NANO LETTERS
Amine Vapor Sensing with Silver Mesowires
2004 Vol. 4, No. 4 665-670
Benjamin J. Murray, Erich C. Walter, and Reginald M. Penner* Department of Chemistry, UniVersity of California, IrVine, IrVine, California 92697-2025 Received January 27, 2004; Revised Manuscript Received February 16, 2004
ABSTRACT Silver mesowires with diameters ranging from 150 to 950 nm and lengths of 100 µm or more were prepared by electrochemical step edge decoration. Upon exposure to ammonia (NH3) vapor, ensembles of these mesowires showed a resistance increase, ∆R/Ro, that was large (up to 10,000%), fast ( 5%. (E) Response of a silver wire array in air during exposure to 5 s pulses of pure NH3 and six other gases as indicated. The response of these arrays to H2S exposure was most often an irreversible increase in R, as shown here.
of metal wire arrays were measured using the circuit shown in the inset of Figure 1C. The current produced by an applied voltage of 2.5-5 mV was measured using a Keithley 428 current amplifier coupled to a Keithley 2000 computerinterfaced multimeter. This study focused primarily on the change in resistance of silver mesowires upon exposure to ammonia vapor at ambient temperature. Ammonia is known to chemisorb on Nano Lett., Vol. 4, No. 4, 2004
Figure 3. (A) Schematic diagram of the circuit used to probe individual silver nanowires using an atomic force microscope (AFM) equipped with a conductive tip in dry nitrogen and NH3. (B) AFM topography image of an array of silver wires embedded in cyanoacrylate. This image is 10 µm × 10 µm. (C) Conductivity map of same area shown in (B) and acquired simultaneously. (D) Resistance versus distance for two silver mesowires probed by AFM. Wire “x”, which was 100 in diameter and responsive to ammonia, showed a resistance of 42.8 kΩ µm-1. Wire “y”, which was 300 nm in diameter and unresponsive to ammonia exposure, showed a resistance of 2.3 Ω µm-1. (E) Plot of R versus time for silver wire “x” during two exposures to 100% ammonia. The measured value of ∆R/Ro for this experiment was close to 100%. Slow convective mixing of 100% NH3 with N2 in the large (≈25 L) chamber enclosing the AFM was the source of the resistance peak seen between the two exposures to NH3.
clean silver surfaces and to desorb molecularly at temperatures ranging from 230 to 180 K depending on the structure of the silver surface.17-20 The silver surfaces of interest here are presumably not clean because they have been exposed briefly to laboratory air. Guo and Madix18 have shown that after the adsorption of oxygen on Ag[110], ammonia binds more strongly and less reversibly than at clean silver surfaces; molecular ammonia has been observed to desorb from these surfaces near 290 K. Reaction products including H2O (350 K), N2 (450 K), and NO (420 K) are also observed to desorb from these oxygen-treated surfaces.17,18 Figure 2 summarizes results for three different types of devices: ensembles of silver mesowires with diameters (in different devices) ranging from 150 nm to 0.5 µm (deposition times of 200-1000 s), ensembles of silver microwires with diameters of 0.9-1.1 µm (deposition time ) 10,000 s), and silver films with thicknesses in range from 400 nm to 1.1 µm. A current versus voltage plot for an ensemble of 200 nm diameter silver wires is shown in Figure 2A. The I-V behavior of this device was ohmic with a resistance of 8.8 kΩ. Exposure to pure NH3 caused a reversible increase in the resistance to 43 kΩ; a ∆R/Ro of 390%. As shown in Figure 2B, the increase in resistance caused by exposure to NH3 occurred in a few seconds. However, after an exposure to NH3, the decay of the resistance to the baseline value in N2 was much slower, requiring 1-2 min. For a particular Nano Lett., Vol. 4, No. 4, 2004
array, the resistance increase depended on the NH3 concentration over the range from 1 to 20% NH3 in N2 (Figure 2C). But, as shown in Figure 2D, the sensitivity of mesowire arrays varied widely, with some showing a ∆R/Ro of just 0.1% and other arrays of nominally identical wires showing ∆R/Ro of 1000% or more. The data summarized in Figure 2A-D support three conclusions. First, most silver mesowire arrays showed a large (∆R/Ro > 5%), rapid (rise time < 5 s), and reversible increase in resistance upon exposure to ammonia at concentrations above 1-2%. Second, the mechanism responsible for the resistance increase is the same for sensors having widely varying wire diameters and widely varying ∆R/Ro responses in NH3. The basis for this conclusion is the observation (Figure 2C) that the sigmoidal NH3 concentration dependence of ∆R/Ro is the same for different sensors and always shows a saturation of the sensor response at approximately 20-25% NH3. The invariance of this curve from sensor to sensor is understandable if the isotherm for ammonia adsorption on the surface of the wire is dictating this response. As shown in Figure 2E, exposure of silver mesowire arrays to CO, hydrocarbons, and water did not cause a change in resistance, but exposure to H2S, which is known to irreversibly chemisorb, caused an irreversible increase in the resistance of silver mesowire arrays. Pyridine and trimethylamine produced resistance changes similar to 667
Figure 4. Activation of a “dead” silver wire array by silver etching. (A) Cyclic voltammogram for silver at an HOPG electrode showing the potential used for slow anodic etching of silver wires. (B) SEM images of silver wires before etching and at two junctures during etching of the same silver wires. Initially, silver is removed uniformly from the surface of wires resulting in the depletion of silver from the interparticle boundaries and the formation of gaps. With continued etching, voids appeared within individual particles. (C) Plot of Ro versus Qetch (red) and ∆R/Ro versus time (blue) for an array of “large” silver wires that initially showed ∆R/Ro < 1%. (D) Plot of ∆R/Ro vs Ro for this array. (E) Plot of Log ∆R/Ro vs Log Ro as per Figure 2D showing properties of this silver wire array before and after etching.
those seen for ammonia. Third, as shown in Figure 2C, the extraordinary resistance increases above 5% in ammonia are limited to wires; films (blue data point) prepared from the same plating solutions consistently showed ∆R/Ro values below 5%. Figure 2D also shows that large mesowires, with diameters approaching 1 µm (green data points), were less likely to exhibit ∆R/Ro above 5% than smaller mesowires prepared using tgrowth of 100-1200 s. The resistance of individual silver mesowires was probed in dry nitrogen and in NH3 using an atomic force microscope (AFM) equipped with a conductive, platinum-coated silicon tip (Figure 3A). The topography of silver mesowires embedded in cyanoacrylate (Figure 3B) and a resistance map of these mesowires (Figure 3C) could be simultaneously obtained using this apparatus. More importantly, the resistance of short segments of a silver mesowire could be isolated between the conductive AFM tip and a second, macroscopic contact to the wire. The first conclusion of these measurements is that sensitivity to ammonia is a property of highly resistive wire segments. In Figure 3D, for example, we plot the measured wire resistance versus the distance from this evaporated contact for two different silver wires, labeled “x” and “y”. In principle, the slopes of these plots are independent of the contact resistance present in this measurement.21 Wire “y” had a diameter of ≈300 nm and it had a resistance per unit length of 2.3 Ω µm-1 -a factor of 5 higher than expected for a hemicylindrical silver wire with this diameter. However, wire “y” showed no change in resistance upon exposure to NH3. Wire “x” (100 nm in diameter) had a resistance of 42.8 kΩ µm-1 and it gave a strong, reversible 668
increase in resistance upon exposure to NH3 as shown in Figure 3E. From AFM investigations of single silver nanowires, we also conclude that the ∆R/Ro observed during exposure to NH3 exposure was not uniformly distributed along the axis of individual mesowires. This conclusion follows from the observation that the majority (≈80-90%) of mesowire segments with lengths of 3-500 µm showed no change in resistance upon exposure to ammonia. Other mesowire segments, like the one probed in Figure 4E, showed values of ∆R/Ro of 100% or more. These observations suggest that the increased resistance of silver mesowires caused by exposure to ammonia is concentrated at a minority of the interparticle boundaries present along the axis of each mesowire. More data will be needed in order to unambiguously reveal the structure and chemical composition of these “chemically responsive interparticle boundaries” or “CRIBs”. One possibility (Figure 5) is that CRIBs are quantum point contacts (QPCs) as described previously by Tao and coworkers.10-12 Nanoconstrictions in gold wires that exhibit quantized conduction also show large increases in resistance upon exposure to thiols and amines that are capable of chemisorbing on gold. A second possibility is that CRIBs consist of an Ag2O layer interposed between metal particles. In this case, the chemisorption of ammonia can modify the barrier to conduction of the Ag2O layer by either n- or p-doping this layer. Either of these candidates for CRIBs can be used to explain the data presented here,22 and further investigations will be required to unambiguously elucidate the physical mechanism of sensing in these polycrystalline silver wires. Nano Lett., Vol. 4, No. 4, 2004
modulation in the resistance would be observed in the presence of NH3. Thus, the existence of CRIBs provides an explanation for the correlation seen (Figure 2D) between ∆R/ Ro measured in NH3 and Ro. This correlation suggests a strategy for “activating” mesowire arrays that are insensitive to ammonia: Create CRIBs in all of the wires of the array removing silver from the wires via anodic etching. Arrays of “large” silver wires showing ∆R/Ro < 1% were etched at +0.1 V vs Ag+/Ago (see Figure 4A). As shown in the SEMs of Figure 4B, etching initially removed silver uniformly from the surfaces of these wires (shown before etching in Figure 4B top image), opening gaps at interparticle boundaries (Figure 4B, middle image, and Figure 4C). With continued etching, individual particles dissolved (Figure 4B, bottom image). Etching consistently increased the sensitivity of silver mesowire arrays to ammonia. Silver mesowire arrays were periodically removed from the etching solution, rinsed with water, air-dried, and the sensitivity to ammonia of these arrays was tested. Ro and ∆R/Ro for exposure to 100% NH3 are plotted versus Qetch for one series of etches in Figure 4C. As silver was removed, both Ro and ∆R/Ro fluctuated initially and fluctuations in ∆R/Ro paralleled the fluctuations observed in Ro. At Qetch ≈ 10-4 C, ∆R/Ro suddenly increased from less than 1% to 30% and Ro increased by a factor of 10. As shown schematically in Figure 5B and 5C, CRIBs also provide an explanation for the insensitivity of silver films to adsorption: The grain boundaries in a film are arranged electrically in parallel with one another. Conduction through this film will favor the lowest impedance current paths which route current around CRIBs (because of their high resistance) instead of through them. In silver mesowires, the opposite is true: CRIBs are arranged electrically in series and a single, highly resistive interparticle boundary can dictate the resistance of the wire. Acknowledgment. This work was funded by the NSF (grant CHE-0111557). Donations of graphite by Dr. Art Moore of GE Advanced Ceramics are gratefully acknowledged. References
Figure 5. (A) Qualitative model of a “chemically responsive interparticle boundary”, or CRIB, consisting of a ballistic quantum point contact located at an interparticle boundary in an silver wire. These structures are likely to have a large impedance. (B) Diagram illustrating conduction through a silver film showing the shunting of current around high impedance CRIBs. The chemical sensitivity of the resistance, imparted by CRIBs, are not observed. (C) Diagram showing the series arrangement of CRIBs in wires. In this case, the DC resistance of the wire is dictated by its most resistive interparticle boundary.
If CRIBs exist, then the insensitivity of some silver mesowire arrays to NH3 could be a consequence of the absence of CRIBs from one or more mesowires in the array. If wires without CRIBs are more conductive than those containing CRIBs, the overall resistance of the array would be dictated by the resistance of the CRIB-free wires and little Nano Lett., Vol. 4, No. 4, 2004
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(13) Walter, E. C.; Murray, B. J.; Favier, F.; Kaltenpoth, G.; Grunze, M.; Penner, R. M. J. Phys. Chem. B 2002, 106, 11407. (14) The silver plating solution was 1 mM silver sulfate (Ag2SO4), 1 mM saccharine, and 50 mM sodium sulfate (Na2SO4). Saccharine was added as a growth inhibitor to slow the deposition kinetics. The platinum plating solution was 1.5 mM H2PtCl6, 0.10 M HCl, and 1 mM saccharine. A typical pulse program employed for FP Pt wire deposition was 5 s at +1.10 V vs SCE, 0.1 s at -0.5 V, 2700 s at -0.10 V. The copper plating solution was 2 mM CuSO4‚5H2O, 0.10 M Na2SO4, and 1 mM saccharine. A typical pulse program employed for FP Cu wire deposition was 5 s at +0.80 V vs Cu2+/Cuo, 0.1 s at -0.80 V, 1200 s at -0.005 V. (15) Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Science 2001, 293, 2227. (16) Walter, E. C.; Favier, F.; Penner, R. M. Anal. Chem. 2002, 74, 1546. (17) Thornsburg, D. M.; Madix, R. J. Surf. Sci. 1989, 220, 268. (18) Guo, X.-C.; Madix, R. J. Surf. Sci. 2002, 510, 37. (19) Ceyer, S. T.; Yates, J. T. Surf. Sci. 1985, 155, 584.
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(20) Gland, J. L.; Sexton, B. A.; Mitchell, G. E. Surf. Sci. 1982, 115, 623. (21) The intercepts on the resistance axis of the plots shown in Figure 3D provide an estimate of the average contact resistance. The contact resistance fluctuated widely in our measurements as is apparent for the two data sets shown in Figure 3D, but although it is much higher for trace “x” than for trace “y” in Figure 3D, it was uncorrelated with the slope of these plots in general. We attribute these fluctuations, and the noise observed in data itself, to the irreproducibility of the contact resistance between the tip and the surface of the nanowire. The degree of irreproducibility, and the amount of noise, varied from tip to tip and seemed therefore to be a function of the particular tip that was used in the measurement. (22) For example, the “activation” of ammonia sensing by anodic etching (Figure 3) could be caused by the formation of narrow metal constrictions as required for quantum point contacts, but anodic etching might also introduce Ag2O at interparticle boundaries.
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