Compact Titanium Oxycarbide: A New Substrate ... - ACS Publications

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Compact Titanium Oxycarbide: A New Substrate for Quantitative Analysis of Molecular Films by Means of Infrared Reflection Absorption Spectroscopy Izabella Brand,*,† Celine Rüdiger,‡,§ Kurt Hingerl,∥ Engelbert Portenkirchner,§ and Julia Kunze-Liebhaü ser*,§ †

Department of Chemistry, University of Oldenburg, Carl von Ossietzky Strasse 7-9, D-26129 Oldenburg, Germany Physik-Department, Technische Universität München, James-Franck-Strasse 1, 85748 Garching, Germany § Institut für Physikalische Chemie, Leopold-Franzens-Universität Innsbruck, Innrain 52c, A-6020 Innsbruck, Austria ∥ Zentrum für Oberflächen- und Nanoanalytik, Universität Linz, Altenbergerstrasse 69, A-4040 Linz, Austria ‡

S Supporting Information *

ABSTRACT: Titanium oxide−titanium carbide (TiOxCy) hybrid materials have tunable electronic properties ranging from semiconductive to semimetallic. They can therefore be employed in solar energy conversion applications and as potential substitute for carbon based electrocatalyst supports for use in fuel cells. Understanding of the optical properties of semimetallic TiOxCy is of great importance. In this paper we report on the optical properties of compact TiOxCy in the mid-IR spectral region. TiOxCy reflects the IR light similarly to metals and is therefore suitable as a new substrate for molecular adsorption studies with infrared reflection absorption spectroscopy. For the first time polarization modulation infrared reflection absorption spectroscopy (PM IRRAS) is applied to study quantitatively at the submolecular level the structure and orientation of fatty acid molecules in monolayers adsorbed on the TiOxCy surface. The analysis of the IR band intensities provides information on the structure, packing, and orientation of the fatty acid molecules in the monolayer. The PM IRRA spectra provide evidence of an interaction between surface atoms and the carboxylic group leading to dissociation of the polar headgroup and bidentate canted coordination of the carboxylate at the TiOxCy surface.

1. INTRODUCTION Functional materials based on titania (TiO2, titania) are technologically important because of their potential application in energy storage and conversion devices such as fuel cells and solar cells1−3 as well as biomedical materials such as implants.4 The largest application obstacle of TiO2 arises from poor electric conductivity due to the large intrinsic band gap (3.2 eV for anatase). Therefore, the development of titanium oxidebased hybrid materials with, for example, carbon5−9 or nitrogen10 that are characterized by their tunable bandgap is of great technological importance. Semimetallic TiOxCy compact films can be synthesized from ∼53 nm thick anodic TiO2 on polycrystalline Ti by a carbothermal reduction process.8 X-ray photoelectron spectroscopy revealed the presence of carbon (mainly graphitic), TiC, TiO2, and substoichiometric titanium oxides (TiO2−x) in the surface layers of these films. Films that are synthesized at the lowest employed annealing temperature (750 °C) exhibited the highest content of TiC and TiO2−x phases and showed electrochemical characteristics similar to glassy carbon, which is an electrically conductive carbon material. The electrocatalytic activity of Pt nanoparticles deposited on semimetallic TiOxCy films toward the ethanol oxidation reaction (EOR) was studied at room temperature (rt) and at elevated temperature. © 2015 American Chemical Society

It has been found that at rt the activity correlates with the annealing temperature during support preparation, with the highest value for TiOxCy films produced at 750 °C, where the support conductivity is highest, and that the activity toward the EOR is strongly enhanced for Pt deposited on TiOxCy films compared to Pt deposited on glassy carbon.8,11 This strong activity increase can be explained by synergistic effects due to catalyst−support interactions, which is currently under investigation. In this context, it needs to be clarified if the TiOxCy surface actively contributes to the EOR, which would result in adsorption phenomena. Such adsorption processes have to be analyzed at the submolecular level in order to understand the role of TiOxCy in the EOR. To the best of our knowledge, the adsorption properties of organic molecules on semimetallic TiOxCy surfaces have not been investigated yet. Only a few fundamental studies investigate adsorption phenomena of nitrogen12 and of methylene blue and methyl orange on the surface of semiconductive TiO2/carbon composite materials.13 The recognition of the adsorption properties of organics on the TiOxCy surface would require the Received: April 14, 2015 Revised: May 20, 2015 Published: May 26, 2015 13767

DOI: 10.1021/acs.jpcc.5b03570 J. Phys. Chem. C 2015, 119, 13767−13776

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The Journal of Physical Chemistry C

phospholipid molecules in model membranes on both substrates are dependent on the interaction of the polar headgroup with the surface atoms. Ramin et al.28 used a SiO2| Au substrate to graft ester-terminated organosilicon polymers on the silica surface to study the structure and orientation of chemisorbed molecules by means of PM IRRAS. In this paper, we report on the optical properties of compact semimetallic TiOxCy, carbothermally produced at 750 °C from anodic TiO2, in the mid-infrared spectral region. Just like in metals, the imaginary part of the refractive index is much higher than the real part, which results in a strong reflection of the IR light from the surface. The surface selection rule is fulfilled, and TiOxCy is tested as a new substrate for IRRAS through investigation of fatty acid monolayers on its surface that are known to have a well ordered structure and orientation.17,31−33 PM IRRAS studies of the structure and orientation of arachidic acid molecules in monolayer assemblies are performed on TiOxCy and on Au surfaces in comparison. Differences in the hydration, in the orientation of the polar carboxylic group, and in consequence, in the orientation of the hydrocarbon chain arise from different surface properties of both substrates.

application of surface analyzing techniques to molecular assemblies prepared at its surface. Infrared spectroscopy (IRS) belongs to one of the most powerful techniques for the analysis of the composition and structure of organic molecules. In addition, IRS is applicable for studying not only the molecular structure in a bulk phase but also the structure of molecular assemblies present at various interfaces.14 In order to investigate the structure of a molecule in a film adsorbed at an interface, reflection based IRS techniques are used. Greenler has demonstrated for the first time that at the air|gold phase boundary the reflectance of the IR light is close to unity at all angles of incidence.15 In the case of perpendicularly polarized (s-polarized, electric field vector perpendicular to the plane of incidence) light, the phase shift leads to the cancellation of the electric field vector at the phase boundary independent of the angle of incidence. In the case of parallel polarized (p-polarized, electric field vector parallel to the plane of incidence) light, the phase shift at grazing angles of incidence leads to an enhancement of the electric field vector at the phase boundary. The phenomenon of this enhancement is called surface selection rule16 and provides the basis for infrared reflection absorption spectroscopy (IRRAS). On metal surfaces, only the normal component of the transition dipole moment vector of a given IR vibration is able to interact with the IR light. Thus, not only the composition but also the molecular scale order in the film adsorbed on the metal surface can be analyzed using IRRAS.16−18 IRRAS was successfully applied for the analysis of molecular films present not only at the air|metal interface15,19 but also at air|water20 and at liquid|metal interfaces.21,22 Materials that fully meet the surface selection rule of IRRAS are limited to metals such as Au, Pt, Ag, or Cu.23 Nonmetallic substrates such as glassy carbon24 and glass25 were also successfully applied in IRRAS. However, the maximal enhancement of the electric field vector at the air|glassy carbon and air|glass surfaces is by a factor of 3.0 24 and 4.5 25 lower than that at the air|gold interface, showing that these substrates reflect the IR radiation more weakly compared with gold, and therefore, their application in IRRAS is more challenging. Thus, one constraint of IRRAS is due to a limited number of substrates fulfilling the surface selection rule, which restricts structural studies of molecular films to those which form stable assemblies on metal, glass, or glassy carbon surfaces. Since the application of IRRAS to complex supramolecular assemblies provides qualitative and quantitative information on the film structure at a submolecular level, the need for development of new IR reflective surfaces overcoming the substrate-based limitation of IRRAS is enormous. As a first step toward supports with alternative surface properties, ultrathin silica26−28 and titania29 films (10−40 nm thick) have been deposited on the IR light reflecting gold surface. It was shown that the underlying gold layer ensures the fulfillment of the surface selection rule of IRRAS. Since titania and silica are commonly used as biomaterials, biologically relevant lipid films have been deposited on their surfaces.27,29 However, this approach is limited to thin layer deposits on metals where the thin film will have different properties than bulk substrates. Particularly attractive for structural studies of organized molecular assemblies at the solid|liquid interface is polarization modulation infrared reflection absorption spectroscopy (PM IRRAS), because of reduction of the background contribution.30 Indeed, the PM IRRAS studies of planar lipid bilayers demonstrated that differences in the hydration and orientation of

2. EXPERIMENTAL PART 2.1. Preparation of Compact TiOxCy. Disks of 1 mm thickness and 15 mm diameter were cut from a 20 mm diameter polycrystalline Ti rod (99.6% purity, temper annealed, Advent Ltd., England). The preparation of the anodic TiO2 film is described in ref 34. In short, the samples were mechanically polished and then electrochemically polished for 3−4 times until a smooth and clean surface was obtained. Compact amorphous TiO2 films were produced on the electropolished Ti by potentiostatic electrochemical anodization in a 0.1 M sulfuric acid (H2SO4, analytical grade, 95−97%, Merck, Germany) electrolyte at 20 V for 600 s. The TiOxCy film was synthesized by carbothermal treatment of the anodic TiO2 in a tubular quartz reactor under controlled gas flow (mass flow controllers from MKS Instruments) according to the following procedure: (1) the reactor was purged for 2 h with a high flow of argon (8Ar, purity 4.8, Linde, Germany) in order to remove air, (2) heat up within 90 min at a constant rate to 750 °C in an Ar gas flow of 200 standard cubic centimeter per minute (sccm), (3) stay for 1 h at 750 °C, (4) add 1 sccm of acetylene (C2H2, solvent-free, Linde, Germany) for 5 min, (5) stay for 1 h at 750 °C in Ar, (6) let oven cool to room temperature. 2.2. Formation of Arachidic Acid Monolayers. Chloroform solutions of arachidic acid (C19H39COOH; eicosanoic acid, C20H40O2; >99.9%; Fluka, Steinheim, Germany) were prepared daily. The concentration of the amphiphilic molecule in the stock solution was close to 1.0 mg mL−1. A few drops of the C19H39COOH stock solution were spread at the water surface in a Langmuir−Blodgett trough (KSV, Helsinki, Finland) equipped with movable barriers and a Wilhelmi plate measuring the surface pressure. The trough was controlled by a computer using KSV LB Instruments software. The temperature of the subphase was kept constant at 20 °C. The C19H39COOH monolayer was compressed to a constant surface pressure of either 40 or 15 mN m−1 and transferred from the air|water interface onto the TiOxCy and Au surfaces. A Langmuir−Blodgett vertical withdrawing was used to produce C19H39COOH monolayers on the solid substrates. At the surface pressure (π) equal to 40 mN m−1, Au and TiOxCy substrates were withdrawn from the aqueous subphase at the speed of 40 and 42 mm min−1, respectively. The transfer ratio 13768


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photoelastic modulator and demodulator (Bruker, Ettlingen, Germany; Hinds Instruments, USA, respectively). The maximum efficiency of the photoelastic modulator was set for half-wave retardation at 2900 cm−1 for the analysis of the CH stretching bands, 1600 cm−1 for the analysis of the CO and CH bending modes, and 1200 cm−1 for the analysis of the CH2 wagging modes. At the half-wave retardation set to 2900 and 1600 cm−1 the PM IRRA spectra were collected with a resolution of 4 cm−1. At half-wave retardation set to 1200 cm−1 the CH2 wagging modes progression was analyzed. In order to distinguish these overlapped, separated by 15−25 cm−1 weak bands, the spectral resolution was set to 2 cm−1. Each spectrum contains 3000 averaged spectra. The angle of incident light was set to 68° (see Figure 3), corresponding to the highest enhancement of the IR light at the air|TiOxCy interface. All IR spectra were collected in dry air atmosphere. The PM IRRA spectra were processed using the software OPUS (Bruker). The enhancement of the electric field, the reflectivity, and phase shift at the air|TiOxCy interface as a function of the angle of incidence of the IR light and the PM IRRA spectra of monolayers for randomly distributed molecules were calculated using a home written program.36 The spectra of randomly distributed molecules in the film were calculated from the optical constants (see section 2.4) of C19H39COOH. The thickness of the analyzed film was set to 2.70 nm,37 corresponding to the thickness of the C 19 H 39 COOH monolayer in a solid state. In the monolayer at the air|water interface at a surface pressure of π of 15 and 40 mN m−1 the average area per C19H39COOH molecule is equal to 0.218 and 0.193 nm2, corresponding to a surface coverage of 86.5% and 97.1%, respectively. In order to calculate the PM IRRA spectra of randomly distributed molecules, these surface coverages were used.

(TR) is defined as the ratio in the decrease in the monolayer area at the air|water interface during the transfer to the area of the substrate. The TR was equal to 1.15 ± 0.23 and 1.05 ± 0.03 on TiOxCy and Au, respectively, indicating a quantitative transfer of a monolayer onto solid substrates. At the surface pressure π = 15 mN m−1, the speed of substrate withdrawal was equal to 18 and 15 mm min−1 on TiOxCy and Au, respectively. The corresponding transfer ratios were equal to 0.98 ± 0.2 and 1.03 ± 0.2. 2.3. IR Ellipsometry. We used a Fourier transform based infrared spectroscopic ellipsometer (rotating compensator setup) of the company Woollam to determine the refractive index of TiOxCy in the near- and mid-infrared spectral range of 2−33 μm (300−6000 cm−1) with a spectral resolution of 4 cm−1. The rotating compensator design allows precision measurements of transparent and opaque substrates, as well as single and multilayer films. In ellipsometry two angles ψ and Δ are measured: By use of the relation (rp/rs) = ρ = tan ψeiΔ, the complex reflectance ratio ρ (ratio of the p and s Fresnel reflectances, which becomes complex, i.e., a phase shift occurs between p- and s-reflected wave, in the cases of absorbing films or with multilayer films) is calculated, which is then either directly inverted in the case of pure materials or fitted for multilayer films. Because the sample has negligible surface roughness (a) (a = 25.3 ± 5.7 nm determined over the surface of 15 × 15 μm2) in comparison to the wavelength (λ) (a ≪ λ), the ellipsometric inversion from the measured angles ψ and Δ to the real and imaginary parts of the dielectric function can be performed without fitting by ε = εr + iεi = (n + ik)2 = sin 2(θ ) + ⎛ 1 − ρ ⎞2 sin (θ ) tan (θ )⎜ ⎟ ⎝1 + ρ ⎠ 2

2

(1)

3. RESULTS AND DISCUSSION 3.1. Reflection of the Electromagnetic Radiation at the Air|TiOxCy Interface. Compact TiOxCy films are obtained by anodic oxidation of titanium to TiO2 and subsequent carbothermal reduction of the oxide to TiOxCy in an argon/ acetylene atmosphere. The TiOxCy film is a semimetal,7 whereas its optical properties are unknown. The optical constants, refractive index (n) and extinction coefficient (k) of TiOxCy in the mid-infrared spectral region, are determined by ellipsometry and shown in Figure 1. In the IR spectral region, the refractive index n of TiOxCy changes from 3.2 at 6000 cm−1 (λ = 1.666 μm) to 8.5 at 1000 cm−1(λ = 10.0 μm). In the investigated spectral region, the extinction coefficient k is higher than the refractive index and changes from 4.4 at 6000 cm−1 to 12.7 at 1000 cm−1. In the mid-IR spectral region, values of the refractive indices of TiOxCy are comparable to refractive indices of Au and glassy carbon, materials that are already used as substrates in IRRAS.23 Large differences between these materials are observed in values of the extinction coefficients. The extinction coefficients of TiOxCy are 3−4 times lower than those of Au;23 however, they are significantly higher than those of glassy carbon.23,24 The high k values of TiOxCy indicate that this material absorbs the IR radiation. Materials that strongly absorb IR light are used as substrates in IRRAS when their reflectivity is close to unity upon IR radiation normal to the surface. The optical properties of TiOxCy are similar to such type of material, which indicates that it reflects IR radiation, which is important for applications as substrate in IRRAS.

The sample is metallic, well described by a Drude dispersion relation for the dielectric function. Fitting this Drude dispersion relation yields a carrier density of 3.1 × 1022 cm−3 and a mobility of 0.66 V cm s−2. 2.4. IR Transmission Measurements. The transmittance spectra of CCl4 (Aldrich, Steinheim, Germany) (background) and ∼1% vol solution of C19H39COOH in CCl4 (analyte) were obtained using a flow cell (Aldrich, Steinheim, Germany) between two ZnSe windows and a Teflon spacer. The IR spectrum of the analyte in CCl4 was used to obtain the optical constants and a PM IRRA spectrum for randomly distributed molecules in the CH stretching mode region. The thickness of the Teflon spacer was equal to 30.7 μm. Sixty-four spectra with a resolution of 4 cm−1 were recorded using a Vertex 70 spectrometer (Bruker, Ettlingen, Germany). Arachidic acid transmission spectra were used to calculate the optical constants according to a procedure described by Allara.17,18 The extinction coefficient k was determined from the transmission spectrum. Next, the refractive index n was calculated from k using a Kramers−Kronig transformation. In the CH stretching mode region of most molecules containing hydrocarbon chains the refractive index at infinite frequency is equal to 1.41.35 The optical constants of C19H39COOH are shown in Supporting Information (section SI.1). 2.5. Polarization Modulation Infrared Reflection Absorption Spectroscopy (PM IRRAS). All the PM IRRA spectra were recorded using a Bruker Vertex 70 spectrometer with the polarization modulation set (PMA 50) equipped with 13769


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s-polarized light takes place, which causes in plane components of the mean square electric field vector (MSEF) to have negligibly small values at all angles of incidence on the TiOxCy surface. The reflectivity of the normal (z) component of ppolarized light decreases with increasing angle of incidence (Figure 2). Simultaneously, at high angles of incidence a phase shift to 90° of the electric field vector of the reflected ppolarized light takes place. These results indicate that the normal component of the MSEF of the p-polarized light on the TiOxCy surface has a significant intensity at high angles of incidence (see Figure 3).

Figure 1. Refractive index n (solid line) and extinction coefficient k (dashed line) of a compact TiOxCy film in the mid-IR spectral region.

Taking into account a phase boundary between air and TiOxCy, the reflectivity and transmittance of a planar electromagnetic wave of 3.44 μm (2900 cm−1) and 6.25 μm (1600 cm−1) length as a function of the angle of incidence (φ) are calculated using the Fresnel equations. The two wavelengths are chosen because a large number of organic molecules absorb IR light around these spectral ranges. The Fresnel equations of reflection (r) and transmission (t) coefficients for p-polarized and s-polarized light at the air|TiOxCy interphase are used to calculate the IR reflectivity as a function of the angle of incidence. The reflectivity of the normal component of ppolarized light (z) and the in plane component of s-polarized light (y) as a function of the angle of incidence of the IR radiation at the TiOxCy surface are shown in Figure 2. Figure 2 shows clearly that TiOxCy strongly reflects the spolarized light at almost all angles of incidence (dashed lines). Upon reflection at the phase boundary, a 180° phase shift of the

Figure 3. Plots of the MSEF normal component of p-polarized light (blue line), and the in plane components of p- (red dashed line) and s(black dashed line) polarized light as a function of the angle of incidence at the wavelengths a) λ = 3.44 μm (2900 cm−1) and b) λ = 6.25 μm (1600 cm−1).

Figure 3 shows that the MSEF has values close to zero for the in plane components of p-polarized (Epx) and s-polarized (Esy) light independent of the angle of incidence. The MSEF of the z-component of p-polarized light at the air|TiOxCy interface has nonzero intensity, and its value strongly depends on the angle of incidence. The MSEF has a maximum of 1.15 and 1.30 for λ = 3.44 μm (2900 cm−1) and 6.25 μm (1600 cm−1), respectively. The corresponding angles of incidence of the IR beam are equal to φ = 65° and φ = 68°. By contrast, at the air| gold interface the MSEF is almost 4 times enhanced at φ ≅ 80°.15 The significantly smaller enhancement of the zcomponent of the reflected p-polarized light at the air|TiOxCy interface is due to a weaker absorbance of the IR light by this material compared to metals like Au. TiOxCy, however, absorbs IR radiation much more strongly than glassy carbon, which is also used as a substrate for IRRAS.24 Therefore, TiOxCy has much higher reflectivity for IR light than glassy carbon. In conclusion, the enhancement of the normal component of the electric field of the p-polarized light and the cancellation of the in plane components of p- and s-polarized light at the phase boundary fulfill the surface selection rule for IRRAS.16 Below we report for the first time the application of TiOxCy as a substrate in IRRAS.

Figure 2. Reflectivity of the IR light at the air|TiOxCy interface as a function of the angle of incidence of the incoming radiation for the following wavelengths: (a) λ = 3.44 μm (ν̃ = 2900 cm−1) and (b) λ = 6.25 μm (ν̃ = 2900 cm−1); solid line, z-component of the p-polarized light; dashed line, y-component of the s-polarized light. 13770


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at (2918.4 ± 0.2) cm−1 and the νs(CH2) mode at (2849.1 ± 0.3) cm−1. Their positions are not dependent on the used substrate. The full width at half-maximum (fwhm) is equal to (14.8 ± 0.5) and (7.8 ± 0.3) cm−1 for the νas(CH2) and νs(CH2) modes, respectively. Both frequencies and fwhm of the methylene stretching modes indicate that the hydrocarbon chains at C19H39COOH molecules in monolayer assemblies exist in a solid state. In order to discuss the conformation of the hydrocarbon chain in C19H39COOH monolayers, the methylene wagging modes are analyzed. They appear at wavenumbers between 1380 and 1100 cm−1, and their number and frequency are dependent on the physical state of the hydrocarbon chain as well as on its length (number of methylene groups involved in the wagging mode progression) and on the conformation.43−45 Figure 5 shows the PM IRRA spectra in the methylene wagging mode region of C19H39COOH monolayers on TiOxCy and on Au substrates.

3.2. Application of TiOxCy as Substrate in IRRAS: Langmuir−Blodgett (LB) Monolayer of Arachidic Acid. Arachidic acid is a saturated fatty acid which forms solid and well packed monolayers at the air|water interface.38 The IRRA spectra of various saturated fatty acids in monolayer assemblies at the air|water and air|solid interfaces are known.17,33,39,40 Therefore, in this study C19H39COOH monolayers serve as a model system to study both the spectroscopic and the adsorption properties of the TiOxCy substrate. PM IRRA spectra of C19H39COOH monolayers on TiOxCy and Au substrates are compared in order to discuss the molecular order in monolayers on solid substrates and to correlate it to the order in the monolayer at the air|water interface. C19H39COOH monolayers are transferred onto both substrates at surface pressures equal to 15 and 40 mN m−1. In accordance with previous studies,31,41 at both surface pressures the arachidic acid monolayer exists in a 2D solid-like state. Figure 4 shows the PM IRRA spectra of LB monolayers of C19H39COOH in the CH stretching mode region on TiOxCy

Figure 5. PM IRRA spectra in the CH2 wagging mode region of LB monolayers of arachidic acid transferred at π = 40 mN m−1 on TiOxCy (solid line) and on Au (dash-dotted line) substrates.

The frequencies of the methylene wagging modes in LB monolayers of C19H39COOH as well as in a crystalline state of the analogue alcohol43are summarized in Table 1. As listed in Table 1, at frequencies lower than 1340 cm−1 the methylene wagging modes have the same frequencies both in the supported monolayer assemblies on Au and TiOxCy and in the dodecanol crystal. Clearly, bands seen in PM IRRA spectra in Figure 5 arise from the CH2 wagging mode progression. A sequence of absorption bands located between 1184 and 1316 cm−1, separated by 15−25 cm−1, is assigned to CH2 wagging modes in an all-trans conformation of the hydrocarbon chain. In the spectral range 1340−1380 cm−1 conformation sensitive CH2 wagging modes appear.45,46 CH2 wagging modes located at 1341, 1354, and 1367 cm−1 are identified with the following conformations of the hydrocarbon chain: end-gauche, gauche− gauche, and gauche−trans−gauche (gtg, kink), respectively.45 In the C19H39COOH monolayer on the TiOxCy surface, no wagging mode is observed at frequencies above 1315 cm−1. This result indicates that the entire hydrocarbon chain has an all-trans conformation. In the arachidic acid monolayer on the Au surface, the δs(CH3) mode at 1378 cm−1 has a shoulder at 1367 cm−1. This mode may arise from the CH2 wagging mode and indicates the presence of a kink, gtg, conformation in a

Figure 4. PM IRRA spectra in the CH stretching mode region of LB monolayers of arachidic acid transferred on (a) TiOxCy and on (b) Au at π = 40 mN m−1 (solid line) and 15 mN m−1 (dashed line).

and on Au substrates. A monolayer of the fatty acid on the TiOxCy surface gives well resolved PM IRRA spectra in the CH stretching mode region. On both substrates, four IR absorption bands originating from the asymmetric CH3 (νas(CH3)), asymmetric CH2 (νas(CH2)), symmetric CH3 (νs(CH3)), and symmetric CH2 (νs(CH2)) stretching modes are clearly seen in all spectra. The νas(CH3) mode in monolayers transferred at π = 15 mN m−1 is a broad asymmetric band centered around 2959 cm−1. In monolayers transferred at π = 40 mN m−1 the νas(CH3) IR absorption band splits into two modes centered at (2963.5 ± 0.3) cm−1 and (2954.3 ± 0.2) cm−1. The splitting of the νas(CH3) mode appears in hydrocarbon chains existing in a crystalline state, when the terminal methyl group has no rotational freedom.42 The high frequency mode arises from the νas(CH3) polarized parallel, whereas the low frequency mode originates from the νas(CH3) polarized perpendicular to the plane of the hydrocarbon chain. The νs(CH3) mode is located at (2878.2 ± 0.6) cm−1. Eighteen methylene groups give rise to the strongest absorption bands observed in the spectra: the νas(CH2) mode 13771


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Table 1. Frequencies of Methylene Wagging Modes in LB Monolayers of C19H39COOH on TiOxCy and on Au and in the Crystal of the Analogue Alcohola wagging band frequencies (cm−1) C19COOH|TiOxCy C19COOH|Au C20OH crystal43 a

k=0

k=1

k=2

k=3

k=4

k=5

k=6

k=7

k=8

k=9

k = 10

k = 11

1184 1185 1184

1201 1201 1201

1217 1217 1219

1234 1234 1235

1252 1252 1253

1261 1269 1270

1287 1286 1285

1302 1302 1301

1316 1316 1315

n.o. n.o. 1328

n.o. n.o. 1341

n.o. 1367 1361

n.o.: not observed.

hydrocarbon chain.45 The hydrocarbon chains of arachidic acid adsorbed in monolayers on TiOxCy and on Au exist in a solid state, but their conformations are different. Figure 6 shows the PM IRRA spectra of C19H39COOH monolayers on TiOxCy and on Au in the spectral region of

PM IRRA spectra in Figure 6 show that the number and intensities of the IR absorption bands in arachidic acid monolayers originating from the carboxylic group region are different on both substrates. On TiOxCy, the IR absorption spectra of the monolayer overlap with the absorption bands from water vapor. The ν(CO) stretching mode of the protonated carboxylic moiety is located at around 1705 cm−1 in C19H39COOH monolayers on TiOxCy and on Au. The ν(CO) mode of the carboxylic moiety in various fatty acid monolayers at the air|water interface has also been reported to appear at around 1705 cm−1.31,32 This shows that the carbonyl group in all investigated monolayers is hydrated and involved in the formation of hydrogen bonds.32 A slight red-shift of the ν(CO) mode in C19H39COOH monolayers transferred at π = 15 mN m−1 indicates that the hydration of the carbonyl group is stronger in films prepared at lower surface pressures. On TiO x C y , the PM IRRA spectra of C 19 H 39 COOH monolayers show an additional IR absorption mode at around 1540 cm−1 which is assigned to νas(COO−) (Figure 6a).17,33 The corresponding symmetric stretching mode of the carboxylic moiety νs(COO−) appears between 1470 and 1415 cm−1, depending on the configuration and binding of the carboxylic group to a solid substrate.17 In the C19H39COOH monolayer transferred onto TiOxCy at π = 40 mN m−1, the νs(COO−) is located at around 1435 cm−1. This band is very weak. In the monolayer transferred at π = 15 mN m−1 the intensity of the νs(COO−) around 1430 cm−1 increases. On the Au surface, the νas(COO−) stretching mode is absent whereas the νs(COO−) mode appears at 1433 cm−1 for C19H39COOH monolayers transferred at π of 15 and 40 mN m−1. Clearly, the presence of the νas(COO−) and νs(COO−) modes in the PM IRRA spectra indicates that a fraction of the carboxylic moieties of the fatty acid molecules is dissociated. Indeed, in monolayer assemblies of fatty acids on metal and metal oxide surfaces, carboxylic acid−carboxylate mixtures have been observed.33 Moreover, the frequency of the νs(COO−) mode indicates that the carboxylate oxygen atoms have a bidentate coordination to the substrate surface. Different intensities of these bands suggest that the terminal carboxylic group has different orientation in both monolayers, as discussed in detail below. 3.3. Quantitative Analysis of the Molecular Order in the Arachidic Acid Monolayer on the TiOxCy Surface. Since the hydrocarbon chains in the C19H39COOH monolayer exist in a solid state and have all-trans conformation, their orientation can be determined from the PM IRRA spectra.17,18 The upper panel in Figure 7 shows the deconvoluted PM IRRA spectrum of the C19H39COOH monolayer on TiOxCy. The bottom panel shows the calculated and deconvoluted PM IRRA spectrum of fatty acid molecules having a random distribution in a monolayer thick (2.7 nm) film. Interestingly, in LB monolayers of C19H39COOH on TiOxCy as well as in the calculated spectrum, the intensities of the νas(CH2) and νs(CH2) stretching modes are comparable. In

Figure 6. PM IRRA spectra from 1800 to 1280 cm−1 of LB monolayers of arachidic acid transferred on (a) TiOxCy and on (b) Au at π = 40 mN m−1 (solid line) and at π = 15 mN m−1 (dashed line).

1800−1300 cm−1. This spectral region provides information on the methyl and methylene bending modes in the hydrocarbon chain as well as on the CO stretching modes of the carboxylic moiety at the polar headgroup of the arachidic acid molecule. First, the IR absorption bands originating from the hydrocarbon chains are described. In the PM IRRA spectrum of the C19H39COOH monolayer transferred onto the TiOxCy surface at π = 40 mN m−1, a weak and asymmetric absorption band around 1470−1460 cm−1 is observed. It originates from the methylene bending mode (δ(CH2)) at the hydrocarbon chain fragment. Because of the low intensity of this mode and an overlap with the water vapor modes, it cannot be analyzed in detail. On the Au surface, the δ(CH2) mode is clearly composed of two contributions at 1471 and 1462 cm−1 (Figure 6). It has been reported that in monolayers of fatty acids at the air|water interface a splitting of the δ(CH2) mode of ∼10 cm−1 takes place.35,47 The number and position of the δ(CH2) modes provide information on the unit cell packing of a hydrocarbon chain in crystalline state.48−50 A splitting of the δ(CH2) mode indicates that the hydrocarbon chains in an arachidic acid monolayer have a vertical orthorhombic or tilted monoclinic unit cell packing.49 13772


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The Journal of Physical Chemistry C cos2 θ =

(3)

The orientation of the hydrocarbon chains in C19H39COOH monolayers on TiOxCy is calculated and compared with its orientation on the Au surface. The procedure described in detail in refs 36, 51, and 52 was used to calculate the tilt of the hydrocarbon chains. The transition dipole vectors of the νas(CH2) and νs(CH2) stretching modes lying in the plane of the methylene group are perpendicular to each other and have an angle normal to the direction of a fully stretched hydrocarbon chain.53 This means that when a hydrocarbon chain adapts an orientation vertical to the interface, the intensity of the methylene stretching modes decreases to zero. In all investigated LB monolayers the methylene stretching modes have nonzero intensities, which shows that the hydrocarbon chains of C19H39COOH have a tilt. This is a different orientation than the average one of hydrocarbon chains in fatty acid monolayers at the air|water interface.31,32,35,54 Table 2 lists the calculated average tilt angles of hydrocarbon chains in LB monolayers at various substrates. In LB monolayers of C19H39COOH on TiOxCy, the hydrocarbon chains have a tilt angle of ∼50° with respect to the surface normal. This value of the tilt angle may indicate that either the hydrocarbon chains are inclined toward the substrate surface or adapt a random orientation in the film. In addition, the surface roughness may strongly influence the orientation of the C19H39COOH molecules in the monolayer. AFM and SEM measurements are performed to determine the surface roughness of TiOxCy (Supporting Information section SI3), which shows that the TiOxCy surface is not flat on an atomic level. SEM images depict that the substrate surface is composed of grains with an average diameter of (16 ± 7) μm. The mean surface roughness of a single grain over an area of (2 × 2) μm2 is (11.4 ± 3.0) nm. The mean surface roughness determined over the area of (15 × 15) μm2, which includes a surface area ranging over different grains, is equal to (25.3 ± 5.7) nm. Clearly, the surface roughness of the TiOxCy surface is significantly higher than the thickness of the C19H39COOH monolayer (2.7 nm). Quantitative analysis of the PM IRRA spectra shows that the orientation of the C19H39COOH molecules on the TiO xCy surface follows the surface morphology, and therefore, the hydrocarbon chains appear to have a random distribution. However, the analysis of other spectral regions confirms that indeed the average orientation of C19H39COOH reflects the morphology of the substrate but individual molecules have a well-defined orientation. In even-numbered long-chain fatty acids the trans conformation along the Cβ−Cα−CO group is the most stable one,55 which is in agreement with our observation of an all-trans confirmation of the entire hydro-

Figure 7. Deconvoluted PM IRRA spectra of arachidic acid monolayers: (a) LB transferred at π 40 mN m−1 to TiOxCy and (b) calculated for a monolayer thick film from optical constants.

monolayers on the Au surface the intensities of the νas(CH2) and νs(CH2) stretching modes depend on the surface pressure of the monolayer transfer and thus on the surface concentration of the fatty acid (Figure 4). In the monolayer transferred at π = 15 mN m−1, the methylene stretching modes have the lowest intensities. Surprisingly, intensities of the methylene stretching modes increase significantly in the monolayer transferred at π = 40 mN m−1 and are higher than in the spectrum of randomly distributed molecules (Supporting Information, section SI.2). In an anisotropic film, the integral intensity of an IR absorption band depends on the surface concentration of an analyzed molecule (Γ) and on the orientation of the transition dipole vector of a given IR absorption mode (μ⃗) in the film with respect to the direction of the electric field vector of the incident light (E⃗ ). The integral intensity of an IR band of molecules adsorbed on the reflector surface (A) is described by eq 2:17,18

∫ A dν ∝ Γ|μ⃗ ·E⃗|2 = Γ |μ⃗ |2 |E⃗|2 cos2 θ

∫ Aexp dν 3 ∫ Acal dν

(2)

The integral intensity of a given absorption band depends on the angle θ between the vectors μ⃗ and E⃗ . The E⃗ vector of the ppolarized light is directed normal to the TiOxCy and to the Au surface. The angle θ may be calculated from the integral intensities of the PM IRRA spectra (Aexp dv) and a calculated spectrum (Acal dv) of randomly distributed molecules in the adsorbed film according to eq 3:17,18

Table 2. Tilt Angles of Hydrocarbon Chains in Saturated Fatty Acid Monolayers on TiOxCy, Au, and Al2O3 film|substrate C19H39COOH|TiOxCy C19H39COOH|TiOxCy C19H39COOH|Au C19H39COOH|Au C19H39COOH|Al2O3 C19H39COOH|Al2O3 C19H39COOH|Au

preparation procedure LB transfer π LB transfer π LB transfer π LB transfer π self-assembly LB transfer π LB transfer π

= = = =

tilt of hydrocarbon chains (deg) 55.2 ± 2.5 48.3 ± 3.6 59.9 ± 5.4 26.2 ± 4.5 10 12 large/not determined

15 40 40 15

= 20 = 20 13773

ref this this this this 17 33 33

work work work work


Article


individual C19H39COOH molecules in LB monolayers is dependent on the chemical nature and the surface morphology of the substrate used.

carbon chain of C19H39COOH. Moreover, on the TiOxCy surface three IR absorption bands arise from the polar headgroup region: the ν(CO) at 1703 cm−1, the νas(COO−) at 1540 cm−1, and the νs(COO−) at around 1435 cm−1. The transition dipole vector of the νs(COO−) lies in the bisector of the COO− moiety, while the νas(COO−) is perpendicular to it.17 The fact that both asymmetric and symmetric carboxylate stretching modes are present in the PM IRRA spectra shows that the transition dipole vectors of both modes have nonzero normal components. Thus, the terminal carboxylic group adapts canted orientation as shown schematically in Chart 1.

4. CONCLUSIONS In this paper we demonstrate that the optical properties of the new compact TiOxCy material in the mid-IR region ensure high reflectance of the linearly polarized IR light from its surface. At the air|TiOxCy interface the surface selection rule of IRRAS is fulfilled, which makes the material applicable as a new substrate for IRRAS. The PM IRRA spectra of the studied fatty acid monolayers show excellent signal-to-noise ratio and allow not only the identification of adsorbed molecules but also the analysis of the molecular structure and quantitative analysis of the orientation of adsorbed species. It was found that the orientation of the long hydrocarbon chains reflects the surface morphology of the TiOxCy substrate. The carboxylic polar headgroup interacts with the surface atoms, which leads to its partial dissociation and bidentate coordination to the surface. Clearly, analysis of the PM IRRA spectra of a molecular film adsorbed on TiOxCy includes the determination of the film composition, structure, packing, orientation of individual molecules in the film, and detection of interactions between the adsorbed fatty acid molecules and the surface atoms of the solid substrate. Since TiOxCy is electrically conductive and is used as support in electrocatalytic oxidation reactions of small organic molecules such as ethanol, it is an interesting material for in situ PM IRRAS studies under electrochemical control.

Chart 1. Orientation of the Arachidic Acid Molecule in the LB Monolayer on (a) TiOxCy and on (b) Au

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Interestingly, on the Au surface the average tilt angle of the hydrocarbon chains in the C19H39COOH monolayer transferred at π = 40 mN m−1 is 59.9° with respect to the surface normal. Low intensities of the methylene stretching modes in the monolayer transferred onto the Au surface at π = 15 mN m−1 correspond to an average tilt angle of (26.2 ± 2.4)° with respect to the surface normal. AFM investigations show that the unmodified polycrystalline Au surface is composed of grains with an average diameter of 63 nm.56 The surface roughness determined over an area of (5 × 5) μm2 is (1.697 ± 0.097) nm. In the case of Au, the surface morphology does not influence the molecular scale order in C19H39COOH monolayers to a large extent. The tilt angle is determined for dry LB films. Dote and Mowery observed that on the Au surface the intensities of the methylene stretching modes in LB monolayers of C17H35COOH transferred at π = 20 mN m−1 are higher than those of the same molecules at the aluminum oxide surface, and they are instable and increase with time.33 Changes in the tilt of the hydrocarbon chains in LB monolayers on Au are more pronounced in the closely packed film transferred at π = 40 mN m−1, suggesting that this tightly packed film is unstable. The analysis of the CH2 wagging modes suggests the presence of gtg kink conformations in the hydrocarbon chains. This conformation of the hydrocarbon chain requires larger average area per molecule than the all-trans conformation of a hydrocarbon chain. In the PM IRRA spectra, the absence of the νas(COO−) absorption mode shows that the transition dipole vector is orientated perpendicular to the E⃗ vector and thus lies in the monolayer plane. This orientation leads to an enhancement of the νs(COO−) mode that is indeed observed in the spectra shown in Figure 6b. The PM IRRAS results suggest that the carboxylic group has an orientation symmetrical to the Au surface with its two O atoms coordinated to the metal as illustrated in Chart 1b. In summary, the orientation of

ASSOCIATED CONTENT

S Supporting Information *

Optical constants of studied fatty acid in mid-IR region, a calculated spectrum of the fatty acid monolayer adsorbed on the gold surface, and determination of the surface morphology and surface roughness of the TiOxCy. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03570.

■

AUTHOR INFORMATION

Corresponding Authors

*I.B.: e-mail, [email protected]; phone, +49441-3973; fax, +49441 798 3979. *J.K.-L.: e-mail, [email protected]; phone, +43-512-50758013; fax, +43-512-507 58199. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the German Science Foundation (DFG) through Project BR 3961/2 is gratefully acknowledged (I.B.). J.K.-L. and C.R. thank the EU RTD Framework Programme FP7 (FP7-NMP-2012-SMALL-6, Project Title DECORE, Project Number 309741) for financial support.

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