A Terahertz-Transparent Electrochemical Cell for In Situ THz

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A Terahertz-Transparent Electrochemical Cell for In Situ THz Spectroelectrochemistry Coleen T. Nemes, John R. Swierk, and Charles A. Schmuttenmaer Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04204 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 16, 2018

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Analytical Chemistry

A Terahertz-Transparent Electrochemical Cell for In Situ THz Spectroelectrochemistry Coleen T. Nemes, John R. Swierk, Charles A. Schmuttenmaer,* Department of Chemistry and Energy Sciences Institute, Yale University, 225 Prospect Street, P.O. Box 208107, New Haven, Connecticut 06520-8107, United States ABSTRACT: Terahertz spectroscopy is broadly applicable for the study of a wide variety of materials, but spectroelectrochemistry has not been performed in the THz range due to the lack of a THz-transparent electrochemical cell. While THz-transparent electrodes do exist, they had never been utilized in a complete three-electrode cell, which is the configuration required for accurate potential control in aqueous media. We have designed and constructed a THz-transparent three-electrode electrochemical cell and have performed THz spectroelectrochemistry of a SnO2 thin film. The cell utilizes a custom-made reference electrode and tubing which allows the composition of electrolyte to be changed during an experiment. THz spectroelectrochemical measurements show a decrease in THz transmission at potentials where SnO2 conduction band states are potentiostatically filled. We also describe a simple method for measuring the uncompensated resistance and RC time constant.

Spectroelectrochemistry (SEC) simultaneously couples spectroscopic and electrochemical measurements to identify chromic changes as a function of applied electrochemical potential.1 Following its initial introduction in the visible region of the spectrum,2 it has been used in conjunction with a wide variety of different spectroscopic methods, including IR,3-5 EPR,6-8 NMR,9 and Raman.1,10 The expansion of SEC to different regions of the electromagnetic spectrum has increased the range of systems that can be studied with SEC; one simply chooses the best spectroscopic method for which the analyte spectrum changes with redox state. The task of combining electrochemical and spectroscopic methods is not trivial. For in situ measurements, an electrochemical cell must satisfy the geometric and transparency requirements of a given spectrometer.1 These requirements have led to several different SEC cell designs with varying degrees of complexity. One example is the opticallytransparent thin layer electrochemical cell (OTTLE) which utilizes a transparent conductive oxide or conductive mesh for the optically-transparent working electrode.11,12 The thin path length through the cell minimizes light absorption from the solvent and offers a short electrolysis time.13 Currently, the breadth of spectroelectrochemical methods does not include terahertz (THz) spectroscopy, despite the utility of THz radiation in studying a wide variety of materials, including biological molecules,14,15 polymers,16 and nanostructured materials.17-19 The THz region lies in the far infrared portion of the spectrum, with most studies probing 0.1 to 10 THz (3.33 to 333 cm-1). Since the transient electric fields of THz pulses are measured, material properties such as complex refractive index and complex conductivity can be obtained. One of the most important aspects of THz spectroscopy is that it is a noncontact probe of a material's conductivity with sub-picosecond temporal resolution. By coupling the ultrafast THz pulses with an optical pumppulse (a method known as optical-pump-terahertz-probe, or

OPTP, spectroscopy), changes in a sample’s THz transmission can be monitored following photoexcitation with subpicosecond resolution. Due to the sensitivity of THz radiation to mobile carriers, THz spectroscopy has been widely used as a non-contact probe of conductivity. This type of measurement has been frequently used to study carrier dynamics in materials related to energy storage and conversion. For example, OPTP measurements have been employed on semiconductor metal oxides with applications in photoelectrochemical cells, such as dye-sensitized solar cells20-25 and water-splitting dyesensitized photoelectrochemical cells.26,27 These semiconductor metal oxide systems have also been studied electrochemically,28-30 and also using spectroelectrochemistry in the visible, near-IR, or mid-IR regions of the spectrum,31-39 as well as by bias-dependent transient absorption spectroscopy.40-42 In contrast to spectroscopic probes in the visible and near-IR, THz radiation is sensitive only to mobile electrons, giving it the ability to easily distinguish between trapped and mobile electrons. Thus THz SEC provides unique and complementary insight into these important semiconductor metal oxide systems. THz spectroscopy can also be used to determine the frequency-dependent conductivity of a sample, which provides insight into the nature of mobile carriers beyond that obtained from a simple OPTP measurement of the amplitude of the transmitted THz pulse.43,44 For example, by fitting the Drude-Smith model to the measured THz conductivity, one can extract carrier density, mobility, and scattering time.45,46 The ability to electrochemically probe the conductivity in situ is a significant advance for studying photovoltaic and catalytic semiconducting oxides. For example, the electrical/conductivity properties of the film can be probed during and following an electrochemical process to better understand how said process alters the film’s attributes as an electron acceptor and transporter. THz SEC will also be useful in studying redox active proteins such as cytochrome-c. The structural motions of cytochrome-c have

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been already been probed via THz time-domain spectroscopy in an ex situ manner, by changing the oxidation state by chemical oxidiation.47 The lack of THz SEC can be attributed to the high THz reflectivity of conductive films used as electrodes. We recently demonstrated that THz-transparent electrodes can be fabricated by patterning fluorine-doped tin oxide (FTO) into a wiregrid configuration.48 In our previous work, these electrodes were used to make THz-transparent dye-sensitized solar cells. Using OPTP measurements, we observed changes in electron trapping dynamics and injection amplitude as a function of applied bias and illumination. While the counter electrode of a DSSC can act as a quasi-reference, this is not the case for electrochemical cells in general. This is especially true for those with aqueous electrolytes, and as a consequence a complete three-electrode cell is required to accurately control the potential of the material of interest. We have developed a THztransparent three-electrode electrochemical cell to enable THz SEC for a variety of materials in virtually any solvent. The THz-transparent cell is constructed without complicated glasswork and is transparent in the THz, UV, visible, and portions of the IR spectral regions. Inlet and outlet PEEK tubing (polyetheretherketone) allows the continuous flow of deoxygenated electrolyte as well as the ability to change the electrolyte composition during experiments. THz SEC is demonstrated measuring changes in THz transmission of a mesoporous SnO2 film resulting from potentiostatic filling of SnO2 conduction band states. Because our cell design is transparent in both the visible and THz range, potential-controlled OPTP experiments are also possible. The areas for improvement of a THz-transparent three-electrode cell are also discussed.

EXPERIMENTAL SECTION Cell Construction. The fully assembled THz-transparent three-electrode cell (THz cell) is depicted in Figure 1. The working electrode is 10 ohm/sqr FTO coated on fused quartz (25.4 mm x 24.5 mm x 1 mm, GM Associates, with FTO coating by Solaronix) with a 5 x 7.5 mm region that is patterned in a wire-grid configuration. Details of the FTO patterning can be found elsewhere48,49 with our modifications described in the supporting information. While a wide range of FTO wire-grid configurations are acceptable,48 we chose 8 μm wide wires spaced with 4 μm gaps. The preparation of the SnO2 paste followed a modified version50 of the method of Ito et al.51 Briefly, the 22-43 nm SnO2 particles (Alfa Aesar) were first hydrated with water and acetic acid. Ethanol was added, and the SnO2 particles were sonicated three times with an ultrason-

icate horn (Branson 450 with ¼ inch tip, 4 minutes with 50% duty cycle on power level 4). Following ultrasonication, αterpineol was added, followed by additional ultrasonication (3x). A solution of ethyl cellulose in ethanol was added, followed by an additional round of ultrasonication (3x). The ethanol was removed by rotary evaporation to yield a paste that was applied by doctor-blading, using Scotch Magic tape as a spacer. The resulting films had a thickness of 3 μm, as measured by profilometry. The counter electrode (Figure 1, B) is prepared from a 10 ohm/sqr FTO coated fused quartz. Two 1.5 mm holes are drilled into the counter electrode plate for the eventual insertion of PEEK tubing (Supelco, 1/16 in OD, 0.03 ID). A 2.75 mm hole is also drilled for the eventual insertion of the reference electrode. The bottom ~2/3 of the FTO is removed via etching with Zn/HCl (details in the supporting information). The primary purpose of removing the FTO is to allow the transmission of THz radiation through the counter electrode, as a continuous sheet of FTO is highly reflective. The counter electrode is then cleaned and platinized by coating with 8 wt% H2Cl6Pt*6H2O/ethanol solution, followed by heating to 450 °C for 30 minutes. The reference electrode is prepared using inexpensive materials since each is permanently epoxied into the cell. A 2.5 cmlong silver wire is first sanded, then placed in concentrated HNO3 for 30 seconds. Following a rinse with water, the Ag wire is electrochemically coated with AgCl in 0.1 M HCl (aq) by holding at a potential 50 mV more anodic than the open circuit voltage for 100 minutes.52 The reference electrode casing comprises a 2 cm-long glass tube (3 mm OD and 1.5 mm ID) with a porous frit (2.8 mm CoralPor, Bio-Logic) that is fixed at the end of the glass tube with PTFE (polytetrafluoroethelene) heat shrink tubing. The reference electrode casing and PEEK tubing are inserted such that they are flush with the front of the substrate and epoxied in place with Loctite epoxy resin (EA 1C). After complete curing of the epoxy, the Ag/AgCl wire is added to the reference electrode casing along with a saturated NaCl solution. The Ag wire is sealed into place with epoxy, and the entire counter electrode plate is immediately immersed in saturated NaCl solution for 24 hours to soak the frit. The counter electrode and working electrode are sealed together with a 60 μm hot melt spacer (Surlyn, Solaronix) by applying mild pressure to the device on a 200 oC hot plate. The resulting inner cell thickness is approximately 60 µm (measured to be 60.45 ± 0.34 µm with profilometry). The electrolyte solution is then immediately injected into the cell. For storage, the cell is filled with water and the open ends of the PEEK tubes are epoxied closed to prevent the reference electrode from drying out. Cells can last for at least two months

Figure 1. THz cell design. A) Working electrode is FTO on a 1 x 25.4 x 25.4 mm fused quartz substrate. A film (gray) is placed on a 5 x 7.5 mm patterned region of FTO consisting of FTO lines that are 8 μm wide wires and spaced by 4 μm. B) Counter electrode is platinized FTO on fused quartz. The Ag/AgCl reference electrode and PEEK tubing inlet and outlet are flush with the surface of the counter electrode plate. C) The assembled cell viewed fromACS direction of incoming THz probe beam, the area of which is represented by the white2 Paragon Plus Environment circle. The working electrode is flipped such that the FTO is inside the cell. The counter and working electrodes are separated by a Surlyn spacer. D) Side view of the assembled cell, not drawn to scale. Parts A-C are drawn to scale.

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Analytical Chemistry when stored in this manner. THz Spectroelectrochemistry. The basics of THz spectroscopy are described in the supporting information and details of the THz spectrometer are reported elsewhere.48 Briefly, 35 fs pulses of 800 nm light are divided into a THz generation beam and THz detection beam. For THz generation, the 800 nm pulses are focused through a 0.100 mm BBO crystal to a spot that forms a plasma. THz pulses emerge from the plasma, which are collimated and then focused using off-axis parabolic mirrors to a 1/e2 diameter of 1.3 mm at the sample position. After passing through the sample, the THz beam is collimated and refocused onto a ZnTe detector crystal, where it coproprogates with the 800 nm read-out beam and is measured using free-space electrooptic sampling.53 The THz cell is mounted with a filter holder that blocks only the bottom ~5 mm of the cell (Figure S-8). For highest THz transmission, the THz cell is oriented such that the FTO wires of the working electrode are perpendicular to the polarization of the THz beam.48 During THz SEC measurements, de-oxygenated 0.1 M KNO3 (aq) electrolyte solution is continuously flowed through the cell using a nitrogen pressure differential (described in the supporting information). For measurements where the maximum transmitted THz amplitude is monitored as a function of applied potential, the THz amplitude and current are acquired simultaneously using an external trigger to initiate data collection for both the potentiostat (Bio-Logic SP-50 with EC-Lab software), which measures current and potential, and the lockin amplifier (Stanford Research, SR830), which measures the transmitted THz amplitude. For THz measurements at fixed potentials, the potential is held constant for at least 1 minute prior to the THz measurement.

pump delay is measured at different applied potentials. Figure 2 displays the high transmittance in the visible and near IR range and also reasonable transmittance in the THz range. Figure 2B shows the transmission through a THz cell filled with water or air. The decrease in transmittance when the cell is filled with water illustrates the need to minimize the path length through the electrolyte. We also note that the optical density of the THz cell does not change significantly when water is replaced with 0.5 M KNO3 (aq) electrolyte. Since patterned FTO is THz-transparent, the transmittance of the air filled THz cell is attributed solely to reflection and absorption losses of the two quartz substrates.

RESULTS AND DISCUSSION Cell Design Validation. Some feel that the use of epoxy and adhesives in an electrochemical cell is a disadvantage, either due to the inability to disassemble the cell54,55 or the possibility of contamination from adhesive materials.56,57 However, for efficient THz transmission through water, the path length must be as short as possible. In other thin cell designs,58 Teflon gaskets have been used to achieve a thin path length without the use of epoxy. While these types of cell designs allow for the possibility of disassembly, they require some degree of machining and engineering to make them leak-free. While our use of a hot melt spacer makes disassembly of the cell difficult, it offers a simple method of achieving short path lengths on the order of tens of microns and therefore high THz transmission. In addition, we find the difficulty in disassembling to be acceptable for a few reasons: 1. The patterned electrodes coated with thin films are not reused, as is there is no guarantee that all of the film can be removed; 2. The use of epoxy and adhesives keeps the design free of complicated glasswork and machining; 3. Each sample resides in its own THz cell, allowing one to measure several samples without having to disassemble and reassemble the cell. If disassembly of the cell is required, it is possible to pry apart the working and counter electrode plates with a razor blade. We also note that the THz cell design uses an epoxy and adhesive that are insoluble in water and chemically resistant to most organic solvents. The THz cell shown in Figure 1 was designed to be transparent in the THz and visible regions to permit both THz SEC, where the THz spectrum can be monitored as a function of potential, as well as potential-controlled OPTP, where the maximum transmitted THz amplitude as a function of optical

Figure 2. A) Percent transmittance of water-filled THz cell over the visible light range. B) Percent power transmitted through the THz cell either filled with water or air.

The reproducibility of the custom-made Ag/AgCl sat. NaCl reference electrode was confirmed with cyclic voltammetry of potassium ferricyanide solutions (Figure S-10). The redox potential was measured to be 214 ± 4 mV versus Ag/AgCl sat. NaCl, which corresponds to 414 ± 4 mV versus NHE. This is within range of the expected potential of 408-430 mV versus NHE.59 The reference electrode is placed in the path of electrolyte flow in attempt to increase the ability to remove bubbles from the surface of the frit.60 The proximity of the reference electrode to the working electrode serves to minimize the uncompensated resistance, or ohmic drop, which is known to be problematic in thin layer cells.57,61-63 In the assembled cell, the counter electrode edge is offset by at least 1 mm from the working electrode edge. This, along with the direction of electrolyte flow that places the CE downstream, prevents products of the counter electrode from migrating to the working electrode.60 The electrochemically active surface area of platinized FTO electrodes was determined by calculating the double layer capacitance64 (additional details in the supporting information). Using a specific capacitance of 17 μF/cm2,65 the electrochemically active area is approximately 3 times that of the geometric area. For the configuration shown in Figure 1, the geometric area of 0.88 cm2


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would correspond to an electrochemically active area of 2.6 cm2. Experiments with alkaline pyrogallol66 were performed to confirm that our cells remain anaerobic with flow of deoxygenated electrolyte (Figures S-12 to S-14). Ohmic Drop and RC Time Constant. THz transparency requirements demand a short path length through the cell. The THz cell can therefore be classified as a thin layer cell, and shares attributes with other thin cell designs that are in the 10100 μm thickness range. Thin layer cells have the advantage of fast bulk electrolysis due to small cell volume, and ideal thin layer cells exhibit symmetric cyclic voltammograms with no peak separation.67 There are two common concerns with thin layer cells that cause deviations from ideal thin layer cell behavior. One is the so-called edge-effect 56,57,62,68 which refers to the analyte diffusing to the working electrode during experiments. The main consequence of the edge-effect is increased peak separation in cyclic voltammograms. This is of no concern in a THz cell measuring solid state analytes, as the film is adhered to the working electrode. The second common concern of thin layer cells is the uncompensated resistance, also referred to as the iR drop, or ohmic drop. The ohmic drop is primarily determined by the solution resistance between the working and reference electrode, however resistance in the working electrode itself also contributes.69 The ohmic drop causes the actual working electrode potential to deviate from the applied potential by

Eactual = Eapplied – iRu

(1)

where i is the current and Ru is the uncompensated resistance. The ohmic drop is usually negligible for cells with aqueous electrolytes, as these are sufficiently conductive. In thin layer cells, however, the geometry of the cell causes the resistance to be high despite having a relatively high solution conductivity. We minimize the ohmic drop in our cell design by placing the reference electrode as close to the working electrode as is reasonably possible. The area of the WE is also minimized in order to maintain the potential across the WE as uniform as possible: points furthest from the reference will experience a greater ohmic drop than the points nearest the reference. This leads to peak broadening in CVs, as demonstrated by Figure S-15. Values of the uncompensated resistance were obtained by fitting equation 2 to the measured current versus time during a linear potential sweep in a purely capacitive region.69

1

(2)

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Figure 3. Current vs. time for linear sweeps (800 mV/s, 0.2 V to 0.3 V vs Ag/AgCl) of a representative THz cell with two concentrations of KNO3 (not de-oxygenated). Red lines are fits of equation 2.

In equation 2, υ is the scan rate, Cdl is the double-layer capacitance, and t0 is the start time of the linear sweep after being held at the initial potential for a few seconds. This method can be used to measure the uncompensated resistance and doublelayer capacitance of each THz cell prior to, or after, experiments. Figure 3 shows the dependence of Ru on electrolyte concentration. While the double-layer capacitance is unaffected, the Ru nearly triples. Both the Ru and double-layer capacitance influence the RC time constant, which dictates the current-response time of the cell. Given a 12 ms RC time constant, as is the case with the 0.1 M KNO3 electrolyte, it will take 36 ms (3RC) for the current to rise to 95% of the final current during a linear potential sweep. The double layer capacitance and RC time constant are minimized by using a working electrode with as small area as possible. Ru can also be minimized by increasing the electrolyte conductivity, as is demonstrated in Figure 3. Additional information regarding the RC time constant measurements is presented in the supporting information. THz Spectroelectrochemistry of SnO2. To demonstrate the utility of the new THz cell, THz SEC was performed on a SnO2 thin film. We chose to demonstrate both potentiodynamic and constant potential measurements. For potentiodynamic measurements, we collect cyclic voltammograms while simultaneously monitoring the transmitted THz peak amplitude. This type of measurement is rapid in comparison to constant potential measurements, where it takes 3.5 minutes to collect the full THz pulse at each selected potential. Because of the rapid acquisition time, potentiodynamic measurements are preferred for samples without a significant frequency dependence. This is because monitoring the changes in the transmitted amplitude does not provide frequency information, but instead monitors the overall average of the frequencies contained in the THz pulse. Figure 4 displays the transmitted THz peak amplitude as the potential is swept from 0.2 V to -0.6 V vs Ag/AgCl at three different scan rates. For these measurements, the THz peak amplitude is monitored over several CV cycles, and the average is taken only after the CVs have become stable (Figure S18). Figure 4 displays the THz peak amplitude averaged over several cycles.


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Analytical Chemistry of the measured potential from the actual potential increases, since the current, and therefore ohmic drop, is greater for faster scan rates. While it is possible to correct for Ru, it is also sufficient to use the overlap of the THz peak amplitude in the forward and reverse scans as an indicator for uncompensated resistance effects, as well as to identify an upper limit for scan rate. If faster scan rates are desired, an appropriate method of compensating for Ru must be utilized. The data shown in Figure 4 can also be viewed as a function of time. Figure 5 shows the transmitted THz peak amplitude as a function of time for the 10 mV/s cyclic voltammogram shown in Figure 4. The maximum dip of the THz peak amplitude is very close to the negative turning potential, as shown by the dashed lines. It is also apparent that the signal to noise ratio is sufficient such that averaging is not required to collect meaningful data for changes of this magnitude.

Figure 4. Transmitted THz peak amplitude (A) and current (B) during cyclic voltammetry sweeps. The 3nd cycle of each cyclic voltammogram is shown. The THz peak amplitude is the average over all cycles, which are shown in Figure S-18.

Since THz radiation is reflected/absorbed by mobile carriers, the transmitted THz peak amplitude decreases as a function of potentiostatic filling of SnO2 conduction band states. Note that the THz peak amplitude remains constant until the potential is more negative than -240 mV, despite an onset of cathodic current starting near -100 mV. Since THz radiation is reflected/absorbed only by mobile electrons, it follows that the anodic current in the region between -100 mV and -240 mV results from potentiostatic filling of trap states. For nanostructured SnO2 films, the presence of trap states at energies below the CB is expected.70 The current onset is also consistent with the SnO2 conduction band potential at pH 7.27,70 Measurements of a blank cell confirms that: 1. The THz response is only present with the SnO2 film and 2. Anaerobic conditions are achieved, as seen by the lack of oxygen reduction current in contrast to measurements without de-oxygenation (Figures S20 to S-22). Three scan rates are shown in Figure 4 to illustrate the effect of uncompensated resistance on the shape of the THz peak amplitude vs. potential curve. Since THz attenuation depends on the density of mobile electrons, we expect the THz peak amplitude to depend solely on the Fermi level of the SnO2 film. Since the Fermi level is determined by the applied potential, the THz attenuation at a given potential should be the same regardless of scan rate or scan direction. For the 10 mV/s measurement, the THz attenuation is essentially the same for the forward and reverse scans, as expected. For the 20 mV/s and 50 mV/s measurements, we observe deviations from the expected behavior, which increase with increasing scan rate. These deviations are an effect of uncompensated resistance, which causes the actual electrode potential to deviate from the measured potential. As the scan rate is increased, the deviation

Figure 5. Transmitted THz peak amplitude and applied potential as a function of time during 10 mV/s cyclic voltammetry sweep.

In addition to measuring the transmitted THz peak amplitude in potentiodynamic measurements, the entire transmitted THz waveform was collected at various fixed potentials. Figure 6A displays these results over the bias potential range of +0.1 to 0.5 V vs Ag/AgCl. As expected, the transmitted THz waveform is identical for +0.1, -0.1, and -0.2 V. While trap state filling has occurred at -0.2 V, there is no THz attenuation since electrons in these states are not mobile. At -0.3 V, however, SnO2 conduction band states are populated, which attenuates the THz pulse. As the magnitude of the negative potential increases, a higher carrier density of electrons populates the conduction band, leading to an increase in THz attenuation. Figure 6B shows the change in optical density (ΔOD) over the range of 0.3 to 2.5 THz. The presence of mobile electrons results in a broad ΔOD feature, which is expected given that mobile electrons have a broad absorption over the THz range.43


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reference, care should be used for long experiment times as the Ag/AgCl electrode can leak water into the non-aqueous electrolyte and serve as a contamination source.69 To avoid this, a Ag/Ag+ pseudo-reference can be fabricated using the same process described above except that a bare silver wire is used, and the sat. NaCl solution is replaced with non-aqueous electrolyte.71 The potential of the pseudo-reference can then be calibrated with an internal standard, such as ferrocene, during or after the experiment.71-73 This thin-layer THz cell also may be used to determine the frequency-dependent THz conductivity of other materials under applied potential. While the established methodology towards extracting optical constants and conductivity of a material within the THz cell is beyond the scope of this work, the ability to perform THz spectroscopy of materials under voltage bias is a significant step forward. The THz thin-layer cell will also be used for bias-dependent optical-pump—THzprobe spectroscopy in the near future.

CONCLUSIONS

Figure 6. A) THz transmission though SnO2 film at various applied potentials. B) Change in optical density relative to the +0.1 V vs Ag/AgCl measurement.

With an Ru of approximately 6 kΩ (Table S-2) for the cell in Figure 6, the actual potential will deviate from the applied potential. For example, a 1 μA current will produce a 6 mV iR drop. Since Ru is known and the current is measured, we can use Equation 1 to determine the iR drop and actual potential during each THz measurement in Figure 6 (Table S-3). The iR drop is not significant for potentials +0.1 through -0.3 V, however is 8 mV and 16 mV for the -0.4 and -0.5 V measurements, respectively. This simple type of post processing iR drop correction can easily be applied for constant potential measurements with a stable and constant current. The relatively large uncompensated resistance of the THz cell leads to one limitation; fast scan rates are to be avoided unless methods for ohmic drop compensation are utilized. Ohmic drop compensation is an option with some potentiostats if fast scan rates are required. Even for samples without the need for potentiodynamic measurements, it is crucial to have the ability to collect reliable CVs with the THz cell. CVs are often used to locate potentials of interest, such as the trap state peak potential in the SnO2 in the present case. The trap state peak of mesoporous SnO2 films have also been observed by us and others to be dependent on sample history.70 It was therefore necessary to collect the CV of the film in the THz cell, rather than using a CV of a different film collected with a standard electrochemical cell. This is the main benefit of using in situ cells in general. Other Applications. The THz cell described here can also be used with non-aqueous electrolytes with small modifications to the reference electrode design. Though it is common to use a Ag/AgCl sat. NaCl reference electrode as a pseudo-

We have designed and constructed a THz-transparent threeelectrode electrochemical cell for THz spectroelectrochemistry. By utilizing a THz-transparent working electrode, the cell can be used for in situ THz spectroelectrochemistry with solid films. This is demonstrated with a SnO2 film under a variety of bias voltages, where THz transmission is measured as a function of potentiostatic filling of trap (non-mobile) and conduction band (mobile) states. This highlights the unique ability of THz radiation to distinguish between mobile and nonmobile electrons, an aspect useful for studies of semiconductor materials. Although the THz cell exhibits an uncompensated resistance, which is an attribute common to thin layer cells in general, it is easily measured and accounted for using constant potential measurements. Since the THz cell is also transparent in the visible spectral range, it can be used for optical-pump—THzprobe spectroscopy. Specifically, it is now possible to include applied potential as a variable in these types of experiments. In regard to the very active area of nanoparticle semiconductor materials, the ability to determine the manner in which trap state filling impacts photocarrier dynamics in these materials is of great interest.

ASSOCIATED CONTENT Supporting Information Images and additional details of THz cell construction. Procedure for etching FTO with Zn/HCl. General explanation of THz spectroscopy and details of spectroelectrochemistry setup. Cyclic voltammograms of potassium ferricyanide in the THz cell. Method of determining the electrochemically active surface area of platinized FTO. Details of alkaline pyrogallol experiments. Additional information on uncompensated resistance and RC time constant measurements. (PDF) The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author

*

[email protected], (203) 432-5049


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Analytical Chemistry

ACKNOWLEDGMENTs This work was funded by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences, under award no. DE-FG02-07ER15909 and by a generous donation from the TomKat Charitable Trust. The authors also thank Gary Brudvig, Robert Crabtree, Kelly Materna, Brian D. McCarthy, and Michael Pegis for helpful discussions.

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