple deposition experiments were made by changing both these parameters. Taking as a reference condition a potential of -1.2 V vs. SCE and a deposition time of 150 seconds, the potential was lowered to -1.3 V vs. SCE while keeping the time constant, and the deposition time was decreased to 100 s while keeping the potential constant. In Figure 2a it can be observed that in the reference conditions the thickness of each layer was ~ 85 nm. By increasing the potential, the thickness increased to ~ 100 nm for each layer, while by decreasing the deposition time the thickness decreased to ~ 50 nm. Both the variations were expected. The increase of thickness with potential was due to the higher amount of HO-


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produced at the surface of the working electrode. The decrease of thickness with deposition time was obvious. Four layers were deposited in the three experimental conditions, while up to ten layers were deposited at -1.3 V vs. SCE and 150 s (Figure 2b) obtaining a total thickness close to 1 m (about 925 nm).

Figure 2. (a) Thickness of the first four layers deposited at -1.2 V for 100 s (black square), -1.2 V for 150 s (red circle), -1.3 V for 150 seconds (blue triangle). (b) Thickness of the first ten layers deposited at -1.3 V 150 s.

In all the three cases the thickness increased linearly with the number of layers. When ten layers were deposited, a slight deviation from linearity was observed starting from the seventh layer, with a negative deviation from the expected thickness. In all the experiments the final results were very uniform and homogeneous coatings were obtained, as shown by the SEM (BSE) image in Figure 3, where a representative 10 layers sample is reported.


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Figure 3. SEM image (BSE). Top view of the first ten layers deposited at -1.3 V 150 s.

The experimental data suggested that: a) the film building process was not affected by the previously deposited layers, at least up to a certain thickness; b) the film was stable against redissolution in the sol when it is re-immersed. c) very good quality films could be obtained. All of the above observations are not trivial. The surface where the deposition of a further layer occurred was moving away from the conducting metal substrate layer after layer, but the availability of hydroxyl ions at the new surface seemed to be unchanged. The reason might be that the film had sufficient porosity to allow water and oxygen reduction to occur on the working electrode and, as a consequence, not to affect the production of HO-. The stability of the deposited multilayer can be easily appreciated by comparison with a simple dip-coating procedure using an acidic sol where the deposited film is re-immersed in the same sol after extraction. The process can be repeated several times, but the thickness does not increase at all, or at least not by an additive process. This was checked in a set of control dipcoating experiments (not shown) where the solution used the electrodeposition experiments was


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employed. The condensation reactions occurring in the layer after dip-coating were evidently not sufficient to get a stable, interconnected gel, as it re-dissolved in the sol upon re-immersion. Only after thermal treatment (or a quite long drying time) a second layer could be deposited on the first one, thus doubling the thickness. What instead happened in the electro-assisted deposition was the formation of the film in a locally basic environment, that is known to increase the rate of condensation, therefore favoring its physical-chemical stabilization. As a matter of fact, the film could be re-immersed in the sol immediately after its extraction, with no dissolution of the gel layer. Pulsed Potential Deposition. In this experiment all the three main parameters which allowed one to modulate the film thickness, i.e. concentration of the precursor in the sol, the applied potential and the total deposition time, were kept constant. However, the set deposition time (300 s) was divided in intervals of varying length (4 - 6 s) by cycling the potential as described in the Experimental section and changing the “ON” (potential at -1.4 V vs. SCE) and the “OFF” (open circuit, OCP) times. This procedure favored the neutralization of the hydroxide ions produced at the substrate surface at the beginning of the deposition by the protons in solution (the pH of the sol is 3), when the accumulation of HO- was still small. Recalling that the diffusion layer was proportional to (Dt)0.5, where D is the diffusion coefficient (estimated to be 4000 m2/s) and t the deposition time, after only 5 s hydroxide ions were capable of reaching distances of 250 m.20 It was observed that setting the potential at -1.4 V vs. SCE, for a deposition time of 300 s (without any interruption) the film exhibited cracks over the entire surface. No cracks were observed, instead, if the total time of 300 s was divided in much shorter intervals. In Figure 4 three cases are presented. In the first case, the deposition time was divided into 75 intervals of 4 s


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each (“ON” time, black square); in the second case the number of intervals was 60, each with a 5 s length (red circle); in the last case the number of intervals was 50, each with a 6 s length (blue triangle). Four different “OFF” times were selected, in such a way to describe a general trend, which is similar in all the three cases.

Figure 4. Thickness of the film as function of the “OFF” time in the three different cases: number of intervals equal to 75 (black square), 60 (red circle), 50 (blue triangle). Applied potential: -1.4 V. Total deposition time: 300 s.

It was observed that: a) using an equal number of intervals, the thickness increased with decreasing “OFF” time; b) using an equal “OFF” time, the thickness increased with decreasing the number of intervals. These results were consistent with the diffusion of HO- ions away from the surface and their neutralization in the acidic sol. At longer “OFF” times the neutralization of the local basic environment was more effective, and the deposition was always starting from an “initial” pH condition. At shorter “OFF” times the hydroxyl concentration increased at the surface because of


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accumulation at each “ON” step, increasing the rate of layer growth. The same happened with increasing “ON” time (or decreasing number of intervals). The quality of the coating was quite good also using this approach.

CONCLUSIONS In this paper, the results of a study aimed at increasing the thickness of a silica coating deposited by the sol-gel electrochemically assisted deposition are described and discussed. Two different approaches were used. With the first one the deposition of multiple layers was obtained by repeated extraction and re-immersion of the sample from the batch. It was demonstrated that the thickness increased linearly with the number of layers, which indicated that the film building process was not affected by the previously deposited layers, at least up to a certain thickness, and that the film was stable against re-dissolution in the sol when upon re-immersion. Furthermore, very good quality films were obtained. In the second experiment a pulsed potential was applied to the sample always immersed in the sol. The thickness was increased in sequential steps, by cycling the potential between a minimum value of -1.4 V (“ON”) and a maximum value of 0 V (“OFF”). It was demonstrated that the thickness could be increased by decreasing the “OFF” time, at constant number of intervals, or by decreasing the number of intervals at constant “OFF” time. The reported results have a practical interest, as they provide different methods for increasing the thickness of the deposited films without affecting their quality, opening perhaps to new application possibilities, while also allowing better understanding of some of the mechanisms of the sol-gel electrochemically assisted deposition method.


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Acknowledgements Funding from University of Padova is gratefully acknowledged.

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TOC GRAPHIC


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Figure 1. Schematic representation of the “intermitted duty-cycles” used for the pulsed potential experiments. 201x141mm (300 x 300 DPI)


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Figure 2. (A) Thickness of the first four layers deposited at -1.2 V for 100 s (black square), -1.2 V for 150 s (red circle), -1.3 V for 150 seconds (blue triangle) . (B) Thickness of the first ten layers deposited at -1.3 V 150 s. 80x33mm (300 x 300 DPI)



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Figure 3. SEM image (BSE). Top view of the first ten layers deposited at -1.3 V 150 s.


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Figure 4. Thickness of the film as function of the “OFF” time in the three different cases: number of intervals equal to 75 (black square), 60 (red circle), 50 (blue triangle). Applied potential: -1.4 V. Total deposition time: 300 s. 201x140mm (300 x 300 DPI)



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Graphical Abstract 286x226mm (150 x 150 DPI)


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Multilayer Deposition of Silica SolâGel Films by Electrochemical

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