Hybrid Gold-Conductive Metal Oxide Films for Attenuated Total

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Hybrid Gold-Conductive Metal Oxide Films for Attenuated Total Reflectance Surface Enhanced Infrared Absorption Spectroscopy Ian R Andvaag, Tyler A Morhart, Osai J.R. Clarke, and Ian J. Burgess ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02155 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on March 5, 2019

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ACS Applied Nano Materials

Hybrid Gold-Conductive Metal Oxide Films for Attenuated Total Reflectance Surface Enhanced Infrared Absorption Spectroscopy Ian R. Andvaag, Tyler A. Morhart, Osai J. R. Clarke and Ian J. Burgess*, † †Department

of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5C9 Canada

*corresponding

author: email ([email protected]), Phone. +1-306-966-4722.

Abstract The use of conductive metal oxide (CMO) films as supporting layers for attenuated total reflectance surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) is treated theoretically and experimentally. The greater mid-infrared transparency of thin layers of indium zinc oxide (IZO), as compared to metals, is verified through IR reflectivity measurements and the Drude model. IZO layers sputtered on silicon micromachined internal reflection elements (Si μIREs) are found to have a thin surface layer with slightly different plasma frequency and electronic scattering time compared to the bulk material. The complex permittivity and refractive index of the IZO can be extracted using the Drude model. This allows application of the Bruggeman effective medium theory to calculate the ATR-SEIRAS response of a layer of gold prolate spheroids supported on an IZO film. Calculated ATR-SEIRAS spectra for a 1 nm thick organic film, modelled as a Lorentz oscillator, predict an order of magnitude improvement in absorbance strength using the IZO film as a base layer compared to a conventional, gold covered internal reflection element. These predictions are qualitatively verified by the electrodeposition of gold nanoparticles on an IZO modified Si internal reflection element and the study of the potential controlled adsorption of a pyridine derivative. The IZO/Au layers are found to be very mechanically stable and can withstand large potential perturbations. This is demonstrated through the in situ study of the repeated reductive desorption of a self-assembled monolayer of 4-mercaptobenzoic acid.

Keywords ATR-SEIRAS, conductive metal oxides, Drude model, Bruggeman effective medium theory, indium zinc oxide (IZO)

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Introduction Infrared spectroscopy is an established tool for providing molecular details on important electrochemical processes. Osawa pioneered the use of electrochemical attenuated total reflectance surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) to overcome many of the experimental limitations of external reflection methods.1,2 In this method, a conductive layer is formed on an infrared transparent internal reflection element (IRE) by depositing a thin (ca. 1020 nm) metal film directly onto the surface of an ATR prism. Suitable prism materials include Ge, ZnSe and Si, with the latter being the most popular choice. Unlike surface enhanced Raman spectroscopy (SERS), enhancement phenomena in ATR-SEIRAS are operative on many metals and the technique is not limited to metal surfaces that generate enhanced, localized, electric fields, Eloc, through excitation of localized surface plasmon polariton (SPP) modes.3-6 Although surface enhancement is weaker for SEIRAS (scales with E2loc) compared to the nonlinear Raman process where it scales with E4loc, experimental signal levels for SEIRAS and SERS are essentially equivalent because the cross sections of dipole allowed IR transitions are many orders of magnitude greater than those of Raman modes. This raises an interesting question: what limits ATR-SEIRAS from becoming a preferred vibrational spectroscopy tool for surface electroanalytical chemistry? In the opinion of the authors, a major limitation of ATR-SEIRAS is the absence of reliable, easy-to-use, and preferably re-useable, substrate platforms. Most metals directly deposited on IRE materials frequently delaminate during an electrochemical experiment. Biasing the working electrode potential to values that lead to hydrogen evolution particularly exacerbates the problem7 as does subjecting the thin film electrode to large extremes in either compressive or tensile stress. Even after a successful experiment, cell disassembly usually results in irrevocable damage to the metal film. Consequently, considerable time and effort

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must be spent preparing (cleaning, polishing, coating, and electrochemically conditioning) the IRE prior to each ATR-SEIRAS experiment. Ohta et al8 reported the use of Au/Ti bilayers for ATRSEIRAS whereby the Ti promoted much greater gold adhesion with the ATR prism. This method is difficult to implement in practise and our attempts to use titanium as a gold adhesion layer yielded, at best, very weak enhancement. We believe there are two main factors that limit this approach. First, the titanium layer induces a smooth gold surface rather than the textured films of quasi-spheroidal particles needed for ATR-SEIRAS. Ohta et al reported that electrochemical annealing by potential cycling in acidic electrolyte can appropriately restructure the gold layer.8 However, in our experience, extensive electrochemical annealing was unable to provide surface enhancement although it did improve the film texture. Secondly, it is known that Ti diffuses rapidly along noble metal grain boundaries9-11 and it is quite possible that the bilayer is really an alloy or, more likely, there is a high concentration of Ti at the Au grain boundaries. The influence of such a composite layer on SEIRAS response is difficult to model but it is likely that both chemical (poor molecular adsorption at the grain boundaries) and electromagnetic (depolarization of the metal ellipsoids by Ti surrounding the gold) effects could be responsible for the poor SEIRAS enhancement from Ti/Au layers. Recently, Lotti et al12 reported a different means to prepare ATR-SEIRAS substrates by sputtering thin layers of a conductive metal oxide (CMO) on an IRE, followed by electrodeposition of platinum. This approach produced an ATR-SEIRAS active Pt layer supported on a thin film of indium tin oxide (ITO) that allowed co-addition of many individual pump-probe measurements. Unlike most metals, CMOs do not exhibit strong interband transitions and have much smaller extinction coefficients throughout the infrared region making them, prima facie, more optically transparent for mid-IR applications.13 Additionally, previous research14 in non-metal materials for plasmonic applications has shown that CMOs may provide much more than an optically transparent platform.15 Thin films of ITO

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support both lateral (localized and propagating) and transverse (epsilon near zero) SPP modes16 as well as hybrid plasmon-diffraction modes.17 These modes are important because large, localized electric fields generated by the excitation of SPP modes are one of the mechanisms that provide surface enhancement in ATR-SEIRAS.18 In order to evaluate the possible utility of CMO layers for ATR-SEIRAS we report herein an indepth characterization of thin films of indium zinc oxide (IZO). We apply effective medium theory to model the optical response of IZO in the ATR-SEIRAS (Kretschmann) configuration with an overlayer of deposited gold and provide a comparison to equivalent calculations for a gold-only SEIRAS film. The contribution details the preparation and characterization of IZO films on micromachined Si IREs19,20 and their modification by gold electrodeposition to generate ATRSEIRAS active layers (see Scheme 1).

Scheme 1 : Preparation of a Si micromachined ATR substrate with a conductive metal oxide layer and subsequent modification through the electrodeposition of gold.

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Experimental Section IZO layer preparation. IZO films were applied to Si micromachined internal reflection elements (Si μIREs, IRUBIS GmbH) by a homebuilt RF magnetron sputtering unit using a power of 30 W and a base vacuum of of 4 × 10-5 torr. The IZO target (Kurt J. Lesker) was a 50.4 mm diameter, 3.2 mm thick disk composed of 90% In2O3 and 10% ZnO by weight. The thickness was monitored by quartz crystal microbalance calibrated by profilometry (KLA Tencor). The conductivity of the films was measured with a 4-point conductivity probe (Lucas Labs Model S-302-4). A correction factor for the finite size of the substrate was applied (obtained by interpolating the correction factors given in Haldor Topsoe).21 The sheet resistances for the 54 nm and 61 nm films were determined to be 332 Ω sq-1 and 254 Ω sq-1, respectively. FTIR Measurements. All IR measurements were made on a dry-air purged Bruker Vertex 70 FTIR spectrometer using an MCT/A detector. Spectra were collected with either 4 cm-1 or 8 cm-1 resolution using a 40 kHz scanning velocity and 2 mm aperture. The spectroelectrochemical cell (Jackfish SEC) was mounted on a VeeMAX III variable angle ATR accessory (PIKE Technologies) and the angle of incidence was set to 55°. Au films directly deposited on the IRE were prepared using the RF magnetron unit described above or a Denton Desk Vacuum IV as described elsewhere.19 Electrochemistry. The cell was filled with 0.1 M NaF (Aldrich) and purged with Ar for 20 min. A coiled Au wire served as the counter electrode and a Ag/AgCl (saturated KCl) reference electrode was used. The working electrode was the IZO-modified Si μIRE surface. Aqueous KAuCl4 (Aldrich) and 4-methoxypyridine (MOP) (97%, Aldrich) were added to give concentrations of 250 µM and 100 μM respectively. All glassware was cleaned in hot 3:1 H2SO4:HNO3 and rinsed with copious amounts of ultrapure water (Millipore). Teflon and viton

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components of the cell were cleaned in fresh piranha solution which had been allowed to cool to room temperature. Au electrodeposition. Beginning at the open circuit potential (ca. +600 mV), the working electrode potential was scanned at 20 mV s-1 between -1000 and +1000 mV. After three cycles were completed, the potential was held at +300 mV, a sample spectrum was collected, and then the potential was stepped to -900 mV and a reference spectrum was collected. At these potentials, MOP is adsorbed and desorbed on Au respectively, and the potential difference absorbance spectrum allows the vibrational signature of MOP adsorbed on electrodeposited Au to be used as a spectroscopic handle for the increasing enhancement factor of the deposited film. Electrodeposition proceeded in this way, by performing three voltammetric cycles between +50 and -1000 mV and collecting a diagnostic potential difference absorbance spectrum, until the MOP signals plateaued. The prepared film was removed from the cell and rinsed with ultrapure water. Thiol reductive desorption. The acid-washed cell was reassembled with the prepared Au/IZOcoated wafer and filled with 0.1 M NaF. Solid p-mercaptobenzoic acid (MBA, Aldrich) was added to saturate the solution (final concentration roughly 0.1 mM). MBA monolayers were allowed to form under open circuit conditions and monitored by IR for fifteen minutes after which time the spectra stopped changing. Scanning to -1000 mV desorbed the MBA monolayer as confirmed by IR measurement. Scanning back to 0 mV (near the OCP of the Au layer) and waiting an additional fifteen minutes completely reformed the monolayer. Three replicate desorptions were collected.

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Results and Discussion Optical modelling of IZO Films Thin films of IZO were deposited on Si internal reflection elements using magnetron sputtering. SEM images of the resulting ~50 nm thick films (see Figure S1a) show a continuous, highly uniform layer with very few defects. The optical properties of conductive metal oxide films in the visible to mid-infrared are very successfully modelled using the Drude model.22 The plasma frequency, ωp, the dampening constant, γ, and the infinite frequency permittivity, ε∞, define the permittivity function in the Drude model;    

 2p  2  i

.

(1)

The plasma frequency depends on the free carrier concentration, N

Nq e2   2p * om

(2)

where m* and qe are the effective mass and charge of an electron and εo is the permittivity of vacuum. For completeness, we note that there are sometimes discrepancies in the literature regarding equation 1, which we address in the Supporting Information. By setting the real part of Equation (1) to zero, one can define the plasma edge, ωedge, in terms of the free carrier density

edge 

 p2  Nq e2  2  2    . *      o m 

(3)

The real part of  is negative for frequencies below the plasma edge and positive for frequencies above the plasma edge. The plasma edge delineates the primarily transparent and reflective optical frequency regions for a conductive material and it is apparent from Equation 3 that decreasing the free carrier concentration leads to a decrease in the plasma edge frequency. The value of N for conductive metal oxides such as ITO is about 1019-1021 cm-2 and roughly 4 orders 7



of magnitude smaller than that of metals.13 Correspondingly, the plasma edge for CMOs lies significantly further toward infrared wavelengths compared to the ultraviolet values of metals. However, these parameters depend on the morphology and grain size of the deposited films and previous reports have indicated that the optical properties of CMOs can be tuned by altering the film preparation conditions23 and post-deposition annealing conditions.24,25 The reflectivity of thin films of IZO sputtered on silicon wafers was used to determine the optical properties of the film.26 The dampening factor and the plasma frequency were adjustable fit parameters used to calculate the permittivity function as per Equation 1. The infinite frequency permittivity was constrained to values near 3.8 as per previous reports.27,28 The real (n) and imaginary (κ) components of the complex refractive index,   n  i , were determined from the relationship between refractive index and permittivity,    . The reflectivity of a stratified interface consisting of air/IZO/Si was computed as a function of angle and frequency using the Fresnel equations, the known refractive index of Si and the computed IZO complex refractive index (see Supporting Information). Although a qualitatively reasonable agreement could be obtained using a single layer model for the IZO, a two-layer approach provided a better fit. Previous studies have shown that the surface layer of CMOs exhibits different optical properties compared to the bulk material and two-layer models are typically required to account for the observed optical response of sputtered CMO films.29 Figure 1 shows the reflectivity fitting results and Table 1 summarizes the results of the fitting analysis. Equation 2 was used to extract the free electron concentration using a value of m*=0.3me, where me is the electron rest mass.30 The dc resistivity, ρ, and the electron mobility, μ, of the bulk and surface IZO were determined using      o 2p  and   e   m*  .31 The resistivity of the IZO surface layer was within a factor of two of the resistivity determined from the four-point conductivity measurement. The

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IZO charge carrier density is about a factor of 3 lower than the reported values for indium tin oxide32 and aluminum-doped zinc oxide (AZO)33 but very comparable to previous reports for the carrier density for IZO.34 Craciun et al34 reported resistivities and charge mobilities between 400 – 2000 μΩ cm and 33-40 cm2 V-1 s-1 respectively for IZO thin films. The former are very similar to the values reported in Table 1, while our measured mobility for bulk IZO is about a factor of two higher. 0.24 0.22 0.20

Reflectivity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.18 0.16 0.14 0.12 0.10 1000

2000

3000

4000

5000

6000

Wavenumber (cm-1)

Figure 1 : Experimental (solid line) and best-fit (dashed line) reflectivity curves from a 54 nm IZO layer supported on a Si wafer at 50 degrees using p-polarized light incident through air. Table 1: Modeled optical parameters of IZO using the Drude model.

(cm-1)

ωp ωp (s-1) γ (cm-1) γ (s-1) Thickness (nm) N (cm-3) ρ (μΩ cm) μ (cm2 V-1 s-1)

Bulk IZO 7151 1.35×1015 320 6.03×1013 49 1.7×1020 375 97

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Surface IZO 7988 1.51×1015 905 1.71×1014 5 2.1×1020 850 34


Figure 2a shows the real and imaginary components of the permittivity function for bulk IZO. The plasma edge frequency is ~ 3700 cm-1, meaning that the IZO becomes increasingly reflective at mid-IR wavelengths longer than about 2.7 μm. Figure 2b plots the real (n) and imaginary (κ) components of the complex refractive index. Although the extinction coefficient increases rapidly for wavenumbers below the plasma edge, the values are rather small compared to metals in the same frequency range. This illustrates a key, potential advantage of using IZO in an ATR configuration for SEIRAS applications.

Figure 2 : a) Complex permittivity function and b) complex refractive index for the bulk IZO layer as determined from the Drude model and the reflectivity fitting. Left ordinate axes are the real components (black solid lines) of the optical parameters and the right ordinate is the imaginary components (red dotted lines).

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Effective medium theory The enhancement mechanism in ATR-SEIRAS is often described in an analogous fashion to the enhancement mechanism in SERS. Specifically, the excitation of LSPPs generates large, localized electric fields at the surface of nanoscale metal particles. It is likely that a similar mechanism applies to SEIRAS active surfaces composed of plasmonically active metals such as Ag and Au as has been shown by recent electromagnetic modelling studies.18 However, ATRSEIRAS enhancement mechanisms are also operative on metals that do not support large LSPP resonances, e.g. Pt35 and Pd.4 Effective medium theory (EMT)36 is an alternative approach to explaining SEIRAS phenomena. Osawa et al37 provided a mathematical basis for calculating ATR-SEIRA spectra by modelling the polarizability of Ag nanoparticles with the Bruggeman effective medium theory.38 Bjerke et al35 used the Bergman EMT to examine the IR spectra of CO adsorbed on roughened Pt surfaces and were able to reproduce inverted and bimodal peaks previously described as abnormal infrared effects (AIREs).39 Pecharromán et al40,41 subsequently explained that the Fresnel equations and changes in film reflectivity due to adsorbate-induced changes in the complex refractive index of the metal-solution interface fully explain AIREs. Within this context, we modified the Bruggeman EMT approach used by Osawa et al37 to calculate ATR-SEIRA spectra of IZO modified Si internal reflection elements. Implementation of the model is described in detail in the Supporting Information. Briefly, the metal surface, treated as a collection of metal prolate spheroids, is embedded in a host medium, which fills the spaces between the metal particles. A thin layer with permittivity  d coats the particles to simulate the presence of an adsorbed organic film. According to Granqvist and Hunderi,42 the Bruggeman effective permittivity,  BR , of a composite layer of metal nanoparticles is

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 BR   h

3 1  F   F 3 1  F   2 F

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(4)

where εh is the permittivity of the host medium (e.g. air or water), F is the volume fraction occupied by the nanoparticles and α is the polarizability of the particles. The polarizability is a function of the volume ratio of the uncoated to coated particles Q, the overall depolarization factor of the bare L1 and coated L2 prolate metal particles, the permittivity of the metal,  M , and if present, the organic coating,  d , 

 d   BR   M L1   d 1  L1   Q  M   d   d 1  L1    BR L2  (5)  d L2   BR 1  L1    M L1   d 1  L1    Q   M   d   d   BR  L2 1  L2 

The depolarization factors are the arithmetic mean of individual depolarization factors along the major and minor semi-axes, computed using the equivalent expressions provided independently by Stoner43 and Osborne44 (see Supporting Information for more details). To allow direct comparison, the fill factor (F = 0.7) and particle aspect ratio (m = 3) were set to the same values reported in Osawa et al’s treatment of ATR-SEIRAS using the Bruggeman EMT.37 To simulate ATR-SEIRA spectra, the effective permittivity function of the gold particle layer (nAu) was computed with and without the presence of the organic layer (modelled as a Lorentz oscillator) as per Equations 4 and 5. The real and imaginary components of the complex refractive index were determined from the permittivity as described above. The reflectivity of the stratified interface was determined using Ohta’s matrix approach45 to calculating the Fresnel equations (see Supporting Information). The absorbance was determined from the negative logarithm of the ratio of the reflectivity of the organic coated gold nanoparticles to that of the bare nanoparticles. Figure 3 overlays calculated spectra for i) a Au/nAu SEIRAS active film ii) an IZO/nAu film and iii) an organic film directly deposited on the Si IRE. The organic layer thickness was 1 nm in all three scenarios. A 10 nm continuous gold layer was used for i) whereas 12


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a 40 nm thick IZO layer (divided into a 36 nm bulk layer and a 4 nm thick surface layer) was used in ii) to more closely simulate experimental conditions (vide infra). Table 2 provides full details for the various layers. The calculated spectra show that the molecular absorbance on IZO/nAu is nearly eight-fold higher compared to Au/nAu and two orders of magnitude greater than the ATR spectrum. The inset in Figure 3 shows the calculated absorbance spectrum for the Bruggeman layer directly deposited on the silicon surface and is quite close in magnitude to the calculated IZO/nAu spectrum.

Figure 3 : Calculated ATR-SEIRAS spectra for a 1 nm thick organic layer i) coating the nAu layer supported on a 10 nm thick layer of continuous gold (red dashed line) and ii) coating a 10 nm thick layer of gold spheroids (nAu) on a 40 nm thick IZO base layer (solid black line) iii) deposited directly on a Si IRE (blue dotted line). The absorbance spectrum in the inset is for the Bruggeman layer directly deposited on the Si IRE. Full details are provided in Table 2. Consideration of the extinction coefficients of the base layers can partly explain the results. At the peak frequency of the Lorentz oscillator, κ has a value of 43 for Au, which is ten times larger than the calculated κ value for the IZO. The attenuation of light as it travels a distance, z, through a lossy material such as a conductor depends on the value of the extinction coefficient

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 4 z  E 2  Eo2 exp   .   

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(6)

Thus, the relative intensities of the electric field reaching the organic layer covered metal nanoparticles for the IZO and Au systems are

E2 E

IZO

2 Au

4 z   .  exp  Au   IZO    

(7)

Table 2: Modeled optical parameters of IZO using the Drude model. Material

Thickness

Air

N/A

Composite layer a. Au spheroids (reference). b. Au spheroids coated with organic film (sample).

10 nm

IZO surface

4 nm

IZO bulk

Si

Model for permittivity

36 nm

N/A

h  1

Fixed Bruggeman EMT model (Eqns 4,5) with  M equal to tabulated permittivities of gold46 and the organic film modelled as a Lorentz oscillator:

 d    ,d 

 2p ,d r2   2  i d

   

   

Parameters

 2p  2  i

 2p   i 2

Fixed

r  1600 cm 1   ,d  1.77  p ,d  173 cm 1  d  20 cm 1 F  0.7

Organic layer thickness = 1nm Major axis diameter = 30 nm Minor axis diameter = 10 nm

   3.8  p  7988 cm 1   905 cm 1

   3.8

 p  7151 cm 1   320 cm 1

  11.6

For z = 20 nm, the field strength at the organic film covered metal nanoparticles is roughly five times larger for IZO and the absorption should be stronger by an equivalent factor. However, this effect is much smaller in Figure 3 as the IZO layer is four times thicker than the continuous Au layer meaning greater transparency does not fully explain the improved calculated absorbance 14


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afforded by the IZO base layer. The Bruggeman EMT does not account for LSPP modes so the larger modelled molecular absorbance cannot be attributed to plasmonic mechanisms. Within the context of EMT, the explanation is more subtle and arises from the fact that the change in the overall reflectivity of the interface between the bare and organic-coated gold nanoparticles is much greater when IZO, rather than continuous Au, forms the base layer upon which the textured gold is deposited. The greater modelled reflectivity changes are a direct consequence of the permittivity functions of the modifying layers and the Fresnel equations as per a “nonplasmonic” interpretation of the ATR-SEIRAS mechanism. The current model does not include LSPP modes but it is important to reiterate that ATR-SEIRAS arising from textured conductive surfaces can also be explained by considering plasmonic phenomena.18,47 Kraack, Hamm and coworkers48,49 demonstrated that thin layers of unmodified ITO only provide an ATR-SEIRAS enhancement factor of about 2. However, earlier work by Franzen et al50 showed that the deposition of thin layers of gold on top of ITO generates new surface plasmon polariton effects which are likely pertinent to the IZO/nAu system used in this study (vide infra). ATR-SEIRAS To generate ATR-SEIRAS active platforms, IZO-coated Si μIREs were further modified by gold electrodeposition in the spectroelectrochemical cell. ATR-SEIRAS active metal films can be formed directly on Si by either electroless deposition or physical vapour deposition. We initially chose to deposit gold on the IZO surface by electrodeposition of AuIII because this method provided a convenient handle to follow the progression of the enhancement in situ. To achieve this, the IZO-modified Si μIRE was assembled in a spectroelectrochemical cell containing an eletrodeposition bath consisting of 0.1 M NaF, 0.25 mM KAuCl4 and 0.1 mM 4methoxypyridine (MOP). The MOP assists the growth of Au nanoparticles on the CMO surface

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and provides a convenient spectroscopic handle.51 Figure S2 shows the voltammetry of the Au deposition. Gold deposits on the IZO surface in the first cathodic scan as evidenced by a pronounced negative current peak at ~ -0.12 V vs Ag/AgCl, a catalytic crossover at ca. +0.10 V vs Ag/AgCl, and the appearance of gold oxide formation in the first return scan to positive potentials. More gold is deposited with each successive negative potential scan and the presence of the MOP in the electrolyte allows for qualitative assessment of the surface enhancement factor as the pyridine derivative desorbs from gold surfaces at sufficiently negative potentials and adsorbs at relatively positive potentials. An absorbance difference spectrum of MOP is generated from the ratio of a single beam spectrum at -0.90 V (reference spectrum) and +0.30 V (sample spectrum). The MOP absorption initially increased as a function of scan number but eventually reached a plateau (see Figure S3). Subsequently, the cell and was disassembled and the modified Si μIRE was rinsed with water. Figure 4a is an electron micrograph of the gold-modified IZO surface of the Si μIRE after gold electrodeposition. The image reveals a compact array of Au particles which is typical for regions of interest near the very centre of the electrodeposited area. However, as shown in Figure S4, the density of the particles decreases slightly with increasing distance from the centre. AFM images of these regions provide an estimate of the Au island thickness. The cross-section of the AFM image shown as an inset to Figure 4b indicates that the Au islands are approximately 5-10 nm high. Although many of the electrodeposited particles are anisotropic in shape, each particle can be characterized by an effective diameter of a sphere of equivalent cross sectional area. The particle diameter histogram, Figure 4c, reveals a mean diameter of ~ 70 nm with a standard deviation of 20 nm. Owing to the complex and disperse distribution of particle shapes and sizes we did not attempt to model the nanoparticle layer using a modified Bruggeman EMT and the

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computed responses in Figure 3 and experimental ATR-SEIRAS results should be compared qualitatively rather than quantitatively.

Figure 4 : a) Scanning electron micrograph and b) AFM image of Au particles electrodeposited on an IZO layer sputtered on a Si internal reflection element. c) Histogram of the particle diameters after equating the SEM measured particle area to an equivalent spherical particle. 17



It is important to note that we also tried to produce ATR-SEIRAS active layers by vacuum sputtering gold on the IZO. Following the deposition conditions detailed by Miyake et al,52 20 nm of Au was magnetron sputtered at a deposition rate of 0.01 nm s-1 on Si μIRE substrates precoated with 50 nm thick IZO layers. Although we measured strong ATR-SEIRAS response on CMO-free Si μIREs prepared using these conditions, no SEIRAS activity was observable from gold vacuum deposited on IZO/Si despite our efforts to optimize deposition conditions. SEM images of vacuum deposited Au (see Figure S5) reveal a much smoother film compared to electrodeposited Au and indicate the importance of percolated metal island networks in the Au film morphology.35 It might be possible that these films can be appropriately textured by extensive electrochemical annealing but we did not observe evidence of surface enhancement over the course of 2-3 hours of potential cycling. The nAu-modified IZO layer was very robust and could easily be reassembled in the spectroelectrochemical cell, after copious rinsing with water, without loss of either the conductive metal oxide film or the electrodeposited gold layer. This represents a major technical advantage as Au films deposited directly on Si tear or delaminate upon cell disassembly. To demonstrate the stability of the film, a fresh solution of 0.1 M NaF and 0.1 mM MOP was added to the cell and the potential was cycled between -1.0 V (vs. Ag/AgCl) and +0.5 V. The voltammograms were found to be essentially invariant with scan number over the course of several hours (see Figure S6). Figure 5 compares the SEIRAS difference spectrum (reference: E = -0.9V; sample: E = +0.1V) to the analogous experiment performed on a Au/nAu coated Si μIRE. As per our previous work on MOP,19,53,54 five medium to strong positive peaks appear between 1650 cm-1 and 1000 cm-1 in the spectrum for the Au/nAu layer. These bands at 1617 cm1,

1510 cm-1, 1309 cm-1, 1209 cm-1, and 1031 cm-1 arise from A′ modes when the pyridine

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derivative absorbs in a vertical orientation.54 Smaller peaks at 1562 cm-1, 1235 cm-1, 1309 cm-1, and 1055 cm-1 are assigned to weaker A′ modes or modes that are largely inactive on the basis of surface selection rules. The spectrum also shows a negative going band at ~ 1650 cm-1 caused by the loss of the water bending vibration due to the replacement of the adsorbed solvent molecules by MOP. The spectrum from the IZO/nAu layer is notable for the approximately four-fold increase in signal levels for the nine bands described above and clearly demonstrates the greater SEIRAS enhancement for the IZO layer compared to a conventional gold film. The rather broad

Au/nAu

0.01 Abs

IZO/nAu 2000

1800

1600

1400

1200

1000

-1

Wavenumber (cm ) Figure 5 : Experimental ATR-SEIRAS spectra for the adsorption of 4-methoxypyridine at +0.10 V (relative to a reference spectrum taken at -0.90 V) using a conventional Au ATR-SEIRAS film (blue spectrum) and an IZO/nAu layer (red spectrum). Electrolyte composition was 0.1 M NaF and 0.1 mM MOP. band at 1106 cm-1 cannot be assigned to any known vibrational modes for MOP and likely originates from potential induced changes in the IZO film or the asymmetric stretching of Si−O− Si arising from interstitial oxygen.55 The apparent downward feature centred at 1800 cm-1 is not a

19



peak but rather an artefact of the skewed baseline. It is highly noteworthy that the bending mode of water is inverted in the IZO/nAu spectrum, some of the lineshapes are asymmetric, and the baseline is not flat. Figure S3 shows that early stages of Au electrodeposition yield Lorentzian lineshapes but the peaks become increasingly asymmetric with increasing amount of electrodeposited gold. Thus, there is an offset between maximizing the enhancement effect (greater metal electrodeposition) and obtaining optimal peak lineshape (less metal electrodeposition). Bjerke et al35 used a more advanced effective medium theory to demonstrate that these effects are correlated with the extent of percolation in metal island films. To demonstrate the robustness of the IZO/nAu layer we chose to perform an ATRSEIRAS experiment that is essentially impossible with a conventional Au/nAu film. The strong adhesion of the IZO layer to the Si surface should overcome experimental limitations of ATRSEIRAS caused by the fragility of the surface enhancing film. For example, the complete reductive desorption of self-assembled monolayer films on polycrystalline gold surfaces requires large negative polarizations to reduce Au-S bonds. The surface stress associated with reductive desorption and coincident hydrogen evolution causes Au films deposited directly on Si surfaces to delaminate.56 To demonstrate the robustness of the Au/IZO layers, we performed ATRSEIRAS experiments in 0.1 M NaF electrolyte saturated with 4-mercaptobenzoic acid (4-MBA). Despite the low solubility of 4-MBA, spectrum I in Figure 6 shows that a self-assembled monolayer (SAM) forms on the Au particles at open circuit potential. The ATR-SEIRAS spectrum of the as-formed SAM is dominated by strong positive absorbance peaks at 1585 cm-1 and ~1400 cm-1. The asymmetric line shape observed near 1400 cm-1 is the result of a poorly resolved peak at ~1417 cm-1 and a weaker band at 1380 cm-1. Three other small, but measureable peaks appear at 1483 cm-1, 1176 cm-1 and 1012 cm-1. The ν8a C-C stretching mode of the

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aromatic ring provides the distinctive vibrational peak at 1585 cm-1. The weaker, low frequency signals at 1012 cm-1 and 1176 cm-1 are assigned to C-H wagging modes of the ring (normal modes ν18a and ν9a) or, possibly the υC-O stretch of the carboxylate headgroup.57,58 Protonated acid self-assembled monolayers provide a carbonyl stretch in the region 1690-1750 cm-1 depending on the extent of hydrogen bonding in the SAM.59,60 Upon, deprotonation, both a symmetric and

Figure 6 : ATR-SEIRAS spectra of a 0.1 M NaF electrolyte saturated with 4-MBA using an IZO/nAu layer. I) Differential absorbance of monolayer formed at open circuit potential II)-IV) successive potential difference spectra using Esample = -1.0V and Ereference = 0.1V. The interface was held at the reference potential for 15 minutes to ensure the SAM was reformed. asymmetric CO2- stretch arises from the carboxylate group. However, assuming the 4-MBA is vertically oriented through a Au-S bond, surface selection rules render the asymmetric vibration infrared inactive and only the lower frequency symmetric stretch is observed near 1400 cm-1. This peak overlaps with another vibrational band and results in a distorted lineshape such as that shown in spectrum I in Figure 6, which makes exact peak assignments somewhat ambiguous.58

21



The overlapping signal near 1400 cm-1 was reported to arise from the ν19B ring mode. However, this is unlikely as this normal mode has a transition dipole moment perpendicular to the molecular axis and, therefore, is IR inactive. Instead, we speculate that the two features near 1400 cm-1 demonstrate a different extent of hydrogen bonding61 and/or ion-pairing with the electrolyte cation.62 Cumulatively, the surface sensitive IR measurement indicates the spontaneous formation of a deprotonated 4-MBA monolayer at open circuit conditions. Spectrum II in Figure 6 was calculated after applying -1.0V to the surface to reductively desorb the 4-MBA SAM. The result is a near perfect inversion of the open-circuit spectrum indicating complete removal of the self-assembled monolayer. Additional signals at 1106 cm-1 and 1660 cm-1 are also observed. The former has already been described as a potential induced change in the IZO film (vide supra) and the latter arises from the replacement of 4-MBA with water upon desorption. As before, the peak associated with this latter absorption change is the inverse of the expected response. After the first reductive desorption, the potential was held at +0.10 V for 15 minutes to reform the 4-MBA SAM. The new monolayer was reductively desorbed to afford spectrum III and the process repeated a third time to yield spectrum IV. The three reductive desorption experiments give essentially identical spectral responses indicating that the IZO/nAu film is undamaged by the monolayer removal and reformation processes. The 4-MBA monolayer also allows a comparison between the signal generated from the modified ATR-SEIRAS surface and an external reflectance experiment. Although Donaldson and Hamm63 have described a 2D correlation spectroscopy method to extract true enhancement factors which can be applied to ATR-SEIRAS,64 a more commonly used metric is the ratio of the IR band intensities of a molecule adsorbed on the ATR-SEIRAS film to those obtained using external reflection

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spectroscopy.65,66 Details of this calculation are provided in the SI, and, after accounting for the apparent roughness factors of the two substrates, the value of G is found to be approximately 60. 0V

0.01 Abs

-0.2 V -0.4 V -0.6 V -0.8 V -1.0 V

1800

1600

1400

1200 -1

1000

Wavenumber (cm ) Figure 7 : Potential dependent ATR-SEIRAS spectra of a 0.1 M NaF electrolyte saturated with 4-MBA using an IZO/nAu layer as measured during a slow (2 mV/s) cathodic sweep. The reference spectrum was measured prior to the scan after holding the potential at 0.0 V for 15 minutes to form the MBA SAM. The reductive desorption was also followed as a function of potential, by collecting spectra during a slow (2 mV/s) cathodic scan from 0.0V to -1.0V. Figure 7 shows a series of ATRSEIRA spectra, averaged over 50 mV windows, during the desorption scan. Evidence of 4-MBA removal is seen starting at ~-0.4V with small absorbance losses in the aromatic ring stretching mode at 1590 cm-1 and the appearance of a small, negative, peak at ~ 1425 cm-1. The peak at 1425 cm-1 is well resolved from the broader feature at 1397 cm-1 compared to the broad, asymmetric lineshape seen in Figure 6 but with increasingly negative potential, the 1397 cm-1

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signal becomes more pronounced. Assuming the two peaks originate from two sources of carboxylate (weakly hydrogen bonded to water versus heavily hydrogen-bonded or ion-paired), then the progression of the two signals with potential indicates that the “free” carboxylate form of MBA is preferentially desorbed from the gold surface. The potential scan during the MBA desorption experiment also provides information on the potential dependent response of the IZO/nAu at high frequencies. The higher frequency spectral region is shown in Figure 8a and is plotted without applying a vertical offset to emphasize the potential dependent shift in the spectral baseline. We note that baseline shifts are qualitatively predicted from the Fresnel reflectivity modelling and the Bruggeman EMT (the offsets were removed in Figure 3 for clarity). The experimental spectra reveal an apparent positive absorbance peak near 4000 cm-1 that shifts to higher wavenumbers with increasingly negative potential. However, interpretation of the spectra is complicated by the shifting baseline and the absorbance changes near 3200 cm-1 and 3653 cm-1. The origin of these features might arise from surface plasmon polariton modes of the IZO/nAu layer as thin ITO-Au hybrid films can exhibit both an epsilon near zero (ENZ) and a surface plasmon polariton (SPP) resonance.16,50 The ENZ mode occurs at frequencies where the real part of the refractive index of the supporting CMO film is near zero whereas the SP mode occurs occur at    p

  and depends on the

permittivity function of the composite layer adsorbed on the IZO film. The screened plasma frequencies of the bulk and surface IZO are indicated with dotted vertical lines in Figure 8a and are seen to lie very close to the aforementioned positive going absorption feature. However, possible SP and ENZ modes overlap with the features at 3200 cm-1 and 3653 cm-1, which are assigned to inverted  OH vibrations of hydrogen-bonded water in accordance with the inverted water bending modes previously described.

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Figure 8 : Potential dependent relative absorbance changes in the high frequency region of the mid-IR during the scan described in Figure 7. Measurements were performed in a) H2O and b) D2O based electrolytes. The reference spectrum was measured at 0.0 V. Dashed lines indicate the screened plasma frequencies of the bulk and surface IZO. To remove the molecular absorption contribution to the high frequency signal, the 4-MBA desorption experiment was repeated in D2O-based electrolyte. The high frequency region is shown in Figure 8b after displacing the spectra to have a common relative absorbance at 5000 cm-1. The use of deuterated solvent shifts the negative absorption feature arising from the solvent to frequencies below 3000 cm-1 and provides a relatively clean spectral window near the

25



predicted positions of the SP and ENZ modes. Both a positive absorbance band and a higher frequency negative band are seen to grow in intensity and shift toward the near IR with increasingly negative applied potential. We speculate that these bands are SP and ENZ modes and that their potential dependence arises because of potential dependent changes in the free carrier concentration67 and consequently, the permittivity function of the IZO (see equations 12). Potential dependent ENZ and/or SPP modes could be exploited for different sensing applications and surface enhancement mechanisms although further investigation is clearly needed to verify the origins of the optical responses observed herein.

Conclusions The present contribution has demonstrated that thin films of conductive metal oxide are highly successful platforms for creating ATR-SEIRAS interfaces. The optical properties of thin layers of In2O3/ZnO (IZO) were determined by combining IR reflection measurements with the Drude model. The extracted optical parameters (complex permittivity and refractive index) verify that IZO is advantageous, as compared to metals such as Au and Ag, because of its increased IR transparency, i.e. a longer wavelength plasma edge. The Bruggeman effective medium theory was used to model the ATR-SEIRAS response of an IZO layer decorated by a layer of gold nanoparticles. When compared to the calculated spectra of an all-gold layer, the IZO/nAu interface produced up to a tenfold improvement in the absorbance of a 1 nm thick organic film. The predicted enhancement arises without the invocation of near-field effects and is non-plasmonic in origin. Experimental verification of the IZO performance was completed through the sputtering of ca. 50 nm films of IZO on Si internal reflection elements and subsequent electrodeposition of Au particles. About a four fold improvement was observed in the absorption intensity of a monolayer of a pyridine derivative which agrees, qualitatively, with the

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Bruggeman EMT modelling. A major limitation of conventional ATR-SEIRAS is the fragility of the surface enhancing metal films and the poor adhesion between the metal and the principal reflecting surface of an internal reflection element. In a series of experiments using a thiol-based self-assembled monolayer, the IZO/nAu films were shown to be remarkably robust. These substrates have also been shown to be amenable for performing ATR-SEIRAS measurements that were hitherto impossible. Although the theoretical treatment of IZO did not include near-field effects it is interesting to consider how further efforts to engineer the IZO layers may lead to even greater enhancement. Conductive metal oxides and CMO-metal hybrids can provide localized and propagating surface plasmon-polariton modes in the high frequency part of the mid-infrared region and there is evidence that these modes may be potential dependent. Further work is necessary to see if these modes are truly operative in the present system. Irrespective, in light of this work, it should be highly rewarding to engineer the CMO layer to provide a synergistic plasmonic effect to the overall ATR-SEIRAS enhancement. It would also be very informative to perform finite element method analysis to determine the local field strength around metal nanoparticles supported on CMO films. Finally, the approach described herein seems very well suited for almost any metal that can be electrodeposited on the conductive metal oxide. This should provide tremendous opportunities to tailor the optical and electrochemical properties of ATR-SEIRAS supporting films.

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Supporting Information The supporting information includes details of modelling calculations, inconsistencies in the Drude model in the literature, SEM characterization of the IZO and Au/IZO films, cyclic voltammetry of the Au deposition onto IZO thin films and the 0.1 mM MOP in 0.1M NaF system, as well as details on the calculation of the enhancement factor, G. This material is available free of charge via the Internet at http://pubs.acs.org/

Acknowledgements This work was funded by a grant from the Natural Science and Engineering Research Council (NSERC) of Canada. TAM acknowledges funding from the NSERC PGS-D program and IRA acknowledges the NSERC USRA program for undergraduate summer research. Professor Robert Johanson from the Department of Electrical Engineering at the University of Saskatchewan is thanked for training and allowing us to use the RF magnetron sputtering unit. Professor Matthew Paige and David Sowah-Kuma are thanked for acquiring AFM images.

TOC Figure

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Hybrid Gold-Conductive Metal Oxide Films for Attenuated Total

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