Electrochemical Modification of Indium Tin Oxide Using Di(4

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Electrochemical Modification of Indium Tin Oxide Using Di(4-nitrophenyl) Iodonium Tetrafluoroborate Matthew R. Charlton, Kristin J. Suhr, Bradley J. Holliday, and Keith J. Stevenson Langmuir, Just Accepted Manuscript • DOI: 10.1021/la503522c • Publication Date (Web): 19 Dec 2014 Downloaded from http://pubs.acs.org on December 23, 2014

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Electrochemical Modification of Indium Tin Oxide Using Di(4-nitrophenyl) Iodonium Tetrafluoroborate Matthew R. Charlton,‡,†,ψ Kristin J. Suhr,‡, § Bradley J. Holliday*,§, and Keith J. Stevenson*,†,§ †

Texas Materials Institute, ψMaterials Science & Engineering Graduate Program, and

§

Department of Chemistry, The University of Texas at Austin, Austin, TX 78712, United States

Optoelectronic applications often rely on indium tin oxide (ITO) as a transparent electrode material. Improvements in the performance of such devices as photovoltaics and light-emitting diodes often requires robust, controllable modification of the ITO surface to enhance interfacial charge transfer properties. In this work, modifier films were deposited onto ITO by the electrochemical reduction of di(4-nitrophenyl) iodonium tetrafluoroborate (DNP), allowing for control over surface functionalization. The surface coverage could be tuned from sub-monolayer to multilayer coverage by either varying the DNP concentration or the number of cyclic voltammetry (CV) grafting scans. Modification of ITO with 0.8 mM DNP resulted in nearmonolayer surface coverage (4.95 x 1014 molecules/cm2). X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of 4-nitrophenyl (NO2Ph) moieties on the ITO surface through the detection of a NO2 nitrogen signal at 405.6 eV after grafting. Further XPS evidence suggests that the NO2Ph radicals do not bond to the surface indium or tin sites, consistent with modification occurring either through bonding to surface hydroxyl groups or through strong physisorption on ITO. CV in the presence of an electroactive probe and electrochemical

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impedance spectroscopy (EIS) were used to investigate the electronic effects that modification via DNP has on ITO. Even at sub-monolayer coverage, the insulating organic films can reduce the current response to ferrocene oxidation and reduction by more than 25% and increase the charge transfer resistance by a factor of 10.

INTRODUCTION Indium tin oxide (ITO) is a ubiquitous transparent conducting oxide material used for electrodes in many electronic applications including photovoltaics,1–4 light-emitting diodes,5–8 and sensors.9–11 In order to improve the performance of such devices, it has become important to develop general and robust strategies to adjust the chemical and electronic properties of these thin film materials through surface modification. A variety of surface functionalization strategies involving polymer films,5,6,9,11 organometallic moieties,12 phosphonic acid derivatization4,13,14 and self-assembled monolayers1,3,4,14 have been investigated. Unfortunately, these approaches often require lengthy, expensive processing steps at high temperatures or low pressures and could produce thick films with irregular surface coverages.15 The method of electrochemically controlled free radical grafting, which can deposit uniform, molecularly thin films at room temperature and pressure, has been widely explored for modifying metals,16–18 semiconductors,18–20 and carbon-based surfaces.18,21–24 While grafting via reduction of diazonium salts has been developed over the past few decades, electrochemical modification via iodonium salts is a relatively new field. The first report of modification by aryl and alkynyl iodonium salts was on glassy carbon by Vase and coworkers in 2005.25 In the years following, electroreduction of various aryl iodonium salts has been used to functionalize carbon


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surfaces.26–30 In 2008, Kanoufi et al. expanded the field by successfully patterning the surface of gold through this method.31 Free radical grafting onto ITO has been investigated, to a lesser extent, using aryl diazonium salts.15 However, due to the higher reactivity of diazonium salts over their iodonium counterparts, formation of undesired nitrogen-based linkages to the surface and azo-linked multilayers can occur from direct diazonium cation attachment.15,32 Breton et al. explained that the major drawback of diazonium salts is a general lack of control over film thickness and organization.33 They mentioned many ways to avoid multilayer formation and obtain controlled film growth, however they involve adding sterically hindered protecting groups on the diazonium or using radical scavengers in the grafting solution.33 Furthermore, spontaneous grafting can occur on metal and semiconductor surfaces during aryl diazonium functionalization, leading to loss of control over the reaction rate and extent of coverage.34 Aryl iodonium salts have inherently more negative reduction potentials than related diazonium salts, providing better control over the deposition rate.25,28 Spontaneous grafting is a function of the chemical potential of the electrode. If the open circuit potential is sufficiently low, as can be the case for carbon,35–37 metals,36–39 and certain semiconductors and oxides,34,40 then diazonium radicals can readily be formed and attach to the surface. However, to date, spontaneous reduction and grafting of an iodonium salt has not yet been observed, nor has it been observed with diazonium salts on ITO. Electrochemical reduction and subsequent dissociation of aryl iodonium salts results in the grafting of an electrically insulating layer of the aryl moiety onto the substrate surface. As shown previously, free radical grafting can form C–C covalent bonds between aryl moieties and graphene or glassy carbon electrodes.22–24,28,41 Currently, the type and surface site of aryl radical bonding on ITO is unknown. Stevenson et al. suggested that the aryl films may be regarded as


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strongly physisorbed to the ITO surface, since X-ray photoelectron spectroscopy (XPS) did not display a significant shift in the In 3d or Sn 3d signals and the complex O 1s spectra transition did not definitively exhibit evidence of covalent bonding of the grafted moieties to the surface.15 In this study, we report the grafting of 4-nitrophenyl (NO2Ph) radicals, formed via the reduction of di(4-nitrophenyl) iodonium tetrafluoroborate (DNP), onto ITO using cyclic voltammetry (CV). To our knowledge, this work is the first time iodonium salts have been used to functionalize a metal oxide surface. The iodonium salt DNP was chosen for this study to enhance the electrochemical and spectroscopic analysis. The nitro groups are beneficial to these primary grafting studies because they are electrochemically active and subsequent reduction after grafting allows for calculation of the surface coverages. Furthermore, the nitro groups are easily resolvable during XPS analysis, allowing for characterization of the chemical properties of the modified electrodes and discussion of possible attachment mechanisms of the NO2Ph radicals onto ITO. Films deposited via reduction of DNP electrochemically insulate the electrode and reduce the current response, prompting our study to elucidate percent passivation using an electroactive redox couple, ferrocene/ferrocenium. Additionally, we report the charge transfer resistance before and after grafting, determined by electrochemical impedance spectroscopy (EIS).

EXPERIMENTAL HPLC-grade acetonitrile (ACN) was dried and stored over activated molecular sieves. All chemicals were used as received from commercial suppliers. DNP [(NO2Ph)2I+BF4–] was prepared using a one-pot synthesis method described by Stevenson et al.30 Unpolished float glass slides (7 x 50 x 0.7 mm) coated on one side with indium tin oxide (ITO) with a resistance of 70 –


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100 Ω/sq (Delta Technologies, Limited) were sonicated sequentially in water, ethanol, acetone, and dichloromethane (5 min. each) before experimental use. All electrochemical experiments were completed under nitrogen in a dry-box using Eco Chemie B. V. software and an Autolab PGSTAT30 Potentiostat. CV and chronoamperometry experiments were completed using GPES software, while FRA software was used for impedance experiments. The electrolyte used was 0.1 M tetrabutylammonium hexafluorophosphate [(n-Bu)4N][PF6] (TBAPF6) in dry ACN. The electrolyte was purified through three recrystallizations from hot ethanol before drying under active vacuum for three days at 100 ºC. All electrochemical experiments were carried out in a 20 mL electrochemical vial using an ITO working electrode, a Ag/AgNO3 (0.01 M AgNO3 and 0.1 M TBAPF6 in dry ACN) non-aqueous reference electrode, and a Pt wire coil counter electrode. All potentials were reported relative to the ferrocene/ferrocenium couple (Fc/Fc+), unless otherwise noted, which was used as an external standard to calibrate the reference electrode. Ferrocene was purified by sublimation at 95 ºC. For all electrochemical experiments, the ITO slide was positioned vertically, with the bottom edge of the slide touching the bottom of the electrochemical vial containing 5 mL of solution. With this setup, a consistent area of the ITO electrode (0.84 cm2) was submerged in solution. Electrochemical modification of the ITO electrodes was carried out in an anhydrous electrolyte solution containing DNP (0.1 – 4.0 mM). In general, grafting was performed with CV by cycling between 0 and -1.3 V vs. Ag/Ag+ for two scans at 50 mV/s. The electrochemical cell was refreshed with DNP/electrolyte solution after grafting one to two ITO samples. After modification, the electrodes were rinsed three times with dry ACN. To observe increased surface coverage and passivation, the grafting of select concentrations was repeated using 10 CV scans


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(0 to -1.2 V vs. Ag/Ag+, 50 mV/s). NO2 reduction voltammograms of the grafted, rinsed ITO electrodes were completed in fresh electrolyte solution by cycling between 0 and -2.0 V vs. Ag/Ag+ for two scans at a scan rate of 100 mV/s. Molecular surface coverages were calculated from the integrated NO2 reduction and oxidation peak currents after polynomial background subtraction. For the reduction currents, only the area associated with the NO2 peak at approximately -1.65 V vs. Fc/Fc+ was integrated. The contribution from the small pre-peak at approximately -1.30 V was not included. To determine the electrochemical passivation, CV of the ITO electrodes before and after grafting was carried out in a 0.5 mM ferrocene/electrolyte solution by cycling between 1.2 and -0.8 V vs. Ag/Ag+ for two scans at 100 mV/s. Percent passivation was calculated by dividing the difference between the ferrocene oxidation peak current (from the 2nd scans) of the unmodified and grafted ITO by that of the unmodified ITO. EIS measurements were performed on unmodified and modified ITO electrodes in 0.5 and 5.0 mM ferrocene/electrolyte solutions at 0 V vs. Fc/Fc+. A 10 mV sine-wave amplitude was used to maintain a linear response, while the frequency was swept from 100 kHz to 50 mHz, with 7 data points taken per decade. All XPS measurements were taken using a Kratos Axis Ultra DLD XPS system with an Al K-α source. Survey scans of each sample were collected from 0-1200 eV binding energy with 1.0 eV resolution, followed by high-resolution scans of the carbon 1s, nitrogen 1s, oxygen 1s, indium 3d, and tin 3d regions at 0.1 eV resolution and 1000 ms dwell time. A charge neutralizer was used and all spectra were charge corrected relative to the aromatic C 1s component at 284.7 eV binding energy. Analysis of XPS spectra was performed with CasaXPS software (version 2.3.15, Casa Software Ltd.). Spectra from high-resolution scans were used to estimate atomic compositions of the modified surfaces. Fits of deconvolved components were made using the


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sum of multiple Voigt functions composed of 30% Lorentzian and 70% Gaussian profiles optimized using the Simplex method.

RESULTS AND DISCUSSION A proposed reaction pathway, adapted from studies by Chan et al. of iodonium salts on graphene is shown in Scheme 1.28 The ITO surface components are based on ITO characterization work by Donley et al.42 Modification of ITO was achieved through a oneelectron reduction and dissociation of DNP dissolved in the electrolyte. The DNP molecule dissociates into 1-iodo-4-nitrobenzene and a 4-nitrophenyl (NO2Ph) radical, which may attach to the ITO surface through the proposed hydrogen abstraction mechanism, shown in Scheme 2. Scheme 1. Proposed grafting mechanism of NO2Ph onto ITO.

Step 1: Electrochemical one-electron reduction of DNP. Step 2: Dissociation of DNP into 1-iodo-4-nitrobenzene and a 4-nitrophenyl radical. Step 3: The radical covalently bonds to the surface hydroxide sites (red) (vide infra). The bulk ITO (blue) and oxygen vacancy sites (white) are also shown.


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Scheme 2. Proposed hydrogen abstraction mechanism. ITO

ITO

ITO O N 2

H

O2N H O2N

O

3a

O2N

O

3b

O

Step 3a: Surface hydroxide hydrogen abstraction by a 4-nitrophenyl radical. Step 3b: Bond formation between a second 4-nitrophenyl radical and a surface oxygen radical.

A representative cyclic voltammogram (showing 10 grafting scans) of ITO in 0.6 mM DNP/electrolyte is shown in Figure 1. The first CV scan exhibits an irreversible reduction peak at -0.86 V vs. Fc/Fc+, corresponding to the formation of the 4-nitrophenyl radical from DNP. In the subsequent CV scans, the current is drastically diminished and a large overpotential appears, shifting the DNP reduction process more negative and outside of the electrochemical window of stability. This response is consistent with typical grafting CVs of iodonium salts throughout the literature.26,27 These results indicate passivation of the electrode by an electrically insulating layer containing NO2Ph moieties. DNP grafting studies were continued using two CV scans with DNP concentrations ranging from 0.1 to 4.0 mM. As the DNP concentration was increased, the initial reduction peak grew larger and the overpotential of the second scan increased, suggesting increased surface coverages and slower electron transfer rates at higher concentrations (Figure S1).


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Figure 1. Ten CV grafting scans of ITO in a 0.6 mM DNP/electrolyte solution. Chan et al. have previously shown that the integrated current (charge) of the grafting reduction peak can be used to calculate the surface coverage of the electrochemically activated species.28 However, this method may overestimate the surface coverage because it assumes that 100% of the reduced iodonium salts lead to bound NO2Ph moieties. In the case of DNP, the nitro groups are electrochemically active. CV of the modified samples and integration of the NO2 reduction peak provides a more precise calculation of the surface density of only the grafted NO2Ph moieties. As expected, the charge measured for the reversible NO2 reduction peak increases with the DNP concentration (Figure 2A), suggesting that under the same electrochemical conditions, a higher concentration of DNP yields a larger number of grafted NO2Ph moieties. The current efficiency, calculated by comparing the integrated currents of the grafting and NO2 scan reduction peaks, is less than 50%, which supports the proposed mechanism in Scheme 2. A graph of surface coverage, based on the NO2 reduction peak, as a function of grafting DNP concentration is shown in Figure 2B. The integrated area under the NO2 reductive peak was used to calculate the surface coverage. The surface coverage after two CV scans displays a linear dependence with concentration from 0.2 up to 0.8 mM DNP. The NO2 reduction peak was not observable at a concentration of 0.1 mM DNP. Grafting at these lower concentrations in the


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linear portion of the curve results in sub-monolayer surface coverages. The end of the linear region (0.8 mM) suggests near-monolayer coverage of the ITO. This same trend of a linear, submonolayer region up to 0.8 mM has been shown before by our group while grafting DNP on glassy carbon.30 However, note that stating monolayer coverage at this point does not imply that the surface coverage is at or near the theoretical close-packed limit for a 4-nitrophenyl monolayer, as calculated by McCreery et al. (vide infra).23 At higher concentrations (1.0 – 3.0 mM DNP), the slope, or change in surface coverage, drastically decreases from that of the linear region, which is consistent with hindered diffusion through the insulating film once monolayer coverage is achieved. The increase in surface coverage at 4.0 mM DNP will be discussed below. Here it must be noted that this CV technique to calculate the surface coverage is an accurate method for determining sub-monolayer, monolayer, and multilayer transitions. However, it cannot be used to accurately observe multilayer surface coverages, as this technique is limited by diffusion through the insulating film. When the same graph in Figure 2B is plotted using the integration of the NO2 oxidative peak, the relationship is still linear up to 0.8 mM DNP (Figure S2). In this linear region, reversible CV responses during the NO2 scan are consistent with those demonstrated for near-monolayer coverages of 4-nitrophenyl films on glassy carbon.23 Again, the linear region was followed by a substantial decrease in the slope, but Figure S2 displays a decreased calculated “surface coverage” at 3.0 and 4.0 mM DNP. At these concentrations, the electron transfer of the nitro groups becomes quasi-reversible and suggests that not all of the NO2Ph moieties are in direct contact with the substrate, as they can no longer all be reoxidized. This electrochemical behavior is consistent with that reported by Stevenson et al. on ITO and Bélanger et al. on glassy carbon for multilayer coverages formed from 4-nitrophenyl diazonium salts.15,22 At these high


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concentrations, reduction of the DNP iodonium salt can result in carbon-carbon bond formation to produce multilayers at the ortho positions of previously grafted phenyl modifiers.43 The potential multilayer coverages were not further explored in this study because multilayer formation was undesired, as it would hinder our electrochemical and spectroscopic analysis of the grafted NO2 groups. The subsequent studies herein are focused on sub-monolayer and monolayer surface coverages.

Figure 2. A) NO2 reduction CVs (2nd scans) of ITO previously grafted (2 CV scans) with a range of DNP concentrations (0.1 – 4.0 mM) in an electrolyte solution. B) Graph of surface coverage, based on the NO2 reduction peak, as a function of grafting DNP concentration. The error bars represent the average standard deviation over multiple measurements at various concentrations.


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The calculated surface coverages from the NO2 reduction peak, based on the geometric electrode area, ranged from 0.77 to 6.98 x 1014 molecules/cm2 (0.2 and 4.0 mM DNP, respectively) using two CV grafting scans. Grafting with 0.8 mM DNP yields a surface coverage of 4.95 x 1014 molecules/cm2, which represents 69% of an ideal close-packed monolayer of 4nitrophenyl moieties (7.23 x 1014 molecules/cm2), as determined by McCreery et al.23 It is interesting to note that even the 4.0 mM sample, which shows multilayer characteristics, has a slightly lower surface coverage than an ideal close-packed monolayer of NO2Ph. This anomaly is similar to results from Downard et al., who observed multilayer formation, on pyrolyzed photoresist films using a 4-nitrophenyl diazonium salt, far before the original monolayer approached the theoretical surface coverage of a close-packed monolayer.44 Using a combination of CV surface coverage data and atomic force microscopy scratching film thickness data they determined that even though their film was 4 layers thick, the surface coverage at the thickness of an NO2Ph monolayer was only 21% of an ideal close-packed monolayer (1.51 x 1014 molecules/cm2). McCreery et al. also reported NO2Ph monolayers deposited on glassy carbon electrodes with surface coverages of 3.91 x 1014 molecules/cm2, which is 54% of the theoretical close-packed monolayer.23 Our studies, along with these literature examples, agree with Downard et al. that film formation becomes self-limiting at surface coverages significantly less than the theoretical close-packed monolayer value.24,44 In fact, there are many sources that show that this theoretical NO2Ph close-packed monolayer is not easily approachable.18,19,24,44 Note that, in general, comparisons of surface coverages between different studies are difficult due to different grafting methods and conditions, substrate properties, and surface coverage analysis techniques.


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The diminished current on the second CV grafting scan in Figure 1 prompted further investigation of DNP grafted ITO with an electroactive redox couple, ferrocene/ferrocenium (Fc/Fc+). CVs of unmodified, 0.2 and 0.6 mM DNP grafted ITO (2 and 10 grafting scans) in a 0.5 mM ferrocene/electrolyte solution are shown in Figure 3. Passivation of the ITO electrodes by the grafted insulating films diminishes the electrochemical response of ferrocene, resulting in smaller peak currents and increased peak separation. To clarify, percent passivation refers to the percentage that the oxidative peak current of ferrocene is diminished after grafting. The percent of surface passivation of the electroactive ITO increases with higher DNP grafting concentrations (2 CV scans: 0.2 mM DNP = 8.7%, 0.6 mM DNP = 28.6%) and with a larger number of CV grafting scans (10 CV scans: 0.2 mM DNP = 61.2%, 0.6 mM DNP = 73.3%). After modification, ferrocene oxidation occurs through electron transfer at non-modified areas (including “pin-holes” at near-monolayer coverage) on the ITO surface and by electron tunneling through the insulating layer. As the deposited film density increases, a higher fraction of electron transfer presumably occurs through tunneling. Since the electron transfer rate at the exposed ITO sites is expected to be more facile than through the insulating film, the increased coverage results in diminished overall current response as well as increased peak splitting, suggesting that the electron transfer rate is reduced significantly. Peak broadening at higher surface coverages also supports two concurrent oxidation mechanisms. It should be noted that because the electrode blocking effect causes quasi-reversible behavior (i.e., larger peak splitting) as the surface coverage increases, the estimation of the surface coverage via integration of peak currents is less precise.


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Figure 3. CVs (2nd scans) of unmodified and grafted ITO in a 0.5 mM ferrocene/electrolyte solution. XPS analyses were completed to further confirm that NO2Ph moieties have been grafted to the ITO surface and to ascertain to which ITO surface sites the NO2Ph radical preferentially attaches. The N 1s spectra of 0.2 and 0.6 mM DNP grafted ITO samples are shown in Figure 4A. While nitrogen is not observed on the pristine ITO surface, the N 1s spectra of the modified electrodes indicate that nitrogen-containing species have been deposited onto the ITO surface during grafting. Higher DNP concentration (0.6 mM) resulted in larger N 1s peaks, corresponding to higher coverage of modifiers on the surface. The N:In atomic ratios, normalized by ITO In 3d intensity (described in the SI), for the 0.2 and 0.6 mM DNP samples are 0.20:1 and 0.36:1, respectively.


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Figure 4. A) N 1s XPS spectra of the electrochemically modified electrodes. B) O 1s spectra of the modified electrodes compared to unmodified ITO. C) Individual components of the O 1s spectrum of unmodified ITO. D) Individual components of the O 1s spectrum of 0.6 mM DNP modified ITO (same legend as in C). The modified samples exhibit multiple nitrogen peaks, which have also been seen in similar studies regarding diazonium and iodonium-based NO2Ph modification.15,20,22,25,41,45 The peak at 405.6 eV can be attributed to the NO2 groups,15,17 while the second main peak at 399.3 eV is consistent with reduced NO2 species, likely primary amine moieties.20,46 Studies grafting with diazonium salts also attributed a N 1s peak around 400.0 eV to the formation of azo-linked multilayers, however this bond structure is not feasible for iodonium salts. A third component comprising less than 10% of the total signal can also be identified at 402.0 eV, which matches the reduced species hydroxylamine (–NHOH).41 Hydroxylamines are intermediate species to the formation of amines, supporting the formation of NH2 upon reduction of the grafted NO2Ph moieties.41 While the origin of the NO2 reduction has not yet been definitively identified, potential sources have been discussed in detail elsewhere.15,22,41 In this case, the most likely chemical sources are reactions with protons released from the hydroxyl-terminated ITO surface during grafting or from exposure to contaminant water in the ACN solvent during the NO2 reduction scans. Garrido et al. have also shown that, given enough time, up to 85% reduction of nitro-functionalized molecular layers can occur during XPS analysis.47 The samples in our study were exposed for 30 minutes in total, so reduction of the films by 30-35% could be considered reasonable, based on previous literature values.45 Since the NO2 to NH2 peak ratios are closer to 50%, at least some of the grafted NO2Ph must be reduced prior to XPS analysis. Further investigation is currently underway to elucidate the specific reduction mechanism.


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Grafting also had a significant effect on the O 1s XPS signal (Figure 4B). The bulk ITO oxygen component in the modified sample spectra centered at 529.6 eV42 decreased relative to that of unmodified ITO, which is consistent with the reduction in overall signal seen in the In 3d and Sn 3d spectra after grafting (Figure S3). Deconvoluted spectra of the oxygen signals of pristine ITO and 0.6 mM DNP grafted ITO samples are shown in Figures 4C and 4D, respectively. In the pristine ITO, components relating to bulk ITO oxygen sites, vacancy adjacent oxygen sites, and surface hydroxide species are all identified. These assignments match the positions determined by Donley et al.42 The second major peak (532.7 eV, orange in Figure 4D), which scales with DNP grafting concentration, is attributed to NO2 groups on the grafted molecules.20,47 In the O 1s spectra, the NO2 component of the modified samples can be identified and fit by maintaining a ratio of 15% between the vacancy adjacent oxygen (530.8 eV) and normal bulk oxygen (529.6 eV) found in the pristine ITO, while maintaining consistent positions and peak widths for all major components. In doing so, it was found that the nitro component comprises 13.2% and 40.8% of the O 1s signal from the 0.2 and 0.6 mM DNP grafted ITO samples, respectively. The presence of the In-O-Ph bonding environment in the O 1s spectrum would confirm covalent bonding of the phenyl radicals to the ITO surface. However, M-O-Ph bonds have been reported around 533.0 eV, too close to the NO2 peak to resolve.20,32 As the surface coverage and O 1s intensity of NO2 (possibly combined with In-O-Ph) increases, the amount of surface hydroxides, shown in green in Figure 4C and 4D, decreases from 58.4% of the total oxygen of the pristine ITO to 33.5% of that of the modified electrode. Relative to the bulk oxygen signal in ITO, the hydroxide concentration drops by 68% after deposition with 0.6 mM DNP. This supports the hypothesis that grafting occurs on the surface


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oxygen sites, replacing the hydroxide terminations, as illustrated in Scheme 2 above. The maximum number of oxygen surface sites on planar ITO, calculated for the bixbyite structure with a lattice parameter of 10.117 Å,48 is on the order of 1015 oxygen sites/cm2, an order of magnitude higher than the close-packed 4-nitrophenyl density calculated by McCreery.23 This is likely the reason for the high concentration of surface hydroxyl groups still present after grafting at 0.6 mM DNP. No shifting in binding energy of the In 3d or Sn 3d spectra was observed upon grafting (Figure S3), suggesting that grafting does not occur on the metal sites. This means that the NO2Ph moieties must either bind to the surface hydroxyl groups or NO2-containing molecules must be physisorbed onto the ITO surface. To check for physisorption, a modified film was rinsed with copious amounts of ACN and ultrasonicated in ACN for 10 minutes. Ultrasonication is a generally accepted method for the removal of physisorbed species from grafted surfaces.21,32,38,49–51 Comparison of the subsequent NO2 reduction CV scans between the ultrasonicated film and a replicate film soaked in ACN for 10 minutes only showed about a 10% decrease in surface coverage after ultrasonication, equivalent to just over one standard deviation difference, calculated from data represented in Figure 2B. This slight decrease can be attributed to physisorbed molecules on the ITO surface, while the remaining 90% of the calculated surface coverage corresponds to either covalently bound NO2Ph moieties or strongly physisorbed NO2-containing molecules. Furthermore, to check for spontaneous grafting, an NO2 reduction scan was completed on a sample of ITO that was allowed to soak in a 0.6 mM DNP/electrolyte solution for 10 minutes. The CV scan showed no observable NO2 reduction peak, indicating that spontaneous grafting did not occur. EIS was used to investigate the electron transfer properties of the ITO after grafting. The impedance response of unmodified ITO was compared to those of 0.2 and 0.6 mM DNP grafted


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ITO using both 0.5 and 5.0 mM ferrocene as the electroactive probe, shown in the Nyquist plots in Figures 5A and 5B, respectively. Both the modified and unmodified electrodes exhibited relatively similar behavior, resulting in a semicircular trace in the complex impedance plane at higher frequencies, eventually leading to a 45° Warburg line at sufficiently low frequencies, due to ion diffusion.

Figure 5. A) and B) Nyquist plots comparing the electrochemical impedance in 0.5 mM and 5.0 mM ferrocene/electrolyte, respectively. C) Bode plot of ITO modified with 0.6 mM DNP and tested in 0.5 mM ferrocene/electrolyte illustrating the methods used to calculate RS, RCT, and Cdl. These characteristic features indicate that, in this frequency range, a Randles equivalent circuit can be used to model the system properties.22 Figure 5C shows a representative Bode plot, taken


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of a 0.6 mM DNP modified electrode in 0.5 mM ferrocene, illustrating how the solution resistance (RS), charge transfer resistance (RCT), and interfacial or double layer capacitance (Cdl) were calculated for each sample. The methods used to determine these resistance and capacitance values are described in the SI.

Table 1. Electrochemical Impedance: Resistance and Capacitance Values RS RCT Cdl (Ω) (Ω) (µF/cm2) 0.5 mM Ferrocene/electrolyte ITO 465 710 3.97 0.2 mM DNP 471 1630 3.43 0.6 mM DNP 487 8910 2.48 5.0 mM Ferrocene/electrolyte ITO 452 58 3.12 0.2 mM DNP 459 154 2.33 0.6 mM DNP 477 450 2.23 Sample

The results of EIS measurements are summarized in Table 1. RS were consistent at 469 ± 12 Ω. Slight variation occurred from sample to sample as a result of small deviations in electrode placement during testing. Modification of the ITO electrodes significantly changed the RCT and Cdl. The concentration of ferrocene also influenced the measurement of RCT. Increasing the concentration ten fold, using similarly prepared electrodes, yielded a reduction in RCT by a factor of 10-20. The exact cause of this phenomenon is not completely understood. It is possible that a higher local concentration of electroactive species along the electrode surface combined with inhomogeneities in both the ITO surface and the grafted film magnify the effect of local areas of high conductivity. It has been shown that near surface Sn dopants in ITO can act as centers for high conductivity and fast electron transfer.52 In both sets of samples, RCT increased with DNP concentration and surface coverage, consistent with the reduced current response as observed in


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the ferrocene CV results. The presence of the insulating films at the surface may necessitate electron tunneling for charge transfer to occur, leading to the increased RCT.

Conclusion Insulating 4-nitrophenyl-containing films were deposited onto the surface of ITO via electrochemical reduction of di(4-nitrophenyl) iodonium tetrafluoroborate, and the resulting modified electrodes were characterized by CV, XPS and EIS. XPS analysis has negated the possibility of covalent attachment to indium or tin sites and resistance to ultrasonication has shown that the NO2Ph radicals either bind to the surface hydroxyl groups or NO2-containing molecules must be strongly physisorbed onto the ITO surface. Sub-monolayer to monolayer surface coverages are linearly dependent on solution concentration up to 0.8 mM DNP, without the observance of spontaneous grafting, resulting in improved control over surface functionalization and modified electrode properties. The grafted moieties form an electronically insulating film, resulting in a decreased current response to ferrocene and increased charge transfer resistance.

ASSOCIATED CONTENT Supporting Information Available: CV grafting scans showing dependence on DNP concentration, graph of surface coverage vs. concentration based upon the NO2 oxidation peak, XPS In 3d and Sn 3d spectra, description of EIS calculations, description of the normalization method used for the N:In atomic ratios. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] Author Contributions ‡

Matthew R. Charlton and Kristin J. Suhr contributed equally towards performing experiments,

analyzing data, and writing. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences on “Understanding Charge Separation and Transfer at Interfaces in Energy Materials” (EFRC:CST, Award Number DE-SC0001091). KJS and BJH also acknowledge the Welch Foundation (Grants F-1529 and F1631, respectively).

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