Reversible Resistance Modulation in Mesoscopic Silver Wires

Anal. Chem. 2005, 77, 5205-5214

Reversible Resistance Modulation in Mesoscopic Silver Wires Induced by Exposure to Amine Vapor B. J. Murray, J. T. Newberg, E. C. Walter, Q. Li, J. C. Hemminger, and R. M. Penner*

Department of Chemistry, University of California, Irvine, California 92697-2025

Ensembles of silver nanowires (AgNEs) with diameters ranging from 200 nm to 1.0 µm have been prepared by electrochemical step edge decoration. These AgNEs showed a rapid ( |-50%|) irreversible decrease in resistance was seen upon exposures to acids including HCl, HNO3, and H2SO4. Based on these and other data, we propose a model in which oxidized constrictions in silver nanowires limit the conductivity of the wire and provide a means for “gating” conduction based on the protonation state of the oxide surface. Gold, silver, platinum, and palladium are ductile and resistant toward oxidation in aqueous solutions over a broad range of pH. These two attributes recommend noble metal nanowires for applications in chemical sensors. However, in contrast to semiconductor nanowires,1,2 the resistance of metal nanowires is not affected by the capacitive coupling of charge carriers to immobilized charge (e.g., ions) at the surface of the wire. Instead, the electric field produced by immobilized ions is screened by the high carrier density within a few angstroms of the surface resulting in no significant change in carrier density or resistance. The resistance of ultrathin metal films is nevertheless increased * To whom correspondence should be addressed. E-mail: [email protected]. (1) Cui, Y.; Wei, Q. Q.; Park, H. K.; Lieber, C. M. Science 2001, 293, 12891292. (2) Hahm, J.; Lieber, C. M. Nano Lett. 2004, 4, 51-54. 10.1021/ac050636e CCC: $30.25 Published on Web 07/01/2005

© 2005 American Chemical Society

by 1-5% upon the chemisorption of molecules (ions or neutrals) at the surfaces of an ultrathin (|< 50 nm) metal film primarily because inelastic electron scattering is facilitated by the localized states associated with a covalent bond.3-10 Bohn and coworkers11-13 have explored this “boundary layer scattering” effect for use in chemical sensors, but this prior work has so far involved metal films, not nanowires. Despite the limitations imposed by the physics of conduction in metals, there are two prior examples involving the use of metal nanowires in chemical sensing: Tao and co-workers14-17 have demonstrated that gold nanowires with a length comparable to the electron mean free path (∼50 nm) show a resistance that is increased by up to 100% by the adsorption from solution of molecules such as thiols and amines. The nanowires in that case were ∼1.0 nm in diameter, and the observed resistance increase is irreversible. We have shown18,19 that palladium nanowires can be used to detect hydrogen gas by a mechanism involving the spontaneous facture of these wires during the first exposure to hydrogen and the reversible closing of the resulting gaps on subsequent exposures to hydrogen. The spontaneous fracturing was caused by hydrogen embrittlement of the palladium coupled with a phase transition of the PdHx involving a (3% volume change; this phase change was also responsible for the opening and closing of fracture gaps during subsequent exposures to hydrogen.18 These two specialized examples are the exceptions (3) Holzapfel, C.; Akemann, W.; Schumacher, D.; Otto, A. Surf. Sci. 1990, 227, 123-128. (4) Holzapfel, C.; Stubenrauch, F.; Schumacher, D.; Otto, A. Thin Solid Films 1990, 188, 7-19. (5) Hsu, C. L.; McCullen, E. F.; Tobin, R. G. Chem. Phys. Lett. 2000, 316, 336342. (6) Lin, K. C.; Tobin, R. G.; Dumas, P.; Hirschmugl, C. J.; Williams, G. P. Phys. Rev. B 1993, 48, 2791-2794. (7) Tobin, R. G. Phys. Rev. B 1993, 48, 15468-15470. (8) Tobin, R. G. Surf. Sci. 2002, 502, 374-387. (9) Winkes, H.; Schumacher, D.; Otto, A. Surf. Sci. 1998, 400, 44-53. (10) Wissmann, P., Ed. Thin Metal Films and Gas Chemisorption; Elseview:New York, 1987. (11) Zhang, Y.; Terrill, R. H.; Bohn, P. W. Anal. Chem. 1999, 71, 119-125. (12) Zhang, Y.; Terrill, R. H.; Bohn, P. W. J. Am. Chem. Soc. 1998, 120, 99699970. (13) Zhang, Y. M.; Terrill, R. H.; Bohn, P. W. Anal. Chem. 1999, 71, 119-125. (14) Li, C. Z.; H. S.; Tao, N. J. Phys. Rev. B 1998, 58, 6775-6778. (15) He, H. X.; Tao, N. J. Adv. Mater. 2002, 14, 161-+. (16) Bogozi, A.; Lam, O.; He, H. X.; Li, C. Z.; Tao, N. J.; Nagahara, L. A.; Amlani, I.; Tsui, R. J. Am. Chem. Soc. 2001, 123, 4585-4590. (17) Li, C. Z.; He, H. X.; Bogozi, A.; Bunch, J. S.; Tao, N. J. Appl. Phys. Lett. 2000, 76, 1333-1335. (18) Walter, E. C.; Favier, F.; Penner, R. M. Anal. Chem. 2002, 74, 1546-1553. (19) Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Science 2001, 293, 2227-2231.

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that prove the rule: Metal nanowires seem to be poor candidates for use in chemical sensors. Recently,20 we reported that the resistance of polycrystalline nanowires composed of silver, copper, gold, and platinum increases upon exposure to the vapor of amines including ammonia, trimethylamine, and pyridine. In the case of silver, this resistance change is rapid ( 18.8 MΩ). Sulfate and nitrate solutions were brought to a pH of ∼1-2 using sulfuric (J. T. Baker, Ultrex II Ultrapure Reagent) or nitric acid (Fisher, Certified ACS Plus), respectively. The acetate and fluoride solutions had pH’s of 6.5 and 5.0, respectively, and were not further acidified. In addition, all plating solutions contained 1 mM saccharine (Aldrich 98%+). The silver nanowires prepared from all four of the electrolytes listed above behaved similarly in sensors; however, we focus attention in this paper on those prepared from sulfate solutions. Nanowire electrodeposition was carried out using a one-compartment, glass electrochemical cell with a volume of 50 mL and a Teflon lid. A small area (30 mm2) of the highly oriented pyrolytic graphite (HOPG) basal plane was exposed to the plating solution using an O-ring and a Teflon holder. In addition to this working electrode, a platinum counter (20) Murray, B. J.; Walter, E. C.; Penner, R. M. Nano Lett. 2004, 4, 665-670. (21) Examples include: PotraSens II (Analytical Technologies Inc., Collegeville, PA), which can detect 0-500 ppm, Pac III (Drager Safety Inc. Leubeck, Germany), which can detect 0-200 ppm, ToxiPro (biosystems, Middletown, CT) can detect 0-100 ppm, and R.M. Technologies Inc. (Mt. Laurel, NJ), which can detect 0-20 000 ppm. “Zero” here is taken to be the limit of detection for the sensor, which is in the 10-25 ppm range in all cases. The response times for these sensors are in the 20-45-s range (T90).

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Analytical Chemistry, Vol. 77, No. 16, August 15, 2005

Chart 1

electrode, and a silver wire reference electrode were used. The potential of the working electrode was controlled using a Gamry PC4-300 potentiostat. Microscopy. Scanning electron microscopy (SEM) and energydispersive X-ray analysis were carried out using a Phillips model XL-30FEG operating at 10-25 keV. Transmission electron microscopy (TEM) was carried out using a Phillips CM-20 at an accelerating voltage of 200 keV. Samples for TEM analysis were prepared by cleaving a thin layer of graphite from the basal plane of a surface on which nanowires had been electrodeposited. This graphite raft was placed onto a gold TEM grid (Ted Pella). Especially at its edges, this graphite raft was transparent to electrons and images of the supported nanowires could be acquired. Conductive Tip Atomic Force Microscopy. The resistance of 5-10-µm sections of individual silver nanowires within transferred AgNEs were probed using an experiment shown in Chart 1. A gold contact was first evaporated across an ensemble of nanowires and an atomic force microscope (AFM, Park Scientific Instruments) was then used in conjunction with a conductive, platinum-coated tip (MicroMasch CSC21/Ti-Pt/15), to electrically isolate 5-10-µm sections of a single nanowire. Surface Analysis. X-ray photoelectron spectra (XPS) were obtained using an Escalab MKII photoelectron spectrometer (VG Scientific). The XPS experiments were performed using an aluminum anode X-ray source (KR ) 1486.6 eV). The energy of the ejected photoelectrons was detected from a ∼1 × 2 mm area of the surface using a 150-mm hemispherical electron energy analyzer. Spectra were energy-corrected using the C(1s) peak at 284.5 eV from the HOPG substrate, and the intensity was adjusted using a Shirley background subtraction. The peak positions used to determine binding energies were determined using a Gaussian fit for the O(1s) peaks and with derivatives for the Ag(3d) peaks. Sensor Fabrication. Silver nanowire arrays were lifted off of the graphite surface and transferred onto a cyanoacrylate-coated glass microscope slide using a previously described procedure.18 Briefly, silver nanowires on the HOPG surface were pressed onto a drop of cyanoacrylate (Special T super glue, Satellite City) supported on the microscope slide. Pressure on the HOPG was maintained for 1-5 min, the cyanoacrylate was permitted to dry for several hours, and finally, the graphite was separated from the cyanoacrylate using a razor blade. Two types of electrical contacts were used: Contacts composed of colloidal silver paste (Ted Pella) were applied manually to the ends of ensembles of nanowires using a light microscope. Alternatively, gold contacts

Figure 1. (a) Schematic diagram of voltage program used to prepare silver nanowires in this study. (b) Cyclic voltammogram of a typical silver plating solution at a HOPG electrode at a scan rate of 20 mV/s. This aqueous solution contained 1 mM Ag2SO4, 1 mM saccharine, and 0.1 M Na2SO4, pH ∼1-2. At the growth potential (Egrowth ∼ -0.16 V), silver was electrodeposited only on the silver nuclei formed during the preceding nucleation pulse. Therefore, during growth, these nuclei increase in size and coalesce to form a continuous silver nanowire. (c) Typical current vs time behavior for the growth of silver mesowires after the application of the nucleation pulse.

were prepared by hot filament evaporation through a shadow mask. No effect of the type of contact on the response of AgNEs was discerned, and the electrical contact itself was not responsible for the responses reported here. This possibility was ruled out by preparing many AgNEs with contacts that were protected by a layer of TorrSeal epoxy. Gas Sensing. Sensing measurements at atmospheric pressure were conducted using an aluminum flow cell with an internal volume of 20 cm3. This flow cell had electrical feedthroughs for the electrical contacts to the sensor. Within this cell, the sensor was constantly exposed to a stream of ultrapure nitrogen gas (Airgas) at a flow rate of 1000 sccm. A 5-mV bias was applied to the nanowire arrays using a Keithley 428 current amplifier, which also amplified the current. Ammonia vapor was obtained from an anhydrous liquefied tank of NH3 (Airgas, grade 4, 99.99%). Flows of NH3, N2, and other gases into the flow cell were regulated using mass flow controllers (MKS 1479A) controlled with a computerinterfaced MKS Instruments 647C. Sensing measurements in a vacuum and variable-temperature measurements of the resistance were conducted within an aluminum chamber that was evacuated using a turbomolecular pump to ∼10-7 Torr. RESULTS AND DISCUSSION A. Nanowire Growth and Sensor Fabrication. Ensembles of silver nanowires were prepared by electrodepositing silver selectively at the step edges present22 on the basal plane of a HOPG electrode surface as previously described.23 Briefly, a threestep procedure involving three sequential voltage pulses (Figure 1a) was employed as follows: First, the graphite surface was electrochemically oxidized at +1.1 V versus Ag/Ag+ for 5 s. Second, a nucleation pulse of -1.0 V × 100 ms was applied to produced silver nanoparticles along each step edge. Finally, these silver nanoparticles were grown at -0.18 V for hundreds of (22) The step edges exploited for nanowire growth are those that are naturally present on these ZYB-grade HOPG surfaces after cleaving with adhesive tape. (23) Walter, E. C.; Murray, B. J.; Favier, F.; Kaltenpoth, G.; Grunze, M.; Penner, R. M. J. Phys. Chem. Soc. 2002, 106, 11407-11411.

seconds until they coalesced into continuous nanowires. In the cyclic voltammogram of Figure 1b, it can be seen that this growth potential (Egrowth) is positive of the onset for silver reduction on the first scan at a clean HOPG surface, but it is just negative of the onset for the second and subsequent scans. Consequently, during “growth”, silver was electrodeposited on the silver nuclei produced during the nucleation pulse but not on the graphite surface (i.e., no new silver nuclei were formed). Nanowire growth was characterized by a constant current as shown in Figure 1c. For hemicylindrical nanowires, this constant current produces a growth law dictating that the nanowire diameter increases in proportion to (deposition time)1/2 24 and this proportionality was observed for silver nanowires ranging from 130 to 260 nm in diameter (Figure 2a). Nanowires in this diameter range were used for most of the sensing experiments to be discussed here. Scanning electron micrographs of these nanowires (Figure 2b-e) revealed a beaded morphology in which diskshaped polycrystalline silver particles were stacked along the axis of the nanowire. This unique morphology was a consequence of the nanowire growth procedure: Each of the beads seen in these wires originated from a nucleus generated during the nucleation pulse; the disk geometry is produced by restricted diffusive transport (restricted because of the proximity of other particles) coupled with efficient radial transport in the directions perpendicular to the water. The silver nanowires prepared using electrochemical step edge decoration (ESED) have two attributes that facilitate the preparation of AgNEs: These nanowires were organized into parallel ensembles containing hundreds of wires (∼2-10 µm-1 lateral density), and these nanowires were up to 1 mm in length. AgNEs were prepared by transferring nanowires from the graphite surfaces on which they were deposited onto glass microscope slides.18 This transfer involved embedding the nanowires in a cyanoacrylate film as described in detail in the Experimental Section. At the end of this procedure, more than 95% of the silver nanowires were transferred from the HOPG surface onto (24) Zach, M. P.; Ng, K. H.; Penner, R. M. Science 2000, 290, 2120-2123.


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Figure 2. (a) Nanowire diameter vs (deposition time)1/2 for the electrodeposition of silver nanowires on freshly cleaved HOPG. Between two and five samples were deposited for each deposition time; wire diameters were measured using the SEM at several locations on the surface, and on multiple wires at each location. Error bars show (1 σ for the size distribution, (b, c) SEMs of silver nanowires grown for 30 s at -0.18 V vs Ag0/Ag+. (d, e) SEMs of wires deposited for 1500 s at -0.18 V vs Ag0/Ag+.

Figure 3. (a) Optical micrograph and (b) SEM of a typical AgNE with silver paint contacts.

the surface of a cyanoacrylate disk 5-10 mm in diameter (Figure 3a). Electrical contactssconsisting of silver paint or evaporated goldswere then applied. As shown in the low-magnification SEM image of Figure 3b, a typical AgNE contained 10-100 nanowires that appeared by SEM to be continuous from contact to contact. The actual number of wires participating in conduction for each AgNE was unknown. B. Measurements of Resistance Modulation of Silver Nanowire Ensembles by Ammonia. Figure 4a shows the resistance of a typical AgNE as it was exposed to 110 s pulses of ammonia vapor at concentrations ranging from 2.4 to 29%. At the highest ammonia concentration we tested in this range, a constant resistance value was not observed with the 110-s pulse duration, but for ammonia concentrations ranging from 2.4 to 17%, the resistance of this AgNE increased linearly (Figure 4b) by up to 18%. AgNEs attained a new, time-invariant resistance within 50 s of a change in the NH3 concentration; this resistance was a true function of the ammonia concentration in the sense that it was not influenced by the NH3 exposures that preceded it (Figure 4b). For NH3 concentrations higher than 20%, AgNEs showed an initial, rapid resistance increase for 20 s followed by a slower increase. A constant resistance value was not observed within 110 s. 5208


Because the onset of the resistance increase for AgNEs occurs in less than 1 s after exposure to NH3, pulses of NH3 as short as 1 s in duration caused a concentration-dependent increase in the resistance. For example, the resistance of a AgNE subjected to 5-s pulses of ammonia (Figure 4c) increased continuously during the 5-s exposure and at a rate that depended on the concentration of ammonia. At the end of the pulse, the resistance decayed exponentially in either N2 or air for 300 s (Figure 4d). For 5-s pulses of NH3, the maximum amplitude of the relative resistance change (∆R/Ro) was linearly correlated with the NH3 concentration over the range from 0.5 to 20% NH3 as shown for three different AgNEs in Figure 4e. The rapid response of AgNEs and the reversibility of the response suggests that AgNEs can function as ammonia sensors over this concentration range. A puzzling aspect of the response of AgNEs to ammonia is illustrated by the data shown in Figure 4e: Different AgNEs showed a wide range of sensitivities to NH3. This variability was observed even for AgNEs prepared from silver nanowires of nearly identical mean diameter. The responses of the three AgNEs for which data are plotted in Figure 4e were normalized and replotted in Figure 4f to demonstrate that the calibration curves for sensors that show a wide range of sensitivities to ammonia have the same

Figure 4. (a) Resistance vs time for a typical AgNE exposed to various concentrations of NH3 in flowing nitrogen as indicated. The resistance increased rapidly during the first 5 s of NH3 exposure followed by a gradual increase to a constant value. (b) The magnitude of this final resistance value varied linearly with concentration from 0 to 30% NH3. (c) AgNEs were exposed to three sequential 5-s pulses at the NH3 concentrations indicated and otherwise were under flowing N2. (d) Resistance vs time of a AgNE showing the resistance recover over roughly 300 s when NH3 was removed from the sensing environment. (e) Response vs NH3 concentration for three unique AgNEs. The order of ammonia exposure was randomly selected and different for each AgNE. Error bars correspond to the standard deviation of three identical exposures. (f) Although the magnitude of the resistance change varied between sensors, when normalized, all sensors showed a similar response range.

Figure 5. Approximately half of the AgNEs with diameters ranging from 100 to 300 nm (a) showing an enhanced response (∆R/R0), defined as larger than 5%sthe maximum resistance change seen in thin metal films. (b) Wires grown for 10 000 s reached a diameter of 0.9-1.1 µm and were unlikely to show an enhanced response. (c) Thin silver films were deposited using multiple nucleation and growth cycles. They all showed the responses expected for thin metal films. (d) The maximum ∆R/R0 when a sensor was first exposed ammonia is plotted vs initial resistance. When the response is plotted on a log scale (e), the enhanced response from the nanowires samples becomes clear.

functional form. This underlying similarity leads to the conclusion that a common mechanism is responsible for the response in all of the AgNEs. The full extent of this sensitivity variability for different AgNEs is shown in Figure 5. Here we compare three types of sensors: (1) AgNEs prepared from nanowires in the 100-300-nm-diameter range (Figure 5a), (2) AgNEs prepared from 900-nm to 1.0-µm-

diameter wires (Figure 5b), and (3) sensors prepared from silver films (Figure 5c). In Figure 5d and e, the normalized resistance response to 100% NH3, ∆R/Ro, is plotted versus the initial resistance of the AgNE in air, Ro. The largest responses are seen for AgNEs containing 100-300-nm-diameter wires, but these AgNEs showed ∆R/Ro values that varied from 0.1 to 3000%! The sensitivity of AgNEs was weakly correlated with Ro, as shown in Analytical Chemistry, Vol. 77, No. 16, August 15, 2005

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Figure 5e, with the most resistive ensembles showing the largest resistance increases upon exposure to NH3. Wires in the 900-nm to 1.0-µm range showed a response to NH3 ranging from ∆R/Ro ) 0.1-20% while silver films, prepared from the same plating solutions as the wires, varied from 0.09 to 2%. For the remainder of this paper, we attempt to understand the mechanism responsible for this resistance modulation and to identify the origin of the variability in the sensitivity to NH3 documented in Figure 5. C. Experimental Results Pertaining to the Mechanism of the Observed. Resistance Modulation. In ultrathin (t < 50 nm) metal films, an increase in the resistance of the film by up to ∆R/ Ro ) +5% has been observed in many independent experiments involving many different metal-adsorbate systems (cf. refs 5-8, 12, 13, and 25-32). In several cases (e.g., refs 7 and 8), it has been convincingly demonstrated that the origin of this resistance increase is an adsorbate-induced decrease in the specularity of electron scattering at the surface of the film where molecules have adsorbed. It is important to note here that this effect has been observed for both single crystalline and polycrystalline metal films. The resistance response seen here for polycrystalline silver films (Figure 5c-e) is fully consistent in direction and magnitude with this mechanism. Certainly, polycrystalline metal nanowires that are less than 100 nm in diameter should manifest a boundary layer scattering effect that is similar in magnitude to that seen in ultrathin polycrystalline metal films. However, the observations summarized in Figure 5 are incompatible in two ways with this sensing mechanism: First, the magnitude of the resistance changes seen for AgNEs is up to 1000 times larger than is seen for the largest boundary layer scattering effects reported for molecular adsorbates in ultrathin metal films. Second, we observe ∆R/Ro values far exceeding 5% even for silver wires with diameters in the 900nm to 1.0-µm rangesa factor of ∼20 larger than the electron mean free path in silver. We propose a different sensing mechanism to account for these results, and this mechanism accounts for the observations reported in Figures 4 and 5 above as well as the results of two other experiments that we discuss next. These experiments are (1) conductive tip AFM measurements of NH3-induced resistance changes for single silver nanowires and (2) measurements of the temperature dependence of resistance for AgNEs: Experiment 1. We conducted measurements of the resistance of 5-10-µm segments of individual silver nanowires within AgNEs during exposures to NH3 using a conductive tip AFM. In these experiments, a 5-10-µm length of an individual silver nanowire located within an NH3-sensitive AgNE was electrically isolated with the aid of a conductive tip AFM as shown in Chart 1, and the resistance of this isolated wire segment was monitored during exposures to NH3 vapor. A typical topographic image of 250-nm(25) Cabrera, A. L.; AguayoSoto, R. Catal. Lett. 1997, 45, 79-83. (26) Cabrera, A. L.; Garrido-Molina, W.; Morales-Leal, E.; Espinosa-Gangas, J. Langmuir 1998, 14, 3249-3254. (27) Cabrera, A. L.; GarridoMolina, W.; Colino, J.; Lederman, D.; Schuller, I. K. Phys. Rev. B 1997, 55, 13999-14004. (28) Cabrera, A. L.; Morales, E.; Hasen, J.; Schuller, I. K. Appl. Phys. Lett. 1995, 66, 1216-1218. (29) Fried, G. A.; Zhang, Y. M.; Bohn, P. W. Thin Solid Films 2001, 401, 171178. (30) Krastev, E. T.; Kuhl, D. E.; Tobin, R. G. Surf. Sci. 1997, 387, L1051-L1056. (31) Lusk, A. L.; Bohn, P. W. J. Phys. Chem. Soc. 2001, 105, 462-470. (32) McCullen, E. F.; Hsu, C. L.; Tobin, R. G. Surf. Sci. 2001, 481, 198-204.

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Figure 6. (a) AFM (10 µm × 10 µm), acquired using a platinumcoated silicon tip, of silver nanowires embedded in a cyanoacrylate film on glass. (b) Resistance map of the same region shown in (a). In this image, the resistance was measured between the AFM tip and an evaporated gold contact located 20 µm from the lower right corner of this image. (c) Resistance versus time plot for a 5-µm section of a silver nanowire during two exposures to pure NH3. (d) Resistance vs distance plot for the section of nanowire interrogated in (c), and for another nanowire section that showed no resistance increase in response to ammonia exposure.

diameter silver nanowires embedded in a cyanoacrylate film is shown in Figure 6a. A resistance map of this same area, recorded simultaneously with the topographic image of Figure 6a, is shown in Figure 6b. Just four wires are seen in this resistance map, indicating that only these three are electrically continuous to the evaporated gold contact located ∼20 µm from the lower, right edge of this image window. Resistance measurements in N2 and NH3 were obtained for 20 wire segments in 15 different AgNEs, all of which showed an overall ∆R/Ro > 5%. Of these 20 data sets, just 2 showed a measurable response to ammonia. The resistance versus time plot for one of these, shown in Figure 6c, shows a resistance increase of +80-100% upon exposure to ammonia. As shown in Figure 6d, this wire segment (labeled i) had a resistance of 42.8 kΩ µm-1, a factor of ∼10 000 higher than the resistance of 1-5 Ω µm-1 seen for wire segments that showed no change upon NH3 exposure (e.g., the trace labeled ii). From these measurements, we conclude that sensitivity of the wire resistance to ammonia is not uniformly distributed along individual silver nanowires, but instead is concentrated in highly resistive wire segments comprising just ∼10%, on average, of the nanowire length.20 Experiment 2. The temperature dependence of the resistance was measured for 20 AgNEs in a vacuum. The electrical resistance of a metal wire, R, is directly proportional to temperature and given by

R ∝ RoR(T - To)

(1)

where Ro and To are a reference resistance and temperature and

Figure 7. (a) Resistance versus temperature for two AgNEs with the indicated sensitivities to ammonia. Here, the resistance of the AgNE, R, was normalized by its resistance at 288 K, R288. (b) Plot summarizing the slope of the measured R vs T for 20 AgNEs. Data for AgNEs that were good NH3 sensors (∆R/Ro > 5%, (O)) and data for AgNEs that showed a weak response to NH3 (∆R/Ro < 5% (b)).

R is the temperature coefficient of resistivity, +3.74 × 10-3 for silver.33 Over the narrow temperature range examined here, linear R versus T in accordance with eq 1 were observed for all AgNEs, as shown in Figure 7a. The sensitivity to ammonia of all 20 AgNEs were also measured. In Figure 7b, R is plotted as a function of the sensitivity to ammonia (∆R/Ro in 100% NH3), for each of the 20 AgNEs. Two features of these data are interesting: First, all 20 AgNEs showed R values smaller than +3.74 × 10-3; however, R varied dramatically from device to device by over 5 orders of magnitude; 10 of the AgNEs had an R that was more than 1 order of magnitude smaller than for bulk silver. Since boundary layer scattering effects cannot be significantly affecting the resistance of wires that are, on average, 200 nm in diameterssome other structural or chemical attribute of these wires is responsible for this behavior. Second, four AgNEs produced a negative, R (as shown in Figure 7a) and all four of these were good sensors for ammonia with ∆R/Ro in the range from 12 to 2300%. A negative R is not observed for metallic conduction but, instead, is characteristic of a thermally activated conduction process and is typical of semiconductors that show a resistance that decreases exponentially with temperature:

R ∝ exp(∆E/kT)

(2)

where ∆E is the energy required for the creation of mobile charge carriers. Both depressed values for R and negative values for R are consistent with the presence of semiconducting silver oxide wire segments in some or all (respectively) of the nanowires in an AgNE. D. Chemically Responsive Interparticle Boundaries (CRIBs). Based on the data presented above and previously,20 we propose the existence of “chemically responsive interparticle boundaries” or CRIBs. A CRIB is a narrow constriction along the axis of a nanowire composed of semiconducting AgO or Ag2O, as shown in Figure 8a. This oxide bridge is formed when a metallic constriction between two silver grains undergoes oxidation in air, or as we shall see, it can be produced on demand by electro(33) Calvert, J. Copper, SIlver, Gold; The University of Denver Press: Denver, CO, 2002.

chemical oxidation in basic aqueous electrolytes. CRIBs have three properties: (1) a large resistance of several hundred kΩ and a negative dR/dT (both characteristic of silver oxides34), (2) a resistance that increases reversibly upon exposure to amines, and, (3) a low “spatial frequency” in nanowires of 0.01-0.1 µm-1 (that is, one CRIB for every 10-100 µm of silver nanowire length). We further hypothesize that the AgO or Ag2O segment is “chemically responsive” because the oxygen-terminated surface of these oxides is basic and negatively charged. As depicted in Figure 8b, this negative charge imposes a coulomb barrier for the transport of electrons through the n-type oxide (both Ag2O35 and AgO36 are intrinsically n-type as a consequence of oxygen vacancies), and the negative charge density on the oxide is increased by deprotonation of the oxide induced by gas-phase amines. An ensemble of nanowires connected in parallel has a resistance that depends disproportionately on the least resistive wire in the ensemble. For example, the resistance of an ensemble, of 100 × 10 kΩ wires is Rtotal ) (10 kΩ/100) ) 100 Ω. But if just 1 of these 100 wires has a lower resistance of 100 Ω, Rtotal is lowered by half to 50 Ω. We therefore expect AgNEs that contain one ammonia-insensitive nanowiresa nanowire having no CRIBs, and a relatively low resistancesto be dominated by the properties of this one “bad” wire both in terms of Ro (it should be relatively less resistive) and in terms of its sensitivity to ammonia (it should be relatively insensitive). CRIBs thereby provide a qualitative rationalization for both the variability of ∆R/Ro that is observed between different AgNEs (e.g., Figure 5e), and the correlation between ∆R/Ro and Ro (also Figure 5e). CRIBs are also consistent with the AFM investigations summarized in Figure 6 above and the temperature dependence data of Figure 7. Specifically, CRIBs composed of Ag2O will contribute a negative R, and the influence of a single CRIB on the temperature dependence of a nanowire will depend on the resistance of the CRIB relative to the total resistance of the wire. At a minimum, the presence of CRIBs would be expected to weaken the otherwise metallic temperature (34) The electronic conductivity of silver metal, Ag2O, and AgO are, respectively: 1.6 × 10-6, (7-30) × 10-5, and 12 Ω‚cm. (35) Tjeng, L. H.; Meinders, M. B. J.; Vanelp, J.; Ghijsen, J.; Sawatzky, G. A.; Johnson, R. L. Phys Rev B 1990, 41, 3190-3199. (36) Farhut, E.; Donnadieu, A.; Robin, J. Thin Solid Films 1975, 29, 319.


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Figure 8. (a) Schematic diagram of a CRIB, (b) Mechanism by which resistance in CRIBs is modulated by bases (e.g., NH3) and acids (e.g., HNO3). This mechanism assumes the CRIB is composed of an n-type oxide, (c) SEM image of a AgNE showing constrictions that are good candidates for CRIBs. Such constrictions are common morphological features that form naturally during the synthesis of these nanowires. (d) High-magnification SEM image showing one such constriction with a diameter of ∼30 nm.

dependence and reduce the apparent R from its bulk valuesas seen in Figure 7b. Indeed, the existence of CRIBs is supported by overwhelming indirect evidence, but not by any direct experimental data. E. Adding CRIBs to Silver Nanowires Using Electrochemical Oxidation. If the CRIB hypothesis is correct and the CRIBs present in electrodeposited silver nanowires consist of Ag2O, then it may be possible to synthesize CRIBs by electrochemical oxidation under conditions favoring the formation of silver oxides. As shown in the Pourbaix diagram of Figure 9a, both Ag2O and AgO are thermodynamically accessible in alkaline solutions with pH >9, and prior work37,38 has confirmed that Ag2O and AgO are generated in succession upon the oxidation of silver in KOH solutions. Cyclic voltammograms of silver nanowires recorded in a pH 12.0 solution (Figure 9b) show reversible waves for Ag2O formation (at +0.30 V) and AgO formation (at +0.55 V) as reported by Kotz and Yeager.39 SEMs of silver nanowires recorded before and after oxidation at 0.70 V (vs Ag+/Ag°) for 10 s at pH 12.0 are shown in the SEM images of Figure 9c. Oxidized nanowires showed increased roughness, increased diameter, and a reduced brightness in the backscattered electron SEM image, compared with nonoxidized silver wires all qualitatively as expected. (37) Dirkes, T. P. J. Electrochem. Soc. 1959, 106, 920. (38) Wales, C. P.; Burbank, J. J. Electrochem. Soc. 1959, 106, 885. (39) Kotz, R.; Yeager, E. J. Electroanal. Chem. 1980, 111, 105-110.

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AgNEs were characterized by XPS both before and after electrochemical oxidation (Figure 9d,e and Table 1). As seen in Figure 9d (sample i), HOPG was devoid of silver upon exposure to the silver electroplating solution without anodization. Freshly synthesized silver nanowires that were exposed to laboratory air for 1 h (sample ii) and 18 h (sample iii) showed an increase in the O(1s) signal with a binding energy (BE) of 532.2 eV (Figure 9e). This oxygen is not associated with silver oxidation and is, instead, attributed to contamination. Although these nanowires showed no evidence of silver oxidation (Table 1; Figure 9d,e), up to ∼5% of an oxide monolayer may have been present and undetected by XPS.40 Wires oxidized at +0.70 V versus Ag0/Ag+ for 10 s (sample iv) showed a Ag(3d5/2) peak that was shifted to lower BE when compared to with freshly deposited AgNEs (sample ii). A new O(1s) peak with a BE of 529.4 eV was also observed on this sample. The BEs seen for Ag(3d5/2) and O(1s) are both consistent with bulk Ag2O.41 A longer oxidation time of 1500 s resulted in larger shifts in Ag(3d) and O(1s) peaks, consistent with further oxidation of the AgNEs, but as shown in Table 1, these shifts are insufficient for AgO and we therefore (40) XPS has a 0.1% atomic resolution. This corresponds to ∼1% of a monolayer (ML). For the samples probed here, electrodeposited silver wires cover ∼20% of the graphite surface area. Thus, the limit of detection for oxide on the silver nanowires is approximately 1.0%/20% ) 5.0% ML. (41) Hammond, J. S.; Gaarenstroom, S. W.; Winograd, N. Anal. Chem. 1975, 47, 2193-2199.

Figure 9. (a) Pourbaix diagram for silver. Indicated here are regions of thermodynamic stability for various silver-containing phases in equilibrium with aqueous 1.0 mM Ag+ as a function of applied potential and pH. At a pH of 12, electrochemically deposited silver metal may be oxidized first to Ag2O and then to AgO. (b) Cyclic voltammogram for silver in 1 mM AgF, 0.01 M NaOH at pH ∼12. Voltammetric waves for silver oxidation to Ag2O and Ag2O oxidation to AgO are observed. (c) SEM images of silver wires prepared from a pH 1 plating solution (top) and similar wires after transfer to pH 12 plating solution and oxidation at +0.70 V vs Ag°/Ag+ for 10 s (bottom). This oxidation process was accompanied by a prompt, visible change in the appearance of the electrode surface from light gray to dark brown. (d, e) Ag(3d) XPS spectra (d) and O(1s) XPS spectra (e) for four nanowire samples, and bare HOPG. These samples were as follows: (i) freshly cleaved HOPG, (ii) freshly deposited, ∼200-nm-diameter silver wires on HOPG, (iii) silver wires on HOPG after exposure to laboratory air for 18 h, (iv) silver wires on HOPG after oxidization at +0.70 V in pH 12, 1 mM Ag+ for 10 s, and (v) silver wires on HOPG after oxidization for 1000 s. (f) A reproduction of Figure 5e with the ∆R/Ro vs Ro data for 16 oxidized AgNEs added (b). The weakest response from among these 16 devices showed a ∆R/Ro ) 25% in 100% NH3.

Table 1. Measured Binding Energies for O (1s) and Ag (3d) Photoelectrons for Electrodeposited Silver Nanowires and Electrochemically Oxidized Nanowires sample silver, fresh silver, 18 h in air 10-s oxidationc 1000-s oxidation

Ag Ag2O AgO

electron level

BE (eV)a

Ag (3d5/2) O (1s) Ag (3d5/2) O (1s) Ag (3d5/2) O (1s) Ag (3d5/2) O (1s)

368.4 ndb 368.4 nd 368.0 529.4 367.8 528.9

Literature Valuesd Ag (3d5/2) Ag (3d5/2) O (1s) Ag (3d5/2) O (1s) O (1s)e

368.3 367.9 529.4 367.5 528.7 531.6

shift 0.0 0.0 -0.4 0.0 -0.6 -0.5 0.0 -0.4 0.0 -0.8 -0.3

a Binding energies were charge corrected using C(1s) ) 284.5 eV for graphite.43 b nd ) none detected, above background contamination. c This oxidation was carried out at +0.7 V vs Ag°/Ag+ in 1.0 mM Ag+, pH 11.7. d Literature values are from Hammond et al. referenced to Ag(3d5/2) ) 368.3 eV.43 e Two peaks are observed in the O 1s spectrum of AgO.44

conclude that a mixed silver(I/II) oxide is produced on these nanowires.

We prepared AgNEs using silver nanowires that were electrochemically oxidized for 10 s. The resistance versus response of such oxidized AgNEs to NH3 exposures were qualitatively identical to those shown for the AgNE arrays in Figure 4. Quantitatively however, etched AgNEs always showed large ∆R/ R0 values in excess of 25%, as shown in Figure 9f. The effect of the 10-s oxidation, then, is to remove much of the response variability for these devices, presumably by ensuring that every wire in the NE contains at least one CRIB. This result supports our original presumption that weak NH3 sensors are those for which one or more wires has no oxidized wire segments and no CRIBs. Electrochemically oxidized AgNEs show a larger response to NH3 because of the introduction of CRIBs into oxidized nanowires, but this result does not tell us anything new about how CRIBs cause an NH3-induced resistance modulation. If the mechanism proposed in Figure 8b is correct, we should expect gas-phase acids to induce a decrease in the resistance for AgNEs. This is experimentally observed, both for AgNEs prepared from asdeposited silver wires (data not shown) and for wires subjected to the etching procedure. Typical data for etched wires are shown in Figure 10. Here three AgNEs were exposed to different strong acids as shown. A large (∆R/Ro > 50%), irreversible decrease in Analytical Chemistry, Vol. 77, No. 16, August 15, 2005

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Figure 10. Normalized change in resistance, ∆R/Ro, for the exposure of AgNEs to the vapors of three strong acids: hydrochloric acid, sulfuric acid, and nitric acid. Large, irreversible decreases in the resistance were invariably observed. In the case of HCl only, exposures to ammonia are also shown both before and after the acid exposure. The response of nonoxidized silver wires to strong acids were indistinguishable from these.

the resistance was observed in each case. The response to acids was always irreversible, but this can be attributed simply to the Henry’s law constants, kH ) ca/Pa, for these acids, which is much largersby more than a factor of 500sthan kH values for the amines we examined.42 SUMMARY AND CONCLUSIONS We have demonstrated that ensembles of silver nanowires prepared by ESED show a resistance that is modulated, reversibly and rapidly, by ammonia and other amines. The amplitude of this resistance change (∆R/Ro up to +3000%) for wires as large as 300 nm in diameter argues against a boundary layer scattering mechanism. Instead, we advance a model involving the presence of Ag2O bridges along these nanowires. Since Ag2O is an n-type (42) Sander, R. Compilation of Henry’s Law Constants for Inorganic and Organic Species of Potential Importance in Environmental Chemistry; 1999, publisher, http://www.mpch-mainz.mpg.de/∼sander/res/henry.html. (43) Chastain, J. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corp.: Eden Prairie, MN, 1992. (44) Bielmann, M.; Schwaller, P.; Ruffieux, P.; Groning, O.; Schlapbach, L.; Groning, P. Phys. Rev. B 2002, 65, -.

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semiconductor (EBG ∼ 1.2 eV), the resistance of these bridges is sensitive to the ionization state of the hydroxyls at the oxide surface: A negatively charged, deprotonated surface is associated with a high resistance state for the bridge whereas a neutral, protonated surface is associated with a low resistance state. We term these Ag2O bridges chemically responsive interparticle boundaries or CRIBs. The existence of CRIBs is inferred from two experiments: (1) conductive tip AFM investigations that show the resistance modulation is confined to isolated sections of nanowires that possess a dramatically elevated electrical resistance and (2) measurements of the temperature coefficient of resistance, R, for AgNEs. Measured R values for AgNEs are always lower than the R for bulk silver. AgNEs that respond strongly to ammonia deviate most strongly from bulk with some AgNEs even exhibiting thermally activated conduction (negative R values). In addition, CRIBS also provide an immediate explanation for the variability in terms of the amplitude of the resistance response since wires without CRIBs can be expected to dramatically attenuate the resistance modulation caused by amines by shunting the current around more resistance wires in an ensemble that contains one or more CRIBs. We further demonstrate that CRIBs can be electrochemically synthesized. Electrochemical oxidation of silver nanowires in alkaline electrolytes induces the removal of silver (etching) at interparticle boundaries followed by Ag2O electrodeposition. This treatment substantially reduces the variability in sensitivity to NH3 with all oxidized AgNEs showing a resistance response (∆R/Ro) to NH3 of more than 25%. This result shows that charge gating can be “artificially” enhanced in metal nanowires. ACKNOWLEDGMENT This work was funded by the NSF (Grant CHE-0111557). J.C.H. acknowledges funding support from the Department of Energy (Grant DE-FG03-96ER45576). Donations of graphite by Dr. Art Moore, formerly of GE Advanced Ceramics, are gratefully acknowledged.

Received for review April 13, 2005. Accepted June 4, 2005. AC050636E

Reversible Resistance Modulation in Mesoscopic Silver Wires

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