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Indium Tin Oxide Film Characteristics for Cathodic Stripping Voltammetry Mary Ensch, Bettina Wehring, Greg Landis, Elias Garratt, Michael Becker, Thomas Schuelke, and Cory A. Rusinek ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22157 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 1, 2019

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ACS Applied Materials & Interfaces

Indium Tin Oxide Film Characteristics for Cathodic Stripping Voltammetry Mary Enscha,b, Bettina Wehringb, Greg Landisa,b, Elias Garratta, Michael F. Beckerb, Thomas Schuelkeb,c, Cory A. Rusinekb* Michigan State University, Department of Chemical Engineering and Materials Science, East Lansing, MI, 488241226, USA b Fraunhofer USA, Inc. Center for Coatings and Diamond Technologies, East Lansing, MI, 48824-1226, USA c Michigan State University, Department Electrical and Computer Engineering, East Lansing, MI, 48824-1226, USA a

KEYWORDS: Cathodic stripping voltammetry, indium tin oxide, manganese detection, x-ray diffraction, electrochemical sensing, physical vapor deposition ABSTRACT: The combination of conductivity, optical transparency, and wide anodic potential window has driven significant interest in indium tin oxide (ITO) as an electrode material for electrochemical measurements. More recently, ITO has been applied to the detection of trace metals using cathodic stripping voltammetry (CSV), specifically manganese (Mn). However, the optimization of ITO fabrication for a voltammetric method such as CSV has yet to be reported, nor have the microstructural properties of ITO been investigated for CSV. Furthermore, CSV does not require optical transparency, thereby allowing non-transparent substrates to be used for deposition. This enables microfabrication procedures to be expanded and simplified compared to glass or quartz. Combining this with the profound importance of sensitive, selective detection of toxic metal ions in environmentally- and biologically-relevant samples, makes ITO especially attractive. In this work, we report a thorough investigation of ITO deposition and processing on silicon (Si) substrates for CSV analysis using Mn as the model analyte. Several ITO process parameters were examined such as heated deposition and post-process annealing. Each ITO film was characterized using a variety of surface, bulk (x-ray diffraction), and electrochemical measurements. While each ITO film type showed electrochemical activity, the heated and annealed (HA) ITO fabrication process yielded superior results for Mn CSV where a limit of detection (LOD) of 0.1 ppb (1.8 nM) was obtained. This work exemplifies new applications of ITO as an electrode material while providing a baseline for trace detection of toxic metals and other contaminants amenable to detection by CSV.

INTRODUCTION Indium tin oxide (ITO) is commonly used as a conductive and optically transparent thin film.1-2 It is found in applications such as liquid crystal displays (LCDs), transparent electrodes for solar cells and photodetectors, surface heaters for automobile windows, touch screens, and spectroelectrochemical applications.1-10 ITO deposition can be performed using chemical vapor deposition (CVD), ion-beam evaporation, spray pyrolysis, and direct current (DC) or radio frequency (RF) sputtering. 1-3, 11-14 Sputtering is most commonly found throughout literature.1, 3, 5-7, 15-17 Depending on the process parameters, ITO films can be crystalline or amorphous with a wide range of optical and electrical properties. The variation in film properties comes from changes in the ordered structure, i.e. the generation of oxygen vacancies.7 Some of these tunable process parameters

include; process gas pressures, substrate temperatures, post deposition annealing temperatures, deposition time, different annealing gas environments such as in argon or air, and film thickness.2, 4-5, 7, 11, 18-19 An amorphous-to-crystalline transition exists for ITO between 100 ºC and 150 ºC.19 This shift is what causes films to have differences in their conducting carrier concentration and their presence of oxygen.1, 7, 19-20 In terms of substrate temperature, the resistivity of the film drops drastically with temperatures above 100 ºC, leading to an increase in carrier concentration and mobility.6, 19 Typically, films sputtered to a thickness between 100-200 nm at room temperature are amorphous, but thicknesses exceeding this range show crystalline growth.4 Changing one or all of these process parameters can create a unique ITO film structure with varying characteristics.2, 4, 7, 11, 19, 21 In addition to deposition process parameters, post deposition processing, such as annealing, can also have an effect on ITO’s

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properties.4 Annealing of DC sputtered films has been shown to increase their grain size, decrease their strain, and decrease their electrical resistivity.4-5, 11, 20 It has been shown that the crystallization temperature when annealing is higher for nonheated substrates than heated substrates.4, 7 Annealing will also change the optical properties by leading to a steeper absorption cutoff and an increased bandgap.4 Annealing studies have been done to determine the effects of annealing in different atmospheres such as air and argon, at different temperatures, and in vacuums.1, 7, 11, 20, 22 For electrochemical applications, ITO has commonly been deployed as an optically transparent electrode (OTE) for spectroelectrochemical measurements.9, 23-26 ITO possesses a large positive potential window that allows it to be used for measurements beyond +1.5 V while retaining high optical transparency in the 390 - 700nm range.23-24, 27 Its lack of reaction in the positive potential range is due to the film already being in its oxidized form.28 It has been used for the spectroelectrochemical detection of several analytes including poly-(Azure A), aqueous iron by in-situ complexation with 2,2’-Bipyridine, total alkaline phosphate in human blood, and ferricyanide.9, 23, 28-29 ITO has also been used for stripping voltammetric measurements of several metals including cadmium (Cd), copper (Cu), lead (Pb), manganese (Mn), mercury (Hg), and cerium (Ce).24, 27, 30-31 In the case of Cd and Cu, attenuated total internal reflectance stripping voltammetry (ATR-SV) was used; the optical signal was monitored as the metals of interest were deposited (electroplated) onto the ITO surface.30 ITO working electrodes and the technique of cathodic stripping voltammetry have also been used for the detection of Ce.24 More recently a paper was published using bare ITO for the simultaneous detection of Pb, Cu, and Hg.31 For Mn analysis, ITO and CSV have been investigated.27 This was completed using both bare ITO and ITO coated with polystyrene-block-poly(ethylene-ran-butylene)-blockpolystyrene-sulfonate (SSEBS).27 Limits of detections (LODs) of 5 nM (0.27 ppb) and 1 nM (0.05 ppb) were reported on the bare and coated ITO, respectively.27 In a follow-up paper, the ability to detect Mn in digested bovine whole blood samples using ITO with CSV was reported; comparable results to that obtained with inductively coupled plasma mass spectrometry (ICP-MS) and graphite furnace atomic absorption spectroscopy (GF-AAS).32 Mn detection is important to insure its concentration in drinking water is below the maximum contaminant level (MCL) of 50 ppb and to monitor concentrations in human blood (4-15 ppb), urine (1-8 ppb), and serum levels (0.4-0.85ppb) due to Mn’s toxic health effects.33-34 Other electrode materials that have been used to study Mn with CSV include palladium (Pd) on copper (Cu), boron-doped diamond (BDD), and various carbon-based materials such as graphite, carbon paste, and carbon nanotubes.35-41 ITO’s advantageous anodic potential window makes it especially beneficial for CSV compared to other materials. While Hg electrodes are not applicable to CSV, it is worth noting that ITO electrodes eliminate toxicity concerns stemming from the electrode material itself. Cu-based Pd sensors studies obtained a relatively high limit of detection (LOD) at roughly 19 ppb which is below the MCL of Mn in drinking water, but is much higher than the levels of Mn typically in blood, urine, and serum.33-34, 36 The BDD study required the use of power ultrasound to be applied during the preconditioning step and was only able to obtain a LOD of 5.50 ppb for Mn in sea water.37

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Although this LOD is appropriate for blood level detection of Mn, it is not sensitive enough for serum analysis. Additionally, BDD is expensive to manufacture and typically has a resistivity value of 2 to 3 orders of magnitude higher than common ITO films, which is disadvantageous in sensor applications.42 A bare carbon paste electrode obtained the same LOD for Mn as the BDD sensor, low enough for blood level measurements. 43 Edge plane pyrolytic graphite was the most promising carbon material used thus far for Mn detection using CSV obtaining a LOD of 0.80 ppb.38 Although these results are promising, the previously mentioned ITO films obtain lower LODs compared to these materials. In Mn CSV, Mn2+ is accumulated onto the electrode surface as manganese dioxide (MnO2) during the deposition step before being reduced back to Mn2+ during the stripping step. Due to this nature, ITO’s metal-oxide surface can be beneficial compared to carbon-based electrodes. It is important to note that metal detection sensor studies using ITO with CSV do not require optical transparency like those being used for spectroelectrochemistry. There are few accounts of ITO deposition on non-optically transparent substrates such as silicon (Si), none of which are applied to CSV.24-25 Additionally, electrode fabrication processes on glass are much more difficult and hazardous than those for Si. They involve wet etching in hydrofluoric acid (HF) due to dry etching processes being slow and hard to control.44 For Si substrates, dry etching processes are preferred, allowing for easier, more precise, and faster etch rates without compromising on fabricator safety. In this work, we report on the optimization of ITO working electrode fabrication processes on non-optically transparent Si substrates for applications in CSV. There are few accounts of ITO deposition on non-optically transparent substrates, but to our knowledge, optimization of ITO fabrication process parameters for an electroanalytical technique such as CSV is yet to be reported.45-46 Several film parameters were investigated, such as heated deposition and post-process annealing. Each scenario was thoroughly characterized and evaluated using topographical, optical, electrical, and electrochemical processes. Lastly, each ITO sample was assessed for its ability to detect Mn by CSV.

EXPERIMENTAL SECTION Chemicals and Materials. Potassium ferrocyanide (K4Fe(CN)6) was purchased from J.T. Baker and a concentrated solution was made using DI water. Manganese (Mn) was purchased from Arcos Organics. A 0.2 M Acetate buffer was prepared from 0.2 M sodium acetate (Sigma Aldrich) in DI water and 0.2 M acetic acid (Fischer Scientific) in appropriate ratios for the desired pH of 5.0. Ferrocene was purchased from Aldrich Chemical Company Inc. and was made into a 1 mM solution with 0.1 M tetrabutylammonium hexafluorophosphate (Aldrich chemistry) in Acetonitrile (Sigma Aldrich). Instrumentation. Atomic force microscopy (AFM) measurements were done using an AFM5100 from Hitachi on tapping mode. An area of 4 um2 was evaluated with a scan frequency of 0.84 Hz. The software Gwyddion was used to analyze the AFM roughness data. All of the roughness values were obtained by drawing three evaluation lines across the top, middle, and bottom of each sample’s topological scan. The three values were then averaged. A cutoff length of 390 nm, roughly 1/5th of the scan length, was used for all measurements.


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(E)

(A)

(C) (F)

(B)

(D)

Figure 1. (Left) Tapping mode AFM images for HA (A), HNA (B), NHA (C) and NHNA (D) and (right) XRD scans for the 222 (E) and 400 (F) peaks on each film. X-ray diffraction (XRD) measurements were performed using a Rigaku SmartLab 3kW Cu source generating a parallel 44 mA beam of x-rays under a 40kV potential. Symmetric (2θ-ω) scanning of out-of-plane diffraction peaks were collected at 4°/min, stepping at 0.02° over a range from 30 90°. A 0D SC-70 photomultiplier tube was used to collect diffracted X-rays though collimating slits set at 0.5° pitch and beam apertures of 20 mm. X-ray beam spot on the sample was limited to a 10 x 12 mm area. All optical measurements were obtained using a UV-VISNIR spectrophotometer from Shimadzu with a wavelength range of 190 – 2500 nm set at a very slow scan speed. Fourpoint probe (4PP) measurements were performed on a 414513-semiconductor parameter analyzer from HewlettPackard. The correction factor used for the four-point probe (4pp) measurements was 4.3 from the table provided in the reference.47 This correction factor is determined based off of the size of the sample relative to how far the spacing is between the individual probes on the instrument.47 All electrochemical measurements were done using an Epsilon Eclipse from Bioanalytical Systems, Inc. (BASi) with a three-electrode cell setup. Each film was used as the working electrode with either a cell placed on top of the film exposing only a circle with a diameter of 4.3 mm to the electrolyte solution, that also housed the counter and auxiliary electrode, or the film was dunked into solution exposing various working areas. A silver/ silver chloride (Ag/AgCl) reference electrode and a platinum (Pt) or graphite auxiliary electrode were used for each experiment. Chronoamperometry (CA) experiments were performed in a ferrocene solution with a silver quasi-reference electrode and a Pt counter electrode. The open circuit potential of each film was measured and used as the starting potential and a cyclic voltammetry (CV) scan was performed first to determine the step potentials of each film. Each CV experiment was performed with 0.1 mM of Fe(CN)64- in 1.0 M KCl. The

potential was scanned from -0.1 V to 0.6 V for most experiments using a traditional triangular waveform. The peak heights obtained for scan rates of 0.1-0.5 mV/s were measured using the extrapolated baseline current method described by Kissinger and Heineman. [45-47] Electrode Fabrication. All electrodes were fabricated in two pulsed DC magnetron sputtering runs of 20 minutes each. These were performed in a Kurt J. Lesker PVD 75 sputtering system. For the first run the substrate temperature was set at 190 ºC and the second run was performed at room temperature. The temperature of 190 ºC was chosen based off literature stating that this temperature allows both an increase in the carrier concentration and the mobility.48 It is also sufficient for the amorphous to crystalline transition discussed previously. Each run used an ITO target (In90:Sn10) with a ratio of oxygen (O2) to argon (Ar) at 1.6%. The power density was set to ~20 W/in. Each run consisted of non-conductive Si wafer pieces and microscope slides to be divided into two groups; those annealed and those not annealed. The microscope slides were added to allow for the determination of optical band gap of each film. The annealing conditions were 400 ºC for 1 hour in ambient air. Total, there were four different samples from the two process runs: heated and annealed (HA), heated and not annealed (HNA), not heated and annealed (NHA), and not heated and not annealed (NHNA). All film thicknesses were between 230 nm and 250 nm.

RESULTS AND DISCUSSION Physical and Optical Characterization: Atomic Force Microscopy. Surface morphology characteristics were evaluated for each electrode film using AFM. The AFM images were processed on non-conductive Si substrates for all films. Figure 1 shows the 2D topography views for HA, HNA, NHA, and NHNA. There are visual similarities between the two heated samples and between the two non-heated samples.



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Table 1: Figures of merit for each ITO film investigated. (n= # of replicates; 1 unless otherwise noted) HA

HNA

NHA

NHNA

RMS (nm) (n=3)

1.45 +/- 0.07

1.22 +/- 0.08

0.62 +/- 0.05

0.54 +/- 0.03

Sheet Resistance (Ω)

9.22

19.22

18.36

43.85

Resistivity (Ω * cm)

2.1E-04

4.4E-04

4.2E-04

10.1E-04

Capacitance (F/cm2) (n=3)

2.7E-05 +/- 9.7E-08

2.2E-05 +/- 1.1E-07

2.7E-05 +/- 2.5E-07

2.0E-05 +/- 1.1E-06

FeCN Ko (cm/s) (n=3)

1.5E-01 +/- 1.7E-02

1.5E-01 +/- 2.8E-02

1.1E-01 +/- 1.1E-02

8.7E-03 +/- 6.4E-05

Direct Optical Band Gap (eV)

4.08

3.96

3.98

3.67

Ohm’s Plot Resistance (Ω)

1,318 +/- 126

2,461 +/- 156

2,941 +/- 172

26,976 +/- 2,187

Mn CSV Sensitivity (A/ppb) (n=3)

-3.4E-08 +/- 1.2E-09

-3.0E-08 +/- 3.4E-09

-3.3E-08 +/- 3.5E-09

N/A

Mn LOD (ppb) (n=3)

0.1 +/- 0.05

0.3 +/- 0.08

0.8 +/- 0.04

N/A

The non-heated samples, NHA (C) and NHNA (D) have smaller grain sizes and are more in-homogeneous in appearance. The heated samples HA (A) and HNA (B) have larger grain sizes that can be easily distinguished. Between the two non-heated samples (C) and (D), annealing causes a significantly greater appearance of taller grains, as can be seen with the increase of lighter color on (C). Annealing the heated sample (A) also brings the appearance of more peaks of higher structure compared to the not annealed (B), but not as significantly. The root mean squared (RMS) roughness values followed a specific trend. NHNA had the lowest value followed by NHA and the two heated films, HNA and HA, had higher values. All of the RMS values are shown in Table 1. While each of the ITO film types are smooth, heating appears to have a greater effect than annealing. Higher microscopic surface area, or roughness, can lead to greater sensitivity and lower limits of detection (LOD) in electrochemical measurements such as CSV.49-51 This analysis suggests that HA and HNA may be more advantageous than the other two films. Physical and Optical Characterization: X-ray Diffraction. Symmetric scans show the presence of multiple peaks in the diffraction pattern, confirming all the samples are polycrystalline. Phase identification using the Crystallography Open Database (COD) diffraction database identifies deposited films are cubic 𝐼𝑎3 ITO.52-55 Moreover, the relative intensity, position, and broadness of the peaks conveys information about relative predominance of specific crystalline phases, strain within the phase, and overall crystalline quality and size of particles. In figures 1(E) and (F), the (222) peak clearly decreases as a function of heat treatment, whether being heated during deposition or annealed after, while the (400) peak increases. The crystal quality parameter, I(222)/I(400), of NHNA is almost 23 times greater than HNA indicating significant, process dependent, transition of preferred phase (Fig. S1). It has been shown in literature that the (400) decreases in size with increasing oxygen content of the films.52 This suggests heated deposition changes the synthesis route of ITO, promoting the (400) phase.

The (222) peak centroid is observed to shift when either independently heating the substrate during deposition or annealing the substrate after a non-heated deposition indicating changes in the relative strain in the (222) phase. Relative to the NHNA condition, the 2θ of the NHA peak centroid is shifted by +0.16° and the 2θ of the HNA by -0.2°. Upon application of both heated deposition and annealing, the 2θ peaks returns to nearly the same 2θ as NHNA, 30.34° (0.06° to NHNA). Further, the full width at half maximum (FWHM) of both the (222) and (400) peaks, in general, decreases as a function of heat treatment, either heated deposition or annealing (Figure S2). This indicates the crystalline quality of both phases improves with the application of heat, either during or after deposition. Thermodynamically, without the application of heat in the ITO synthesis process, the mobility of indiumoxygen clusters and tin is restricted. This limits the likelihood of misorientations, or faulting in crystalline stacking order to heal, leading to bond distortions and restricting coherent crystallization of the film. This situation accounts for the broad FWHM in NHNA, showing higher probability of a partially amorphous film. Based on the observed improvement in crystalline phases, the application of heat above the ‘crystallization threshold’ during deposition (~ 150 °C) appears to enable mobility of the film constituent indiumoxide clusters and tin.56 This allows for a greater fraction of the deposited materials to form long-range crystal order during deposition, decreasing the FWHM in general. An exception to this is the jump in FWHM from NHNA (400) to NHA (400), and the swap of the (222) and (400) FWHM positions of HNA. For NHA, this may be the result of frozen indium-oxide cluster and tin positions at the room temperature deposition. While annealing can unfreeze these constituents, enabling them to re-form into a stable lattice, their diffusion through the film is limited by the surrounding material, restricting their ability to occupy energetically favorable lattice positions. Therefore, the formation of longrange (400) phases is subdued. In contrast, the mobility of the indium-oxide clusters with tin during heated deposition in HNA is enhanced, allowing the constituent atoms a greater


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degree of freedom since the surrounding material has not yet been deposited. Statistically, this increases the probability of long-range order and stable bond formations leading to decrease in the FWHM. Combining these two heat treatments in HA supports these conclusions showing the narrowest FWHM, largest crystallite size from the (400) (Fig S3), and least amount of relative stress or strain of any sample. This could lead to the best film structure for Mn detection due to the larger grain size leading to less scattering of the charge carrier and increased conductivity.19 This would also suggest that HNA could potentially be the best at Mn detection seeing the trends in FWHM, and crystallite size, are mirror those of HA depending on the (222) or (400) orientation. Interestingly, the I(222)/I(400) of HNA is slightly lower than HA which could play an important role in which film ultimately behaves better as a sensor for Mn using CSV. Physical and Optical Characterization: UV-Vis Spectroscopy. While this study is primarily focused on ITO deposited on Si, microscope slides were included since optical measurements can provide insight on the charge carriers in each film. That could help explain why one film would be more or less advantageous for CSV. The transmittance (T%) spectra of each electrode and a reference microscope slide is displayed in Figure 2. When comparing the four films, it is seen that the T% scans of the not annealed (HNA, and NHNA) samples are similar and the annealed (HA, and NHA) samples are similar to one another, this is different from the visual trend with the AFM that suggests similarities between the heated and not heated samples. It has been found in literature that annealing shifts the absorption edge toward a higher energy.4 This phenomenon is observed with these ITO films presented here; HA, and NHA are shifted to higher energies. It is also seen that HNA exhibits this shift, suggesting that heating the substrate during deposition has a similar effect. This push to higher energy would suggest there being a lower deft density in these films compared to NHNA and thus a higher crystallinity.4

Figure 2: Transmittance spectra of each film deposited on a microscope slide.

At higher wavelengths (1500 – 2500 nm) there is a distinction between each film’s T% spectra. NHNA has the highest T% in this range followed by HNA and NHA. HA has the lowest transmittance in this range. This leads to the assumption that HA has the highest carrier concentration. It has been shown that a higher carrier concentration leads to this lower transmittance value between the wavelengths of 15002500 nm.4, 48 The increase in carrier concentration would be beneficial in an electrode sensor application. The direct optical bandgap of each film was determined due to the fact that a shift to crystallinity would increase the band

gap due to the decrease in the density of localized states.57-58 This allows a correlation between the band gap and crystallinity of the film, that with the highest band gap would have the highest crystallinity. To perform this measurement, the energy gap, Eg, of each film was determined using the mathematical relation in equation 1:59-60 αhv = (hv-Eg)r (1) where hv is the photon energy, r is the transition coefficient that has a value of 2 for indirect band gap and ½ for the measurement of direct band gap, and α is the absorption coefficient given by relationship (2). T = (1-R) exp(-αd) (2) where d is the thickness of the film. The direct optical band gap was determined by extrapolating the linear region of the Tauc graph, αhv2 vs hv, to obtain the highest coefficient of determination. This graph is provided as Figure S4, and all of the fitting parameters are displayed in Table S1. The lowest optical bandgap was observed on the NHNA sample with the highest belonging to the HA sample. Both heating and annealing increased the band gap, and therefore the crystallinity. This trend can be explained by the density of states model by Mott and Davis.60 This shows that in the process of heating or annealing the defects that are present in amorphous films are annealed out producing more saturated bonds and a higher optical band gap.60 This is also seen with the general increasing FWHM trends from the XRD analysis. This trend is consistent with the optical T% spectrum in that the shift to a higher energy would also be an indication of a shift to a higher bandgap. The optical bandgap data is shown in Table 1. HA has the highest bandgap followed by NHA and HNA, making them highly applicable for electrode sensing applications. All of the films fall into the range of commonly reported values (3.5 – 4.2 eV) for the direct optical bandgap of ITO.5, 7, 21 Electrical Properties. For electrochemical sensing applications, a film with a low resistance is desired. All resistivity and sheet resistance data are summarized in Table 1. The highest sheet resistance and resistivity was exhibited by the NHNA film with 43.8 Ω and 1.008 mΩ*cm respectively. The lowest sheet resistance and resistivity was found on the HA film with 9.22 Ω and 0.21 mΩ*cm respectively. Both the NHA films and the HNA films showed very similar values for both their resistivity and sheet resistance. These results are consistent with literature findings in that the resistivity of the ITO film drops when heated to temperatures above 100 ºC and when annealed post deposition.4, 7, 11, 19 A carbon film electrode used for Mn detection using CSV had a resistance much lower than these ITO films at 2 Ω, but heavily doped BDD sensors used for a similar application are not as conductive as these films with resistivity values in the 10-2 Ω *cm range.42, 61-62 The electrical properties are consistent with the optical properties in showing that HA should be expected to behave as the best electrochemical sensor, potentially followed by HNA. Electrochemical Characterization. To investigate how each non-optically transparent ITO film behaves electrochemically, cyclic voltammetry (CV) experiments were performed and Ohm’s resistance plots were generated. Capacitance values for each film were determined using CV. An electrolyte of 1 M potassium chloride (KCl) was used. Scan rates ranged from 0.1 – 0.4 V/s and the current value at

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0.0V was recorded. This current value and corresponding scan rate were graphed to determine the capacitance of the film.

(A)

(B)

(C)

Figure 3: (A) CV i-E curves of 0.1 mM Fe(CN)63-/4- in 1.0 M KCl. Scan Rate: 0.1 V/s. (B) Potential window study in pH 5.0 acetate buffer. (C) Mn CSV in pH 5.0 acetate buffer. [Mn]: 25 ppb; deposition time: 3 min; deposition potential: +1.2 V. The slope provided the capacitance in units of A*s/V or F. Dividing by the exposed area provided the final obtained value in F/cm2. Capacitance graphs are provided in Figure S5. Potassium ferrocyanide (K4Fe(CN)6) was chosen as the redox couple to evaluate each surface due to it having an inner sphere electron transfer mechanism.63 Some sources state that the rate of reaction that Fe(CN)63-/4- displays on the surface does not vary significantly with surface oxide coverage, but rather is due to the monolayer adsorption that is occurring.63 It was still chosen to be used here due to its surface sensitive nature and the ease of comparison to other electrode materials that have been studied using it. The CV for each film at 0.1 V/s is shown in Figure 3A. The peak separations (∆Ep) at 100 mV/s (0.1 V/s) were 90 mV for HA, 83 mV for HNA, 89 mV for NHA, and 95 mV for NHNA. These are lower than the peak separation for ITO reported previously at 160 mV.64 Boron-doped diamond (BDD) is known to show peak separations of 70-85 mV for this redox couple.65 Glassy carbon has been reported to have peak separations at 61-65 mV and closer to 145 mV in some cases.64, 66 Carbon electrodes in general are known to display different properties depending on the pretreatment used.66-67 The ITO in this study have similar ∆Ep’s to the high range commonly seen with BDD. The rate constants for Fe(CN)63-/4- were determined using the Nicholson Method for low peak separations.68 Exact psi values were used by linear interpolation of the tabulated data.68 From Figure 3A, it is clear that HA had the highest peak current out of the four films. NHNA had the lowest with the largest peak separation. HNA and NHA have very similar responses, but from their similar resistance and optical bandgap this is expected. The peak height trend obtained corresponds to the conductivity of each film with HA being the most conductive, HNA and NHA having similar conductivities, and NHNA being the least conductive out of the four. The tabulated rate constants are displayed in Table 1. The fastest rate constant was obtained on both HA and HNA; the slowest was provided by NHNA. In general, for an electrochemical sensor, the highest current response and the fastest reaction rate would be desired. Since the fastest rate constant was obtained with HA and HNA (and HA exhibited higher peak current), the results suggest that HA or HNA are favorable for a potential sensor-based application.

Ohm’s resistance plots were generated for each film by plotting the change in peak potential (Ep) vs the change in peak current (ip). The slope of these graphs was linear and carry units of (V/A), which is equal to resistance () in the electrochemical cell. These plots are shown in Figure 4. The 4PP measurements only measure the resistance of the film through the distances between the probes. The Ohm’s plot measurements allow the resistance to be measured when the surface is exposed to an electrolyte solution, as it will be in a practical application. Each ITO film type abides by Ohm’s law and has a constant resistance when the temperature is constant. Since they all obey Ohm’s law, they are considered to be ohmic resistors. NHNA had the highest resistance and HA has the lowest resistance; NHA and HNA were again very similar in value to one another and fell in between NHNA and HA with HNA being slightly closer to that of HA. All of the resistance values obtained from the Ohm’s plots are displayed in Table 1. The general trend that is seen in resistance is expected from the results of the 4PP data.

Figure 4: Ohm’s resistance plots for all ITO films calculated from the CV i-E curves in Figure 3. To further evaluate the electrochemical characteristics of each film, chronoamperometry (CA) measurements were performed (data not shown). CA allows for the measurement of the microscopic surface area using an applied potential step.49 The resulting data could then be analyzed with the Cottrell Equation to solve for the exposed surface area of the working electrode.49 A constant diffusion coefficient of 2.24 x 10-05 cm/s was used for all calculations.69 The microscopic surface areas of HA, HNA, NHA, and NHNA were all statistically similar circa 0.20 cm2. This is almost identical to the 0.196 cm2 geometric surface area that was used for the


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electrochemical measurements. The similarities in these values is likely due to the smoothness of each film shown in

(A1)

(B1)

(C1)

(A2)

(B2)

(C2)

Figure 5: Manganese stripping voltammograms for HA including the full range (A1) and small range (A2), for HNA including the full range (B1) and small range (B2), and for NHA including the full range (C1) and small range (C2) in pH 5.0 acetate buffer. Deposition potential: 1.2 V; deposition time: 3 min. Table 1. This result suggests that the surface roughness of each film may not be a crucial parameter in this investigation. The potential window of each ITO film was also studied and the CV i-E curves are shown in Figure 3B. A potential sweep was started at 0.0 V before scanning anodically to +2.5 V. The sweep polarization was then reversed and scanned to 1.75 V before stopping at 0.0 V. In the anodic scan, it can be seen that each ITO film exhibits an excellent potential window. In the cathodic (reverse) scan, several peaks are obtained due to the electrochemical reduction of ITO. Liu et al. also studied the electrochemical reduction of ITO and attributed these peaks to oxygen reduction and the reduction of ITO to metallic indium (In) and tin (Sn).70 The peak ca. 0.3 V is likely due to the reduction of oxygen in each of the films and the magnitude is the largest on HA. This supports the previous claim regarding the reduction of the (400) XRD peak with increasing oxygen in the ITO film (HNA exhibited a slightly higher (400) peak to HA). Looking at all four scans, however, it is apparent that both heating and annealing of ITO may increase oxygen content. The cathodic peaks obtained ca. -1.2V are related to the reduction of ITO to metallic In and Sn.70 These peaks are also largest and sharpest in HA. This may indicate that the HA process yields the most In and Snterminated crystallite phases. This in combination with the larger crystallite size (as determined by AFM and XRD) clearly plays a role in electrode performance (as indicated by Figure 3A). Cathodic Stripping Voltammetry of Manganese. For further discernment of the ITO films, CSV of Mn was completed. Using 25 ppb Mn, a deposition potential of +1.2 V was used for 3 minutes based on our previous experience with Mn CSV on ITO.27 Experiments on each film were

completed in triplicate and the average is plotted in Figure 3C. No Mn response was obtained on NHNA, while the other three films exhibited sharp and well-resolved peak responses. A similar response on both HNA and HA was observed. The peak seen on NHA peak was comparable in visual area (to HA and HNA) but was smaller in current magnitude. The peak current responses for each ITO film type are shown in Table 1. Combining these results with those obtained throughout the electrochemical characterization further suggests that heated deposition is the most beneficial for a CSV application. Further investigation of HA and HNA is still needed to determine the optimum ITO film type. Calibration and Figures of Merit. Calibration curves were generated to determine and compare several figures of merit for the different ITO film types. Concentrations of 1, 5, 10, 25, and 50 ppb of Mn were used; all experiments were completed in triplicate. The stripping voltammograms for HA, HNA, and NHA can be seen in Figure 5. Calibration curves for the total concentration range are provided in Figure S6. A Linest function was used to determine the value and uncertainty of the slope, and y-intercept, and the uncertainty in the y and R2 values. To find the uncertainty in the LOD, the equation for propagation of uncertainty for fitting the best fit line of each calibration curve was used.71 It is immediately apparent that this application was not feasible for the NHNA film. There is no linearity in its calibration curve with an R2 value of 0.663, and hardly any response was measured for any level of Mn. Due to this, NHNA was left out of Figure 5. NHA had an overall average response of [I (A) = (-3.3E-08 ± 3.5E-09)c (A/ppb) + (1.2E-07 ± 8.9E-08) (A) R2= 0.9670] and a LOD (3σ/m) of 0.8 ppb. The lowest concentration of 1 ppb was hard to distinguish from the background for this film


7


(Figure 5 (C2)), and the background shifted slightly throughout the measurements. HNA had an overall response of [I (A) = (-3.0E-08 ± 3.4E-09)c (A/ppb)+ (1.4E-07 ± 8.6E08) (A) R2=0.9637] and a LOD (3σ/m) of 0.3 ppb. Although 1 ppb is still relatively hard to distinguish from the background (Figure 5(B2)), the background level shifts less and the peaks appear more symmetric than those obtained on NHA. Although the sensitivity of NHA is higher than HNA, HNA has much more reproducibility shown with a significantly lower standard deviation at 1 ppb. HA provided the lowest LOD, highest sensitivity, and greatest linear correlation (R2) out of the four films examined. This film’s response for the full range is [I(A) = (-3.4E-08 ± 1.2E-09)c (A/ppb) + (8.2E-08 ± 3.1-08) (A) R2= 0.9962] with a LOD (3σ/m) of 0.1 ppb. This LOD is 3x lower than that of HNA, and this is the only film where an analytically acceptable linear calibration is obtained for the full concentrations studied. 0.1 ppb is also lower than the 0.27 ppb LOD previously recorded in literature for Mn detection on bare ITO.27 When considering 1 ppb, there is a clear distinct peak separated out from the background of this film (Figure 5 (A2)). The background is also the flattest of the four films and remains stable throughout the whole calibration curve. These peaks also appear to be the sharpest. When comparing three calibration sets run on each film, HA was the most reproducible overall. The LODs obtained on HA and HNA are significant due to it being below the maximum contaminant level (MCL) of 50 ppb for Mn in drinking water and in the detection of Mn in blood (4-15 ppb), urine (1-8 ppb), and serum (0.4-0.85 ppb).3334 The LOD of NHA is below three of these concerning levels; allowing for all three films to have the desired performance for real life measurements. It is also worth noting that the ITO films investigated here did not undergo significant optimization of the deposition time. Further increase in the deposition time can lead to even lower LODs, where necessary. When considering interferences, the method of CSV is advantageous due to some metal contaminant’s inability to form soluble oxides are higher oxidation states. In addition a previous study, by members of our team, showed that the typical Fe2+ levels in drinking water are low enough to not be of interference concern on ITO sensors, and that other metal ions interference can be removed with the addition of a SSEBS membrane to the ITO surface.27 An interference study using a carbon paste electrode also showed that interferences could be eliminated by masking with sodium citrate.72 This could be applicable to the detection on ITO as well. Based on these studies, interferences during Mn detection using CSV can be eliminated and is of little concern.

Conclusion The performance of four different ITO film types (HA, HNA, NHA, and NHNA) were investigated to determine the optimum electrode material, specifically for Mn CSV. For the heated samples, a substrate temperature setting of 190 ºC was used. For annealing, 400 ºC for 1 hour in air was used. The films were thoroughly characterized using topographical, crystallographic, and electrochemical measurements. The AFM and XRD analysis revealed similar trends for the samples that were heated compared to those that were not heated. Both heated samples, annealed (HA) and not annealed (HNA), showed higher roughness values, crystallite sizes, and

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(400) peaks, and lower full width half max values compared to those that were not heated. Optical measurements that were performed to obtain valuable information about the film’s optical band gap and carrier concentration. These results showed that the annealed samples had higher band gap values and carrier concentrations than the not annealed samples. All optical band gaps were in typical literature ranges for ITO with HA having the highest; displaying the highest crystallinity in correlation to the XRD analysis. For electrical property comparison, a four-point probe was used to determine the sheet resistance and resistivity of each film. NHNA had the highest of both of these values, NHA and HNA were vary comparable to one another, and HA had the lowest, indicating favourable performance as an electrode material. For the electroanalytical detection of Mn by CSV, HA, HNA, and NHA exhibited favourable response. No response was obtained on NHNA. While HA, HNA, and HNA yielded the sensitivity needed for the trace detection of Mn, only HA displayed a calibration curve that could be considered analytically acceptable with a linear correlation coefficient (R2) of 0.996. Each of the other ITO films had R2 values < 0.970. As such, it can be concluded that the HA ITO fabrication process is the optimum for behaviour as an electrode material and more specifically, CSV of Mn. It appears that a film’s roughness or crystallite size, resistivity, and I(222)/I(400) peak ratio may play a key role. This work expands on the application and understanding of ITO as an electrode material and lays a foundation for other analytes available for detection by CSV.

ASSOCIATED CONTENT Funding Sources Internal funding was provided for this research by Fraunhofer USA Inc. Center for Coatings and Diamond Technologies.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Supplementary information includes full width half max XRD trends, I(222)/I(400) trends, and Crystallite size trends for all films. Tauc plot used to determine optical band gap and corresponding fit values. The graph of the current vs scan rate curves used to determine the capacitance values. The calibration curves for the Mn detection on the HA, HNA, and NHA films.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Phone: 01-517-884-8694. Fax: 01-517-432-8167

ORCHID Mary Ensch: 0000-0002-4718-7517 Elias Garratt: 0000-0002-0331-3876 Cory Rusinek: 0000-0002-6852-0219

ACKNOWLEDGMENT The authors would like to acknowledge Ed Drown (Fraunhofer CCD) for his assistance with the AFM images, Nina Baule (Fraunhofer CCD) for her helpful discussions, and Shengyuan Bai (MSU) for his work with the XRD.


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