SiO 2 Nanoparticles Derived from Arundo Donax L. Ash Composite with TiO 2 Semiconductor for Ecient Photocatalytic Dye Reduction

In this report, the biomass derived silicon dioxide (SiO 2 ) nanoparticles composite with titanium dioxide (TiO 2 ) semiconductors used as ecient photocatalyst for degradation of Rhodamine B (RhB) dye molecules under UV-visible light irradiation is proclaimed. At rst SiO 2 derived from Arundo donax L. ash and TiO 2 synthesized using titanium (IV) isopropoxide by co-precipitation method and then their different compositions prepared by wet impregnation method were exampled to various optical and atomic level fundamental studies. The amorphous and crystalline nature of SiO 2 and TiO 2 ratify from XRD and here it is found that the crystalline nature decreased in their compositions as compared to TiO 2 . 293 nm UV photons harvesting SiO 2 observed which could be due to more impurity states presence on surface is further accomplished red shift after composition with TiO 2 lead to moving photons harvesting nature towards visible region. The band gap increases in SiO 2 /TiO 2 composites as for TiO 2 composition is rapport well with the aforementioned redshift value. Out of all samples the low recombination rate is procured in 50 wt% SiO 2 /50 wt% TiO 2 composite sample. The separated ~ 100–200 nm sized TiO 2 nanoparticle and aggregated tiny SiO 2 nanoparticles availability in composite sample is authentically substantiated from electron microscopic studies. The presence of Si, O and Ti elements in composite samples probed by XPS. Following the fundamental studies, the photocatalytic degradation ability of the as-prepared samples has been scrutinized against the degradation of Rh B dye in which the pronounced photocatalytic degradation eciency 93.7% is successfully achieved on 50 wt% SiO 2 /50 wt% TiO 2 nanocomposite photocatalyst. the 50 wt% SiO 2 /50 wt% TiO 2 composite sample deliver good visible light absorption and separation of charge carrier as for UV-visible and PL analyses. The enhanced photocatalytic degradation activity observed in 50 wt% SiO 2 /50 wt% TiO 2 nanocomposite sample due to improved light absorption, charge carrier separation by changing of electronic structure. The scavenging investigation shows that the *OH radicals contributed more while the degradation of Rh B dye by photocatalytic removal.


Introduction
From last decade wastewater from various coloring/dying industries is a signi cant ecological issue, particularly in the developing countries. It is account for that approximately 1-20 % of production of the colors/dyes is directly releasing into natural surface water as material e uents [1,2]. The angst is that these could be poisonous to microorganisms, oceanic life, and individuals and being a grim bother for both climate and living being [3]. Therefore, globally water pollution turns into a paramount issue in recently due to ~ 70% of the colors/dyes from the various enterprises straightforwardly mixed with the naturally available water bodies as above said [4]. The basic coloring industries speci cally the material industry is widely utilizing textile dyes to coloring their items which are photo catalytically stable and intractable to chemical oxidation process [5][6]. Since this industry likewise utilizes considerable measure of water in their cycles to frame profoundly coloring, the most part of it has been dangerous impact to our biological system because the existence of synthetic compounds [7]. Typically, the traditional physical techniques such as reverse osmosis, ultra ltration, ion exchange on arti cial adsorbent resins, adsorption on the activated carbon materials and coagulation by chemical agents are widely used for the elimination of dye pollutants [8]. These approaches are merely successful in transferring organic mixtures from water phase to another phase, here the secondary e uence formation is inevitable. On the other hand, these methods will need a further step to take care of solid-wastes and renewal of the adsorbent which will further unnecessarily upsurge the cost to the method. So, it is imperative to implement another approach to eliminate the pollutants prior to releasing them into the water bodies [9].
The different traditional methods presently applied in the evacuation of hued e uents in industrial water.
Notwithstanding these could be a classical approach, the prompt entire elimination of the colors is still elusive [10,11]. Ongoing studies have been dedicated to the utilization of photocatalysis in the expulsion of colors from wastewaters, especially, due to the capacity of this strategy to thoroughly degrade the objective toxins [12,13]. TiO 2 is a semiconductor with several unique traits such as non-toxicity, acidic and/or basic chemical stability, long-term reliability, and high oxidation power [14]. TiO 2 NPs have been addressing environmental problems as an effective photocatalyst for more than two decades by removing organic pollutants into H 2 O and CO 2 in water. As TiO 2 irradiated by photons during dye degradation process it decomposes the water molecules into hydroxyl radicals (*OH), result in degrading of water's organic pollutants [15]. Despite TiO 2 is of a superb photocatalyst, it has a limited photocatalytic activity due to its wide discontinuous energy region in E-K space provides easy and rapid charge carrier separation.
The green synthesis of nanoparticles is an economical, eco-friendly and simple method for preparing metal oxide nanoparticles under mild conditions. The combustion of leaves produces considerable quantities of ultra ne/nanoparticles formed largely by mineral transformation through the high temperature ignition method [16]. Recently research on silica has intensive on the preparation of nanoparticles and this can be derived from either organic chemicals or biomass [17]. Relatively, the usage of biomass is low when compared to chemical precursors due to lack of an assessment and valuation on the reputation of silica from biomass. The natural resources-based silica nanoparticles have gained decisive attention in the eld of materials science due to its eminent properties such as large availability, bioactivity, eco-friendliness and cost effectiveness. It is knowing that biomass is an alternate source for commonly used organic precursors. However, the use of biogenic based silica nanoparticles for supporting SiO 2 has been reported by elsewhere [18] .
Silica derived from natural plants is one the attractive routes because it is a green synthesis and huge source availability around [19]. Rice husks, sugarcane bagasse, peanut shells and agricultural materials have been widely used till date for deriving silica materials [19,20]. Just two years back, I. Fatima et al., found that bamboo leaves are also very suitable attractive leaves for extracting SiO 2 material [19] According to our knowledge, it has never seen before that use of Arundo donax L. leaf to produce SiO 2 material. Here it is successfully done for the rst time and obtained SiO 2 is composited with TiO 2 in order to nd its photocatalytic performance.
So, in this work rst SiO 2 derived from Arundo donax L. ash, TiO 2 synthesized by simple precipitation method and prepared different compositions of SiO 2 and TiO 2 (25/75, 50/50, 75/25 in wt%) by wet impregnation method. The photophysical and electronic structure properties of samples were investigated continuously using following instrumental analyses XRD, FTIR, UV, PL, SEM, EDX and mapping, HRTEM and XPS and their properties will be comprehensively described in the upcoming result and discussion's part along with displaying obtained spectra and images. In the end, the photocatalytic degradation performance against the Rh B dye molecules using SiO 2 nanoparticles and TiO 2 semiconductors and their three different composition samples was explored. From this, it has found that 50 wt% SiO 2 /50 wt% TiO 2 nanocomposite photocatalyst shows higher photocatalytic degradation activity than other composition ratios as well as TiO 2 . 10 g of calcined ash transferred into 250 ml conical ask which already has 2.5 N of sodium hydroxide (NaOH) solution. This reaction solution re uxed at 60°C for 3 h under identical stirring speed. The silica which presents in ash was dissolved and converted into sodium silicate.

Experimental
The resulting solution was cooled and ltered to get sodium silicate solution. Ash residues was discarded. The sodium silicate solution was acidi ed using 2 N HCl, here the gel silica formation was formed particularly at around 10-11 pH value. Further, in order to achieve ne particle solution, HCl was added drop by drop into above reaction solution until pH reach 2. Continuously, the ammonia solution was added in the above reaction mixture to increase the pH to 8-9. Na 2 SiO 3 +2HCl → SiO 2 +2NaCl+H 2 O Then prepared silica gel was left for 48 h for getting age. This resultant solution was washed using look warm DD water to remove the impurities. Finally, the silica particles were dried in oven at 60 ° C for 24 h to obtain impurity free silica material.

Preparation of TiO 2 nanoparticles
The TiO 2 NPs was prepared by precipitation method. For that, rst 0.2 mole of titanium (IV) isopropoxide and NaOH were dissolved separately in 25 ml of DD water under continuous stirring. These separated solutions were mixed together gently and continued stirring for 240 min with constant stirring rate. The obtained solution was let it be for 24 h to get age. This solution was centrifuged and washed with DD water several time and then dried at 80°C to obtain the TiO 2 NPs. This yield product was calcined at 600°C for 3 h eventually.

Preparation of different weight percentage of SiO 2 /TiO 2 nanocomposite
The various weight ratios of SiO 2 -TiO 2 nanocomposites (25:75, 50:50, and 75:25) were prepared through wet impregnation method. These different weight percentages of SiO 2 and TiO 2 nanoparticles are combined into a 30 ml ethanol solution. Following that, the solution was stirred at 50°C for 2 h. After that, the obtained reaction solution was kept at 80°C for 12 h.

Characterization
The structural, morphological and optical behaviors of the as-prepared samples were thoroughly examined through various instrumental analysis. The X-ray diffraction (XRD) study was used to found the crystalline nature and phase purity of SiO 2 , TiO 2 and SiO 2 /TiO 2 samples using Riguku smart lab diffractometer in with Cu Kα radiation used (λ = 1.54046 Å) and recorded data in the 2θ range of 10 to 80°. A Fourier transform infrared spectroscopy (FTIR) was obtained using a Perin Elmer spectrophotometer in the range of 4000 to 400 cm − 1 for studying functional groups which avail in our samples. To know morphology and elemental composition ratio in our prepared samples, Carl Zeiss Supra 55 eld emission scanning electron spectroscopies (FESEM) used. Further, the precise morphology of highly photocatalytic sample was investigated by Tecnai T20 high resolution transmission electron microscopy (HR-TEM). The chemical composition was veri ed by a Thermo Fisher Scienti c ESCALAB Xi + X ray Photo Spectroscopy (XPS). The optical absorption spectra were obtained by Ocean Optics USB4000 photo spectrometer. Photoluminescence property of the prepared samples was obtained by Perkin Elmer LS45 photo spectrometer. Finally, UV-visible absorption spectrums of the Rh B solutions were obtained by Epoch-2 microplate reader.

Evaluation of photodegradation over Rh B dye
The photocatalytic degradation performance of the prepared samples was evaluated against Rh B dye molecule under the visible light irradiation. For that 0.01 g of the prepared samples was rst dispersed in 100 ml of Rh B dye molecule (25 ppm) separately. Before light irradiation, the reaction solution stirred and kept in dark room for 1 hr. to attain ad/desorption equilibrium. During the irradiation using a xenon lamp (300 W), 1 ml of Rh B solution was collected each 25 min of time break and centrifuged at 6000 rpm to eliminate the photocatalyst particles. The residual absorption changes were measured by UV-visible spectrophotometer. To investigate the stability of the e cient photocatalyst, the degradation experiments were repeated for 5 times in the identical reaction conditions. Finally, the photocatalyst particles were separated from the dye solution by centrifugation process and dried in an oven. These dry photocatalyst particles were further used for repetitive cycle. To nd the role of main active species in the degradation process different scavengers were used in the catalytic process. The triethanolamine (TEOA), benzoquinone (BQ) and isopropyl alcohol (IPA) were used as h + , *O 2 − and *OH scavengers respectively. Figure 1. shows the XRD pattern of the SiO 2 , TiO 2 and SiO 2 /TiO 2 samples. In Fig. 1 (a)  This result is well matched with the anatase phase of body centered tetragonal structure of TiO 2 (19).

Results And Discussion
Here, the recorded diffraction patterns of SiO 2 and TiO 2 are have good harmony association with the previous reports [21]. The calculated d spacing values of (110) and (004)   The recorded UV-visible absorption spectra for SiO 2 , TiO 2 and SiO 2 /TiO 2 nanocomposite samples is given in Figure. 3 (a-e). In TiO 2 semiconductor, the 2p level of O 2valence band (VB) to 3d levels of Ti 4+ conduction band (CB) makes energy discontinues region in the crystal structure which allowing the absorption of photons. The absorption peak observed around 400 nm for TiO 2 indicates the wide band gap nature of the material. Here, the observed sharp absorption peak presumably indicates the good crystalline behavior which strongly con rmed through XRD.
Only the marginal shift in absorption edge of TiO 2 is observed after SiO 2 composition. The separate and magni ed absorption spectrum of SiO 2 shows an unclear absorption edge around ~ 293 nm. Generally, amorphous SiO 2 has absorption edge value is at far UV region [24]. The observed absorption edge value is also quietly agreed with the reports [25]. The reason could be more impurity states exist in the prepared SiO  It is well known that the photoluminescence (PL) spectroscopy has been widely used to examine the recombination rate of excited electron-hole pairs and existed defect states in semiconductors [7].
Generally, in wide band gap material the emission band at UV region is attributed to the discontinuous energy region which existed in the Brillouin zone boundary of material leads to exciton formation. Typically, this is called it as material characteristic peak. On the other hand, other emission bands which particularly seen in visible region are recognized as defect states. Figure 5 shows observed relative PL spectra of the prepared samples at an excitation wavelength of 320 nm. In the PL spectrum of pure SiO 2 , the emission band shows a strong, intensi ed and broad PL peak at 410 nm wavelength indicates the electrons and holes recombine process happen rapidly. The emission peak intensity of 50 % SiO 2 -50 % TiO 2 sample is weakened as compare to other samples strongly demonstrating that the recombination of photogenerated charge carriers is suppressed signi cantly. Further, the avail deep level emissions from TiO 2 and composite samples belongs to the oxygen, Ti vacancies and surface oxygen vacancies (SOVs).
In PL analysis, the eventual conclusion is that the composition of SiO 2 with TiO 2 lead to alter the electronic structure of the material and these changes make favorable photogenerated electron and hole separation which further would be effectively improving the photocatalytic performance when go for the photo degradation of dye molecule applications.
The morphologies of the as-prepared samples were evaluated by FESEM (Fig. 6) In Fig. 6 (a) and (b), the captured SiO 2 and TiO 2 nanoparticles images are shown. The as prepared SiO 2 samples show highly agglomerated tiny nanoparticles this may be due to cluster mechanism involved during synthesis as for identical synthesis conditions.
The TiO 2 nanoparticles are uniform, smooth with average particle size of 80-120 nm. After composite formation (50 wt% SiO 2 /50 wt% TiO2) the nanoparticles are highly agglomerate compared to the pristine TiO 2 sample. Similarly, no morphological changes were observed much more of TiO 2 and SiO 2 indicating the dispersion of SiO 2 and TiO 2 nanoparticles homogeneously in the composite sample. The high distribution of the as-prepared composite may improve its adsorption capacity as well as the active sites of the prepared sample.
The energy-dispersive X-ray spectroscopy (EDX) and mapping study of SiO 2 and 50 wt% SiO 2 / 50 wt% TiO 2 samples were shown in Fig. 7 and Fig. 8 respectively. The observed major elements are related to Si, Ti and O which con rms that presence of TiO 2 and SiO 2 in composition sample.
In order to probe an internal precise morphology, generally the transmission electron microscopy analysis is employed. It can be seen from Fig. 9 (a) and (b) is that tetragonal spherical like TiO 2 and sprinkle like SiO 2 morphologies of TEM images. These have well agreement with the early discussed FESEM analysis.
Further, the HRTEM image was taken on spherical like TiO 2 nanoparticle. This shows interplanar spacing value of 0.24 nm which corresponding to the 004-crystal plane. The polycrystalline nature of TiO 2 semiconductor is con rmed by SAED pattern.
XPS analysis was used to investigate the chemical bonds, exact composition, and oxidation state of the compounds. 50 wt% SiO 2 /50 wt% TiO 2 nanocomposite sample was subjected to XPS analysis and observed spectra are displayed in Fig. 10. The high-resolution spectra of prepared samples Fig. 10 (c-d) shows characteristic peaks of Ti 2p, Si 2p and O 1s indorsing the presence of Ti, Si and O shown in respectively. The deconvolution peak provides the necessary proof of the synergistic interface of the prepared element in the nanocomposite. Furthermore, the high-resolution spectra of Ti 2p shows two shake-up satellites located at 458. 3 Figure 11 demonstrates the photodegradation of Rh B in the certain irradiation time intervals and in the presence of aforementioned photocatalysis.
As shown in Fig. 11 (d), the intensity of the 554 nm absorption peaks decreased rapidly due to the decreases of Rh B chromophore under the UV light irradiation in the existence of our prepared photocatalyst almost disappeared after 150 min signi es that the degradation of Rh B dye molecules. It is noticed that the absorption of the dye decreases with increasing of irradiation time. Compared to pure TiO 2 and SiO 2 , the SiO 2 /TiO 2 nanocomposite samples show high decolorization e ciency of Rh B dye molecules and eventually the observed higher photocatalytic degradation e ciency is in 50 wt% SiO 2 /50 wt% TiO 2 nanocomposite photocatalyst which may be due to higher light absorption as well as photogenerated charge carriers' separation. This would be strongly con rmed in the early UV-Visible absorption and Photoluminescence spectra studies.
These photocatalytic reaction rates were found by the pseudo rst-order equation by monitoring the absorption of the dye molecules (Fig. 13). The pseudo rst-order rate constants (k obs ) for the photocatalytic degradation response of Rh B were found by using the plots of ln(C/C 0 ) against irradiation time (t), in which C and C 0 are the maximum absorptions of Rh B dye at a certain time and the initial time respectively, which were identi ed from the sequential absorbance variations in the UV-visible absorption spectra. In Fig. 13, all the calculated R 2 values were larger than 0.95, demonstrating that the data tted well with the straight lines (Table-1). Among all prepared photocatalysis, the 50 wt% SiO 2 /50 wt% TiO 2 composite photocatalysis exhibited the best photo-degradation e ciency. The recyclability is one of the furthermost features for real use of prepared photocatalysts materials. Hence, the recyclability of the as-prepared 50 wt% SiO 2 -50 wt% TiO 2 nanocomposite was investigated up to four successive cycle and the outcomes are shown in Fig. 14. In each cycle, a fresh Rh B solution was used to examine the stability of the photocatalyst material. The 50 wt% SiO 2 -50 wt% TiO 2 nanocomposite shows its stable photocatalytic action for the four successive runs. Subsequently four repeated cycles, we noticed only a minor change in the photocatalytic capability compared to the rst cycle. This observed loss in degradation e ciency may be due the drops of catalyst particles during the collecting process.
The photocatalytic performance of active species in the degradation process is needs to be recognize the better reaction mechanism of the 50 wt% SiO 2 /50 wt% TiO 2 nanocomposite photocatalytic sample.
Therefore, various scavengers were used to found the role of active species in the degradation. So, here we used benzoquinone (BQ), triethanolamine (TEOA), and isopropyl alcohol (IPA) as a *O 2 − , hole and *OH scavengers, respectively. The experimental results are displayed in Fig. 15. After the addition of the trapping agents to the photocatalysis reaction solution, the Rh B degradation e ciency is in the order of TEOA > BQ > IPA. This trapping experimental results suggest that the degradation of Rh B in the presence of 50 wt% SiO 2 /50 wt% TiO 2 composite photocatalysis was most interfered with the existence of IPA it shows that *OH radicals is the primary active species in the dye degradation process. Also, the adding of BQ scavenges *O 2 shows notable decrease in the degradation e ciency it shows that the superoxide radicals also play signi cant part in the degradation. The radical trapping outcomes reiterate the role of active species in the photocatalytic degradation of Rh B was in the order of *OH>*O 2 > h + . This result eventually tells us apart from doping, manipulating different morphologies in TiO 2 semiconductor, the composites formation along with amorphous materials is also a promising route to nd out better photocatalytic active materials.

Conclusion
In summary, the SiO 2 , TiO 2 and their different compositions (SiO 2 /TiO 2 ) in wt% ratios were successfully synthesized and prepared in which amorphous SiO 2 derived rst time from Arundo donax L. ash. The result of the structural and morphological analyses demonstrate that strong interactions exist between TiO 2 and SiO 2 when make composition. It would strongly nd that formation of the 50 wt% SiO 2 /50 wt%