chemical sensing and catalysis by one - dimensional metal - oxide nanostructures

15 Mar 2004 17:40 AR AR218-MR34-05.tex AR218-MR34-05.sgm LaTeX2e(2002/01/18) P1: FHD AR REVIEWS IN ADVANCE10.1146/annurev.matsci.34.040203.112141 (Some corrections may occur before final publication online and in print) R E V I E W S I N A D V A N C E Annu. Rev. Mater. Res. 2004. 34:151–80 doi: 10.1146/annurev.matsci.34.040203.112141 Copyright c  2004 by Annual Reviews. All rights reserved CHEMICAL SENSING AND CATALYSIS BY ONE-DIMENSIONAL METAL-OXIDE NANOSTRUCTURES Andrei Kolmakov and Martin Moskovits Department of Chemistry and Biochemistry, University of California, Santa Barbara, email: akolmakov@chem.ucsb.edu, mmoskovits@ltsc.ucsb.edu Key Words one-dimensional nanostructures, sensors, catalysis ■ Abstract Metal-oxide nanowires can function as sensitive and selective chemical or biological sensors, which could potentially be massively multiplexed in devices of small size. The active nanowire sensor element in such devices can be configured either as resistors whose conductance is altered by charge-transfer processes occurring at that their surfaces or as field-effect transistors whose properties can be controlled by applying an appropriate potential onto its gate. Functionalizing the surface of these entities offers yetanother avenue forexpanding their sensingcapability. In turn,because charge exchange between anadsorbate and thenanowire can change the electrondensity in the nanowire, modifying the nanowire’s carrier density by external means, such as applying a potential to the gate, could modify its surface chemical properties and perhaps change the rate and selectivity of catalytic processes occurring at its surface. Although research on the use of metal-oxide nanowires as sensors is still in early stages, several encouraging experiments have been reported that are interesting in their own right and indicative of a promising future. INTRODUCTION Chemical and biological sensors have a profound influence in the areas of per- sonal safety, public security, medical diagnosis, detection of environmental toxins, semiconductor processing, agriculture, and the automotive and aerospace indus- tries (1–4 and references therein). The past few decades has seen the development of a multitude of simple, robust, solid-state sensors whose operation is based on the transduction of the binding of an analyte at the active surface of the sensor to a measurable signal that most often is a change in the resistance, capacitance, or temperature of the active element. The evolution of gas sensors closely parallels developments in microelectronics in that the architecture of sensing elements is influenced by design trends in planar electronics, and one of the major goals of the field is to design nano-sensors that could be easily integrated with modern electronic fabrication technologies. For 1531-7331/04/0804-0151$14.00 151 15 Mar 2004 17:40 AR AR218-MR34-05.tex AR218-MR34-05.sgm LaTeX2e(2002/01/18) P1: FHD AR REVIEWS IN ADVANCE10.1146/annurev.matsci.34.040203.112141 152 KOLMAKOV  MOSKOVITS Figure 1 A cartoon of a nanowire-based electronic nose. The nanowire surfaces are functionalized with molecule-selective receptors. The operation is based on molecular selective bonding, signal transduction, and odor detection through complex pattern recognition. example, the current goal is to replace the large arrays of macroscopic individual gas sensors used for many years for multicomponent analysis, each having its associated electrodes, filters, heating elements, and temperature detection, with an “electronic nose” embodied in a single device that integrates the sensing and signal processing functions in one chip (5–8). Multicomponent gas analysis with these devices is accomplished by pattern recognition analogous to odor identifica- tion by highly evolved organisms (Figure 1) (9–11). By increasing the sensitivity, selectivity, the number of sensing elements, and the power of the pattern recogni- tion algorithms, one can envision a potent device that can detect minute quantities (ultimately one molecule) of an explosive, biohazard, toxin, or an environmentally sensitive substance against a complex and changing background, then signal an alert or take “intelligent” action. However, this requires an increase in the sensitiv- ity and selectivity of active sensor elements despite the loss of active area and the increased proximity of neighboring individual sensing elements as the individual components are miniaturized. Recent progress in materials science and the many new sensing paradigms originating out of nanoscience and technology, particu- larly from bottom-up fabrication, makes one optimistic that these goals are within reach. Metal oxides possess a broad range of electronic, chemical, and physical prop- erties that are often highly sensitive to changes in their chemical environment. Because of these properties metal oxides have been widely studied, and most commercial sensors are based on appropriately structured and doped oxides. Nevertheless, much new science awaits discovery, and novel fabrication strate- gies remain to be explored in this class of materials by using strategies based 15 Mar 2004 17:40 AR AR218-MR34-05.tex AR218-MR34-05.sgm LaTeX2e(2002/01/18) P1: FHD AR REVIEWS IN ADVANCE10.1146/annurev.matsci.34.040203.112141 NANOWIRES, SENSORS, AND CATALYSTS 153 on nanoscience and technology. Traditional sensor fabrication methods make use of pristine or doped metal oxides configured as single crystals, thin and thick films, ceramics, and powders through a variety of detection and transduction prin- ciples, based on the semiconducting, ionic conducting, photoconducting, piezo- electric, pyroelectric, and luminescence properties of metal oxides (4, 12–14). Chemical and biological sensors having nanostructured metal oxides and espe- cially metal-oxide nanowires benefit from the comprehensive understanding that exists of the physical and chemical properties of their macroscopic counterparts (15). This review is limited primarily to semiconducting devices with quasi-one- dimensional nanostructures such as nanowires and nanobelts. Likewise, we restrict ourselves to two related device configurations: conductometric elements and field- effect transistors. A few issues relating to real-world sensors and sensor arrays are also discussed. Numerous quasi-one-dimensional oxide nanostructures with useful properties, compositions, and morphologies have recently been fabricated using so-called bottom-up synthetic routes. Some of these structures could not have been created easily or economically using top-down technologies. A few classes of these new nanostructures with potential as sensing devices are summarized schematically in Figure 2. These achievements in oxide one-dimensional nanostructure synthesis and characterization were recently reviewed by Xia et al. (16) and others elsewhere (17–19). Much work has also been published on the use of carbon nanotubes, individually or as arrays, as sensors (20–25). Although we do not refer to this work (which has also been thoroughly reviewed) (26–30), the great progress made to date in understanding the electronic properties of carbon nanotubes, their Figure 2 A schematic summary of the kinds of quasi-one-dimensional metal- oxide nanostructures already reported (see reviews 16, 17). (A) nanowires and nanorods; (B) core-shell structures with metallic inner core, semiconductor, or metal-oxide; (C) nanotubules/nanopipes and hollow nanorods; (D) heterostructures; (E) nanobelts/nanoribbons; (F) nanotapes, G-dendrites, H-hierarchical nanostructures; (I) nanosphere assembly; (J) nanosprings. 15 Mar 2004 17:40 AR AR218-MR34-05.tex AR218-MR34-05.sgm LaTeX2e(2002/01/18) P1: FHD AR REVIEWS IN ADVANCE10.1146/annurev.matsci.34.040203.112141 154 KOLMAKOV  MOSKOVITS reactivity toward gases, photochemical properties, junction effects, and perfor- mance when configured as transistors certainly informs the discussion of all quasi- one-dimensional systems. We therefore acknowledge the great debt we owe to that literature in establishing and clarifying many of the key questions pertaining to quasi-one-dimensional nanostructures. The properties of bulk semiconducting oxides have been extensively studied and documented. Not so those of quasi-one-dimensional oxide nanostructures (i.e., systems with diameters below ∼100 nm), which are expected to possess novel characteristics for the following reasons: (a) A large surface-to-volume ratio means that a significant fraction of the atoms (or molecules) in such systems are surface atoms that can participate in surface reactions. (b) The Debye length λ D (a measure of the field penetration into the bulk) for most semiconducting oxide nanowires is comparable to their radius over a wide temperature and doping range, which causes their electronic properties to be strongly influenced by processes at their surface. As a result, one can envision situations in which a nanowire’s conductivity could vary from a fully nonconductive state to a highly conductive state entirely on the basis of the chemistry transpiring at its surface. This could result in better sensitivity and selectivity. For example, sensitivities up to 10 5 -fold greater than those of comparable solid film devices have already been reported for sensors on the basis of individual In 2 O 3 nanowires (31). The signal-to- noise ratio obtained indicates that ∼10 3 molecules can be reliably detected ona3-µm-long device. By shortening the conductive channel length to ∼30 nm, the adsorption of as few as 10 molecules could, in principle, be detected. (c) The average time it takes photo-excited carriers to diffuse from the interior of an oxide nanowire to its surface (∼10 −12 –10 −10 s) is greatly reduced with respect to electron- to-hole recombination times (∼10 −9 –10 −8 s). This implies that surface photoinduced redox reactions (Figure 3) with quan- tum yields close to unity are routinely possible on nanowires (assuming reactants reach the surfaces rapidly enough and interfacial charge transfer rates are not limiting). The rapid diffusion rate of electrons and holes to the surface of a nanostructure provides another opportunity as well. The recov- ery and response times of conductometric sensors are determined by the adsorption-desorption kinetics that depends on the operation temperature. The increased electron and hole diffusion rate to the surface of the nanode- vice allows the analyte to be rapidly photo-desorbed from the surface (∼a few seconds) even at room at temperature. (d) Semiconducting oxide nanowires are usually stoichiometrically better de- fined and have a greater level of crystallinity than the multigranular oxides currently used in sensors, potentially reducing the instability associated with percolation or hopping conduction. 15 Mar 2004 17:40 AR AR218-MR34-05.tex AR218-MR34-05.sgm LaTeX2e(2002/01/18) P1: FHD AR REVIEWS IN ADVANCE10.1146/annurev.matsci.34.040203.112141 NANOWIRES, SENSORS, AND CATALYSTS 155 Figure 3 A summary of a few of the electronic, chemi- cal, and optical processes occurring on metal oxides that can benefit from reduction in size to the nanometer range. (e) Nanowires are easily configurable as field-effect transistors (FETs) and potentially integratable with conventional devices and device fabrication techniques. Configured as a three-terminal FET, the position of the Fermi level within the bandgap of the nanowire could be varied and thus used to alter and control surface processes electronically. (f) Finally, as the diameter of the nanowire is reduced, or as its materials prop- erties are modulated either along its radial or axial direction, one can expect to see the onset of progressively more significant quantum effects (32). Surface Reactions on One-Dimensional Oxides, Gas Sensing, and Catalysis The exploration of the metal-oxide nanostructures as a platform for chemical sens- ing is a recent event. Yang and coworkers fabricated and tested the performance of individual SnO 2 single-crystal nanoribbons configured as four-probe conduc- tometric chemical sensors both with and without concurrent UV irradiation (33). Photoinduced desorption of the analyte can lead to rapid detection and reversible operation of a sensor even at room temperature. A detection limit ∼3 ppm and re- sponse/recovery times of the order of seconds were achieved for NO 2 . Comparing the performance of the ohmic nanoribbon sensors with those that showed rectifi- cation led the authors to conclude that the nanoribbons themselves dominate the photo-chemical response and not thephenomena occurring at the Schottky barriers. 15 Mar 2004 17:40 AR AR218-MR34-05.tex AR218-MR34-05.sgm LaTeX2e(2002/01/18) P1: FHD AR REVIEWS IN ADVANCE10.1146/annurev.matsci.34.040203.112141 156 KOLMAKOV  MOSKOVITS Figure 4 Top: TEM, HRTEM, SEM images of an individual SnO 2 nanoribbon: (A) low magnification, (B) atomically resolved, and (C) deposited on previously prepared Au electrodes. Bottom: The conductance response of the nanoribbon to NO 2 pulses in air with simultaneous 365 nm irradiation (after Law et al. 33). A wide array of potentially useful one-dimensional metal-oxide nanostructures, including nanobelts, were synthesized and characterized in Wang’s group (19, 34) and in other laboratories (see 16, 17 and references therein). Comini et al. (35) configured groups of the SnO 2 nanobelts between platinum interdigitated elec- trodes and assessed their behavior at 300–400 ◦ C under a constant flux of synthetic air. The nanobelt sensors showed excellent sensitivity toward CO, ethanol, and NO 2 .NO 2 could be detected down to a few parts per billion. Individual SnO 2 and ZnO 2 single-crystalline nanobelts (30–300 nm width and 10–30 nm thickness) (34) were configured as FETs and studied by Arnold et al. 15 Mar 2004 17:40 AR AR218-MR34-05.tex AR218-MR34-05.sgm LaTeX2e(2002/01/18) P1: FHD AR REVIEWS IN ADVANCE10.1146/annurev.matsci.34.040203.112141 NANOWIRES, SENSORS, AND CATALYSTS 157 (36). The electrical properties of these individual nanobelts in vacuum, in air, and under oxygen, as a function of thermal treatment, suggested that the oxygen adsorption and desoption dynamics depends sensitively on the concentration of surface oxygen vacancies, which, in turn, affect the electron density in the nanobelt. CdO nanowires, nanobelts, and nanowhiskers are prospective active elements for LEDs and lasers from nanostructures. The Zhou group (37) showed that in vacuum, as-prepared CdO nanowires have a carrier concentration of ∼1.3 × 10 20 cm −3 arising from oxygen vacancies and interstitial Cd. Temperature- dependent conductance measurements indicate an activated process with E a ∼ 13.3 meV at high temperature, switching to tunneling conductance below 30 K. The conductance of single nanowires exposed to 200 ppm of NO 2 (an oxidizing gas) at room temperature dropped by ∼30%. Kolmakov et al. used nanoporous alumina as a template for synthesizing arrays of parallel Sn nanowires, which were converted to polycrystalline SnO 2 nanowires of controlled composition and size (38). Conductance measurements on these in- dividual nanowires were carried out in inert, oxidizing, and reducing environments in the temperature range ∼25–300 ◦ C (39). At high temperatures and under an inert or reducing ambient, the nanowires behaved as highly doped semiconductors or quasi-metals with high conductances that depended weakly on temperature. When exposed to oxygen, the nanowires were transformed to weakly doped semiconduc- tors with a high conductance activation energy. The switching between the high and low conductance states of the nanowires was fully reversible at all tempera- tures. Configured as a CO sensor, a detection limit of ∼a few 100 ppm for CO in dry air and at 300 ◦ C was measured with these SnO 2 nanowires, with sensor response times of ∼30 s. The above observations can be largely accounted for in terms of mechanisms developed over many years to explain the function of polycrystalline metal-oxide gas sensors (40–43). This mechanism is outlined below, using SnO 2 nanowires in the presence of oxygen (an electron acceptor) and CO (an electron donor) as a model system for oxide semiconductor systems moregenerally. Specific departures from this general picture are pointed out for individual cases and for other surface adsorbate molecules when necessary. The surface of stoichiometric tin oxide (a large bandgap semiconductor) is rel- atively inert. Even moderate annealing in vacuum, or under an inert or reducing atmosphere, causes some of the surface oxygen atoms to desorb, leaving behind oxygen vacancy sites (Figure 5). Likewise, exposure to UV results in oxygen pho- todesorption (or of other surface species) even at low temperatures. Essentially, all experiments carried out to date on metal-oxide nanowires (or other nanostructures) indicate that the role of oxygen vacancies dominates their electronic properties along much the same lines as they do in bulk systems. Each vacancy results in the formation of a filled (donor) intragap state lying just below the conduction band edge (Figure 5c). The energy interval between these states (or at least some) and the conduction band is small enough that a large fraction of the electrons in the donor states is ionized even at low temperatures, thus converting the material into 15 Mar 2004 17:40 AR AR218-MR34-05.tex AR218-MR34-05.sgm LaTeX2e(2002/01/18) P1: FHD AR REVIEWS IN ADVANCE10.1146/annurev.matsci.34.040203.112141 158 KOLMAKOV  MOSKOVITS Figure 5 (a) Stoichiometric SnO 2 (110) surface, (b) partially reduced SnO 2 with missing bridging oxygens. Molecular oxygen binds to the vacancy sites as an electron acceptor. CO molecules react with preadsorbed oxygens. Electron are released back to the nanowire [a,b after Kohl (14) with modifications]. (c) Oxygen vacancies make SnO 2 into an n-type semiconductor. (d ) When the Debye length is comparable to the radius of the nanowire, adsorption of electron acceptors shifts the position of the Fermi level away from the conduction band. an n-type semiconductor. At a given temperature the conductance of the nanowire, G = πR 2 eµn/L, is determined by the equilibrium conditions determining the rel- ative concentrations of (singly or doubly) ionized surface vacancy states, which determine the electron concentration in the bulk of the material. (Surface defects can also migrate into the interior resulting in bulk defects that are clearly much less responsive to surface processes, and their low diffusion constant implies that they are normally not important participants in the material’s sensing action, which re- quires a response time faster than the inverse diffusion rate. However, bulk defects do contribute to a sensor’s long-term stability.) The conductance of SnO 2 changes rapidly with gas adsorption as a result of a (usually) multistep process wherein the first is the adsorption of a molecule (for ex- ample, with O 2 , might dissociate into two surface oxygen ions after chemisorption) with a consequent molecule-to-SnO 2 charge transfer (or vice versa). With oxygen as the adsorbate, the afore-mentioned surface vacancies are partially repopulated, which results in ionized (ionosorbed) surface oxygen of the general form O −α β S . The resulting (equilibrium) surface oxygen coverage, θ, depends on the oxygen partial pressure and the system temperature through the temperature-dependent adsorption/desorption rate constants, k ads/de , on the concentration of itinerant 15 Mar 2004 17:40 AR AR218-MR34-05.tex AR218-MR34-05.sgm LaTeX2e(2002/01/18) P1: FHD AR REVIEWS IN ADVANCE10.1146/annurev.matsci.34.040203.112141 NANOWIRES, SENSORS, AND CATALYSTS 159 electrons, n, and the concentration of unoccupied chemisorption (vacancy) sites, N s . β 2 O gas 2 + α · e − + N s   O −α βs k ads · N s · n · p β/2 O 2 = k des · θ (where α, β = {1,2} accounts for the charge and molecular or atomic nature of the chemisorbed oxygen (44)). In forming ionosorbed oxygen, electrons become lo- calized on the adsorbate, creating a ∼30–100-nm-thick, electron-deficient surface layer corresponding approximately to the Debye length for SnO 2 (in the tempera- ture range 300–500 K), which results in band bending in the surface region of bulk samples. For 10–100 nm diameter nanowires, the charge-depletion layer encom- passes the entire nanowire resulting in a so-called flat-band conditions wherein the relative position of the Fermi level shifts away from the conduction band edge not only at the surface but throughout the nanowire (Figure 5d). Ultimately, a new kinetic equilibrium among the free electrons and the neutral and ionized vacancies is re-established. Under these nearly flat-band conditions at moderate tempera- tures and for electron momenta directed radially, electrons can reach the surface of the nanowire with essentially no interference from the low electrostatic barrier. As a result, the electrons become distributed homogeneously throughout the entire volume of the nanowire. Accordingly, the charge conservation condition simplifies to N s · θ = R 2 · (n − n m ), where n m is the density of itinerant electrons remaining in the nanowire after exposure to the adsorbate. The accompanying electron depletion n = 2N s θ/R results in a significant drop in conductance: G = π R 2 eµ L · 2N s θ R and the corresponding depopulation of the shallow donor states results in an in- crease in activation energy (39). [We neglect the dependence of the mobility on the surface coverage, a reasonable approximation at small bias voltages and when the electron diffusion length (∼1 nm) is much smaller than the diameter of the nanowire (∼50 nm) (45)]. Upon adsorbing a reducing gas such as CO, the following surface reaction takes place with the ionoadsorbed oxygen β · CO gas + O −α β S → β · CO gas 2 + α · e − which results in the reformation of the adsorption (defect) sites and the redonation of electrons to the SnO 2 (Figure 5b). It can be shown that under flat-band condi- tions the increase in electron concentration, n CO ∼ p β α+1 CO , and therefore in the 15 Mar 2004 17:40 AR AR218-MR34-05.tex AR218-MR34-05.sgm LaTeX2e(2002/01/18) P1: FHD AR REVIEWS IN ADVANCE10.1146/annurev.matsci.34.040203.112141 160 KOLMAKOV  MOSKOVITS conductivity of the nanowire, G CO ∼ e · n CO (T ) · µ(T ), increases mono- tonically with CO partial pressure (44). This was confirmed experimentally on nanowires assuming O − (α, β = {1}) to be the dominant reactive surface species (39). The foregoing simple mechanism is able to account for the operation of tin-oxide nanowire sensors under ideal ambients consisting of dry oxygen and a combustible gas such as CO. In a real-world environment, a large array of other molecules (chief among them, water) complicates the picture. Surface hydrox- yls and hydrocarbons can temporarily or permanently react with adsorption sites modifying or adding to the possible reaction pathways. A consequence of being able to shift the position of the Fermi level of the oxide nanowire by applying an external field or by doping the nanowire is the possibility of controlling molecular adsorption onto its surface (resulting in the oscillation of the adsorbate between an electron donor and acceptor). An interesting instance of this was reported recently (46) with In 2 O 3 nanowires exposed to NH 3 . For nano- wires with a low density of oxygen vacancies (corresponding to a Fermi level lower in energy within the bandgap), the adsobate behaved as an electron donor causing the resistivity of the nanowire to increase upon exposure to ammonia. With a higher oxygen vacancy density (the Fermi level nearer to the lower edge of the conduction band) the NH 3 behaved as an acceptor, quenching the nanowire’s conductance (Figure 6). Single Nanowire FETs The architecture of a typical nanowire-based FET is shown in Figure 7. The nanowire acts as a conducting channel that joins a source and drain electrode. The entire assembly rests on a thin oxide film, which, itself, lies on top of a conducting (in this case p-type Si) gate electrode. (This is a so-called back gate configuration. A top gate can also be deposited on the nanowire as an alternative.) Tuning a nanowire’s properties by configuring it as the conductive channel of a FET was Figure 6 Alternating donor (right plot) versus acceptor (left plot) behavior of NH 3 adsorbate as a function of the doping level of an In 2 O 3 nanowire (taken from Zhang et al. 46). [...]... electronically controllable selectivity and sensitivity (reactivity) PHOTOCHEMICAL PROPERTIES OF INDIVIDUAL METAL- OXIDE NANOWIRES The photochemical and photophysical properties of metal- oxide nanoparticles and materials fabricated from them have been studied extensively, especially in the context of solar energy conversion In a similar vein, small diameter, quasi -one- dimensional oxide nanostructures are promising... recently (16, 64–66) Many applications including chemical sensing and catalysis rely on achieving a high surface- to-bulk ratio in the active nanosystem and therefore do not necessarily require high crystallinity This potentially expands the range of strategies for producing technologically viable, low-cost devices based on quasi -one- dimensional oxide nanostructures Here we discuss a few recent accomplishments... nanostructure to self-organize Arrays of free-standing metal/ metal -oxide nanowire arrays were fabricated using porous alumina (PAO) templates Briefly, highly ordered and periodic nanopores in PAO were produced by a two-step anodization (67) By varying the anodizing conditions, the electrolyte temperature, the anodizing voltage, and/ or the nano-texture of the aluminum substrate, the diameter, length, and the density... 18:71–113 Barsan N, Grigorovici R, Ionescu R, Motronea M, Vancu A 1989 Mechanism of gas-detection in polycrystalline thick-film SnO2 sensors Thin Solid Films 171:53–63 Barsan N, Weimar U 2001 Conduction model of metal oxide gas sensors J Electroceram 7:143–67 Barsan N 1994 Conduction models in gas -sensing SnO2 layers—grain-size 15 Mar 2004 17:40 AR AR218-MR3 4-0 5.tex AR218-MR3 4-0 5.sgm LaTeX2e(2002/01/18) P1:... semiconductors and FET-based chemical sensors fabricated by traditional technologies (53–57) The difference between what one expects with bulk systems and with nanowires, however, is one of extent The combination of factors such as the comparability of the Debye length and nanowire radius, the small number of carriers present in the nanowire (typically ∼105 electrons), the high surface-to-bulk ratio, and its... including enhanced hydrogen dissociation on titania defects and on the Pt electrodes (followed by hydrogen spill-over from the Pt) and hydrogen-induced modification of the Schottky barriers formed between percolating nanotubules High-quality metal- oxide nanostructures have also been fabricated using a thin film deposition technique that couples glancing-angle, collimated, ballistic vapor deposition (GLAD)... unusually fast (∼30 ms) By modifying this vapor deposition method to allow for reactive ballistic deposition, a high-surface-area surface consisting of feather-like MgO filaments could be grown (85) CONCLUDING REMARKS This review is a status report Progress in the synthesis of quasi -one- dimensional nanostructures nanowires and nanotubules—is currently in active ferment The range of materials and the scope for... 17:40 AR AR218-MR3 4-0 5.tex AR218-MR3 4-0 5.sgm LaTeX2e(2002/01/18) P1: FHD AR REVIEWS IN ADVANCE10.1146/annurev.matsci.34.040203.112141 NANOWIRES, SENSORS, AND CATALYSTS 177 ACKNOWLEDGMENTS We thank Drs Y Zhang, G Cheng, and Y Lilach for their crucial contribution to this work and Profs H Metiu and E McFarland for helpful discussions and for loaning us some equipment This work was supported by a DURINT... PA 1996 Surface Science of Metal Oxides Cambridge/New York: Cambridge Xia YN, Yang PD, Sun YG, Wu YY, Mayers B, et al 2003 One- dimensional nanostructures: synthesis, characterization, and applications Adv Mater 15:353– 89 Wang ZL 2003 Nanowires and Nanobelts: Materials, Properties and Devices Dordrecht/London/New York: Kluwer Bandyopadhyay S, Nalwa HS, eds 2002 Quantum Dots and Nanowires Stevenson Ranch,... migration of electrons and holes to the nanostructure surface where they can participate in chemical reactions before recombining Although these sorts of applications of oxide quasi -one- dimensional nanostructures are in their infancy, a few compelling studies have already appeared Dai and coworkers recently reported molecular photodesorption from single-walled carbon nanotubes and its dramatic influence . reserved CHEMICAL SENSING AND CATALYSIS BY ONE-DIMENSIONAL METAL-OXIDE NANOSTRUCTURES Andrei Kolmakov and Martin Moskovits Department of Chemistry and Biochemistry,. (32). Surface Reactions on One-Dimensional Oxides, Gas Sensing, and Catalysis The exploration of the metal-oxide nanostructures as a platform for chemical sens- ing