An Advanced Experiment for Studying Electron Transfer and Charge

Laboratory Experiment pubs.acs.org/jchemeduc

An Advanced Experiment for Studying Electron Transfer and Charge Storage on Surfaces Modified with Metallic Complexes Diego González-Flores* and Mavis L. Montero Centro de Electroquímica y Energía Química (CELEQ) y Escuela de Química, Universidad de Costa Rica, 2060 San José, Costa Rica S Supporting Information *

ABSTRACT: Data-storage devices can be fabricated by modifying different surfaces with metallic complexes. The required time to write information and the time that information remains is crucial for the performance of the electronic device. We illustrate this technological application of molecules by adsorbing a polyoxometalate on graphite and characterization by ac voltammetry and open circuit potential amperometry. A charge-transfer constant from the surface to the electrode of 118 s−1 and a charge-storage half-life of 42 s were calculated by these methodologies. This laboratory can be easily performed and adapted in any advanced electrochemistry or materials science laboratory.

KEYWORDS: Graduate Education-Research, Upper-Division Undergraduate, Analytical Chemistry, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulatives, Electrochemistry, Kinetics, Surface Science

S

Some previous methodologies had been developed to calculate electron-transfer constants, for example, Laviron’s treatment10 based on Butler−Volmer theory relates rate constants to overpotential. However, this treatment usually required high overpotential and sweep rates, or highly irreversible systems. In contrast, ac techniques have the advantage of easier and more extended applicability to diverse systems. Impedance describes the resistance in an ac circuit. When a circuit possesses a capacitive component, the total resistance of the system is also influenced by this element and is dependent on the frequency. Any electrochemical cell can be modeled by a Randles circuit in which each process of the cell is described by an electric element. The system used in this experiment consists of an electrochemically active electrode and can be modeled by the circuit shown in Figure 1.11 The solution

urface science is a dynamic research area in nanotechnology. Even though in some cases specialized equipment is necessary for the study of surface phenomena (such as STM or AFM1), this Journal has published articles that make surface science accessible to students. Experiments in selfassembly2,3 and study of surface properties by contact angle measurements4,5 are easily performed; however, electrochemistry experiments in surface science as a probe of molecular behavior are rarely introduced.6 Electrochemistry is a valuable characterization technique for materials used in electronics. The modification of surfaces with metallic complexes has shown the possibility of obtaining hybrid materials with application in data-storage devices with higher density of states, lower power consumption, and longer retention times.7,8 Electrochemistry constitutes a powerful tool for the study of electron-transfer processes on surfaces, making it an ideal instrument for the determination of different kinetic constants. Smestad and Grätzel,9 for example, introduced electrochemistry as a tool for studying electron-transfer processes in solar cells. With the aim of introducing advanced students in electrochemistry and material science to these methodologies, a simple experimental procedure with high conceptual content and applicability was designed. The experimental procedure combines ac voltammetry with open circuit potential amperometry (OCPA). After completion of the experiment, students will be familiar with the concepts of electron transfer, charge retention-time constants, and modeling of electrolytic cells with Randles equivalent circuits. These techniques are easily adaptable to diverse electrode systems. © XXXX American Chemical Society and Division of Chemical Education, Inc.

Figure 1. Randles circuit for the experimental system: CDL is the double-layer capacitance, Rsol is the resistance of the solution, Cads is the adsorption capacitance, and RCT is the resistance for the charge transfer.

A

dx.doi.org/10.1021/ed3004885 | J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

resistance is Rsol, the capacitance corresponding to the ionic double layer on the surface of the electrode is described by CDL, and the resistance for the electron transfer during the redox faradic process is described by RCT. It is also important to include a Cads factor that corresponds to the adsorption capacitance for the ions adding onto the interface. AC voltammetry is a fundamental technique for studying interfaces processes. The excitation signal consists of a linear dc voltage sweep with a superposed oscillating voltage of a certain frequency and amplitude (Figure 2). A back-and-forward scan

amperometry (OCPA) that overcomes the inconveniencies imposed by the capacitive currents.13 The OCPA technique requires the determination of the open circuit potential (EOCP) of the system prior to each measurement. Once the EOCP is known, the pulse sequence is applied (Figure 3). A reducing potential (ER) pulse is applied until

Figure 2. Exitation signal for ac voltammetry.

is represented in Figure 2; however, during the laboratory experiment, the scan is performed only in one direction. The response signal is automatically measured by the equipment as the admittance, which is the inverse of the impedance, normalized by the angular frequency (Y/ω). Because voltammetric currents are expected to scale with angular frequency, the normalization diminishes background effects and aids in measuring the amplitude of the ac voltammetry peaks.12 The maximum of the ac voltammetry peak corresponds to the point where the dc component of the applied potential E equals the redox potential of the electrochemical process, Eo, associated to the electron transfer in the metallic complex. The amplitude of the ac voltammetry peaks is calculated by dividing the maxima of the peak over the Ybackground. The Ybackground is obtained by tracing a line across the base of the signal and taking the value at E = Eo potential (this can be done with a sofware such as OriginPro 8). When this amplitude is plotted with respect to the measured frequency, a sigmoidal curve is obtained. The data can be modeled using the Randles circuit in Figure 1. When this model is applied at the potential were E = Eo, the current ratio Ipeak/Ibackground equals the admitance ratio Ypeak/Ybackground and eq 1 can be derived.11 Ypeak Ybackground

⎛ 2 (2πf )2 ⎞1/2 ⎜ ρ + 4k 02 ⎟ =⎜ 2 ⎟ ⎜ 1 + (2πf2) ⎟ ⎝ 4k 0 ⎠

Figure 3. Excitation signals and the corresponding current responses for the OCPA technique: td1 and td2 correspond to two different delay times and tm1 and tm2 correspond to two different measurement times.

point a, then the counter electrode is switched off for some time td1 in which the charging current decays, then the voltage is taken to VOCP and connected again at point b. The faradic current decay is measured for a time tm1 until it reaches to zero (point c). Then, the procedure is repeated for different delay times, for example, in Figure 3, td2 > td1. As the delay time is increased, the maximum current for each measurement diminishes.14 The charge for each curve can be calculated by the numerical integration of the current over the time. The maximum charge for each curve can be then approximately modeled with respect to the delay time following a first-order decay function, ln Q = −ktd + ln Q 0

(2)

where Q represents the charge density, Q0 represents the maximum current density when td = 0, td is the delay time, and k is the corresponding kinetic constant. The half-life (t1/2) can be calculated from eq 2 as ln 2/k.

(1)

In this equation, ρ is given by 1 + Cads/CDL, k0 represents the electron transfer constant, and f the frequency. Equation 1 can be used for modeling the obtained data by adjusting just two parameters. For the fabrication of an electronic device, two important criteria are how quickly the layer charges and how long the charge remains. Some chronoamperometric techniques have been used for the characterization of the charge’s decay; however, most of those techniques have the problem of the charging currents that add to the faradic decay process and affects the result of the measurements. Roth and co-workers developed a technique named open circuit potential

■

EXPERIMENTAL PROCEDURE This 4-h experiment (one lab session) was performed in a small class with students working in pairs. However, due to the difference in terms of theory and operation between ac voltammetry and OCPA, it was preferable to split the experiment into two shorter lab sessions. To prepare the electrode, a graphite pencil bar was wrapped with Teflon leaving a known area exposed for the electrochemistry. The electrode was dipped in a 0.5 mol/L phosphomolybdic acid B




Figure 4. (A) Student plot of Y/ω as a function of potential for the signal at 0.23 V. (B) Student plot of |Ypeak/Ybackground| vs frequency in log scale.

electrochemical techniques required for this characterization with teaching purposes, hence the interest in developing this experiment using phosphomolybdic acid modified graphite as the working electrode. This electrode has been described previously16 and is based on a graphite pencil bar modified with this polyoxoanion. The adsorption of polyoxoanions on graphitic surfaces has already been extensively studied17 and has many advantages in the study of electrical properties such as irreversible adsorption and reliable electrochemical behavior. Typical student ac voltammetry measurements obtained at eight different frequencies are shown in Figure 4A. When students perform this experiment, they easily observe the effect of the frequency on the shape of the voltammograms. As the frequency is increased, the baseline admittance diminishes and the amplitude of the signal decreases. When the maximum amplitude is divided by the baseline and plotted as a function of frequency, the sigmoidal curve in Figure 4B is obtained.12 The Ypeak/Ybackground ratio decreases as the frequency becomes faster than the electron transfer process. The data can be fitted to eq 1 by hand or by using a program such as Maple or OriginPro 8 (a detailed procedure for the Maple fitting is described in the Supporting Information). Even though phosphomolybdic acid exhibits three different signals, only the signal at 0.23 V was used for the analysis. Based on the parameters employed, an electron-transfer constant of 118 s−1 was directly obtained from the value of k0 calculated from the fitting of eq 1. This constant is closely related to how fast the electron transfer takes place on the surface when the redox process occurs. For example Roth and co-workers18 describe larger electron-transfer constants for zinc(II) porphyrins anchored over silicon (on the order of 104 s−1). If the modified system is viewed as a data-storage device, larger transfer constants are related to faster information writing processes, and therefore faster memory devices. In the second part of the experiment, students become familiar with chronoamperometric techniques and the concepts of charge retention and open circuit potential. The student

solution in water for 10 s. The electrode was then placed in the three-electrode cell with a 0.5 mol/L H2SO4 solution. AC Voltammetry

Using Autolab 8-series potentiostat equipment, the measurements were performed using the FRA (frequency response analyzer) interface program. Every voltammogram was obtained from 0.0 to 0.3 V for each frequency. The frequencies ranged from 1 Hz to 50,000 Hz. Open Circuit Potential Amperometry

For the OCPA, the same electrode and supporting electrolyte were used. The OCP was determined prior to the measurement. Some software packages, such as NOVA 1.6, have a tool for determining the OCP (in this case it was around 0.4 V). Once the OCP was determined, a chronoamperometric method was used for the rest of the measurements. A −0.2 V pulse was applied for 0.02 s, then the cell was switched off for a delay time, the cell was again reconnected and set to the previously calculated OCP and the current was recorded for 0.05 s (it could be also for 0.1 s). The procedure was repeated for every delay time calculating the OCP prior to each measurement. Delay times can vary from 1 to 100 s.

■

HAZARDS Phosphomolybdic acid is very hazardous in case of skin contact (corrosive), of eye contact (irritant), of ingestion, of inhalation. Inhalation of dust will produce irritation to gastrointestinal or respiratory tract, characterized by burning, sneezing and coughing. Sulfuric acid is corrosive and contact can cause severe damage to skin and eyes. Phosphomolybdic acid and H2SO4 should be handled with gloves and eye protection.

■

RESULTS AND DISCUSSION Even though the modification of silicon surfaces with metallic complexes and its corresponding electrochemical characterization have acquired an increased relevance in the last years,15 there is a lack of description of the most advanced C




means less frequent refreshing cycles and more importantly, lower energy consumption.

results of the faradic current decay measurements are shown in Figure 5. To guarantee an appropriate performance of the experiment, students have to be aware of observing a decrease in the current as the waiting time is increased.

■

SUMMARY As was demonstrated in these experiments, the combination of ac voltammetry with OPCA leads to a fully characterization of the charging and discharging process in an electro-active modified surface. These experiments constitute an easy first approach to advanced electrochemical techniques with important applications in modern research areas as nanotechnology and surface science.

■

ASSOCIATED CONTENT

S Supporting Information *

Experimental procedure for the students and instructor notes. This material is available via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: diego.gonzalezfl[email protected]. Notes

The authors declare no competing financial interest.

■

Figure 5. Current decay curves obtained by students for the phophomolybdic acid electrode at different td delay times.

ACKNOWLEDGMENTS We acknowledge CRUZA-CSIC for the funding and Rafael Rodriguez Arguedas for his contributions to the manuscript.

The integration of the obtained charge is shown in Figure 6A. The total charge delivered by the system can by calculated as the charge at 0.05 s, where all the curves reach a plateau. The modeling of the charge decay as a function of frequency using a first-order kinetics (according to eq 2) is shown in Figure 6B. The kinetic constant corresponds to 0.0165 s−1 and the half-life (t1/2) corresponds to 42 s. The maximum stored charge corresponds to Q0 (according to eq 2), which in this case has a value of 1.72 × 10−5 C/cm2. In the experiment of Roth and coworkers,18 t1/2 was on the order of 20 and 150 s for zinc(II) porphyrins over silicon. High t1/2 constants are related to systems with longer storage times. In terms of memories, this

■

REFERENCES

(1) Aumann, K.; Muyskens, K. J. C.; Sinniah, K. J. Chem. Educ. 2003, 80, 187. (2) Levine, M. J.; Schwarzt, J. A. J. Chem. Educ. 1988, 65, 638. (3) Keating, C. D.; Musick, M. D.; Keefe, M. H.; Natan, M. J. J. Chem. Educ. 1999, 76, 949. (4) Dionísio, M.; Sotomayor, J. J. Chem. Educ. 2000, 77, 59. (5) Craig, V. S. J.; Jones, A. C.; Senden, T. J. J. Chem. Educ. 2001, 78, 345. (6) Holder, G. N. J. Chem. Educ. 1999, 76, 1478.

Figure 6. (A) Integration of OCPA current transients as a function of delay time and (B) ln Q as a function of delay time. D




(7) Cummings, S. P.; Savchenko, J.; Ren, T. Coord. Chem. Rev. 2011, 255, 1587−1602. (8) Lindsey, J. S.; Bocian, D. F. Acc. Chem. Res. 2011, 44, 638. (9) Smestad, G. P.; Grätzel, M. J. Chem. Educ. 1988, 75, 752. (10) Laviron, E. J. Electroanal. Chem. 1979, 101, 19. (11) Hortholary, C.; Coudret, C. C. R. Chim. 2008, 11, 702. (12) Creager, S. E.; Wooster, T. T. Anal. Chem. 1998, 70, 4257. (13) Roth, K. M.; Dontha, N.; Dabke, R. B.; Gryko, D. T.; Clausen, C.; Lindsey, J. S.; Bocian, D. F.; Kuhr, W. G. J. Vac. Sci. Technol., B 2000, 18, 2359. (14) Roth, K. M.; Lindsey, J. S.; Bocian, S. F.; Kuhr, W. G. Langmuir 2002, 18, 4030. (15) Sánchez, A.; Urcuyo, R.; González-Flores, D.; MontalberthSmith, R.; León, C.; Pineda, L. W.; Montero, M. L. Surf. Sci. 2012, 606, 527−535. (16) Martel, D.; Sojic, N.; Kuhn, A. J. Chem. Educ. 2002, 79, 349. (17) Klemperer, W. G.; Wall, C. G. Chem. Rev. 1998, 98, 297. (18) Roth, K. M.; Yasseri, A. A.; Liu, Z.; Dabke, R. B.; Malinovskii, V.; Schweikart, K.-H.; Yu, L.; Tiznado, H.; Zaera, F.; Lindsey, J. S.; Kuhr, W. G.; Bocian, D. F. J. Am. Chem. Soc. 2003, 125, 505.

E


An Advanced Experiment for Studying Electron Transfer and Charge

Recommend Documents