Fluoroalkylsilanes with Embedded Functional ... - ACS Publications

Jun 11, 2015 - Department of Chemical Sciences, University of Padova and INSTM, ... University of Bologna, Viale del Risorgimento 2, I-40136 Bologna, ...
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Fluoroalkylsilanes with Embedded Functional Groups as Building Blocks for Environmentally Safer Self-Assembled Monolayers Barbara Ballarin,*,† Davide Barreca,‡ Maria Cristina Cassani,*,† Giorgio Carraro,§ Chiara Maccato,§ Adriana Mignani,∥ Dario Lazzari,*,⊥,# and Maurizio Bertola⊥ †

Department of Industrial Chemistry “Toso Montanari”, UdR INSTM of Bologna and Center for Industrial Research−Advanced Applications in Mechanical Engineering and Materials Technology (CIRI−MAM), Alma Mater Studiorum, University of Bologna, Viale del Risorgimento 4, I-40136 Bologna, Italy ‡ CNR-IENI and INSTM, Department of Chemical Sciences, University of Padova, Via Marzolo 1, I-35131 Padova, Italy § Department of Chemical Sciences, University of Padova and INSTM, Via Marzolo 1, I-35131 Padova, Italy ∥ Center for Industrial Research−Advanced Applications in Mechanical Engineering and Materials Technology (CIRI−MAM), University of Bologna, Viale del Risorgimento 2, I-40136 Bologna, Italy ⊥ Miteni S.p.A., Località Colombara 91, I-36070 Trissino, Vicenza, Italy S Supporting Information *

ABSTRACT: The fabrication of silane-based fluorinated self-assembled monolayers (FSAMs) on indium tin oxide (ITO, a transparent electrode) was carried out making use of the following fluoroalkylsilanes (FAS): 2,2,3,3,4,4,5,5,6,6,6-undecafluoro-N-[3-(trimethoxysilyl)propyl]hexanamide (1; RF = C5F11) and 1,1,2,2,3,3,4,4,4-nonafluoro-N-[3-(trimethoxysilyl)propyl]butane-1-sulfonamide (2; RF = C4F9), containing an embedded amide and a sulfonamide group, respectively, between the short perfluoroalkyl chain (RF with C < 6) and the syloxanic moiety. The obtained FSAM-modified/ITO systems were characterized by X-ray photoelectron spectroscopy (XPS), contact angle (CA), surface energy measurements, and electrochemical impedance spectroscopy (EIS) and compared to ITO modified with a 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyltriethoxysilane (3; RF = C6F13), with the perfluoroalkyl group linked to the syloxanic moiety through a simple hydrocarbon chain. The results obtained show that the presence of the −NHCO− and −NHSO2− groups have a different mode of action and, with the former, despite the short perfluoroalkyl chain, the ITO−1 system presents a CA (Θwater = 113.5°) and surface energy (γl = 14.0 mJ m−2) typical of amphiphobic materials. These properties can be exploited in a variety of applications, such as self-cleaning, anti-fouling, and anti-fingerprint coatings, and in advanced microelectronic components.

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INTRODUCTION

Nevertheless, the concern toward long perfluorinated alkyl chains owing to their bioaccumulation, toxicity, and environmental persistence has fueled the attention on environmentally safer building blocks with short perfluorinated carbon chains (RF), involving no more than six carbon atoms. Materials satisfying this requirement do not deliver perfluoroctanoic acid (PFOA), a substance of very high concern, via oxidative degradation. As a consequence, they significantly reduce the environmental footprint, with important consequences for bioorganisms, food quality, and water safety.9−12 Despite the reduction of RF chain lengths decreases the toxicity and bioaccumulation, it leads to a detrimental performance decrease in applications where a large content of F atoms is mandatory to achieve a significant low surface

Over the past decade, alkoxysilanes with fluorinated organic substituents (FAS) have attracted a considerable interest to satisfy the demanding requests of advanced applications in different fields, such as sensing, catalysis, and environmental protection.1 Attention has also been devoted to the synthesis of new fluorinated materials for the fabrication of optical waveguides with improved properties, absorbents for oil and heavy metals, gelling polymers, and finally, advanced electronic components.2,3 In addition, because such fluorinated compounds are both hydrophobic and lipophobic materials,4−6 efforts have been devoted to surface modification for the production of self-cleaning, anti-scratch, and anti-fouling systems, mimicking the well-known lotus effect.7,8 In this regard, the development of novel fluorinated building blocks containing suitable functional groups represents an important target from an applicative point of view. © XXXX American Chemical Society

Received: April 21, 2015 Revised: June 11, 2015

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produced FSAMs. The key aspect of this work is to examine if the presence of an embedded functional group (able to promote intermolecular hydrogen bonding among the anchored molecules),21,30,31 can counterbalance the shorter perfluorinated chain in terms of performances expected from the deposited layer. Moreover, we exploited the concomitant presence of trimethoxysilane, more reactive than ethoxysilane, as a monolayer-forming agent.32 To the best of our knowledge, very few examples of fluorinated self-assembled FSAMmodified/ITO are reported in the literature up to date,33,34 and this investigation is the first one devoted to the behavior of silanes containing short, environmentally safer perfluorinated groups.

energy and hydrophobicity. As a consequence, advanced synthetic strategies in this field are actually focused on a complete molecule redesign, to meet all of the pending requirements simultaneously.13−18 The selection of a specific method for chemical surface modification with FAS is influenced by several parameters, and to date, numerous methods have been proposed, including plasma surface modification, physical vapor deposition, chemical adsorption, sol−gel processes, and layer-by-layer (LbL)19,20 and fluorinated self-assembled monolayer (FSAM) deposition methods.21−23 In particular, surface modification with alkylsilane is one of the most commonly used method to prepare monolayers on oxides, 22 and a self-assembled monolayer (SAM) is an amenable, fast, versatile, and costeffective way to prepare modified SiO2 surfaces by grafting molecules via −OH groups that typically terminate SiO2 surfaces after wet chemical cleaning.5,24−27 In the present work, thanks to the combined effort of industrial and academic research groups, we have investigated the properties of indium tin oxide (ITO) glass surfaces modified with environmentally safer FAS (RF < 6 carbon atoms) via a self-assembled method. The ITO substrate has been chosen thanks to its conducting and transparent properties that allow for an easy optical and electrochemical characterization.28 Two FAS produced by Miteni S.p.A., 29 2,2,3,3,4,4,5,5,6,6,6-undecafluoro-N-[3(trimethoxysilyl)propyl]hexanamide (1) and 1,1,2,2,3,3,4,4,4nonafluoro-N-[3-(trimethoxysilyl)propyl]butane-1-sulfonamide (2), containing an embedded amide and sulfonamide group, respectively (Scheme 1), have been used for this purpose. For

2. EXPERIMENTAL SECTION 2.1. Chemicals. Perfluoromethyl ester C5F11C(O)OMe16 and perfluorohexansulfonyl fluoride C4F9SO2F35 were produced by Miteni S.p.A. via electrochemical fluorination (ECF);36,37 (3-aminopropyl)trimethoxysilane, 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyltriethoxysilane (3), diiodomethane, K3[Fe(CN)6], K4[Fe(CN)6], KNO3, and sodium citrate were purchased from Sigma-Aldrich and used without further purification. Ultrapure water purified by the Milli-Q plus system (Millipore Co., with resistivity over 18 MΩ cm) was used in all cases. The prepared derivatives 1 and 2 were characterized by chromatographic and spectroscopic methods; all of the organosilanes are stored under nitrogen prior to use. Nuclear magnetic resonance (NMR) spectra were recorded at 25 °C using a Varian Gemini XL 300 (1H, 300.1 MHz; 13C, 75.5 MHz; 19F, 282.3 MHz) instrument. Infrared (IR) spectra were recorded in the 4000−650 cm−1 range by means of a Fourier transform infrared (FTIR) spectrometer PerkinElmer Spectrum 2000, operating in standard transmittance mode; samples were prepared as either liquid films on a KBr disk as supporting material (neat) or ground into a fine powder and pelletized with KBr (pellet). Gas chromatography−mass spectrometry (GC−MS) analyses were performed using an Agilent 5973 instrument. GC analysis were performed using a Hewlett-Packard HP-5890 instrument. 2.1.1. Synthesis of 2,2,3,3,4,4,5,5,6,6,6-Undecafluoro-N-[3(trimethoxysilyl)propyl]hexanamide) (1). A 1.0 L round-bottom flask fitted with a condenser and overhead stirrer and previously filled with nitrogen (3 purging cycles), was charged with C5F11C(O)OMe (770.0 g, 2.35 mol). Subsequently, (3-aminopropyl)trimethoxysilane (440.3 g, 2.45 mol) was trickled into the reactor with concomitant control of the reaction temperature between 20 and 25 °C. The final mixture was stirred for 1 h at 25 °C. The clean product 1 (1108.0 g, 2.33 mol; yield = 99%) was obtained as a pale yellow liquid (d = 1.367 g/cm3), after elimination of the volatiles under vacuum (first, 60 mmHg and then 18 mmHg; temperature ≤ 30 °C). 1H NMR (300.1 MHz, CDCl3) δ: 7.14 (bs, 1H, NH), 3.58 (s, 9H, OCH3), 3.39 (m, 2H, NCH2−), 1.73 (m, 2H, NCH2CH2−), 0.69 (t, 3J = 9.0 Hz, 2H, −CH2Si). 13C NMR (75.5 MHz, CDCl3) δ: 160.2 (bs, CO) 130.0− 103.0 (m, C5F11), 50.6 (OCH3), 42.2 (NCH2−), 21.8 (NCH2CH2−), 6.4 (−CH2Si). 19F NMR (282.3 MHz, CDCl3) δ: −80.8 (t, 3JF,F = 8.5 Hz, CF3), −119.9 (t, 3JF,F = 12.7 Hz, CF2), −122.6 (m, CF2), −122.9 (m, CF2), −126.2 (m, CF2). MS (EI), m/z (relative abundance, %): 444 ([M − OCH3]+ •, 15), 178 ([N(H)(CH2)3Si(OMe)3]+ •, 38), 121 ([Si(OMe)3]+ •, 100), 91 ([Si(OMe)2H]+·, 26). IR (neat, cm−1): 3325 (vs, νNH), 2948, 2846 (vs, νCH), 1704 (vs, νCO), 1548 (δNH), 1100−1350 (vs, νSi−O and νC−F overlapping bands).38,39 2.1.2. Synthesis of 1,1,2,2,3,3,4,4,4-Nonafluoro-N-[3(trimethoxysilyl)propyl]butane-1-sulfonamide (2). A 500 mL round-bottom flask fitted with a condenser and overhead stirrer and previously filled with nitrogen (3 purging cycles) was charged with NaOCH3 (16.0 g, 0.296 mol), methanol (300 mL), and perfluorohexansulfonyl fluoride (93.0 g, 0.308 mol). After the batch was heated to 50 °C, (3-aminopropyl)trimethoxysilane (50.0 g, 0.279 mol) was slowly added over a period of 30 min. After the reaction mixture was stirred for 2 h, methanol was removed under reduced pressure

Scheme 1. Chemical Structure of the Monomers Used for the FSAM Deposition

comparison, bare ITO or ITO modified with 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyltriethoxysilane (3), in which the perfluoroalkyl group is linked to the syloxanic moiety through a simple hydrocarbon chain, has been studied. The different FSAMs were characterized by complementary techniques to investigate the interplay between the molecular structure of the FAS molecules and the properties of the B

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Langmuir (200 mmHg, 50 °C), yielding 144 g of a raw solid mixture. Subsequently, 400 mL of tetrahydrofuran (THF) was added, and the suspension was filtered to remove NaF. The filtrate was evaporated (20 mmHg, 25 °C) to dryness to afford the final product as a yellowish solid (126.0 g, 0.273 mol; yield = 98%). 1H NMR (300.1 MHz, acetone-d6) δ: 7.4 (bs, 1H, NH), 3.27 (s, 9H, OCH3), 2.43 (bs, 2H, NCH2−), 1.93 (bs, 2H, NCH2CH2−), 0.89 (bs, 2H, −CH2Si). 13C NMR (75.5 MHz, acetone-d6) δ: 130.0−104.0 (C4F9), 50.5 (OCH3), 47.8 (NCH2−), 25.9 (NCH2CH2−), 7.1 (−CH2Si). 19F NMR (282.3 MHz, acetone-d6) δ: −81.87 (t, 3JF,F = 9.5 Hz, CF3), −115.42 (m, CF2), −122.07 (m, CF2), −126.66 (m, CF2). IR (KBr pellet, cm−1): 3522 (vs, νNH), 2935 (vs, νCH), 1250 (vs, νSi−O, νC−F, νasSO overlapping bands), 1137 (vs, νsimSO). 2.2. Substrate Preparation. ITO substrates (purchased from Optical Filters, Ltd., U.K.; surface resistivity = 12 Ω cm; 1.05 cm2 geometric area) were preliminary cleaned in warm acetone for 5 min and then in a 1:1 Milli-Q water/isopropanol mixture in an ultrasonic bath for 30 min, finally rinsed thoroughly with Milli-Q water, and dried under flowing N2. 2.3. FSAM-Modified/ITO Preparation. To modify the substrate surface, freshly cleaned ITO slides were immersed into a 5% (v/v) solution of the chosen perfluoroalkylsilane in isopropanol for 24 h in air at room temperature, followed by rinsing with pure isopropanol and heating at 100 °C for 10 min.22,28 2.4. Characterization. X-ray photoelectron spectroscopy (XPS) analyses were run on a PerkinElmer Φ 5600ci spectrometer at a working pressure lower than 10−8 mbar, using a non-monochromatized Al Kα excitation source (hν = 1486.6 eV). The spectrometer was calibrated by assigning to the Au 4f7/2 line the binding energy (BE) of 84.0 eV with respect to the Fermi level. Charging correction was performed by assigning to the C 1s line of adventitious carbon a value of 284.8 eV.40 The estimated standard deviation for BEs was ±0.2 eV. After a Shirley-type background subtraction,41 raw spectra were fitted by means of a nonlinear least-squares deconvolution program, using Gaussian−Lorentzian peak shapes. Atomic percentages were evaluated using sensitivity factors provided by Φ V5.4A software. The samples were introduced directly into the analysis chamber by a fast entry lock system. Electrochemical impedance spectroscopy (EIS) spectra were recorded between 10 mHz and 100 kHz using a μAUTOLAB type III controlled by a personal computer via NOVA 1.6 software. Each measurement was repeated 3 times. A 0.1 M KNO3 solution containing 1 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) mixture was used. All electrochemical measurements were conducted in a conventional three-electrode cell using a saturated calomel electrode (SCE) reference electrode and a Pt wire counter electrode in a N2 atmosphere at room temperature. Contact angle (CA) measurements were determined by the average of 10 measurements at room temperature using the sessile drop method on a Dataphysics OCA 15 plus goniometer under ambient conditions at room temperature. Typically, 1 μL water droplets were used for CA measurements. Values of the drop CA images were recorded with a charge-coupled device (CCD) camera, after adjusting the contrast, magnification, and focus. Free surface energies for all samples and their disperse and polar components were calculated according to the theory of the Owens− Wendt−Rabel and Kaelble (OWRK) method,42 using water and diiodomethane as standard testing liquids.43 The liquid surface tension of water and diiodomethane used for the calculation and the static CAs of diiodomethane are reported in Table S1 and Figure S1 of the Supporting Information.

F in CFx species (see Figure S3a of the Supporting Information).40 This finding is also supported by a detailed analysis of the C 1s peak that can be decomposed into five contributing bands (see, as a representative example, Figure 1a): (I) at BE = 284.8 eV, assigned to both adventitious carbon

Figure 1. Surface XPS photopeaks for (a) C 1s (ITO−2), (b) N 1s (ITO−1), (c) N 1s (ITO−2), and (d) S 2p (ITO−2).

contaminations and C−C bonds from the used polymeric matrices, (II) at BE = 286.4 eV, attributed to C−O bonds, (III) at BE = 288.8 eV, (IV) at BE = 291.5 eV, and (V) at BE = 293.8 eV, ascribed to various CFx mojeties (CF2−CH2, CF2−CF2, and CF3−CF2).25,40 All samples possessed similar spectra features as concerns the main silicon (Si 2p) signal (BE = 102.3 eV) that could be attributed to Si in the siloxane chain (see Figure S3b of the Supporting Information).40 With regard to the N 1s photopeak, as reported in panels b and c of Figure 1, both ITO−1 and ITO−2 showed a band (I) located at 400.0 eV attributed to H−N−C groups.40,44 In addition, specimen ITO−2 also showed a second contribution (II, BE = 401.5 eV) that could be ascribed to nitrogen species bound to various surface oxygen sites (such as adsorbed NOx).45 Sample ITO−2 showed the presence of the S 2p peak (BE = 168.5 eV) that could be assigned to the presence of S in the sulfonamide group (Figure 1d).40,44 More spectroscopic details on XPS are presented in paragraph 2 of the Supporting Information, . Figure 2 displays ultraviolet−visible (UV−vis) optical spectra and the photographs of FSAM/ITO samples. As observed, the transmittance of all FAS-treated samples was around 85% in the 350−800 nm spectral range, with results similar to the bare ITO. This finding highlights that the FSAM surface modification do not alter the glass optical properties and that compounds 1 and 2 are good candidates for all of the applications where transparency is demanded.46 CA measurement is one of the most sensitive probes for the analysis of the surface wettability and, consequently, for the study of the hydrophilic/hydrophobic properties of a modifying

3. RESULTS AND DISCUSSION In this study, efforts were initially dedicated to the analysis of the system chemical composition by XPS. As a general observation, survey spectra of FSAM/ITO samples (see Figure S2 of the Supporting Information) indicate the presence of carbon, oxygen, tin, and indium and clearly showed the F 1s peaks at a BE of ≈688.8 eV, in agreement with the presence of C

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test liquids (Table S1 and Figure S1 of the Supporting Information), and the results are reported in Table 1.55 Table 1. Surface Free Energies (γl) with Their Disperse (γld) and Polar (γlp) Fractions According to Laplace−Young Fitting sample

γl (mJ m−2)

γld (mJ m−2)

γlp (mJ m−2)

bare ITO ITO−1 ITO−2 ITO−3

41.0 14.0 25.7 24.1

37.8 13.6 25.5 22.8

3.2 0.4 0.2 1.4

As observed, a dramatic change in surface free energies occurs with the FSAM-modified surfaces and the lower γl value is obtained when compound 1 is employed, whereas ITO−2 and ITO−3 have analogous values. For comparison, the value of poly(tetrafluoroethylene) (PTFE, Teflon) is 18.5 mJ m−2, whereas for the above-mentioned silane FAS17, the reported γl is 12 mJ m−2.53 It is well-known that, when the critical surface tension decreases below 20 mJ m−2 as in the ITO−1 case, the surface is considered amphiphobic, i.e., both oleophobic and hydrophobic. Amphiphobic materials, because of their unique waterrepellency and self-cleaning abilities, are employed in several applications that include areas, such as anti-fouling coatings, anti-fingerprint coatings in screens of portable electronic devices, and microscale electronics, where functional molecules are employed as a substitute for present silicon-based circuits.56−58 EIS is a powerful technique that employs a small-amplitude, alternating-current (AC) signal to probe the impedance characteristics of a cell. Interpretation of EIS measurements is usually performed by fitting the impedance data to an equivalent electrical circuit, which should be representative of the physical processes taking place in the system under investigation. In particular, EIS enables us to follow how the modification of ITO electroactive surface affects the charge transfer by a redox mediator, such as K4[Fe(CN)6]/K3[Fe(CN)6]. In a typical Nyquist plot, the semi-circle portion corresponds to the charge-transfer resistance (Rct) at higher frequency range (the higher the diameter, the higher the charge-transfer resistance of the electroactive surface), whereas a linear part at lower frequency range represents the diffusion-limited process.59 Figure 4 shows representative Nyquist diagrams of the electrochemical impedance spectra of bare ITO and FSAM/ITO samples. ITO showed a very small semi-circle in the high-frequency region and a straight line in the lowfrequency region, indicating that the electron transfer process of the redox couple is essentially diffusion-controlled. On the contrary, the modification of the ITO surface with compounds 1 and 3 generated a considerable enlargement of the semicircles (vide inf ra) because of the increase in charge-transfer resistance (increase of the blocking behavior of the selfassembled layers). This behavior is also supported by the change of the cyclic voltammogram (CV) paths, as reported in Figure S5 of the Supporting Information. A standard Randle’s equivalent circuit (inset Figure 4A) was used to fit the impedance data.28 The parallel combination of the electron-transfer resistance (Rct) and the double-layer capacitance (Cdl) give rise to a variation in the semi-circle in the Nyquist plots, because of the dielectric and insulating features

Figure 2. (Top) UV−vis trasmittance spectra of FSAM/ITO glasses. The curve pertaining to bare ITO is also reported for comparison. (Bottom) Photographs of FSAM/ITO and ITO samples.

layer.47−51 The static CAs obtained with water for the FSAM/ ITO samples are reported in Figure 3. As expected, an increase of the CA was observed for FSAM-modified ITO.27

Figure 3. Static CAs of water (1 μL) on the ITO and FSAM/ITO monolayers.

ITO−1 exhibits a CA value of 113.5°, higher than those observed for ITO−2 and ITO−3 samples. Although CA measurements also depend upon the substrate nature, this value is similar to that reported for a Si wafer surface covered by a densely packed 1H,1H,2H,2H-perfluorodecylsilane [CF3(CF2)7CH2CH2Si(OMe)3, FAS17; CA = 111.5°], even if the latter has a longer perfluorinated chain.25,52,53 Surface free energies (γl, mJ m−2) with their disperse (γld) and polar (γlp) fractions54 were calculated according to the OWRK method,42 using water and diiodomethane as standard D

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significantly higher, with Rct for ITO−1 10 times higher than that of ITO−3. These data suggest that the attachments of compounds 1 and 3 on the ITO surface generated an insulating layer, acting as a barrier for the interfacial electron transfer, and that ITO−1 possesses better insulating properties (compare the pertaining resistance values in Table 2). In contrast, as concerns ITO−2, the semi-circle at higher frequency became very small, indicating the occurrence of an easier electronic transport. These data are in agreement with the increase of the current signal observed in the CV (see Figure S5 of the Supporting Information). The impedance data were also used to calculate the surface coverage (Θ%) of the FSAM/ITO (Table 2), by assuming that the current is due to the presence of pinholes and defects within the monolayer. As reported by Ganesh et al., Θ% was calculated using the equation62 Θ% = (1 − R 0ct /R ct) × 100

(1)

where R0ct is the charge-transfer resistance of the bare ITO electrode. The Θ% found was ≈98 and 70% for ITO−1 and ITO−3, respectively, and these results are in agreement with the higher reactivity toward hydrolysis of trimethoxysilane.32

4. CONCLUSION Fluoroalkylsilane-coated indium tin oxide (FSAM/ITO) systems with good optical properties have been prepared making use, in line with the new environmental requests, of FAS containing a short perfluoroalkyl tail (RF with C < 6) and an embedded amide (1, RF = C5F11) and a sulfonamide group (2, RF = C4F9). The obtained results show that the presence of the amide group plays an important role and can balance the lower fluorine content. Indeed, it was found that the ITO−1 system presents a CA and a surface energy similar to FAS with RF > 6 and typical of amphiphobic materials. Therefore, compound 1 can be considered an effective and efficient replacement in all of the technological applications where oil- and water-repellent coatings are requested. On the contrary, the presence of the sulfamide group in ITO−2 does not bring a variation in terms of CA and surface free energy compared to ITO−3 (without an embedded functional group), although it has a positive effect on electronic transport compared to bare ITO. This kind of behavior, in line with previous reports concerning alkanethiols containing a sulfone (−SO2−) group, has been explained taking into account that the expected perturbation in the monolayer is compensated for by the ordering induced through dipole− dipole interactions among the sulfone groups.21

Figure 4. (A) Nyquist plots obtained in 0.1 M KNO3 solution containing 1 mM Fe(CN)63−/4− solution at E = 0.22 V versus SCE and 25 °C for ITO−1, ITO−2, ITO−3 and bare ITO. (Inset) Scheme of standard Randle’s equivalent circuit with Rct = charge-transfer resistance, Rs = ohmic resistance of the electrolyte solution, Cdl = double-layer capacitance, and ZW = Warburg impedance. (B) Enlargement at higher frequencies.

at the sample/electrolyte interface. χ2 values of the order of 6 × 10−5 for all fitting results were calculated, giving an excellent correlation between the experimental and simulated data (Table 2). All of the capacitive elements of the equivalent circuit have been substituted with CPE, namely, the “capacitance dispersion” expressed in terms of a constant phase element. The CPE represents a circuit parameter with limiting behavior as a capacitor for n = 1 or a resistor for n = 0, where n is the exponent of CPE;60,61 in our case, n is very close to 1. When the semicircular part of the EIS spectrum of ITO−1 and ITO−3 (Rct = 13 000 and 1013 Ω, respectively) is compared with respect to bare ITO (Rct = 305.0 Ω), the diameter of the semi-circle obtained for the FSAM/ITO results Table 2. EIS Parameters for FSAM/ITO Systemsa sample bare ITO ITO−1 ITO−2 ITO−3

Rct (Ω−1) 305.0 13000.0 50.0 1013.5

± ± ± ±

4.9 658.2 2.7 52.0

CPE (μF cm−2) 12.4 1.9 30.5 3.3

± ± ± ±

0.1 0.1 0.3 0.2

n(CPE)

Θ%

± ± ± ±

97.65 ± 0.13 b 69.91 ± 1.02

0.997 0.997 0.993 0.997

0.001 0.001 0.001 0.001

a 0.1 M KNO3 + 1 mM K4Fe(CN)6/K3Fe(CN)6 (1:1 molar ratio), Eap = +0.22 V versus SCE, and χ2 values of the order of 6 × 10−5 for all fitting results. bIn this case, eq 1 is not applicable because it would lead to a negative value.

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ASSOCIATED CONTENT

S Supporting Information *

CA, surface energy, XPS, field emission scanning electron microscopy (FE-SEM), and electrochemical characterization. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01416.

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: +39-0512093704. Fax: +39-0512093694. E-mail: [email protected]. *Telephone: +39-0512093700. Fax: +39-0512093694. E-mail: [email protected]. *E-mail: [email protected]. Present Address #

ICAP-SIRA S.p.A., via Corridoni 19, I-20015 Parabiago, Milano, Italy. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Adriana Mignani thanks the CIRI−MAM, University of Bologna, for financial assistance. Giorgio Carraro, Chiara Maccato, and Davide Barreca acknowledge the financial support under the FP7 projects “SOLAROGENIX” (NMP4-SL-2012310333), Padova University ex-60% 2012−2013, PRAT 2010 (CPDA102579), and Regione Lombardia−INSTM ATLANTE. The authors are grateful to Guia Guarini of the Institute of Science and Technology for Ceramics, National Research Council of Italy (ISTEC CNR) for CA measurements.

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REFERENCES

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