Hydrogen Detection by Polyaniline Nanoﬁbers on Gold and Platinum Electrodes Jesse D. Fowler, † Shabnam Virji, † Richard B. Kaner, ‡ and Bruce H. Weiller* ,† Materials Processing and EValuation Department, Physical Sciences Laboratory, The Aerospace Corporation, P.O. Box 92957/M2-248, Los Angeles, California 90009, and Department of Chemistry & Biochemistry and California NanoSystems Institute, UniVersity of California, Los Angeles, Los Angeles, California 90095-1569 ReceiVed: NoVember 29, 2008; ReVised Manuscript ReceiVed: January 27, 2009 Polyaniline nanoﬁbers were deposited on either gold or platinum electrodes and used as resistive sensors for the detection of hydrogen. In earlier work (J. Phys. Chem. B 2006, 110, 22266-22270), we showed that hydrogen interacts directly with polyaniline nanoﬁbers to induce a small resistance decrease (-3%) at low concentrations of hydrogen (1%) using gold electrodes. This work showed that the sensor response on gold electrodes is due to hydrogen interaction with the polyaniline nanoﬁbers. However, with platinum electrodes a much larger resistance increase (+65%) is observed under the same conditions. The sensor response on platinum electrodes is due to hydrogen interaction with platinum at the polyaniline-platinum interface. Hydrogen facilitates the formation of a Schottky barrier between platinum and polyaniline through a change in work function as platinum is converted to platinum hydride. The work function of polyaniline nanoﬁbers was measured, and a model for sensor response is presented based on the relative work functions of the platinum, platinum hydride, and polyaniline nanoﬁbers. Platinum hydride formation is fully reversible with the introduction of oxygen that converts the platinum hydride to water. The greater sensitivity of the platinum sensor can be used to detect hydrogen at a concentration of 10 ppm. Introduction Hydrogen is an explosive rocket fuel that takes very little energy to ignite at concentrations greater than 4% in air. Given its tendency to rapidly disperse, it is important to sense hydrogen at much lower levels for leak detection. A number of different sensors for hydrogen have been demonstrated using sensitive layers based on palladium, 1 platinum, 2 and polyaniline, 3-5 as well as combinations of these materials. 6-8 Polyaniline is a conducting polymer that shows orders of magnitude change in conductivity when exposed to gases that can dope or dedope the polymer. It is particularly well suited as an atmospheric sensor, since it is one of the more stable conducting polymers, 9 and it can be readily synthesized in nanoﬁber form. Recently we have discovered simple methods for the chemical synthesis of polyaniline nanoﬁbers dispersed in water. We have shown that the nanoﬁber form shows greatly enhanced response as a sensor for detecting strong acids, 10 ammonia, 10 methanol, 11 hydrogen sulﬁde, 12 and various other toxic industrial chemicals. 11,13 We have previously shown that hydrogen interacts directly with doped polyaniline nanoﬁbers to induce a small change in the conductivity of polyaniline nanoﬁbers. This small increase in conductivity can be used to detect hydrogen gas. 3 Direct mass uptake of hydrogen by the nanoﬁbers was also observed using a quartz crystal microbalance (about 3% relative to the nanoﬁber mass). One of the most interesting results we found is that the interaction of polyaniline with hydrogen was completely sup- pressed in the presence of humidity. Also a signiﬁcant deuterium isotope effect in both the electrical and mass response was observed when D 2 was used. The interaction of hydrogen with polyaniline is of great interest not only for sensing applications but also as a potential hydrogen storage material. The mechanism of the interaction is not clear, but one possibility is the scheme previously presented by MacDiarmid, 14 which we have also discussed. 3 Brieﬂy, hydrogen interacts with doped polyaniline at the charged amine nitrogen sites. Dissociation of H 2 follows with formation of new N-H bonds to the amine nitrogen of the polyaniline chain. Subsequent charge transfer between adjacent amine nitrogens returns the polyaniline back to its polaronic, doped, emeraldine salt state with a release of hydrogen. As part of an effort to further understand the hydrogen-polyaniline nanoﬁber sensor mechanism, we have examined the response with sensor substrates made with platinum electrodes that also have the ability to be heated internally. The response to hydrogen with platinum electrodes is dramati- cally different from that with gold electrodes. As described earlier for gold, we ﬁnd a small decrease in resistance upon exposure to hydrogen. However, with platinum we ﬁnd a large increase in resistance with hydrogen exposure. Thus with a simple change in the electrode metal, the response to hydrogen is much larger and with opposite polarity. This is not the case with other gases we have studied, such as ammonia or hydrogen chloride, and this phenomenon appears to be unique to hydrogen detection. In this study, hydrogen sensing by polyaniline nanoﬁbers using gold and platinum electrodes is investigated and compared by monitoring resistance changes. The Pt-polyaniline interface is the origin of the hydrogen sensitivity, so a careful cleaning and conditioning regime was developed. The gold-polyaniline nanoﬁ- ber sensors are completely inhibited by humidity, while the platinum-polyaniline nanoﬁber sensors are not. The platinum sensors are somewhat desensitized in the presence of oxygen, but the response recovers fully and quickly (they are fully reversible) while the gold sensors are unaffected by oxygen. The platinum sensors show a much greater sensitivity than the gold sensors for detecting low concentrations of hydrogen (e10 ppm). We ﬁnd that with gold electrodes the current versus voltage plots are linear (ohmic) for the sensors under hydrogen exposure. With platinum * To whom correspondence should be addressed. E-mail: bruce.h.weiller@ aero.org. † The Aerospace Corporation. ‡ University of California, Los Angeles. J. Phys. Chem. C 2009, 113, 6444–6449 6444 10.1021/jp810500q CCC: $40.75  2009 American Chemical Society Published on Web 03/26/2009