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. 2024 Aug 7;40(33):17536–17546. doi: 10.1021/acs.langmuir.4c01751

Photoelectrochemistry of Redox-Active Self-Assembled Monolayers Formed on n-Si/Au Nanoparticle Photoelectrodes

Kayla M Mancini †, Yousef Khatib †, Lauren Shahine †, Glen D O’Neil †,‡,*
PMCID: PMC11340028  PMID: 39110768

Abstract

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Controlling the chemistry of the electrode–solution interface is critically important for applications in sensing, energy storage, corrosion prevention, molecular electronics, and surface patterning. While numerous methods of chemically modifying electrodes exist, self-assembled monolayers (SAMs) containing redox-active moieties are particularly important because they are easy to prepare, have well-defined interfaces, and can exhibit textbook photoelectrochemistry. Here, we investigate the photoelectrochemistry of redox-active SAMs on semiconductor/metal interfaces, where the SAM is attached to the metal site instead of the semiconductor. n-Si/Au photoelectrodes were fabricated using a benchtop electrodeposition procedure and subsequently modified by immersion in aqueous solutions of (ferrocenyl)hexanethiol and mercaptohexanol. We explored the relevant preparation conditions, finding that after optimization, we were able to obtain canonical cyclic voltammetry for a surface-bound redox molecule that could be turned on and off using light. We then characterized the optimized electrodes under varying illumination intensities, finding that the heterogeneous electron transfer kinetics improved under higher illumination intensities. These results lay the foundation for future studies of semiconductor/metal/molecule interfaces relevant to sensing and electrocatalysis.

Introduction

Chemical control over the electrode/solution interface is crucial for designing electrodes with variable properties.1,2 Redox-active self-assembled monolayers (SAMs) are widely used to control the electrode/solution interface because they offer well-ordered, precise control of the chemical environment of the redox molecule.3,4 These modified electrode surfaces have a broad range of applications in sensing,5−10 energy storage and conversion,11−13 corrosion prevention,14 molecular electronics,15−19 surface patterning,20,21 and for tuning interfacial energetics.22 Redox-active SAMs are also useful for testing fundamental theories of electron transfer in both metals23−26 and semiconductors.27−31 Consequently, there are numerous approaches to understanding (and modeling) the voltammetry of redox molecules adsorbed on surfaces, which are useful for extracting kinetic and thermodynamic parameters from experiments.30−35

While modifying metallic, and in particular Au, electrodes with redox-active SAMs can be accomplished on the benchtop, modification of semiconductor, particularly Si, surfaces is more challenging.36 However, modification of Si with redox-active SAMs is particularly important for the electronics industry, where the need for miniaturization is driving advances in molecular-level interface control.37 The main challenge in forming well-ordered, electrochemically well-behaved monolayers on Si is preventing oxide growth, necessitating multistep syntheses in air- and water-free environments.38,39 Modifying Si photoelectrodes with metal films, nanoparticles, or 2D materials is another common approach for controlling the energetics and chemistry of the Si interface.40,41 These types of interfaces are widely used as diodes in the electronics industry and as water-splitting photoelectrodes for energy storage applications.42−44 For electrochemical applications, an advantage of modifying semiconductor surfaces with metals is that the electrochemical properties of the metal are largely conserved, essentially creating metallic electrodes that can be turned on/off with light. One drawback of this approach is the lack of direct chemical control over the metal surfaces, which often change dramatically during electrolysis.45

In this study, we investigate the photoelectrochemistry of redox-active SAMs on semiconductor/metal junctions, where the chemical modification occurs at the metal site instead of the semiconductor. To the best of our knowledge, there are no previous reports of redox-active SAMs formed on semiconductor/metal interfaces, although there are numerous examples of semiconductor/molecule/metal junctions in the literature (reviewed in ref (16)). As a first demonstration, we modified n-Si electrodes with electrodeposited Au NPs, onto which we formed a redox-active SAM consisting of (ferrocenyl)hexanethiol (FcHT) and mercaptohexanol (MCH). We characterized the electrodes using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV). The important new result is that under optimized conditions, the photoelectrochemistry of the n-Si/Au/FcHT photoelectrodes showed fast electron transfer and near-ideal figures of merit for surface-bound redox molecules under illumination, but no redox activity in the dark. We explored how the preparation conditions (i.e., FcHT concentration, metal morphology, and SAM deposition time) affect the observed photoelectrochemistry. Finally, we measured how illumination intensity changes the voltammetry, and in particular the electron transfer kinetics, of the redox-active SAM. This work is part of a larger effort within our group to expand the scope and application of semiconductor/metal light-addressable electrochemical (LAE) sensors; attainment of that goal necessitates understanding the photoelectrochemistry of tethered redox species to these interfaces. This work lays the foundation for future studies in sensing and electrocatalysis with modified semiconductor/metal photoelectrodes.

Experimental Section

Materials and Solutions

6-(Ferrocenyl)hexanethiol (FcHT) and 6-mercaptohexanol (MCH; 97%) were purchased from Sigma-Aldrich. Phosphate-buffered saline (PBS) reagent tablets, KCl, K2SO4, HClO4, and H2SO4 were from Fisher and were of ACS Reagent grade. Ferrocene methanol (FcMeOH; 97%) was obtained from Acros Organics. Ammonium fluoride (NH4F; 40% m/m, semiconductor grade) was from Honeywell. Hydrogen tetrachloroaurate(III)trihydrate (HAuCl4; Au 49% min, crystalline) was purchased from Alfa Aesar. All solutions were prepared using 18.2 MΩ·cm DI water (Millipore Simplicity).

Electrode Preparation

The photoelectrodes used in this study were prepared using n-type Si (100) from Pure Wafer (San Jose, California, USA). The n-type wafers were doped with phosphorus (resistivity 1–5 Ω·cm). Control experiments were conducted with p+-Si (100) wafers (resistivity <0.005 Ω·cm) that were highly doped with boron. Si wafers were diced into ∼1 cm2 samples by scoring the unpolished side with a diamond-tipped pen and breaking the wafer along the scratch line. Each sample was cleaned with Piranha solution (a 3:1 v/v mixture of concentrated H2SO4 to 30% H2O2) for 30 min at 105 °C. Caution: Piranha solution is highly corrosive and reacts dangerously with organics. The samples were thoroughly rinsed with DI water before being submerged in a 40% NH4F solution (previously deoxygenated with Ar for 30 min) for 10 min to remove the oxide on both sides of the sample. H-termination of the Si was confirmed by placing a small droplet of water on the surface to confirm hydrophobicity. Ohmic back connections were prepared by contacting a Cu wire using indium solder. The back contacts were insulated by sealing the entire assembly in 3 M Electroplater’s tape (3 M 470), with a 3 mm-diameter circular opening that allowed exposure of the polished front Si surface to the electrolyte. This protocol was performed in batches of ≈20 electrodes that were stored in the dark until future use.

The exposed Si surface was etched in a deaerated 40% NH4F solution for 10 min at room temperature to remove the native oxide. The electrode was rinsed with copious amounts of DI water prior to electrodeposition. Au NPs were electrodeposited onto the polished front surface of the Si in order to establish a semiconductor/metal junction and increase the electronic coupling between the semiconductor and redox species, using a modified procedure described by Allongue et al.46 To minimize oxidation, the electrode was biased to −1.945 V vs SCE before being dipped into the electrodeposition solution. A glassy carbon rod was used as the counter electrode during electrodeposition to prevent contamination of the n-Si with Pt. The electrodeposition solution consists of 0.2 mM HAuCl4, 1 mM KCl, 0.1 M K2SO4, and 1 mM H2SO4. Electrodeposition was carried out with the room lights on using a deposition time of 5 min. After electrodeposition, the samples were rinsed with copious amounts of DI water.

Electrodes were modified with a self-assembled monolayer consisting of FcHT by immersing electrodes in an aqueous FcHT solution in 0.02 M PBS for 60 min. Electrodes were rinsed with copious amounts of DI water and then were immediately incubated in 30 mM MCH solution for 90 min to passivate unbound electrode surface. Incubations were performed in sealed 50 mL conical tubes (Falcon).

Photoelectrochemical Measurements

Electrochemical measurements were performed using a CH Instruments 760E potentiostat. All electrochemical measurements were carried out in a 30 mL electrochemical cell with a borosilicate glass window with a three-electrode configuration. A saturated calomel electrode (SCE) served as the reference and a glassy carbon rod as the counter. The semiconductor was illuminated with either a Boling Super LED (amazon.com) or a white-light LED from AmScope. The intensity of the white light was measured to be ≈80 mW/cm2. All optical components were housed inside an optical enclosure purchased from Thorlabs.

CV measurements were carried out under direct illumination and in the dark in an electrolyte containing 0.1 M HClO4 over a potential range from approximately −0.4 to 0.4 V vs SCE. EIS for Mott–Schottky measurements were carried out in the dark in 0.1 M HClO4 over a potential range, −1 to 0 V vs SCE. When required, light intensity was modulated using a series of neutral density filters from ThorLabs (NEK01S) and calibrated using a ThorLabs PM161-121 USB power meter. In all the photoelectrochemical measurements presented herein (unless otherwise stated), the illumination intensity (≤80 mW cm–2) is sufficiently high to have the response of the electrode be governed by the electrochemistry rather than carrier generation/transport. Reproducibility was evaluated by performing all measurements with multiple independently prepared samples (n ≥ 3). The reported figures of merit are reported as the mean ± uncertainty, which was represented by the standard deviation (or 95% confidence interval) of the mean. Where appropriate, statistical comparisons were made using either a Student’s t test or ANOVA with Bonferroni correction. We used the Peak Analyzer toolkit in OriginPro 8.5 for determining FWHM values and for performing deconvolution of the voltammograms in Figures 5 and 6.

Figure 5.

Figure 5

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(a) Cyclic voltammograms of 1 mM FcHT monolayers in 0.1 M HClO4 with 0.2 mM, 0.5 mM, and 1 mM n-Si/Au NP LAES. AFM images (3 × 3 μm) of samples prepared with (a) 0.2 mM, (b) 0.5 mM, and (c) 1 mM HAuCl4 in the electrodeposition bath. Experimental conditions: v = 0.4 V s–1; reference = SCE; counter = glassy carbon rod.

Figure 6.

Figure 6

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CVs of n-Si/Au/FcHT electrodes prepared by incubating in 1 mM FcHT solutions for 15 (black trace), 30 (red trace), or 60 min (blue trace). A representative dark trace is also shown in gray. Electrolyte: 0.1 M HClO4; reference: SCE; counter: glassy carbon rod; v = 0.4 V s–1.

Physical Characterization

We performed AFM using a Bruker Dimension Icon AFM to characterize the surface morphology of the electrodeposited Au NPs. The images were collected in ScanAsyst mode using a SCANASYST-AIR-HPI probe (7 nm radius, k = 0.25 N/m, f = 55 kHz). The imaging rate was 0.5 Hz with a resolution of 512 samples/line. All images were 3 μm × 3 μm, making each pixel ≈6 nm, similar to the probe radius. XPS was performed on a Thermo Scientific K-Alpha XPS at the Surface Analysis Facility at the University of Delaware. Survey scans were conducted over the range 0 to 1360 eV. Single-element scans for Au, Si, Fe, S, O, and C were also performed.

Results and Discussion

The goal of this study was to characterize the photoelectrochemistry of semiconductor/metal/molecule interfaces formed by depositing redox-active SAMs on the metal surface of a semiconductor/metal interface. While the modification of metal and silicon surfaces with redox-active SAMs is common in the literature, we were unable to find any examples of redox-active SAMs formed on semiconductor/metal interfaces.

Fabrication of n-Si/Au/FcHT Interfaces

We prepared n-Si/Au electrodes by electrodeposition at −1.945 V vs SCE for 5 min from an electrolyte containing HAuCl4, 0.1 M K2SO4, 1 mM KCl, and 1 mM H2SO4 (see Experimental Section for details).46−49 These electrodes were subsequently modified with redox-active SAMs consisting of FcHT and MCH (Figure 1) by incubating the electrodes in an aqueous phosphate buffer solution containing FcHT for a fixed time to allow the formation of the SAM.50 The electrodes were thoroughly rinsed and then submerged in a 30 mM solution of MCH to passivate unreacted Au sites and form a mixed monolayer. We hypothesized that due to sulfur’s higher affinity for Au than Si, the thiolated redox molecules would selectively bind to the Au surfaces via a gold–thiol bond. We systematically studied the effect of FcHT concentration, SAM incubation time, and morphology of the Au layer to understand how each of these factors contributes to the observed voltammetry.

Figure 1.

Figure 1

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A schematic showing the preparation of the self-assembled monolayers on n-Si/Au junctions.

Physical Characterization of n-Si/Au NP/SAM Interfaces

We characterized the morphology and elemental composition of n-Si/Au and n-Si/Au/FcHT interfaces using AFM and XPS, respectively. Figure 2a shows a representative AFM height image (3 μm × 3 μm) of an n-Si/Au sample prepared as described above. The images presented in Figure 2 are representative of multiple images acquired at multiple locations on three separate samples. However, even with multiple imaging locations on multiple samples, we are only characterizing ≈4 × 10–4% of the electrode surfaces. The AFM height image confirms that the electrodeposition procedure produces heterogeneous n-Si/Au surfaces that are largely composed of small, closely spaced NPs. Figure 2b shows the corresponding AFM peak force error image, which is a measure of the peak force deflection during ScanAsyst imaging and is caused by different tip–substrate interactions between dissimilar materials. The measurements in Figure 2b are not quantitative and are only used to help visualize the contrast of the height image in Figure 2a.

Figure 2.

Figure 2

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AFM characterization of the n-Si/Au photoelectrodes prepared by electrodepositing Au from an electrolyte containing 0.2 mM HAuCl4, 0.1 M K2SO4, 1 mM H2SO4, and 1 mM KCl in (a) height mode and (b) peak force error mode. The measurements in part (b) are not quantitative and are only used to help visualize the contrast of the height image in part (a). AFMs were acquired over 3 μm × 3 μm in ScanAsyst mode using SCANASYST-AIR-HPI probes (7 nm radius, k = 0.25 N/m, f = 55 kHz).

We performed XPS to confirm that the FcHT was successfully attached to the Au NP. Figure 3 shows single-element XPS data for n-Si/Au (black traces) and n-Si/Au/FcHT (red traces) interfaces. Survey scans (Figure S1) and single-element scans for the C 1s and O 1s (Figure S2) are presented in the Supporting Information. Figure 3a shows Si 2p XPS spectra, with two peaks evident. The peak at ≈99 eV corresponds to elemental Si, and the broad peak between 102 and 105 eV corresponds to oxidized silicon species.51 It is unknown if the oxide forms during electrodeposition or if the Si was oxidized in the lab ambient between preparation and analysis (approximately 5 days). However, the presence of the oxide does not seem to dramatically impact the ability of the electrodes to perform fast electron transfer (vide infra). Figure 3b shows the Au 4f5/2 (≈88 eV) and 4f7/2 (≈84 eV) spectra, consistent with the formation of gold nanoparticle films. The intensity of both gold spectral peaks is attenuated after the formation of the monolayer, consistent with SAM formation on Au surfaces. The ratio of the peak intensities of the 4f7/2/4f5/2 is nearly identical for both systems (1.23 for n-Si/Au samples and 1.25 for n-Si/Au/FcHT samples), suggesting a similar average oxidation state for the gold particles before and after SAM formation.52 The XPS suggests that all of the Au is present as Au(0), consistent with other Au-SAM interfaces.52Figure 3c shows Fe 2p3/2 and 2p1/2 peaks at ≈708 and ≈721 eV, respectively. These peaks are associated with the Fe2+ oxidation state of the neutral ferrocene molecule.53 Importantly, the n-Si/Au samples without a SAM showed no evidence of Fe in the XPS. Figure 3d shows the high-resolution S 2p span with peaks at ≈162 and ≈168 eV. Note that the intensity of these peaks is very low compared with the other elements studied. The peak at ≈162 eV is associated with bound thiols, while the peak at ≈168 is suggestive of oxidized sulfur species.54 We observed the peak at ≈168 eV in the samples prepared without a monolayer. Given that the electrodeposition bath contains significant amounts of sulfate (>0.1 M), we tentatively assign this peak to residual sulfate present in the Au layer. Ion incorporation into thin films of other metals has been previously demonstrated.55 Importantly, there is no evidence of unbound thiol, which would be present at ≈164 eV.54 The lack of evidence for unbound thiol suggests that a monolayer of FcHT and MCH is formed under the preparation conditions. Taken together, the data in Figures 2 and 3 show that the electrodeposited nanoparticles are gold and that FcHT/MCH can be attached to the n-Si/Au surface through the formation of gold–thiol bonds.

Figure 3.

Figure 3

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Elemental characterization of n-Si/Au and n-Si/Au/FcHT photoelectrodes using XPS. (a) Si 2p region; (b) Au 4f region; (c) Fe 2p region; (d) S 2p region. The black traces are n-Si/Au electrodes, and the red traces are n-Si/Au/FcHT-MCH electrodes.

Characterization of the Junction Energetics of n-Si/Au/FcHT Electrodes

Characterizing the energetics (i.e., the positions of the flat-band potential (Efb), the conduction edge (Ecb), and the valence band edge (Evb)) of the n-Si/Au and n-Si/Au/FcHT junctions is an important first step toward understanding the photoelectrochemistry of the interfaces, because it determines the applied potential range where n-Si will be depleted of minority carriers.56 When the semiconductor is depleted of minority carriers, the electrochemical reaction can be turned on/off using light. We compared n-Si/Au electrodes with and without an FcHT monolayer to understand if the presence of the monolayer influenced the band energetics. Figure S3 in the Supporting Information shows representative plots of reciprocal square capacitance versus the applied potential for n-Si/Au and n-Si/Au/FcHT interfaces submerged in a deaerated 0.1 M HClO4 solution. Qualitatively, the two sets of data are nearly identical. The calculations for extracting the various parameters is detailed in the Supporting Information, Section S2, and a table summarizing the results is presented in Table 1. We estimated Efb of the n-Si/Au and n-Si/Au/FcHT samples to be −0.36(±0.05) and −0.37(±0.01) V vs SCE, respectively (all data reported as mean ± s; n ≥ 3). Using the slope of the linear portion of the C–2 vs E plot, we estimated the charge carrier densities, Nd, to be 9(±3) × 1014 and 7(±4) × 1014 cm–3 for n-Si/Au and n-Si/Au/FcHT samples, respectively. These carrier concentrations correspond to a wafer resistivity of ≈5 Ω·cm (estimated using reference57), which is consistent with the manufacturer’s stated value (1–5 Ω·cm). We estimated Ecb to be −0.62(±0.05) and −0.64(±0.02) V and Evb to be 0.48(±0.05) and 0.46(±0.02) for samples without and with the SAM, respectively. None of the values are statistically different at the 95% confidence limit (two-tailed Student's t test).

Table 1. Summary of the MS Figures of Merita.

sample Efb (V) vs SCE Nd (cm–3) Evb (V) vs SCE Ecb (V) vs SCE
n-Si/Au –0.36(±0.05) 9(±3) × 1014 0.48(±0.05) –0.62(±0.05)
n-Si/Au/FcHT –0.37(±0.01) 7(±4) × 1014 0.46(±0.02) –0.64(±0.02)
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a

Measurements are presented as the average of three individually prepared samples with the uncertainty represented by one standard deviation.

The data above provide an estimate of the band energetics of the semiconductor/metal and semiconductor/metal/molecule interfaces. The first important result is that FcHT binding does not appear to influence the energetics of the semiconductor/metal junction; each figure of merit is statistically similar when comparing n-Si/Au and n-Si/Au/FcHT interfaces. This result is surprising given that Fc-containing SAMs are known to alter the work function of metallic Au films.58 We suspect that the consistent Efb, Evb, and Ecb values are observed because the potential of the redox molecule is close to the valence band edge.59 The second important result is that for all applied potentials greater than Efb (approximately −0.36 V), n-Si will be in depletion (and hence photoactive). Given that E0 is approximately 0.3 V for FcHT on Au, the electrochemistry should be light-addressable. The third important observation is that Efb, Ecb, and Evb are all significantly more positive than our previously reported n-Si/Au photoelectrodes.48,49 In our previous studies, we characterized the band energetics in neutral (or near-neutral) pH solutions containing 0.1 M KNO3 and found samples to have Efb of approximately −0.7 V vs SCE.48,49,59 We suspect that the shift in flat-band potential may be caused by a shifting of the Si bands with pH, since the Si band positions shift toward more positive values in acidic solutions.60

Electrochemical Behavior of n-Si/Au/FcHT Electrodes

As described in detail elsewhere,4,61 a SAM containing a well-behaved redox-active species should display several distinct characteristics: (i) the oxidation and reduction waves should have Gaussian shapes with peak currents that are directly dependent on the scan rate; (ii) there should be no difference in the anodic and cathodic peak potentials (i.e.; ΔEp = Ep,a – Ep,c ≈ 0 mV); (iii) the full width at half-maximum of the peaks should be 90.6 mV; (iv) only one wave should be observed in the forward and reverse scans. In general, these characteristics can be observed when the redox-active sites do not interact with each other in the SAM and when the heterogeneous electron transfer (HET) kinetics are fast compared with the CV scan rate.

We used CV to characterize the electrochemical behavior of n-Si/Au/FcHT electrodes in the presence and absence of light to determine if the electrochemistry was activated by light. Figure 4a shows CVs of a n-Si/Au/FcHT electrode immersed in a 0.1 M HClO4 solution in the absence and presence of ≈80 mW cm–2 white light at 0.1 V s–1. CVs collected in the dark (black trace) show negligible current, which is consistent with n-Si being in depletion. Under illumination, electron–hole pairs are generated within the Si. The holes are transported to the Au surface due to band bending induced by the metal particles and react with the FcHT bound to the Au surface. The generation, transport, and collection of photogenerated carriers effectively “turns on” the electrochemical response of the n-Si/Au/FcHT electrodes, as seen in the red trace in Figure 4a. Qualitatively, the illuminated CV shows characteristics of a surface-bound redox species, notably Gaussian peak shapes with very little separation between anodic and cathodic peak potentials. ΔEp between the anodic and cathodic scans was ≈2 mV for this sample at 0.1 V s–1, which is very close to the theoretical value of 0 mV and suggests fast, near-reversible charge transfer from the n-Si/Au to the surface-bound Fc. The FWHM value of the anodic peak was 93 mV at 0.1 V s–1, which is also consistent with fast kinetics. The position of the redox wave is approximately −0.05 V vs SCE, less than the standard reduction potential of FcHT on Au (≈0.3 V vs SCE). The cathodic shift in potential is consistent with the semiconductor being in depletion and is caused by energy from the absorbed light shifting the electrode potential to more anodic values than those applied by the potentiostat.

Figure 4.

Figure 4

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(a) CVs of n-Si/Au/FcHT electrodes in 0.1 M HClO4 at 0.1 V s–1 in the absence (black trace) and presence (red trace) of ≈80 mW cm–2 illumination. (b) Plot of peak current versus scan rate for n-Si/Au/FcHT electrodes. The data points and error bars represent the mean and the 95% confidence interval, respectively, of four independent samples.

In some samples (estimated as ≈1/3 of hundreds of samples fabricated over more than 6 months), we observe peak splitting at slower scan rates (v ≤ 0.1 V s–1; representative examples in Figure S4). Split peaks are often observed on ferrocene-containing SAMs formed on electrodes, and they are typically attributed to intermolecular reactions between adjacent Fc molecules.61,62 It is unclear why only some of our samples display this behavior, but we suspect that inhomogeneities in the electrodeposition and/or SAM formation are the cause. We discuss nonideal responses in more detail below.

We also prepared SAMs on p+-Si/Au electrodes, which are highly doped and not photoactive (Figure S5 in the Supporting Information). The data in Figure S5 are qualitatively similar to Figure 4a, but the peaks are sharper and the currents are larger. We suspect that the differences between samples prepared with n-Si and p+-Si are caused by differences in the metal morphology of the two samples leading to changes in the interactions between FcHT molecules in the SAM. We used the difference in anodic peak position between the illuminated n-Si samples and the p+-Si samples to estimate the photovoltage (Voc) of the n-Si/Au/FcHT junctions, which is approximately 0.35 V. This value is consistent with our previous results using similar n-Si/Au photoelectrodes.48,49 Recently, we observed that samples have consistent photovoltage when E0 of the redox species is more positive than the potential of the valence band edge.59

We performed several control experiments to confirm that the observed voltammetry was due to the FcHT SAM attaching to the Au NPs. First, we performed CV at several different scan rates between 0.1 and 1.0 V s–1. Figure 4b shows a plot of anodic peak current versus scan rate current for n-Si/Au/FcHT samples. The relationship between the current and scan rate is linear (R2 = 0.997), confirming that the FcHT is attached to the surface.63 Next, we incubated freshly etched n-Si electrodes (without Au) in 1 mM FcHT and subsequently in 30 mM MCH. Figure S6a presents CVs of these electrodes in 0.1 M HClO4. The electrodes show a small peak around −0.2 V, suggesting that the thiol can react with the freshly etched Si surface, albeit to a much lesser extent than when Au is present (Figure S6b). Interestingly, the peak position for the FcHT on freshly etched n-Si is approximately 0.15 V more cathodic than observed on the n-Si/Au/FcHT samples, suggesting that Voc is determined by the energy difference between the n-Si and FcHT in the absence of Au. Recent studies have shown that SAMs can form spontaneously between freshly etched Si and thiols (and disulfides).51 As an additional control experiment, we polarized freshly etched n-Si electrodes in the electrodeposition bath without the HAuCl4 and incubated them sequentially in FcHT and MCH. We observed only background redox activity after incubating the prepolarized electrodes in the thiol solutions (Figure S7). Next, we prepared SAMs containing only MCH on n-Si/Au. Figure S8 shows CVs of a n-Si/Au/MCH electrode in 0.1 M HClO4 in the presence and absence of illumination. These electrodes show almost no electrochemical activity, confirming that the supporting electrolyte and the backfilling MCH are not the cause of the observed voltammetry. Finally, Figure S9 shows CVs of n-Si/Au/FcHT electrodes in 0.1 M HClO4 before (Figure S9a) and after (Figure S9b) vortex mixing in DI water. We hypothesized that vigorous vortex mixing would dislodge any weakly bound FcHT molecules. Figure S9 shows only a small decrease in current after vortex mixing, suggesting that the SAM is strongly tethered to the Au surfaces.

To confirm that the FcHT is forming a monolayer on the surface, we determined the charge passed during oxidation by integrating the anodic scan of the CV and dividing by the scan rate. The surface coverage was determined using the following equation:

graphic file with name la4c01751_m001.jpg 1

where Γ is the surface coverage of the molecule (in mol cm–2), q is the charge passed during the oxidation wave (in C), n is the number of electrons transferred in the redox reaction (=1), A is the electrochemically active surface area (ECSA; in cm2), and F is Faraday’s constant (= 96,485 C mol–1). We employed ECSA because the electrodes consist of high surface area Au NPs, which increase the geometric area by ≈2–3 times, depending on the electrodeposition conditions (vide infra). ECSA was measured by performing illuminated CV in 0.5 M H2SO4 over the potential range from −0.2 to 1.6 V vs SCE and integrating the Au oxide reduction peak around 0.5 V on the cathodic sweep (see Figure S10 in the Supporting Information for example CVs in H2SO4). The charge under the oxide reduction peak was converted to surface area using the well-known conversion factor (= 390 μC cm–2 Au).64 We measured surface coverages of 4(±2) × 10–10 mol cm–2 (mean ± s; n = 3 independent samples). Assuming that Fc is close-packed with a diameter of 0.66 nm, the theoretical monolayer coverage for ferrocene on Au is ≈4.5 × 10–10 mol cm–2,24 which is in excellent agreement with our samples.

Factors that Impact the Formation of FcHT SAMs on n-Si/Au Photoelectrodes

We next sought to understand how three preparation factors impact SAM formation and thus the observed electrochemical response. We investigated (1) the role of FcHT concentration for a fixed time, (2) the concentration of HAuCl4 in the electrodeposition bath, and (3) the incubation time at a fixed FcHT concentration.

Effect of the FcHT Concentration during Incubation

To determine how the concentration of FcHT impacted the electrochemical response, we prepared n-Si/Au/FcHT electrodes by incubating n-Si/Au electrodes in 1 or 2 mM solutions of FcHT for 60 min followed by a 90 min incubation in MCH. Figure S11 shows CVs of SAMs containing 1 mM FcHT (black) and 2 mM FcHT (red) tested in 0.1 M HClO4. CVs for samples prepared with 2 mm FcHT show a large increase in peak current compared with samples prepared with 1 mM FcHT. These samples also show evidence of a diffusional current (i.e., the current does not return to baseline at the anodic limit). The surface coverage of samples prepared with 2 mM FcHT is 2.2(±0.4) × 10–9 mol cm–2, approximately 5× larger than the films prepared with 1 mM FcHT. This surface coverage is also 5× larger than the theoretical monolayer coverage for Fc on Au, suggesting that multilayer films are formed under these conditions. Previous work has shown that self-assembled multilayers are formed for Fc-terminated alkanethiols when the concentration of the Fc-alkanethiols is elevated during incubation.65,66 Because higher FcHT concentrations produced multilayer films, we performed the remaining studies using 1 mM FcHT in the incubation solution.

Effect of HAuCl4 Concentration in the Electrodeposition Solution

Next, we investigated how the concentration of HAuCl4 in the electrodeposition solution influenced the voltammetry of the SAM. Note that in all previously discussed data, the concentration of HAuCl4 in the electrodeposition bath was 0.2 mM. Based on our previous work using n-Si/Pt electrodes, we suspected that changing the concentration of the metal precursor would change the morphology and loading of the metal NPs.67 Recently, Lazenby and co-workers showed that surface roughness and morphology of Au electrodes and microelectrodes had a dramatic impact on the performance of electrochemical aptamer-based sensors based on SAMs of DNA aptamers.68 We prepared electrodes by electrodepositing Au from an electrolyte containing 0.2, 0.5, or 1 mM HAuCl4 (along with 0.1 M K2SO4, 1 mM KCl, and 1 mM H2SO4) for 5 min at −1.945 V vs SCE. The electrodes were subsequently incubated sequentially in 1 mM FcHT for 60 min followed by 30 mM MCH for 90 min.

Figure 5 shows representative CVs of samples prepared with 0.2 mM (black), 0.5 mM (red), and 1 mM (blue) HAuCl4. Qualitatively, we observe that the peak current, peak position, and peak separation all increase as the concentration increases. Representative figures of merit extracted from the data in Figure 5 are summarized in Table 2. As the concentration of HAuCl4 in the electrodeposition solution increases, we observe a statistically significant increase in ECSA between 0.2 and 0.5 mM/1 mM, but there was no difference in ECSA between 0.5 and 1 mM (Student’s t test, 95% confidence). This suggests that the surface is saturated with Au particles when 0.5 or 1 mM HAuCl4 is used in the electrodeposition bath. From each voltammogram, we calculated the surface coverage of Fc using eq 1 and found that all samples were consistent with monolayer formation (i.e., the true value of a monolayer, 4.5 × 10–10 mol cm–2, falls within the 95% confidence interval). However, the mean surface coverage for samples prepared with 1 mM HAuCl4 was ≈50% larger than the true value of a monolayer. We also observed a shift in E1/2 from 3(±11) to 82(±44) mV as the HAuCl4 concentration increased from 0.2 to 1 mM. We suspect that this shift is caused by the thickness of the Au layer increasing, leading to less light being absorbed by the semiconductor and in turn, decreased photovoltage.60

Table 2. Summary of the CV and AFM Figures of Merit Obtained with n-Si/Au/FcHT Photoelectrodesa.
[Au] (mM) ECSA (cm2) Γ (mol cm–2) E1/2(V) vs SCE ΔEp (mV) FWHM (mV) dparticle(nm) RMS roughness (nm)
0.2 0.14 ± 0.03 4(±2) × 10–10 0.003 ± 0.011 12 ± 4 89 ± 11 45 ± 10 6.6
0.5 0.20 ± 0.02 4(±4) × 10–10 0.029 ± 0.017 29 ± 11 86 ± 13 47 ± 12 10.4
1 0.20 ± 0.05 6(±4) × 10–10 0.082 ± 0.044 41 ± 16 50 ± 22 61 ± 26 10.6
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a

Values are reported as the mean ± standard deviation of three independently prepared samples from CVs recorded at 0.4 V s–1 in 0.1 M HClO4.

We observed an increase in the ΔEp (from 12 ± 4 to 41 ± 16 mV) and a decrease in FWHM (from 89 ± 11 to 50 ± 22) as the gold concentration increased from 0.2 to 1 mM. Both changes suggest that the monolayers are becoming more disordered with increasing Au concentration. When samples are prepared with 0.2 mM HAuCl4 in the electrodeposition bath (as shown in Figures 4 and 5 and Table 2), the samples are nearly ideal, but at higher concentrations, they deviate significantly from ideal behavior.

Comparing the data from Figure 5 with the CV in Figure 4, it is immediately clear that there are multiple redox environments present when higher concentrations of HAuCl4 are present in the electrodeposition bath. To qualitatively understand the behavior of these multiple redox environments, we deconvoluted the anodic traces of the three CVs present in Figure 5 using a Gaussian–Lorentzian peak fitting protocol described by Lee et al.69 Briefly, capacitance was subtracted from the data using a linear background subtraction and the data was fit with two peaks, a Gaussian peak at more cathodic potentials and a Lorentzian peak at more positive potentials. No limits were placed on peak widths, heights, or positions. Figure S12 shows the deconvoluted voltammograms along with the background subtracted experimental data and Table S1 presents the peak parameters for each subpeak. In all cases, we observed excellent agreement between the sum of the component peaks and the experimental data. Peak 1 is present at more cathodic potentials and has an FWHM of approximately 90 mV (range 87–99 mV). Peak 2 is present at more anodic potentials and has a narrower FWHM (range: 28–38 mV). In all three cases, peak 1 had a much larger area than peak 2 (2.3–3.6 × ). The fits show an anodic shift in the peak centers of each subpeak and an increase in peak current with increasing HAuCl4 concentration (also observable in Figure 5). We note that while deconvolution can provide some additional insight into the nature of the chemical environment, these results should not be viewed as quantitative. One significant drawback of this approach is that a fit for a given set of experimental data can converge with vastly different subpeaks. Figure S13 shows three examples of fits obtained for the same set of data with vastly differing subpeaks.

To further understand the photoelectrochemical data, we performed AFM topography imaging of the surfaces to characterize changes in morphology and roughness. Figure 5b, c, and d shows representative 3 μm by 3 μm AFM height images of n-Si/Au/FcHT samples prepared with 0.2, 0.5, and 1.0 mM HAuCl4 in the electrodeposition bath, respectively. As the concentration of the HAuCl4 in the electrodeposition solution increases, the morphology of the film becomes more disordered. At low concentrations, we observe a dense array of small particles. At higher concentrations, we observe larger particles and aggregates (>100 nm) and more clustering of the particles on the surface. From the AFM images, we determined the mean particle diameter and the RMS roughness of the surfaces (summarized in Table 2). The mean particle diameter increased from 45 ± 10 to 61 ± 26 nm and the RMS roughness increased from 6.6 to 10.6 nm as the HAuCl4 concentration increased from 0.2 to 1.0 mM, respectively. The standard deviation in the particle size (10 and 26 nm for 0.2 and 1.0 mM HAuCl4, respectively) and RMS roughness both suggest that the surfaces are becoming more disordered with increasing HAuCl4 concentration.

Effect of FcHT Incubation Time

We next investigated how the incubation time impacted the voltammetry of the n-Si/Au/FcHT photoelectrodes. In these experiments, we incubated the n-Si/Au electrodes in aqueous solutions of 1 mM FcHT for 15, 30, or 60 min followed by a 90 min incubation in 30 mM MCH. Representative CVs for these samples are shown in Figure 6. For samples incubated for 15 min (black trace), there are three redox signals apparent: two irreversible waves at −0.04 and 0.01 V and a quasi-reversible oxidation peak at 0.05 V vs SCE. The peak at 0.05 V has a height of approximately 5.6 μA and a ΔEp of approximately 25 mV. After 30 min (red trace), the two waves at −0.4 and 0.01 V combine to form a quasi-reversible peak around 0.02 V with a second larger peak at 0.06 V. Both anodic peaks have corresponding reduction peaks with similar peak separations (ΔEp ≈ 24 and ≈23 mV for 0.02 and 0.06 V peaks, respectively). After 60 min (blue trace), only a single quasi-reversible peak is evident at 0.01 V with ΔEp ≈ 14 mV. As the FcHT incubation time increases from 15 to 60 min, the FcHT surface coverage (Γ) increases from 1.3 × 10–10 to 4.5 × 10–10 mol cm–2, demonstrating that the growth of the monolayer is incomplete at short times. These data present a “snap shot” of the growth and organization of the FcHT SAMs and suggest that the FcHT requires significant time to form an organized monolayer. At shorter time scales, multiple peaks are observed, suggesting different chemical environments for the surface-bound Fc moieties. However, after 60 min, the monolayer is sufficiently organized to yield a single redox peak. As previously mentioned, ≈1/3 of the samples prepared with optimized conditions (0.2 mM HAuCl4 in the electrodeposition solution and 1 mM FcHT incubated for 60 min) still exhibit multiple peaks. It is plausible that these samples may have required additional time for the SAM to organize.

We performed peak fitting analysis of the data in Figure 6 (vide supra) where a Gaussian–Lorentzian fit of the data was used to qualitatively understand the changes in each subpeak.69 We obtained suitable fits of the experimental data using three peaks for the 15 and 30 min incubation times, but only two peaks for the 60 min incubation time. Fits of the data and a summary of the results are presented in Figure S14 and Table S2, respectively.

Summary of Preparation Effects

Taken together, the data in Figures S5, S6, and S10 suggest that the chemical environment of the Fc moiety is dramatically impacted by preparation conditions. As shown in Figures 2 and 5, our samples are composed of Au NPs, with a heterogeneous distribution of size and shape, which are randomly dispersed on the surface of Si (100). Perhaps unsurprisingly, this disorder can lead to nonideal voltammetry—especially when higher concentrations of HAuCl4 and shorter FcHT incubation times are used to prepare the SAM. Previously, Nijhuis and co-workers performed detailed spectroscopic and electrochemical measurements along with molecular dynamics simulations to untangle how disorder in Fc-containing SAMs on metallic Au impacted the electrochemistry.61 Their findings (and others’)69 suggest that for SAMs where Fc is separated from the Au surface by six methylene groups (the case for this study), multiple redox peaks originate from interactions between Fc and the electrolyte, Fc and other Fc molecules, and Fc buried in the SAM (and thus shielded from the electrolyte). We suspect that for the rough, disordered surfaces used herein, the SAM is likely to be less well-ordered, leading to more intermolecular interactions between Fc moieties. This hypothesis is supported by the work of Landis and co-workers, who investigated redox-active SAM formation on nanoporous Au, finding that changes in the porosity of the nanoporous Au had substantial effects on the intermolecular interactions between Fc molecules in the SAMs.62 Both of these studies were performed with metallic Au electrodes, and therefore, no photoeffects were considered. In the samples prepared here, variations in the photovoltage are expected due to variations in Au nanoparticle size. The Boettcher group has characterized such variations in photovoltage on other semiconductor/metal systems for water-splitting applications.70 Ciampi and co-workers observed that for Fc-containing SAMs on Si, nonideal behavior (multiple peaks and/or FWHM not equal to ≈90 mV) originates from variations in the photovoltage across the semiconductor surface.71 Interestingly, Ciampi’s group observed that peak splitting was dependent on illumination intensity (vida infra).

We hypothesize that the heterogeneity of the Au layer can contribute to nonideal behavior in two ways: first, by contributing to the disorder in the SAM as discussed by Nijhuis et al.,61 Landis et al.,62 Lee et al.,69 and Dempsey et al.72 Second, the distribution of NP sizes is also likely to affect Voc across the semiconductor/metal junction.70,73,74 We explore this in more detail below.

Impact of Light Intensity on n-Si/Au/FcHT Electrode Behavior

Recent work has shown that the illumination intensity impacts the kinetics and photovoltage of ferrocene-containing redox-active SAMs attached directly to n-Si. Gooding and co-workers showed that n-Si modified with monolayers of Fc displayed a cathodic shift of ≈160 mV under illumination intensities ranging from ≈3 to 90 mW cm–2.28 Concurrently, the standard electron transfer rate constant increased by a factor of ≈3 with the same range of intensities. The number of photogenerated holes is thought to directly influence the rate of the reaction in two ways: first, oxidation is limited by the number of available holes, and increasing the intensity increases the number of holes. Second, the driving force for hole transfer from Si to Fc is increased with increasing intensity. More recently, Ciampi and co-workers observed peak splitting using similar n-Si/Fc electrodes illuminated at low intensities (≈1 mW cm–2). This peak splitting was attributed to variations in the local photovoltage due to the n-Si surface and was not present at illumination intensities >10 mW cm–2.71 A fundamental difference between those previous studies and those herein is that to oxidize ferrocene, holes must first be transferred from Si to Au before being transferred to Fc. This experimental arrangement decouples the solid-state interfacial charge transfer from the molecular HET.

To understand how illumination intensity impacts the kinetics of the n-Si/Au/FcHT samples, we performed CV at scan rates between 0.25 and 25 V s–1 under illumination intensities ranging from 0.81 to 77 mW cm–2. Based on the results of the optimization experiments described above, we prepared samples by electrodepositing Au onto freshly etched n-Si from a 0.2 mM HAuCl4 solution and subsequently incubating the electrodes in 1 mM FcHT solutions for 60 min followed by a 90 min incubation in MCH. Figure 7a shows CVs of n-Si/Au/FcHT electrodes in 0.1 M HClO4 acquired using 0.81, 7.8, 25, and 77 mW cm–2 light at 0.25 V s–1. CVs collected at various scan rates for the four different intensities are presented in Figure S15 in the Supporting Information. Qualitatively, decreasing the illumination intensity shifted the voltammograms toward more anodic potentials, increased ΔEp, and broadened the FWHM of the voltammograms. Figure 7b shows the effect of illumination intensity on the peak current for 0.25 V s–1 (black circles) and 25 V s–1 (red squares). For each scan rate, there was no statistical difference between the means (p > 0.05; one-way ANOVA with Bonferroni correction). This behavior was expected because the magnitude of the peak is governed by the amount of FcHT adsorbed to the Au surfaces, in addition to the nature of the kinetics of the electrochemical reaction (i.e., reversible redox waves will have larger peaks than quasi-reversible or irreversible waves). At the lowest illumination intensities, we observed that the anodic traces became limited by light when probed at higher scan rates (Figure S15c,d).

Figure 7.

Figure 7

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Changing the illumination intensity has an impact on the kinetics of HET reactions on n-Si/Au/FcHT electrodes. (a) CVs of n-Si/Au/FcHT in 0.1 M HClO4 collected with 0.81 (orange trace), 7.8 (blue trace), 25 (red trace), and 77 (black trace) mW cm–2 white light. CVs were recorded at 0.25 V s–1 in an electrolyte containing 0.1 M HClO4. Plots showing the effect of illumination intensity on (b) peak current, (c) peak position (E1/2), and (d) peak potential separation (ΔEp). *p < 0.05; **p < 0.01; ***p < 0.001.

Figure 7c shows that the peak position (E1/2) shifts toward more cathodic potentials with increasing intensity, consistent with results from the Gooding and Ciampi groups.28,71 There was no change in E1/2 with the scan rate. The shift in peak position is caused by changes in Voc of the n-Si/Au/FcHT electrodes; as the illumination intensity increases, more carriers are generated in the semiconductor, which changes the splitting of the electron and hole quasi-Fermi levels.60 The quasi-Fermi level splitting will be increased with increasing intensity, leading to larger Voc. The magnitude of the change in Voc is approximately 100 mV over the range of intensities studied here. The oxidation peaks for the highest (77 mW cm–2; black trace) and lowest intensities (0.81 mW cm–2; orange trace) are separated along the potential axis. One possible application of this observation is the opportunity to use light intensity to multiplex electrochemical sensors with identical redox tags, although more thorough investigations are required.

Figure 7d shows the impact of illumination intensity on ΔEp, which is related to the electron transfer rate. At 0.25 V s–1, ΔEp is consistent between ≈8 and 77 mW cm–2 but broadens significantly at 0.8 mW cm–2. At 25 V s–1, ΔEp increases significantly between 8 and 77 mW cm–2 (note that above 1 V s–1, a limiting current was observed on the anodic trace under 0.8 mW cm–2 illumination). These data demonstrate that the electron transfer kinetics are impacted by the illumination intensity, with higher intensities leading to faster kinetics. However, these effects are most pronounced when higher scan rates are used. We suspect that higher scan rates are required to observe these effects with n-Si/Au/FcHT photoelectrodes because both the charge transfer between n-Si/Au and the electrochemical kinetics of FcHT oxidation on Au are each extremely fast.

As previously mentioned, Ciampi and co-workers observed peak splitting at low illumination intensities that were attributed to photovoltage variations across the interface.71 While we did not observe peak splitting, we did observe peak broadening with decreasing intensity. The FWHM of samples increased from 92 to 134 mV as the illumination intensity decreased from 77 to 0.81 mW cm–2. Given that the SAM is unlikely to be reorganized on the time scale of the experiments (∼2 s), it is plausible that the broadening we observe may be caused by variations in Voc across the surface. This scenario seems especially likely given the distribution of Au particle sizes on these samples (Figure 2).

Conclusions

Here, we investigated the photoelectrochemistry of redox-active SAMs on semiconductor/metal junctions, where the chemical modification occurs at the metal site instead of the semiconductor. We prepared n-Si/Au photoelectrodes using a benchtop electrodeposition procedure and subsequently formed mixed SAMs consisting of FcHT and MCH by sequential incubation in aqueous solutions of each thiol. We investigated how three preparation factors (FcHT concentration, FcHT incubation time, and Au electrodeposition precursor concentration) impact SAM formation and thus the observed electrochemical response. We characterized the sensors using physical and electrochemical methods, finding that under certain conditions, nearly ideal cyclic voltammetry for a surface-confined redox molecule was obtained. We also investigated the effect of illumination intensity on the observed response, finding that higher illumination intensity improves the HET kinetics for FcHT oxidation.

Under certain conditions (e.g., when the HAuCl4 or FcHT concentrations are high or when the incubation time is <1 h), we observed irregular voltammetry characterized by multiple redox peaks, which originate from variations in the chemical environment(s) of the surface-bound ferrocene moieties. We suspect the differences come from variations in the SAM ordering,61 Fc–Fc interactions,62 and photovoltage.71 Unfortunately, the measurements presented here do not definitively suggest one mechanism over another and these samples will require more careful study with scanning probe techniques to characterize the relative importance of each variable. These data highlight how caution should be exercised when fabricating these types of sensors, because under many conditions the voltammetry is not ideal.

The measurements presented herein are the first to demonstrate the photoelectrochemistry of a semiconductor/metal/molecule interface. The results are significant because they open up new avenues for future inquiry. First, because the interfaces have excellent photoelectrochemical behavior, they can be used as a test bed for future studies of electron transfer at semiconductor/metal interfaces. While this study investigated a relatively simple redox system (FcHT/MCH), there is considerable scope for expansion to probe the effect of linker length, counterion, redox mediator, etc. Second, the formation of the SAM using Au–S chemistry can be extended to other systems including biomolecules, catalysts, etc., which may enable new applications in semiconductor/metal photoelectrochemistry.

Acknowledgments

We would like to thank the US National Science Foundation (NSF CBET-1944432) and the Research Corporation for Science Advancement (Cottrell Scholar) for supporting our research. G.D.O. thanks the NSF Major Research Instrumentation Program (CHE-2215861), which provided support for the AFM used in this study. The authors acknowledge Dr. Xu Feng at the Surface Analysis Facility at the University of Delaware for XPS measurements. The XPS was sponsored by a grant from the US National Science Foundation Major Research Instrumentation Program (CHE-1428149). Any opinions, findings, conclusions, or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.4c01751.

  • Additional XPS scans; electrochemical impedance spectroscopy (EIS) characterization; additional CV data and control experiments; deconvolution of redox waves containing multiple redox environments; and additional CVs of variable light intensity data (PDF)

The authors declare no competing financial interest.

Supplementary Material

la4c01751_si_001.pdf (709.1KB, pdf)

References

  1. Wrighton M. S. Surface Functionalization of Electrodes with Molecular Reagents. Science 1986, 231 (4733), 32–37. 10.1126/science.231.4733.32. [DOI] [PubMed] [Google Scholar]
  2. Murray R. W. Chemically Modified Electrodes. Acc. Chem. Res. 1980, 13 (5), 135–141. 10.1021/ar50149a002. [DOI] [Google Scholar]
  3. Love J. C.; Estroff L. A.; Kriebel J. K.; Nuzzo R. G.; Whitesides G. M. Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105 (4), 1103–1170. 10.1021/cr0300789. [DOI] [PubMed] [Google Scholar]
  4. Eckermann A. L.; Feld D. J.; Shaw J. A.; Meade T. J. Electrochemistry of Redox-Active Self-Assembled Monolayers. Coord. Chem. Rev. 2010, 254 (15–16), 1769–1802. 10.1016/j.ccr.2009.12.023. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Chaki N. K.; Vijayamohanan K. Self-Assembled Monolayers as a Tunable Platform for Biosensor Applications. Biosens. Bioelectron. 2002, 17 (1–2), 1–12. 10.1016/S0956-5663(01)00277-9. [DOI] [PubMed] [Google Scholar]
  6. Beitollahi H.; Khalilzadeh M. A.; Tajik S.; Safaei M.; Zhang K.; Jang H. W.; Shokouhimehr M. Recent Advances in Applications of Voltammetric Sensors Modified with Ferrocene and Its Derivatives. ACS Omega 2020, 5 (5), 2049–2059. 10.1021/acsomega.9b03788. [DOI] [PMC free article] [PubMed] [Google Scholar]
  7. Mayall R. M.; Renaud-Young M.; Gawron E.; Luong S.; Creager S.; Birss V. I. Enhanced Signal Amplification in a Toll-like Receptor-4 Biosensor Utilizing Ferrocene-Terminated Mixed Monolayers. ACS Sens. 2019, 4 (1), 143–151. 10.1021/acssensors.8b01069. [DOI] [PubMed] [Google Scholar]
  8. Wu Y.; Belmonte I.; Sykes K. S.; Xiao Y.; White R. J. Perspective on the Future Role of Aptamers in Analytical Chemistry. Anal. Chem. 2019, 91 (24), 15335–15344. 10.1021/acs.analchem.9b03853. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Schoukroun-Barnes L. R.; Macazo F. C.; Gutierrez B.; Lottermoser J.; Liu J.; White R. J. Reagentless, Structure-Switching, Electrochemical Aptamer-Based Sensors. Annual Review of Analytical Chemistry 2016, 9 (1), 163–181. 10.1146/annurev-anchem-071015-041446. [DOI] [PMC free article] [PubMed] [Google Scholar]
  10. Lubin A. a; Plaxco K. W. Folding-Based Electrochemical Biosensors: The Case for Responsive Nucleic Acid Architectures. Acc. Chem. Res. 2010, 43 (4), 496–505. 10.1021/ar900165x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  11. Blakemore J. D.; Gupta A.; Warren J. J.; Brunschwig B. S.; Gray H. B. Noncovalent Immobilization of Electrocatalysts on Carbon Electrodes for Fuel Production. J. Am. Chem. Soc. 2013, 135 (49), 18288–18291. 10.1021/ja4099609. [DOI] [PubMed] [Google Scholar]
  12. Loget G.; Mériadec C.; Dorcet V.; Fabre B.; Vacher A.; Fryars S.; Ababou-Girard S. Tailoring the Photoelectrochemistry of Catalytical Metal-Insulator-Semiconductor (MIS) Photoanodes by a Dissolution Method. Nat. Commun. 2019, 10, 3522. 10.1038/s41467-019-11432-1. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Aroonratsameruang P.; Pattanasattayavong P.; Dorcet V.; Mériadec C.; Ababou-Girard S.; Fryars S.; Loget G. Structure–Property Relationships in Redox-Derivatized Metal–Insulator–Semiconductor (MIS) Photoanodes. J. Phys. Chem. C 2020, 124 (47), 25907–25916. 10.1021/acs.jpcc.0c08971. [DOI] [Google Scholar]
  14. Feng Y.; Teo W.; Siow K.; Gao Z.; Tan K.; Hsieh A. Corrosion Protection of Copper by a Self-Assembled Monolayer of Alkanethiol. J. Electrochem. Soc. 1997, 144 (1), 55–64. 10.1149/1.1837365. [DOI] [Google Scholar]
  15. Xiang D.; Wang X.; Jia C.; Lee T.; Guo X. Molecular-Scale Electronics: From Concept to Function. Chem. Rev. 2016, 116 (7), 4318–4440. 10.1021/acs.chemrev.5b00680. [DOI] [PubMed] [Google Scholar]
  16. Vilan A.; Cahen D. Chemical Modification of Semiconductor Surfaces for Molecular Electronics. Chem. Rev. 2017, 117, 4624–4666. 10.1021/acs.chemrev.6b00746. [DOI] [PubMed] [Google Scholar]
  17. McCreery R. L. Carbon-Based Molecular Junctions for Practical Molecular Electronics. Acc. Chem. Res. 2022, 55, 2766. 10.1021/acs.accounts.2c00401. [DOI] [PubMed] [Google Scholar]
  18. Tao N. J. Probing Potential-Tuned Resonant Tunneling through Redox Molecules with Scanning Tunneling Microscopy. Phys. Rev. Lett. 1996, 76, 4066–4069. 10.1103/PhysRevLett.76.4066. [DOI] [PubMed] [Google Scholar]
  19. Li Y.; Wang H.; Wang Z.; Qiao Y.; Ulstrup J.; Chen H. Y.; Zhou G.; Tao N. Transition from Stochastic Events to Deterministic Ensemble Average in Electron Transfer Reactions Revealed by Single-Molecule Conductance Measurement. Proc. Natl. Acad. Sci. U.S.A. 2019, 116 (9), 3407–3412. 10.1073/pnas.1814825116. [DOI] [PMC free article] [PubMed] [Google Scholar]
  20. Kumar A.; Biebuyck H. A.; Whitesides G. M. Patterning Self-Assembled Monolayers: Applications in Materials Science. Langmuir 1994, 10 (5), 1498–1511. 10.1021/la00017a030. [DOI] [Google Scholar]
  21. Singh M.; Kaur N.; Comini E. Role of Self-Assembled Monolayers in Electronic Devices. J. Mater. Chem. C 2020, 8, 3938–3955. 10.1039/D0TC00388C. [DOI] [Google Scholar]
  22. Heimel G.; Romaner L.; Zojer E.; Bredas J.-L. The Interface Energetics of Self-Assembled Monolayers on Metals. Acc. Chem. Res. 2008, 41 (6), 721–729. 10.1021/ar700284q. [DOI] [PubMed] [Google Scholar]
  23. Chidsey C. E. D. Free Energy and Temperature Dependence of Electron Transfer at the Metal-Electrolye Interface. Science 1991, 251, 919–922. 10.1126/science.251.4996.919. [DOI] [PubMed] [Google Scholar]
  24. Chidsey C. E. D.; Bertozzi C. R.; Putvinski T. M.; Mujsce A. M. Coadsorption of Ferrocene-Terminated and Unsubstituted Alkanethiols on Gold: Electroactive Self-Assembled Monolayers. J. Am. Chem. Soc. 1990, 112 (11), 4301–4306. 10.1021/ja00167a028. [DOI] [Google Scholar]
  25. Sachs S. B.; Dudek S. P.; Hsung R. P.; Sita L. R.; Smalley J. F.; Newton M. D.; Feldberg S. W.; Chidsey C. E. D. Rates of Interfacial Electron Transfer through π-Conjugated Spacers. J. Am. Chem. Soc. 1997, 119 (43), 10563–10564. 10.1021/ja972244y. [DOI] [Google Scholar]
  26. Smalley J. F.; Finklea H. O.; Chidsey C. E. D.; Linford M. R.; Creager S. E.; Ferraris J. P.; Chalfant K.; Zawodzinsk T.; Feldberg S. W.; Newton M. D. Heterogeneous Electron-Transfer Kinetics for Ruthenium and Ferrocene Redox Moieties through Alkanethiol Monolayers on Gold. J. Am. Chem. Soc. 2003, 125 (7), 2004–2013. 10.1021/ja028458j. [DOI] [PubMed] [Google Scholar]
  27. Yang Y.; Ciampi S.; Choudhury M. H.; Gooding J. J. Light Activated Electrochemistry: Light Intensity and pH Dependence on Electrochemical Performance of Anthraquinone Derivatized Silicon. J. Phys. Chem. C 2016, 120, 2874–2882. 10.1021/acs.jpcc.5b12097. [DOI] [Google Scholar]
  28. Yang Y.; Ciampi S.; Gooding J. J. Coupled Thermodynamic and Kinetic Changes in the Electrochemistry of Ferrocenyl Monolayers Induced by Light. Langmuir 2017, 33, 2497–2503. 10.1021/acs.langmuir.6b04106. [DOI] [PubMed] [Google Scholar]
  29. Ahmad S.; Ciampi S.; Parker S.; Goncales V. R.; Gooding J. J. Forming Ferrocenyl Self-Assembled Monolayers on Si(100) Electrodes with Different Alkyl Chain Lengths for Electron Transfer Studies. ChemElectroChem. 2019, 6, 211–220. 10.1002/celc.201800717. [DOI] [Google Scholar]
  30. Vogel Y. B.; Zhang L.; Darwish N.; Gonçales V. R.; Le Brun A.; Gooding J. J.; Molina A.; Wallace G. G.; Coote M. L.; Gonzalez J.; Ciampi S. Reproducible Flaws Unveil Electrostatic Aspects of Semiconductor Electrochemistry. Nat. Commun. 2017, 8, 2066. 10.1038/s41467-017-02091-1. [DOI] [PMC free article] [PubMed] [Google Scholar]
  31. Vasquez R.; Waelder J.; Liu Y.; Bartels H.; Maldonado S. A Gauss’s Law Analysis of Redox Active Adsorbates on Semiconductor Electrodes: The Charging and Faradaic Currents Are Not Independent. Proc. Natl. Acad. Sci. U. S. A. 2022, 119 (36), e2202395119 10.1073/pnas.2202395119. [DOI] [PMC free article] [PubMed] [Google Scholar]
  32. Laviron E. General expression of the linear potential sweep voltammogram in the case of diffusionless electrochemical systems. J. Electroanal. Chem. 1979, 101, 19–28. 10.1016/S0022-0728(79)80075-3. [DOI] [Google Scholar]
  33. Smith C. P.; White H. S. Theory of the Interfacial Potential Distribution and Reversible Voltammetric Response of Electrodes Coated with Electroactive Molecular Films. Anal. Chem. 1992, 64 (20), 2398–2405. 10.1021/ac00044a017. [DOI] [PubMed] [Google Scholar]
  34. Tender L.; Carter M. T.; Murray R. W. Cyclic Voltammetric Analysis of Ferrocene Alkanethiol Monolayer Electrode Kinetics Based on Marcus Theory. Anal. Chem. 1994, 66 (19), 3173–3181. 10.1021/ac00091a028. [DOI] [Google Scholar]
  35. Santangelo P. G.; Lieberman M.; Lewis N. S. Cyclic Voltammetry of Semiconductor Photoelectrodes III: A Comparison of Experiment and Theory for n -Si and p -Si Electrodes. J. Phys. Chem. B 1998, 102 (24), 4731–4738. 10.1021/jp980073t. [DOI] [Google Scholar]
  36. Gooding J. J.; Ciampi S. The Molecular Level Modification of Surfaces: From Self-Assembled Monolayers to Complex Molecular Assemblies. Chem. Soc. Rev. 2011, 40 (5), 2704. 10.1039/c0cs00139b. [DOI] [PubMed] [Google Scholar]
  37. Aragonès A. C.; Darwish N.; Ciampi S.; Sanz F.; Gooding J. J.; Díez-Pérez I. Single-Molecule Electrical Contacts on Silicon Electrodes under Ambient Conditions. Nat. Commun. 2017, 8 (1), 15056. 10.1038/ncomms15056. [DOI] [PMC free article] [PubMed] [Google Scholar]
  38. Fabre B. Functionalization of Oxide-Free Silicon Surfaces with Redox-Active Assemblies. Chem. Rev. 2016, 116, 4808–4849. 10.1021/acs.chemrev.5b00595. [DOI] [PubMed] [Google Scholar]
  39. Li T.; Dief E. M.; Lyu X.; Rahpeima S.; Ciampi S.; Darwish N. Nanoscale Silicon Oxide Reduces Electron Transfer Kinetics of Surface-Bound Ferrocene Monolayers on Silicon. J. Phys. Chem. C 2021, 125 (50), 27763–27770. 10.1021/acs.jpcc.1c07788. [DOI] [Google Scholar]
  40. Zhang Z.; Yates J. T. Band Bending in Semiconductors: Chemical and Physical Consequences at Surfaces and Interfaces. Chem. Rev. 2012, 112, 5520–5551. 10.1021/cr3000626. [DOI] [PubMed] [Google Scholar]
  41. Tung R. T. The Physics and Chemistry of the Schottky Barrier Height. Applied Physics Reviews 2014, 1, 011304 10.1063/1.4858400. [DOI] [Google Scholar]
  42. Sze S. M.; Ng K. K.. Physics of Semiconductor Devices; 3rd ed., Hoboken N. J.; Wiley-Interscience: 2007. [Google Scholar]
  43. Esposito D. V.; Levin I.; Moffat T. P.; Talin A. A. H2 Evolution at Si-Based Metal-Insulator-Semiconductor Photoelectrodes Enhanced by Inversion Channel Charge Collection and H Spillover. Nature materials 2013, 12 (6), 562–568. 10.1038/nmat3626. [DOI] [PubMed] [Google Scholar]
  44. Scheuermann A. G.; Lawrence J. P.; Kemp K. W.; Ito T.; Walsh A.; Chidsey C. E. D.; Hurley P. K.; McIntyre P. C. Design Principles for Maximizing Photovoltage in Metal-Oxide-Protected Water-Splitting Photoanodes. Nat. Mater. 2016, 15, 99–105. 10.1038/nmat4451. [DOI] [PubMed] [Google Scholar]
  45. Lin F.; Boettcher S. W. Adaptive Semiconductor/Electrocatalyst Junctions in Water-Splitting Photoanodes. Nature materials 2014, 13 (1), 81–86. 10.1038/nmat3811. [DOI] [PubMed] [Google Scholar]
  46. Prod’Homme P.; Maroun F.; Cortès R.; Allongue P. Electrochemical Growth of Ultraflat Au(111) Epitaxial Buffer Layers on H-Si(111). Appl. Phys. Lett. 2008, 93 (17), 21–24. 10.1063/1.3006064. [DOI] [Google Scholar]
  47. Chen Q.; Switzer J. A. Photoelectrochemistry of Ultrathin, Semitransparent, and Catalytic Gold Films Electrodeposited Epitaxially onto n-Silicon (111). ACS Appl. Mater. Interfaces 2018, 10 (25), 21365–21371. 10.1021/acsami.8b06388. [DOI] [PubMed] [Google Scholar]
  48. Terrero Rodríguez I. M.; Borrill A. J.; Schaffer K. J.; Hernandez J. B.; O’Neil G. D. Light-Addressable Electrochemical Sensing with Electrodeposited n-Silicon/Gold Nanoparticle Schottky Junctions. Anal. Chem. 2020, 92, 11444–11452. 10.1021/acs.analchem.0c02512. [DOI] [PubMed] [Google Scholar]
  49. Arthur E. G.; Ali H.; Hussain A.; O’Neil G. D. Square-Wave Voltammetry Enables Measurement of Light-Activated Oxidations and Reductions on n-Type Semiconductor/Metal Junction Light-Addressable Electrochemical Sensors. Anal. Chem. 2023, 95 (24), 9219–9226. 10.1021/acs.analchem.3c00630. [DOI] [PMC free article] [PubMed] [Google Scholar]
  50. Dauphin-Ducharme P.; Arroyo-Currás N.; Kurnik M.; Ortega G.; Li H.; Plaxco K. W. Simulation-Based Approach to Determining Electron Transfer Rates Using Square-Wave Voltammetry. Langmuir 2017, 33 (18), 4407–4413. 10.1021/acs.langmuir.7b00359. [DOI] [PMC free article] [PubMed] [Google Scholar]
  51. Dief E. M.; Vogel Y. B.; Peiris C. R.; Le Brun A. P.; Gonçales V. R.; Ciampi S.; Reimers J. R.; Darwish N. Covalent Linkages of Molecules and Proteins to Si–H Surfaces Formed by Disulfide Reduction. Langmuir 2020, 36 (49), 14999–15009. 10.1021/acs.langmuir.0c02391. [DOI] [PubMed] [Google Scholar]
  52. Bourg M.-C.; Badia A.; Lennox R. B. Gold–Sulfur Bonding in 2D and 3D Self-Assembled Monolayers: XPS Characterization. J. Phys. Chem. B 2000, 104 (28), 6562–6567. 10.1021/jp9935337. [DOI] [Google Scholar]
  53. Grosvenor A. P.; Kobe B. A.; Biesinger M. C.; McIntyre N. S. Investigation of Multiplet Splitting of Fe 2p XPS Spectra and Bonding in Iron Compounds. Surface & Interface Analysis 2004, 36 (12), 1564–1574. 10.1002/sia.1984. [DOI] [Google Scholar]
  54. Castner D. G.; Hinds K.; Grainger D. W. X-Ray Photoelectron Spectroscopy Sulfur 2p Study of Organic Thiol and Disulfide Binding Interactions with Gold Surfaces. Langmuir 1996, 12 (21), 5083–5086. 10.1021/la960465w. [DOI] [Google Scholar]
  55. Moffat T. P.; Wheeler D.; Edelstein M. D.; Josell D. Superconformal Film Growth: Mechanism and Quantification. IBM J. Res. & Dev. 2005, 49, 19–36. 10.1147/rd.491.0019. [DOI] [Google Scholar]
  56. Gelderman K.; Lee L.; Donne S. W. Flat-Band Potential of a Semiconductor: Using the Mott–Schottky Equation. J. Chem. Educ. 2007, 84 (4), 685. 10.1021/ed084p685. [DOI] [Google Scholar]
  57. Masetti G.; Severi M.; Solmi S. Modeling of Carrier Mobility against Carrier Concentration in Arsenic-, Phosphorus-, and Boron-Doped Silicon. IEEE Trans. Electron Devices 1983, 30 (7), 764–769. 10.1109/T-ED.1983.21207. [DOI] [Google Scholar]
  58. Méndez De Leo L. P.; de la Llave E.; Scherlis D.; Williams F. J. Molecular and Electronic Structure of Electroactive Self-Assembled Monolayers. J. Chem. Phys. 2013, 138 (11), 114707 10.1063/1.4795575. [DOI] [PubMed] [Google Scholar]
  59. Hussain A.; Mancini K.; Khatib Y.; O’Neil G. D. Characterizing and Understanding the Photovoltage in N-Si/Au Light-Addressable Electrochemical Sensors. Analyst 2024, 149, 3716. 10.1039/D4AN00768A. [DOI] [PubMed] [Google Scholar]
  60. Walter M. G.; Warren E. L.; McKone J. R.; Boettcher S. W.; Mi Q.; Santori E. A.; Lewis N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446–6473. 10.1021/cr1002326. [DOI] [PubMed] [Google Scholar]
  61. Nerngchamnong N.; Thompson D.; Cao L.; Yuan L.; Jiang L.; Roemer M.; Nijhuis C. A. Nonideal Electrochemical Behavior of Ferrocenyl–Alkanethiolate SAMs Maps the Microenvironment of the Redox Unit. J. Phys. Chem. C 2015, 119 (38), 21978–21991. 10.1021/acs.jpcc.5b05137. [DOI] [Google Scholar]
  62. Patel D. A.; Chevalier R. B.; Weller A. M.; Shakespeare C. C.; Soares E. J.; Landis E. C. Porosity Effects on the Ordering and Stability of Self-Assembled Monolayers on Nanoporous Gold. J. Phys. Chem. C 2020, 124 (49), 26851–26863. 10.1021/acs.jpcc.0c08685. [DOI] [Google Scholar]
  63. Bard A. J.; Faulkner L. R.; Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons, Inc.: New York, 2001. [Google Scholar]
  64. Trasatti S.; Petrii O. A. Real Surface Area Measurements in Electrochemistry. Pure Appl. Chem. 1991, 63 (5), 711–734. 10.1351/pac199163050711. [DOI] [Google Scholar]
  65. Viana A. S.; Abrantes L. M.; Jin G.; Floate S.; Nichols R. J.; Kalaji M. Electrochemical, Spectroscopic and SPM Evidence for the Controlled Formation of Self-Assembled Monolayers and Organised Multilayers of Ferrocenyl Alkyl Thiols on Au(111). Phys. Chem. Chem. Phys. 2001, 3 (16), 3411–3419. 10.1039/b101435h. [DOI] [Google Scholar]
  66. Viana A. S.; Kalaji M.; Abrantes L. M. Electrochemical Quartz Crystal Microbalance Study of Self-Assembled Monolayers and Multilayers of Ferrocenylthiol Derivatives on Gold. Langmuir 2003, 19 (22), 9542–9544. 10.1021/la0300333. [DOI] [Google Scholar]
  67. Hernandez J. B.; Epright Z. B.; Terrero Rodríguez I. M.; O’Neil G. D. Electrodeposition Parameters Dramatically Influence the Morphology, Stability, and Performance of n-Si/Pt Light Addressable Electrochemical Sensors. ChemElectroChem. 2023, 10, e202300400 10.1002/celc.202300400. [DOI] [Google Scholar]
  68. Ritz A. J.; Stuehr O. M.; Comer D. N.; Lazenby R. A. Controlling Gold Morphology Using Electrodeposition for the Preparation of Electrochemical Aptamer-Based Sensors. ACS Appl. Bio Mater. 2024, 7, 1925. 10.1021/acsabm.3c01254. [DOI] [PubMed] [Google Scholar]
  69. Lee L. Y. S.; Sutherland T. C.; Rucareanu S.; Lennox R. B. Ferrocenylalkylthiolates as a Probe of Heterogeneity in Binary Self-Assembled Monolayers on Gold. Langmuir 2006, 22 (9), 4438–4444. 10.1021/la053317r. [DOI] [PubMed] [Google Scholar]
  70. Laskowski F. A. L.; Oener S. Z.; Nellist M. R.; Gordon A. M.; Bain D. C.; Fehrs J. L.; Boettcher S. W. Nanoscale Semiconductor/Catalyst Interfaces in Photoelectrochemistry. Nat. Mater. 2020, 69. 10.1038/s41563-019-0488-z. [DOI] [PubMed] [Google Scholar]
  71. Zhang S.; Lyu X.; Hurtado Torres C.; Darwish N.; Ciampi S. Non-Ideal Cyclic Voltammetry of Redox Monolayers on Silicon Electrodes: Peak Splitting Is Caused by Heterogeneous Photocurrents and Not by Molecular Disorder. Langmuir 2022, 38 (2), 743–750. 10.1021/acs.langmuir.1c02723. [DOI] [PubMed] [Google Scholar]
  72. Dhar D.; McKenas C. G.; Huang C.-W.; Atkin J. M.; Dempsey J. L.; Lockett M. R. Quantitative Effects of Disorder on Chemically Modified Amorphous Carbon Electrodes. ACS Appl. Energy Mater. 2020, 3 (8), 8038–8047. 10.1021/acsaem.0c01434. [DOI] [PMC free article] [PubMed] [Google Scholar]
  73. Boettcher S. W.; Strandwitz N. C.; Schierhorn M.; Lock N.; Lonergan M. C.; Stucky G. D. Tunable Electronic Interfaces between Bulk Semiconductors and Ligand-Stabilized Nanoparticle Assemblies. Nat. Mater. 2007, 6 (8), 592–596. 10.1038/nmat1943. [DOI] [PubMed] [Google Scholar]
  74. Nellist M. R.; Laskowski F. A. L.; Qiu J.; Hajibabaei H.; Sivula K.; Hamann T. W.; Boettcher S. W. Potential-Sensing Electrochemical Atomic Force Microscopy for in Operando Analysis of Water-Splitting Catalysts and Interfaces. Nature Energy 2018, 3, 46–52. 10.1038/s41560-017-0048-1. [DOI] [Google Scholar]

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