Elsevier

Progress in Materials Science

Volume 66, October 2014, Pages 112-255
Progress in Materials Science

SnO2: A comprehensive review on structures and gas sensors

https://doi.org/10.1016/j.pmatsci.2014.06.003Get rights and content

Abstract

Metal oxides possess exceptional potential as base materials in emerging technologies. In recent times, significant amount of research works is carried out on these materials to assess new areas of applications, including optical, electronic, optoelectronic and biological domains. In such applications, the response and performance of the devices depend crucially, among other factors, on the size, shape and surface of the active oxide materials. For instance, the electronic and optical properties of oxides depend strongly on the spatial dimensions and composition [1]. The large number of atoms on the surface, and the effective van der Waals, Coulombic and interatomic coupling significantly modify the physical and chemical properties of the low dimensional oxide materials vis-á-vis its bulk counterparts. As a result, low dimensional oxide materials, such as nanoparticles, nanospheres, nanorods, nanowires, nanoribbon/nanobelts, nanotubes, nanodisks, nanosheets evoke vast and diverse interests. Thermal and physical deposition, hydro/solvothermal process, spray-pyrolysis, assisted self-assembly, oil-in-water microemulsion and template-assisted synthesis are regularly employed to synthesis one-, two- and three-dimensional nanostructures, which have become the focus of intensive research in mesoscopic physics and nanoscale devices. It not only provides good scopes to study the optical, electrical and thermal properties in quantum-confinement, but also offers important insights for understanding the functional units in fabricating electronic, optoelectronic, and magnetic devices of nanoscale dimension. Tin oxide (SnO2) is one such very important n-type oxide and wide band gap (3.6 eV) semiconductor. Its good quality electrical, optical, and electrochemical properties are exploited in solar cells, as catalytic support materials, as solid-state chemical sensors and as high-capacity lithium-storage. Previously, Chopra et al. [2] reviewed different aspects of transparent conducting SnO2 thin films. Wang et al. [3] discussed device applications of nanowires and nanobelts of semiconductor oxides, including SnO2. Batzill et al. [4] discussed about the surface of single crystalline bulk SnO2. However, it is understood that neither there is any comprehensive review on various crystallographic phases, polymorphs, bulk modulus, lattice parameters and electronic states of SnO2, nor there is any updated compilation on the recent progress and scope on SnO2 nanostructures. Therefore, the proposed review covers the past and recent progress on the said topics and is summarized in the following manner. The available theoretical and experimental works on crystal structures, bulk modulus, lattice parameters are reviewed in details. The electronic states and the band structures of these phases are discussed next. Active crystal surfaces of SnO2 play vital roles in its many interesting properties, including sensing and catalytic applications. So, a short review is written on its different surfaces, its electronic structures and density of states. The discussion on the importance of morphological variations on the properties of SnO2 is followed by a review on different methods for obtaining such structures. A detail survey on the existing literature on techniques and mechanisms for the growth of nanostructures are included. SnO2 is efficiently employed in gas sensing applications. A review on such applications is compiled based on the role of morphology and performance. The future course of SnO2 as an important material in the contemporary research is also discussed.

Introduction

It was not long ago, Rutherford came up with an accurate description of atomic configuration that, over the time, went through rigorous assessments on various scientific platforms. However, what was mostly unthinkable at that time, is possible at present through the progress of modern technologies. Nowadays, advance science has made it possible for us to see, and even move atoms using the scanning tunneling, or atomic force microscopes. Modern technology envisions a scenario where it can control and manipulate materials at nanometer scale, can command the chemical–physical phenomena and realize devices at atomic scale. The physical, or chemical manipulation over materials help engineers create superstructures by selectively arranging atoms and placing it exactly where it is desired. These techniques allow processing and manipulation at atomic scale, and direct the materials grow in size up to the micron, or nano order. It is termed as bottom-up engineering in nanotechnology. Various physical, chemical deposition techniques and most of the chemical processes that design and grow molecular networks and templates, come under this category. This is different from the top-down engineering, where a piece of raw material is broken into many small pieces of the order of atomic scale. Photo-lithography and lithography using electron beams (E-beam lithography) are two of the most widely used top-down techniques. The technology that deals with the manipulation of atoms and molecules in nanoscale dimension is called nanotechnology. Nanotechnology borrows liberally from physics, chemistry, materials science and biology, and connects the gap between the fundamental knowledge and the prowess of microstructural engineering. It deals with macroscopic particles in smaller dimension, where intermolecular forces, such as van der Waals play larger roles than its gravitational counterparts. In this regime of quantum science, electrons no longer flow through conductors as particle, but behave more like wave. It can hop or tunnel across insulating layers that would have originally prevented the passage if conventional macro scale physics were considered. In these small dimensions, surface of the materials is also a crucial factor in deciding how a material will eventually behave. While a bulk solid material will typically have less than 1% of its atom on the surface, a nanoparticle can possess over 90% of its atom on the surface. The high surface-to-volume ratio of nanoparticles makes them inherently more reactive. In this light of event, SnO2 is a competent and suitable candidate in the contemporary research and development programmes for exploring its various properties both in bulk and nano-dimensions. It has also been a popular oxide material for various chemical and physical applications.

SnO2 conforms to the Odouble bondSndouble bondO structure and is an n-type, wide band gap (3.6 eV) semiconductor oxide [5], [6]. Interestingly, the simultaneous occurrence of transparency and conductivity of SnO2 is a unique feature among the Group-IV elements of the periodic table. For example, its superior optical transparency is suitable for optically passive component in number of devices [7], [8], [9]. The study of SnO2 is triggered by its impressive range of applications in solar cells [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], as catalytic support materials [26], [27], [28], [29], [30], [31], [32], [33], [34], as solid-state chemical sensors, etc. [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50]. The key for understanding various aspects of SnO2 is its surface properties which eventually are governed by the dual valency of Sn. The dual valency facilitates a reversible transformation of the surface composition from stoichiometric surfaces with Sn4+ surface cations into a reduced surface with Sn2+ surface cations depending on the oxygen chemical potential of the system [4]. Medvedeva et al. [51] formulated criteria for the successful combination of high electrical conductivity with complete transparency in the visible range and emphasize the significant correlation between their structural attributes with the electronic and optical properties. In the direct band structure of SnO2, the top of the valence band mostly consists of O(p) states, while the bottom of the conduction band has an anti-bonding character arising from the Sn(4s) and O(p) states. Simple analytical expression of the band gap can be obtained by the effective-mass approximation (EMA), where simple Coulomb potential of the electron–hole pair is considered [52], [53], [54], [55]. Other effects such as the Coulomb attraction between the electron and the hole, or the polarization of the nanocrystals are treated as perturbations. However, the accurate representation of the electronic structures of nanocrystals is achieved by replacing the exchange–correlation self-energy by its linear expansion in the dynamically screened Coulomb interaction [56], [57]. Density-functional theory (DFT) is used to calculate the total energy and electronic structure of a compound by employing local-density approximation (LDA) and generalized gradient approximation (GGA) [58]. Tight binding approximation (TBA) on the other hand, considers the entire valence and conduction bands in order to describe the metal oxides semiconductors and heterostructures [59]. Unfortunately, theoretical predictions of the band gap failed to explain the experimentally obtained band gaps. In order to account for this discrepancies that exist between the theoretical and experimental observations, empirical correctional terms are added. The ground-state energy is calculated as a function of unit-cell volume, using a first-principles periodic Hartee–Fock linear combination of atomic orbitals (LCAO) approximation [60], [61]. Perdew et al. [62] suggested an alternative approach to consider the improved values for the total energy, which incorporates dominant self-interaction corrections (SIC). Schleife et al. [63] have recently reported plane-wave GW method to correctly predict the electronic structures as well as band gap of SnO2. It has been shown that self-interaction corrections to LDA can be very important for a quantitative description of a system with strongly localized states.

Doped SnO2 nanostructures that are used in optoelectronic devices generally possess high carrier concentrations of ∼1020 cm−3. At these carrier concentrations, however, light absorption by free carriers alters the optical constants significantly in a range extending from the near-infrared to visible region [64], [65]. Polycrystalline indium (In) and lithium (Li) doped SnO2 [66] show strong electrical properties, and they are optically transparent. The reason for the coexistence of electrical conductivity and optical transparency is mostly unclear. It was explained [67] that transparent conductivity is related to the existence of shallow donor levels near the conduction band, formed by a large concentration of oxygen vacancies. To understand the phenomenon of transparent conductivity in SnO2, Kílíc et al. [68] carried out first-principles calculations of formation energies and electrical (donor, acceptor) levels for various intrinsic defects, such as oxygen vacancy (VO), tin interstitial (Sni), tin antisite (SnO), tin vacancy (VSn) and oxygen interstitial (Oi) in different charge states and under different chemical potential conditions in SnO2. The calculations were performed in the framework of DFT within LDA using the Ceperley–Alder exchange correlation potential. It has been shown that Sni and VO dominate the defect structure of SnO2 due to the multivalence of Sn, explaining the natural nonstoichiometry of this material. That these defects produce shallow donor levels explains n-type conduction in undoped SnO2. This also explains that SnO2 can have a high carrier concentration with minor effects on its transparency. Medvedeva et al. [51] discussed the new class of transparent conducting oxides (TCO) using full-potential linearized augmented plane-wave method (FPLAPW) within the screened exchange LDA approach [69], [70].

The conductivity of pure nonstoichiometric SnO2 is determined by oxygen vacancies whose concentration is usually difficult to control. A number of earlier reports [71], [72], [73] emphasized on the conductivity of the doped films. Doping in SnO2 is done in order to introduce electron degeneracy. It is known that the addition of group III elements decreases conductivity and group V elements increases conductivity [74]. In such films, the resistivity (ρ) initially decreases with increasing dopant concentration, but starts increasing at even higher dopant concentration. The optimum concentration ranges between 0.4 and 3 mol%. Common dopants for increasing n-type conductivity of SnO2 are antimony (Sb), fluorine (F), and molybdenum (Mo) [75]. Terrier et al. [76] showed that introduction of Sb atoms in SnO2 decreases the porosity of the gel by increasing the density of cross-linked junctions. In some cases, the doping results in a larger grain size, affecting the preferred orientation of the crystal lattice with no apparent change in the lattice parameters [77], [78]. Randhawa et al. [79] reported ρ of Sb doped SnO2 thin films ∼5 × 10−4 Ω cm; with transparency (Tr) ≈95%. According to Kim et al. [80], ρ  9.8 × 10−4 Ω cm while Tr  88% along with large shift in the fundamental absorption edge. The effective mass (me) of conduction electrons also increases from 0.1 me for pure SnO2 to 0.29 me for films with 3 at.% Sb. Such a large change in me indicates non-parabolic conduction band [2]. Mishra et al. [78] calculated the electronic structure and associated properties of Sb doped SnO2 using both the self-consistent field scattered wave molecular orbital cluster approach and augmented spherical wave supercell band structure approach. The metallic nature of the electron led to the conclusion that the conductivity is due to the Sn 5s-like band. Additionally, the conductivity of this material will increase with thermal excitations of electrons to the Sn like bands. F doped SnO2 are polycrystalline and retain the rutile structure with no change in lattice parameter [81]. It generally shows higher mobilities of about 25–50 cm2 V−1 s−1 than Sb doped SnO2. Agashe et al. [82] reported very weak dependence of grain size on the concentration level (viz. 0–350 at.%) of the dopant. Manifacier et al. [83] reported a slight decrease in mobility (μη) when SnO2 is doped with F [2]. Thangaraju et al. [81] reported the ρ and μη for F doped SnO2 thin films in the range of 10−4–10−3 Ω cm and 7–17.2 cm2 V−1 s−1. Electronic devices employing Sb doped SnO2 nanowires exhibited low turn-on/threshold voltages [84], [85]. Bhise et al. [86] observed that the high emission current density, good current stability and mechanically robust nature of the In doped SnO2 nanowires offer unprecedented advantages as promising cold cathodes for many potential applications based on field emission, where the electron density lies between 1.3 × 1020 and 13.2 × 1020 cm−3.

Over the years the synthesis and characterization of the low dimensional inorganic nanostructured materials have received much attention because of their possible applications in a wide range of technological fields. More so because the preparation of the nanocrystals with different morphologies such as nanoparticles, nanorods, nanobelts, nanowires or thin films provide an opportunity to explore the comparative studies of their physical and chemical properties with sizes and morphologies. Nanostructures are routinely obtained by hydro-solvo-thermal, spray-pyrolysis, vapor–liquid–solid, sol–gel, microemulsion, thermal evaporation methods [87], [88], [89], [90], [91], [92], [93], [94], [95], [96], [97], [98], [99]. SnO2 nanorods, nanowires, nanoparticles, nanodisks, nanosheets, nanoribbon, and nanobelts have been synthesized and their properties are studied intensively [91], [95], [99], [100], [101], [98], [102], [103], [104], [105], [106]. Fujihara [107], Zhu [108] and Das et al. [109] used hydrothermal technique to prepare nanocrystalline SnO2 possessing high thermal stability. However, these techniques generally suffer from a poor control of the grain morphology and surface that affect the conductance of materials. Many improved methods, such as, a controlled water/anisole mixture that leads to the formation of a suspension of monodisperse nanocomposites of Sn/SnOx in a colloidal solution is preferred [110], [111]. The high degree of mechanical stability of SnO2 open up routes for potential applications, where the quality and the structure of the surface of nanoparticles play crucial role in determining the performance [112], [113].

It is known for a long time that the absorption of gas on the certain oxide semiconductor surface can cause a considerable change in the electrical resistivity of the sample [114]. Thus, a change in the surrounding gaseous environment easily leads to the change in the conduction (or, the resistance) of the semiconductor. Based on this phenomenon, metal oxide gas sensors are playing an important role in detecting toxic pollutants, such as CO, H2S, NOx, SO2 and hydrocarbons (e.g. CH4, LPG, etc.). At present nanostructural SnO2 is one of the most used materials as gas sensors [115], [116], [117]. The large surface area of nanostructures helps in the superior interaction between the analytes and the sensing part [118], [119]. The size, crystallinity and the concentration of defects in these nanostructures also determine the effective gas response of the sensors [120]. Xu et al. [117] calculated the change in conductivity as a function of the chemisorbed reducing gas species of single grain transitions. The gas sensing mechanism is described as the adsorption–desorption process of oxygen on the surface of the sensing materials. The formation of oxygen adsorbates (O2- or O) leads to a space charge region on the surface of the metal oxide, resulting in an electron depleted surface layer due to electron transfer from the surface to oxygen. The change in the conductivity of the sensing element with the reduction in grain size can be thus attributed to the penetrability of the depletion layer into the interconnecting grains. Although semiconductor gas sensors based on SnO2 have already been in the market, there is vast room for modifications of the sensing properties, such as sensitivity and selectivity to meet its ever-expanding demands. Both sensitivity and selectivity also depend on the distribution, chemical state, and cluster size of the added noble metals and, therefore, depend on the synthesis process [121]. The sensing materials between two electrodes can be assumed as a large grain cluster whose each and every constituting elementary grain contributes to the conductivity and thus the total system acts as a single mean grain. The sensitivity and selectivity, to some extent, have been tuned by using different catalysts, promoters and by varying the operating temperature [122], [123], [124]. The role of different dopants on the gas sensing behavior of SnO2 have also been studied to obtain high-performance sensors [125], [126], [127], [128]. For example, indium (In) doping was used to enhance the selectivity of SnO2 gas sensor [129], [130]. Sb doped SnO2 nanowire based gas sensors have promising application for ethanol sensing with low resistance, quick response and recovery times [131]. It was found that introduction of small quantities of noble metals (such as palladium (Pd) or platinum (Pt)) increases the sensitivity towards some gases (i.e. improves selectivity) and the sensor can be operated at lower sensing temperatures (i.e. improving stability) [115], [132], [133], [134], [135]. Cu doped SnO2 nanowire is good for H2S sensing [136]. Moreover, many reports described the mechanism behind the gas sensing capabilities of nanowires, nanorods, and other nanoforms, and those will be discussed in detail in this review [119], [137], [138], [139], [140], [141], [142], [143].

Thus, present article emphasizes on an extensive reviewing of the past research works on bulk SnO2 and its nanostructures. The discussions involved crystal structures, active crystal surfaces, band gap assignments, physical properties and potential uses of SnO2 as gas sensors. A comprehensive analysis is presented on high pressure induced crystallographic phases of SnO2, the changes in lattice and elastic parameters. It is observed that the electronic and optical properties of a material depend strongly on its spatial dimensions and composition. So, an elaborate account of the theoretical and experimental studies on the band gap with the help of density functional theory, tight binding approximation, etc. is presented. It was found that one way to control the physics of a material is to alter its dimension and morphologies. The size reduction also incorporates significant surface related defects, disorder and randomness in the system, which eventually determines physical and chemical features of that material. Creation of the nanocrystals and designing small nanostructures are the initial steps toward a faster technology. Nanostructures, such as spheres, cubes, tetrahedral, or octahedral, wires, or belts, or tubes, random, or aligned rods as well as mesoporous morphologies have become the focus of intensive research owing to their possible applications in mesoscopic physics. We have compiled a detail survey on the existing literature and discussed different technical, experimental details and theoretical conclusions to ascertain the fundamental mechanism that directs such formations. It not only provides a scope to study the electrical and thermal transport in the regime of quantum confinement, but also is expected to play an important role in both interconnection and functional units in fabricating electronic, optoelectronic, and sustainable energy devices with nanoscale dimension.

Section snippets

Crystal structure

SnO2 possesses several polymorphs such as the rutile-type (P42/mnm), CaCl2-type (Pnnm), α-PbO2-type (Pbcn), pyrite-type (Pa3), ZrO2-type orthorhombic phase I (Pbca), fluorite-type (Fm3m), cotunnite-type orthorhombic phase II (Pnam) with ninefold coordination [144], [145], [146], [147], [148]. All these structures are sequentially obtained when the most commonly available and stable rutile phase is subjected to high mechanical pressure.

Thus, the most important form of naturally occurring SnO2

Lattice parameters

The lattice parameters in such pressure driven phase structures are determined by fitting XRD data with Rietveld or other refinement methods (see Fig. 7). The Rietveld method refines user-selected parameters to minimize the difference between an experimental pattern (observed data) and a model based on the hypothesized crystal structure as well as instrumental parameters (calculated pattern). The whole fitting process involves adjusting the unit cell and peak shape parameters to acquire the

Bulk modulus versus pressure and crystallite size

Fig. 10 shows the pressure dependence of the relative volume for both bulk and nanocrystal SnO2. The experimental PV data were fitted with the Birch–Murnaghan equation of state [173] as described earlier (see Eq. (8)). Haines et al. [154] calculated B0205(7)GPa whereas B0 stands as 7.4(2.0). The data for the CaCl2 type phase lie on the extrapolated equation of state of the rutile phase [180]. The bulk modulus of α-PbO2-type SnO2 was found to lie between 199(4) GPa B0=7 and 208(2) GPa B0=4.

Electronic states

It is noted that there is no dearth of the theoretical and experimental works on rutile SnO2, yet there are very few and almost none on other SnO2 polymorphs. The zero pressure values of the band gap energies of the SnO2 is given by rutile-type as 3.57 eV. SnO2 crystals possess a rutile structure with symmetry of D4h-l4, belonging to the tetragonal system. The lattice parameters are: α=β=γ=90°, a = 0.473 nm, c = 0.318 nm. The unit cell contains two molecules with the Sn atoms at the positions (0, 0, 0)

DOS of bulk SnO2

The projected density of states (DOS) for the bulk SnO2 was calculated at the optimized structures with all-electron basis sets incorporating d-functions. As mentioned earlier, 5p binary SnO2 belong to one class of materials that combine high electrical conductivity with optical transparency, and thus suitable for optoelectronic applications. The calculation of DOS explains the inherent features of the materials to exhibit these dual features of transparent conducting oxides, viz. the optical

Surface

SnO2 is known for its application in gas sensors, as oxidation catalyst and as transparent conductor. The surface activities is crucial for various chemical and physical properties that make SnO2 an important material for these applications. Moreover, the growth and shape of the nanostructures particularly depend on the thermodynamic energy of the surface and growth occurs in a bid to minimize the energy. In the present section, the stoichiometric and non-stoichiometric surfaces, the

Nanostructures: synthesis and morphology

The ability to manipulate the shape and size of a material offers researchers to tailor its mechanical, chemical, magnetic and electronic properties. As the size reduces to the quantum regime, hitherto unknown features of a material in the nanoscale dimension offer possibilities for better and newer application in many field of science. The physical and chemical arrangements of atomic arrays, production of shaped materials in micro and nanometer scale and the ability to design molecules aiming

SnO2 as sensor

Sensor technology has gained popularity as the need for physical, chemical and biological recognition systems. Gas sensors are used to detect gas, to discriminate odor, or generally to monitor changes in the ambient gas atmosphere. Metal oxide based gas sensors devices are in the use for the last couple of decades. However, an appropriate choice of the materials for a particular gas is very important, for not all materials are suitable for all kind of gas detection. For this, the analysis of

Acknowledgements

Authors are thankful to Dr. Sreemanta Mitra and Dr. Biswajit Mandal of Indian Association for the Cultivation of Science, Kolkata, India for sharing their views on the present topics and providing priceless study materials.

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