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Figure 3. Unit cell of the tetragonal cassiterite structure of the SnO2. The Sn is represented by the orange spheres and O is represented by the blue spheres.

Figure 3 Unit cell of the tetragonal cassiterite structure of the SnO2. The Sn is represented by the orange spheres and O is represented by the blue spheres.

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One possibility to overcome this drawback is based on tuning parameters during the synthesis and preparation of these nanomaterials. For instance, Babu et al. demonstrated that it is possible to modify the band structure of SnO2 quantum dots by varying both the CB and VB potentials and the SnO2 bandgap value from the UV region to the visible wavelength region, simply by varying the quantum dots annealing temperature, as shown in Figure 1. This bandgap shift may have a beneficial impact on the possibility of using solar radiation to activate the semiconductor [13]. Figure 1. VB and CB edge potentials with bandgaps of SnO2 quantum dots annealing with different temperatures. In this figure, the acronym SQD relates to the annealing temperature at °C. For instance, the sample SQD-200 was annealed at 200 °C. Reproduced with permission from Ref. [13]. Copyright 2018 Elsevier. Figure 1. VB and CB edge potentials with bandgaps of SnO2 quantum dots annealing with different
One possibility to overcome this drawback is based on tuning parameters during the synthesis and preparation of these nanomaterials. For instance, Babu et al. demonstrated that it is possible to modify the band structure of SnO2 quantum dots by varying both the CB and VB potentials and the SnO2 bandgap value from the UV region to the visible wavelength region, simply by varying the quantum dots annealing temperature, as shown in Figure 1. This bandgap shift may have a beneficial impact on the possibility of using solar radiation to activate the semiconductor [13]. Figure 1. VB and CB edge potentials with bandgaps of SnO2 quantum dots annealing with different temperatures. In this figure, the acronym SQD relates to the annealing temperature at °C. For instance, the sample SQD-200 was annealed at 200 °C. Reproduced with permission from Ref. [13]. Copyright 2018 Elsevier. Figure 1. VB and CB edge potentials with bandgaps of SnO2 quantum dots annealing with different
Figure 2. Representation of the Valence Band (VB) and Conduction Band (CB) for the undoped (left), n-type (centre), and p-type (right) doped semiconductors. Notice the relative position between VB, CB, and the Fermi energy level (Ep) in all three cases. Additionally, the p- and n-doping create intermediate energy levels between the VB and CB, changing the relative position of the VB and CB in relation to Fermi energy level (Ep), as shown in Figure 2. In the case of a n-doped semiconductor, the intermediate level is created in the bottom of the CB, and it is called the donor level, whereas in the case of the p-doped semiconductor, the intermediate level is created on the top of the VB, and it is called the acceptor level. The doping and the creation of these intermediate states makes less likely that electrons can be directly excited from the VB to the CB, like it would happen in an intrinsic or undoped semiconductor [25].
Figure 2. Representation of the Valence Band (VB) and Conduction Band (CB) for the undoped (left), n-type (centre), and p-type (right) doped semiconductors. Notice the relative position between VB, CB, and the Fermi energy level (Ep) in all three cases. Additionally, the p- and n-doping create intermediate energy levels between the VB and CB, changing the relative position of the VB and CB in relation to Fermi energy level (Ep), as shown in Figure 2. In the case of a n-doped semiconductor, the intermediate level is created in the bottom of the CB, and it is called the donor level, whereas in the case of the p-doped semiconductor, the intermediate level is created on the top of the VB, and it is called the acceptor level. The doping and the creation of these intermediate states makes less likely that electrons can be directly excited from the VB to the CB, like it would happen in an intrinsic or undoped semiconductor [25].
Figure 4. Band alignment scheme for Sny_,Pb,O2, where x = 0.00, 0.06, and 0.12. All energies are given relative to an O 1s state. Adapted with permission from Ref. [50]. Copyright 2016 the Royal Society of Chemistry.
Figure 4. Band alignment scheme for Sny_,Pb,O2, where x = 0.00, 0.06, and 0.12. All energies are given relative to an O 1s state. Adapted with permission from Ref. [50]. Copyright 2016 the Royal Society of Chemistry.
Figure 5. The bandgap of some photocatalysts concerning the redox potential of different chemical species, measurements conducted at a pH of 7. Adapted with permission from Ref. [68]. Copyright 2015 Elsevier. Furthermore, it is still relevant that these semiconductors have stability as well as a particle size that allows their catalytic activity by creating active centers that promote the adsorption of reagent molecules on their surface to provide its activation [67]. Figure 5 illustrates some of the most common semiconductor materials used in photocatalysis and the reduction potentials required for converting CO? into different products.
Figure 5. The bandgap of some photocatalysts concerning the redox potential of different chemical species, measurements conducted at a pH of 7. Adapted with permission from Ref. [68]. Copyright 2015 Elsevier. Furthermore, it is still relevant that these semiconductors have stability as well as a particle size that allows their catalytic activity by creating active centers that promote the adsorption of reagent molecules on their surface to provide its activation [67]. Figure 5 illustrates some of the most common semiconductor materials used in photocatalysis and the reduction potentials required for converting CO? into different products.
Figure 6. Illustration of the influence of oxygen vacancies in the energy levels and the excitation of the electron during the photocatalytic activity of doped SnO2. Adapted from reference [2]. Copyright 2020 Faculty of Technology, University of Novi Sad.
Figure 6. Illustration of the influence of oxygen vacancies in the energy levels and the excitation of the electron during the photocatalytic activity of doped SnO2. Adapted from reference [2]. Copyright 2020 Faculty of Technology, University of Novi Sad.
Figure 7. Crystal structure of SnO2 and optimized structures of Ce-SnO2. (R1, R2) represent different replace positions. The Ce atom easily replaces the Sn atom in the R1 position and the O atom connected to the Ce atom is easily lost. Reproduced with permission from Ref. [84]. Copyright 2021 Elsevier. Figure 7. Crystal structure of SnO, and optimized structures of Ce-SnO). (R1, R2) represent different Song et al. reported another interesting study exploring the positive effect of the vacancy defects promoted by the doping of SnO with Ce ions, as depicted in Figure 7. They applied the Ce-doped-SnO) catalyst to NO removal under visible light irradiation. The synthesized samples were prepared in three different conditions: pristine SnO2, Ce-doped- SnO>-Ar (calcinated in argon atmosphere), and Ce-doped-SnO>-Air (air atmosphere). A higher amount of oxygen vacancies was observed in Ce-doped-SnO)-Ar, followed by Ce- doped-SnO)-Air and SnQ). As a result, they observed that Ce-doped-SnO -Ar presented enhanced photoactivity, being capable to oxidize 70% of the NO- flow, while the undoped SnO, attained stabilization in 10%. The Ce-doped SnO,-Ar catalyst also remained stable in different humidity conditions [84].
Figure 7. Crystal structure of SnO2 and optimized structures of Ce-SnO2. (R1, R2) represent different replace positions. The Ce atom easily replaces the Sn atom in the R1 position and the O atom connected to the Ce atom is easily lost. Reproduced with permission from Ref. [84]. Copyright 2021 Elsevier. Figure 7. Crystal structure of SnO, and optimized structures of Ce-SnO). (R1, R2) represent different Song et al. reported another interesting study exploring the positive effect of the vacancy defects promoted by the doping of SnO with Ce ions, as depicted in Figure 7. They applied the Ce-doped-SnO) catalyst to NO removal under visible light irradiation. The synthesized samples were prepared in three different conditions: pristine SnO2, Ce-doped- SnO>-Ar (calcinated in argon atmosphere), and Ce-doped-SnO>-Air (air atmosphere). A higher amount of oxygen vacancies was observed in Ce-doped-SnO)-Ar, followed by Ce- doped-SnO)-Air and SnQ). As a result, they observed that Ce-doped-SnO -Ar presented enhanced photoactivity, being capable to oxidize 70% of the NO- flow, while the undoped SnO, attained stabilization in 10%. The Ce-doped SnO,-Ar catalyst also remained stable in different humidity conditions [84].
Figure 8. (a) SnO2 nanobelts, (b) SnO2 nanofibres, (c) SnOz nanowires, (d) SnO, nanoplates, (e) SnOz hollow nanospheres, (f) SnOz porous nanotubes. Adapted from references [89-93]. Copyright 2003, 2009, 2011, 2013, 2020 Elsevier. For nanostructures, the control of the size and specific surface area is essential to en- hance some properties that depend on the electron-hole pair transport to the nanostructure surface. In this sense, the SnO2 morphological control is very necessary to be carefully managed during the synthesis of the nanostructures. This control is significantly associated with the method used as it depends on the precursors, solvents, surfactants, templates, and other factors, such as temperature, atmosphere, time, and pH. The synthesis of the undoped and d-SnQ} is obtained in several nanomorphologies (Figure 8), such as particles, wires, belts, rods, and sheets [85]. It can be assembled in hierarchical structures to obtain, for example, hollow spheres [86], fibres [87], and tubes [88].
Figure 8. (a) SnO2 nanobelts, (b) SnO2 nanofibres, (c) SnOz nanowires, (d) SnO, nanoplates, (e) SnOz hollow nanospheres, (f) SnOz porous nanotubes. Adapted from references [89-93]. Copyright 2003, 2009, 2011, 2013, 2020 Elsevier. For nanostructures, the control of the size and specific surface area is essential to en- hance some properties that depend on the electron-hole pair transport to the nanostructure surface. In this sense, the SnO2 morphological control is very necessary to be carefully managed during the synthesis of the nanostructures. This control is significantly associated with the method used as it depends on the precursors, solvents, surfactants, templates, and other factors, such as temperature, atmosphere, time, and pH. The synthesis of the undoped and d-SnQ} is obtained in several nanomorphologies (Figure 8), such as particles, wires, belts, rods, and sheets [85]. It can be assembled in hierarchical structures to obtain, for example, hollow spheres [86], fibres [87], and tubes [88].
Table 1. Some references for doped SnOo, their synthesis approach and morphological aspects.
Table 1. Some references for doped SnOo, their synthesis approach and morphological aspects.
Figure 9. Synthesis routes of various SnO2 nanostructures using hydrothermal treatment. Repro- duced with permission from Ref. [85]. Copyright 2022 Elsevier.
Figure 9. Synthesis routes of various SnO2 nanostructures using hydrothermal treatment. Repro- duced with permission from Ref. [85]. Copyright 2022 Elsevier.
Table 1. Cont. One-dimensional SnO? nanostructures, specially nanobelts and nanowires, are synthe- sized mainly using chemical vapour approaches for undoped and doped, as reported for Sb doped SnO, [113 tures presenting As observed, the . Nanorod structures were obtained for copper doped SnQ? synthesized by a novel one-step microwave irradiation method in solution [114]. The doping effect can be observed in comparison with undoped SnO) final morphology. Despite the doped struc- higher dimensional length, as nanorods, it is composed of nanocrystals. crystallite size was reduced when copper was present in the structure. The photocatalytic activity was monitored via the degradation of organic dyes. It was found that doped - materials showed better photocatalytic activity than the undoped SnO). The superior activity for doped nanostructure was attributed to the effective electron-hole separation by surface modification.
Table 1. Cont. One-dimensional SnO? nanostructures, specially nanobelts and nanowires, are synthe- sized mainly using chemical vapour approaches for undoped and doped, as reported for Sb doped SnO, [113 tures presenting As observed, the . Nanorod structures were obtained for copper doped SnQ? synthesized by a novel one-step microwave irradiation method in solution [114]. The doping effect can be observed in comparison with undoped SnO) final morphology. Despite the doped struc- higher dimensional length, as nanorods, it is composed of nanocrystals. crystallite size was reduced when copper was present in the structure. The photocatalytic activity was monitored via the degradation of organic dyes. It was found that doped - materials showed better photocatalytic activity than the undoped SnO). The superior activity for doped nanostructure was attributed to the effective electron-hole separation by surface modification.
“Here “SnO” denotes a Sn” oxyhydroxide species. * Baruch et al. obtained similar results in the process of electroreduction of CO2 with SnOz and SngO4(OH)4 electrocatalysts, where they related the production of formate to a new intermediate, which is formed only superficially, the carbonate bound to tin (Sn- CO3-bidentate and Sn-CO3-monodentate), Figure 10. Furthermore, it suggests that the electrocatalytic activity in reducing CO2 from SnOy? is linked to the formation of tin(II) oxyhydroxide species on the surface [129].
“Here “SnO” denotes a Sn” oxyhydroxide species. * Baruch et al. obtained similar results in the process of electroreduction of CO2 with SnOz and SngO4(OH)4 electrocatalysts, where they related the production of formate to a new intermediate, which is formed only superficially, the carbonate bound to tin (Sn- CO3-bidentate and Sn-CO3-monodentate), Figure 10. Furthermore, it suggests that the electrocatalytic activity in reducing CO2 from SnOy? is linked to the formation of tin(II) oxyhydroxide species on the surface [129].
Figure 11. Proposed density of electronic states (DOS) for (i) SnO, (ii) 1 at% Ni-SnOz, and (iii) 5 at% Ni-SnO, NPs. Reproduced with permission from Ref. [130]. Copyright 2020 Elsevier.
Figure 11. Proposed density of electronic states (DOS) for (i) SnO, (ii) 1 at% Ni-SnOz, and (iii) 5 at% Ni-SnO, NPs. Reproduced with permission from Ref. [130]. Copyright 2020 Elsevier.
Figure 12. (a) Possible schemes for electron-hole separation and transport at composite interface and (b) photocatalytic activities of materials on CO2 photoreduction under simulated sunlight irradiation. Reproduced with permission from Ref. [133]. Copyright 2015 Elsevier. Thus, He et al. [133] studied the formation of a composite formed between SnO2 and g-C3N, and verified its activity in the CO2 photoreduction process and in the photodegra- dation of organic pollutants. The authors observed that the formation of the junction between the two materials leads to the obtaining of a photocatalyst with high activity when compared with pure materials and with P25, a classic photocatalyst. The results indicated the formation of a Z-type heterojunction between SnO and g-C3N4, which hin- dered the recombination of electron-hole pairs, consequently increasing their photoactivity (Figure 12a).
Figure 12. (a) Possible schemes for electron-hole separation and transport at composite interface and (b) photocatalytic activities of materials on CO2 photoreduction under simulated sunlight irradiation. Reproduced with permission from Ref. [133]. Copyright 2015 Elsevier. Thus, He et al. [133] studied the formation of a composite formed between SnO2 and g-C3N, and verified its activity in the CO2 photoreduction process and in the photodegra- dation of organic pollutants. The authors observed that the formation of the junction between the two materials leads to the obtaining of a photocatalyst with high activity when compared with pure materials and with P25, a classic photocatalyst. The results indicated the formation of a Z-type heterojunction between SnO and g-C3N4, which hin- dered the recombination of electron-hole pairs, consequently increasing their photoactivity (Figure 12a).
Figure 13. A representative scheme of electron transfer in the heterojunction (Re-IO-TiO2_, /SnO2) in the CO2 photoreduction process. Adapted with permission from Reference [134]. Copyright 2021 Elsevier.
Figure 13. A representative scheme of electron transfer in the heterojunction (Re-IO-TiO2_, /SnO2) in the CO2 photoreduction process. Adapted with permission from Reference [134]. Copyright 2021 Elsevier.
Figure 14. Mechanism of the formation of ZnTcPc/SnO2 composites. Reproduced with permission from Ref. [147]. Copyright 2018 Elsevier.
Figure 14. Mechanism of the formation of ZnTcPc/SnO2 composites. Reproduced with permission from Ref. [147]. Copyright 2018 Elsevier.
Table 2. d-SnO2 samples for which antibacterial activity through ROS production has been reported Table 2 includes the ZOI of the undoped SnO 2 samples used as controls in the an- tibacterial treatment. In all cases, regardless of the metal used in the doping, an increase in the ZOI of the d-SnO, samples was observed in relation to the undoped sample, which shows the improvement in the antimicrobial action with doping. The SnO2 doping process favours antibacterial activity because of the reduction it causes in the bandgap energy This bacterial inactivation process has been widely reported in the literature. Table 2 presents some of the most recent works in which this mechanism was identified as the main responsible for the antibacterial activity. The materials’ zone of inhibition (ZOI), used to determine the extent of antimicrobial activity, is also indicated in the table for comparison.
Table 2. d-SnO2 samples for which antibacterial activity through ROS production has been reported Table 2 includes the ZOI of the undoped SnO 2 samples used as controls in the an- tibacterial treatment. In all cases, regardless of the metal used in the doping, an increase in the ZOI of the d-SnO, samples was observed in relation to the undoped sample, which shows the improvement in the antimicrobial action with doping. The SnO2 doping process favours antibacterial activity because of the reduction it causes in the bandgap energy This bacterial inactivation process has been widely reported in the literature. Table 2 presents some of the most recent works in which this mechanism was identified as the main responsible for the antibacterial activity. The materials’ zone of inhibition (ZOI), used to determine the extent of antimicrobial activity, is also indicated in the table for comparison.
Figure 15. Schematic diagram of the antibacterial activity from generation of ROS induced by Co-doped SnO, NPs. Figure reprinted from Reference [152]. Copyright 2017 Elsevier. ENE SOSaSASaeeS Ayes “SESt SS ara ine ae RD a ee aaa a Figure 15 shows how Co- -doped SnO, NPs can n induce the ROS production process for bacterial cell destruction [152]. Co-doped SnOz NPs accumulate on the surface of the bacterial strain by direct or electrostatic forces; the respective metal ions of the NPs (i.e., Co?* and Sn**) penetrate the cell oxidizing the cell walls by getting themselves reduced into more stable ones. The oxidation process causes intense oxidative stress giving rise to ROS, the species responsible for attacking the cell and killing it.
Figure 15. Schematic diagram of the antibacterial activity from generation of ROS induced by Co-doped SnO, NPs. Figure reprinted from Reference [152]. Copyright 2017 Elsevier. ENE SOSaSASaeeS Ayes “SESt SS ara ine ae RD a ee aaa a Figure 15 shows how Co- -doped SnO, NPs can n induce the ROS production process for bacterial cell destruction [152]. Co-doped SnOz NPs accumulate on the surface of the bacterial strain by direct or electrostatic forces; the respective metal ions of the NPs (i.e., Co?* and Sn**) penetrate the cell oxidizing the cell walls by getting themselves reduced into more stable ones. The oxidation process causes intense oxidative stress giving rise to ROS, the species responsible for attacking the cell and killing it.
Figure 16. (a) ZOIs in antifungal test and (b) MIC test of plant extract, un-doped SnO7 NAPs, Co- doped SnO, NPs and standard drug against fungal strains. Fluorescence microscopy with Calcofluor black shows the development of hyphae and pseudo-hyphae in controls (¢c,e) and Co-doped SnO NPs treated cells (d,f). TEM image of untreated Candida albicans cells (g) and treated with Co-doped SnO» NPs (h) showing the changes detected in the cellular nucleus (n) and mitochondria (m). Figure reprinted from Reference [166]. Copyright 2018 Elsevier. Aspergillus niger, Aspergillus flavus, and Candida albicans. The measurement of zones of inhibition (ZOIs) of samples of Co-SnO2 NPs were higher than those of undoped SnO2 NPs, standard drug, and plant extract for all fungal strains while minimal inhibitory con- centrations (MICs) were minimal. For example, the zones of inhibition (ZOIs) of NPs of Co-doped SnO? were of 17 + 0.04 mm, 23 + 0.08 mm and 26 + 0.06 mm for A. niger, A. flavus, and C. albicans, while for the standard drug were obtained ZOIs of 14 + 0.08 mm, 19 + 0.06 mm and 20 + 0.09 mm, respectively (Figure 16a). Minimum inhibitory concentra- tions (MICs) of 0.61 + 0.04 mg mL~!, 0.49 + 0.07 mg mL~! and 0.55 + 0.03 mg mL~! and 0.68 + 0.03 mg mL~! were obtained against the fungi A. niger, A. flavus, and C. albicans when using Co-doped SnO NPs while values of 0.59 + 0.09 mg mL~! and 0.74 + 0.08 mg mL“! respectively were obtained with the standard drug. black shows the development of hyphae and pseudo-hyphae in controls (c,e) and Co-doped SnO
Figure 16. (a) ZOIs in antifungal test and (b) MIC test of plant extract, un-doped SnO7 NAPs, Co- doped SnO, NPs and standard drug against fungal strains. Fluorescence microscopy with Calcofluor black shows the development of hyphae and pseudo-hyphae in controls (¢c,e) and Co-doped SnO NPs treated cells (d,f). TEM image of untreated Candida albicans cells (g) and treated with Co-doped SnO» NPs (h) showing the changes detected in the cellular nucleus (n) and mitochondria (m). Figure reprinted from Reference [166]. Copyright 2018 Elsevier. Aspergillus niger, Aspergillus flavus, and Candida albicans. The measurement of zones of inhibition (ZOIs) of samples of Co-SnO2 NPs were higher than those of undoped SnO2 NPs, standard drug, and plant extract for all fungal strains while minimal inhibitory con- centrations (MICs) were minimal. For example, the zones of inhibition (ZOIs) of NPs of Co-doped SnO? were of 17 + 0.04 mm, 23 + 0.08 mm and 26 + 0.06 mm for A. niger, A. flavus, and C. albicans, while for the standard drug were obtained ZOIs of 14 + 0.08 mm, 19 + 0.06 mm and 20 + 0.09 mm, respectively (Figure 16a). Minimum inhibitory concentra- tions (MICs) of 0.61 + 0.04 mg mL~!, 0.49 + 0.07 mg mL~! and 0.55 + 0.03 mg mL~! and 0.68 + 0.03 mg mL~! were obtained against the fungi A. niger, A. flavus, and C. albicans when using Co-doped SnO NPs while values of 0.59 + 0.09 mg mL~! and 0.74 + 0.08 mg mL“! respectively were obtained with the standard drug. black shows the development of hyphae and pseudo-hyphae in controls (c,e) and Co-doped SnO
Figure 17. LFPs observed in different locations of a currency bill stained with Eus* doped SnOp (7% Eu** in mol), when illuminated by UV radiation with A = 254 nm. (A-C) show different regions of the bill when illuminated by the UV radiation. The parts stained, illuminated with the UV light, and analysed are shown within red dashed circles on the bill pictures in the upper left and the central left panels of the figure. Reprinted with oermission from Reference [177]. Copyright 2019 American Chemical Society.
Figure 17. LFPs observed in different locations of a currency bill stained with Eus* doped SnOp (7% Eu** in mol), when illuminated by UV radiation with A = 254 nm. (A-C) show different regions of the bill when illuminated by the UV radiation. The parts stained, illuminated with the UV light, and analysed are shown within red dashed circles on the bill pictures in the upper left and the central left panels of the figure. Reprinted with oermission from Reference [177]. Copyright 2019 American Chemical Society.
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