Sulfur-Doped TiO 2 : Structure and Surface Properties

A comprehensive study on the sulfur doping of TiO2, by means of H2S treatment at 673 K, has been performed in order to highlight the role of sulfur in affecting the properties of the system, as compared to the native TiO2. The focus of this study is to find a relationship among the surface, structure, and morphology properties, by means of a detailed chemical and physical characterization of the samples. In particular, transmission electron microscopy images provide a simple tool to have a direct and immediate evidence of the effects of H2S action on the TiO2 particles structure and surface defects. Furthermore, from spectroscopy analyses, the peculiar surface, optical properties, and methylene blue photodegradation test of S-doped TiO2 samples, as compared to pure TiO2, have been investigated and explained by the effects caused by the exchange of S species with O species and by the surface defects induced by the strong H2S treatment.


Introduction
Titanium dioxide (TiO 2 ) is widely used for photocatalysis.It has attracted considerable attention because of its characteristics, including optical properties, reactivity and chemical stability, as well as its non-toxicity [1,2].In particular, TiO 2 -based photocatalysts have been used for significant applications, such as antibacterial actions [3], medical research [4], drug delivery [5], and self-cleaning fields [6].Most of all, this material is widely used in the degradation of pollutants in air and water by the decomposition of organic compounds [7,8].
Despite its outstanding photocatalytic properties, TiO 2 is only able to absorb a small range of the UV portion of the solar spectrum [9], because of its relatively high band gap.To solve this problem, the most-used strategy is the engineering and shift of the TiO 2 band gap to the visible light region, in such a way to enhance its photocatalytic activity.In this regard, the surface modification obtained by anchoring selected species, such as MoS 2 or graphene-like systems [10,11], or by the incorporation of metal or non-metal dopants into the TiO 2 structure [12], allows one to harvest the visible spectrum or to increase the reactivity in the UV spectrum.It has been found that metals are able to induce a desired band gap shift, but also induces recombination centers, thus reducing the photocatalysis capability in combination with thermal instability [13].
Indeed, sulfur-doped TiO 2 has attracted much attention due to the fact that increasing quantities of S can reduce the band gap [23], as well as show a strong absorption in the visible light [24].
or could be embedded within the TiO 2 lattice, thus creating S-Ti-O bonds [31].
Therefore, to better understand and describe the properties and then the possible applications of the so-obtained S-doped TiO 2 , it is fundamental to investigate the nature of the H 2 S interaction with the TiO 2 surface.
Moreover, a further aspect to take into account is the role of H 2 S as a reactant in catalytic hydro-treatment and Claus reactions [32,33].It is noteworthy that, nowadays, the emission limits of SO x are becoming very rigorous, because air pollution has become a serious global problem.Oil and gas extraction sites are one of the main sources of H 2 S emissions, and they are usually removed by means of the well-known Claus process.The reaction occurs via dissociative adsorption of H 2 S on a metal oxide [34], mainly on Al 2 O 3 but also on TiO 2 , used as catalyst.
In both processes, H 2 S was found to modify the surface properties of the metal oxide support catalyst.According to the literature, many theoretical studies concerning H 2 S reaction and adsorption on TiO 2 are known, particularly those on anatase and rutile phases [35][36][37][38][39].
To our knowledge, only a few experimental studies have focused on the effects originated by H 2 S dosage on TiO 2 surface, concerning the relationship among surface, structure, and morphology properties.Our study aims to contribute to these themes, as it reports a quite extensive chemical and physical characterization of the surface properties of S-doped TiO 2 , obtained after H 2 S treatment.The samples are investigated by X-ray diffraction (XRD), and high resolution transmission electron microscopy (HRTEM), in addition to Raman, Fourier Transform-Infrared (FTIR) and UV-visible (UV-Vis) spectroscopies.The obtained results are compared to those of pure TiO 2 .

XRD Analysis
The XRD patterns of TiO 2 before and after H 2 S dosage at 673 K for 1 h are shown in Figure 1 (black and red lines, respectively), together with the typical crystalline features of anatase (PDF card # 21-1272) and rutile (PDF card # 21-1276) phases, as highlighted by blue and green lines (anatase and rutile, respectively).
From the pattern of TiO 2 after the sulfidation procedure, no considerable modifications of the peculiar peaks of anatase and rutile are observed, thus remarking that the amount of S species inside the S-doped TiO 2 does not affect the lattice under the adopted preparation conditions.We shall return to this point by analyzing the amount of sulfur via elemental analysis, as illustrated in Figure S1.

Raman Spectroscopy
Figure 2 shows the Raman spectrum of S-doped TiO2 compared with pure TiO2, used as a reference material, both recorded with a 514 nm laser line.

Raman Spectroscopy
Figure 2 shows the Raman spectrum of S-doped TiO 2 compared with pure TiO 2 , used as a reference material, both recorded with a 514 nm laser line.

Raman Spectroscopy
Figure 2 shows the Raman spectrum of S-doped TiO2 compared with pure TiO2, used as a reference material, both recorded with a 514 nm laser line.In detail, concerning the spectrum of pure TiO 2 (black curve), the four bands at 144, 396, 514, and 636 cm −1 are ascribed to the E g , B 1g , A 1g , E g Raman active modes, respectively, of the anatase phase, as described in the literature [10,18,40].Furthermore, the shoulder at 608 cm −1 and the small peak at 444 cm −1 (labelled by asterisks) are due to the A 1g and E g modes of the rutile phase [40].As for the S-doped TiO 2 sample, as obtained after the sulfidation step (red curve), the typical Raman fingerprints of TiO 2 are still present, even if a clear explanation of the erosion of the weak modes labelled by asterisks remains unclear [29,35]   In detail, concerning the spectrum of pure TiO2 (black curve), the four bands at 144, 396, 514, and 636 cm −1 are ascribed to the Eg, B1g, A1g, Eg Raman active modes, respectively, of the anatase phase, as described in the literature [10,18,40].Furthermore, the shoulder at 608 cm −1 and the small peak at 444 cm −1 (labelled by asterisks) are due to the A1g and Eg modes of the rutile phase [40].As for the S-doped TiO2 sample, as obtained after the sulfidation step (red curve), the typical Raman fingerprints of TiO2 are still present, even if a clear explanation of the erosion of the weak modes labelled by asterisks remains unclear [29,35].In particular, rutile (Figure 3a) and anatase (which reveals its diffraction pattern from [111] zone axis) nanoparticles (Figure 3b), which have a well-defined structure and shape, show extended faces with highly regular terminations, together with sharp corners and edges.Conversely, in Figure 3c,d, S-doped TiO2 particles with rounded shapes are shown, as highlighted by the white arrows.
From Figure 3d, it can be observed that the borders of the particle appear to be completely indented, with corners and edges sensitively smoothed, which is caused by the formation of local defective In particular, rutile (Figure 3a) and anatase (which reveals its diffraction pattern from [111] zone axis) nanoparticles (Figure 3b), which have a well-defined structure and shape, show extended faces with highly regular terminations, together with sharp corners and edges.Conversely, in Figure 3c,d, S-doped TiO 2 particles with rounded shapes are shown, as highlighted by the white arrows.From Figure 3d, it can be observed that the borders of the particle appear to be completely indented, with corners and edges sensitively smoothed, which is caused by the formation of local defective regions at the atomic level.The explanation of this phenomenon could be plausibly ascribed to the action of H 2 S, whose strong acidic character could lead to remarkably defective surfaces of TiO 2 particles.
For a deeper understanding of the effects of H 2 S treatment, the TiO 2 and S-doped TiO 2 samples were investigated by low resolution TEM (insets in Figure 3a,c, respectively).From the comparison of the related particle size distributions shown in Figure S2 (Supplementary Materials), a slight increment of particle dimensions for S-doped TiO 2 is detectable, as confirmed by the mean crystal sizes provided by the anatase (101) and rutile (110) XRD peak broadenings (Scherrer's equation) shown in Table S1 (Supplementary Materials).
Moreover, the slight decreasing of the specific surface area (38 m 2 /g) of S-doped TiO 2 observed, as compared with TiO 2 (50 m 2 /g), in addition to the slight increasing of the particle size in the case of S-doped TiO 2 samples, can be explained by the moderate sintering effect due to annealing treatment conditions occurring at 673 K under an H 2 S atmosphere.
Furthermore, the impressive action of H 2 S on the nature of TiO 2 particles has been also confirmed by FTIR investigation (vide infra).

FTIR Spectroscopy
FTIR spectra collected at 100 K at decreasing CO coverage on TiO 2 were compared to a similar sequence on the S-doped TiO 2 sample (Figure 4a,b).The spectra were acquired by increasing an initial CO dose (70 Torr) to reach equilibrium conditions, i.e., the maximum CO coverage, after which the system was progressively outgassed up to the complete removal of the adsorbed CO molecules.
Catalysts 2017, 7, 214 5 of 11 regions at the atomic level.The explanation of this phenomenon could be plausibly ascribed to the action of H2S, whose strong acidic character could lead to remarkably defective surfaces of TiO2 particles.
For a deeper understanding of the effects of H2S treatment, the TiO2 and S-doped TiO2 samples were investigated by low resolution TEM (insets in Figure 3a,c, respectively).From the comparison of the related particle size distributions shown in Figure S2 (Supplementary Materials), a slight increment of particle dimensions for S-doped TiO2 is detectable, as confirmed by the mean crystal sizes provided by the anatase (101) and rutile (110) XRD peak broadenings (Scherrer's equation) shown in Table S1 (Supplementary Materials).
Moreover, the slight decreasing of the specific surface area (38 m 2 /g) of S-doped TiO2 observed, as compared with TiO2 (50 m 2 /g), in addition to the slight increasing of the particle size in the case of S-doped TiO2 samples, can be explained by the moderate sintering effect due to annealing treatment conditions occurring at 673 K under an H2S atmosphere.
Furthermore, the impressive action of H2S on the nature of TiO2 particles has been also confirmed by FTIR investigation (vide infra).

FTIR Spectroscopy
FTIR spectra collected at 100 K at decreasing CO coverage on TiO2 were compared to a similar sequence on the S-doped TiO2 sample (Figure 4a,b).The spectra were acquired by increasing an initial CO dose (70 Torr) to reach equilibrium conditions, i.e., the maximum CO coverage, after which the system was progressively outgassed up to the complete removal of the adsorbed CO molecules.TiO 2 spectra show the typical features due to the adsorption of CO on the different Ti sites on activated TiO 2 surfaces, as discussed in the literature (Figure 4a) [41][42][43].
As can be seen from Figure 4a, the intense main peak observed at 2179 cm −1 can be explained by the building up of lateral interactions among an array of parallel CO oscillators, adsorbed on Ti 4+ sites on flat (101) surfaces [41], while that at 2155 cm −1 is due to CO molecules interacting by hydrogen bonds with residual OH groups.The main band, in the 2178 cm −1 -2192 cm −1 range (Figure 4a), undergoes a frequency shift upon decreasing CO pressure, due to the changing of the lateral interactions between CO oscillators on the TiO 2 surface [43,44].Notice that a higher frequency shift is indicative of a highly regular and extended face.
Coming back to OH groups, which remain on the surface despite the activation treatment at 673 K, their presence is confirmed by the stretching modes observed at 3718 cm −1 and 3672 cm −1 (inset of Figure 4a).These features are shifted to lower frequency as a consequence of CO adsorption, thus giving rise to a broad and more intense feature centered at 3562 cm −1 .Notice that the spectral features of the OH groups are then completely restored at a lower CO pressure together with the progressively disappearing and shifting of the 2155 cm −1 band to 2160 cm −1 .These two events indicate that the complete and reversible CO desorption from OH groups is occurring [42,45].
The sharp band at 2140 cm −1 is easily associated to physically adsorbed CO, which forms a multi-layer surface when the liquid nitrogen temperature brings CO to a "liquid-like" state [46].On the other hand, the band at 2146 cm −1 that, by decreasing CO coverage, gradually shifts upward and merges with the 2155 cm −1 band, is due to the CO species on rutile facelets present in TiO 2 P25 [41].
The weak feature at 2166 cm −1 was assigned to CO adsorbed on Ti Lewis centers on (001) surfaces, where Ti sites along Ti-O rows are strongly bound to two oxygens, which cause a screened electrostatic potential at these Ti sites, as concluded by Mino et al. [47] in a combined FTIR/Density Functional Theory (DFT) study on the CO adsorption on anatase (001) and (101) facets.
The weak band at 2208 cm −1 was assigned to CO adsorbed on highly acid Ti Lewis sites located on defective situations such as edges, steps, and corners, thus exhibiting very low coordination [41].
After H 2 S dosage (Figure 3b), some modifications can be highlighted.In particular, a slightly wider Full Width at Half Maximum (FWHM) for the main feature at 2180 cm −1 is observed due to a more disordered system, caused by the presence of S species interrupting the regularity of the extended faces.
Notice that the 2180 cm −1 -2191 cm −1 main band for S-doped TiO 2 (Figure 4b) undergoes a quite similar frequency shift upon decreasing CO pressure, as compared to pure TiO 2 (Figure 4a).
The 2166 cm −1 band, previously assigned to Ti Lewis centers on flat (001) faces and strongly bound to O anions, presents an increased intensity and can be ascribed to a reduced screening electrostatic potential due to the O → S exchange.In fact, the presence of sulfur could cause an increased acidity followed by the observed increased intensity (Figure 4b).Conversely, the reduced intensity of the 2160-2155 cm −1 envelope can be explained by the weaker interaction of CO molecules with the residual OH groups (Figure 4b).
Moreover, notice that the band at 2207 cm −1 , ascribed to CO on highly cus (coordinatively unsaturated) Ti sites on S-doped TiO 2 [41], shows higher intensity with respect to pure TiO 2 and is the last one to disappear by outgassing.It can be hypothesized that the larger S atoms replace the smaller O ones, thus favoring the formation of defects such as edges, steps, and corners.
These results can be confirmed by the wide band observed in the 3400-3200 cm −1 region, before CO dosage, due to the formation of surface H 2 O as a consequence of oxygen-sulfur exchange (inset of Figure 4b), as explained in the next paragraph.
On the basis of studies concerning the dissociative adsorption of H 2 S on TiO 2 [48], it was found that above a temperature of 593 K, the Ti-SH bond is still strong, being -SH irreversibly adsorbed on the surface of TiO 2 .However, by increasing the temperature, S-H bonds become weaker and finally break.H moves to a neighboring O, forming -OH groups.Finally H 2 O is formed at the surface and S moves to oxygen vacancy positions [29].
Catalysts 2017, 7, 214 7 of 11 Moreover, besides the dissociative adsorption, H 2 S molecules can also interact with surface -OH groups (bonded to Ti 4+ ) to give rise to hydrogen bonds.

UV-Vis Spectroscopy
The UV-Vis spectrum of S-doped TiO 2 (red curve) as compared to pure TiO 2 (black curve), is shown in Figure 5. From this, a wide shift of the absorption edge for the S-doped TiO 2 sample towards higher wavelengths (lower energies) is clearly detectable, together with an additional wide absorption centered at around 390 nm.
break.H moves to a neighboring O, forming -OH groups.Finally H2O is formed at the surface and S moves to oxygen vacancy positions [29].
Moreover, besides the dissociative adsorption, H2S molecules can also interact with surface -OH groups (bonded to Ti 4+ ) to give rise to hydrogen bonds.

UV-Vis Spectroscopy
The UV-Vis spectrum of S-doped TiO2 (red curve) as compared to pure TiO2 (black curve), is shown in Figure 5. From this, a wide shift of the absorption edge for the S-doped TiO2 sample towards higher wavelengths (lower energies) is clearly detectable, together with an additional wide absorption centered at around 390 nm.In particular, the band gap shift is emphasized by the Tauc plot (see inset in Figure 5), where the intercepts with the abscissa axis of the extrapolation of the linear part of the curves clearly highlight a red shift of the absorption edge for S-doped TiO2 (see black and red dotted lines for TiO2 and S-doped TiO2, respectively).
In general, this energy red-shift is associated with a change of TiO2 electronic structure, after the treatment with H2S, which causes the S → O exchange at the surface of TiO2 [50,51].Taking into consideration this aspect, it is well known that the formation of doping states involves additional electronic levels that can be formed between the valence (VB) and the conduction (CB) bands, thus reducing the electron transition energy [21,52].Along this line, the features in the UV-visible range can be ascribed to the presence of additional electronic states above the valence band edge of pure TiO2, due to S species, as well supported by XPS measurements [52,53].These states can be attributed to sulfur 3p atomic orbitals mixing with the VB of TiO2.In addition, another plausible mechanism to explain the observed absorption shift could also involve O vacancies caused by the thermal treatment at 673 K under vacuum conditions [54].
The S-doped TiO2 sample was testedfor the photodegradation of methylene blue (MB) in water solution under solar light irradiation as compared to the TiO2 P25 benchmark (Figure S4, Supplementary Materials).In this figure, the MB C/C0 vs. time plots of S-TiO2 and of the TiO2 P25 are shown.Although the MB photodegradation of S-doped TiO2 is definitely high, under solar light irradiation its photocatalytic activity is lower than that of pure TiO2 P25.We think that the In particular, the band gap shift is emphasized by the Tauc plot (see inset in Figure 5), where the intercepts with the abscissa axis of the extrapolation of the linear part of the curves clearly highlight a red shift of the absorption edge for S-doped TiO 2 (see black and red dotted lines for TiO 2 and S-doped TiO 2 , respectively).
In general, this energy red-shift is associated with a change of TiO 2 electronic structure, after the treatment with H 2 S, which causes the S → O exchange at the surface of TiO 2 [50,51].Taking into consideration this aspect, it is well known that the formation of doping states involves additional electronic levels that can be formed between the valence (VB) and the conduction (CB) bands, thus reducing the electron transition energy [21,52].Along this line, the features in the UV-visible range can be ascribed to the presence of additional electronic states above the valence band edge of pure TiO 2 , due to S species, as well supported by XPS measurements [52,53].These states can be attributed to sulfur 3p atomic orbitals mixing with the VB of TiO 2 .In addition, another plausible mechanism to explain the observed absorption shift could also involve O vacancies caused by the thermal treatment at 673 K under vacuum conditions [54].
The S-doped TiO 2 sample was testedfor the photodegradation of methylene blue (MB) in water solution under solar light irradiation as compared to the TiO 2 P25 benchmark (Figure S4, Supplementary Materials).In this figure, the MB C/C 0 vs. time plots of S-TiO 2 and of the TiO 2 P25 are shown.Although the MB photodegradation of S-doped TiO 2 is definitely high, under solar light irradiation its photocatalytic activity is lower than that of pure TiO 2 P25.We think that the explanation for this lower photocatalytic performance in the degradation of MB is the balance of two opposite effects.The first one, associated with the sulfur doping which allows the solar light harvesting, is beneficial.However, the defective surface, as a result of the heavy H 2 S treatment, has a strong, definitely detrimental effect in the charge separation efficiency.It is safely concluded that, upon the adopted H 2 S conditions (673 K, vacuum), the non-reversible surface modification does not help to increase the photocatalytic efficiency with respect to the TiO 2 P25 powder used as a starting material.

Materials and Methods
TiO 2 (P25, Evonik), in pellet form, was activated at 673 K for 30 min under dynamic vacuum, then oxidized in an oxygen (40 Torr) atmosphere at the same temperature for 30 min, twice.By keeping the temperature at 673 K, the obtained sample was sulfided in an H 2 S atmosphere (30 Torr) for 1 h, twice, and then outgassed.The sample was further sulfided following the same method.
FTIR spectra of CO adsorbed at 100 K on TiO 2 and S-doped TiO 2 at decreasing coverages were obtained in an IR cell designed for liquid nitrogen flowing, and were recorded by means of a Bruker IFS-28 spectrometer, equipped with a Mercury Cadmium Telluride (MCT) cryogenic detector, with a resolution of 4 cm −1 (64 interferograms were averaged for each spectrum).The spectra were acquired in the 4000-400 cm −1 interval, where the fundamental vibration modes are observed.
Raman spectra were recorded using a Renishaw Raman InVia Reflex spectrophotometer equipped with an Ar+ laser emitting at 514 nm, using both static and rotating configurations.
UV-Vis measurements were collected by using a UV-Vis-NIR spectrophotometer (Varian Cary 5000, equipped with a reflectance sphere.Due to their strong optical absorption, the samples were diluted in BaSO 4 powder. X-ray diffraction patterns were collected by means of a diffractometer (PANalytical PW3050/60 X'Pert PRO MPD) with a Ni-filtered Cu anode, working with a reflectance Bragg-Brentano geometry, by using the spinner mode.The mean crystal sizes were calculated from XRD measurements by Scherrer's equation: L = Kλ/β cosθ (λ is the X-ray wavelength, β is the full width at half maximum (FWHM) of the diffraction line corrected by the instrumental broadening, θ is the diffraction angle, and K is a constant assumed to be 0.9).Peak fitting of XRD patterns was adopted, using the Pseudo-Voigt function of anatase (101) and rutile (110) XRD peaks.
High resolution transmission electron microscopy images were acquired with a JEOL 3010-UHR instrument operating at 300 kV, equipped with a 2 k × 2 k pixels Gatan US1000 CCD camera.
N 2 adsorption-desorption experiments were carried out at 77 K (Micromeritics ASAP 2020 instrument) to determine the Brunauer-Emmett-Teller (BET) surface area.The surface area of the samples was determined after outgassing at RT, overnight.
For the photodegradation test, the same quantities of S-doped TiO 2 and TiO 2 powder (used as a reference) were dispersed in aliquots of a methylene blue (MB) water solution 12.5 mg/L and kept in the dark for 1 h at RT.After exposure, for increasing times, to a solar lamp (SOL2/500S lamp, Honle UV technology, Munchen, Germany) ranging from ultraviolet to infrared radiation (295-3000 nm), the dispersions were centrifuged for 30 min at 10,000 rpm.Photocatalytic degradation of MB was investigated by means of UV-Vis spectroscopy in the transmission mode.The integrated intensity of the adsorbed MB manifestations (C) was used to obtain C/C 0 vs. time plots, where C 0 is the concentration at the initial intensity before illumination (Figure S4).

Conclusions
In this work, S-doped TiO 2 samples were synthesized by means of H 2 S treatment at 673 K. From several ex situ investigations, including XRD, Raman spectroscopy, and HRTEM, the structure and morphology of samples were obtained.In particular, even if XRD and Raman analyses, due to detection limits, did not give sensitive information concerning the effects of H 2 S dosage on TiO 2 , HRTEM images showed remarkable changes of TiO 2 particle shapes as a consequence of H 2 S, which causes strong erosion of the faces, corners, and edges of the nanoparticles.This impressive action was also confirmed by FTIR spectra, where remarkable differences in the relative intensity of all peaks in S-doped TiO 2 , when compared to pure TiO 2 , were observed.Moreover, the changes in the OH groups range can be ascribed to oxygen-sulfur exchange phenomena, explained with the dissociative adsorption of H 2 S on TiO 2 .
Finally, UV-Vis spectroscopy demonstrated how also the electronic structure of TiO 2 can be deeply modified by H 2 S. The red shift of the S-doped TiO 2 absorption edge can be explained with additional extrinsic electronic levels introduced by the sulfur doping.This affected the optical properties of TiO 2 , whose absorption edge was extended to the visible-light region.
From all these considerations, it can be safety concluded that H 2 S treatment of TiO 2 , to achieve an S-doped TiO 2 material, has a deep and strong effect on TiO 2 , such as on the morphological, surface, electronic, and optical properties.However, the photocatalysis efficiency of TiO 2 in the degradation of methylene blue is not improved by the H 2 S treatment at the adopted conditions, due to the strongly defective surface of S-doped TiO 2 which decreases the charge separation efficiency.

Supplementary Materials:
The following are available online at www.mdpi.com/2073-4344/7/7/214/s1, Figure S1: EDAX spectrum of TiO 2 /H 2 S, Figure S2: Particle size distributions (PSDs) of TiO 2 P25 (left panel) and of S-TiO 2 (right panel), Figure S3: Time dependence of C/C 0 upon solar light exposure of S-TiO 2 (red line) as compared to the TiO 2 P25 (black dotted line), for photodegradation of methylene blue, Figure S4: FTIR spectra, recorded before CO dosage, of TiO 2 (black curve) and TiO 2 /H 2 S (red curve), Table S1: Mean crystal sizes of anatase and rutile nanoparticles in TiO 2 P25 and S-TiO 2 samples, as obtained from XRD peak broadening (black and red patters in Figure 1).

Figure 2 .
Figure 2. Raman spectra of TiO 2 and S-doped TiO 2 (black and red lines, respectively).Raman fingerprints of rutile are marked by asterisks. .

Figure 2 .
Figure 2. Raman spectra of TiO2 and S-doped TiO2 (black and red lines, respectively).Raman fingerprints of rutile are marked by asterisks.

Figure 3 .
Figure 3. TEM images of TiO2 (a,b) and S-doped TiO2 (c,d).In the insets of (a,c) low resolution images of TiO2 and S-doped TiO2 are shown, while in the insets of (b,d) two selected regions are Fast Fourier Transform (FFT) imaged.

Figure 3 .
Figure 3. TEM images of TiO 2 (a,b) and S-doped TiO 2 (c,d).In the insets of (a,c) low resolution images of TiO 2 and S-doped TiO 2 are shown, while in the insets of (b,d) two selected regions are Fast Fourier Transform (FFT) imaged.

Figure 4 .
Figure 4. FTIR spectra of CO adsorbed at 100 K on (a) TiO 2 and (b) S-doped TiO 2 at decreasing coverages (black bold line, maximum coverage: p CO = 70 Torr, grey bold line, minimum coverage: p CO → 0); (c) scheme modelling of Ti sites on two different faces interacting with CO molecules and the frequency values of the corresponding bands at maximum coverage (black bold line).

Figure 5 .
Figure5.UV-Vis spectra of S-doped TiO2 (red curve) and pure TiO2 (black curve) used as a reference material.The Tauc plot of both samples is shown in the inset.An example of a Tauc Plot is reported in ref[49].

Figure 5 .
Figure5.UV-Vis spectra of S-doped TiO 2 (red curve) and pure TiO 2 (black curve) used as a reference material.The Tauc plot of both samples is shown in the inset.An example of a Tauc Plot is reported in ref.[49].