One-Dimensional Oxide Nanostructures as Gas-Sensing Materials: Review and Issues

In this article, we review gas sensor application of one-dimensional (1D) metal-oxide nanostructures with major emphases on the types of device structure and issues for realizing practical sensors. One of the most important steps in fabricating 1D-nanostructure devices is manipulation and making electrical contacts of the nanostructures. Gas sensors based on individual 1D nanostructure, which were usually fabricated using electron-beam lithography, have been a platform technology for fundamental research. Recently, gas sensors with practical applicability were proposed, which were fabricated with an array of 1D nanostructures using scalable micro-fabrication tools. In the second part of the paper, some critical issues are pointed out including long-term stability, gas selectivity, and room-temperature operation of 1D-nanostructure-based metal-oxide gas sensors.


Introduction
In 1962, Seiyama et al. discovered that the electrical conductivity of ZnO could be dramatically changed by the presence of reactive gases in the air [1]. Since then, there have been tremendous reports on the applications of semiconducting metal oxides as gas sensors due to their small dimensions, low cost, and high compatibility with microelectronic processing. Recently, one-dimensional (1D)

Single 1D Nanostructure Gas Sensors
Law et al. [2] have found that individual single-crystalline SnO 2 nanoribbons have strong photoconducting response and thus detect ppm-level NO 2 at room temperature by illuminating the nanoribbons with UV light of energy near the SnO 2 bandgap (E g = 3.6 eV at 300 K). Photogenerated holes recombine with trapped electrons at the surface, desorbing NO 2 and other electron-trapping species: h + + NO 2 -(ads) → NO 2(gas) . The space charge layer thins, and the nanoribbon conductivity rises. Ambient NO 2 levels are tracked by monitoring changes in conductance in the illuminated state. The larger and faster response of individual nanoribbon sensors with 365 nm illumination than that with 254 nm illumination suggested that the presence of surface states plays a role in the photochemical adsorptiondesorption behavior at room temperature.
Wang and co-workers demonstrated the gas sensing ability of field-effect transistors (FETs) based on a single SnO 2 nanobelt [3]. SnO 2 nanobelts were doped with surface oxygen vacancies by annealing in an oxygen-deficient atmosphere. Then the source-drain current of SnO 2 nanobelt FETs could respond and recover with exposure and removal of oxygen in ambient nitrogen at 200 °C. Later, they improved the device performance of the SnO 2 nanobelt FETs [4]. Low-resistance RuO 2 /Au Ohmic contacts on the SnO 2 nanobelts led to high-quality n-channel depletion mode FETs with well-defined linear and saturation regimes, large on current, and on/off ratio as high as 10 7 . The FET characteristics show a significant modification upon exposure to 0.2% H 2 . The channel conductance in the linear regime increases by around 17% at all gate voltages. The hydrogen reacts with and removes the oxygen adsorbed on the metal oxide surface and thus increases the electron concentration and the conductance of the nanobelt channel [5]. Qian et al. [6] reported a CO sensor based on an individual Au-decorated SnO 2 nanobelt.
Wang and co-workers presented a high sensitivity humidity sensor based on a single SnO 2 nanowire [7]. The SnO 2 nanowire based sensor had a fast and sensitive response to relative humidity in air from a wide range of environments at room temperature. In addition, it had relatively good reproducibility, and its linear response to 30-90% RH makes it easy to calibrate. The sensitivity of the single SnO 2 nanowire based sensors to CO, CH 4 and H 2 S gases at 250 °C was improved by 50-100% through surface functionalization with ZnO or NiO nanoparticles [8]. The heterojunction between the surface coating layers and SnO 2 (i.e., n-n junction for ZnO-SnO 2 and p-n junction for NiO-SnO 2 ) and the corresponding coupling effect of the two sensing materials played a critical role in controlling device sensitivity. Besides heterojunctions, many other factors such as the size and crystalline state of surface additives and the concentration change of structure defects in the nanowires might bring a pronounced influence on the gas sensing performance of the SnO 2 nanowire based device. Thus, it was difficult to use a uniform model to completely elucidate the nature of the surface additives. Despite this, it was clear that surface functionalization is a good strategy to improve the sensitivity and selectivity of the SnO 2 -based nanosensor. Kumar et al. [9] reported highly sensitive H 2 S sensors based on homogeneously Cu-doped SnO 2 single nanowires. By Cu doping, the sensitivity of SnO 2 single nanowire sensors could be increased by up to 10 5 .
Recently, Wang and co-workers reported gigantic enhancement of sensitivity in a single ZnO (E g = 3.37 eV at 300 K) nanowire based gas sensor with asymmetric Schottky contact [10]. The device was composed of a single ZnO nanowire mounted on Pt electrodes with one end in Pt:Ga/ZnO Ohmic contact and the other end in Pt/ZnO Schottky contact (Figure 2a). An ultrahigh sensitivity of 32000% was achieved using the Schottky contacted device (SCD) operated in reverse bias mode at 275 °C for detection of 400 ppm CO mixed with dry air, which was four orders of magnitude higher than that obtained using an Ohmic contact device (OCD) under the same conditions ( Figure 2b). The local Schottky barrier height of the small contact area between the nanowire was tuned through the responsive variation of the surface chemisorbed gases at the junction area (Figures 2c-2e), which serves as a "gate" for controlling the transport of charge carriers [11,12]. In addition, the response time and reset time were shortened by a factor of seven. Liao et al. [13] showed that the sensitivity of gas sensor based on a single ZnO nanowire to H 2 S in air at room temperature could be modulated and enhanced by He + irradiation at an appropriate dose. Choi et al. [14] have developed a new smart ZnO nanowire gas sensor based on the commercially available 0.35 μm complementary metal-oxide-semiconductor (CMOS) process to improve the sensing performance with better resolution and to evaluate the reliability of the single ZnO nanowire gas sensor. Zhou and co-workers reported ultrasensitive single In 2 O 3 nanowire sensors for NO 2 and NH 3 at room temperature [15]. The devices exhibited far superior performance compared to previously reported results. For instance, the devices exhibited sensitivities (defined as the resistance after exposure divided by the resistance before exposure) of 10 6 for NO 2 and 10 5 for NH 3 , which are four or five orders of magnitude better than results obtained with thin-film based sensors. Response times (defined as time duration for resistance change by one order of magnitude) as short as 5 s for 100 ppm NO 2 and 10 s for 1% NH 3 have also been achieved. The lowest detectable gas concentrations were 0.5 ppm for NO 2 and 0.02% for NH 3 . In addition, UV illumination of our devices can dramatically enhance the surface molecular desorption kinetics and thus lead to substantially reduced recovery time. They have further developed the device performance of the single In 2 O 3 nanowire sensors [16], demonstrating a detection limit of NO 2 at ~20 ppb, which is the lowest detectable concentration ever achieved with all types of metal oxide nanowire sensors and all conventional solid-state NO 2 sensors working at room temperature ( Figure 3). Recently, Zeng et al. [17] demonstrated a highly sensitive and selective H 2 S nanosensor by using a single In 2 O 3 nanowire transistor. The nanosensor worked at room temperature without UV-assisted desorption and exhibited a detection limit of 1 ppm for H 2 S. The response and recovery are both very fast at ~50 s. Moreover, the nanosensor demonstrates an extremely weak response to NH 3 and total insensitivity to CO, which is highly promising for practical application for detecting low concentration of H 2 S. Sensing response of a single nanowire device to NO 2 diluted in air. The normalized conductance change (∆G/G 0 ) is plotted as a function of time with the nanowire sensor exposed to NO 2 of various concentrations. Recovery was made by UV light (254 nm) desorption of NO 2 . At point A, the first cycle was taken with UV illumination. The nanowire conductance kept rising until the UV light was turned off at point B. 20 ppb NO 2 was introduced to the airflow at point C (Reprinted from reference [16] with permission from American Chemical Society).
Moskovits and co-workers have intensively studied the electron-transport properties of single SnO 2 nanowires configured as FETs over a wide temperature range in various atmospheres comprised of mixtures of N 2 /O 2 /CO [18][19][20]. Owing to their large surface-to-volume ratios, the bulk electronic properties of the nanowires were found to be controlled almost entirely by the chemical processes taking place at their surface, which could in turn be modified by controlling the gate potential. Thus, the rate and extent of oxygen ionosorption and the resulting rate and extent of catalytic CO oxidation reaction on the nanowire's surface could be controlled and even entirely halted by applying a negative enough gate potential, presenting the prospect of tuning catalysis or other surface reactions entirely through electronic means [20]. Moskovits and co-workers have shown enhanced gas sensing of single SnO 2 nanowires configured as resistive elements by surface decoration with metal nanoparticles such as Pd [21] and Ag [22]. For Pd-decorated SnO 2 nanowires, the 500-1000% improvement in sensitivity toward oxygen and hydrogen was attributed to the enhanced catalytic dissociation of the molecular adsorbate on the Pd nanoparticle surfaces and the subsequent diffusion of the resultant atomic species to the oxide surface (spillover effect). For Ag-decorated SnO 2 nanowires, the significant improvement in sensitivity toward ethylene was due to the modification of the Schottky junction formed between the Ag particles and the tin oxide resulting from the surface chemical processes involving ethylene and oxygen occurring exclusively on the silver nanoparticles' surface (electronic effect). (c) I-V curves obtained at 50 °C for Pd-decorated VO 2 nanowire after various exposure times to hydrogen gas (5 sccm), added to the background argon stream (10 sccm). (d) The change in current for a Pd-decorated VO 2 nanowire biased at 10 V as a function of time of exposure to hydrogen gas. Initially the current increases gradually with hydrogen exposure time and then at ~7 min increases dramatically by ~3 orders of magnitude (5 × 10 -6 A → 6 × 10 -3 A) in the absence of the series resistor (Reprinted from reference [24] with permission from American Chemical Society).
Recently, Stelecov et al. [23] have demonstrated gas sensors based on single VO 2 nanowires, where the pressure dependent onset of metal-insulator transition in single crystal suspended VO 2 nanowires was used as a sensor signal. Moskovits and co-workers have reported that exceptionally sensitive hydrogen sensors were produced using Pd-nanoparticle-decorated, single VO 2 nanowires [24]. The high sensitivity arose from the large downward shift in the insulator to metal transition temperature following the adsorption on and incorporation of atomic hydrogen, produced by dissociative chemisorption on Pd, in the VO 2 , producing ∼1000-fold current increases (Figure 4).
Wang and co-workers have studied oxygen sensing properties of room-temperature single nanowire gas sensors based on various oxides such as ZnO [25], β-Ga 2 O 3 [26], and ZnSnO 3 [27]. Single β-Ga 2 O 3 (E g = 4.9 eV at 300 K) nanowires exhibited a very fast oxygen response time of ~1 s in 254 nm UV illumination [26], providing a route for realizing room-temperature fast-response oxygen sensors.
β-Ga 2 O 3 has very low carrier density and the oxygen sensing only appeared under the UV illumination. This is different from semiconducting nanowires including SnO 2 , In 2 O 3 , and ZnO, which initially have oxygen sensing properties due to the high carrier density without UV illumination, have long response time of several minutes under the UV illumination. Extremely high oxygen sensitivity about six orders of magnitude was realized from single ZnSnO 3 nanowires with abundant grain boundaries [27]. Such a drastic sensing was ascribed to grain boundary barrier modulation, demonstrate a promising approach to realize miniaturized and highly sensitive oxygen sensors.
Morante and co-workers have provided a systematic study on effects of contact resistances and the nanowire diameter size on the CO and humidity measurements using a single SnO 2 nanowire [28]. Controlled AC impedance measurements revealed that the single SnO 2 nanowire sensor had CO detection threshold smaller than 5 ppm and measurement instability lower than 4% at 295 °C. They we have demonstrated ultralow power consumption of self-heated single SnO 2 nanowire gas sensors [29]. For instance, the response of the sensors to 0.5 ppm NO 2 without heater (I m = 10 nA) was the absolute equivalent to that with a heater (T = 175 °C) ( Figure 5). These devices required extremely low optimal conditions for NO 2 sensing with less than 20 μW to both bias and heat them, which was significantly lower than the 140 mW required for the external microheater. Furthermore, they have demonstrated the equivalence between thermal and room-temperature UV light-assisted responses of single SnO 2 nanowire gas sensors [30,31] (Figure 6). For instance, the response of the sensors to 0.5 ppm NO 2 at room temperature under UV light illumination was the absolute equivalent to that operating at 175 °C in dark conditions. The experimental results revealed that nearly identical responses, similar to thermally activated sensor surfaces, could be achieved by choosing the optimal illumination conditions. Besides SnO 2 , In 2 O 3 , and ZnO nanowires or nanobelts, gas sensors based on single ZnO nanorods [32][33][34][35], single SnO 2 nanotubes [36], single TiO 2 and WO 2.72 nanowires [37], and single NiO nanowires [38] have been reported. Liao et al. [39] have presented that the gas sensitivity of a single CeO 2 (E g = 3.2 eV at 300 K) nanowire sensor to CO, H 2 , ethanol, gasoline, and H 2 S at room temperature could be significantly increased by incorporation of Pt nanoparticles on a CeO 2 nanowire. In comparison to conventional metal oxide sensors, the Pt-sensitized single CeO 2 nanowire sensor had an obvious advantage in selective detection of CO gas. However, the exact origin of the selectivity is still in question.    [44][45][46] or spectrometric method for NH3 [47,48]). Although the single 1D nanostructure sensors can be much cheaper than the high instrumental systems, the practical application of the nanosensor technique might be possible after several key issues such as long-term stability, gas selectivity and low-temperature operation are resolved. These issues will be addressed in detail in Section 3 of this paper. Peng et al. [52] fabricated a gas sensor based on an assembly of porous silicon nanowires (SiNWs) by making electrical contacts on the top portion of as-prepared nanowires by thermal evaporation or other methods through a mask. The sensors made from the porous SiNWs assembly showed fast response and excellent reversibility to ppb-level NO concentration. The excellent sensing performance coupled with scalable synthesis of porous SiNWs could be a viable mass-production of sensor chips.

Critical Issues
The use of 1D metal oxide nanostuctures as gas sensors has potential advantages compared to conventional thin film devices due to the intrinsic properties of 1D nanostructures such as high surface-to-volume ratio and high crystallinity. However, the use of nanowires in real devices is still in a preliminary stage. Thus, how to integrate them with low-cost and high-yield mass production processes has become a major challenge for the future. Here, we discuss technological issues of longterm stability, selectivity, and room-temperature operation which act as bottleneck to the massive use of 1D metal oxide nanostructures in commercial gas sensors [53].

Long-Term Stability
Sysoev et al. [54] reported a comparative study of the long-term gas-sensing performance of gas sensors based on randomly oriented single crystal SnO 2 nanowire mats and thin layers of pristine SnO 2 nanoparticles. The sensing elements composed of percolating nanowires demonstrate excellent sensitivity and long-term stability toward traces of 2-propanol in air. The superior initial sensitivity of the nanoparticle layer deteriorated during the first month of the operation and approached to one observed steadily in the nanowire mats. The better stability of the nanowire mats sensors was explained in framework of reduced propensity of the single crystal nanowires to sinter under realworld operation conditions with respect to nanoparticle thin film. The stability of the percolating paths, analyte delivery and transduction mechanism in nanowire network sensing elements was defined at the microscopic level. Hernadez-Ramirez et al. [53] showed the advantages of individual nanowire-based sensors comparison to porous-film sensors that the absence of nooks and crannies in nanowire-based devices facilitates direct adsorption/desorption of gas molecules, improving the dynamic behavior of these prototypes to various gases. Especially, single-nanowire sensors have no contribution of necks and grain boundaries to the device operation, leading to good stability. However, the well-controlled manipulation and characterization of nanowires should be developed to produce stable and device-quality 1D nanostructure-based sensors.

Gas Selectivity
Ideal gas sensors are the ones that respond to only target gas molecules. Unfortunately, metal-oxidebased gas sensors respond in a similar way toward different oxidizing (or reducing) gas molecules, even though sensitivities might be different depending on the type of gas molecules. A variety of methods were proposed in metal-oxide gas sensors to improve the selectivity, such as particular bulk or surface doping, the application of gas specific prefilters, etc. [55][56][57]. . re san excellent performance as a gas sensor and e-nose system capable of promptly detecting and reliable discriminating between ethanol, 2-propanol and CO in air at a ppb level of concentration which cannot be detected by common olfactometer.

Room-Temperature Operation
Adsorption and desorption of gas molecules on the surface of metal oxides are both thermallyactivated processes, which cause the response and recovery times to be usually very slow at room temperature. Thus, gas sensors based on 1D oxide nanostructures operate at high temperature (200--500 C) to enhance the surface molecular adsorption/desorption kinetics and continuously clean the surface. Development of room-temperature gas sensors might have very important advantages such as low power consumption, simple system configuration, reduced explosion hazards, and longer device lifetime.
Desorption of gas molecules typically requires much higher activation energy than adsorption. Law et al. first demonstrated SnO 2 -nanoribbon-based gas sensor operating at room-temperature by desorbing attached NO 2 gas molecules using ultraviolet (UV) irradiation [2]. UV-assisted desorption of NO 2 was explained as follows: before UV illumination, oxygen species are adsorbed on the SnO 2 nanoribbon surface, taking free electrons from the n-type SnO 2 nanoribbon and forming a depletion region that extends into the thin nanoribbon. When the SnO 2 nanoribbon is illuminated by UV light with wavelength shorter than the bandgap energy of SnO 2 , electron-hole pairs are generated. The positive holes discharge the negatively charged oxygen ions chemisorbed on the nanoribbon surface and eliminate the depletion region. Electrons produced at the same time increase the conductivity of the SnO 2 nanoribbon. More recently, UV irradiation was reported to influence adsorption of gas molecules on the surface of SnO 2 or ZnO nanostructures as well as desorption. Under illumination, photons partially desorb oxygen species from the surface, providing an increased number of adsorption sites available for gas molecules. Thus, the gas response of SnO 2 or ZnO nanostructures with UV light irradiation was about 120 times higher than that without UV light irradiation [31,62].
Fan et al. reported highly sensitive room-temperature chemical sensors for detection of NO 2 and NH 3 based upon ZnO nanowire field-effect transistors [63]. The electric field applied over the back gate electrode modulates the carrier concentration, which in turn significantly affects adsorption and desorption behaviors of gas molecules or gas sensitivity. A strong negative field was utilized to refresh the sensors by an electrodesorption mechanism.

Summary
In this article, we review gas sensor application of 1D metal-oxide nanostructures with major emphases on the types of device structure and issues for realizing practical sensors. In the initial stage, gas sensors based on individual 1D nanostructure were successfully fabricated using electron-beam lithography and demonstrated excellent gas-sensing capability. As a result, the individual 1D-nanostructure gas sensors have been a platform technology for fundamental research. Recently, gas sensors with practical applicability were proposed, which were fabricated with an array of 1D nanostructures using scalable micro-fabrication tools such as conventional optical lithography. Some critical issues were addressed including long-term stability, gas selectivity, and room-temperature operation of the 1D-nanostructure-based metal-oxide gas sensors.