Anal. Chem. 1987, 59, 217-218 (15) S 0 r h M.; Smith, G. P.; Norvell, V. E.; Mamantov, G.; Klan, L. N. J . Electrochem. Soc. 1981, 128, 333. (16) Norvell, V. E.; Tanemoto, K.; Mamantov. G.; Klatt, L. N. J . Elecfrochem. SOC. 1981, 128, 1254. (17) Chapman, D. M.; Smith, 0 . P.; Sarlle, M.; Petrovic, C.; Mamantov, G. J . €lecfrmhem. SOC. 1984, 131, 1609. (18) brward, B. L.; Kian, L. N.; Mamantov, G. Anal. Chem. 1985, 57, 1773. (19) Chapman, D. M.; Buchanan. A. C.; 111; Smith, G. P.; Mamantov, G. J . Am. Chem. SOC. 1988, 108, 654. (20) Brewster, J. D.; Anderson, J. L. Anal. Chem. 1982, 5 4 , 2560. (21) Pyun, C.-H.; Park, S.-M. Anal. Chem. 1988, 58, 251. (22) DeGuibert, A.; Plichon, V. J . Necfroanal. Chem. Inferfacial Necfrochem. 1978, 9 0 , 399.
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(23) DeGuibert, A.; Plichon, V.; Badoz-Lambling, J. J . Nectroanal. Chem. Interfacial Necfrochem. 1979, 105, 143. (24) Wiikes, J. S.; Levisky, J. A.; Wilson, R. A.; Hussey, C. L. Inorg. Chem. 1982, 21, 1263. (25) Norvell, V. E.; Mamantov, G. Anal. Chem. 1977, 49, 1470. (26) Fannln, A. A., Jr.; Floreani, D. A.; King, L. A.; Landers, J. S.; Plersma, B. J.; Stech, D. J.; Vaughn, R. L.; Wiikes, J. S.; Wlillams, J. L. J . Phys. Chem. 1984, 88, 2614.
RECEIVED for review July 7, 1986. Accepted September 8, 1986. This work was supported by the National Science Foundation, Grant No. CHE-8412730.
Preparation of Strongly Adherent Platinum Black Coatings C a r l A. M a r r e s e Sensor R&D Group, Bacharach, Inc., 625 Alpha Drive, Pittsburgh, Pennsylvania 15238 Platinized electrodes, in which a film of high surface area Pt black is formed, have found utility in many areas of chemistry. In our laboratories, we frequently employ platinized platinum wire for various applications in gas sensors (I). The advantage of platinized wires is the increased surface area of the Pt black coating. The high surface area improves electrical contact to gas diffusion electrodes, as well as improving the stability of reference potentials. Unfortunately, one of the problems encountered with Pt black coatings prepared from typical literature preparations (2)is the relative frailty of the Pt black matrix. Often we have noticed, during sensor operation, that the Pt black coating is completely absent from the wire after several weeks. In an effort to circumvent this loss of Pt black, we have sought a method to prepare platinized wire of high surface area and durability. One method of preparing durable platinum black coatings is to sinter the platinum particles of the coating after platinizing. However, this is an additional time-consuming procedure which can also decrease the surface area. We have found that platinizing with simulatneous ultrasonic agitation produces durable platinum black films of high surface area. The technique is quick (1-3 min) and, with no subsequent treatment, such as sintering, constitutes an advantage over present platinizing procedures. It was first introduced in 1938 by Cupr (3)who employed ultrasonic agitation to assist in the electroplating of copper. The technique has since then been used for tin and silver electrodepositions ( 4 ) and for the electroplating of smooth, bright platinum on copper (5). The platinum wires platinized with ultrasonic agitation for use in our laboratories were much more durable than nonultrasonically plated wires. In the strict Darwinian sense, we are platinizing by “natural selection”: Only those platinum particles that can endure the harsh ultrasonic agitation survive and become part of the electrode. EXPERIMENTAL S E C T I O N Equipment. Cyclic voltammograms were generated with an EG&G Princeton Applied Research Model 273 galvanostat/potentiostat. Recordings were made on a Hewlett-Packard Model 7044A x-y recorder. Electrochemical plating was achieved with a Hewlett-Packard Model 6202B dc power supply. The ultrasonic cleaner was purchased from Sonicor, Model SC-BOTH. The plating cell was a 50-mL beaker, whereas the voltammetric cell wa4 a conventional three-electrode cell with counter and reference compartments isolated from the working compartment by fine glass frits. Materials. Platinum electrodes were fabricated from 0.005 in. diameter wire (Sigmund Cohm Corp.) sealed in soft glass. The 0003-2700/87/0359-0217$01.50/0
exposed wire was cut to 5.0 mm yielding a geometric area of approximately 0.02 cm2.Electrical contact was made with mercury and copper wire. A Ag/AgCl (saturated KCl) reference electrode, with cracked bead tip, was employed. The counter electrode was braided Pt wire, coiled on a glass rod for support. The electrolyte, for area measurements, was 1M H2S04employing Fisher brand HPLC grade water. The platinic acid solution (1.4% PtCb2- and 0.02% Pb2+in dilute aqua regia) was prepared from 7.0 g of the 0.005 in. Pt wire in 100 mL of aqua regia (one part HN03, three parts HCl) diluted to 500 g of solution with the HPLC water. To this dilute solution, 0.1 g Pb(N03)2was added. (The chloride content was low enough to not cause the precipitation of PbC12.) K3Fe(CN)6was from Fisher Scientific and used without further purification. Platinizing Procedure. The plating apparatus, Figure 1, was constructed by cementing the 50-mL beaker to the bottom of the 1-L ultrasonic tub with Dow Corning Silastic 730 RTV [4], A braided, platinized Pt wire [6], serving as the counter electrode, was secured to the outside of the plating cell and connected to the positive pole of the dc power supply. The electrodes to be plated were immersed in the cell to an approximate depth of 0.25 in. [5], measured from the glass (electrode)tip. The temperature of the bath was 23 2 “C. To plate platinum, the ultrasonic cleaner was turned on, followed by the power supply, which was adjusted to 2 V. After the designated time, both power supply and cleaner were turned off and the electrodes were rinsed several times with the HPLC water. The areas were determined electrochemically by integration of the hydrogen adsorption processes in the cyclic voltammogram at 0.1 V/S.
*
R E S U L T S AND DISCUSSION Figure 2 illustrates the characteristic surface voltammetry of platinum in sulfuric acid electrolyte for ultrasonically platinized (A) and unplatinized (B) platinum wires (note current scales). The voltammograms clearly exhibit the hydrogen adsorption reductive processes (from +0.15 to -0.18 V), uncomplicated by impurity adsorption, such as Cl-. It is evident from the area under the H adsorption processes of the platinized wires that the technique produces a Pt black coating of high surface area. The small oxidation at +0.35 V is probably due to Pb, as P b is present in most electrodepositions of platinum (2). (The “Pb” oxidation is not present in the voltammogram for the unplatinized wire, and the peak current is linear with potential sweep rate and decreases with increasing time for soaking in concentrated HN03.) In every instance, wires that have been platinized with the aid of ultrasonic agitation yield electrochemical areas that are larger than those obtained without the agitation, Figure 3. As one can see, the ultrasonic method can yield platinized wires with areas as much as 6 times larger than their nonultrason0 1986 American Chemical Society
218
ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
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20 1918
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Flgwe 1. Schematic diagram of the ultrasonic plating apparatus: (1) walls of the Sonicor ultrasonic tub; (2) 50-mL beaker; (3) platinic acid plating solution; (4) Dow Coming Silastic 730 RTV cement; (5) soft glass encasement and Pt wire electrode; (6) braided, platinized, platinum counter electrode; (7) water level in the ultrasonic tub. I
,
Figure 3. Comparlson of electrochemicallydetermined surface areas of ultrasonically platinized wires (0)to nonultrasonically platinized wires .).(
Table I. Comparison of Pt Black Film Durabilitya D
plating method
area before ultrasonic cleaning,* cm2
area after ultrasonic cleaning: cm2
% loss
ultrasonically platinized 10.7 (2.81) 10.4 (2.58) 2.5 (0.7) nonultrasonically platinized 3.93 (1.58) 0.64 (0.26) 82 (9.5) 12
1.0
0.8
0.6
EIVoltr
VI
0.4
0.2
,
O!O -0.2
A~/A~CI)
Figwe 2. Cyclic vdtammetric current potential cwves for ultrasonically platiriized, A, and unplatinized, B, wire electrodes in deaerated 1 M H,SO,: current scale for A, 100 pA per division; current scale for B, 1 p A per division.
ically plated counterpart. This consistent increase in area is undoubtedly due, in part, to the increased flux of the PtCh2ion from the ultrasonic agitation ( 4 , 5 ) . Such an increase in flux is observed with ferricyanide reduction at an unplatinized wire electrode; the “limiting” current one obtains for linear potential sweep voltammetry under ultrasonic agitation is nearly 10 times the peak current in the absence of agitation, both at 0.1 VIS. Still, what is more important is the durability of the ultrasonically platinized wire electrodes at these high surface areas. In Table I, we compare the two plating techniques for the durability of the Pt black coatings. Wires platinized by their respective techniques were subsequently subjected to ultrasonic cleaning in the HPLC water for the length of time the electrodes were subjected to plating, e.g., 2.5 min. The wire electrodes prepared by the ultrasonic platinization procedure retain approximately 97% material, as judged by the electrochemically determined areas. The wires not subjected to
“Areas and Values for 70 loss are an average of four individual electrodes. Numbers appearing in parentheses represent first standard deviation. Determined electrochemically. Ultrasonic cleaning in HPLC grade H20 for 2.5 min. the ultrasonic platinization had on the average an 82% decrease in area. This loss in area is a loss of material, as we can observe Pt black dislodging from the wires during the ultrasonic cleaning treatment. The platinum wires, platinized with the aid of ultrasonic agitation, invariably exhibit high surface areas and show no evidence of failure due to the loss of the platinum black coating, even after several months in operation. The technique is very quick and requires no more sophisticated equipment other than an ultrasonic cleaner and a common dc power supply. Registry No. Pt, 7440-06-4; Pb(NO&, 10099-74-8.
LITERATURE CITED (1) Chand, R. US. Patent 4 152233, 1979. (2) Feltham, M.; Spiro, M. Chem. Rev. 1971, 77, 177. (3) Cupr, V. Chem. Llsty 1938, 32, 215. (4) Rich, R. Platlng (East Orange, N . J . ) 1955, 4 2 , 1407. (5) Kukoz, I.: Lukoz, L. A. Zh. prikl. Khim. (Leningrad) 1968, 3 9 , 7 0 5 .
RECEIVED for review May 29, 1986. Accepted September 8, 1986.
