In Fig. 2, X- ray diffraction (XRD) XRD patterns of TiO2, ST, CST and modified ST are displayed. Two principal crystalline planes (1 0 1) and (1 1 0) were identified corresponding to anatase and rutile in the TiO2 sample. However, the patterns of the particles obtained after the hydrothermal treatment, pointed to the fading of the characteristic peaks of P25 powder and new peaks corresponding to ST appeared. The diffraction peaks ~ 10, 24, 28 and 48 2θ, could be clearly identified and are in agreement with a titanate layered structure (Fig. 2(a-e). The broad peaks at 10 and 28 2θ correspond to interlayer spacing in layered titanates and revealing the nanotube structure (Hernández and Medina 2015; Vattikuti et al. 2018). The peaks at 24 and 48 2θ indicate the presence of hydrogen and sodium trititanates. Considering these facts, the XRD patterns can be assigned to trititanate (H,Na)2Ti3O7 with different interlayer spacing; for example, Ti6O13, Ti6O12, and Ti9O18. Diffractogram of thermally treated ST was similar to that of non-calcined ST (Fig. 1(d)). There is a modification in the low angle, 10° 2θ corresponding to a partial collapse in the tubular structure from thermal treatment (Lee et al., 2007). The deterioration of the tubular structure may be attributed to the dehydration of inter-layered OH radical which led to contraction and breaking of tubes structures. During the annealing treatment, the chemical bonds such as H2O and –OH are removed from the titanate structure, driving to the change of crystalline form and, at the same time, degrading the nanotube morphology. Figures 2(a) and 2(b) present the diffractograms of the bare and catalysts loaded with Ag or Zn (ST + Zn5 and ST + Ag5), where sodium titanate was the main observed phase. Due to the low concentration of co-catalysts, there is an absence of Ag or Zn species reflections in the patterns. In the case of ST + Ag5 pattern the widening of signals at low angles occurs. However, signals associated to Ag in the diffractogam at 38 and 44° were not spotted (Barrocas et al., 2016). A change at 10° signal was observed in the ST + Zn5 diffractogram that could be attributed to incomplete transformation of structural nanotubes; though, there is an increase in 28° signal of (3 1 0) plane characteristic of Zn. At 2θ of 47º a characteristic plane (2 0 0) of anatase disappeared. Three planes (0 1 1), (3 0 1) and (2 0 1) evidence the transformation of TiO2 to ST, respectively. Titanate specie was modified changing the NaOH concentration from 2.5 to 5 M and time of 24 to 48 hours (Fig. 1).
SEM analysis
According to Fig. 3 the morphology of ST depends on NaOH concentration and time in the hydrothermal treatment. Agglomerated particles were appreciated in the Figs. 3a and 2b corresponding to ST synthesized from 2.5 M NaOH solution and 24 hours at 170°C. In contrast, Figs. 3c and 3d show tubular particles of ST. Those samples were obtained with high NaOH concentrations during 48 hours of hydrothermal treatment. Tubular aspects are related with an inter-layer coordination of non-coordinate Ti and O atoms (Saponjic et al. 2005; Morgan et al. 2008); these micrographs exhibit formation of tubular structures. The length of the obtained ST is up to few micrometers and ~ 200 nm width. The Figs. 3e and 3f correspond to ST + Ag5 and ST + Zn5, respectively. It can be noted that in both cases, there is a loss of the tubular structure due to thermal treatment during the impregnation process.
EDX analysis is showed in the Table 2. Theoretical, Na2Ti3O7 and Na2Ti7O13 have a Ti/Na = 1.5 and 3, respectively, while the O/Ti are 2.3 and 2.16. Values of Ti/Na and O/Ti in the Thermal treatment did not display a significant effect in the composition of ST, although, phase transition to anatase and rutile has been previously reported (Nguyen and Bai, 2015), that could be related to the loss of Na atoms from the ionic substitution of Na+ by H+ when the washing is carried out to reach a pH of 7; although, the washing was conducted only with distilled water. When Ag is impregnated in the ST material, Ti/Na increased while O/Ti decreased pointing to an ionic exchange of Na from Ag. When ST is modified with Zn, Ti/Na increased possibly from the ionic exchange of Na by Zn; however, O/Ti kept indicates the formation of ZnO in the ST. XPS analysis was conducted to verify this hypothesis (Fig. 4).
Table 2
Elemental composition of ST synthesized determined by EDX.
Sample
|
Elemental composition, %
|
Ti/Na
|
O/Ti
|
Ti/M
M = Ag, Zn
|
Ti
|
Na
|
O
|
Ag
|
Zn
|
TiO2
|
35.00
|
0
|
65.00
|
0
|
0
|
-
|
1.85
|
--
|
ST
|
45.03
|
8.9
|
46.07
|
0
|
0
|
5.05
|
1.02
|
--
|
CST
|
45.79
|
7.59
|
46.61
|
0
|
0
|
6.03
|
1.01
|
--
|
ST + Ag5
|
47.10
|
8.26
|
39.48
|
5.16
|
0
|
5.70
|
0.83
|
9.12
|
ST + Zn5
|
45.57
|
5.63
|
46.58
|
0
|
2.22
|
8.09
|
1.02
|
20.52
|
ST and CST XPS survey spectra are similar indicating that chemical changes do not occur from thermal treatment of ST. For ST + Ag5 and ST + Zn5 signals of Ag and Zn appeared in a significant intensity. Quantification by XPS of ST samples is displayed in Table 3.
Table 3
Elemental composition of ST determined from XPS.
Samples
|
Elemental composition, %
|
Ti/Na
|
O/Ti
|
Ti/M
|
Na
|
Ti
|
O
|
Ag
|
Zn
|
M = Ag, Zn
|
ST
|
13.67
|
27.96
|
58.37
|
0
|
0
|
2.04
|
2.08
|
--
|
CST
|
13.64
|
28.60
|
57.76
|
0
|
0
|
2.09
|
2.01
|
--
|
ST + Ag5
|
11.82
|
22.36
|
57.70
|
8.10
|
0
|
1.89
|
2.58
|
2.76
|
ST + Zn5
|
15.19
|
26.25
|
50.81
|
0
|
7.75
|
1.72
|
1.93
|
3.40
|
Values of Ti/Na and O/Ti ratios in ST indicate the predominant formation of sodium trititanate being the main phase from the hydrothermal treatment applied. XPS analysis of modified ST indicates the above in Table 3: ionic exchange of Na+ by Ag+ and ZnO formation. As expected, the values of elemental composition of Table 3 are different with respect to Table 2 due to the difference in the volume of analysis in each technique. Analysis from XPS guarantees the surface inspection of the samples. Surface composition change in the calcined and modified ST was expected; however, in CST composition practically this change did not occur. In the modified ST a slight decrease in the Ti/Na ratio was observed due to migration of Na atoms from the inner part of the sample to the surface originating an increase of Na composition in the surface and a change in the surrounding chemical (Fig. 5).
Characteristic signals of Ti at 459 and 465 eV that corresponds to 2p3/2 and 2p1/2 are showed in Fig. 5. Signals area attributed to Ti4+ ions forming an octahedral structure of titanates (Kurra et al. 2019). There is a doublet signal in the Fig. 5a that correspond to Ti2p3/2 and Ti2p1/2 with binding energy at 465 eV (Ti2p3/2) and 459 (Ti2p1/2) (Coelho et al., 2016). Na substitution by Ag and Zn was corroborated by XPS. In Fig. 5a the Ti signals of ST and CST appear in the same binding energy while the Ti signals in ST + Ag5 and ST + Zn5 are moved to lower energies indicating that the incorporation of the mentioned atoms in the ST occurred. The same behavior was observed in the case of Na spectra analysis which shows a slight shift for modified ST. At 530 eV the signal of O 1s appears in all of the ST pointing to the formation of O2− oxygen-titanium bonds (Hernández-Hipólito et al. 2014). In Fig. 5a a shifting in the Ti2p band corresponding to ST + 5Ag; approximately, 0.49 eV units are moved to low energy in comparison to ST and CST. This shifting is attributed to ions substitution of Na by Ag indicating from the movement of electrons in a longer distance at Ti nucleus in the atomic expansion of Ti (Barrocas et al. 2016). In the case of ST and CST signals appear at 1072 eV indicating the presence of ions Na in the titanate (Marciniuk et al. 2014). Moreover, in the case of the Ag3d a difference of 6eV between Ag3d5/2 (368 eV) and Ag3d3/2 (374 eV) is characteristic to metallic state of Ag (Duan et al., 2017). In Fig. 5d the XPS spectra of ST + 5Zn is showed emphasizing the signals at 1021 and 1044 eV. Division of orbitals of Zn2p in the ZnO is observed (Wang et al. 2009). A shifting of 0.81 eV in the Ti2p occurs pointing to a change in the Ti environment in coordination with Zn being modified the ionic radius of Ti. In this case, the presence of octahedral and tetrahedal titanium is possible (Cho et al. 2014; Wang et al. 2009). However, the samples in this study only show the octahedral structure where the metallic atoms do not modify the principal structure of ST. XPS analysis show that ST is not affected in the elemental composition from the thermal treatment at 400°C. SEM images and the EDS analysis corroborate the above mentioned, although, the tubular structure in the ST collapses. ST + 5Ag and ST + 5Zn display a different behavior because the binding energies indicate that the ST are not forming binding to the metallic elements. Additionally, the ionic exchange of Na by Zn and Ag occurs. In TEM images are appreciated the morphology of the samples prepared herein (Fig. 6).
TEM images shows the tubular structure of ST (Fig. 5a) although this morphology collapses when is treated at 400°C (Fig. 6b). In the case of modified ST, small particles are appreciated in each case demonstrating the formation of nano particles from the ionic exchange. In order to explain the collapse of morphology thermogravimetric (TGA) analysis was employed. In Fig. 7 the TGA/DSC analysis of the samples is showed.
Thermogravimetric curves of the obtained ST materials are displayed in Fig. 7. The greater part of the weight loss occurs at lower temperatures, where the dehydration of physisorbed water takes place. The single stage weight loss of roughly 10% occurs up to 200°C. This process is reflected as an endothermic peak on a DSC curve. After this temperature the weight slowly and continuously decreases nearly up to 700°C. At T ≤ 300°C the dehydration of interlayered OH groups could reduce the interlayer distance but does not destroy the tubular shape. When temperature > 300°C, the dehydration of interlayered OH groups induced the change of crystalline form and, at the same time, the nanotube morphology is destroyed. A broad exothermic peak in a temperature range from 300 to 800 C on the DSC curve could indicate that the synthesized ST, loose interlayered OH groups in a broader range while interlayered OH groups remain in the structure up to 600°C or the cleavage of both type of OH groups occurs simultaneously. Between 200–300°C an exothermic reaction occurs attributing this process to hexa-tri-titanate transformation (Lee et al. 2007). At 400–600°C an endothermic process is observed that means the collapse of tubular structure forming spherical particles.
A comparison of prepared ST using UV-vis spectroscopy demonstrates the absorbance of ST with and without modifications in a range of 600 − 200 nm (Figure not showed). Figure 8 shows the UV-vis spectroscopy modified with the Kubelka-Munk function showing a high absorption between 325–340 nm to ST and CST while for ST + Ag5 and ST + Zn5 the absorption is observed at 355 nm. TiO2 spectra shows the absorption at 350 nm indicating a shifting of bands for the prepared ST from the dispersion of photons by defects in the crystals of ST (Benzarouk et al. 2012). Eg-values are observed in the Table 3.
Table 3
Optical Eg obtained from the Kubelka-Munk function applied to DRS spectroscopy of prepared ST.
Sample
|
Eg (eV)
|
TiO2
|
3.5
|
ST
|
3.8
|
CST
|
3.6
|
ST + Ag5
|
3.4–3.5
|
ST + Zn5
|
3.4–3.5
|
Eg-TiO2 values resulted modified when it was transformed to sodium titanate from 3.5 to 3.8 eV. Moreover, Eg-CST value decreased at 3.6 due to recrystallization and morphology change from the thermal treatment. Impregnation of ST with Ag and Zn promoted a slight change of Eg due to the Burstein-Moss effect. The change in the absorption bands was caused by increasing of carrier charges and the blocking of low energy transitions from the doped and calcined treatment of ST. This promoted an increasing of Fermi level modified the Eg (Achour et al. 2007). It is preferable that the energetic levels decreasing to active the photocatalyst making more large the interval of light that can be useful, UV and visible.
NO abatement
According to reactions 1 and 2, NO was produced by mixing certain amounts of Cu and HNO3 obtaining a maximum concentration of 641 ppm in average. NO photo-degradation using ST are showed in the Fig. 9 and compared with the use of TiO2.
Figure 9a shows the NO concentration in function of amount of copper in the reaction 2. Practically, there is not degradation of NO (conversion to NO2) for the exposition under UV light (253 nm). It is important to note that the NO concentration proved in the present work were higher than those reported in previous reports, for instance, Ma et al., (2015) achieved the photocatalysis process with 400 ppb of NO; Duan et al., (2017) used 450 ppb of NO. Concentration tested herein was selected considering the maximal concentration of NO emission in a car with an air:fuel ratio 14.64:1 (by mass) that is less than the stoichiometric level (Dey and Mehta, 2020). TiO2 exhibited a degradation percentage of around 50% with an initial charge of 0.2 and 0.4 g, although, when the amount of photocatalyst increased, the NO degradation remains unchanged. This means that TiO2 showed a capacity to decrease the NO concentration until 320 ppm being significance in the total emissions in a car with the above characteristic of air:fuel ratio. Moreover, higher charges of TiO2 are not favorable for the photo-reduction of NO: with 0.6, 0.8 and 1 g of TiO2 an increase of C/Co occurred possibly by phenomenon of adsorption. Namely, when NO is on the photoreactor TiO2 adsorb it blocking sites for the photocatalysis process. Carrera et al., (2007) reported a similar behavior: when the time of exposition passed the NO concentration increased possibly also by the blocking of photocatalytic sites in the TiO2 surface when a low NO concentration (50 ppm) was tested. Other possible reason is the non-homogeneous distribution of the sample in the photo reactor that prevented the usage of total of the area of the TiO2. However, this explanation is opposite with the other photocatalyst materials because ST, with and without modification, showed higher performance increasing the NO photo-degradation with higher charges of photocatalyst. The most efficient synthesized photocatalyst was ST using 1.0 g in the experiments. NO degradation occurred in stages and is relatively similar to other photocatalyst used (65% percentage of degradation). Compared with the same material load, CST degraded 62% of NO, while TS + Ag5 and TS + Zn5 showed a degradation percentage of 45% and 40%, respectively. Major percentage of NO degradation is attributable at higher surface area in ST than other photocatalysts. Possibly, the thermal treatment in the modified ST affected the performance reducing the surface area and the active sites to achieve the photocatalytic process. This is better related to the band gap of the ST; it is consistent to say that the band gap, together with the surface area of the sample, have a greater influence on the photocatalytic effect of the synthesized materials. Then, the thermal treatment in the ST was not suitable to degrade NO.
One of the possible routes for NO photoreduction is described as catalytic sites which tetrahedrally coordinated Ti is found on the surface. This species has been reported by Anpo et al. (1997) as a catalytic site that favors the decomposition of NO to N2 and O2 using Ti-modified zeolites. Since most of the Ti ions on the surface of the photocatalyst have a 5 coordination, a single oxygen vacancy could lead to the formation of a reducing site. Wu and van de Krol (2012) described NO reduction mechanism where NO molecules are captured in sites where oxygen vacancies working as photo-catalytic sites. In the work of Nguyen and Bai (2015) photo degradation of NOx was pointed out oxidant the NO and NO2 using ST treated with acid while in the present work the photo reduction of NO was demonstrated. Moreover, the adsorption of NO did not occurr in the conditions herein. Experimental data shows the highest degradation percentages for synthesized photocatalysts.