Photocatalytic Degradation of Organic Dye under UV-A Irradiation Using TiO2-Vetiver Multifunctional Nano Particles

The properties and photocatalytic performance of anatase nanoparticles of pure TiO2 and a core–shell structure of TiO2 on calcined vetiver grass leaves have been compared. Samples were fabricated by sol-gel and heating at 450 °C for 5 h. The comparison was based on data for X-ray diffraction (XRD), UV-Vis spectrophotometry, photoluminescence, transmission electron microscopy, specific surface area measurement, pore volume assessment, and methylene blue degradation testing. The results showed that the pure TiO2 consisted of agglomerated equiaxed nanoparticles of individual grain sizes in the range 10–20 nm. In contrast, the TiO2-vetiver composite exhibited a core–shell structure consisting of a carbonaceous core and TiO2 shell of thickness 10–15 nm. These features influenced the photocatalytic performance in such a way that the lower cross-sectional area, greater surface area, and higher pore volume of the TiO2 shell increased the number of active sites, reduced the charge carrier diffusion distance, and reduced the recombination rate, thereby improving the photocatalytic activity. This improvement derived from morphological characteristics rather than crystallographic, semiconducting, or optical properties. The improved performance of the TiO2-vetiver core–shell was unexpected since the X-ray diffraction data showed that the crystallinity of the TiO2 was lower than that of the pure TiO2. These outcomes are attributed to the reducing effect of the carbon on the TiO2 during heating, thereby facilitating the formation of oxygen vacancies, which enhance charge separation and hence photocatalysis by TiO2.


Introduction
Water is an indispensable part of life because it is essential for various purposes, including drinking, public hygiene, energy, agriculture, and industry. However, freshwater resources currently are diminishing due to the increasing growth of global population, over-exploitation, and water pollution [1]. In order to deal with water scarcity, wastewater treatment and reclamation are favourable

Preparation of Vetiver Adsorbent
Vetiver grass was collected from the field in Phitsanulok Province, Thailand. Only the leaves of the vetiver grass were used to synthesise the adsorbent while other parts were removed. After cleaning with water to remove dirt, the vetiver leaves were dried in a muffle furnace at 105 • C for 12 h in order to remove moisture. The dried vetiver leaves were crushed with a porcelain mortar and pestle and sieved to a particle size in the range 0.1-0.6 mm, after which they were pyrolysed in the same muffle furnace at 600 • C for 3 h, followed by natural cooling. After pyrolysis, the residue was stored in an airtight container. The main inorganic components of the solid residue of vetiver leaves were determined previously to be potassium (45 wt%) and silicon (27 wt%) [35]. The physical and chemical properties of dried vetiver and adsorption mechanism were reported in the previous work [35].

Synthesis of Pure TiO 2 and TiO 2 -Vetiver Core-Shell
The pure TiO 2 and TiO 2 -vetiver core-shell was synthesised using a sol-gel method reported elsewhere [36]. For the latter, 1 g of calcined vetiver adsorbent was added to a mixture of 12.5 mL of titanium isopropoxide (TTIP), 80 mL of 2-propanol, and 3 mL of deionised (DI) water, after which blending was achieved by magnetic stirring for 4 h. The resultant suspension was filtered, washed by double distilled water, and dried at 105 • C for 12 h. The dried product then was calcined in the muffle furnace at 450 • C for 5 h in (3 • C/min) order to recrystallise the TiO 2 , followed by natural cooling. After calcination, the residue was stored in an airtight container. Pure TiO 2 was prepared by the same procedure but without the addition of calcined vetiver.

Characterisation
For both pure TiO 2 and the TiO 2 -vetiver core-shell, the following characterisation was undertaken. The mineralogies were determined by X-ray diffraction (XRD, Philips X' Pert PRO PW 3719). The crystallite size of particle was calculated using the Scherrer's equation [37]. The band gaps were calculated from data obtained by UV-Vis spectrophotometry from the diffuse reflectance spectra (DRS, Shimadzu UV-3600) with an integrating sphere attachment (Shimadzu ISR-3100) and the application of the Kubelka-Munk equation [38]. Photoluminescence spectra (PL) were recorded at 300 nm at room temperature a using spectrophotometer (Horiba Jobin Yvon Fluoromax-4). The morphologies were assessed by transmission electron microscopy (TEM, JEOL JSM-2010). The specific surface areas and pore size distributions were determined by N 2 gas adsorption-desorption using the Brunauer-Emmett-Teller (BET) method (Quantachrome Adtosorb 1 MP).

Photocatalytic Performance Testing
Methylene blue (MB, Sigma-Aldrich (Singpore), ≥95%) was used as the organic dye pollutant. A typical batch test involved mixing 0.01 g of the synthesised core-shell or pure TiO 2 with 50 mL of MB aqueous solution of 10 −5 M concentration and magnetic stirring for 12 h in dark conditions (without UV-A irradiation) in order to adsorb the MB maximally on the particle surfaces. The suspensions then were irradiated by UV-A (two 20 W black lights, 370 nm) for 4 h. The irradiation was from above and the distance to the suspension surface was 15 cm. At periodic intervals of irradiation, a small sample of liquid was removed using a vacuum filter. The removed aliquot then was characterised by UV-Vis spectrophotometry using the standard absorbance intensity of λ max = 664 nm using the UV-visible spectrophotometer. Figure 1 shows the XRD patterns for pure TiO 2 and the TiO 2 -vetiver core-shell. The sharp peaks at 25.2 • , 37.6 • , 48.0 • , 54.9 • , 62.6 • , 70.3 • , and 75.0 • 2θ for pure TiO 2 show that it consists solely of the anatase polymorph [39]. In contrast, the TiO 2 -vetiver core-shell peaks also indicate anatase but of lower intensities, which are attributed to the effect of the amorphous residue of the vetiver grass leaves. The reduced intensities, which derived from reduced crystallinity of the anatase, probably resulted from the one or more of the following causes. First, silicon is known as a grain growth inhibitor for TiO 2 [40], which may have had a similar effect in suppressing nucleation and hence growth. Second, the crystal radii of K and Si in sixfold coordination (0.152 nm and 0.054 nm, respectively) are considerably different from that of Ti (0.0745 nm) [41] and the interstitial site (0.0782 nm) [42]. Consequently, if these dissolved in the TiO 2 lattice, they would have served to destabilise it. Third, carbon is a very strong reducing agent, so the Ti 4+ (0.0745 nm) could have been reduced partially to Ti 3+ (0.081 nm) [41], which could have expanded the lattice at these sites and contracted the lattice at the sites of the charge-compensating oxygen vacancies, which also would have had the potential to destabilise the lattice. resulted from the one or more of the following causes. First, silicon is known as a grain growth inhibitor for TiO2 [40], which may have had a similar effect in suppressing nucleation and hence growth. Second, the crystal radii of K and Si in sixfold coordination (0.152 nm and 0.054 nm, respectively) are considerably different from that of Ti (0.0745 nm) [41] and the interstitial site (0.0782 nm) [42]. Consequently, if these dissolved in the TiO2 lattice, they would have served to destabilise it. Third, carbon is a very strong reducing agent, so the Ti 4+ (0.0745 nm) could have been reduced partially to Ti 3+ (0.081 nm) [41], which could have expanded the lattice at these sites and contracted the lattice at the sites of the charge-compensating oxygen vacancies, which also would have had the potential to destabilise the lattice. The data used to calculate the optical indirect band gaps, which are shown in Figure 2, indicate that these are 3.58 eV and 3.66 eV for pure TiO2 and the TiO2-vetiver core-shell, respectively. The band gap of the former was higher than the commonly reported value of 3.2 eV [9]. However, the band gap of anatase TiO2 has been reported to cover a wide range of 3.23-3.59 eV [39]. Although the determined value is within this range, it is possible that it reflects silicon contamination, which is known to increase the band gap [9,43]. The even higher band gap of the TiO2-vetiver core-shell is likely to derive from the previously mentioned effect of carbon in the partial reduction of TiO2. Consequently, the XRD and band gap data are mutually supportive in suggesting that the presence of carbon caused lattice destabilisation and partial amorphisation. This is supported by modeling of the band gaps of both reduced TiO2, which contains oxygen vacancies [44] and of amorphous TiO2 [45], which were substantially higher than that of the stoichiometric crystalline analogue. Figure 3 shows photoluminescence (PL) spectra of pure TiO2 and the TiO2-vetiver core-shell. Since PL emission occurs upon the recombination of the photogenerated electrons and holes [46], then the higher PL intensity of pure TiO2 and that of the TiO2-vetiver core-shell suggests that the latter experienced enhanced charge separation and so exhibited a reduced recombination rate [47,48]. These effects would increase the probability for electrons and holes to form ROS and hence increase the photocatalytic activity [36,48]. The data used to calculate the optical indirect band gaps, which are shown in Figure 2, indicate that these are 3.58 eV and 3.66 eV for pure TiO 2 and the TiO 2 -vetiver core-shell, respectively. The band gap of the former was higher than the commonly reported value of 3.2 eV [9]. However, the band gap of anatase TiO 2 has been reported to cover a wide range of 3.23-3.59 eV [39]. Although the determined value is within this range, it is possible that it reflects silicon contamination, which is known to increase the band gap [9,43]. The even higher band gap of the TiO 2 -vetiver core-shell is likely to derive from the previously mentioned effect of carbon in the partial reduction of TiO 2 . Consequently, the XRD and band gap data are mutually supportive in suggesting that the presence of carbon caused lattice destabilisation and partial amorphisation. This is supported by modeling of the band gaps of both reduced TiO 2 , which contains oxygen vacancies [44] and of amorphous TiO 2 [45], which were substantially higher than that of the stoichiometric crystalline analogue. Figure 3 shows photoluminescence (PL) spectra of pure TiO 2 and the TiO 2 -vetiver core-shell. Since PL emission occurs upon the recombination of the photogenerated electrons and holes [46], then the higher PL intensity of pure TiO 2 and that of the TiO 2 -vetiver core-shell suggests that the latter experienced enhanced charge separation and so exhibited a reduced recombination rate [47,48]. These effects would increase the probability for electrons and holes to form ROS and hence increase the photocatalytic activity [36,48].  For the TiO2-vetiver core-shell, it is likely that reduction by carbon and the consequent generation of oxygen vacancies are responsible for this since it has been reported that oxygen vacancies can enhance charge separation by acting as electron donors [49,50]. It is unlikely that dissolved K or Si played a role since (1) it is highly unlikely that K would dissolve either substitutionally or interstitially owing to size considerations; (2) Si dissolved substitutionally would not affect ionic charge compensation and hence create no oxygen vacancies; and (3) Si dissolved interstitially is likely to have resulted in ionic charge compensation by the formation of Ti vacancies. While Ti vacancies could have acted as hole donors, it is considered that the displacement of Ti 4+ by Si 4+ and the resultant formation of Ti vacancies are energetically unfavorable. It has to be noted that, in this work, the increasing of oxygen vacancy has not been directly observed. The increase of oxygen vacancy is implied from the increasing of surface area, which can lead to increasing the surface defects and oxygen vacancy [51][52][53]. Figure 4 shows TEM images of pure TiO2 and the TiO2-vetiver core-shell. The pure TiO2 was equiaxed and of particle size in the range 10-20 nm, albeit highly agglomerated. It has to be noted that the particle size is very close to the crystallite size, as shown in Table 1. It can be said that the Optical indirect band gaps calculated from DRS data for pure TiO 2 and TiO 2 -vetiver core-shell.  For the TiO2-vetiver core-shell, it is likely that reduction by carbon and the consequent generation of oxygen vacancies are responsible for this since it has been reported that oxygen vacancies can enhance charge separation by acting as electron donors [49,50]. It is unlikely that dissolved K or Si played a role since (1) it is highly unlikely that K would dissolve either substitutionally or interstitially owing to size considerations; (2) Si dissolved substitutionally would not affect ionic charge compensation and hence create no oxygen vacancies; and (3) Si dissolved interstitially is likely to have resulted in ionic charge compensation by the formation of Ti vacancies. While Ti vacancies could have acted as hole donors, it is considered that the displacement of Ti 4+ by Si 4+ and the resultant formation of Ti vacancies are energetically unfavorable. It has to be noted that, in this work, the increasing of oxygen vacancy has not been directly observed. The increase of oxygen vacancy is implied from the increasing of surface area, which can lead to increasing the surface defects and oxygen vacancy [51][52][53]. Figure 4 shows TEM images of pure TiO2 and the TiO2-vetiver core-shell. The pure TiO2 was equiaxed and of particle size in the range 10-20 nm, albeit highly agglomerated. It has to be noted that the particle size is very close to the crystallite size, as shown in Table 1. It can be said that the For the TiO 2 -vetiver core-shell, it is likely that reduction by carbon and the consequent generation of oxygen vacancies are responsible for this since it has been reported that oxygen vacancies can enhance charge separation by acting as electron donors [49,50]. It is unlikely that dissolved K or Si played a role since (1) it is highly unlikely that K would dissolve either substitutionally or interstitially owing to size considerations; (2) Si dissolved substitutionally would not affect ionic charge compensation and hence create no oxygen vacancies; and (3) Si dissolved interstitially is likely to have resulted in ionic charge compensation by the formation of Ti vacancies. While Ti vacancies could have acted as hole donors, it is considered that the displacement of Ti 4+ by Si 4+ and the resultant formation of Ti vacancies are energetically unfavorable. It has to be noted that, in this work, the increasing of oxygen vacancy has not been directly observed. The increase of oxygen vacancy is implied from the increasing of surface area, which can lead to increasing the surface defects and oxygen vacancy [51][52][53]. Figure 4 shows TEM images of pure TiO 2 and the TiO 2 -vetiver core-shell. The pure TiO 2 was equiaxed and of particle size in the range 10-20 nm, albeit highly agglomerated. It has to be noted that the particle size is very close to the crystallite size, as shown in Table 1. It can be said that the pure TiO 2 is single grain particle. More interestingly, the TiO 2 -vetiver core-shell exhibited a core-shell structure, where the vetiver acted as the core and the TiO 2 acted as the shell. The size of the TiO 2 -vetiver core-shell was variable, depending on the nature of the vetiver substrate. The TiO 2 shell was continuous and of thickness in the range 10-15 nm. pure TiO2 is single grain particle. More interestingly, the TiO2-vetiver core-shell exhibited a coreshell structure, where the vetiver acted as the core and the TiO2 acted as the shell. The size of the TiO2-vetiver core-shell was variable, depending on the nature of the vetiver substrate. The TiO2 shell was continuous and of thickness in the range 10-15 nm. Such core-shell structures have gained considerable interest owing to their ability to enhance both charge separation and resistance to photocorrosion. Analogues of the present core-shell structure exist in the form of TiO2 on the surfaces of carbon nanotubes (CNT) [54], TiO2 coated on amorphous carbon [55], and the converse arrangement of a TiO2 core and graphitic carbon shell [56].

Results and Discussion
N2 adsorption-desorption isotherms of the pure TiO2 and TiO2-vetiver core-shell were also studied, as shown in Figure 5. The gas-adsorption isotherm is reported as the volume of gas adsorbed as a function of P/Po. The results suggest that the prepared samples own typical Type IV isotherm. The gap between equilibrium adsorption and desorption pressures above P/Po of about 0.4, further confirming the characteristic of the mesoporous material. The results for the specific surface area and pore volume of pure TiO2 and the TiO2-vetiver coreshell are given in Table 1. It can be seen that the values for the pure TiO2 are less than half those of the TiO2-vetiver core-shell. The evidence for the reason for these differences lies in the nature of the TiO2 shell of the TiO2-vetiver core-shell. While the pure TiO2 appears to have a considerably greater specific surface area, even with the agglomeration, as well as greater pore volume, the TiO2 shell of the TiO2-vetiver core-shell appears to be smooth. This apparent contradiction can be explained by closer examination of the TiO2 shell, which reveals a granular nanostructure that exhibits both surface Such core-shell structures have gained considerable interest owing to their ability to enhance both charge separation and resistance to photocorrosion. Analogues of the present core-shell structure exist in the form of TiO 2 on the surfaces of carbon nanotubes (CNT) [54], TiO 2 coated on amorphous carbon [55], and the converse arrangement of a TiO 2 core and graphitic carbon shell [56]. N 2 adsorption-desorption isotherms of the pure TiO 2 and TiO 2 -vetiver core-shell were also studied, as shown in Figure 5. The gas-adsorption isotherm is reported as the volume of gas adsorbed as a function of P/Po. The results suggest that the prepared samples own typical Type IV isotherm. The gap between equilibrium adsorption and desorption pressures above P/Po of about 0.4, further confirming the characteristic of the mesoporous material. pure TiO2 is single grain particle. More interestingly, the TiO2-vetiver core-shell exhibited a coreshell structure, where the vetiver acted as the core and the TiO2 acted as the shell. The size of the TiO2-vetiver core-shell was variable, depending on the nature of the vetiver substrate. The TiO2 shell was continuous and of thickness in the range 10-15 nm. Such core-shell structures have gained considerable interest owing to their ability to enhance both charge separation and resistance to photocorrosion. Analogues of the present core-shell structure exist in the form of TiO2 on the surfaces of carbon nanotubes (CNT) [54], TiO2 coated on amorphous carbon [55], and the converse arrangement of a TiO2 core and graphitic carbon shell [56].
N2 adsorption-desorption isotherms of the pure TiO2 and TiO2-vetiver core-shell were also studied, as shown in Figure 5. The gas-adsorption isotherm is reported as the volume of gas adsorbed as a function of P/Po. The results suggest that the prepared samples own typical Type IV isotherm. The gap between equilibrium adsorption and desorption pressures above P/Po of about 0.4, further confirming the characteristic of the mesoporous material. The results for the specific surface area and pore volume of pure TiO2 and the TiO2-vetiver coreshell are given in Table 1. It can be seen that the values for the pure TiO2 are less than half those of the TiO2-vetiver core-shell. The evidence for the reason for these differences lies in the nature of the TiO2 shell of the TiO2-vetiver core-shell. While the pure TiO2 appears to have a considerably greater specific surface area, even with the agglomeration, as well as greater pore volume, the TiO2 shell of the TiO2-vetiver core-shell appears to be smooth. This apparent contradiction can be explained by closer examination of the TiO2 shell, which reveals a granular nanostructure that exhibits both surface The results for the specific surface area and pore volume of pure TiO 2 and the TiO 2 -vetiver core-shell are given in Table 1. It can be seen that the values for the pure TiO 2 are less than half those of the TiO 2 -vetiver core-shell. The evidence for the reason for these differences lies in the nature of the TiO 2 shell of the TiO 2 -vetiver core-shell. While the pure TiO 2 appears to have a considerably greater specific surface area, even with the agglomeration, as well as greater pore volume, the TiO 2 shell of the TiO 2 -vetiver core-shell appears to be smooth. This apparent contradiction can be explained by closer examination of the TiO 2 shell, which reveals a granular nanostructure that exhibits both surface topography and a network of spherical pores in the sub-nanometre size range. The unexpected presence of the pores is likely to have resulted from the oxidation of the carbon during annealing, which would have disturbed the establishment of the solid nanostructure upon recrystallisation of the TiO 2 . Since the pores appear to be spherical, this suggests gas entrapment during the process. It also is possible that the pores were ruptured and hence continuous, which would increase the specific surface area further. Figure 6a shows the complete data for MB adsorption followed by MB degradation. These data confirm that the pure TiO 2 has less than half the surface area of the TiO 2 -vetiver core-shell. Figure 6b shows the normalised data for MB degradation, where it can be seen that the extent of MB degradation by the TiO 2 -vetiver core-shell effectively was complete within 2 h. The pure TiO 2 appears to have experienced a 1 h time lag, after which the kinetics of MB degradation were similar to those of the TiO 2 -vetiver core-shell, although the process does not appear to have reached steady-state after 4 h. It is clear that both photocatalyst types perform considerably better under UV-A light. Figure 6a shows the complete data for MB adsorption followed by MB degradation. These data confirm that the pure TiO2 has less than half the surface area of the TiO2-vetiver core-shell. Figure 6b shows the normalised data for MB degradation, where it can be seen that the extent of MB degradation by the TiO2-vetiver core-shell effectively was complete within 2 h. The pure TiO2 appears to have experienced a 1 h time lag, after which the kinetics of MB degradation were similar to those of the TiO2-vetiver core-shell, although the process does not appear to have reached steadystate after 4 h. It is clear that both photocatalyst types perform considerably better under UV-A light.
The preceding data suggest that the photocatalytic activity of the pure TiO2 is somewhat inferior to that of the TiO2-vetiver core-shell. This observation is consistent with the findings for the photoluminence (Figure 3) and specific surface area data (Table 1), but it contradicts the data for the X-ray diffraction (Figure 1) and the band gap ( Figure 2). From this, it is concluded that the controlling factors in the performance depend more on a reduced recombination rate (photoluminescence) and the number of surface active sites (specific surface area) rather than the crystallinity (X-ray diffraction) or the semiconducting and optical properties (band gap). In addition, the charge carrier diffusion distance in the TiO2-vetiver core-shell would be relatively short owing to the 10-15 nm shell thickness and the possibility that it could be even less owing to the thinner walls resulting from the porosity. The diffusion distance in the pure TiO2 would be longer owing to the larger 10-20 nm (solid) grain size. It is well known that the recombination rate decreases with decreasing charge carrier diffusion distance. Figure 7 shows the kinetic rate of MB removal. It can be seen that after irradiation the MB was completely remove in 2 h by TiO2-vetiver core-shell. Since the TiO2-vetiver core-shell has higher surface area and volume of pores, then this can result higher adsorption (Figure 6a). Then, during the photocatalytic mechanism (after irradiation), the MB at the surface could be decomposed quickly so the surface can adsorb more MB. It can be said that the co-mechanisms of adsorption by adsorbent and photocatalysis lead to higher performance compared to only photocatalytic of pure TiO2.  The preceding data suggest that the photocatalytic activity of the pure TiO 2 is somewhat inferior to that of the TiO 2 -vetiver core-shell. This observation is consistent with the findings for the photoluminence (Figure 3) and specific surface area data (Table 1), but it contradicts the data for the X-ray diffraction (Figure 1) and the band gap ( Figure 2). From this, it is concluded that the controlling factors in the performance depend more on a reduced recombination rate (photoluminescence) and the number of surface active sites (specific surface area) rather than the crystallinity (X-ray diffraction) or the semiconducting and optical properties (band gap). In addition, the charge carrier diffusion distance in the TiO 2 -vetiver core-shell would be relatively short owing to the 10-15 nm shell thickness and the possibility that it could be even less owing to the thinner walls resulting from the porosity. The diffusion distance in the pure TiO 2 would be longer owing to the larger 10-20 nm (solid) grain size. It is well known that the recombination rate decreases with decreasing charge carrier diffusion distance. Figure 7 shows the kinetic rate of MB removal. It can be seen that after irradiation the MB was completely remove in 2 h by TiO 2 -vetiver core-shell. Since the TiO 2 -vetiver core-shell has higher surface area and volume of pores, then this can result higher adsorption (Figure 6a). Then, during the photocatalytic mechanism (after irradiation), the MB at the surface could be decomposed quickly so the surface can adsorb more MB. It can be said that the co-mechanisms of adsorption by adsorbent and photocatalysis lead to higher performance compared to only photocatalytic of pure TiO 2 . Data provides the surface area-normalised degradation constants. The comparison of normalised degradation constants excluded the influence of surface areas. In order to investigate the effect of surface area from the adsorbent on the degradation activity of TiO2, the surface areanormalised degradation values against light irradiation time were plotted as shown in Figure 8 and the calculated surface area-normalised rate constants are presented in Table 1. The results clearly suggested that surface area of the catalyst has a crucial impact on the activity of degradation in this study because the surface area-normalised rate constants of TiO2-vetiver core-shell are increased from the pure TiO2.  Data provides the surface area-normalised degradation constants. The comparison of normalised degradation constants excluded the influence of surface areas. In order to investigate the effect of surface area from the adsorbent on the degradation activity of TiO 2 , the surface area-normalised degradation values against light irradiation time were plotted as shown in Figure 8 and the calculated surface area-normalised rate constants are presented in Table 1. The results clearly suggested that surface area of the catalyst has a crucial impact on the activity of degradation in this study because the surface area-normalised rate constants of TiO 2 -vetiver core-shell are increased from the pure TiO 2 . Data provides the surface area-normalised degradation constants. The comparison of normalised degradation constants excluded the influence of surface areas. In order to investigate the effect of surface area from the adsorbent on the degradation activity of TiO2, the surface areanormalised degradation values against light irradiation time were plotted as shown in Figure 8 and the calculated surface area-normalised rate constants are presented in Table 1. The results clearly suggested that surface area of the catalyst has a crucial impact on the activity of degradation in this study because the surface area-normalised rate constants of TiO2-vetiver core-shell are increased from the pure TiO2.  The schematic representation of the photocatalytic mechanism is present in Figure 9. It can be explained that during the adsorption process MB can be adsorbed at the surface of TiO 2 -vetiver core-shell as can be seen in Figure 5a. After irradiation, the photocatalytic at the surface of TiO 2 -vetiver The schematic representation of the photocatalytic mechanism is present in Figure 9. It can be explained that during the adsorption process MB can be adsorbed at the surface of TiO2-vetiver coreshell as can be seen in Figure 5a. After irradiation, the photocatalytic at the surface of TiO2-vetiver core-shell can occur immediately. This results in enhancing the photocatalytic performance of MB degradation.

Conclusions
The present work reports a comparison between nanoparticles of pure TiO2 and a core-shell structure of TiO2 on calcined vetiver grass leaves. The samples were fabricated using a sol-gel method that involved heating at 450 °C for 5 h, which yielded the anatase polymorph of TiO2. The comparison was based on data for X-ray diffraction, UV-Vis spectrophotometry, photoluminescence, transmission electron microscopy, specific surface area measurement, pore volume assessment, and methylene blue degradation testing.
The results showed that the pure TiO2 consisted of agglomerated equiaxed nanoparticles of individual grain sizes in the range 10-20 nm. In contrast, the TiO2-vetiver composite exhibited a coreshell structure consisting of a carbonaceous core and TiO2 shell of thickness 10-15 nm. The TiO2 shell formed a continuous coating with topography and porous network at a sub-nanometre scale. These features appear to have had a significant influence on the photocatalytic performance in that, compared to the pure TiO2, the lower cross-sectional area, greater surface area, and higher pore volume of the TiO2 shell potentially increased the number of active sites, reduced the charge carrier diffusion distance, and reduced the recombination rate, thereby improving the photocatalytic activity. This improvement derived from morphological characteristics rather than crystallographic, semiconducting, or optical properties.
The improved performance of the TiO2-vetiver core-shell was unexpected in light of the X-ray diffraction data, which showed that the crystallinity of the TiO2 was lower than that of the pure TiO2. It also was unexpected in consideration of the unusually high band gap of the TiO2 shell. However, both of these outcomes are likely to have been generated through the reducing effect of the carbon on the TiO2 during heating, thereby facilitating the formation of oxygen vacancies, which enhance charge separation and hence photocatalysis by TiO2. In this sense, the effect of charge separation dominated those of crystallinity and band gap. It has to be noted that, in this work, the increasing of

Conclusions
The present work reports a comparison between nanoparticles of pure TiO 2 and a core-shell structure of TiO 2 on calcined vetiver grass leaves. The samples were fabricated using a sol-gel method that involved heating at 450 • C for 5 h, which yielded the anatase polymorph of TiO 2 . The comparison was based on data for X-ray diffraction, UV-Vis spectrophotometry, photoluminescence, transmission electron microscopy, specific surface area measurement, pore volume assessment, and methylene blue degradation testing.
The results showed that the pure TiO 2 consisted of agglomerated equiaxed nanoparticles of individual grain sizes in the range 10-20 nm. In contrast, the TiO 2 -vetiver composite exhibited a core-shell structure consisting of a carbonaceous core and TiO 2 shell of thickness 10-15 nm. The TiO 2 shell formed a continuous coating with topography and porous network at a sub-nanometre scale. These features appear to have had a significant influence on the photocatalytic performance in that, compared to the pure TiO 2 , the lower cross-sectional area, greater surface area, and higher pore volume of the TiO 2 shell potentially increased the number of active sites, reduced the charge carrier diffusion distance, and reduced the recombination rate, thereby improving the photocatalytic activity. This improvement derived from morphological characteristics rather than crystallographic, semiconducting, or optical properties.
The improved performance of the TiO 2 -vetiver core-shell was unexpected in light of the X-ray diffraction data, which showed that the crystallinity of the TiO 2 was lower than that of the pure TiO 2 . It also was unexpected in consideration of the unusually high band gap of the TiO 2 shell. However, both of these outcomes are likely to have been generated through the reducing effect of the carbon on the TiO 2 during heating, thereby facilitating the formation of oxygen vacancies, which enhance charge separation and hence photocatalysis by TiO 2 . In this sense, the effect of charge separation dominated those of crystallinity and band gap. It has to be noted that, in this work, the increasing of oxygen vacancy has not been directly observed. The increase of oxygen vacancy is implied from the increasing of surface area, which can lead to increasing the surface defects and oxygen vacancy.