Semiconductor Nanowires and Nanotubes: From Fundamentals to Diverse ApplicationsView this Special Issue
One-Dimensional SnO2 Nanostructures: Synthesis and Applications
Nanoscale semiconducting materials such as quantum dots (0-dimensional) and one-dimensional (1D) structures, like nanowires, nanobelts, and nanotubes, have gained tremendous attention within the past decade. Among the variety of 1D nanostructures, tin oxide (SnO2) semiconducting nanostructures are particularly interesting because of their promising applications in optoelectronic and electronic devices due to both good conductivity and transparence in the visible region. This article provides a comprehensive review of the recent research activities that focus on the rational synthesis and unique applications of 1D SnO2 nanostructures and their optical and electrical properties. We begin with the rational design and synthesis of 1D SnO2 nanostructures, such as nanotubes, nanowires, nanobelts, and some heterogeneous nanostructures, and then highlight a range of applications (e.g., gas sensor, lithium-ion batteries, and nanophotonics) associated with them. Finally, the review is concluded with some perspectives with respect to future research on 1D SnO2 nanostructures.
One-dimensional (1D) nanoscale materials, such as nanotubes, nanowires, and nanobelts, have attracted significant attention due to their unique size- and dimensionality-dependent electrical, optical, chemical, and mechanical properties and promising applications as interconnection and functional components in designing nano-sized electronic and optical devices [1, 2]. 1D semiconductor nanostructures represent an important and broad class of nanoscale wire-like structure, which can be rationally and predictably synthesized in single crystalline form with controlled chemical composition, diameter, length, and doping level with high precision. The availability of nanostructures has enabled a wide-range of proto-type devices and integration strategies [3, 4].
It is well known that materials behave differently at the nanoscale than their bulk counterparts. Low-dimensional nanoscale materials, with their large surface areas and possible quantum confinement effects, exhibit superior mechanical, thermal, chemical, electrical, and optical properties distinct from their bulk counterparts. The manipulation of well-controlled precise dimensions, crystallinity, and composition of 1D nanostructures gives rise to unique properties, thus enabling a variety of applications that would not be possible with materials with bulk dimensionality. Doubtlessly, a thorough understanding of the fundamental properties of the 1D nanostructures system is indisputably the prerequisite of research and development towards practical applications .
As a n-type direct wide-band semiconductor ( eV at 300 K), SnO2 is transparent in the visible light region and useful as optoelectronic devices , catalyst supports , transparent conducting electrodes , antireflective coatings , and a proto-type material for metal oxide sensors . One-dimensional tin oxide nanostructures have been synthesized by a variety of techniques, including vapor transport , carbothermal reduction , laser ablation of pure tin in an oxidizing Ar/O2 atmosphere , oxidation of electrodeposited tin wires , oxidation of tin vapors at elevated temperatures , solvothermal synthesis , and electrospinning , and so forth, Our group reported a molecule-based chemical vapor deposition (MB-CVD) to synthesize 1D SnO2 nanowires [18, 19], nanowire arrays [20, 21], and heterostructures  by the decomposition of a single-molecular precursor [Sn(O t Bu)4] containing preformed Sn-O bonds on gold nanoparticles, on a Si, Al2O3, and TiO2 substrates via the well-known vapor-liquid-solid (VLS) growth mechanism. Meanwhile it is precisely controlled over chemical composition, morphology, and deposition rate.
This article will provide a helpful review of the state-of-the-art research activities focused on synthesis and devices of 1D SnO2 nanostructures. The first section introduces typical 1D nanostructures obtained on SnO2, including nanowires, nanotubes, nanobelts, and some special nanostructures. Next, some important electronic and optoelectronic devices built on 1D SnO2 nanostructures are presented, which include gas sensor, lithium-ion batteries, and nanophotonics. This review will then conclude with some perspectives and outlook on the future developments in the 1D SnO2 nanostructures and related research areas.
2. Typical 1D SnO 2 Nanostructures
SnO2 nanowires have been synthesized by several physical and chemical methods, for example, vapor-solid (VS) synthesis by SnO evaporation under pulsed flow conditions led to the formation segmented wires . In addition, thermal evaporation of SnO or SnO2 was also used to the formation of nanowires in high quantity . Further, SnO2 nanowires have been obtained by self-catalyzed growth mechanism via carbothermal synthesis , the laser ablation of pure tin in oxidising Ar/O2 atmosphere , and oxidation of electrodeposited tin wires within structure directing AAO membranes  or through oxidation of tin vapours at elevated temperatures . The catalyst-assisted molecule-based CVD growth is a generic approach towards controlled synthesis of metal oxide nanostructures and had been demonstrated for SnO2, as shown in Figure 1. It is revealed that the average radial dimension of the SnO2 wires is approximately 0.9–1.1 μ m. The indexing of the SAED pattern shows that the SnO2 nanowires grow along the direction with an interplanar spacing of 0.334 nm, which corresponds to the plane of SnO2 in the rutile phase . Large surfaces covered with tin oxide nanorods could be achieved by an aqueous approach controlling ionic strength and pH of precursor solutions .
Considerable attention has also been paid to semiconducting nanotubes due to high surface-to-volume ratios compared with their solid wire counterparts since the first report of carbon nanotubes by Iijima in 1991 . Nanotubes are often obtained for materials with layered or pseudolayered structures . For the preparation of SnO2 tubular structures, the template-assisted approach has been proven to be an effective route, which generally involves the use of removable templates, such as preobtained nanorods , cellular fibers , porous membranes , and carbon nanotubes (CNTs) . However, the morphologies of the tubular structures are rather limited due to the difficulty in fabricating templates with diverse morphologies. Recently, 1D silica mesostructures are used as sacrificial templates to synthesize SnO2 nanotubes with preserved morphologies via a simple hydrothermal route (Figure 2(a)), resulting in the formation of well-defined SnO2 nanotubes with different lengths and unique helical SnO2 nanotubes with a wealth of conformations (Figures 2(b) and 2(c)) . In addition, SnO2 nanotubes with controlled diameter and length were synthesized using an electrochemical method at room temperature. The length and wall thickness of the nanotubes increased monotonically with the deposition time, and the diameter of the nanotubes was altered by varying the pore size of the scaffolds .
Nanobelts (or nanoribbons), an independent family in the realm of 1D nanomaterials, are regarded as an ideal system to fully understand dimensionally confined transport phenomena and may act as valuable units to construct nanodevices owing to their well-defined geometry . SnO2 nanobelts have been successfully synthesized by thermal evaporation of different source materials. Structurally perfect and uniform single crystalline SnO2 nanoribbons, with widths of 30–200 nm, width-to-thickness ration of 5–10, and length of several hundred micrometers to a few millimeters, were synthesized by simple thermal evaporation of SnO2 or SnO powder at high temperatures. It is worth to notice that wire-like nanostructures can be observed in the first stage of growth process due to the presence of strain. The profile of the fringes implies that the geometrical shape of wire-like nanostructures is likely to be a ribbon . SnO2 nanobelts also were synthesized by mixture of Sn foil and SnO powders with the same growth mechanism . A simple rapid oxidation of elemental tin was proposed for large-scale production of SnO2 nanoribbons. The width of the nanostructure was sensitive to the reaction temperature. When the growth temperature was increased to 1350°C, the diameter range of these nanostructures increased markedly to 50–350 nm (Figure 3(b)) compared to the wire-like nanostructures grown at 1080°C (Figure 3(a)). The lengths of the nanoribbons were up to several hundreds of micrometers, and the typical width and thickness were in the range of 30–150 nm and 10–30 nm (Figure 3(c)), respectively. The as-synthesized SnO2 nanoribbons appeared to be single crystalline and exhibited  and  growth directions (Figures 3(d)–3(e)) . Recently, surfaces of zigzag SnO2 nanobelts have been synthesized by a VS process, which are reduced surfaces terminated by Sn atoms and the Sn terminated surface is a nonpolar surface. As both reduced SnO and stoichiometric SnO2 (101) surfaces are neutral terminations, the polar surface model is no more suitable for the (101) type surface of rutile SnO2 nanobelts .
2.4. Other SnO 2 1D Nanostructures
Synthesis and assembly of SnO2 1D nanostructures with special morphologies, shapes, and compositions have attracted great interests very recently because they process unique properties and functionalities that are not accessible in the single-component materials, due to the combination of material classes such as metals, metal oxides, semiconductors, and polymers. They may also be used to fabricate special electronic and optoelectronic devices which cannot be fulfilled using simple 1D nanostructures [40, 41]. Branched 1D SnO2-V2O5 heterostructures were fabricated by growing vanadium oxide nanostructures on as-grown SnO2 backbones (Figure 4(a)). HR-TEM images of the SnO2-V2O5 interface revealed a strong alignment of the two different crystal lattices, resulting in a heteroepitaxial growth of vanadium oxide on tin oxide. The single-crystalline nature and growth direction of the V2O5 were indexed to be (Figure 4(b)) . Hierarchical nanostructures with SnO2 backbones and ZnO branches are successfully prepared in a large scale by combining the vapor transport and deposition process (for SnO2 nanowires) and a hydrothermal growth (for ZnO). The ZnO nanorods grow epitaxially on the SnO2 nanowire side faces mainly with a four-fold symmetry. The number density and morphology of secondary ZnO nanostructure can be tuned by adjusting the baking condition, such as the salt concentration, reaction time, and additives . In addition, SnO2 nanobelt/CdS nanoparticle core/shell heterostructures are successfully prepared via a simple and efficient sonochemical approach. The CdS nanoparticles are nearly spherical in shape and have typical sizes in the range of 10–20 nm. The measured spacing of the crystallographic planes is 0.315 nm, which corresponds to the lattice plane of the hexagonal structured CdS crystal .
3. Applications of 1D SnO 2 Nanostructures
As an important group of wide-band gap semiconductors with tunable conductivity and high transparency, 1D SnO2 nanostructures have been used to fabricate nanoscale gas sensor, electronic and optoelectronic devices.
3.1. Gas Sensor
Gas sensing using nanomaterials is an attractive, versatile application for important molecule species, environmental and security-checking purposes [44, 45]. The advantages of using 1D nanostructures for chemical sensing are diverse. With a large surface-to-volume ratio and a Debye length comparable to the nanowire radius, the electronic property of the nanowire is strongly influenced by surface processes, yielding superior sensitivity than their thin film counterpart. The sensing mechanism of metal oxides is mainly governed by the fact that the oxygen vacancies on the oxide surfaces are electrically and chemically active, therefore, the conductivity (or resistivity) of oxide nanomaterials is strongly affected by the adsorbed molecules. SnO2 is n-type semiconductor with oxygen deficiency. The lattice oxygen is evaporated in the form of gas, which makes the doubly ionized oxygen vacancy and electrons. If tin dioxide is heated at 300–400°C, the oxygen in the atmosphere is adsorbed at the surface of SnO2 with the negative charge. Because electron is provided from the surface of crystal, the surface of tin oxide becomes an electron depletion layer. This means the formation of potential barrier near the grain boundary. If CO is present in the atmosphere, the CO oxidizes into CO2 with the reaction of adsorbed oxygen and remaining electron returns to the SnO2 crystal. This increases the electrical conductivity to a great extent. So, we can detect CO by measuring the remarkable resistance decrease upon CO reaction. Generally speaking, the reduction in resistance occurs mainly at the surface and can be explained by the reduction of potential barrier .
The first SnO2 nanobelt chemical sensor was made on an alumina substrate by dispersing SnO2 nanobelts atop prefabricated platinum interdigitated electrodes on a substrate. The sensor was employed to detect CO and NO2 for environmental polluting species and ethanol for breath analysis. A single-SnO2-nanobelt sensor integrated with microheaters to sense dimethyl methylphosphonate (DMMP) was fabricated. The conductance of the SnO2 nanobelt sensor was reduced by 5% when the sensor was exposed to 78 ppb of DMMP . It was also reported that light photoactivates molecule absorption and desorption as an alternative way to the use of temperature. An explosive environment is the typical application where the use of heated sensors is not favorable. The strong photoconducting response of individual single-crystalline SnO2 nanoribbons makes it possible to achieve equally favorable adsorption-desorption behavior at room temperature by illuminating the devices with a ultraviolet (UV) light of energy near the SnO2 bandgap. The active desorption process is thus photoinduced molecular desorption . Kolmakov et al. realized an array of parallel nanowires of SnO2 by using self organized highly ordered porous alumina templates. Conductance measurements were performed on isolated individual nanowires with vapor-deposited Ti/Au microcontacts. They analyzed the behavior of nanowire in N2 and N2 + 10% O2. CO reacts with preadsorbed oxygen species on SnO2 to form carbon monoxide thus donating few electrons back into the bulk resulting in an increased conductivity, which depends monotonically on the gas phase partial pressure of CO (Figure 5) . Single SnO2 metal oxide nanowires are used at the nanoscale level as individual monocrystal for the electrical transduction of the gas interaction with these sensing materials. Electrical contact characteristics and resistance variations under different gas ambient are analyzed from two- and four-probe measurements of individual nanowires. CO and humidity behaviors are reported for single SnO2 nanowires with CO detection threshold smaller than 5 ppm and measurement instability lower than 4% (Figure 6) . A technical approach to fabricate practical devices by coupling a single-crystal SnO2 nanowire sensing element with a microhotplate gas sensor platform was also presented .
In order to tune the sensitivity and selectivity of SnO2 nanowires-based sensors, many techniques have been explored. The addition of a small amount of noble metals such as Au, Pd, Pt, and Ag and Ni speeds up surface reactions and improves selectivity towards target gas species . Kolmakov examined the influence of the surface sensitization with catalyst particles of Ni/NiO and Pd . Wan reported the use of Sb doping to tailor the resistivity of SnO2 nanowires deposited by thermal evaporation process starting from a mixture of Sn and Sb powders in the ratio 10 : 1 . Meanwhile, Pan et al. modified SnO2 nanowires by Ar/O2 plasma treatment through preferential etching of the lattice oxygen atoms, which produced nonstoichiometric surface compositions that imparted a many-fold higher sensitivity toward gas absorption on such surfaces (Figure 7) .
3.2. Anode Materials for Lithium-Ion Batteries
Rechargeable lithium-ion batteries (LIBs) have been recognized as an attractive power source for popular mobile devices. At present, LIBs are efficient, light-weight, and rechargeable power sources for consumer electronics such as laptop computers, digital cameras, and mobile phones. Moreover, they have been extensively studied for use as power supplies of electric vehicles (EVs) and hybrid electric vehicles (HEVs), which require high energy and power densities .
As a promising anode material for LIBs, tin-based material has attracted growing attention owing to the extraordinary electrochemical behavior, such that the initial irreversible capacity induced by Li2O formation, and the abrupt capacity fading caused by volume variation could be effectively reduced when in nanoscale form, the high theoretical capacity (992 mAhg−1) and the higher operating voltage comparing to traditional carbonaceous anode active materials [56, 57]. The self-catalytically grown SnO2 nanowires could provide more reaction sites on the surface and enhance the charge transfer in the electrochemical reactions. Moreover, Sn particles at the tips of nanowires also contributed to the Li+ storage and prevented the capacity loss that is induced by the existing metal catalysts. SnO2 nanowires showed an initial Coulombic efficiency of approximately 46.91% and improved cyclic performance and a higher reversible specific capacity of over 300 mAhg−1 up to the 50th cycle as shown in Figure 8 . In addition, SnO2 nanowire electrode without any buffer layer was used as rechargeable LIBs and exhibited superior electrochemical performance stable cycling behaviors and delivered a high specific discharge capacity of 510 mAhg−1, even at the 50th cycle .
SnO2 nanorod arrays represent an excellent geometry because they can offer direct channels for efficient electron transport. SnO2 nanorod array electrodes can be produced in a one-pot template-free alkaline hydrothermal process. The array electrode showed good performance as a LIB anode in properties such as capacity retention (580 mAhg−1 after 100 cycles at 0.1 C) and rate capability (stable 350 mAhg−1 at 5 C) .
SnO2-based heterostructure is a very promising strategy to achieve high-power-density and high-energy-density LIBs. A high electrochemical activity of V2O5 loaded on SnO2 nanowire-based electrode showed a very-high rate capability. The thin V2O5 layer is beneficial for fast Li+ intercalation/deintercalation, while the SnO2 core nanowire provides a fast path for electron transportation and also increases the electrochemical utilization of V2O5. SnO2/V2O5 core/shell nanowires could deliver a high power density of 60 kWkg−1 while the energy density remains as high as 282 Whkg−1. A better rate capability was achieved at high temperature (Figure 9) . Carbon-coated SnO2 nanorod array also revealed excellent cycling stability (stable 320 mAhg−1 at 3000 mAg−1) and rate capability (585 mAhg−1 after 50 cycles at 500 mAg−1) .
Nanowires represent attractive building blocks for active nanophotonic devices, including light-emitting diodes (LEDs), lasers, and detectors [63, 64]. Significantly, the ability to assemble and electrically drive nanoscale sources and detector blocks could allow for fully integrated nanophotonic systems for use in applications ranging from biodetection through information processing. Nanowires of binary oxides have been employed throughout this work because of the variety of beneficial properties, including extreme mechanical flexibility and chemical stability.
SnO2, has recently been shown to act as an excellent subwavelength waveguide because of its defect-related PL bands at 2.5 eV (green) and 2.1 eV (orange) (Figure 10) . Nonresonant waveguiding (i.e., subbandgap light) in these structures can be achieved by simply focusing laser diodes on the end facet of the nanowire. SnO2 wires have dimensions between 100 nm and 400 nm, an optimal size range to efficiently guide visible and UV wavelengths because of the high index of refraction of SnO2 (). Optical linkages between active nanowires (GaN and ZnO) and passive nanowires (SnO2) can be formed via tangential evanescent coupling (Figure 11). It has been shown that a staggered side-by-side configuration, in which the active and passive elements interact over a few microns, outperforms bridged, or direct end-to-end coupling. Weaker coupling is achieved by staggering structures with a thin air gap (several hundred nanometers) between them, allowing communication via tunneling of evanescent waves . With further integration, it should be possible to create more functional geometries, such as branched optical hubs and Mach-Zehnder interferometers (optical modulators) that use the electro-optic effect for phase shifting. The integration of high-frequency electrically driven lasers with passive nanowire waveguides is the next step toward effectively transducing and routing packets of optical information within an optical computer or communication device . However, the goal of room temperature, electrically driven nanolasers, remains an active area of current research.
4. Summary and Perspective
In summary, we provide a comprehensive review of the state-of-the-art research activities focused on rational synthesis (e.g., nanowires, nanobelts, nanotubes, and heterostructures) and potential applications (e.g., gas sensor, lithium-ion batteries, and nanophotonics) of 1D SnO2 nanostructures. The fascinating achievements, till now, towards the device applications of 1D SnO2 nanostructures should inspire more and more research efforts to address the remaining challenges in this interesting field in the future.
Controlled manipulation of matter at the nanoscale will lead to fascinating and novel behavior and also impact device properties. The ability to rationally control key 1D SnO2 nanostructures parameters during growth, fabricate intra-NW heterostructures with well defined , and atomically abrupt crystalline interfaces should further increase the versatility of 1D SnO2 nanostructure-based electronic and photonic devices and also reduce the load for subsequent assembly processes.
Such a synthetic control will be extremely useful for creating precisely defined systems to investigate the effects of quantum confinement on electronic and optical properties of nanostructures resulting from the modification of the electronic density of states and also aid in developing novel nanophotonic devices.
Looking into the future, with extensive research in nanostructures synthesis to accurately control dimension and composition, a critical understanding of the modified properties of materials at the nanoscale, and the hierarchical assembly of nanostructures with exquisite spatial control, progress will be made, and new and interesting nanosystems will create the technologies of the future.
The authors graciously thank the scientists cited in this paper for their contributions to the research.
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