Electronic Structure of SnO2: Comparison
Please note this is a comparison between Version 2 by Catherine Yang and Version 1 by Protima Rauwel.

Tin oxide (SnO2) is a versatile n-type semiconductor with a wide bandgap of 3.6 eV that varies as a function of its polymorph, i.e., rutile, cubic or orthorhombic. Bulk SnO2 has a bandgap of ~~3.6 eV; however, experimental bandgaps range from 1.7 eV to 4 eV, thereupon widening its range of applications to photovoltaics and photocatalysis. Bandgap engineering is widely studied in SnO2, as it belongs to the family of transparent conducting oxides (TCO). Additionally, bandgaps can be controlled via parameters, such as synthesis routes and the application of a substrate-induced strain for thin-film growth that simultaneously produce intrinsic defects and structural changes. 

  • SnO2
  • nanomaterials
  • synthesis
  • polymorphism
  • band gap

1. Bandgap Engineering in SnO2

Direct bandgaps are systematically located at the high-symmetry Γ point. Besides, the crystalline quality of thin films related to defects and impurities has been shown to influence the bandgap [80][1]. The phase transition of SnO2 to higher-pressure-induced phases also encourages a decrease in the direct bandgap, which is the result of a more compact lattice, ensuing higher orbital overlapping [81][2]. In agreement with these observations, Table 1 compiles the structural and electronic properties of all the SnO2 polymorphs. However, experimentally, the synthesis and stabilization of higher-symmetry SnO2 polymorphs is complicated. Few studies report the presence of other SnO2 phases for Fe-doped SnO2, owing to the substitution of Sn by Fe ions [82][3], where the rutile phase co-exists with a small amount of α-PbO2 (Pbcn) secondary phase. After annealing at 800 °C, they observed that only traces of the SnO2 orthorhombic phase remained, which confirms the low stability of the orthorhombic phase at high temperatures compared to the SnO2 rutile structure. In addition, the possible presence of iron oxide lowers the SnO2 band gap to 2–3 eV [83][4]. In fact, according to the crystal field theory, Fe3+ ions are placed in an octahedral configuration in the presence of a weak field ligand (oxide) in a high-spin configuration. In addition, Fe3+ ions possess an ionic radius (0.645 Å) that is slightly smaller than Sn4+ ions (0.69 Å), which decreases the lattice parameters and, consequently, increases orbital overlapping between Sn4+ and O2− ions, leading to a lower bandgap. Radaf et al. succeeded in stabilizing the orthorhombic SnO2 structure by adding Cr3+ dopant [84][5]. The crystallite size decreased with the increase in Cr concentration, and the bandgap consequently decreased from 3.6 eV for the undoped SnO2 thin film to 3.28 eV with 5% of Cr. While the addition of those metals leads to the stabilization of the orthorhombic phase, Keskenler et al. [85][6] have demonstrated that W incorporation also stabilizes the Pbca cubic phase until a doping threshold of 2.0 at. %. Here, W6+ is likely to substitute Sn4+, which shrinks the lattice owing to the lower ionic radius. When the W concentration exceeds 2.0 at. %, lower oxidation states of tungsten could also substitute Sn4+ sites, which counteract the unit-cell shrinkage [85][6]. This can be explained by the Moss–Burstein effect, where materials with high carrier concentration, such as W, fill unoccupied states deep within the conduction band. Consequently, the Fermi level of the n-type SnO2 shifts into the conduction band. The increase in the optical bandgap is due to the excited electrons transitioning from the valence band to empty states in the conduction band localized at higher energy levels [85,86][6][7].
Table 1.
Crystal structure of SnO
2
polymorphs, volume of the unit cell (Å
3
), its density (in g.cm
−3
) and direct bandgaps (eV).

2. Point-Defect Engineering in SnO2

The n-type conductivity of undoped rutile SnO2 materials can be explained by the defects present in the structure. Among the four different intrinsic defects, i.e., oxygen vacancy VO, tin interstitial Sni, tin antisite SnO and oxygen interstitial Oi, the predominant and combined occurrence of VO and Sni leads to electron donor properties [50,89][12][13]. SnO2 nanostructures exist in diverse morphologies (e.g., nanorods, nanocubes, nanosheets, nanowire and nanospheres) as a result of the synthesis route. Interstitial and vacancy-mediated defects are specific to each morphology, as the shape and size of the nanoparticle influence the surface and volume defects generated [90,91][14][15]. Hence, engineering SnO2 nanoparticles via controlled synthesis conditions allows the tailoring of their size, shape, morphology, intrinsic and surface defects. These properties play an important role in their electrochemical properties and redox mechanisms, especially for LiB applications. Defect engineering in semiconductors, more particularly in nanomaterials, is important for several applications. In fact, surface defects in nanomaterials are capital for surface-related phenomena in catalysis. Surface-defect engineering of SnO2 has already been studied for photocatalytic [92][16] and gas sensing [93,94][17][18] applications. Furthermore, oxygen-related defects generated in an oxygen-poor environment create exposed Sn4+ cations, as well as oxygen vacancies at the surface leading to abundant reactive sites [95][19]. In general, point defects such as surface-oxygen vacancies are common in nanomaterials owing to the high surface-to-volume ratio [96,97][20][21]. These defects are tailored via synthesis conditions, i.e., oxygen-rich or oxygen-poor conditions, synthesis temperature and annealing atmospheres. In addition, synthesizing faceted nanoparticles and exposing certain crystal facets to enhance catalytic activity are important topics in catalysis [98][22]. Furthermore, doping with foreign atoms to create VO and VSn, as explained before, stabilizes higher-symmetry polymorphs through the production of oxygen vacancies in the structure. On the other hand, in optoelectronic and electronic applications, passivating surface defects is necessary for enhancing the conductivity of SnO2, as in the case of F-doped SnO2 [99][23]. The principle is to eliminate defect states within the bandgap of the material by reducing these surface traps and, consequently, increasing the charge mobility and conductivity of SnO2. Since an oxygen anion is doubly ionized, the depletion of oxygen leads to a general enhancement of the charge-carrier concentration. These oxygen vacancies, i.e., VO, can have three different charge states, commonly termed as neutral VOx, singly ionized VO and doubly ionized oxygen vacancy VO●● in Kröger–Vink notation. Two types of stoichiometric defects can occur inside the SnO2 lattice, i.e., Frenkel and Schottky defects, which do not influence the conductivity of the material, as the stoichiometry remains the same and charges remain in equilibrium. Schottky defects involve the simultaneous presence of charge-equivalent metal vacancies VSn⁄⁄⁄⁄ (quadruply negatively charged Sn vacancy) and oxygen vacancies VO●●. The mechanism consists of one Sn4+ ion leaving its lattice site (SnSnx), along with two oxygen atoms leaving their lattice sites (OOx) simultaneously and diffusing within the crystal in order to create charged vacancies, VSn⁄⁄⁄⁄ and two VO●●, respectively, whereas Frenkel defects are a type of point defect, where SnSnx or OOx leaves its original lattice site and occupies an interstitial site.
Stoichiometric defect mechanisms do not interfere with electronic properties of SnO2 nanoparticles, unlike nonstoichiometric defects. Surface defects in bulk materials have an insignificant influence on their physical and chemical properties because of their low proportion. However, surface defects in nanomaterials can drastically change catalytic and electronic properties, as a result of their high surface-to-volume ratio. Formation of VSn⁄⁄⁄⁄/VSn⁄⁄, VO●●/VO/VOx, Sni and Oi depends on the oxygen environment during synthesis [100][24]. In the case of oxygen-deficient SnO2 nanomaterials, oxygen vacancies are formed by the transfer of oxygen atoms from their site (OOx) to the ambient because of an oxygen-poor environment during synthesis. This leads to a metastable state where oxygen vacancies are filled with the remaining two electrons of O2− (VO×) that then maintain the charge neutrality of the structure. However, this intermediate state is still unstable and leads to the subsequent release of electrons into the surroundings. The electrons released during the formation of the ionized oxygen vacancies are transferred to the Sn-5s state of the conduction band and ionize the Sn cation. These vacancies play a critical role as acceptors, whereupon they form new energy levels deep within the bandgap of the material. However, VO, as usual, couples with Sni in oxygen-deficient conditions, whereupon complex defects are formed, i.e., VO●● + Sni⁄⁄, giving rise to the n-type conductivity of SnO2 with electron mobility from the SnSn⁄⁄ to SnSn⁄⁄⁄⁄ sites. The mechanism for n-type conductivity involves the hybridization of the Sn-5s and O-2p states near such vacancies, facilitating electron transfer from the valence to the conduction band [101][25]. The only possibility to obtain p-type conductivity is via the introduction of VSn⁄⁄⁄⁄ + 4 h. In the case of Sn-deficient SnO2, VSn are the predominant defects that are created. An opposite reaction occurs where four electrons are taken from the valence band to form holes in order to generate interstitial site SnSn⁄⁄⁄⁄. The production of Sn vacancies can be mediated in Sn-poor conditions or by doping SnO2 with tri-valent elements substituting the SnSn× that create VSn [102,103][26][27]. Simultaneous doping with elements, such as N, creates acceptor states that then facilitate p-type conductivity. The computational and experimental studies on co-doping suggest the replacement of approximately four Sn atoms by four Al atoms and one O atom by one N atom [104][28]. In the case of metal excess, the defect equation governed by this mechanism involves the formation of Sn interstitial atoms, Snix, which further act as electron donors and can be successively doubly ionized to Sni●● or quadruply ionized to Sni●●●●. These doubly ionized Sn2+ states can act as traps that restrict the possibility of transition from the conduction band minimum to holes just above the valence band maximum. Therefore, passivation of the Sni●●●● on the surface of the SnO2 nanoparticles tends to enhance the oxygen-vacancy-related transitions. Lastly, in oxygen-rich conditions, Oi⁄⁄ have the lowest formation energy and are therefore abundant.
Among all these point defects described above, oxygen vacancies caused by oxygen-poor conditions are the most abundant intrinsic defects occurring in SnO2 nanomaterials because of the lowest formation enthalpy [100][24]. Moreover, many studies [105,106,107,108,109,110][29][30][31][32][33][34] have probed these new energy levels via photoluminescence (PL) spectroscopy. As previously mentioned, unstable VO× vacancy acts as a donor level and is located at 0.03 eV, just under the conduction band. In addition, ionized VO is also considered a shallow donor, as it is located 0.15 eV below the conduction band, while VO●● is an acceptor level located at 1.4 eV above the valence band [109,110][33][34]. In SnO2 nanomaterials, the surface-oxygen vacancy is doubly ionized (or VO●●) and is the most dominant emission [111][35]. Wang et al. have investigated the photoluminescence mechanisms under a 255 nm excitation wavelength, resulting in band-to-band and defect excitations. Each peak was successfully identified and energy levels in the band diagram of SnO2 also corroborate them. For example, the transition between the VOx donor level to the VO●● acceptor level is attributed to the 467 nm (2.65 eV) emission peak, whereas the electron transition from VO to the valence band can be assigned to the 439 nm (2.83 eV) emission peak. In addition, Snix is a shallow donor, as Sn interstitials tend to occur exclusively in the +4 state located very near the conduction band, contributing to the n-type semiconductor properties of SnO2, even though Sn interstitials are not abundant. The band-to-band transition is identified by the 328 nm emission, corresponding to an energy of 3.78 eV. Habte et al. [112][36] have demonstrated that the addition of Zn2+ cations to the SnO2 lattice leads to PL emission peak shifts, toward lower energies. There could be two reasons for the optical bandgap reduction. Since Zn2+ cations are smaller, their insertion should promote orbital overlapping because of a reduction in the lattice parameter. The other reason could be the shift toward longer wavelengths corresponding to the ZnO bandgap (3.37 eV). However, they highlighted that the lattice structure remains unchanged with Zn2+; therefore, the decrease in the optical bandgap can be attributed to the presence of Zn-O complexes. Salem et al. [113][37] have observed similar changes in the bandgap with Ni-doped ZnO. Nevertheless, the addition of Zn2+ should also enhance emission peak intensities, since the substitution of a smaller and lower valency cation encourages the formation of oxygen vacancies.
Since SnO2 is an n-type intrinsic semiconductor, the most prominent defects are, therefore, VO and Sni because of the lowest formation enthalpy. They are present in the volume of the material and contribute to the electronic conductivity. In nanomaterials, these defects are present on the surface and are instrumental in several catalytic reactions, including oxygen evolution reaction, hydrogen evolution reaction, gas sensing or electrocatalytic CO2 reduction [114][38]. On the other hand, for applications in electronic devices, these surface states are detrimental to the device’s functional properties. Photoluminescence spectroscopy is commonly used to identify these defects by providing information on optical transition between defect levels and band edges [115][39]. Depending on the application, these surface defects need to be either passivated or exacerbated. The importance of doping SnO2 with acceptors lies in the possibility of obtaining a p-type semiconductor that would eventually lead to a SnO2 homojunction diode. In general, surface defects act as trap states that enhance defect-level emission from the bandgap states. Furthermore, these defect states also extend the photo absorption of the materials to the visible region. Consequently, several new applications in LED, visible light detectors and photocatalysis are likely.

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