Assessment of the superior photocatalytic properties of Sn2+-containing SnO2 microrods on the photodegradation of methyl orange

A microporous Sn2+-containing SnO2 material presenting microrod morphology and a surface area of 93.0 m2 g–1 was synthesized via a simple hydrothermal route. Sn2+ ions were detected in the interior of the material (15.8 at.%) after the corrosion of a sample through sputtering. The material’s optical properties have demonstrated the absorption of a considerable fraction of visible light up to wavelengths of 671 nm, due to the presence of Sn2+ states in the material’s band structure. The analysis of the internal crystalline structure of a single microrod was carried out with the aid of a focused ion beam microscope and indicated that the material is mesocrystalline down to nanoscale level. It was proposed that the Sn2+ ions occupy intergranular sites in the highly defective crystalline structure of the material and that Sn2+ states, as well as its relatively large surface area, are responsible for the material’s superior photoactivity. The synthesized material was tested as a photocatalyst to decompose hazardous contaminants in water. The photocatalytic performance of the material was much higher than those of commercial TiO2 and SnO2 materials, decomposing nearly all methyl orange (an azo-dye model) content in water (10 mg L–1) in 6 min under UV irradiation for a photocatalyst dose of 5.33 g L–1. The photodegradation of methyl orange was also verified under visible light.

Equipment and characterization methods.High-resolution scanning electron microscopy (SEM) images were collected in a JEOL Field-effect electron microscope, model 7500F.The x-ray photoelectron spectrometry (XPS) analysis was carried out in a spectrometer Omicron Sphera with 7 channeltrons.Since this technique can only probe the chemical composition within few nanometers deep in the surface of samples, the microrods underwent a sputtering process with Ar + ions, so to expose its interior, before the collection of the photoelectron spectra.The peaks of Sn 3d 3/2 and 3d 5/2 , O 1s and C 1s were collected in high-resolution.The C 1s peak from adventitious carbon was used as reference for the calibration of the spectra, being set at 285 eV.Diffraction patterns were collected in a diffractometer model D8 Advance Eco from Bruker.The radiation used was the line Kα from Cu, and the equipment was operated at 25 mA and 40 kV.The 2θ range was 5 to 90°, being scanned at angular steps of 0.02°.The acquisition time at each step was 0.21 s.The micro-Raman spectroscopy analysis took place at a spectrometer model Lab RAM HR from Horiba Jobin Yvon using a laser with a wavelength of 633 nm.The Raman shift range varied from 100 to 1500 cm -1 at a resolution of 0.3 cm -1 .High-resolution transmission electron microscopy (HR-TEM) images were collected in a FEI microscope, model Tecnai G 2 F20, at an acceleration voltage of 200 kV.Prior to the TEM analysis, a single microrod of the sample was deposited on a Si substrate and had part of its surface coated by protective C and Pt layers in a focused ion beam (FIB) microscope (FEI, model Helios Nanolab 600I).Then, the ion beam of the microscope was used to thin out the SnO 2 /Sn 2+ microrod until a longitudinal section of it was obtained (Supplementary Fig. S1a and b-in the Supplementary Material), which was later welded to a FIB lift-out grid (Supplementary Fig. S1c).Once the microrod section was immobilized at the FIB lift-out grid, it was further thinned out (Supplementary Fig. S1d) so to enable probing both the interior and the subsurface of the microrod (in the region opposite to the C and Pt layers) by TEM.The material's specific surface area was determined through the Brunauer-Emmett-Teller (BET) method using the nitrogen adsorption-desorption data collected at 77 K in an ASAP 2010 equipment from Micromeritics.The porosity of the material was evaluated in terms of the Barret-Joyner-Halenda (BJH) method using the data from the desorption branch.The UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) analysis was carried out in a spectrophotometer Lambda 1050 (Perkin Elmer) equipped with accessories for reflectance measurements.The assessment of the MO content and of the intermediate species in the aqueous samples were performed at negative mode in a UV-Vis spectrophotometer Cary 60 from Agilent Technologies and in a liquid chromatography equipment (model Accela from Thermo) hyphenated with LCQ-Fleet mass spectrometers from Thermo (LC-MS/MS).

Photodegradation experiments.
The photodegradation experiments were carried out in a closed photocatalytic reactor containing a multi-position stirrer and light lamps positioned inside the reactor (top part).The photodegradation tests were carried out using a stock solution of 10 mg L -1 MO dye, previously aerated during 30 min.Afterwards, 20 mg of a material were added into 30 mL of the dye solution, remaining under stirring for 90 min to reach the adsorption equilibrium of MO dye.Samples were collected at the end of the dark regime, and then, sequentially, under illumination regime at pre-determined times, and stored in 50-mL Falcon tubes.The tubes were centrifuged at 10,000 RPM, and the supernatants were collected for analysis via UV-vis spectrophotometry.The experiments were carried out under a controlled temperature of 20 °C, under continuous air bubbling.Two types of lamps were employed in the experiments: 18 W black light lamps (Philips; TL-D BLB), and 20 W white light lamps (Philips; T10 plus).

Results and discussion
Morphological and surface characteristics of the SnO 2 /Sn 2+ material.Figure 1 exhibits the morphological features of the synthesized material.The images (a) and (b) of this figure indicated that the hydrothermal synthesis method induced the strict formation of microrods, which tended to exhibit hexagonal cross sections, as noted from the red arrows shown in Fig. 1b and c.The preponderant widths of the microrods were of few micrometers, while their lengths extended to tens of micrometers.The size distribution of the particles showed that 89% of the particles resided within the length range from 6 to 18 µm, whereas 94% of them present widths from 0.7 to 4.2 µm (Supplementary Fig. S2a and b, respectively).Nevertheless, lengths and widths of nearly 30 and 7 µm, respectively, were also observed.The majority of the particles presented a very rough surface, although it was also possible to detect few particles with smooth surface (Fig. 1c).It was also possible to observe a layered structure along the length of some particles (yellow arrows in Fig. 1c). Figure 1d and e present magnified views of a microrod, where it is possible to verify its rough texture.Furthermore, Fig. 1e suggests that nanometric spherical particles coalesced to originate the structures of the microrods.Additionally, evidence of grain boundaries could be verified at the surface of the particles by SEM analysis carried out at very high magnifications (100 kx and 500 kx).The Supplementary Fig. S3a demonstrated that even a "smooth surface" microrod exhibited plenty of irregularities in its texture (suggesting that the microrods may not be monocrystalline), while Supplementary Fig. S3b indicated the approximate size of such grains extend up to tens of nanometers.The fact that they strictly formed microrods with hexagonal cross sections (such as in a monocrystal) suggests that these microstructures might be mesocrystalline.
To determine the material's specific surface area and porosity, the nitrogen isotherms found in Supplementary Fig. S4a were collected.As can be noted from this figure, the material provided type-II isotherms with no evident hysteresis between the adsorption and desorption branches.This type of isotherm is typically assigned to nonporous materials 28 ; however, the BJH method allowed verifying the presence of pores with average diameter of 4.0 nm and also with dimensions smaller than 2 nm (Supplementary Fig. S4b), thus demonstrating the material's microporous nature.Furthermore, its BET specific surface area was determined as 93.0 ± 1.5 m 2 g -1 , which is considerably high.Therefore, the results from nitrogen adsorption demonstrated that the material's surface properties are suitable for catalytic applications, considering the availability of adsorption sites provided by its relatively high surface area and porosity.

Chemical composition.
To elucidate the chemical composition of the produced material, XPS analysis was performed and demonstrated that the material is composed by Sn, O and C, as noted from Supplementary Fig. S5a.By deconvoluting the peaks of each of these elements, it was verified that the Sn 3d 3/2 and Sn 3d 5/2 peaks were composed by two components each, as illustrated in Fig. 2. Whereas the curves centered at 495.20 and 486.75 eV were ascribed to Sn 4+ states, those at 493.58 and 485.02 eV were associated to Sn 2+ states.The integration of the areas under each component curve allowed us determining the Sn 2+ :Sn total atomic ratio as 0.158, indicating that the produced material contained a considerable amount of Sn 2+ in its interior, despite exhibiting the crystalline structure of rutile SnO 2 (stoichiometrically composed by Sn 4+ ), as discussed in "Crystalline structure" section.We believe that the presence of citrate ions in the synthesis medium partially prevented the oxidation of Sn 2+ from the tin precursor into Sn 4+ in SnO 2 , provided that citrate possesses reducing properties, being employed as a reducing agent in the synthesis of metallic Ag nanoparticles 29 .The deconvolution of the O 1s spectrum provided three component curves centered at 530.46, 531.15 and 533.46 eV, which were respectively assigned to oxygen atoms in the SnO 2 lattice, to oxygen species exposed by the sputtering process, and to H 2 O or hydroxides (Supplementary Fig. S5b) 30 .According to the literature, the O 1s component at 531.15 eV should be ascribed to chemisorbed oxygen species 30 .Nonetheless, the SnO 2 /Sn 2+ material had its surface corroded prior to the XPS analysis; therefore, we assigned this component to lattice oxygen species exposed by the sputtering process.Lastly, the C 1s peak was assigned solely to adventitious carbon, with contributions from C-C and C-O at 285 and 286 eV, respectively (Supplementary Fig. S5c) 31 .
The surface of the SnO 2 /Sn 2+ material with a non-sputtered surface was also characterized by XPS.The survey of its external surface provided the same composition as noted for its interior, exhibiting peaks of Sn, O and C (Supplementary Fig. S5d).Nonetheless, the high-resolution spectra of these elements presented some discrepancies in comparison with the internal composition.With respect to the Sn 3d peaks (Supplementary Fig. S5e), it was noted that their deconvolution led to a smaller contribution from the Sn 2+ ions (Sn 2+ :Sn total ratio = 0.043), in comparison with the results of the sputtered microrods.We believe that this observation is due to the further oxidation of the Sn 2+ ions at the material's surface by air after the synthesis.Concerning the O 1s peak (Supplementary Fig. S5f), its total area was found higher than that measured for the sputtered sample, also pointing out to a more oxidized surface.Moreover, while the area of the oxygen atoms at the lattice remained roughly the same, the areas of the oxygen atoms related to chemisorbed species and to H 2 O/hydroxides were found greater, in comparison with the corresponding contributions obtained for the sputtered sample.Finally, concerning the C 1s, an extra contribution assigned to carbonate was noted at 289 eV (Supplementary Fig. S5g), which may be due to the oxidation of the carbon compounds at the surface of the material, as well as to residual citrate from the synthetic medium.All these results demonstrated that the surface of the material is at a more oxidized state than its interior, consisting of a reasonable observation.
Crystalline structure.The x-ray diffractometry (XRD) analysis of the synthesized material was carried out, and the collected diffractogram can be seen in Fig. 3a.Along with the experimental diffraction patterns  www.nature.com/scientificreports/(in black), one can also find the standard diffraction pattern of tetragonal rutile SnO 2 (PDF #41-1445-in red), whose space group is P4 2 /mnm (no.136).As can be noted, nearly all rutile SnO 2 standard peaks demonstrated a good correspondence with the diffraction peaks observed for the synthesized material concerning both, position and intensity, therefore indicating the crystalline structure of rutile SnO 2 for the synthesized material.
It is worth noting that, although the individual particles of the material exhibited micrometric sizes, the diffraction peaks observed for the synthesized material were considerably broad.This observation indicates that the microrods are constituted by numerous nanosized grains.To ascertain such assumption, we employed HR-TEM to probe the crystalline features at the subsurface of a microrod and in its interior.To do so, a section of a single microrod was extracted and thinned out in a FIB microscope (as described in detail in the "Materials and methods" section), and then this section of the microrod was analyzed via HR-TEM, whose results can be found in Fig. 4. In Fig. 4a, the dark field image of the internal part of a microrod shows numerous bright dots caused by the diffraction of the electron beam of the microscope.Each of these dots correspond to the constituent crystal grains of the microrod.In the Supplementary Fig. S6, there is the corresponding bright field image of the same region, where it is possible to visualize the constituent grains of this single microrod.As can be noted, the grains presented sizes of few nanometers and demonstrated no well-defined shape and organization.
As one analyzes the HR-TEM image in Fig. 4b, it is possible to observe several crystalline planes extending for few nanometers, which exhibited varied orientation.The interplanar distance measurements of these planes provided ~ 2.1 Å, in agreement with the distance reported for the planes (210) of rutile SnO 2 phase (PDF #41-1445).
Additionally, according to Fig. 4c, the selected area electron diffraction (SAED) analysis provided well-defined ring patterns, as it is characteristic of polycrystalline materials.These rings are evidenced by the superposition of the SnO 2 /Sn 2+ x-ray diffractogram and could be indexed to the planes (110), ( 101) and (211) of rutile SnO 2 .The d-spacing between crystalline planes were determined with basis on the diffraction pattern of the rings in Fig. 4c, providing distances of 3.01, 2.68 and 1.86 Å for these respective planes, whereas, according to PDF#45-1445, the corresponding distances are 3.35, 2.64 and 1.76 Å.Thus, a 10-% decrease was noted for the (110) interplanar distance.In the Supplementary Fig. S7a, it is represented the tetragonal crystalline structure of rutile SnO 2 , along with the planes (110) and (1-10).As can be observed from these planes, the atomic density along the first plane is greater than that of the second, therefore, a contraction of the unit cell along the direction [110] is more probable, as represented by the blue arrows in this figure.Besides, the decrease of the interplanar distances between the planes (110) could reflect in the dilation of the unit cell in other directions, and, in fact, the interplanar distance between the planes (211) increased ~ 5%.Therefore, the angles of the originally tetragonal unit cell must have changed to values other than 90°, becaming either monoclinic or even triclinic.This effect is represented in the Supplementary Fig. S7b, as seen from the top of (001) plane.As can be verified in the Supplementary Fig. S8, the angles found in the SnO 2 /Sn 2+ microrods exhibit an approximate correspondence with those estimated for a distorted SnO 2 unit cell.
This discussion on the morphology of the microrods suggests that their growth process in the reaction medium was similar to that of a monocrystal (nucleation followed by crystal growth), rather than by the oriented attachment 32 of single grains to give rise to the mesocrystalline structure noted for the synthesized material.As could be noticed from the SEM images of the material, some microrods containing no spherical particles attached to their surfaces could be found among the particles of the material.Then, it is possible that all mesocrystalline microrods initially grew rapidly in the form of smooth surface microrods, when the tin concentration was still high, from the condensation of the hydroxylated tin ions (tin precursors) formed from hydrolysis reactions with tin cations.As soon as most of the tin precursors were consumed during the growth of the microrods, the kinetics of formation of the tin oxide decreased, and the residual tin precursors began to originate the nanospheres from the surface defects on the microrods to attribute them their characteristic rough texture.Concerning the role of the citrate, it is reasonable to believe that it acted as a capping agent, posing a steric hindrance after its preferential adsorption on favorable facets of the growing microrods, then inhibiting their growth in this particular direction, while directing their growth via facets which the citrate adsorbed at a smaller extent 32,33 .A schematic representation of their growth can be found in the Supplementary Fig. S9.
Still concerning the TEM analysis, images were also collected at the subsurface of the material and provided analogous results, as demonstrated in Supplementary Fig. S10a to c.The polycrystalline character of this material was also demonstrated for a single nanosphere found at the surface of a microrod via SAED (Supplementary Fig. S11).
A Raman spectroscopy analysis of the material was also performed to check for surface impurities, such as SnO and Sn 3 O 4 (natively Sn 2+ -containing phases), and the collected spectra can be found in Fig. 3b.As one can note, the Raman spectra of the synthesized material exhibited only a broad band between 400 and 700 cm -1 , where lie the peaks associated to SnO 2 34 .This band showed a maximum at 588 cm -1 , indicating the presence of in-plane oxygen vacancies in the SnO 2 /Sn 2+ material 35 .The presence of O vacancies are expected in highlydefective crystalline structures, and, in the case of the SnO/Sn 2+ material, the defects can be ascribed to the Sn 2+ cations residing in the SnO 2 crystalline structure.The fact that individual well-defined characteristic SnO 2 peaks could not be identified further corroborates that the materials demonstrated low crystallinity, in alignment with the interpretation of the XRD, TEM and SAED results.Moreover, no peaks associated to SnO and Sn 3 O 4 were detected between 100 and 350 cm -1 , thus supporting the high purity of the produced material.
In summary, as it was possible to verify from the XRD, Raman spectroscopy, TEM and SAED results, the synthesized material presented a strong polycrystalline character down to nanoscale level.In addition, if any Sn 2+ -containing phases, such as SnO or other intermediate tin oxide phases (Sn 2 O 3 , Sn 3 O 4 etc.), coexisted with the SnO 2 phase, they should have been detected by Raman spectroscopy even at very low concentrations, since these phases scatter light more efficiently than SnO 2 36 .Therefore, we propose that the Sn 2+ ions detected in the interior of the SnO 2 /Sn 2+ material through XPS analysis induced the formation of its highly defective internal crystalline structure, as schematically represented in Fig. 5.According to this figure, the SnO 2 /Sn 2+ microrods are composed by numerous nanosized crystallites composed by Sn 4+ ions, while the Sn 2+ ions reside at the grain boundaries of the crystallites (as terminal cations at the edges of their lattices), where they act disrupting the long-range periodicity of the SnO 2 crystalline structure.Based on this model, it is expected that the smaller the average size of the constituent crystallites, the greater the Sn 2+ content to be accommodated within the grain interfaces, thus justifying the relatively large Sn 2+ content in the interior of the synthesized material.This model may also justify the presence of the micropores in the material, as noted from the surface area measurements ("Morphological and surface characteristics of the SnO 2 /Sn 2+ material" section).Opto-electronic properties.As it is reported, due to the high band gap of SnO 2 (~ 3.6 eV), this phase is transparent to visible light 37 , nonetheless, once in powdered form (and provided that its particles are sufficiently small), it becomes white due to the scattering of ambient visible light.Contrastingly, the obtained material was evidently yellow, as shown in the embedded picture of Fig. 6a, indicating that the presence of Sn 2+ ions altered the absorption properties of SnO 2 .To assess the optoelectronic properties of the SnO 2 /Sn 2+ material, we employed UV-vis DRS, and the results, expressed as Kubelka-Munk and Tauc plots, are provided in Fig. 6a and  b, respectively.
According to the Kubelka-Munk plot (Fig. 6a), the SnO 2 /Sn 2+ material possesses a strong absorption within the visible-light spectral range, presenting a maximum at ~ 410 nm and an absorption edge at 671 nm.Since it is theorized that SnO 2 possesses a direct band gap 38,39 , we estimated the direct band gap of the SnO 2 /Sn 2+ material by means of the Tauc plot, as shown in Fig. 6b.According to the intersection of the red lines extrapolated to the x axis in this figure, the slope indicating 3.7 eV is in good agreement with the band gap energy of ~ 3.6 eV reported for undoped SnO 2 40 .Nevertheless, another slope can be distinguished pointing to 2.2 eV, suggesting that the SnO 2 /Sn 2+ material behaves as a self-doped material, and that its enhanced visible-light absorption is related to Sn 2+ defect states introduced into the band structure of SnO 2 .The band structure of the SnO 2 /Sn 2+ material is represented in the embedded diagram of Fig. 6b, which is based on theoretical calculations for Sn 2+ -containing tin oxide phases 38,41 .According to these calculations, the presence of Sn 2+ cations in SnO and mixed-valence tin oxide phases induces an upshift of their VB maximum and a consequent redshift in their photoabsorptive response, relative to pristine SnO 2 ¸ due to the filling of extra Sn s and p states with the additional electrons from Sn 2+ .Such predictions were supported by Tanabe et al. 's results 42 , which determined a shallower VB maximum for Sn 3 O 4 (+ 2.5 V vs. Standard Hydrogen Electrode-SHE), compared with that determined for SnO 2 (+ 3.6 V vs. SHE).
For comparison, we also analyzed the optical properties of commercial SnO 2 .To ensure that the Sn 2+ content in this material was totally oxidized into Sn 4+ , it was annealed in air at 1000 °C for 2 h.According to the Kubelka-Munk and Tauc plots obtained for this material (Supplementary Fig. S12a and b, respectively), the SnO 2 sample exhibited an absorption edge at 340 nm, with a band gap energy of 3.8 eV, in alignment with the values found in the literature for SnO 2

40
. This result evidenced that SnO 2 containing virtually none Sn 2+ ions cannot use visible light to drive photocatalytic processes, in contrast with the material SnO 2 /Sn 2+ .It is worth highlighting that such enhanced absorption of visible light may promote the application of the SnO 2 /Sn 2+ material in solardriven photocatalysis and photosensing, for instance.
Photodegradation experiments.Considering the potential application of the synthesized material in photocatalysis, its photocatalytic performance was tested in terms of the photodegradation of MO dye, a persistent pollutant model in wastewaters.Figure 7a compares the effect of light presenting different spectral emissions on the photocatalytic discoloration of a 10 mg L -1 MO dye solution.The adsorption equilibrium was attained after 90 min of contact in the dark, and ~ 10% of the MO content was adsorbed on the SnO 2 /Sn 2+ material, as demonstrated in Supplementary Fig. S13a.No noticeable degradation via photolysis occurred, as shown in Supplementary Fig. S13b.According to Fig. 7a, the SnO 2 /Sn 2+ material manifested a high photocatalytic activity under UV light, reaching 97% of MO degradation in 12 min.Although with a lower photodegradation performance, the material proved to be photosensitive to visible light (Fig. 7a).The photodegradation of MO under UV irradiation was repeated for a photocatalyst dose of 5.33 g L -1 , and the decomposition of the MO content nearly reached completion in 6 min (Supplementary Fig. S13c).The results from the LC-MS/MS analysis demonstrated that no photodegradation by-products were detected after the photodegradation of MO, indicating that its complete decomposition was attained (Supplementary Fig. S14a-c).
For comparison, the performance of the photocatalytic degradation of MO dye was further tested using anatase TiO 2 (the most studied material in heterogeneous photocatalysis) and commercial SnO 2 .Figure 7b shows that both materials exhibited poor photocatalytic activity under UV irradiation, when compared to the SnO 2 / Sn 2+ material's efficiency (it is worth noting that black light lamps provide UVA, a less energetic component within the UV spectral range).These results indicated that the incorporation of Sn 2+ ions in SnO 2 associated with a relatively large surface area increased the photocatalytic activity of the synthesized material dramatically, being able to outperform TiO 2 at less energetic irradiation conditions.

Conclusions
A mixed-valence tin oxide material was synthesized via simple hydrothermal method.The material presented a polycrystalline nature down to nanoscale.Its particles exhibited a well-defined microrod morphology, indicating that they were, in fact, mesocrystalline and grew through nucleation followed by crystal growth (in the same manner as a monocrystal).It was verified that the microrods possessed a significant amount of Sn 2+ cations residing within the grain boundaries.The investigation on the material's crystalline structure indicated that the material could be tetragonal rutile SnO 2 , however, the abundant Sn 2+ defects may have distorted the originally tetragonal crystalline structure of SnO 2 into a monoclinic or triclinic crystalline structure.Furthermore, differently of SnO 2 , the synthesized material absorbed visible light very efficiently due to the insertion of Sn 2+ states within its band gap structure, harnessing photons with wavelengths up to nearly 700 nm.Photodegradation experiments indicated that the material is able to decompose the azo-dye methyl orange, a pollutant model in water, under either UV or visible light, thus exhibiting promising photocatalytic properties to undertake the decontamination of waters and wastewaters under solar light.The photodegradation efficiency of the Sn 2+ -containing SnO 2 material is substantially higher when compared with those of commercial TiO 2 (anatase) and SnO 2 .Its superior photocatalytic performance is probably due to the efficient charge separation in its band structure as well as to its relatively high surface area (93.0 m 2 g -1 ).

Figure 1 .
Figure 1.SEM images of the synthesized material exhibiting (a) an overview of the microrods' morphology and size distribution, (b) and (c) the hexagonal cross section of the particles and their layered nature, (d) the surface texture of a microrod and (e) a close-up of its surface.

Figure 2 .
Figure 2. XPS spectrum of the Sn 3d 3/2 and Sn 3d 5/2 peaks for the synthesized material after the sputtering process.

Figure 3 .
Figure 3. Results of the XRD (a) and Raman spectroscopy (b) analyses for the synthesized material.The Raman spectra were collected in two different regions for the same sample.

Figure 4 .
Figure 4. Dark field TEM (a) and HR-TEM (b) images along with the SAED analysis (c) of the internal structure of a SnO 2 /Sn 2+ microrod.

Figure 6 .
Figure 6.Kubelka-Munk (a) and Tauc (b) plots for the SnO 2 /Sn 2+ material.The inset in (a) is a picture of the synthesized material, whereas that in (b) represents its band structure.

Figure 7 .
Figure 7. Photocatalytic discoloration of a 10 mg L -1 MO dye solution under UV and visible light irradiation at a photocatalyst dose of 0.40 g L -1 of the SnO 2 /Sn 2+ material (a).In (b), it is possible to find the results from the tests with anatase TiO 2 and SnO 2 .