Fabrication and Characterization of Nanostructured Pd Hydride pH

Nov 25, 2005 - School of Chemistry, University of Southampton, Southampton, SO17 .... Bard, A. J., Mirkin, M. V., Eds.; Dekker: New York, 2001; pp 397...
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Anal. Chem. 2006, 78, 265-271

Fabrication and Characterization of Nanostructured Pd Hydride pH Microelectrodes Toru Imokawa,† Kirsty-Jo Williams,‡ and Guy Denuault*,‡

School of Chemistry, University of Southampton, Southampton, SO17 1BJ, U.K., and Steel Research Laboratory, JFE Steel Corporation, 1 Kokan-cho, Fukuyama, Hiroshima 721-8510, Japan

Novel pH microsensors were made by electrodepositing mesoporous Pd films onto Pt microdisks, electrochemically loading the films with hydrogen to form the r+β Pd hydride phase, and then switching to the potentiometric mode to monitor pH. To create a nanostructure, the films were deposited within a molecular template formed by the self-assembly of surfactant molecules, a technique known as true liquid crystal templating. The films retain the micrometer size of the Pt microdisk but offer electroactive areas up to 900 times larger. Optimum hydrogen loading conditions were determined, and the mesoporous Pd microdisks were found to have excellent potentiometric properties. From pH 2 to 12, their potential was Nernstian, highly reproducible, very stable ((1.2 mV over 2 h), and without hysteresis. Their response time was better than 1 s. However, the presence of oxygen reduced their lifetime significantly, thereby requiring frequent reloading. These microelectrodes do not require calibration before and after measurements, a procedure normally essential for potentiometric pH microsensors. To our knowledge, these are the first results where nanostructured materials made by the true liquid crystal templating method have been used in the potentiometric mode; moreover, these are the first results demonstrating the application of nanostructured microdisks in the potentiometric mode.

subsequently showed that a similar methodology was possible with OH- concentrations by recording their steady-state oxidation current,10-13 and this was also used for SECM experiments.14,15 However, this amperometric approach is limited to acidic/alkaline conditions. Moreover, the current reflects the total acidity (basicity), because acid-base equilibria provide a source of H+ (OH-) to the Faradaic process. In addition, the amperometric route requires different microelectrodes depending on the pH range, Pt being the most reliable for the reduction of H+ and Au being the most reliable for the oxidation of OH-. We have also tried to monitor local pH variations by recording the SECM tip current for several pH-dependent reactions, e.g., the very low currents due to the partial formation-reduction of an oxide layer on a microdisk electrodes.8 Although qualitatively useful, none of these routes yielded quantitative results that could be translated into absolute pH. Most potentiometric pH microsensors are made with either glass,16 liquid (or polymeric) membranes,3,4 or metal oxide films.17-21 Their response is far from ideal (E-pH characteristics often vary between electrodes) and require calibrations before and after measurements.22 E-pH plots often possess different slopes in different pH regions and intercepts vary unpredictably. For example, antimony oxide microdisks21 yield a linear E-pH relationship, but it is unstable and the electrode requires regular preconditioning. The stability, accuracy, and reproducibility of the potentiometric response worsen as the sensor size decreases. This

The research reported in this article is driven by the desire to fabricate robust and reliable pH microdisk electrodes to perform spatially resolved pH measurements in scanning electrochemical microscopy1 (SECM) and determine pH gradients in crevice corrosion,2 sediments,3 or plant research.4 Previously we sought to monitor pH changes with the amperometric response of microelectrodes. H+ concentrations can be calculated from their steady-state reduction current at microdisks, and we had applied this to SECM experiments.5-9 We

(8) Yang, Y. F.; Denuault, G. J. Chem. Soc., Faraday Trans. 1996, 92, 37913798. (9) Yang, Y. F.; Denuault, G. J. Electroanal. Chem. 1998, 443, 273-282. (10) Abdelsalam, M. E.; Denuault, G.; Baldo, M. A.; Bragato, C.; Daniele, S. Electroanalysis 2001, 13, 289-294. (11) Abdelsalam, M. E.; Denuault, G.; Baldo, M. A.; Daniele, S. J. Electroanal. Chem. 1998, 449, 5-7. (12) Daniele, S.; Baldo, M. A.; Bragato, C.; Abdelsalam, M. E.; Denuault, G. Anal. Chem. 2002, 74, 3290-3296. (13) Daniele, S.; Baldo, M. A.; Bragato, C.; Denuault, G.; Abdelsalam, M. E. Anal. Chem. 1999, 71, 811-818. (14) Abdelsalam, M. E. Ph.D., University of Southampton, Southampton, 2000. (15) Liu, B.; Bard, A. J. J. Phys. Chem. B 2002, 106, 12801-12806. (16) Thomas, R. C. J. Physiol. London 1974, 238, 159. (17) Bezbaruah, A. N.; Zhang, T. C. Anal. Chem. 2002, 74, 5726-5733. (18) Hassan, S. S. M.; Marzouk, S. A. M.; Badawy, N. M. Anal. Lett. 2002, 35, 1301-1311. (19) Marzouk, S. A. M.; Ufer, S.; Buck, R. P.; Johnson, T. A.; Dunlap, L. A.; Cascio, W. E. Anal. Chem. 1998, 70, 5054-5061. (20) Wipf, D. O.; Ge, F. Y.; Spaine, T. W.; Bauer, J. E. Anal. Chem. 2000, 72, 4921-4927. (21) Horrocks, B. R.; Mirkin, M. V.; Pierce, D. T.; Bard, A. J.; Nagy, G.; Toth, K. Anal. Chem. 1993, 65, 1213-1224. (22) Denuault, G.; Nagy, G.; Toth, K. In Scanning Electrochemical Microscopy; Bard, A. J., Mirkin, M. V., Eds.; Dekker: New York, 2001; pp 397-444.

* To whom correspondence should be addressed. E-mail: [email protected]. † JFE Steel Corp. ‡ University of Southampton. (1) Bard, A. J., Mirkin, M. V., Eds. Scanning Electrochemical Microscopy; Marcel Dekker, Inc.: New York, 2001. (2) Wolfe, R. C.; Weil, K. G.; Shaw, B. A.; Pickering, H. W. J. Electrochem. Soc. 2005, 152, B82-B88. (3) Zhao, P. S.; Cai, W. J. Anal. Chim. Acta 1999, 395, 285-291. (4) Jones, J. I.; Eaton, J. W.; Hardwick, K. Aquat. Bot. 2000, 67, 191-206. (5) Frank, M. H. T.; Denuault, G. J. Electroanal. Chem. 1993, 354, 331-339. (6) Frank, M. H. T.; Denuault, G. J. Electroanal. Chem. 1994, 379, 399-406. (7) Yang, Y. F.; Denuault, G. J. Electroanal. Chem. 1996, 418, 99-107. 10.1021/ac051328j CCC: $33.50 Published on Web 11/25/2005

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is a general problem of microelectrodes operated in the potentiometric mode. Even a 10-µm-diameter Ag/AgCl microdisk does not behave as expected.23 It follows a Nernstian behavior with chloride activity but with an erratic offset. Whereas the amperometric properties of microelectrodes are known to depend on their characteristic dimension,24 the potentiometric properties are not thought to be size dependent. This may be so down to ∼50 µm where the response of the surface surpasses that of the edge, but with smaller electrodes, imperfections (cracks at the metalglass interface) become critical and seriously affect the electrode potential. To address these problems, we have sought to increase the microdisk electroactive area while retaining its original geometric dimensions. For this, we modified the microdisk with a nanostructured Pd film electrodeposited from a hexagonal liquid crystal template according to the procedure reported previously.25 A solution of amphiphiles with controlled composition and temperature adopts a liquid crystal phase, which is then used as a molecular template to direct the deposition. The template is tailored by selecting the composition from the phase diagram and the alkyl chain length of the molecule or using a swelling agent to increase the size of the basic unit. The self-assembly of liquid crystal rods is the most common molecular arrangement used and TEM characterization of the films shows deposits with a hexagonal array of cylindrical pores. The films are termed H1-e to indicate that they were electrodeposited from the H1 lyotropic liquid crystalline phase. The pore diameters reflect those of the liquid crystal structure and typically range between 2 and 5 nm. Their walls are between 2 and 5 nm thick while their depth depends on the electrochemical charge passed during deposition and can reach micrometers. The film’s electroactive area is up to 3 orders of magnitude greater than that of the bare electrode, indicating that the pores are entirely accessible to the solution. For nanostructured Pd films, the surface area-to-volume ratio, ∼107 cm2 cm-3, corresponds to a specific surface area of 91 m2 g-1 while for Pt it is of the order of 22 m2 g-1.26 The pH microsensors were produced by electrochemically charging the nanostructured palladium films with hydrogen to form the R+β Pd hydride phase, which is known to behave as a hydrogen electrode and therefore yield a Nernstian dependence of potential on pH.27 The potential of a R+β Pd hydride electrode is independent of the hydride composition (its hydrogen loading)28 and holds a steady value of +0.050 V versus RHE. Hence, R+β Pd hydride electrodes have long been used as reference and pH electrodes. Pd hydride pH sensors yield an almost theoretical E-pH response with a slope of -0.059 V/pH (25 °C). However, to our knowledge, there is only one report of a Pd hydride microdisk pH sensor,2 probably because (1) the potentiometric response is generally worse than that of large Pd hydride electrodes, (2) loading hydrogen into the tip of a Pd microdisk is slow, and (3) (23) Denuault, G.; Frank, M. H. T.; Peter, L. M. Faraday Discuss. 1992, 23-35. (24) Montenegro, M. I., Queiro´s, M. A., Daschbach, J. L., Eds. Microelectrodes: Theory and Applications; Kluwer Academic Publishers: New York, 1991. (25) Attard, G. S.; Bartlett, P. N.; Coleman, N. R. B.; Elliott, J. M.; Owen, J. R.; Wang, J. H. Science 1997, 278, 838-840. (26) Bartlett, P. N.; Gollas, B.; Guerin, S.; Marwan, J. Phys. Chem. Chem. Phys. 2002, 4, 3835-3842. (27) Ives, D. J. G.; Janz, G. J. Reference Electrodes; Academic Press: New York, 1961. (28) Lewis, F. The Palladium Hydrogen System; Academic Press: London, 1976.

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the potential is not stable as hydrogen continually diffuses down the wire. Here we show that the high surface area-to-volume ratio nanostructured Pd electrode circumvents these problems. In this article, we report the fabrication of the nanostructured microdisks, their characterization, loading with hydrogen, and study of their potentiometric response. We demonstrate a significant improvement in the potentiometric properties of microelectrodes and show the prospect of reliable micrometer size potentiometric pH probes. EXPERIMENTAL SECTION All reagents were used as received without further purification. Water (98%, Fluka), or Brij 56 (C16H33(OCH2CH2)nOH, C16EOn, n ) 4-12, Aldrich), 2 wt % heptane (99%, Lancaster), and 39 wt % purified water. Note that Brij 56 is ∼88 times less expensive than C16EO8; having observed no significant differences in performance, most experiments were run with Brij 56. Water (1.17 g) and (NH4)2PdCl4 (0.36 g) were placed in a glass container, the surfactant (1.41 g) and heptane (0.06 g) were then added, and the mixture was stirred for 15 min using a glass rod. Very viscous at room temperature, the mixture was then homogenized by stirring for 15 min at 35 °C and allowed to equilibrate at 25 °C for 2 h. The hexagonal (H1) phase was confirmed by observing the characteristic charcoal texture29 with a polarizing microscope. H1-e Pd films were deposited using a (29) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975.

Figure 1. Scanning electron micrographs of a 25-µm-diameter Pt disk before (a) and after deposition of Pd films made with (b) C16EO8, (c) Brij 56, and (d) no surfactant. The potential was stepped from +0.4 to +0.1 V vs SCE until a charge of 11 µC had passed.

two-electrode configuration with a Pt microdisk WE and an SCE as the reference/counter electrode. The plating mixture was placed at the bottom of a glass cell thermostated at 25 °C. Each microdisk was polished (Al2O3 1.0 and 0.3 µm) and cycled in 1 M H2SO4 before the deposition. Potential cycling was repeated until the voltammogram stabilized, and the electrode was repolished if it showed unusual voltammetry. The electrode was then removed from the 1 M H2SO4 solution, rinsed with purified water, left to dry, and placed in the plating mixture. The film was plated by stepping from +0.3 to + 0.1 V versus SCE until the desired charge had been passed (typically 5.5-44 µC). The electrode was returned to +0.3 V, disconnected, removed from the plating mixture, rinsed with purified water, and soaked in purified water for at least 24 h to remove the surfactant from the film. Plain Pd films (without nanostructure) were deposited on 25-µm-diameter Pt microdisks with a 40 mM (NH4)2PdCl4 plating bath.30 The WE was pretreated as described above, placed in the bath, and its potential stepped from +0.4 V versus SCE to various deposition potentials (+0.1, +0.2, +0.3 V). After 11 µC had passed, the electrode was removed from the bath and rinsed with a copious amount of water. The radius of the films was estimated from the limiting current for the reduction of Ru(NH3)63+ taken on a linear sweep voltammogram in a deaerated 10 mM Ru(NH3)6Cl3, 0.2 M KCl solution. (The equation for the steady-state current at a microdisk, i ) 4nFDca, was used with D ) 8.8 × 10-6 cm2 s-1.) pH was measured with a combination pH electrode (InLab409) and a pH meter (320), both from Mettler Toledo. Values obtained were used as standards when testing the potentiometric pH response of H1-e Pd hydride microelectrodes, and care was taken to calibrate the pH meter over the range of interest. All measurements were conducted at 25 °C. The output (mV) was recorded (30) Guerin, S. Ph.D. thesis, University of Southampton, 1999.

after it had reached a stable value (20-60 s) and then converted to pH using a calibration curve, obtained as follows. The combination pH electrode was placed in commercial buffers (pH 1.679 ( 0.030, Fluka; and pH 4.00 ( 0.01, 7.00 ( 0.01, and 10.00 ( 0.01, Aldrich) thermostated at 25 °C. The output (V) was then recorded and plotted against the buffer pH. The linearity of the pH electrode response was excellent, with a typical r2 value of 0.999 98. From the calibration curves, the pH measured with the combination pH electrode was estimated to have an error of (0.01 pH unit within the pH range (1.68-10.00). Scanning electron micrographs (SEMs) were taken with the “wet mode” of an XL30 ESEM (Philips), which allows a gaseous atmosphere in the chamber. Images were taken (25 kV, gaseous secondary electron detector, 0.5-0.6 Torr water pressure) before and after Pd deposition, taking particular attention to changes in Pd film morphology caused by lattice expansion upon hydrogen absorption. RESULTS Deposition and Characterization of the Films. Figure 1 illustrates the films formed on the microdisk electrodes. The texture is much smoother with C16EO8 than with Brij 56. With the later, SEM images show crevices and pits, some of which go all the way through the thickness of the film. A plating potential in the kinetically controlled region ensured that the texture and geometry of the films remained as shown in Figure 1. This was confirmed by systematically observing the electrodes with the SEM at all stages of production. The films were further characterized by voltammetry in acid to identify the features characteristic of surface reactions, Figure 2. In the presence of the nanostructure, the cyclic voltammograms have the distinctive voltammetric fingerprint for Pd in acid, namely, surface oxide formation/stripping, H adsorption/desorpAnalytical Chemistry, Vol. 78, No. 1, January 1, 2006

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Figure 2. Cyclic voltammograms recorded in 1 M H2SO4 at 20 mV s-1 with a 25-µm-diameter Pt disk modified with a H1-e Pd films (Brij 56) (solid line) and a plain Pd film (dashed line). Deposition charge 11 µC. (a, b) H adsorption-desorption, (c, d) H absorption-desorption for the R hydride phase, (e, f) onset of H absorption-desorption for the β hydride phase, and (g, h) oxide formation-stripping.

tion, and H absorption/desorption for the R and β hydride phases. The voltammograms obtained with the nanostructured microdisks are identical to those observed with nanostructured Pd films deposited on millimeter-size electrodes,26 indicating that deposition on the microdisk has not altered the nanostructure. Moreover, the magnitude of the peak currents suggests that the electroactive area of the films is several orders of magnitude larger than that of the original microdisk. Indeed a roughness factor of 248, Table 1, is obtained using the conversion factor of 424 µC cm-2 estimated by Rand and Woods31,32 from the charge under the oxide stripping peak, after subtraction of the double layer charge. Interestingly, the films made with C16EO8 present the same thickness under the SEM as those made with Brij 56 but, despite appearing smoother, offer an electroactive area twice as large. The important point is not the 2-fold increase in roughness between H1-e Pd (Brij 56) and H1-e Pd (C16EO8) but the 50-fold increase between the plain film and H1-e Pd (Brij 56). With both surfactants, the electroactive area increases almost linearly with the deposition charge, thus indicating that the whole of the nanostructure is accessible to solution. The plain Pd film gives a roughness factor as expected for good quality plating. As with H1-e Pt,33,34 the nanostructured Pd microelectrodes retain the diffusional characteristics of conventional microdisks. Thus, at low scan rates, the modified electrodes yield steady-state sigmoidal current-voltage curves analogous to that obtained with the bare microdisks (in contrast, at high scan rates, the modified electrodes show a significant charging current indicative of an enhanced electroactive area normally associated with much larger electrodes). The diffusion-controlled limiting current for the reduction of Ru(NH3)63+ was used to check the diameter of the film, and results were found to be in agreement with those obtained by SEM. By suitably choosing the deposition potentials in the foot of the wave rather than on the plateausto avoid the diffusion-controlled regime and restricting the deposition charge, the film covers the underlying microdisk without extending (31) Rand, D. A. J.; Woods, R. J. Electroanal. Chem. 1972, 35, 209. (32) Rand, D. A. J.; Woods, R. J. Electroanal. Chem. 1971, 31, 29. (33) Evans, S. A. G.; Elliott, J. M.; Andrews, L. M.; Bartlett, P. N.; Doyle, P. J.; Denuault, G. Anal. Chem. 2002, 74, 1322-1326. (34) Birkin, P. R.; Elliott, J. M.; Watson, Y. E. Chem. Commun. 2000, 16931694.

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significantly beyond its edge. For the thickness used in later experiments, ∼1 µm, most films protrude by 1-2 µm over the glass. Hydrogen Loading. The modified microdisks were loaded with hydrogen to form Pd hydride. This was done by stepping the potential from the double layer region (-0.2 V) to a value, EL, in the hydrogen region and holding this potential for a set time or until the current had decayed to zero. Typical current transients for loading hydrogen into the nanostructure are shown in Figure 3a-c. The curves start with a large current, which rapidly decays to a plateau whose magnitude decreases and duration increases when the loading potential becomes more positive. The current further decays to a lower plateau, which is significant only for the most negative loading potentials. To study the different chronoamperometric regimes, the transient recorded at EL ) -0.80 V was investigated further. The loading charge, Figure 3d, was calculated by integration of the current transient and compared with the stripping charge, Figure 3e. The latter was obtained by recording a stripping voltammogram after loading at -0.80 V for a set time and repeating for different loading times. The important result is that after 35 s the stripping charge remains constant whereas the loading charge increases continuously with the loading time. This indicates that H absorption is completed at the end of the first plateau on the current transient. Hence, the initial current decay is thought to correspond to double layer charging, hydrogen adsorption, and R Pd hydride formation while the first plateau is thought to reflect the completion of the β hydride phase. The second plateau is thought to correspond to hydrogen gas evolution. The precise rate of loading is not self-evident. Since the voltammetry suggests pores filled with solution, the current could reflect the insertion and transport of hydrogen within the Pd lattice. However, the transient time scale implies that this is not the case because H should diffuse through 1.4 nm, half the wall thickness of the nanostructure, under 40 ns (DH,R ∼ (2-3) × 10-7, DH,β ∼ (9-10) × 10-7 cm2 s-1).35 Moreover, the amount of H+ needed for a H/Pd of 0.6, characteristic of the β phase, is so large that the pores are rapidly depleted in H+. (Assuming a 1-µm-deep, 4-nm-diameter pore, with 3-nm-thick walls, the concentration of H+ in the pore would need to be ∼160 M to provide all the hydrogen needed to load the walls to H/Pd ) 0.6 without having to rely on the diffusion of H+ from the solution outside the pore. The number of Pd atoms in the walls is estimated from the density of Pd and the volume of Pd left after subtraction of the pore volume, assuming a hexagonal close pack distribution of cylindrical pores.) It is therefore possible that the current transients reflect H+ diffusion from bulk solution to the Pd film. If this were the case, the limiting current at the microdisk would be ∼4.8 × 10-7 A (using ilim ) 4nFDca, with DH+,solution ) 7.7 × 10-5 cm2 s-1 36 and [H+] ) 13 mM as calculated from the pH). In fact, the limiting current would be greater because HSO4- releases H+ and the concentration of protons available for the Faradaic process is larger than that predicted by the pH.8 The plateau currents have the correct order of magnitude but are lower than expected for diffusion in solution. This may be because transport within the pores is affected by the diffuse layer whose thickness (∼4 Å for (35) Millet, P. Electrochem. Commun. 2005, 7, 40-44. (36) Mills, R.; Lobo, V. M. M. Self-Diffusion In Electrolyte Solutions, a Critical Examination of Data Compiled from the Literature; Elsevier: New York, 1989.

Table 1. Estimated Roughness Factor and Average Film Thicknessa,b deposition of films

substrate electrode type polished Pt H1-e Pd(A) film H1-ePd(B)

plain Pd film polished Pd

experimental results

calculated parameters

nominal charge charge density electroactive geometric surface roughness average material diameter/µm surfactant QDep/µC area/10-3 cm2 area/10-3 cm2 factor RF thickness/µm C cm-2 Pt Pt Pt Pt Pt Pt Pt Pt Pd Pt Pt Pd

25 25 25 25 25 25 25 25 25 10 25 25

C16EO8 C16EO8 C16EO8 Brij 56 Brij 56 Brij 56 Brij 56 Brij 56 Brij 56

11 22 44 5.5 11 22 44 11 1.8 11

2.2 4.5 9.0 1.1 2.2 4.5 9.0 2.2 2.2 2.2

0.013 3.6 6.7 12.3 0.87 1.8 3.0 6.5 1.7 0.25 0.035 0.010

0.0050 0.0070 0.0097 0.0140 0.0055 0.0072 0.0092 0.0141 0.0070 0.0015 0.0068 0.0049

2.6 508 688 877 157 248 324 463 242 163 5.2 2.1

0.9 1.4 1.9 0.6 0.9 1.4 1.9 0.9 0.7 0.7

a The latter was calculated from the geometric surface area (as seen by SEM), assuming a Faradaic efficiency of 98%. b Q Dep, deposition charge. H1-e Pd(A), nanostructured Pd films made with C16EO8. H1-e Pd(B), nanostructured Pd films made with Brij 56.

Figure 3. Current (curves a-c) and charge (curves d, e) transients recorded while loading and stripping hydrogen from an H1-e Pd film (Brij 56, QDep ) 44 µC) on a 25-µm-diameter Pt disk in a 0.5 M Na2SO4 + H2SO4 solution (pH 1.88). The potential was stepped from -0.20 V to (a) EL ) -0.73 V, (b) EL ) -0.75 V, and (c) EL ) -0.80 V. Curve c was integrated to give the loading charge, curve d. Curve e shows the corresponding stripping charge obtained for set loading times.

a 0.5 M 1:1 electrolyte according to the Gouy-Chapman theory37) is significant relative to the pore diameter. Alternatively, this could result from different loading rates at the top and bottom of the pore. Anyhow, loading in a nanostructured film is much faster than in a Pd film plated without the surfactant. In the absence of nanostructure, H needs to diffuse through the thickness of the film, the loading plateau is much smaller and poorly defined, and the current slowly decays without indicating the completion of the loading.38 A full interpretation of the shape of the loading transients in H1-e Pd was outside the scope of this study, but the results presented here suggest that the rate-determining step is more complicated than simply diffusion of H+. Hydrogen loading was further characterized by stripping voltammetry after each loading transient. This was repeated for different film thicknesses, loading potentials, and times. The stripping charge was normalized with the charge used to deposit the film, and the ratio of hydrogen to palladium atoms was plotted (37) Bard, A. J.; Faulkner, L. R. Electrochemical methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001. (38) Imokawa, T. Ph.D. University of Southampton, Southampton, 2003.

Figure 4. H/Pd ratio as a function of the loading potential for H1-e Pd (Brij 56) films deposited (b 11 µC, 2 44 µC) on 25-µm-diameter Pt microdisks. Loading carried out in a 0.5 M Na2SO4 + 0.05 H2SO4 solution (pH 1.88). The H/Pd ratios were estimated from stripping charges after subtracting the double layer and hydrogen adsorption charges and accounting for a 98% Faradaic efficiency for deposition.26

against the loading potential, Figure 4. Except for the potential shift due to the pH difference, the plot is identical to that with large electrodes in 1 M H2SO4.26 Around -0.625 V, the H/Pd ratio reaches a small plateau of ∼0.025, close to the maximum solubility of hydrogen (0.03) in the R phase. Around -0.71 V, the transition from the R to the β phase leads to a sharp rise in H/Pd ratio. At lower potentials, the H/Pd ratio slowly increases to 0.6, the bulk value for the β Pd hydride. The plot does not depend on the thickness of the film thus confirming that the whole of the nanostructure is loaded with hydrogen. However, after repeated use, any loss of Pd, by dissolution when the film is taken to its oxide formation region26 or during loading, means that the deposition charge no longer reflects the amount of Pd in the film. (After 20 repetitive loadings at -0.75 V, an H1-e Pd(Brij 56) film (11 µC) absorbed ∼85% of the hydrogen amount it took on the first loading.) To circumvent this problem, the hydrogen stripping charge was normalized by the oxide stripping charge. The latter reflects the electroactive area, and since the surface area-to-volume ratio of these films is huge (∼107 cm2 cm-3),26 it is also a very good measure of the amount of Pd in the film. Plots of the hydrogen stripping-oxide stripping charge ratio versus the H loading potential were found to be independent of the amount of Pd left in the film and very similar to Figure 4. This is remarkable Analytical Chemistry, Vol. 78, No. 1, January 1, 2006

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Figure 5. Potential transient recorded with a H1-e Pd (Brij 56) film deposited (11 µC) on 25-µm-diameter Pt microdisk. Experiment carried out in a deaerated 0.5 M Na2SO4, 0.05 M H2SO4 solution immediately after hydrogen was loaded at -0.75 V for 60 s (H/Pd ) 0.62).

and indicates the very good reliability of the electrode provided some nanostructure is present. It is also very useful since at any time it is possible to assess the hydrogen loading without having to know the amount of Pd left in the film. Two practical conclusions can be drawn from the results. First, the H/Pd ratio can be controlled by varying the loading time, tL. Second, the amount of hydrogen loaded can be estimated from the charge passed during loading, on the condition that tL is in the first plateau region. Like a normal Pd electrode, the nanostructured Pd film deposited on the Pt microdisk can therefore be charged to a desired level of H. The process is reproducible, but after repeated loading, the amount of H1-e Pd is observed to decrease. Potentiometric Response. Once loaded with hydrogen, the microdisks were tested for their potentiometric properties. A typical potential transient has two characteristic times, t1 and t2, in Figure 5. The shape of the transient is similar to that reported for Pd black electrodes39 and is consistent with a gradual loss of hydrogen. The time taken to stabilize, t1, was found to depend on H loading. β films with H/Pd of 0.59 and 0.51, respectively, took 15 and 10 min to stabilize. R+β films with H/Pd between 0.42 and 0.28 reached a stable potential almost instantaneously, while the potential of an R film with a H/Pd of 0.1 rose continuously. Between t1 and t2, the plateau potential did not vary by more than (1.2 mV, which is again remarkable for a microelectrode. After t2, the potential was found to rise significantly. A comparison of stripping voltammograms38 recorded just after loading H and after the transient in Figure 5 clearly shows that, despite using ultrapure reagents and deaerated conditions, there is virtually no H left in the film after 180 min. Two mechanisms could be responsible, the recombination of H to form H2 and the oxidation of H by trace impurities. Indeed, in aerated conditions, t2 drops down to ∼5 min. Clearly this will depend on the size and loading of the H reservoir. Preliminary experiments in deaerated conditions indicate a lifetime (t2 - t1) independent of solution pH but roughly proportional to the geometric diameter and thickness of the film (1-1.5 h for 11 µC, 2-3 h for 22 µC for films deposited on 25-µm-diameter Pt microdisks and loaded to H/Pd ∼0.6). The potentiometric-pH response of H1-e Pd hydride microelectrodes was carefully investigated via titrations and calibrations in buffered media. Before each experiment, an H1-e Pd (Brij 56) film was loaded with hydrogen at -0.75 V, in a 0.5 M Na2SO4 + 0.05 M H2SO4 solution. Its open circuit potential (EPd-H) was monitored in the same solution, and titrations were started once (39) Jasinski, R. J. Electrochem. Soc. 1974, 121, 1579-1584.

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Figure 6. (a) Potential transients recorded with a pH meter (top) and a H1-e Pd (Brij 56) film (bottom) deposited (11 µC) on a 25-µmdiameter Pt microdisk and loaded with H at -0.75 V in deaerated 0.5 M Na2SO4, 0.05 M H2SO4 solution. The solution was continuously deaerated with Ar; aliquots of 1 M NaOH and 1 M H2SO4 were used to modify the pH. (b) E vs pH calibration curve obtained from titrations with different H1-e Pd (Brij 56) films (11 µC for O and b, 22 µC for 0, and 44 µC for 3). O, 0 and 3, forward titrations; b, reverse titration. Data at pH 6.32 were recorded before and after the titrations in a 0.5 M Na2SO4 + 0.05 M phosphate buffer.

EPd-H reached the stable value corresponding to the R+β phase. Diluted NaOH or H2SO4 was added to raise or lower the pH. Ar was bubbled continuously during the titration to deaerate and stir the solution. Throughout the experiment, EPd-H and the output of a combination glass pH electrode were monitored simultaneously. An example of a titration is shown in Figure 6a. After each addition of NaOH or H2SO4, the microdisk potential responded quickly and maintained a very stable value. The calibration curve, Figure 6b, shows an outstanding reproducibility between different electrodes and an absence of hysteresis between forward and reverse titrations. Moreover, the response is linear (EPd-H ) -0.0587pH - 0.5891 V vs SMSE, with r2 ) 0.999 86) through the whole pH range, including neutral conditions. This is unique among pH microelectrodes. The slope is in good agreement with the theoretical value (-2.303RT/F ) -0.0592 V pH-1 at 298 K), and the intercept (equivalent to +0.051 V vs SHE) agrees well with the plateau potential for conventional R+β Pd-H electrodes of +0.050 V versus RHE in various solutions. To assess the error from liquid junction potentials at the reference electrode, two calibration curves were recorded against a SCE in deaerated buffer solutions (pH 2.07, 4.00, 6.86, 10.00, and 11.88) with known liquid junction potentials for the SCE (ranging from -0.0018 to -0.0030 V). Throughout, great care was taken

Table 2. Exchange Current Densities (I0) for H+ + e a H (Pd-H) on Pd Hydride Microelectrodes in 0.5 M Na2SO4 + 0.05 M H2SO4 (pH 1.88) Solutionsa Pd film deposited

i/E slope/µA V-1

i0/nA

I0/A m-2

H1-e Pd(C16EO8) H1-e Pd(Brij56) plain Pd

3.7 2.6 0.22

94 67 6

133 95 8

a Exchange currents (i ) were estimated from the limiting form of 0 the Butler-Volmer equation at low overpotentials and the i/E slopes obtained from voltammograms recorded at 10 mV s-1. All films were deposited with a charge of 11 µC on 25-µm-diameter Pt disks. I0 was calculated with the geometric surface of area of the film by assuming a disk of 15-µm radius.

to obtain reliable measurements: the EPd-H was first read in a 0.5 M Na2SO4 + 0.05 M H2SO4, then in the buffer test solution, and again in the sulfate solution to check that it had recovered its starting value. The electrode was rinsed before each transfer, and all solutions were thermostated to 25 °C. Although EPd-H reached a stable value under 1 s following transfer, in each case, it was monitored for 3 min. Excellent linearity and reproducibility was found (EPd-H ) -0.0586pH - 0.1871 V vs SCE, with r2 ) 0.999 97). Again the slope is in agreement with the theoretical value, while the intercept is equivalent to +0.054 V versus SHE. The calibration and the theoretical expression (E ) - (2.303RT/ F)pH + 0.050 V vs SHE) differ by 5 mV at pH 2 and 11 mV at pH 12. Considering the major errors, namely, the stability of the microdisk ((1.2 mV) and of the SCE ((1 mV), the temperature fluctuations within the cell ((1 °C equivalent to (0.7 mV), and the liquid junction potential ((3 mV), it can be concluded that the potentiometric response of the H1-e Pd hydride microdisks is almost theoretical and that the accuracy is better than (0.1 pH unit. Advantage of the Nanostructure. For comparison, experiments were run with polished Pd microdisks (25-µm diameter) and with plain Pd films on Pt microdisks. Both were characterized and loaded with H as described above for the H1-e Pd films. Not surprisingly, the former never produced a stable open circuit potential, and even with large loading charges (70 µC), the potential was always greater than that expected. This is due to the diffusion of H within the Pd microwire, and it would be necessary to load the whole length of the wire as in ref 2 to obtain a useful pH microelectrode. The plain Pd films, deposited in similar conditions except for the absence of surfactant, had the same geometric dimensions, Figure 1d, as the H1-e Pd films but much smaller electroactive areas, Table 1. They were loaded with H, and their potentiometric transients followed the shape obtained with the nanostructured films. Their plateau potential was a few millivolts below the expected value but with an appreciable slope, and the transition to the R phase occurred after ∼40 min.38 In addition, the plateau potential was not reproducible between electrodes. Despite having the same thickness, the plain Pd films could not be used as reliable pH sensors, and it is clear that the presence of the nanostructure confers outstanding potentiometric properties to the films. The huge electroactive area provides many sites where H+ from the solution can equilibrate with H in the Pd. The rate of hydrogen absorption is greatly enhanced by the nanostructure

which ensures that no point in the metal is more than a few nanometers from the surface. Diffusion within the lattice is expected to be so fast as not to be rate limiting and the H composition to be rapidly uniform within the film. Moreover, the film decorates the underlying defects of the Pt microdisk, (e.g., cracks between the microwire and the glass) and reduces the effect of the large perimeter to surface area ratio. Pt prevents the diffusion of hydrogen down the microwire and maintains the hydrogen-topalladium ratio within the film. Overall, shorter times and less negative potentials are required to load H1-e Pd films with H. Although a gradual loss of surface activity under open circuit conditions cannot be excluded, the poor potentiometric performance of plain Pd hydride films reflects a lesser ability to define the potential. This is demonstrated by a significant difference in exchange current densities, Table 2. Hence, the high rate of the potential-determining reaction contributes to the stability, reproducibility, accuracy (theoretical response), and fast potentiometric response of H1-e Pd hydride microelectrodes. CONCLUSIONS Nanostructured H1-e Pd films were electrodeposited on Pt microdisk electrodes using a molecular template created by a hexagonal lyotropic crystalline phase. While retaining micrometer dimensions, these electrodes possess huge electroactive surface areas with typical roughness factors around 300. Hydrogen absorption into the H1-e Pd films was shown to be very fast. Pd hydrides with a given H/Pd ratio could be readily prepared by controlling the electrode potential and the duration of the deposition. The nanostructured Pd hydride microelectrodes showed excellent potentiometric response over the pH range tested, 2-12. The potentiometric pH response was rapid, stable, reproducible, and almost theoretical in deaerated solutions. These properties, clearly superior to conventional pH microsensors, are thought to result from the combination of (1) the R+β Pd hydride phase, which is known to show almost theoretical potential with normal size electrodes, and (2) a nanostructured film with a huge surface area and a rapid potential-determining process (H+ + e a H(Pd-H)). H1-e Pd hydride microelectrodes have a lifetime of 1-3 h in deaerated solution because of the continuous removal of hydrogen under open circuit conditions. Although the lifetime is too short for continuous process monitoring, it is sufficient for many analytical applications. The electrode can be reloaded with hydrogen quickly, which makes it possible to perform pH measurements repeatedly. To our knowledge, these are the first applications of liquid crystal templated nanostructured materials in potentiometric application and pH monitoring. ACKNOWLEDGMENT The authors acknowledge the financial support of the JFE Steel Corp. and of the EPSRC (GR/M92430/01, and award of a DTA account for K.-J.W.) and are grateful to Phil Bartlett, Jan Marwan, and Derek Pletcher for helpful discussions.

Received for review July 26, 2005. Accepted October 26, 2005. AC051328J

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