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INSTRUMENTATION . . . .
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John T. McDevitt, David R. Riley, and Steven 0. Haupt Department of Chemistry and Biochemistry The University of Texas at Austin Austin, TX 78712-1167
electron-transfer phenomena at temperatures below T,.In this INSTRUMENTATION article we will introduce high-?', electrochemistry, summarize the more significant developments, and discuss several important opportunities for future research.
The discovery in 1986 of high-temperature superconductivity i n Lal.8,Ba,l,Cu0, (T.= 30-35 K)was thought by many to be the beginning of a new era in which cuprate superconductors would find widespread application in areas ranging from ultrafast electronics to magnetic levitation in transportation systems. The prospect of high-T, supercond u c t o r s such as YBa,Cu,O,_, (T. = 92 K), Bi,Sr,Ca,Cu,O,,,,, (T,= 110 K),and Tl,Ba,Ca,Cu,O,,, (T.= 127 K) operating at liquid nitrogen temperature (77 K)generated a tremendous amount of hope that many applications previously considered impracticable would become commercially viable. However, as our understanding of high-temperature superconductors has increased, it has become clear that many obstacles must be surmounted before high-T, superconductors will be commercially available. The poor physical properties exhibited by these brittle ceramic compounds; their propensity to degrade chemically when exposed to water, acids, CO, and CO,; and the difficulty associated
Fabrication and response of high-T, electrodes Before the properties of high-T, electrodes can be studied, methods for preparing functional superconductor electrodes must be identified. Because cuprate ceramic samples are quite brittle, porous, and chemically reactive, they lack many of the desirable properties normally associated with electrode materials. Consequently, the response of an unmodified ceramic electrode (an electrode made simply by attaching a wire to a bulk ceramic pellet) is dominated by resistive and capacitive artifacts associated with highly porous electrode materials. Special methods are required to fabricate electrodes from these ceramic samples ( 1 ) . Figure 1 illustrates some common electrode geometries that have been used successfully for the study of high-Tc electrochemistry. In the simplest embodiment, a high-T, sample is encapsulated in an epoxy matrix (2)to fill the majority of the pores on the exterior of the pellet (Figure la). Electrodes of this
0003-2700~3/0365-535A/$o4.00/0. ": : . . . @ 1993 American Chemical Society :""':: I . . . . .
with synthesizing phase-pure materials are only a few of the problems that have hampered their application. Thus the study of high-temperature superconductivity provides new challenges and opportunities for electrochemists. Electroanalytical investigations have already provided important insight into the environmental reactivity problems associated with cuprate compounds, and much progress has been made in developing alternative processing methods for preparing useful forms of high-T' samples. Equally important, with the discovery of hight e m p e r a t u r e superconductivity comes the possibility of studying
ANALMICALCHEMISTRY, VOL. 65, NO. 11, JUNE 1,1993 * 535 A
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type are prepared easily and display reasonably good electrochemical characteristics. Moreover, if the interface becomes damaged, a fresh superconductor surface can be exposed simply by polishing away the degraded layer. A single superconducting macroelectrode can therefore be used for many experiments. Although the epoxy-encapsulated electrodes have been used extensively to acquire information related to high-To electrochemistry, the surfaces of these ceramic electrodes are not well defined. The complex nature of the rough, polycrystalline metal oxide electrode-solution interface .
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makes it difficult to interpret the electrochemical results acquired at these electrodes. The development of alternative methods to prepare high-?', electrodes with better defined surfaces has been an important area of research. To study metal plating phenomena and to examine high-T, electrode decomposition kinetics, Miller and coworkers (3-5)have developed procedures for preparing rotating ringdisk electrodes from cuprate phases (Figure lb). Ceramic pellet inserts are mounted on rods, potted in epoxy, and coupled to a tube equipped with either a F't or a Au ring. The electrode assemblies are polished with a 1-um Al,O. abrasive slurrv prior to US
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The availability of superconductors with transition temperatures above 77 K makes it possible, in principle, to study superconductor electrochemistry below T.. At such low temperatures, solvent resistance values are extremely high. To circumvent this problem, Murray and co-worker6 (6, 7)developed two different methods for preparing superconductor-based microelectrodes. In their first procedure ( 6 ) , superconductor microdisk electrodes (Figure le) are generated by using the same basic methodologies as for the epoxyencapsulated macroelectrodes, except that the sanding procedure is completed in a n electrolytic bath while monitoring the electrode assembly potentiometrically. This
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Insulating matrix Superconductor
0
Rdisk
E vs. SCE (7 Flgure 1. Schematic illustration of the various superconductor electrode geometries. (a) Macmeledrode. (b) mtating riw-disk i W r o d e , (c) mkmdisk elecirode. and (dl m h b a n d electrode.
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Figure 2. Cyclic voltammetry of a 2.5-mM TCNQ solution in 0.1 M EtNBF, CH,CN (100 mWs) recorded at various electrode materials. (a) Platinum. (b) ~s%%%, (c) h.&ro.,&uo4. (d) L B ~ . ~ ~ , , C (e) T I ~ . s ~ C u 0 5 . aand 7 , (I)Nd,.,Ceo.&uOe Voltammetry ..TCN(Z"- warns is shown. All mafkers are equal to 20 pA.
536 A * ANALYTICAL CHEMISTRY, VOL. 65, NO. 11, JUNE 1,1993
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procedure allows determination of the precise instant during which the superconductor surface is exposed, thereby yielding a very small electrode area. During their second procedure (7). well-behaved superconductor microband electrodes are prepared by depositing a film of a superconductor, such as Bi,Sr,CaCu,O,, onto a single crystalline MgO substrate and then overcoating it with insulating layers of either Al,03 or AIN (Figure Id). To complete the electrode fabrication, the structure is cleaved to expose a band of pristine superconductor sandwiched between two insulating regions. Cyclic voltammetry is a popular tool that has been exploited to characterize the mom-temperature properties of these high-T, electrodes. Choice of solvent system, method of surface preparation, and identity of the high-T, phase are critically important in dictating the electrochemical behavior exhibited by these systems. In aqueous solutions, parasitic corrosion reactions occur that result in the chemical degradation of the host electrode. All metal cations in
the YBa,Cu,O, system, except for copper, possess only a single stable oxidation state and are not electrochemically active. Consequently, the response of the electrode is dominated by the redox chemistry of the copper ions in the corrosion phase assemblage (8,9). In the absence of water, however, high-T, phases behave well as electrode materials (2.3,10). Reproducible voltammetry similar in appearance to that which can be acquired at noble metal electrodes such as P t and Au can be obtained readily for a large number of redox couplehigh-To electrode material combinations. Representative examples of TCNQ voltammetry recorded a t several high-T, epoxy-encapsulated macroelectrodes and at P t are shown in Figure 2. Both the TCNQo'-' and TCNQ-"-' waves are well resolved for all five superconductor electrodes, which indicates that these materials can be used successfully as working electrodes. Furthermore, in the absence of solution-dissolved redox couples, no evidence of any oxidative or reductive waves associated with the YBa,Cu,O, host electrode is ob-
served for background scans (on the cyclic voltammetric time scale) in the potential window of 1.3 to -1.4 V versus SCE (2,3). Working electrodes composed of either P t or Au exhibit similar behavior with only slightly larger background limits. Electrochemically assessed corrosion reactivity of high-T. phases The high reactivity displayed by cuprate superconductors is a particularly troublesome problem that must be solved before these materials can be used. Several analytical methods have been exploited to study the surface chemistry of cuprate superconductors. Problems with oxygen loss from high-T, samples complicate analyses completed with classical surface science methods such as Auger electron spectroscopy, X-ray photoelectron spectroscopy, UV photoemission spectroscopy, and scanning electron mimsoopy (SEM). However, electrochemistry is a n ambientpressure, surface-sensitive technique that can be exploited to study surface chemistry of oxide superconductors. The first reports in this area. by Miller and co-workers ( 4 , 5 ) , involved the use of high-T: rotating ring-disk electrodes. By using such electrodes, researchers acquired information related to the stoichiometry of the dissolution products for YBa,Cu,O, samples exposed to aqueous acids. From the lowest acid concentration measured, 1mM HC1, the dissolution process was found to occur nonselectively. Even u p to 100 mM HCI in 1M NaCl, the dis~olution p m s s yielded concentrations of Y', Ba2+, and Cu'+ in the molar ratios of 1:2:3, characteristic of the bulk material. In this concentration range, the etching rate is directly proportional to the concentration of H+ and suggests this reaction
YBa$,cusO,
+
+ 2Baa + 2Cu2-'+ Cu%+ 7%0 (1) 14€l++?
The presence of Cu" was not detected electrochemically a t the ring electrode, but the evolution of molecular oxygen suggested that a rapid reaction occurs in solution CUS Flguru J. electmde (a) Schematic illustration deplning the rapid electrcm transfer lhat wun berween a Solution-dissolved redox muple and a piistine high-To electrode surfaca and corresponding voltammstry with peak splitling values, A h , cbsa to 59/n (mv).(b) Sluggish electron transfer m u r s when thin insulating layers collect on the suds and corresponding volammelry has A€, values much greater than 59/11(mQ
+ (1/2)%O +
Cuz' + € +I(114) + 0, (2) Recent MS experiments (11) have indicated t h a t t h e high-T, lattice serves as the source of evolved oxygen rather than water. This observation is consistent with the notion
ANALYTICAL CHEMISTRY, VOL. 65, NO. 11. JUNE 1.1993 * 537 A
INSTRUMENTATION that the Cu-0 bonding interactions in cuprate compounds are highly covalent: the positive charge is shared more or less evenly between the oxygen and the copper components. In another method for examining the reactivity of cuprate compounds, electrodes are prepared from high-T, ceramic pellets and cyclic voltammetry for solution-dissolved redox couples is recorded (3, 12).The splitting between the cathodic and the anodic peak potentials (i.e., AEJ is used to acquire diagnostic information related to the surface quality of the high-T, electrodes. In the absence of corrosivereagents, reversible voltammetry is obtained with AEp values close to 59/11 (mV) (Figure 3a). However, a thin insulating layer can form on the electrode surface from a chemical reaction between the superconductor and the reagents in solution. Under such circumstances, the heterogeneous electron exchange rate becomes sluggish. The method of Nicholson and Shain (13) shows that this leads to AEp values that are significantly greater than 59/n (mv) (Figure 3b), thereby yielding a broad cyclic voltammetric response. When the prototypical superconductor, YBa,Cu,O,, is exposed to water the following reactions are thought to occur (14) 2YB%Cu,07 (s) + 3H,O +
ing scan number. This behavior is consistent with the formation of insulating layers on the YBa,Cu,O,, electrode surface, as described in Equations 3 and 4. This procedure provides a convenient and versatile method for monitoring surface cormsion reactions. To evaluate the relative reactivities of six of the most common highT. phases, voltammetry h a s been performed in different solutions containing varying amounts of water. Figure 5 shows the cyclic voltammograms acquired at three of the six high-temperature superconductor phases in both a dry acetonitrile solution (Figure 5a, 5c, and 5e) and in solutions of acetonitrile containing 4.5% water (by volume) (Figure 5b, 5d, and 5f). In the absence of any intentionally added water, the cyclic voltammetry is well resolved; peak splitting is close to 59 mV for all three electrode materials. However, when water is added to the electrolytic fluid, the voltammetric response for the three materials differs substantially. Voltammetry that is performed at YBa,Cu,O,, (Figure 5b) becomes totally unresolved, whereas voltammetry acquired at Nd,.,,Ceo.l,CuO, (Figure 5f) remains well behaved. In Figure 5d, intermediate behavior is observed for Lal.,,Sro.,,Cu04, which
Y@aCu05(s) + 5CuO (a) + 3Ba(OW2+ (112) 0,(g) (3) Ba(OH), + CO,
+ BaCO, (s) + H,O (4)
A s a result of these reactions, YBa,Cu,O, is converted into a series of insulating phases that cover the surface of the electrode material and alter its electrochemical response. To monitor high-T, electrode decomposition reactions, electrochemical studies with cuprate electrodes in t h e presence of water were conducted. Figure 4 illustrates the evolution of AEp values versus scan number recorded at a YBa,Cu,O,, electrode in a series of acetonitrile solutions containing varying amounts of water (3).Data acquired at a Pt electrode are also included for comparison. Although the presence of water does not alter the voltammetry at the noble metal electrode (Figure 4d), at YBa,Cu,O,, electrodes (Figure 4a-4c) the higher water content leads to larger values of AEpr which increases monotonically with i n creasing water content and increas-
can tolerate a modest amount of water for a short period of time. Electrochemical studies of this type in a number of corrosive solutions yield the following surface reactivity scale
Interestingly, more traditional bulk reactivity methods based on X-ray powder diffraction and SEM measurements confirm exactly this same reactivity trend (15).
Measurementand control of the copper oxidation level I t is commonly accepted t h a t the oxygen stoichiometry of cuprate superconductors is one of the most crucial parameters controlling the conductive and superconductive properties of these compounds. Oxygen content for YBa,Cu,O,, varies over the range 0 e x e 1 for normal synthetic conditions. Within t h e YBa,Cu,O,, system, superconductivity is typically found for orthorhombic materials with oxygen contents between 6.35 and 7.0. Samples t h a t have oxygen contents e 6.35 tend to display either semiconducting or insulating behavior. Systematic studies of YBa,Cu,O,, samples with different oxygen contents have shown that superconductive transition temperature values tend to cluster around 90 K for 6.84 < 7 - x e 7.0 and near 60 K for 6.40 e 7 - x e 6.60. The YBa,Cu,O,, materials normally are synthesized by using standard solid-state methods in which metal oxide and carbonate salts are heated to temperatures > 900 "C. Under such conditions, the samples form initially as oxygen-deficient tetragonal compounds with oxygen content close to 6.0. To transform them into their superconducting orthorhombic forms, the specimens are annealed at 500 "C in an oxygenrich environment. The treatment increases the oxygen content to 7.0 oxygen atoms per unit cell. In spite of the importance of the oxygen stoichiometry, to date there are only a few reliable methods that can be exploited t o measure this quantity accurately. A common procedure involves dissolving the ceramic sample in acid and using iodometric titration or electrochemical procedures to measure the oxidation state of copper (16). The oxygen content can be inferred on the basis of the principle of charge neutrality
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Figure 4. Values of A E recorded ~ at 100 mVls as a 1uncl;on 01 scan number lor a 2.5-mM TCNO solution in 0.1 M Bu,NBF,lacetonitrile containing different amounts 01 water. (a) YBB,Cu,O,..electfcde woln 250 mM 10 45% oy volbme) addea water. (b) YSa&d3%, eienme wlth t o mM (0018% by w a n e ) added water. (c) Y@a,Cd,O,.. e enrode with no inlemonally added water. an0 (d) PI elglrcde with 254 mM (045% by vo m e ) added w&r.
538 A * ANALYTICAL CHEMISTRY, VOL. 65, NO, 11, JUNE 1,1993
and knowledge of the copper valence. Similarly, gravimetric procedures have been developed that rely on the change in mass that occurs when cuprate samples are heated in a stream of hydrogen and reduced. Unfortunately, these tests are destructive and do not lend themselves to the analysis of superconductor thin films. However, a number of research groups have recently used solid-state electrochemical procedures to measure and control the oxygen content in cuprate superconductor materials (17-21). These methods provide a nondestructive way to measure the oxygen content.
Moreover, the solid-state electrolytes can be used to analyze the properties of thin-film specimens. The main advantages of using the solid-state electrolytic approach are that it provides much higher oxygen activities than those obtained by using high gas-pressure methods and it is a convenient, versatile method by which to coulometrically tailor the oxygen content of the samples. For example, a t 700 "C a potential bias of 0.13 V across a solidstate electrolyte, measured against a n air reference, can produce a n equivalent oxygen partial pressure of -100 atm (18). Thus, the procedure
Electrochemical processing of high-T, structures Several barriers must be overcome before cuprate superconductors can be used commercially on a large scale. Most important in this regard is the development of new processing methods that can be used to prepare bulk and thin-film samples of highT. materials. Electrochemical synthesis provides an attractive alternative for the preparation of a variety of high-T, superconductor structures. Electrochemical techniques are inherently simple and can be performed on a large scale. As an alternative to the popular powder-in-tube technique for the development of high-T, wires and tapes, Bhattacharya and co-workers (22,23) reported an electrodeposition method that can be exploited for the preparation of high-quality T1-BaCa-Cu-0 superconductors. Electrodeposition reactions a r e completed by using dimethyl sulfoxide ' l T Ba", Ca2+, solutions containing , and Cu2+ salts. For this purpose, SrTiO, or MgO substrate materials coated with 500 8, of a silver conductor are used as the electrode surface. Potential excursions to - 4.0 V versus Ag/AgNO, are used to ensure the reductive deposition of all the metal components. Concentrations of the individual components in the bath are adjusted empirically to ensure proper stoichiometry in the final material. The electrodeposition procedure yields a highly reactive film in which individual metal components are
"i 4-
-$' 1
0.7
can be used as a synthetic tool to tailor the oxidation level of high-T. samples. In addition, high-TJsolidstate electrolyte experiments have been exploited to measure the oxygen diffusion coefficient accurately as a function of temperature and oxygen concentration. Although the use of the solid-state electrolytes for the study of high-T, electrochemical processes normally is completed a t temperatures > 400 "C, Cahen and co-workers (20, 21) demonstrated that the oxygen content of porous ceramic samples of can be altered a t room YBa&u,O,, temperature in a controllable and reproducible fashion by using nonaqueous solvents. At such low temperatures, structural rearrangements involving the lattice framework are suppressed. Consequently, the group has been able to generate metastable structures that do not appear to be accessible through conventional high-temperature routes.
0 4.2
0.7
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0 -0.2
E vs. SCE (Vl
Flgure 5. Cyclic voltammetry at 100 mV/s for a 2.5-mM TCNQ solution. (a) ~Cu,O,,elenr& in dry CH,CN 10.1 M ELNBF,, (b) same as (a) with 4.5% Water (by volume) added m the B(Bctmlytic solution, (cl La, &ro d2uO. elenrode in dry CH.CN 10.1 M ELNBF., Id) same as (ci w.m 4.5% water (by voILme) d d i o iheelamtylic sokmon. ie) NEI, &e, &UO. elenmde in dry CH&N 10.1 M ELNBF.. and (0same as (e) with 4.5% water (bq volume) aoded m Ihe elecholytii solution. All markers am eqdal m I O #A. . . . ..
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..
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ANALYTICALCHEMISTRY, VOL. 65,NO. 11, JUNE I, 1993
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INS7RUMENTA7ION mixed on a n atomic scale. The resulting mixture can he transformed into a superconducting film i n a short period of time (i.e., minutes) by heating the amorphous precursor film to temperatures of 850-900 "C. The heat treatment serves to crystallize and to oxygenate the film. The electrodeposition method can be used to produce thick films with uniform coverage. The technique is relatively simple and inexpensive, offers high (micrometer per minute) deposition rates, and can be used to fabricate long conductor segments. Moreover, the procedure has led to the preparation of T1-Ba-Ca-Cu-0 thick films that exhibit transition temperatures above 100 K with critical currents of 56,000 A/cmz at 76 K in zero field and 20,000 A/cmz at 76 K i n a magnetic field of 1.0 T (23). This procedure has yielded the highest critical current value for a T1Ba-Ca-Cu-0-based tape condudor to date. The electrodeposition technique shows strong promise as a method for the fabrication of commercially viable and technologically useful high-T, wires and tapes. Norton and Tang (24, 25) developed and optimized a molten salt electrocrystallization procedure for the preparation of single crystals of the bismuth-based superconductor, Ba,,K,BiO,. According to t h e i r method, KOH is heated above its melting temperature (- 180 "C) and Ba(OH), 8H20 and Bi20, are dissolved in the molten salt in a Teflon crucible. A water-saturated nitrogen atmosphere is maintained above the vessel to avoid the spontaneous air oxidation of the molten flux that occurs at elevated temperatures. The Teflon cell i s equipped with Ag working, F't auxiliary, and Bi reference electrodes. Upon sedimentation of the undissolved excess Bi,O, material, electrolysis i s initiated thmugh the application of an applied potential of 0.6 to 0.9 V versus the Bi reference, Consequently, electrochemical oxidation of the Bi3+occurs, causing Ba,,K,BiO, crystalswhich a r e not very soluble i n KOH-to precipitate onto the surface of the working electrode. In this context, the electrocrystallization method is particularly powerful because the process selects only the most conductive phases. The collection of insulating phases onto the surface of the electrode serves to hamper all subsequent electrontransfer processes. Therefore electrochemical growth of such insulators is self-limited. However, electrodeposited conductive compounds can serve 540 A
as active electrode surfaces and foster the further growth of such materials. Control of the molten salt composition, contacting atmosphere, electrochemical potential, current density, and crystallization temperature affords the opportunity to tailor the properties of the crystals. Figure 6 illustrates a n array of Ba,_,K,BiO, crystals that were electrocrystallized onto a silver wire for 40 min at 240 "C. With longer crystallization times, crystal facets with dimensions > 5 mm long can he obtained. The crystals display unusually high transition temperatures with onset values of 32 K and transition widths of 2 K, measured magnetically. These quantities are superior to those obtained for samples prepared with conventional ceramic powder methods (24, 25). Moreover, Miller and co-workers reported that refinements of this procedure have resulted in the production of large crystals suitahle for the fabrication of tunnel junctions (26).
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Superconductor-based electrochemistry below T. One of the most exciting areas of high-T, electrochemical research involves the study of superconductorbased electron-transfer phenomena that oeeur at or below T,.The many unusual phenomena associated with superconductors (271,such as the
Flgure 6. scanning electron micrographs of Ba, ,KxBi03 crystals. (a) Crystals gmwn onto a 0.5-mm-diameter Silver wire at 240 "C for 40 min under wnslant current conditions (9 mA/cm2).(b) Higher magnification of the same electrcde assembly showing individual superconductor crystals, many Of which display ruin StNCtURS. (Reprinted with permission fmm References24 and 25.)
ANALYTICAL CHEMISTRY, VOL. 65, NO. 11, JUNE 1,1993
Meissner effect, Josephson tunneling, and the proximity effect, suggest that interesting and novel discoveries might result from research in this area. To make such measurements possible, new strategies have been developed for performing ultralow temperature electrochemical experiments. Whereas cuprate superconductors display transition temperatures much greater t h a n those of conventional superconductors, these cornpounds still must he chilled t o temperatures below 127 K (-148 "C) to achieve superconductivity. Significant progress h a s been made r e cently toward such cryo-electrochemical experiments. A t temperatures below T., the electronic charge carriers within superconducting materials undergo an unusual transformation i n which sets of electrons form loosely associated pairs. Consequently, a large number of paired electrons settle into a single state with energy below that of the analogous superposition of isolated electrons. The cooperative interaction between these Cooper pairs leads t o t h e formation of a macroscopic quantum state that we call superconductivity. J u s t at Tc,superconductivity is weak because only a small percentage of the conduction electrons condense into Cooper pairs. As the temperature is further lowered, however, a larger fraction of the electrons becomes paired. Thus t h e currentcarrying capacity is increased in the superconductor as the temperature falls below T.. The Cooper pairs can extend spatially somewhat beyond the physical dimensions of the superconductor. This behavior is responsible for the Josephson tunneling phenomenon in which electron pairs tunnel without resistive losses between two closely spaced superconductor segments separated hy a thin insulator region. Application of small potential biases across the junction serves to differentiate the two superconducting segments, unlocks the phase coherence of the two sides, and alters the flow of supercurrent through the device. The search for molecular analogies of Josephson tunneling is one area of research t h a t may be addressed through the study of low-temperature superconductor electrochemistry. Multiple electron-transfer reactions may be accomplished at a faster rate or at less extreme potentials by using a superconducting electrode. A number of research groups have recently begun to develop strategies to search for such
superconducting quantum electrochemical phenomena. To conduct electrochemical measurements by using high-?'. electrcdes at temperatures below 120 K, a number of formidable technical problems must be overcome. Lorenz and co-workers (28-31) developed the first successful strategy by using solid electrolyte systems such as Agdoped p"-alumina in contact with a variety of high-T. ceramic samples. Following potential step excursions, the quasi-dc limits of the resultant current flow amss the superconductor-electrolyte interface were monitored as a function of temperature. Interestingly, increases in the current were noted at temperatures that correlate well with To.The group demonstrated that this electmhemical procedure can be used for reliable measurements of the superconducting transition temperature. Bockris and Wass (32) confirmed these resulta in studies of proton reduction t h a t were recorded at a YBa,(Cu,.,Pd,.,)O, electrode contacted by frozen HClO, * 5.5 H,O electrolyte. More recent results (31) acquired by using electrochemical impedance spectroscopy w i t h assemblies RbAgJ, / YBa,Cu,O,, suggest t h a t both t h e interfacial charge-transfer resistance and the double-layer capacitance values are slightly reduced at temperatures very close to T.. Superconducting quantum electrochemical phenomena are thought t o be responsible for such effecta (28-31). This approach possesses the tremendous advantage of allowing measurements t o be conducted over a very large temperature window. Obvious disadvantages, however, include ill-defmed contact between the solid electrolyte and the ceramic and the fact that only a limited number of redox systems can be examined. Fluid molecular interfaces offer the distinct advantages (33)of providing better fluid-superconductor contact and yielding greater ultimate flexibility to examine a variety of molecular-phase charge-transfer reactions. To make possible solution electrochemical measurements at superconductor electrodes below Tc, the superconducting transition temperature had to increase and the low temperature limit for solution electmchemistry had to decrease (Figure 7). The world's record for reproducible superconductivity is 125 K for the Tl,B~Ca&u,O,, phase. Murray and co-workers (34, 35) reported the discovery of three remarkable electrolytic solvent mix.
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tures that have made possible solution electrochemical measurements at temperatures below 90 K Through theuseofsuchfluids,solutionvoltammetry h a s been acquired over the temperature range of 115-180 K (36).Moreover, the same solvent systems have been exploited to obtain measurements of double-layer capacitance and charge-transfer resistance values for numerous Tl-BaCa-Cu-0 samples with different transition temperatures (33). From such studies, abrupt reduction in the magnitude of the doublelayer capacitance was noted at temperatures that correlate well with To. Alterations in the charge-transfer resistances were also noted over the same temperature range; however, these results were less reproducible. The data represent the fwst observation of the onset of superconductivity at a molecular fluid/high-T, interface. An alternative strategy for t h e study of superconductor-based electron-transfer reactions is based on t h e use of conductive polymer/ high-T, structures. Although a number of research groups have used high-temperature superconductors to grow polymer f h s at mom temperature (10, 37), our group (38, 39) has begun to explore electron transfer between conductive polymers and high-T. structures below T.. From these experiments, the exchange of electrons between conduc-
tive polymers and superconductors can be explored. Because experimenta of this type can be carried out completely in t h e solid state, the temperature window over which such measurements can be acquired is greatly extended. Moreover, the use of conductive polymers provides the capacity to alter the doping level in a controllable fashion and affords the flexibility to systematically alter the nature of the host molecular conductor. The availability of numerous highly delocalized electrons in the doped form of the conductive polymer should faeilitate the exchange of electrons between t h e molecular phase and the supercondudor. Before discussing the conductive polymer-superconductor interactions, we should consider the behavior of metal-superconductor structures. When normal metals and superconductors are brought into intimate contact with each other, a leakage of the Cooper pairs from the supercondudor to the normal metal can occur along with quasiparticle (i.e., isolated electron) penetration from the metal to the superconductor. This phenomenon, the proximity effect, is most pronounced directly at the interface between the two conductors. For sufficiently thin metal films atop thick superconductor samples, a normally nonsuperconducting metal can be driven completely into the superconducting state by virtue of the
//
CohButyronitrile m lhfeatn/e/- 8 8 K
69% Pmpioniile
Bmmoethane Butyronilnle lsopentane MeWcyclopentane 115 K
-
31%Butyronltrile 155 K
Bromoethane Dffihlorornethane Bulymnitrile -180K 128 K Pmpionltrile
-
B%:2 170 K
Figure 7. Comparison of the low temperature limits for solution electrochemistry that are associated with various solvent systems with the superconducting transition temperatures for selected cuprate high-T, phases. ANALYTICAL CHEMISTRY, VOL. 65, NO. 11, JUNE 1,1993 * 541 A
INSIRUMENIAIION effect. Conversely, thin superconductor specimens revert to a normal metallic state when placed on top of thick metal samples. In our studies, the conductive polymer layer is designed to function as the normal metal layer. One of the simplest methods for examining the proximity effect is to study the temperature dependence of the contact resistance above and below To.A method similar to that of Meissner (40) can be used to acquire contact resistance values by taking the difference between four- and three-point resistance measurements. The former provides the resistance of the sample (RJ and the latter the resistance of one contact (RJ plus R.. The advantage of this technique is that it uses easily prepared bulk materials and is relatively simple to perform. Poly(3-hexylthiophene) contacts on superconducting samples of
the four-point sample resistance with an onset temperature near 110 K and zero resistance close to 85 K are noted (Figure Ea, right axis). Almost identical sample resistance results are acquired with the use of four silver contacts on the same specimen. The poly(3-hexylthiophene) / Pbo:3Bi,., Srl.6Ca&u301a contact resistance (Figure 8a, left axis) displays activated behavior; the contact resistance increases as the temperature is lowered from room temperature to 110 K. As T. (onset) is approached, however, the contact resistance decreases dramatically. Moreover, we have observed similar behavior when using poly(3-hexylthiophene) contacts to samples of
YBa,Cu,O,_, (T,= 92 K) a n d (T. = 94 K).In each GdBa,Cu,O,, case, t h e contact resistance decreases at a temperature that correlates well with the transition temp e r a t u r e for t h e u n d e r l y i n g superconductor. As an important control, the wntact resistance behavior of several poly(3-hexylthiophene)/normal metal interfaces was also investigated (Figure 8b). Unlike the measurements made with the superwnducting templates, no decreases in the contact resistance values were noted near 100 K. One possible explanation for the decrease of the poly(3-hexy1thiophene)high-T.superconductor contact resistance is
Pb0.3Bi1.7Sr1.~Ca2.4cu~010+~ (Tc
110 K) were used t o obtain threeand four-point measurements (39), as illustrated in Figure 8. We infer that good electrical connection is made between the polymer and the supercondudor because decreases in
1.1
0.7
0.3
-0.1
-0.5
combinations. (a) P~,,Bi,,~r,,.Ca,.,Cu,0,,, ceramic pellet sample poly(3.hexylthiophene) cornan* and (b) poly(3-hexylthiophene) film film contacls. . . .
542 A
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ANALYTICAL CHE
superconductor sandwich assembly and the corresponding cyclic voltammetry (5 mWs) recorded at room temperature in 0.1 M Et,NBF,/CH,CN for a YBa,Cu30,, thin-film electrode assembly coated with polypyrrole. NO.lI,JUNE1.1993
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. .
.. .. . ,
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,
that superconductivity is induced at the polymerlsuperconductor interface caused by the proximity effect (39). Measurements of this type do not address the distance over which superconductivity penetrates into the polymer. These results may represent the first evidence for the induction of superconductivity into an organic polymer. Superconductor-based macromolecular electronic devices The use of molecular materials for the development of novel electronic devices has been the topic of much scientific literature (41, 42). Molecule-based devices offer prospects for enhanced sensitivity and selectivity that are not possible with conventional solid-state materials. In virtually all previous macromolecular devices, the active elements have been fabricated by organizing molecular systems onto a metal or a semiconductor template surface. Electrochemical methods have been used for the fabrication and analysis of such systems. With the recent discovery of the high-temperature superconductivity, new opportunities exist for the development of electrochemically fabricated hybrid molecule-superconductor devices. The first two examples of hybrid molecule-superconductor devices have been reported recently. In the first device (43), the light-absorbing properties of a molecular dye were combined with a Superconductor’s large temperature dependence of resistance near Toto produce a colorspecific light sensor. For the second device (381, a method for modulating the superconducting transition tem-
Temperature (K)
oure 1 ‘SUS temperature curves for a 100pm-wide x 3-mm-long microbridge created from a film of YBa&u,O,. (a) Microbridge coated with
-
2-pn Rim of doped polypynole, (b) the same polywrrole-coated
microbridge following its room-temperature BlecImchemical raducbion, and (c) uncoated
microbridge.
perature in an electrochemically deposited conductive polymerlhigh-T, thin-film sandwich structure was developed. In both cases, devices were fabricated by using high-T, films of YBa&u,O,, prepared via laser ablation (44). Superconductor microbridges were created on the surface of the f h s and the molecular materials were deposited by using either sublimation techniques (for the dye layers) or electmehemical procedures (for the conductive polymer f h s ) . For the conductive polymerlsuperconduetor sandwich structures, electrochemical procedures are used to cycle the polymer between its neutral-insulating and oxidized-conductive forms. The well-behaved voltammetry observed for a polypyrrole film coated onto a superconductor film demonstrates that electrical flow between the polymer and the superconductor occurs readily (Figure 9). When the electrical properties of the composite structures were examined, it was observed for the first time that the polymer oxidation state dramatically affects the transition temperature and critical current of the underlying superconductor (38). Whereas t h e neutral polypyrrole only slightly influences the electrical properties of the YBa,Cu,O,, film as compared to the pristine (uncoated) sample, the oxidized polymer depresses T. by more than 15 K (Figure 10). Moreover, the polymer can be cycled several times between its conductive and insulating forms, yielding similar behavior. Control studies conducted without a polymer layer .. led to no reversible modulation of T A possible explanation for t h e modulation of superconductivity in the hybrid polymerlsuperconductor structures is that the polymer causes a proximity effect. Accordingly, previous studies of semiconductor1 superconductor s t r u c t u r e s have shown t h a t t h e magnitude of t h e proximity effect is highly dependent on the free carrier concentration (45, 46). Highly conductive metals with large carrier concentrations normally produce large proximity effects, whereas the effect is not normally found when insulators are coated onto superconductor structures. The polymerlsuperconductor sandwich structures exhibit a new element of flexibility in which the polymer free carrier concentration can be electrochemically modified in a relatively simple fashion. Thus a new type of molecular switch for controlling superconductivity has been demonstrated.
The reactive nature of cuprate superconductors has significantly hampered the fabrication of both proximity and Josephson weak-link devices. It may be possible to incorporate an organic conduetor into such systems and thus eliminate some of the degradation of the superconductor that occurs when conventional materials such as Si, Cu, Sn, Al, Pb, or In are used to make contact with the cuprate materials. These common conductors are readily oxidized by the cuprate compounds, leading to the formation of nonconducting degradation products at the interface between the two conductors. In contrast, oxidatively doped conductive polymers appear to be chemically compatible with the high-T, materials (38, 39). Thus, the enhanced processing ability available with the use of organic polymers may lead to the development of new types of polymerlsuperconductor circuits, devices, and sensors.
Conclusions Recent advances in the understanding of superconductor electrochemical phenomena have led to the development of new high-To processing methods and to more comprehensive knowledge of the surface chemistry of cuprate compounds. Moreover, electrochemical studies conducted below Tohave yielded new insights into t h e area of superconducting quantum electrochemistry, thereby opening a new chapter of electrochemical research. Although the area is only in ita infancy, a number of experiments have demonstrated the influence of the superconducting state on electron-transfer chemistry. Examples of electrochemically tailored hybrid moleculelsuperconductor devices have also been demonstrated. Additional electrochemical experiments are needed to acquire a more complete understanding of superconductorlmolecular phase interactions. The authors gratefully acknowledge support provided by the National Seience Foundation grant no. DMR-8914476 and by the Welch Foundation.
References (1) Riley, D. R.; Manthiram, A,; McDevitt, J. T. Chem. Mdw. lsSa, 4, 1176. (2) McDevitt, J. T.; Lon ire, M.; Gollmar, R.; Jernigan, J. CDalton, E. F.; McCarley, R.; Murray, R. W.; Little, W.A.; Yee, G. T.;Holcomb, M. J.; Hutchinson, J. E.;Collman, J. P. I. Elechoanal. Chem. 1968,243,465. (3) Rosamilia, J. M.; Miller, B.; Schneemeyer, L. F.; Waazczak, J. V.; O’Bryan, H. M., Jr. I. Electrockem. SOC.1987, 134, 1863.
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(4) Magee, V. M.; Rosamilia, J. M.; Kom e t a n i , T . Y . ; S c h n e e m e y e r , L. F . ; Waszczak, J. V.; Miller, B. J. Electrochem. SOC.1988, 135, 3026. ( 5 ) Rosamilia, J. M.; Miller, B. J, Electrochem. SOC.1988, 135, 3030. (6) Gollmar, R. 0.; McDevitt, J. T.; Murray, R. W.; Collman, J . P.; Yee, G. T.; Little, W. A. J. Electrochem. Soc. 1989, 136, 3696. (7) McDevitt, J. T.; Murray, R. W.; Shah, S. I. J Electrochem. SOC.1991, 138, 1346. (8) Rochani, S.; H i b b e r t , D. B.; Dou, S. X.; Bourdillon, A. J.; Liu, H. K.; Zhou, J . P.; Sorrell, C. C. J. Electroanal. Chem. 1988.248. 461. (9) Rosamilia, J. M.; Miller, B. 1.Electroanal. Chem. 1988,249, 205. (10) McDevitt, J . T.; McCarley, R. L.; Dalton, E . F.; Gollmar, R . ; M u r r a y , R. W.: Collman, J . P.: Yee. G. T.: Little. W. A. In Chemistry of High-Temperature Superconductors II; Nelson, D. L.; George, T. F., Eds.; ACS Symposium Series 377; American Chemical Society: Washington, DC, 1988; Chapter 17. (11) Shafer, M. W.; de Groot, R. A,; Plechatv, M. M.; Scilia, G. J. Phvsica C (Amsterdam) 1988, 153-55, 836. (12) McDevitt, J. T.; Riley, D. R.; Zhou, J . P. Proceedings for the 1st International Symposium on Advanced Materials; N a tional Association of Corrosion Engineers: San Diego, CA, April 1991; 38-1. (13) Nicholson, R.; Shain, I. Anal. Chem. 1964, 36, 206. (14) Yan, M. F.; Barns, R. L.; O’Bryan, H . M.; G a l l a g h e r , P. K.; Sherwood, R. C.; J i n , S. Appl. Phys. Lett. 1987, 51, 532. (15) Zhou, J. P.; Riley, D. R.; Manthiram, A.; McDevitt, J . T. Appl. Phys. Lett., i n press. (16) Rosamilia, J. M.; Miller, B. Anal. Chem. 1989, 61, 1497. (17) MacManus, J. L.; Fray, D. J.;Evetts, J. E . Supercond. Sci. Technol. 1989, 1 , 291. (18)Yugami, H.; Watanabe, T.; Suemoto, T.; Shin, S.; Sobajima, S.; Ishigame, M. Jpn. J. Appl. Phys. 1989, 28, 1411. (19) Beyers, R.; Ahn, B. T.; Gorman, G.; Lee, V. Y.; Parkin, S. P.; Ramirez, M. L.; Roche, K. P.; Vazquez, J. E.; Gur, T. M.; Huggins, R. A. Nature (London) 1989, 340. 619. (20) Schwartz, M.; Cahen, D.; Rappaport, M.; Hodes, G. Solid State Ionics 1989, 32/33, 1137. (21) Schwartz, M.; Scolnik, Y.; Rappaport, M.; Hodes, G.; Cahen, D. JMater. Chem. 1991, 1, 339. (22) Bhattacharya, R. N.; Noufi, R.; Roybal. L. L.: Ahrenkiel. R. K. I. Electrochem. Soc: 1991. 138. 1643: (23) Bhattacharya, R. N.; Parilla, P. A,; Noufi, R.; Arendt, P.; Elliot, E. J. Electrochem. Soc. 1992, 139, 67. (24) Norton, M. L.: Tang, H. Y. Chem. Mater. 1991, 3, 431. (25) Norton, M. L. Mat. Res. Bull. 1989, 24, 1391. (26) Rosamilia, J. M.; G l a r u m , S. H.; Cava, R. J.; Batlogg, B.; Miller, B. Physica C (Amsterdam) 1991. 182. 285. (27) Kresin, V. Z.; Wolf,’ S. A. Fundamentals of Superconductivity; Plenum: New York, 1990. (28) Pinkowski, A,; Doneit, J . ; J u t t n e r , K.; Lorenz, W. J.; Saemann-Ischenko, G.; Zetterer, T.; Breiter, M. Electrochimica Acta 1989, 34, 1113. (29) Pinkowski, A.; Doneit, J.; Juttner, K.; Lorenz, W. J . ; Saemann-Ischenko, G.; Breiter, M. Europhys. Lett. 1989, 9, 269. ~
(30)Pinkowski. A,: Doneit. J.: Jiittner. K.;Lorenz, W: J.; Saemann-Ischenko; G.; Zetterer, T.; Breiter, M. Pkysica C (Amsterdam)1989,162-64,1039. (31)Pinkowski, A.;Jiittner, K.; Lorenz, W. J. J. Electroanal. Ckem. 1990, 287, m?nR (&j-Bockris, J. 0.;Wass, J. J Electroanal Ckm. 1989,267,329. (33) Peck, S.R.; Curtin, L. 5.; McDevitt, J. T.; Murray, R. W.; Collman, J. P.; Little, W. A.; Zetterer, T.; Duan, H. M.; Dong, C.; Hermann, A. M. J. Am. Ckem. Soc. 1992,114, 6771. (34)McDevitt, J. T.;Ching, S.; Sullivan, M.; Murray, R. W. J. Am. Ckem. Soc. 1989,Ill, 4528. (35) Ching, S.;McDevitt, J. T.; Peck, S. R.; Murray, R. W. I. Electrochem. Sot. 1991,138,2308. (36) Curtin, L. 5.;Peck, S. R.; Tender, L. M.; Murray, R. W. Anal. Chem. 1993, 65, 386. (37)Osteryoung, J. G.; Magee, L. J., Jr.; Carlin, R.T. 1.Electrochem. Sot. 1988, i. ..w,.i.m,, 2 13. .5_. % . _ (38) Hau t, S. G.; Riley, D. R.; Jones, C. T.; Zfao, J.; McDevitt, J. T. J. Am, Ckm. SOC. ISSS,115,1196. (39)Haupt, 5. G.; Riley, D. R.; Zhao, J.; Kim,S.; McDevitt, J. T., submitted for publication in Pkp Ckem (40) Meissner, Phys. Rev 1960,117, fi79-m
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John T. McDevitt received his B.S. degree in chemistryfiom California Polytechnic State University, San Luis Obispo, in 1982 and his F'h.D. in physical chemistv from S t a ~ r dUniversityin 198% His IP search interests include electrochemical investigationsof high-temperaturesuperconductors and laser ablation deposition of electronic materials.
. David R. Riley (lep) received his B.S. d e greefrom West Virginia University in 1989 and will receive his B.D. from the University of Texas at Austin in August 1993.His research interests include electrochemistry, corrosion, cuprate and OYganic superconductors, conductive polymers, and composite high-TJmolecular devices. Steven G.Haupt (right) received his B.S. degreefrom the US.Naval Academy in 1981 and sewed as a research assistant at the Naval Research Laboratory studyin# the synthesis and characterizationof several conducting phthalocyanines. He is currently working toward a Ph.D. i n chemistry at the University of Texas at Austin, where his research interests include the elucidation of the electrical, optical, and superconductingproperties of conductive orgonit matmals.
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