Electrical properties of tin oxide materials

Electrical properties of tin oxide materials

3

Pedro H. Suman Department of Physical-Chemistry, São Paulo State University (UNESP), Araraquara, Brazil Chapter outline 3.1 Electrical properties of undoped and n- and p-doped SnO2 materials  41 3.2 Electrical properties of stannous oxide (SnO)  44

3.2.1 Ambipolar behavior of SnO  46 3.2.2 Two-dimensional SnO materials  49

3.3 Electrical properties of mixed-valence tin oxide: the Sn3O4  50 3.4 Conclusions and future prospects  52 Acknowledgments  52 References  54

3.1 Electrical properties of undoped and n- and p-doped SnO2 materials Tin dioxide (SnO2), also known as stannic oxide, is the most abundant form of tin oxide on earth extracted from the cassiterite mineral and has been widely used in a range of technological applications including transparent conductors [1, 2], chemical sensors [3, 4], high-efficiency solar cells [5], and catalysts [6, 7]. The most chemical and thermal stable crystalline structure of SnO2 at ambient pressure is the rutile-type structure with tetragonal unit cell (lattice constants a = b = 4.7382 Å and c = 3.1871 Å) and space group P42/mnm [8], where the tin atoms are sixfold coordinated to threefold coordinated oxygen atoms. However, the formation of orthorhombic and cubic structures can also be favored in high-pressure conditions [9, 10]. Due to the presence of intrinsic defects (oxygen vacancies (VO) and interstitial tin (Sni)) in the tetragonal lattice acting as charge donor sites, SnO2 presents typical n-type conductivity with wide direct bandgap transition of 3.6 eV located at the Γ-point of the Brillouin zone and optical transparency in the visible spectrum [11, 12]. The deep understanding of these features in terms of the crystal and electronic structures and nature and amount of defects are notably relevant to control the electrical conductivity of SnO2 materials aiming practical applications. As undoped SnO2 is a native n-type semiconductor, the electrical conduction model used to describe its electrical properties is mainly associated with its intrinsic donor defects. Interstitial tin donor level is fully ionized in the conduction band (CB) [13], whereas shallow (singly ionized oxygen vacancy) and deep (doubly ionized oxygen Tin Oxide Materials­: Synthesis, Properties, and Applications. https://doi.org/10.1016/B978-0-12-815924-8.00003-7 © 2020 Elsevier Inc. All rights reserved.

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Tin Oxide Materials: Synthesis, Properties, and Applications

vacancy) donor levels associated to the oxygen vacancies are present in the bandgap (0.03 and 0.15 eV below the bottom of the CB, respectively [14, 15]). It means that the oxygen vacancies are expected to modulate the electron concentration (ne) resulting from the ionization of these two donor levels, and the ionized electrons are enabled to move through the solid lattice via temperature or other driving force [16]. In this way, the small-polaron hopping mechanism is used to explain the electron conduction as a function of the temperature in metal oxide semiconductors (SMOx) with a high degree of defects [17, 18]. Considering this model for undoped SnO2 where the electron conduction takes place in a nonadiabatic regime at high temperature (T > θD/2, where θD is the Debye temperature, which is 500 K for SnO2 [19, 20]), the electron conductivity (σ), the electron concentrations (ne), and the electron mobility (μe) in a nonstoichiometric system are related by the following equations:

σ = ene µe µe =

µ0  E  exp  − H  T 3/ 2  kT 

(3.1) (3.2)

where e is the electron charge, μ0 indicates the pre-exponential factor, T is the temperature, EH is the charge hopping energy, and k is the Boltzmann constant. Both equations reveal that μe and, consequently, σ are temperature-dependent. The influence of the oxygen vacancies on the electrical conductivity of SnO2 has been demonstrated experimentally by measuring the conductivity of SnO2 at different temperatures. The conductivity was found to vary inversely with the oxygen partial pressure, i.e., the increasing of the oxygen content leads to lower conductivities [14, 21]. However, this behavior is not supported by some first-principles calculations, which attributes the n-type conductivity of SnO2 to hydrogen acting as the unintentional donor [22–24]. Porte et al. examined the influence of key growth variables (e.g., growth temperature and oxygen pressure in the chamber) on the structural and electronic properties of SnO2 films grown by pulsed-laser deposition (PLD) method [25]. Fig. 3.1A shows the variation of the carrier concentration and the bandgap energy as a function of the temperature deposition of the SnO2 thin films. As the temperature increases, the n-type carrier concentration was reduced by three orders of magnitude, indicating that the density of n-type donor defects in SnO2 is lower at high temperatures. Fig. 3.1B displays the change of the optical bandgap (Eg) and the Fermi energy (EF) in terms of the temperature. Above 400°C, constant values of 3.6–3.7 eV and 4.5–4.6 eV were found to the bandgap and the Fermi energies, respectively, whereas abrupt shifts in the bandgap energy (3.2–3.3 eV) and the Fermi energy (4.7–4.8 eV) values were observed at 300°C due to the band tailing effects present in amorphous films [26]. The impact of the oxygen pressure during the deposition (PD) on the carrier concentration and the bandgap was also studied. A three-orders of magnitude reduction in the carrier concentration was obtained by increasing PD (Fig. 3.2A), which is related to the decrease of VO and Sni or the presence of Oi or VSn to balance the n-type defects. At oxygen-rich atmosphere, the low formation energy of Oi or VSn defects favors its presence, while in this same condition, the high formation energy of VO and Sni prevents their creation, once SnO2 films approach its stoichiometry due to the greater ox-

Electrical properties of tin oxide materials43

Fig. 3.1  (A) Carrier concentration measured by AC Hall effect measurements and (B) optical bandgap estimated from Tauc plot of UV-Vis spectra and Fermi levels as a function of the temperature deposition of SnO2 thin films. Reproduced with permission from Porte Y, Maller R, Faber H, AlShareef HN, Anthopoulos TD, McLachlan MA. Exploring and controlling intrinsic defect formation in SnO2 thin films. J Mater Chem C 2016;4:758–65. Published by The Royal Society of Chemistry.

Fig. 3.2  (A) Carrier concentration measured by AC Hall effect measurements and (B) optical bandgap estimated from Tauc plot of UV-Vis spectra and Fermi levels as a function of the background oxygen pressure for the deposition of SnO2 thin films. ⁎Bandgap values obtained from Batzill et al. [28]. Reproduced with permission from Porte Y, Maller R, Faber H, AlShareef HN, Anthopoulos TD, McLachlan MA. Exploring and controlling intrinsic defect formation in SnO2 thin films. J Mater Chem C 2016;4:758–65. Published by The Royal Society of Chemistry.

ygen incorporation in the lattice [27]. No considerable variation in the bandgap energy (3.6–3.7 eV) was observed as a function of the oxygen pressure. However, it was found an abrupt change in the EF above 150 mTorr (Fig. 3.2B). Overall, the authors demonstrated the ability to accurately control the mobility and concentration of the charge carrier by controlling the experimental parameters for the fabrication of the films.

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Tin Oxide Materials: Synthesis, Properties, and Applications

The doping process is an extensively used approach for tuning the electrical properties of SnO2 materials by introducing electron degeneracy. However, for efficient doping, the ionic radius of the dopant has to be close to or smaller than the host Sn4+ ion. P-type conductivity in SnO2 is commonly achieved by adding group III-A elements with lower valence than Sn4+, including Al [29], In [30], and Ga [31, 32] acting as acceptor dopants. These elements are incorporated in the Sn sites and produce shallow acceptor, decreasing the intrinsic n-type conductivity by increasing the hole concentration [22]. Li [33] and Mg [34, 35] are other impurities also used for this purpose. The incorporation of group-VA impurities (N, P, As, and Sb) would be another strategy to reach p-type doping in SnO2. These elements have one less valence electron than O and one more valence electron than Sn and, therefore, they are supposed to exhibit amphoteric behavior in SnO2 [36]. It means that they work as acceptors for O site substitution (p-type doping), but also as donors when incorporated on the Sn site (n-type doping). However, first-principles electronic-structure calculations demonstrated that the group-V impurities preferentially incorporate on tin sites (except for N), and thus n-type doping is more likely to occur [36]. Taking advantage of these properties, several physical and chemical methods have been used to produce different SnO2 structures that include 3D architectures [37–39], thin films, [40, 41], and nanomaterials (nanowires, nanotubes, nanobelts, and nanoparticles) [42–45]. However, the preparation and characterization of low-dimensional inorganic nanomaterials have received considerable attention over the years due to the outstanding electrical properties and remarkable potential for many applications. Esro et  al. examined the impact of the Sb doping on the electronic properties of highly transparent SnO2 films composed of nanocrystals and prepared by the van der Pauw technique [46]. Both the electron concentration (ne) and the electron mobility (μe) increased following an increase in the Sb content from 0% to 2%, reaching maximum values of 6.4 × 1020 cm−3 and ~32 cm2 V−1 s−1, respectively for 2% of Sb loading (Figs. 3.3C and D). Inversely, the resistivity ρ and sheet resistivity Rs reached minimum values of 7.35 × 10−4 Ω cm and 32 Ω sq.−1, respectively in the corresponding Sb loading (Fig. 3.3A and B). These parameters exhibited an opposite behavior above 2% of Sb doping.

3.2 Electrical properties of stannous oxide (SnO) Stannous oxide or tin monoxide (SnO) is a metastable phase of Sn-O system [47] typically used as gas sensors, anode material in sodium and lithium-ion batteries and catalysts [48–51]. At ambient pressure, SnO possesses PbO-type layered crystalline structure with a tetragonal unit cell (litharge crystal structure with space group P4/nmm) [52–54] where each tin atom is located at the top a pyramid structure with four oxygen neighbors in the base position. However, SnO materials can also be grown from the orthorhombic structure [55]. Stannous oxide has intrinsic p-type conductivity owing to the presence of tin vacancies (VSn) and oxygen interstitials (Oi), with a superior contribution of VSn due to its low energy formation, which introduces acceptor defects near to the valence band maximum (VBM) [52]. Similarly to the α-PbO,

Electrical properties of tin oxide materials45 103

Charge carrier concentration cm–3

(A)

(C)

101 1021

r (Ω cm)

102

10–3

10–4

(B)

37 32

1020

Mobility (cm2/Vs)

Sheet resistivity (Ω/sq)

10–2

27 22 17 12 7

0.00 0.1 1 10 [Sb3+]/[Sb3++Sn4+] (%)

(D)

2 1 10 0.00 0.1 [Sb3+]/[Sb3++Sn4+] (%)

Fig. 3.3  (A) Sheet resistivity, (B) resistivity, (C) carrier concentration, and (D) electron mobility of Sb-doped SnO2 as a function of the Sb loading. Reproduced with permission from Esro M, Georgakopoulos S, Lu H, Vourlias G, Krier A, Milne WI, et al. Solution processed SnO2:Sb transparent conductive oxide as an alternative to indium tin oxide for applications in organic light emitting diodes. J Mater Chem C 2016;4:3563–70. Published by The Royal Society of Chemistry.

litharge SnO materials can also present n-type conductivity, as confirmed by electrical measurements in SnO nano- and micro-structures synthesized in reducing ­synthesis atmosphere [48, 56]. Stannous oxide exhibits a large direct optical bandgap (2.5–3.0 eV) [28, 57] that provides high transparency in the visible region coupled with a small indirect bandgap (0.7 eV) [57, 58], which allows the Fermi level to be shifted from the VBM to the vicinity of the conduction band minimum (CBM) via element or electrostatic doping, favoring the ambipolar behavior [59].

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Tin Oxide Materials: Synthesis, Properties, and Applications

3.2.1 Ambipolar behavior of SnO Semiconducting metal oxides (SMOx) have attracted considerable attention to be employed as active layers in thin-film transistors (TFTs) alternatively to conventional silicon-based materials for the next generation of transparent and flexible electronics, such as flexible displays and smart sensors [60–62]. Most of SMOx used as the active element in TFT devices typically present n-type conduction, including ZnO and amorphous InGaZnO (α-IGZO), which can exhibit high field-effect mobility (~40 cm2 V−1 s−1), elevated on/off ratio (~109), and high optical transparency in the visible region [63–65]. In contrast, TFTs based on p-type SMOx such as CuO, Cu2O, and NiO commonly present low field-effect mobility and on/off ratio, once their deep VBM is composed of O 2p orbitals with strong directivity and large electronegativity, which limits the hole transport [66]. Thus, the practical use of p-type channels in high-performance TFTs is restricted, especially for the fabrication of complementary metal oxide semiconductor (CMOS) circuits where both n- and p-type semiconductors are required [67–70]. Continuous efforts have been made to fabricate high-performance devices using p-type and ambipolar oxide semiconductors to play a similar function of the CMOS circuits, but with simpler technology and more compact architecture. Organic semiconductors, silicon-based materials, and carbon nanotubes have exhibited ambipolar behavior. However, it is hard to find an oxide semiconductor able to operate in both n-type and p-type modes. It occurs due to its usual large fundamental bandgap and high density of subgap states that prevents the balance of injection and transport of holes and electrons [71–73]. SnO has been a rare example of p-type semiconductor with high hole mobility and on/off ratio (6.5 cm2 V−1 s−1 and >105, respectively, which is considered high for p-type semiconductors [74, 75]) able to operate in the ambipolar mode in TFTs and CMOS devices [66, 76–80]. Usually, the VBM of SMOx (e.g., SnO2 in Fig. 3.4A) is composed of localized and anisotropic O 2p orbitals. For SnO, Sn 5s, and O 2p orbitals are equally near to the VBM (Fig. 3.4B), and the formation of hybridized Sn 5s–O 2p orbitals results in the high hole mobility [72, 81]. The ambipolar nature of SnO is explained in terms of its electronic configuration (Fig.  3.5). As previously mentioned, the fundamental bandgap of SnO (0.7 eV [57, 58]) is typically smaller than other p-type semiconductors (usually larger than 1.7 eV). Its ionization potential (the energy difference between the VBM and the vacuum level) is approximately 5.8 eV [66, 82], which is similar to other p-type materials. It means that the electron affinity of SnO (~5.1 eV) is comparable to n-type semiconductors. Therefore, considering these parameters and the small effective mass of the charge carriers in SnO (~0.4 m0 for the electrons and ~0.6 m0 for the holes) [83], the carrier polarity (electrons or holes) can be controlled contributing to the ambipolar behavior. Nomura et al. [76] reported for the first time in 2011 the ambipolar nature of SnO, and then many research groups have used p-type SnO film as the active layer to operate in the ambipolar mode in TFTs devices. Luo et  al. demonstrated the conversion of SnO TFTs operation from unipolar p-type to the ambipolar mode through back-channel passivation [85], whereas Li et al. analyzed the ambipolar behavior of SnOx TFTs fabricated by reactive sputtering followed by post-annealing treatments in different temperatures as a function of the film morphology and composition [86].

Electrical properties of tin oxide materials47

Fig. 3.4  Energy band structures of (A) SnO2 and (B) SnO materials. The VBM of SnO is composed of Sn 5s and O 2p hybridized orbitals. Reproduced with permission from Fortunato E, Barros R, Barquinha P, Figueiredo V, Park S-HK, Hwang C-S, et al. Transparent p-type SnOx TFTs produced by reactive rf magnetron sputtering followed by low temperature annealing. Appl Phys Lett 2010;97:52105.

Fig. 3.5  (left) Band alignment of n- and p-type oxide semiconductors measured by UPS [84]. The dashed lines indicate the Fermi level. (right) The energy levels of SnO were experimentally determined by hard X-ray-photoemission spectroscopy [72]. Reproduced with permission from Hosono H, Ogo Y, Yanagi H, Kamiya T. Bipolar conduction in SnO thin films. Electrochem Solid-State Lett 2011;14:H13–6.

In both cases, the density of subgap states in the SnO channel was reduced either by the surface passivation or by the excess of Sn on the sidewalls of the microgrooves that composes the film structure, which allows shifting the Fermi level from the valence band to the conduction band by changing the polarity and magnitude of gate voltage. Yan Liang et  al. described the ambipolar characteristics of TFTs using SnO as the channel with balanced electron and hole field-effect mobilities [59]. The output curves (Fig. 3.6A) present the ambipolar nature of the devices, which is related to the type of carrier that is modulated by the magnitude and polarity of the gate (VG) and

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Tin Oxide Materials: Synthesis, Properties, and Applications

Fig. 3.6  (A) Output curves of the ambipolar SnO TFT under p- and n-channel operations. The inset exhibits a schematic illustration of the SnO TFTs. (B) Transfer curves at VDS < 0 (left) and VDS > 0 (right). (C) Schematic band diagrams of the transistor at VG = −18 V (red), 0.8 V (black), and 20 V (blue), corresponding to the points A, B, and C in the left part of B. (D) Dependence of Von on both negative and positive VDS. Reproduced with permission from Yan Liang L, Tao Cao H, Bo Chen X, Min Liu Z, Zhuge F, Luo H, et al. Ambipolar inverters using SnO thin-film transistors with balanced electron and hole mobilities. Appl Phys Lett 2012;100:263502.

drain (VDS) voltages. A linear current signature (diode-like) is observed in low VG and high VDS and is associated to the injection of the opposite carrier. Fig. 3.6B shows the transfer curves for different VDS. The calculated field-effect mobility in the saturation and linear regimes were 0.16 cm2 V−1 s−1 and 0.32 cm2 V−1 s−1 (for p-type channel) and

Electrical properties of tin oxide materials49

0.63 cm2 V−1 s−1 and 1.02 cm2 V−1 s−1 (for n-type channel), respectively, which means that the injection of both holes and electrons are similar. To maximize the balanced injection of both carriers in the channel layer, it is crucial the selection of materials with small bandgap coupled with the appropriate band alignment between the S/D electrodes and the semiconductor channel contact. Fig. 3.6C shows a schematic band diagram for different VG (point A, B and C in Fig. 3.6B). For point A, the holes injection is favored into the channel once the band bends significantly at the source, and so the VBM of the SnO channel aligns with the source. At point C, only the electron injection is promoted resulting from the alignment of the CBM of the SnO channel with the drain. An equilibrium hole and electron injection can be considered at point B as the gate voltage produces the minimum current (onset voltage Von). Independently of the polarity of VG, the onset voltages shift linearly with the applied VDS (Fig. 3.6D).

3.2.2 Two-dimensional SnO materials Two-dimensional (2D) materials with one or few atomic layers in thickness have arisen in the last few years as new attractive candidates to the next-generation of electronic and optoelectronic devices. Each monolayer in these materials is composed of atoms connected by strong covalent bonds, while weak van der Waals forces are responsible for linking multiple layers [49]. Due to its thin depth and large lateral size, 2D materials present a unique high surface area structure, resulting in novel properties favorable to applications that have surface-dependent performance. Graphene, which is composed of a single graphite layer, was the first studied 2D material, and due to its excellent electrical and thermal transport properties, it is certainly the most used 2D material in a wide range of applications [87–89]. Despite its exceptional high carrier mobility (105 cm2 V−1 s−1 at room temperature), the lack of a bandgap limits its usage in electronic devices [90]. Layered 2D materials from transition metal dichalcogenides (TMDCs) are semiconductors considered alternative building blocks to graphene [91–93] with particular attention to layered 2D metal oxides due to its higher stability in air [94]. Tin monochalcogenides (SnX, where X = O, S, Se, or Te), for example, is a promising class of material to be explored in the 2D limit [90, 95–97]. Theoretical calculations show that the tetragonal structure of SnO is composed of layered structures of Sn-O-Sn with the c-axis length of 4.84 Å ([001] direction), and a van der Waals gap of 2.52 Å between the layers [98–100] resulting from the dipole-dipole interaction of the Sn 5s2 lone pair electrons. Fig. 3.7 presents the top and side views of the SnO monolayer structure. Different approaches, including mechanical exfoliation [101–103], template-free hydrothermal growth method [104], and PLD technique [90] have been used to produce 2D SnO. Saji et al. studied the electronic transport properties of field-effect transistors (FETs) using different amounts of SnO layers deposited by PLD as the active channel [90]. Cross-section TEM and AFM analyses were performed to confirm the number of layers as a function of the time deposition. The transfer curve showed that the drain current of the SnO FETs increases by increasing the negative voltage applied to the gate terminal, while the output curves revealed the typical p-type conductivity of the devices independently of the channel thickness. It was also found that the

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Tin Oxide Materials: Synthesis, Properties, and Applications

Fig. 3.7  (A) Top view and (B) side view of the structure of SnO monolayer. Yellow and green balls represent Sn and O atoms, respectively. The dashed lines indicate a SnO monolayer unit cell. Reproduced with permission from Du J, Xia C, Liu Y, Li X, Peng Y, Wei S. Electronic characteristics of p-type transparent SnO monolayer with high carrier mobility. Appl Surf Sci 2017;401:114–9.

field-effect mobility increases as the thickness of the channel is extended from 5 to 12 SnO monolayers. Further increasing the number of SnO layers, the mobility starts to decrease and reaches ~0.1 cm2 V−1 s−1 using a 30 SnO monolayers. The authors also studied the optical properties of the 2D SnO. The direct optical bandgap varied from 2.80 to 2.95 eV changing from nine monolayers to bulk films, which is in good agreement with values usually reported for SnO [28, 57]. However, it increases to 3.53 eV using 2 SnO monolayers film [90].

3.3 Electrical properties of mixed-valence tin oxide: the Sn3O4 SnO2 followed by SnO are undoubtedly the well-known crystalline phases of tin oxide, and due to their particular physical-chemical properties, they have received considerable attention for a long time in several applications, including chemical and biological sensors, solar cells, lithium-ion batteries, catalysts, and TFTs [4, 5, 48, 57, 105–108]. Intermediated tin oxides with composition SnO2−x (0 < x < 1) are expected to be present between SnO2 and SnO according to the Sn-O phase diagram [109], and their existence was theoretically and experimentally demonstrated [110, 111]. An attractive characteristic of these materials is their layered crystal structure composed of alternating stacking of tin and oxygen atomic layers with a certain amount of vacant oxygen sites [110, 112], assigning them unique properties when compared to SnO2 and SnO. Theoretical calculations performed by Wang et al. [112] revealed that the crystal structures of a series of layered tin oxides with composition SnxOy (Sn2O3, Sn3O4, Sn4O5, Sn5O6, Sn7O8, Sn9O10, and Sn11O12) consist of the combination of three basic layer types: distorted SnO, Sn3O4, and Sn2O3. Additionally, these primary components can be arbitrarily stacked to engineer other SnxOy structures. Different Sn2+ and Sn4+ ratio and specific formation energy are associated with the establishment of each one of these structures. The electronic structure of these mixed-valence tin oxides was also analyzed, and their bandgap energies were found to change from 1.56 to 3.25 eV by stacking the monolayers properly. These values result from the Sn2+–O

Electrical properties of tin oxide materials51

and Sn2+–Sn2+ interactions at the layer interface, which are linearly dependent on the interlayer distance. Among these several possibilities of nonstoichiometric tin oxide materials, Sn3O4 is unquestionably the most studied intermediated phase that was firstly investigated by Lawson [113] as the product of the disproportionation process of SnO [114]. Sn3O4 is an unusual n-type semiconductor with a bandgap in a visible region (2.5–2.9 eV) [28, 115], which is very attractive from the viewpoint of optical applications, especially for catalytic reactions toward hydrogen generation and dye degradation [116, 117]. However, Sn3O4 has also been used as gas sensors and anode material for lithium-ion batteries alternatively to the SnO2 [56, 118, 119]. We experimentally demonstrated the growth of layered Sn3O4 nanobelts by the carbothermal reduction method using controlled synthesis parameters [118]. Fig. 3.8A shows the one-dimensional characteristics of the obtained layered Sn3O4 structures resulting from the fast growth of the material, and the selected area electron diffraction (SAED) in the inset exhibits the typical single-crystalline pattern. Each layer is proposed to act as a substrate for the epitaxial growth of the next layer, as observed in the regions with color contrast. The indexed interplanar distance in the high-­resolution TEM image (Fig. 3.8B) is associated with the triclinic structure of the Sn3O4 phase (JCPDS card #16-0737). The EDS analysis confirmed that the nanobelts are composed only by Sn and O atoms without any impurity, and a cross-section TEM analysis identified the interplanar distance of 8.2 Å, which is the plane parallel to the layers [118]. This distance is a lattice parameter of the crystal according to the XRD analysis of the Sn3O4 material. Although metastable mixed-valence tin oxides were predicted from theoretical calculations to present unique properties that differ from SnO2 and SnO, it has been challenging to obtain well-stabilized Sn3O4 phase aiming to examine its electrical

Fig. 3.8  (A) TEM image of a layered Sn3O4 nanobelt. The inset shows the SAED pattern. (B) High-resolution TEM image of the belt.

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Tin Oxide Materials: Synthesis, Properties, and Applications

properties. Our group has reported, for the first time, the gas sensing properties of single-crystalline layered Sn3O4 nanobelts synthesized by the carbothermal reduction process [118]. The changes in the electrical properties were initially monitored after cyclic exposure to different oxygen levels using nitrogen as baseline gas. A more detailed study presented the gas sensing performance of Sn3O4 nanobelts toward different analyte gases (NO2, H2, CO, and CH4) operating in different working temperatures [56] (Fig.  3.9A–C). This material exhibited both remarkable sensor signals toward different concentrations of NO2 and excellent selectivity against potential interferent gases such as H2, CO, and CH4 at 200°C in comparison to the SnO and the extensively studied SnO2. Furthermore, the low surface area presented by all the nanobelts indicates that it should not be the determining factor in their relative sensor signal (Fig. 3.9D). The resulting sensor response of the nanobelts was supposed to be a synergic effect of the exposed surface, material size, and the presence of active lone pairs on the SnO and Sn3O4 surface. Given the attractive electrical properties of Sn3O4 nanobelts for gas detection, more studies still have to be addressed to a deeper understanding of its sensor response. Furthermore, it is import to analyze the thermal and chemical stability of the nonstoichiometric tin oxides in order to grow structures in a controlled way aiming real technological application in several areas.

3.4 Conclusions and future prospects In this chapter, it was discussed the electrical properties of SnxOy materials, focused on SnO2, SnO, and Sn3O4 stoichiometries. As demonstrated, their electrical properties are highly dependent on the structural, morphological, and chemical characteristics, and the careful control of these parameters is essential for practical applications. The growth of SnO2 materials has been achieved with substantial progress in the last decades, while the preparation of SnO and mainly Sn3O4 structures still requires significant effort, particularly due to its thermal and chemical instability in high temperatures. Once SnO and Sn3O4 present layered structures, the electrical performance of these materials can be tuned by controlling the number of layers, which is notably exciting in many technological applications such as gas sensors, catalysts, and energy conversion. Furthermore, in terms of electrical characterization, in situ and operando studies in SnxOy materials could enable a strong understanding of the conduction mechanisms associated with the surface chemistry and electronic properties of these unusual stoichiometries of tin oxide.

Acknowledgments Pedro H. Suman acknowledges the São Paulo Research Foundation (FAPESP) (Procs. 2009/13491-7, 2012/11139-7, 2013/18511-1, and 2016/20808-0) for funding support of his research.

Electrical properties of tin oxide materials53

Fig. 3.9  (A) Sensor signal of tin oxide nanobelts as a function of the operating temperature after exposure to 50 ppm NO2 in dry air. The inset shows the response time to 50 ppm of NO2 at different operating temperatures. (B) Sensor signal as a function of the time at 200°C for cyclic exposure to NO2 pulses with concentrations ranging from 1 to 50 ppm. (C) Sensor signal after exposure to 50 ppm of NO2, H2, CO, and CH4 at 200°C. (D) Nitrogen adsorption/desorption isotherms for tin oxide nanobelts. Reproduced with permission from Suman PH, Felix AA, Tuller HL, Varela JA, Orlandi MO. Comparative gas sensor response of SnO2, SnO, and Sn3O4 nanobelts to NO2 and potential interferents. Sensors Actuators B Chem 2015;208:122–7.

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