Synthesis, characterization of porphyrin and CdS modified spherical shaped SiO2 for Reactive Red 120 degradation under direct sunlight

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Introduction
Semiconductor nanoparticles of different materials and shapes are investigated according to their viz., size dependent electronic and photocatalytic properties. One may understand that the electronic, magnetic and optical characteristic one-dimensional nanostructures such as nanowires, nanotubes and nanofibers are prerequisite as they have varying different physico-chemical properties compared to the bulk material. CdS is an important II-VI semiconductor material with a wide band gap (Energy 2.53 eV), marvellous features in luminescence and photochemistry. Mixing SiO 2 with CdS nanoparticles is an apt candidate method to prevent decomposes, and improving luminescence performance and further functionalization [1][2][3][4][5][6][7][8][9][10][11][12][13]. The dual functional microencapsulated phase change materials modified CdS/SiO 2 dual-layered shell effectively utilized for degradation of methylene blue (MB) under natural sunlight illumination [14]. The n-eicosane@SiO 2 /CdS microcapsules revealed an excellent solar photocatalytic activity to MB dye degradation under the natural sunlight, and approximately 90 percent degradation was observed at 240 min. Moreover, the same nanocomposite exhibited good performance in solar thermal energy storage, thermoregulation and phase-change reliability. A sequence of magnetic Fe 3 O 4 /SiO 2 /CdS nanocomposites has been synthesized by adopting a facile and convenient method by Eskandari and Kazemi [15]. Initially, Fe 3 O 4 /SiO 2 was prepared followed by surface modifications with 3-(mercaptopropyl) trimethoxysilane (MPTMS). The surface modified Fe 3 O 4 /SiO 2 was coated with CdS and effectively utilized for Photocatalytic reduction of nitrobenzene. In another study, Byungwoo Park's group examined Surface-plasmon-enhanced photoluminescence of CdS nanoparticles with Au/SiO 2 nanocomposites [5].
Porphyrins have an acute absorption in the 400-450 nm regions (Soret band) and weak absorption in the 500-700 nm regions (Q bands). More importantly, and when the Soret and Q bands are red-shifted, that improves the overlap between the absorption and solar energy distribution on the earth [16][17][18][19][20][21]. Porphyrin compounds play a pivotal role in the domains viz., photosynthesis, transport and storage of oxygen, and electron transfer. From the point of view, we have prepared two porphyrins with the reported procedures [22], and used to modify SiO 2 along with CdS. The structure of prepared porphyrins such as meso-tetra-phenyl-porphyrin (TPP) and 5,10,15,20-meso-tetra-(para-amino)-phenyl-porphyrin (TPAPP) are given in Fig. 1.
People working for water purification and environment are concerned for our water resources constituting both groundwater and polluted wastewater. Now a day, with increasing revolution in science and technology there is a high demand in industries for newer chemicals to be used to control water pollution.
Regarding photocatalytic degradation of toxic dyes, it has brought a new revolution to reduce water pollution [23][24][25][26]. Recently, Lotfi et al., [27] reported PAMAM/SiO 2 (polyamidoamine/ silica) nanohybrid as new and appropriate adsorbent for pharmaceuticals. It can be attributed that the contaminant removal was occurred in homogenous surface cites of the PAMAM/SiO 2 . Mahy et al., [28] reported hydrodechlorination and absolute degradation of chlorinated compounds with the concerted action of Pd/SiO 2 and Fe/SiO 2 catalysts. The catalytic activity of the dual catalysts has been examined by taking hydrodechlorination of 2,4,6-trichlorophenol (TCP) in water. The complete degradation of TCP was followed by two different mechanisms viz., hydrodechlorination with Pd/SiO 2 catalyst and photo-Fenton phenol degradation with Fe/SiO 2 catalyst. Since SiO 2 is an insulator, it is not directly used as a photocatalyst for degradation experiments until mixed with semiconducting materials. In the process of continuous exploration on environmental remediation [20,21], the present work is focused on the modification of SiO 2 with CdS semiconductor and two different porphyrins.
The prepared composites effectively utilized for Reactive Red 120 (RR 120) degradation under direct sunlight.

Preparation of SiO 2
The sol-gel method was adopted for the synthesis of SiO 2 . About 4 mL of water and 6 in 90 mL of 2-propanol under vigorous stirring. The formed colloidal white precipitate was stirred continuously over night at room temperature. The gel attained was filtered, washed with DI (distilled water) and ethanol followed by a process of drying in an air over at 90 ο C for 6 h .
Further, the sample was calcinated at 450 ο C in a muffle furnace for 6 h.

Surface modification of CdS/SiO 2
About 500 mg of CdS/SiO 2 composite was dispersed in 20 mL of ethanol under stirring followed by the addition of 500 mg of Glycidoxypropyltrimethoxy silane (GPTMS), is a an exemplary coupling agent and it is able to chemically bond in both metal oxides and amino terminated organic molecules [21]. Then, it was stirred for 6 h at 50 ᴼC, the GPTMS modified CdS/SiO 2 composite was filtered, washed several times with ethanol, to absolutely eradicate the unreacted GPTMS, and made dry in an air oven at 80 ᴼC for 4 h.

Photodegradation experiments
Photocatalytic activiy of the as-obtained composites was assessed by the deterioration of RR 120 azo dye in direct sunlight. Solar photocatalytic degradation experiments were performed under similar conditions on sunny days between 11.30 a.m. and 2.30 p.m. It was found that the solar intensity remained almost constant during the experiments [29]. In each examination, the reaction suspension consisting of 10 mg of the composites in 10 ppm of RR 120 solution (100 mL), has been magnetically stirred in the darkness for 30 min to ascertain the adsorption/desorption equilibrium between the composite particles and RR 120 dye. Then the suspension was brought out to the direct sunlight irradiation, and aliquots (3 mL) were taken at given time intervals and centrifuged to eradicate composite particles. The supernatants were analyzed by registering the absorbance variations of an absorption band (533 nm) in the UV-Visible spectrum of RR 120.

Characterization of the prepared composites
The prepared composites CdS/SiO 2 , CdS/SiO 2 -TPP & CdS/SiO 2 -TPAPP were characterized by X-ray diffractometry (XRD), Raman, Fourier transform infrared spectroscopy (FT-IR), filed emission scanning electron microscopy (FE-SEM) along with energy dispersive spectra (EDS), diffuse reflectance spectroscopy (DRS) and photoluminescence spectra (PL).  (Fig. 3), no peak of SiO 2 was detected, suggesting that SiO 2 was amorphous. Although in our previous work [21], the XRD pattern of SiO 2 /ZnO contains both SiO 2 and ZnO peaks, the broad peak of amorphous SiO 2 between degrees 19 and 30 may be merged with CdS peaks appeared at the same region in SiO 2 /CdS composites (Fig. 3). The peaks appeared at 26.28, 43.57 and 51.83 degrees corresponds to (111), (220) and (311) diffraction peaks of CdS [4]. The crystallinity including homogeneity and surface conditions of semiconducting materials could be studied using Raman spectroscopy.
Crystalline materials give sharp peaks whereas very broad peaks can be observed for amorphous or polycrystalline material. Raman spectra of the prepared materials are exhibited in Fig. 4. The intense sharp peak observed at around 300 cm −1 and less intense peak at 600 cm −1 is ascribed to the CdS fundamental longitudinal optical mode phonon (1LO) and its overtone (2LO), respectively [30]. There was no peak observed for SiO 2 in SiO 2 /CdS composites, although SiO 2 gave most noticeable bands at 436, 716, 1265, 1392, 1665, 1704 and 1760 cm -1 [31], this can be caused by amorphous nature of SiO 2 and also very less intense bands only observed for pure SiO 2 [31]. As shown in Figs. 4b and 4c, the Raman spectra provide additional piece of evidence for the strong interaction between the porphyrins (TPP and TPAPP) and the surface of SiO 2 /CdS composite, as indicated by the distinctive differences between the Raman spectra of the free FT-IR spectroscopy is used to examine the chemical bonds and to identify the organic species present in the compounds or materials. The presence of chemical bonds in prepared composites is studied by FT-IR spectrometer in the range of 400 cm −1 -4000 cm −1 at room temperature. Fig. 5 shows the FT-IR spectra of (a) CdS/SiO 2 , (b) CdS/SiO 2 -TPP and (c) CdS/SiO 2 -TPAPP. The bands appeared between 800 and 1110 cm -1 corresponds to Si-O-Si bonds [21]. A peak observed between 430 and 490 cm −1 is ascribed to the presence of Cd-S bond, peaks observed this region is assigned to Metal-Sulfur bond [33]. A peak observed between 1600 to 1660 cm −1 is ascribed to H-O stretching vibration that might have been from adsorbed water during preparation or from the atmosphere. The GPTMS altered samples (Fig. 5b and c) exhibit the peaks in the range 2850-2990 cm - [34,35]. In porphyrins modified samples, the emission wavelength does not vary with dopants ie., porphyrins as may be seen from Fig. 10. This implies that the energy level of defect states/sulfur vacancies (in CdS) related to the valance band nearly keeps unchanged. It is also clear from Fig. 10 that luminescence intensity is slightly enhanced on doping with porphyrins. This may be due results in enhanced surface defects result in soar up in the PL intensity of porphyrins modified composites. This indicates that porphyrins seem to be acting as sensitizing agent, and thus enhances the radiative recombination processes. Hence, the fluorescence efficiencies of porphyrins modified samples are prominent than that of the pristine CdS/SiO 2 [36].

Photocatalytic degradation of RR 120 dye
The research in the area of water purification technology and environmental remediation are concerned for our water resources constituting both groundwater and polluted wastewaters.
Nowadays, with increasing revolution in science and technology there is a high demand in industries for newer chemicals to be used to control water pollution. In the point of view, the prepared composites are effectively utilized for an azo dye RR 120 degradation under direct solar light. It was already reported that RR 120 dye with stands to self-photolysis under direct solar light [37]. RR 120 underwent 60.7 and 67% of degradation occurred with CdS/SiO 2 -TPP and CdS/SiO 2 -TPAPP, respectively at 30 min irradiation (Fig. 11). Under the same conditions, unmodified CdS/SiO 2 gave only 50% of degradation. Both porphyrins modified composites were noticed to be very efficient toward RR 120 degradation when compared with pristine CdS/SiO 2 . The order of activity is CdS/SiO 2 -TPAPP > CdS/SiO 2 -TPP > CdS/SiO 2 .
A mechanism for the photocatalytic degradation of RR 120 with porphyrins modified composites under solar lights is proposed (Fig. 12). When the porphyrins modified composites are irradiated by solar light, the porphyrins (TPP & TPAPP) molecules can be induced along with CdS [20,21] TPAPP can oxidize RR 120 to its radical cation either directly or through a primarily formed • OH produced by the oxidation of ubiquitous water. The same type of mechanism was proposed the degradation of dye by porphyrins modified semiconductors [21,38,39].

Conclusions
The synthesis and structural confirmation of porphyrins modified CdS/SiO 2 composites and their photoactivity was discussed. These prepared composites were evaluated in the deterioration of RR 120 dye in water under solar light irradiation. The method adopted for this synthesis is simple, eco-friendly and no individual tool is required. FT-IR spectra reveal that the band between 430 and 490 cm −1 is allotted to Cd-S (Metal-Sulfur) bond.

Research Highlights
Sphere shaped SiO 2 was prepared by a simple sol-gel method without any template Photoluminescence intensity of CdS/SiO 2 is slightly enhanced on doping with porphyrins Both porphyrins modified composites were found to be very efficient toward RR 120 degradation when compared with pristine CdS/SiO 2

Impact Statement
Initially, sphere shaped SiO 2 was prepared by a simple sol-gel method without any template. Later, the sphere like SiO 2 was modified with cadmium sulfide (CdS) semiconductor and porphyrins. The formation of composites (CdS/SiO 2 -porphyrins) was confirmed by different characterization techniques such as XRD, Raman, FT-IR, FE-SEM, EDS, DRS and PL measurements. The photocatalytic activity of CdS/SiO 2 was slightly influenced by porphyrins such as meso-tetra-phenyl-porphyrin (TPP) and 5,10,15,20-mesotetra-(para-amino)-phenyl-porphyrin(TPAPP). Regarding photocatalytic degradation of toxic dyes, it has brought a new revolution to reduce water pollution. Since SiO 2 is an insulator, it is not directly used as a photocatalyst for degradation experiments until mixed with semiconducting materials. In the process of continuous exploration on environmental remediation, the present work is focused on the modification of SiO 2 with CdS semiconductor and two different porphyrins. The prepared composites effectively utilized for Reactive Red 120 (RR 120) degradation under direct sunlight. So, we feel that this work may be suitable for this journal.