NANO LETTERS
Proton Adsorption and Electrical Double-Layer Formation Inside Charged Platinum Nanochannels
2002 Vol. 2, No. 12 1433-1437
Kun-Lin Yang and Sotira Yiacoumi* Georgia Institute of Technology, 200 Bobby Dodd Way, Atlanta, Georgia 30332-0512
Costas Tsouris* Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6181 Received August 21, 2002; Revised Manuscript Received October 9, 2002
ABSTRACT This study is focused on the formation of the electrical double layer (EDL) inside uniform platinum nanochannels. Strong hysteresis of adsorption/desorption of protons observed in cyclic voltammetry experiments suggests that adsorption is controlled by ion diffusion through the charged nanochannels whereas desorption is controlled by the electrical potential. It was determined that the nanostructured platinum electrodes prepared in this study were selective for halogen ions. The sorption capacity decreased in the order of F- > Cl- > Br-.
Introduction. Understanding the electrical double-layer (EDL) formation, ion adsorption, and ion transport inside charged nanochannels is critical to many electrochemical1 and biophysical problems.2,3 It may also provide valuable knowledge for such emerging fields as nanotechnology and bionanotechnology, including self-assembly,4 molecular transport,5 and synthesis of biomacromolecules.4 However, the classical Gouy-Chapman theory describing the EDL is not applicable at the nanoscale level3 because molecular-level effects such as the adsorption of water molecules on the charged surface play important roles in the formation of the EDL. A direct study of EDL formation and adsorption of ions in nanochannels is now possible because of recently developed methods for synthesizing long-range-ordered mesoporous materials. Typical mesoporous materials with long-range ordering, such as MCM-41 (Mobil crystalline material),6 were prepared by the liquid-crystal template technique in which surfactant is added in an aqueous solution to form a self-assembly. This assembly is subsequently used as a template for the synthesis of nanostructured materials. When the reaction is completed, the template surfactant can be easily removed, yielding a high-surface-area, uniformly nanostructured material with long-range order.6 The pore structure of the mesoporous material reflects the original morphology of the surfactant assembly. For example, when the hexagonally packed phase (HI) of the surfactant is used, * Corresponding authors. E-mail:
[email protected]; Phone: (404) 894-2639; Fax: (404) 894-8266. E-mail:
[email protected]; Phone: (865) 241-3246; Fax: (865) 241-4829. 10.1021/nl0257622 CCC: $22.00 Published on Web 10/29/2002
© 2002 American Chemical Society
the product contains an array of hexagonally packed and noninteracting cylindrical nanochannels. The inner diameter of these nanochannels can be tailored by using template surfactant molecules that have different chain lengths. Mesoporous materials with a high surface area and a controllable pore-size distribution have many applications in electrochemistry, separations, and catalysis.7 Among these materials, nanostructured platinum films are of special interest because platinum is an excellent electrode material that has been used extensively in many electrochemical applications.8 A mesoporous platinum film with a welldefined long-range order of hexagonally packed cylindrical nanochannels (HI-ePt) was first prepared by the electrochemical reduction of platinum salts in the presence of a template of the nonionic surfactant octaethyleneglycol monohexadecyl ether.9-11 The pore diameter of the platinum film was found to be 2.5 nm, making it very suitable for the study of EDL formation at the nanoscale level. Another advantage of mesoporous platinum electrodes is their high surface area compared with that of the polished platinum electrodes. As a result, the electrical current response is expected to be much stronger, and the signal-to-noise ratio can be increased. This feature is very useful in the study of slow reactions and/or specific adsorption, situations that usually generate low electrical current. According to the Gouy-Chapman theory,12 the EDL is composed of the Stern and the diffuse layers. The former is a layer of immobilized ions directly in contact with the charged surface, whereas the latter is a layer of mobile ions in the solution phase, the distribution of which can be
determined by the Poisson-Boltzmann equation. It has also been demonstrated that the pore size has a strong effect on the formation of the EDL inside nanopores, in particular when the pore size is less than 20 Å. At such a small length scale, the strong overlapping of the diffuse layer is considerable for low-concentration electrolyte systems. For example, the overlapping effect may cause the exclusion of ions from nanopores (also known as Donnan exclusion),13 a finding that may have important applications in separations.14,15 Current interest in mesoporous materials, however, is focused primarily on electrochemical applications, and studies of EDL are usually carried out at high electrolyte concentrations.11 Under such conditions, the properties of the EDL are totally controlled by the Stern layer, which means that the overlapping of the diffuse layer is not significant. Thus, the purpose of this study is to investigate the behavior of the diffuse layer and to understand ion adsorption, desorption, and exclusion by charged nanochannels for a dilute system, an area that has received much less attention in the past than investigations of concentrated systems. The results of this study may have important applications in novel separation processes such as electrosorption of trace heavy-metal ions.14-16 Materials and Methods. Nanostructured Platinum Electrode. The platinum electrodes used in the experiments were prepared by the liquid-crystal template method proposed by Attard et al.9 Briefly, 29 wt % hexachloroplatinic acid (H2PtCl6), 29 wt % deionized water, and 42 wt % octaethyleneglycol monohexadecyl ether (C16EtO8) were mixed vigorously at 45 °C to make a homogeneous solution. Electroplating was then carried out at -0.1 V versus the saturated calomel electrode (SCE) for 200 s on a 1.6-mmdiameter polished gold electrode. The total current was determined to be 5.45 ( 0.12 C. After the electroplating experiment, the homemade platinum electrode was rinsed and immersed in deionized water for 3 days to remove the residual surfactant before the cyclic voltammetry experiments were conducted. Cyclic Voltammetry Experiments. All experiments were performed with a Bioanalytical Systems (BAS, West Lafayette, IN) voltammetric analyzer (CV-50W) connected to a BAS C2 cell stand. The reference electrode used was the BAS model RE-1 (Ag/AgCl electrode immersed in 3 M NaCl). The auxiliary electrode was a gauge platinum wire, and the working electrode was the homemade nanostructured platinum electrode, which was cleaned electrochemically by using 1 M sulfuric acid solution between 1.2 and -0.2 V at 200 mV s-1 for several cycles before the experiments. Aqueous solutions used in the cyclic voltammetry experiments were prepared by using E-Pure (Barnstead, Dubuque, IA) deionized water, and the electrolyte used was at least 99% pure. Nitrogen was used to purge the solution for 10 min; it then flowed continuously over the top of the solution during the experiment. Results and Discussions. Characterization of the Nanostructured Platinum Electrode. TEM images of the nanostructured platinum electrodes prepared by the liquid-crystal template technique of Attard and co-workers9 have shown a hexagonally packed structure. The diameter of each nanochan1434
Figure 1. Cyclic voltammograms recorded in a 2 M H2SO4 solution at 200 mV s-1 between -0.15 and +1.25 V (vs Ag/AgCl). (s) Nanostructured platinum electrode prepared by a liquid-crystal template method followed by electroplating of H2PtCl6 onto a polished gold electrode with a diameter of 1.6 mm; (- -, in the neighborhood of 0 µA) polished platinum electrode of the same diameter as that of the nanostructured electrode.
nel was found to be 2.5 nm, similar to that reported in the literature,9 which can be characterized as that of a mesopore. The platinum electrodes were then characterized by electroanalytical methods. Cyclic voltammograms of the nanostructured and polished platinum electrodes in 2 M H2SO4 at 200 mV s-1 from 1.25 to -0.15 V (vs Ag/AgCl) are shown in Figure 1. Because the nanostructured platinum electrode has a much higher surface area, it also has a current response approximately 100 times higher than that of the polished platinum electrode. Similar results were obtained in other studies reported in the literature11 in which the roughness factor of the nanostructured platinum electrode was determined to be 210 ( 13. The high surface area and the uniform pore structure of the nanoporous platinum electrode make it very sensitive in detecting small electrical currents that are associated with low concentrations of ions. Effects of Proton Adsorption/Desorption and DissolVed CO2. The prepared nanostructured platinum electrode was used to study the capacitance of the EDL (Cdl) and the adsorption of protons inside nanochannels. The electrolyte solution 0.01 M NaF was chosen for use in the experiment because it shows minimum specific adsorption compared with that of other electrolytes. Moreover, because the diffuse layer is more important than the Stern layer under these conditions, a low electrolyte concentration was used. Therefore, the electrical current measured is primarily the charging current of the diffuse double layer, which is small and does not surpass the electrical current that is associated with faradaic reactions. Cyclic voltammograms recorded at 1 mV s-1 in 0.01 M NaF are shown in Figure 2. The dominant rectangular-shaped curve with two peaks at the negative and positive ends is the EDL charging current. On the basis of these results, the capacitance of the EDL is estimated to be 330 µF (in the range of 0.1 to -0.4 V). In addition to the double-layer charging current, two peaks, A and B, evolving at the negative and positive scans, respectively, were also noted. It is assumed that these peaks (A and B) result from the adsorption and desorption (or reduction and oxidation) of the protons, respectively. This assumption is supported by a comparison of Figures 1 and 2 and by the calculation of the reduction potential of proton adsorption/desorption. Nano Lett., Vol. 2, No. 12, 2002
Figure 2. Cyclic voltammograms recorded in a 0.01 M NaF solution at 1 mV s-1 between -0.6 and +0.4 V (vs Ag/AgCl). The solution was covered by N2 during the experiment, and the concentration of the dissolved CO2 decreased with time. Peaks A, B, and C are shown, with the magnitudes of A and C decreasing with time and the magnitude of B increasing with time. Peaks A and B are caused by the adsorption and desorption of protons, respectively.
Figure 3. Estimation of the reduction potential of proton adsorption onto a nanostructured platinum electrode at equilibrium. Each point represents the peak current for each negative scan and its corresponding potential. The manually imposed fitting curve of these points was extrapolated to -340 mV, which is the possible reduction potential of the adsorption of protons at pH ) 6.3. This value compares favorably with the theoretical calculation of the shift of the reduction potential, -0.41 V.
Figure 1 is a well-characterized cyclic voltammogram for platinum electrodes;1 the double peak evolving at 0 V during the negative scan is known to be a result of the strong adsorption of protons (4.0 M) followed by the reduction of protons.1 In Figure 2, however, the proton concentration is only 5.0 × 10-7 M (pH ) 6.3), which results in a shift in the reduction potential. According to the Nernst equation, the reduction potential, E, associated with adsorption of protons (H+ + e- + Pt f Pt-H) can be expressed as E ) E0 - 0.059 log[H+]-1
(1)
where E° is the standard reduction potential of this reaction. On the basis of eq 1, when the proton concentration is reduced from 4.0 to 5.0 × 10-7 M, the theoretical reduction potential of adsorption will shift from 0 to -0.41 V. As shown in Figure 2, the potential corresponding to the maximum current also gradually shifts toward -0.34 V before the peak diminishes. This is better illustrated in Figure 3, in which the peak current at each negative scan is plotted versus the potential at which the peak occurs. The fitting curve of the peak current is extrapolated to an equilibrium potential of -0.34 V, which corresponds to the reduction Nano Lett., Vol. 2, No. 12, 2002
potential at the equilibrium state. This value compares favorably with the theoretical calculation of the shift of the reduction potential, -0.41 V. Therefore, it is believed that peaks A and B are caused by the adsorption and desorption of protons, respectively. Desorption peak B shows less dependency on potential compared with adsorption peak A, and apparently, it occurs at a more positive potential (i.e., -0.2 V). Meanwhile, the strong hysteresis of adsorption/desorption cannot be observed in Figure 1, suggesting that this phenomenon is a direct consequence of the nanostructure of the electrode. Because protons have to diffuse through the nanochannels to reach adsorption sites, it is assumed that adsorption inside nanochannels is a diffusion-controlled process. However, the surface of the nanochannels is fully saturated with protons before desorption begins; therefore, desorption is primarily a potential-controlled process. This point is also supported by the more negative threshold desorption potential that is observed when desorption occurs. On the basis of eq 1, the desorption peak at -0.2 V corresponds to a proton concentration of 1.6 × 10-3 M, which is 3 orders of magnitude higher than the bulk proton concentration, implying that the protons are more concentrated by a factor of 1000 inside the nanochannels when the desorption process begins. This feature suggests that the nanochannels have a very high capacitance for proton adsorption, which is probably caused by the strong pore-proton interaction inside the nanopores. Figure 2 also shows dynamic behaviors of peaks A, B, and C. For each cycle, the magnitudes of peaks A and C decrease with time whereas that of peak B increases with time. This behavior can be explained by the pH and concentration changes of dissolved CO2 in the solution. When CO2 is dissolved in the aqueous solution, a weak carbon acid is formed (CO2 + H2O T H2CO3), and protons are then released (H2CO3 T H+ + HCO3-); therefore, the initial solution is always acidic (pH ) 6.3). After the solution is purged with nitrogen gas, the pH increases until a level of 7 is reached. For example, according to pH measurements, the final pH after 20 cycles was 6.93. The increasing pH accelerates the desorption (oxidation) and hinders the adsorption (reduction) of protons. Therefore, the magnitude of peak B increases, and that of peak A decreases. The total number of protons desorbed, however, is greater than that of protons adsorbed because residual protons, which were deposited on the surface during the liquid-crystal template process using H2PtCl6, are also released. (The number of protons released from the platinum film is small compared with that of the bulk solution and will not affect the pH value.) For the same reason, the decrease in peak C is attributed to the residual Cl- of the solution (resulting from the preparation of the material) or to the decreasing OH- concentration during the experiment. After the system reaches equilibrium (about 20 cycles), peak A diminishes, and only peak B can still be observed. This feature is repeatedly observed in all experiments using the nanostructured platinum electrodes under different conditions. Ion SelectiVity and Molecular SieVe Property. To examine the effect of pore size on ion selectivity and Donnan 1435
Figure 4. Comparison of cyclic voltammograms recorded at 1 mV s-1 in three different 0.01 M electrolyte solutions. (s) NaF; (- -) NaCl; (- -) NaBr. The positive scan in the positive potential range shows that the capacitance follows the order F- > Cl- > Br-.
Figure 5. Effect of the scan rate: two cyclic voltammograms were recorded at 5 mV/s (s) and 1 mV/s (- -), respectively. The electrolyte solution used is 0.01 M NaF. Results show that the double-layer capacitance depends only slightly on the scan rate.
exclusion in the nanopores,13 three electrolytes, namely, NaF, NaCl, and NaBr (each at 0.01 M), were used. The resulting cyclic voltammograms are shown in Figure 4. Because a common ion (Na+) was used in all of the experiments, no significant differences were observed in the negative potential range. At high positive applied voltages, however, the electrical current strongly depends on the type of electrolyte, and its magnitude follows the order iNaF > iNaCl > iNaBr. This phenomenon occurs because of Donnan exclusion in the nanochannels. For larger anions such as Br-, it is more difficult to enter the charged nanochannels; therefore, the double-layer capacitance and current response are lower than those of smaller ions such as F-, which can easily diffuse into the nanochannels. This feature implies that the nanostructured platinum film used in the experiments may hold potential for applications in selective separations such as molecular sieves and can be used in electroanalytical techniques. Effect of Scan Rate. In our earlier work,17 it was assumed that the mesoporous capacitance (for pore diameter >2 nm) is independent of the scan rate at a relatively low electrolyte concentration compared with the microporous capacitance (for pore diameter
