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Silver Oxide Microwires: Electrodeposition and Observation of Reversible Resistance Modulation upon Exposure to Ammonia Vapor B. J. Murray, Q. Li, J. T. Newberg, J. C. Hemminger, and R. M. Penner* Department of Chemistry, UniVersity of California, IrVine, IrVine, California 92697-2025 ReceiVed July 26, 2005. ReVised Manuscript ReceiVed September 19, 2005
Wires composed of AgxO (1 < x < 2) with diameters ranging from 0.7 to 1.1 µm were prepared by electrochemical step edge decoration on highly oriented pyrolytic graphite (HOPG) electrode surfaces. AgxO microwires were obtained by the oxidative electrodeposition of AgxO from a pH ) 6 acetatebased plating solution. Step edge selectivity, coupled with the high nucleation density necessary for wire formation, required that the electrodeposition be carried out within a potential window of just 10 mV. The resulting microwires were characterized by scanning electron microscopy and electron diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. AgxO microwires were transferred from the HOPG surface to a glass surface, and electrical contacts were applied to ensembles of between 5 and 100 transferred microwires. The resistance of these microwire ensembles increased reversibly by up to 5000% upon exposure to NH3 vapor, whereas an irreversible decrease in the wire resistance was seen upon exposure to the vapors of strong acids. We propose that the mechanism responsible for this resistance modulation is identical to that proposed recently [Murray et al., Anal. Chem. 2005, 77, 5205] to account for resistance modulation by ammonia of electrodeposited silver metal nanowire ensembles.
I. Introduction Metal oxide films composed of TiO2, SnO2, Ga2O3, and other oxides have been used extensively in resistive gas sensors.1 While enormous effort has been expended to synthesize and structurally characterize nanowires composed of metal oxides including SnO2,2-5 ZnO,6-11 TiO2,12-15 and many others, just a handful of publications report the evaluation of these nanowires in chemical sensing applications: Moskovits and co-workers have prepared SnO2 (1) Pallas-Areny, R.; Webster, J. G. Sensors and Signal Conditioning; John Wiley & Sons: New York, 2001. (2) Dai, Z. R.; Gole, J. L.; Stout, J. D.; Wang, Z. L. J. Phys. Chem. B 2002, 106, 1274. (3) Liu, Z. Q.; Zhang, D. H.; Han, S.; Li, C.; Tang, T.; Jin, W.; Liu, X. L.; Lei, B.; Zhou, C. W. AdV. Mater. 2003, 15, 1754. (4) Nguyen, P.; Ng, H. T.; Kong, J.; Cassell, A. M.; Quinn, R.; Li, J.; Han, J.; McNeil, M.; Meyyappan, M. Nano Lett. 2003, 3, 925. (5) Wang, Z. L.; Kong, X. Y.; Ding, Y.; Gao, P. X.; Hughes, W. L.; Yang, R. S.; Zhang, Y. AdV. Funct. Mater. 2004, 14, 943. (6) Chang, P. C.; Fan, Z. Y.; Wang, D. W.; Tseng, W. Y.; Chiou, W. A.; Hong, J.; Lu, J. G. Chem. Mater. 2004, 16, 5133. (7) Jie, J. S.; Wang, G. Z.; Chen, Y. M.; Han, X. H.; Wang, Q. T.; Xu, B.; Hou, J. G. Appl. Phys. Lett. 2005, 86, 031909. (8) Shen, G. Z.; Cho, J. H.; Yoo, J. K.; Yi, G. C.; Lee, C. J. J. Phys. Chem. B 2005, 109, 5491. (9) Wang, Y. C.; Leu, I. C.; Hon, M. H. J. Cryst. Growth 2002, 237, 564. (10) Yang, P. D.; Yan, H. Q.; Mao, S.; Russo, R.; Johnson, J.; Saykally, R.; Morris, N.; Pham, J.; He, R. R.; Choi, H. J. AdV. Funct. Mater. 2002, 12, 323. (11) Zhang, Y. S.; Yu, K.; Jiang, D. S.; Zhu, Z. Q.; Geng, H. R.; Luo, L. Q. Appl. Surf. Sci. 2005, 242, 212. (12) Miao, L.; Tanemura, S.; Toh, S.; Kaneko, K.; Tanemura, M. Appl. Surf. Sci. 2004, 238, 175. (13) Miao, Z.; Xu, D. S.; Ouyang, J. H.; Guo, G. L.; Zhao, X. S.; Tang, Y. Q. Nano Lett. 2002, 2, 717. (14) Park, I. S.; Jang, S. R.; Hong, J. S.; Vittal, R.; Kim, K. J. Chem. Mater. 2003, 15, 4633. (15) Yang, H. G.; Zeng, H. C. J. Am. Chem. Soc. 2005, 127, 270.
nanowires and investigated the properties of these for detecting CO;16-19 ZnO nanowires have been used in sensors for O2,20,21 and humidity.11 Recently,22,23 we reported that ensembles of silver nanowires (100-900 nm in diameter) prepared by electrochemical step edge decoration (ESED) show a rapid ( 20%).22,23 Exposure of these silver nanowires to acid vapors causes an irreversible resistance decrease that is similar in magnitude to the resistance increase caused by ammonia. Collectively, these and other experiments are consistent with the presence of trace AgxO on these nanowires, and we have proposed a mechanism, shown in Scheme 1, in which AgxO plays an integral role.22 If this mechanism is correct, then wires composed of AgxO might (16) Kolmakov, A.; Zhang, Y. X.; Cheng, G. S.; Moskovits, M. AdV. Mater. 2003, 15, 997. (17) Kolmakov, A.; Moskovits, M. Annu. ReV. Mater. Res. 2004, 34, 151. (18) Zhang, Y.; Kolmakov, A.; Chretien, S.; Metiu, H.; Moskovits, M. Nano Lett. 2004, 4, 403. (19) Zhang, Y.; Kolmakov, A.; Lilach, Y.; Moskovits, M. J. Phys. Chem. B 2005, 109, 1923. (20) Fan, Z. Y.; Lu, J. G. Appl. Phys. Lett. 2005, 86, 123510. (21) Li, Q. H.; Liang, Y. X.; Wan, Q.; Wang, T. H. Appl. Phys. Lett. 2004, 85, 6389. (22) Murray, B. J.; Newberg, J. T.; Walter, E. C.; Li, Q.; Hemminger, J. C.; Penner, R. M. Anal. Chem. 2005, 77, 5205. (23) Murray, B. J.; Walter, E. C.; Penner, R. M. Nano Lett. 2004, 4, 665.
10.1021/cm051647r CCC: $30.25 © 2005 American Chemical Society Published on Web 11/23/2005
6612 Chem. Mater., Vol. 17, No. 26, 2005 Scheme 1. Proposed Mechanism by which the Resistance of AgxO Wires and Silver Metal Nanowires Is Increased by Amines and Decreased by Acids
Scheme 2. Synthesis by ESED of AgxO Microwires
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II. Experimental Procedures Wire Electrodeposition. AgxO microwires were electrodeposited in one step using ESED (Scheme 2) in conjunction with an electrodeposition procedure based on the prior work of Switzer et al.25 for preparing films of AgO. This procedure was altered slightly to enable the electrodeposition of microwires on highly oriented pyrolytic graphite (HOPG). The plating solution of 0.2 M sodium acetate (99.995%, Sigma-Aldrich) and 15 mM silver acetate (99.99%, Sigma-Aldrich) was prepared using Barnstead Nanopure water (F >17.5 MΩ) and had a pH of 6.4. Microwires were electrodeposited using a one-compartment glass electrochemical cell with a volume of 50 mL and a Teflon lid. A small circular area (30 mm2) of the 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 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) was carried out using a Phillips model XL-30FEG operating at 10-25 keV. Selected area electron diffraction (SAED) was carried out using a Phillips CM-20 Transmission electron microscope (TEM) at an accelerating voltage of 200 keV. Samples for TEM analysis were prepared by detaching a few layers of graphite with affixed AgxO nanowires from the basal plane of a HOPG electrode. 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 wires could be acquired. XPS. XPS spectra 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 ejected photoelectrons were collected from an ∼1 × 2 mm area of the surface and energy analyzed using a 150 mm hemispherical electron energy analyzer. Spectra were energy corrected using the C(1s) peak at 284.5 eV26 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 derivatives.
be expected to show a resistance modulation that mimics that seen in silver wires, but this hypothesis has remained untested because we have been unsuccessful in our attempts to synthesize AgxO wires, until now. Here we describe an ESED-based synthesis for AgxO microwires (Scheme 2), and we present characterization data for these wires. Our data provide direct evidence for the presence of Ag(II)O in the wires; however, these data do not exclude the possibility that Ag(I)O is also present and X-ray photoelectron spectroscopy (XPS) data for these microwires imply the presence of Ag(I)O, so throughout the text we shall refer to the deposited material as AgxO. We further demonstrate that AgxO wires can be transferred onto glass microscope slides using a previously described embedding procedure.24 Electrical contacts were applied to ensembles of AgxO microwires, and the resistance response of these ensembles toward the ammonia and vapors of acids was observed. (24) Walter, E. C.; Favier, F.; Penner, R. M. Anal. Chem. 2002, 74, 1546.
Raman Spectroscopy. Raman spectra were recorded using an optical microscope fitted with a spectrometer and a nitrogen-cooled charge-coupled device (CCD) detector. The sample was illuminated in air at room temperature by a Coherent Innova 90-6 argon ion laser operating at 514.5 nm and 50 mW. This excitation beam was focused to a diameter of 0.5 mm and directed, external to the collection optics, onto the graphite surface at an angle of 65° from its normal. Light scattered from the surface of the sample was collected by a 20× Zeiss EpiPlan objective a few millimeters above the sample. Scattered excitation was rejected using a holographic notch filter (Kaiser Notch-Plus), and the remaining inelastically scattered light was directed via an f4 lens into an imaging spectrograph (Chromex 250IS, 1200 grove mm-1 holographic grating, 500 nm blaze) equipped with a liquid nitrogen-cooled CCD (Princeton Instruments model LN/1024EUV) with 1024 × 256 pixels. The signals from the 256 pixels perpendicular to the dispersion direction were binned, effectively creating a linear detector with 1024 channels and a dispersion of 2.2 cm-1 per pixel. The spectrograph was calibrated using solid MoS2 (383 cm-1, 408 cm-1)27 and neat liquids of chloroform (260 cm-1, 364 cm-1),27 (25) Breyfogle, B. E.; Hung, C. J.; Shumsky, M. G.; Switzer, J. A. J. Electrochem. Soc. 1996, 143, 2741. (26) Chastain, J. Handbook of X-ray Photoelectron Spectroscopy; PerkinElmer Corp.: Eden Prairie, MN, 1992. (27) Frost, K. J.; McCreery, R. L. Appl. Spectrosc. 1998, 52, 1614.
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Figure 1. (a) CV at 20 mV s-1 for the HOPG basal plane immersed in aqueous 15 mM silver acetate (AgCH3COO) and 0.20 M sodium acetate (NaCH3COO) at a pH ≈ 6.4. (b) Detail of the AgxO deposition wave in the potential range from 0.5 to 0.9 V vs Ag0/Ag+ and (inset) detailed view of the region of the deposition wave where silver oxide (AgxO) was preferentially grown at the step edges of HOPG. (c) Current versus deposition time for the electrodeposition of silver oxide at each of the four potentials indicated in part b.
methylene chloride (288 cm-1),28 and cyclohexane (384 cm-1, 426 cm-1).29 A collection time of 300 s was employed for all spectra. Sensor Fabrication. Silver oxide wire arrays were lifted off of the graphite surface and transferred onto a cyanoacrylate-coated glass microscope slide using a previously described procedure.24 Briefly, silver wires 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. Contacts composed of a colloidal silver paste (Ted Pella) were applied manually to the ends of ensembles of wires using a light microscope. Gas Sensing. Sensing measurements were conducted at atmospheric pressure using an aluminum flow cell with an internal volume of 20 cm3. This flow cell had electrical feed-throughs 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 across the wire arrays using a Keithley 428 current amplifier which also amplified the current. The current was then recorded using a computer interfaced Keithley 2000 multimeter. 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 computer-interfaced MKS Instruments 647C.
III. Results and Discussion The synthesis of AgxO wires using ESED involved the step edge selective electrodeposition of AgxO on HOPG electrode surfaces, as shown in Scheme 2. The synthesis method for AgxO wires employed here, based on the prior work of Switzer and co-workers,25 involved the anodic electrodeposition of AgxO from aqueous 5 mM silver acetate in 0.2 M sodium acetate at pH ≈ 6. The cyclic voltammogram (CV) for a HOPG electrode immersed in this plating solution is virtually identical to the CV reported by Switzer et al. for the same solution at a platinum electrode.25 Of particular interest are potentials in the range from +0.50 V (28) Frey, G. L.; Tenne, R.; Matthews, M. J.; Dresselhaus, M. S.; Dresselhaus, G. Phys. ReV. B 1999, 60, 2883. (29) The McCreeryGroup. Raman Shift Frequency Standards. http:// www.chemistry.ohio-state.edu/∼rmccreer/shift.html (accessed May 2005).
to +0.90 V vs Ag/Ag+ shown in Figure 1b. The anodic process with an onset near +0.75 V was determined by Switzer et al.25 to be Ag+ + H2O f AgO + 2H+ + 1e-
(1)
The step edge selective electrodeposition of AgxO was observed when the electrodeposition reaction was carried out potentiostatically at potentials located at the foot of this voltammetric wave, as shown in the inset for Figure 2b. Within the narrow potential range from 0.69 to 0.72 V, current versus time transients (Figure 1c) were characterized by an induction time that decreased as the potential was made more positive over this range. This behavior suggests that on HOPG electrodes, the electrodeposition of AgxO is autocatalytic: the overpotential for AgxO growth is lower on the nascent AgxO deposit than on bare regions of the HOPG surface. The peaked current response seen especially for the 0.72 V response is likely caused by the combined effects of concentration polarization and ohmic drop across the cross section of the growing AgxO wire. Shown in Figure 2 are SEM images of a HOPG electrode after electrolysis in this solution for 1000 s at the specified potentials. Only in the 10 mV range from 0.7 to 0.71 V were continuous nano- and microstructures observed at step edges. These structures consisted of ensembles of unfaceted grains each with a diameter in the 200-600 nm range. In contrast to the morphology of noble metal wires prepared by ESED, in which individual metal grains are stacked in a collinear fashion along a step edge and nucleation occurs only at the onset of wire growth, here a much greater degree of disorder is apparent in these SEM images suggesting that the nucleation of AgxO occurs progressively during the growth of these structures. Control over the wire diameter was limited to the range from 700 nm to 1.1 µm as shown in Figure 3. Wires in the 600-800 nm range were obtained by electrodepositing at +0.71 V for approximately 300 s. Shorter deposition times produced structures that appeared to be continuous for just a few micrometers. The wire diameter self-limited at approximately 1.1 µm at a deposition time of 2000 s. The electrodeposition current at this point had decayed to nearly zero, presumably as a consequence of the excessive ohmic drop across the diameter of these wires.
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Figure 2. SEM images at low and high magnification of the HOPG electrode surface after the electrodeposition for 1000 s of silver oxide (AgxO) at the indicated potentials. The plating solution is the same as specified in Figure 1. The formation of continuous wire structures was observed only at 0.70 and 0.71 V.
Figure 3. (a) Wire diameter vs deposition time for the electrodeposition of silver oxide on freshly cleaved HOPG. Wire diameters were measured using the scanning electron microscope on multiple wires at several locations on the graphite surface and on multiple places on each wire. Error bars show (1 σ for the diameter distribution. (b, c, d) SEMs of silver oxide wires grown at +0.71 V vs Ag0/Ag+ for 300, 600, and 2000 s, respectively, as indicated in part a.
Especially in view of the sensing behavior and mechanism to be discussed below, it is important to qualify the definition of wire “diameter” we have used above. From the SEM images shown in Figure 3, it is apparent that while the largest diameter of these AgxO wires is in the 700 nm to 1.1 µm range, the highly disordered and polycrystalline morphology of these wires ensures that much smaller, nanometer-scale
constrictions and contacts exist along the axis of each wire, between AgxO grains. The composition of the deposited microwires and microparticles was analyzed by energy-dispersive X-ray microanalysis (EDX), SAED, XPS, and Raman spectroscopy. Microparticles were included in XPS analyses because of our inability to exclude them whereas EDX and SAED data
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Figure 4. (a) O(1s) XPS spectra of the (i) AgO pellet, (ii) electrodeposited AgxO microwires, (iii) electrodeposited silver nanowires. The silver oxide pellet was pressed from silver(II) oxide powder (Aldrich). The silver wires were electrodeposited as previously described from an acidic silver sulfate solution.23,30 (b) SAED of electrodeposited wires corresponding to cubic AgO (JCPDS 76-1489).39 (c) Raman spectrum of silver oxide wires acquired at room temperature with an excitation of 514.5 nm. The peak locations (cm-1) and relative intensities measured in multiple samples are all characteristic of AgO.33,40
were acquired for microwires selectively. EDX and XPS survey scans (data not shown) revealed that both silver and oxygen were present in the deposited material. The XPS O(1s) region is shown in detail in Figure 4a (spectrum ii), together with the measured XPS O(1s) of freshly electrodeposited silver wires23,30 (spectrum iii) and spectra for AgO powder purchased from Aldrich (Aldrich 22,363-8, purity unspecified, spectrum i). The spectrum of the AgO powder shows two peaks with binding energies of 529.0 and 530.7 eV that compare with literature values31 for AgO of 528.7 eV and 531.6. Ag2O shows a single peak in this region at 529.4 eV.32 It is reasonable to expect that some Ag2O contaminates this AgO powder because AgO is known to transform slowly and spontaneously into Ag2O at room temperature.32 This can be the origin of the 529.0 eV peak energy, intermediate between the values expected for AgO and Ag2O. The O(1s) XPS for silver wires (spectrum iii), on the other hand, shows a single, broad peak centered at 532 eV that we have previously22 assigned to contamination and traces of oxygenated species (e.g., CdO, CsOH, etc.) at defects. Neither AgO nor Ag2O is detected by XPS on these surfaces. Spectrum ii was acquired for the material synthesized here. It shows both a weak, narrow peak at 529 eV (that is consistent with the low binding energy peak in the AgO reference spectrum shown in Figure 4c) together with a broader peak centered at 531.5 eV. We assign the broad peak at 531.5 eV to a combination of the higher energy AgO band and the contamination peak observed also in spectrum iii. Because the low energy peak is at the same energy as that seen in sample i, it is again reasonable to conclude that this spectrum is derived from a sample containing both Ag2O and AgO. The electron diffraction pattern shown in Figure 4b is most consistent with cubic AgO (JCPDS 76-1489), but it must be noted that diffraction from cubic Ag2O (JCPDS 43-0997) is (30) Walter, E. C.; Murray, B. J.; Favier, F.; Kaltenpoth, G.; Grunze, M.; Penner, R. M. J. Phys. Chem. B 2002, 106, 11407. (31) Bielmann, M.; Schwaller, P.; Ruffieux, P.; Groning, O.; Schlapbach, L.; Groning, P. Phys. ReV. B 2002, 65, 235431. (32) Hammond, J. S.; Gaarenstroom, S. W.; Winograd, N. Anal. Chem. 1975, 47, 2193.
virtually indistinguishable, differing essentially in the number of silver atoms present in the trigonal holes of the bodycentered cubic oxygen sublattice. There is no evidence in either the SAED pattern or the Ag(3d) XPS for the presence of elemental silver in these microwires. We also employed Raman spectroscopy to characterize the electrodeposited material (both microwires and particles). A typical Raman spectrum is shown in Figure 4c. This spectrum is assignable (as indicated) to cubic AgO; no Raman scattering transitions are seen for Ag2O in this energy range.33 When all these characterization data are considered collectively, we conclude that the electrodeposited silver oxide is predominantly AgO; however, our data do not permit us to exclude the presence of some Ag2O, and the measured XPS O(1s) binding energies provide indirect evidence for the presence of Ag2O in addition to AgO in these wires. This is the reason we refer to the oxide throughout this paper as AgxO. Recently,22,23 we have reported that electrodeposited silver wires show large, reversible changes in resistance upon exposure to ammonia and other amines. On the basis of a thorough study of this system, we have concluded that this resistance response is caused by the presence of silver oxide bridges interposed between some of the silver grains of these wires.22 How do these oxide bridges impart a sensitivity to amines? As shown in Scheme 1, exposure to amines causes the deprotonation of the hydroxyls present on the oxide surface resulting in the formation of a negative surface change distribution that coulombically impedes the transport of electrons through the n-type silver oxide bridge, thereby increasing the resistance of the wire. Conversely, acid vapor protonates the hydroxyls, eliminates the negative surface change, and produces a low resistance state for these same silver wires, also as seen experimentally.22 If the hypothesized mechanism represented by Scheme 1 were correct, we would expect that microwires composed of AgxO would exhibit very similar resistance modulation effects upon exposure to amines and acids. (33) Waterhouse, G. I. N.; Bowmaker, G. A.; Metson, J. B. Phys. Chem. Chem. Phys. 2001, 3, 3838.
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Figure 5. Resistance responses during exposures to NH3 vapor for a typical ensemble of AgxO microwires. (a) Resistance versus time for various concentrations of NH3 in N2. (b) The resistance measured at the end (i.e., 630 s) of the exposures plotted in part a, plotted vs NH3 concentration. (c) Resistance versus time during exposure to 10 s pulses of NH3 at the indicated concentrations. (d) [bottom] The relative change in resistance, ∆R/R0, vs ammonia concentration. ∆R/R0 was approximately proportional to the NH3 concentration over the range from 0-30% NH3. [top] Signal-to-noise ratio where the signal is defined as ∆R/R0 and the noise is measured as the relative standard deviation of the baseline resistance (i.e., in N2) sampled during a 30 s window immediately preceding each NH3 exposure.
We directly tested this hypothesis by transferring the AgxO microwires described above to a glass microscope slide by embedding these wires in cyanoacrylate, as described in the experimental section and previously.24 Electrical contacts of colloidal silver paint were then applied to ensembles of between 5 and 100 wires, and the current flowing through this wire ensemble in response to an applied bias of 5 mV was monitored as the wires were exposed to ammonia in a background of pure nitrogen gas. Typical data are summarized in Figure 5. In Figure 5a is shown the resistance versus time for a AgxO microwire ensemble exposed to the ammonia concentration program, spanning concentrations ranging from 1 to 50%, plotted at the bottom of the figure. The resistance response of the AgxO microwires, plotted at the top of Figure 5a, shows a resistance increase of between 2 and 13 kΩ over this concentration range, corresponding to relative resistance changes (∆R/R0) of 15-100%. This maximum ∆R/R0 value would place this device within the top 10% of the nearly
100 silver wire ensembles examined in our prior work.22,23 We further note that the resistance of the AgxO microwire ensembles stabilized within 300 s for concentrations of ammonia above 20% whereas at lower NH3 concentrations a stable resistance response was not seen during the 630 s duration of exposure. In Figure 5b we plot the final resistance observed at the end of the 630 s pulses as a function of the ammonia concentration. These data show that the resistance of the AgxO microwire ensemble, measured at the end of a 630 s exposure, is a true function of the ammonia concentration in the sense that it was not influenced by the NH3 exposures that preceded it. The temporal properties of the resistance response to NH3 seen here are identical to those we have previously reported22,23 for ensembles of silver metal nanowires. The onset of this resistance increase upon NH3 exposure is fast (20%), irreversible decreases in the resistance. The vapor of these strong acids was produced simply by sparging N2 (UHP, Airgas) through concentrated solutions of hydrochloric acid (36.5-38%, EMD, Inc.) or sulfuric acid (9598%, EMD, Inc.).
not, apparently, been investigated previously, there is prior work by Madix and co-workers37,38 involving the chemisorption of ammonia onto “oxygenated” Ag(110) surfaces. In that work, the adsorption of NH3 at room temperature was dissociative, resulting in the formation of surface OH and NHx (x ) 1 or 2). We believe that the reproducibility of the response to NH3 seen for AgxO microwires argues against this sort of dissociative mechanism because consecutive exposures to NH3 would progressively alter the surface chemistry (and the sensor response) by causing a buildup of NHx species, but this mechanism cannot be excluded by the data presented here. A second unresolved issue involves the likely role played by surface-adsorbed water in the mechanism. Conclusions In this work, we demonstrate the extension of the ESED wire growth method to the preparation of microwires composed of AgxO. Microwires of this material were obtained by anodically electrodepositing AgxO from acetate electrolyte under conditions favoring the nucleation of AgxO at the step edges present on a HOPG electrode. The necessary degree of selectively existed within a narrow potential window spanning just 10 mV. The resulting AgxO wires were electrically continuous over hundreds of micrometers; however, AgxO is a brittle material, and these wires proved to be difficult to manipulate, transfer, and otherwise incorporate into sensor structures for evaluation of their electrical properties because of difficulties involving wire fracture. (37) Guo, X.-C.; Madix, R. J. Surf. Sci. 2002, 510, 37. (38) Thornsburg, D. M.; Madix, R. J. Surf. Sci. 1989, 220, 268. (39) Stehlik, B.; Weidenthaler, P. Collect. Czech. Chem. Commun. 1959, 24, 1416. (40) Waterhouse, G. I. N.; Bowmaker, G. A.; Metson, J. B. Appl. Surf. Sci. 2001, 183, 191.
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When this transfer operation could be successfully carried out and electrical contacts are applied to ensembles of AgxO wires, we found that the resistance of AgxO microwires was strongly modulated by exposure to the vapors of strong acids and the base ammonia. Ammonia caused a rapid and reversible increase in resistance whereas the vapor of strong acids such as HCl and HNO3 caused a large, irreversible decrease in resistance. These responses were indistinguishable from those we have seen previously for ensembles of silver nanowires.22,23 This striking similarity suggests a common mechanism for this resistance modulation in these two types of wires, thereby adding to the body of evidence
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implicating trace AgxO on silver nanowires in the mechanism of the acid/base-induced resistance modulation in these nanostructures. Acknowledgment. This work was funded by the National Science Foundation (Grant CHE-0111557) and the Petroleum Research Fund of the American Chemical Society (Grant 40714AC5). J.C.H. acknowledges funding support from the Department of Energy (Grant DE-FG03-96ER45576). Graphite for this work was supplied by a grant from the EU Commission FP6 NMP-3 Project No. 505457-1 ULTRA-1D. CM051647R
