Facile preparation of new nanohybrids for enhancing photocatalytic activity toward removal of organic dyes under visible light irradiation

Journal of Physics and Chemistry of Solids 140 (2020) 109271

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Facile preparation of new nanohybrids for enhancing photocatalytic activity toward removal of organic dyes under visible light irradiation Saviz Zarrin, Felora Heshmatpour * Department of Chemistry, Faculty of Science, K.N. Toosi University of Technology, P.O. Box 16315-1618, Tehran, 15418, Iran

A R T I C L E I N F O

A B S T R A C T

Keywords: TiO2/Nb2O5/SnO2/RGO TiO2/Ceramic/SnO2/RGO Reduced graphene oxide Visible light irradiation Organic pollutant

In the present investigation, we have designed highly active titanium dioxide nanohybrids modified by various additives such as niobium oxide (Nb2O5), tin (IV) oxide (SnO2), reduced graphene oxide (RGO) and ceramic (SiO2/Fe3O4/ZrO2) by combining the hydrothermal and sol-gel methods. The prepared samples i.e. TiO2/Nb2O5/ SnO2/RGO, TiO2/Ceramic/SnO2/RGO, TiO2/Nb2O5/SnO2 and TiO2/Ceramic/SnO2 were identified by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), transmission electron microscopy (including high-resolution imaging-HRTEM), Brunauer–Emmett–Teller (BET), ultra­ violet–visible spectroscopy (UV–Vis), Diffuse reflectance spectroscopy (DRS) and fourier-transform infrared spectroscopy (FT-IR). Moreover, the photocatalytic performances of the newly designed nanohybrids were studied toward the photodegradation of crystal violet (CV) and methyl orange (MO) dyes under visible light illumination and the results were compared with those of previously reported TiO2/Nb2O5, TiO2/Ceramic and TiO2 nanoparticles. In this regard, the removal of chemical oxygen demand (COD) obeys the photodegradation. The TiO2/Nb2O5/SnO2/RGO sample displays a photocatalytic performance significantly larger than those of the other samples and also effectively decreases the recombination rate of electrons and holes.

1. Introduction In the past few decades, along with the societal development, the environmental hazards caused by industrial contaminants have been more serious and important. Photocatalytic degradation of organic pollutants in water or air is one of the promising methods to prevent the proliferation of industrial wastes. Organic dyes do to their stability and large degree of organic present in them, pose sever ecological difficulties by depleting the dissolved oxygen content of water and releasing toxic materials that endanger the aquatic life [1]. Numerous methods such as filtration, sedimentation, adsorption and photocatalysis are used for the removal of these contaminated chemicals. Photocatalytic degradation, in which the organic pollutants and dyes are degraded through photo­ catalytic oxidation and reduction reactions in the presence of a semi­ conductor as a photocatalyst, is one of the most promising processes used for water purification due to their widespread applications [2–9]. Photocatalytic efficiency of semiconductor involves interactions of the photoexcited charge carriers leading to the generation of reactive oxy­ gen species which result in degradation of toxic materials in water. When a photocatalyst is excited with sun light, absorption of photons

with energy equal or higher than the band gap energy levels leads to the formation of electron-hole pairs. These photoexcited charge carriers react with the adsorbed water molecules and oxygen molecules leading to the creation of superoxide radicals (�O2 ) and hydroxyl (�OH) radi­ cals, respectively. These reactive oxygen species interact with the adsorbed toxic materials and pollutants in water resulting in their degradation [7]. Among many examined photocatalysts, titanium di­ oxide (TiO2) as a semiconductor has widely been employed as a pho­ tocatalyst due to its exceptional properties such as interesting physio-chemical stability, corrosion resistance, environmental friendli­ ness, low cost, non-toxicity and high photocatalytic efficiency [1,10]. The photocatalytic process by TiO2 has been applied in air purification [11], hydrogen production [12], wastewater behavior [13] and etc. However, in spite of the mentioned advantages, the application of pure TiO2 nanoparticles, similar to other metal oxide semiconductors, is restricted because of its large bandgap, fast recombination of electron-hole, easy agglomeration and low adsorption capability [14]. These limitations remarkably decrease the photocatalytic activities and consequently industrial applications of the pure TiO2 [15]. As the pho­ tocatalytic activity depends on the effective transfer and separation of

* Corresponding author. E-mail address: [email protected] (F. Heshmatpour). https://doi.org/10.1016/j.jpcs.2019.109271 Received 24 August 2019; Received in revised form 27 October 2019; Accepted 15 November 2019 Available online 23 January 2020 0022-3697/© 2020 Elsevier Ltd. All rights reserved.

S. Zarrin and F. Heshmatpour

Journal of Physics and Chemistry of Solids 140 (2020) 109271

photogenerated charge carriers, it is important to inhibit their recom­ bination for achieving higher activity [5]. To enhance the photocatalytic efficiency of the TiO2, numerous approaches have been developed like doping with metal and nonmetal ions [16,17], and immobilization of TiO2 onto natural and porous materials (e.g. sepiolite [18], montmo­ rillonite [19], kaolin [20], attapulgite [21] and diatomite [22]). In this way, TiO2 has been mixed with the other oxides [23], or combined with some new carbon-based nanomaterials having exclusive structures like graphite oxide [24], carbon nanotubes [25] and graphene [26]. To increase the photocatalytic activity, the heterojunction oxide structures (e.g. TiO2/WO3 [27], TiO2/ZnO [28], TiO2/Nb2O5 [23] and TiO2/SnO2 [29]) have been further employed for photodegradation of organic pollutants which efficiently reduce the recombination of pho­ togenerated charge carriers. Qin et al. [30]. prepared CdS/TiO2 heter­ ojunction nanofibers via a facile electrospinning method and examined their photocatalytic performances in selective oxidation of “benzyl alcohol to benzaldehyde” under visible light irradiation. They showed that the presence of an appropriate amount of CdS makes a high pho­ tocatalytic activity which arises from decrease in the recombination of electron-hole pairs. In recent years, many works has been focused on the use of niobium oxide and n-type semiconductors as photocatalyst [31]. Ferraz et al. [32]. prepared Hexagonal-Nb2O5/Anatase-TiO2 nanohybrid for photo­ degradation of “methylene blue” under UV-light, Visible-light and Visible-light plus H2O2 irradiation. They concluded that the presence of Nb2O5 impurity not only prevents the Anatase-to-Rutile phase trans­ formation in TiO2, but also significantly enhances the photocatalytic performance of the pure TiO2. Ceramic nanostructures have been the most explored materials for photocatalytic degradation of organic dyes due to their unique semi­ conducting properties and their enhanced chemical activities [33]. Ceramic is a porous mineral with the main component of amorphous SiO2 [34]. Originally, it is not pure and contains many mineral impu­ rities [35]. Ceramics seem to be attractive candidates due to their low density, highly porous structure, high specific surface area, high adsorption ability and chemical stability [36–38]. Using a modified sol-gel method, Wang et al. [34]. synthesized TiO2/Ceramic nano­ hybrids which were applied in photodegradation of “Rhodamine B00 under UV light. The nanohybrids showed high photocatalytic activities compared with pure TiO2, being probably attributed to the presence of ceramic. Besides, Padmanabhan et al. [39]. provided TiO2/ceramic nanohybrids annealed at different temperatures by thermal hydrolysis of titanium oxysulfate in aqueous suspension of the ceramics. They also investigated their photocatalytic properties toward the photo­ degradation of “Rhodamine B00 dye. The sample calcined at 500 � C showed the highest photodegradation efficiency due to its large specific surface area, mesoporous structure and small crystallite size. The ob­ tained results confirmed that immobilization of TiO2 on ceramic sub­ strate makes an increase in the photocatalytic activity. Modification of TiO2 by SnO2 as a semiconductor is an effective method in order to enhance the photodegradation efficiency, using the transfer and separation of photo-produced carriers between two semi­ conductors with distinct energy gaps. Ahn et al. [40]. initially prepared SnO2 hollow nanofibers by electrospinning method and then applied them as substrates to grow TiO2 nanorods by a hydrothermal process. The produced samples were used as photocatalyst for photodegradation of “Reactive Black 5 (RB5) and Rhodamine B00 dyes. The nanofibers exhibited excellent activity because of their high specific surface area and appropriate hetero-nanostructure band behavior. Moreover, their high photocatalytic activities can be related to the increased charge mobility, decreased electron-hole recombination and increased effi­ ciency for charge separation. Among the various carbon-based nanomaterials, graphene has a special dignity. Due to its superior thermal/chemical stability, excellent electronic conductive properties and high specific surface area, gra­ phene is frequently used in photodegradation of organic pollutants [41].

It is widely used as an electron transfer medium which is able to inhibit the recombination of photo-generated electrons and holes through increasing the lifetime values of photo-generated electrons [42]. More­ over, due to its large specific surface area, graphene can be a supporting material to enhance the photocatalytic activity of TiO2. Mixing the TiO2 with graphene or reduced graphene oxide (RGO) is one of the effective methods to improve the photocatalytic activities of nanostructured TiO2. The TiO2/graphene nanohybrid exhibits a lower charge recom­ bination rate in comparison to the pure TiO2. Sohail et al. [43]. reported synthesis of TiO2/reduced graphene oxide nanohybrid and investigated its photocatalytic properties for photodegradation of “methylene blue”. Their results indicated that reduced graphene oxide particles are promising materials for dispersion of TiO2 nanoparticles. No cluster formation or agglomeration was detected for TiO2 in the nanohybrids. Herein, the new TiO2/Nb2O5/SnO2/RGO, TiO2/Ceramic/SnO2/ RGO, TiO2/Nb2O5/SnO2 and TiO2/Ceramic/SnO2 nanohybrids have been produced by combining the facile hydrothermal and sol–gel methods. The surface area, particle size, morphology, structure, phase composition and photocatalytic ability of the nanohybrids have also been studied. The small amount of each sample was employed to examine the photocatalytic activity using the photodegradation of crystal violet (CV) and methyl orange (MO) organic dyes under visible light irradiation. Afterwards, the obtained results were compared with those of previously reported TiO2/Nb2O5, TiO2/Ceramic and TiO2 nanoparticles. 2. Experimental section 2.1. Materials Urea (CO(NH2)2), sulfuric acid (H2SO4), Potassium persulfate (K2S2O8), Sodium hydroxide (NaOH) and Phosphorus pentoxide (P2O5) were purchased from Sigma-Aldrich. Graphite powder (99.95%, 325 mesh) was purchased from Alfa Aesar. The precursor complexes of ti­ tanium (IV) ethoxide (Ti(OC2H5)4), titanium sulfate [Ti(SO4)2], niobium ethoxide (Nb2(OC2H5)10), Tin(IV) chloride pentahydrate (SnCl4.5H2O), tetraethyl orthosilicate (TEOS)(Si(OC₂H₅)₄) and also Zirconium (IV) acetylacetonate (Zr(C₅H₇O₂)₄) were purchased from Merck. Also, the other reagents, including hydrochloric acid (HCl), hexane (C6H14), ammonium hydroxide(NH4OH), ethanol (C2H5OH), crystal violet (CV) and methyl orange (MO) were also purchased from Merck. Polyethylene glycol (PEG, with average molecular weights of 200) was used in the reaction. 2.2. Synthesis of pure TiO2 At first, 35 mL of Ti(SO4)2 solution with the concentration of 0.5 mol l 1 was provided. Then, resulting 0.2 g (3.3 mmol) of CO(NH2)2 with the concentration of 1 mol l 1 and 7 mL of PEG (20%) were added to above solution. The solution was stirred for 2 h at room temperature. Then, the suspension was placed in a 70 mL Teflon-sealed autoclave and main­ tained at 120 � C for 2 h and then the precipitates were centrifugally separated and washed with deionized water three times. The obtained white powder was dried at room temperature. 2.3. Synthesis of Fe3O4 nanoparticles At first, 0.85 mL HCl, 25 mL water, 5.20 g FeCl3.6H2O, and 3.83 g FeCl2.4H2O were mixed in a beker. Separately, 15 g NaOH was dissolved in 250 mL of deionized water. Then, the former solution was added dropwise into the NaOH solution, under continuous magnetic stirring. The obtained Fe3O4 nanoparticles were separated by a magnet (Neo­ dymium block magnet, Q-30-07-2.5-HE (Webcraft, GmbH)), washed with water for three times and dried at room temperature. Finally, the obtained brown powder was ground in a ceramic mortar. 2

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Journal of Physics and Chemistry of Solids 140 (2020) 109271

2.4. Synthesis of ceramic nanohybrids

2.11. Characterization of synthesized nanohybrids

0.25 g of Fe3O4 nanoparticles was ultrasonically dispersed in 40 mL anhydrous ethanol for 1 h. Then, 4.5 mL concentrated ammonium hy­ droxide solution was added to the above solution. Then, 0.8 mL TEOS and 0.25 g Zr(C5H7O2)4 were added under vigorous stirring. Afterwards, the resulting solution was stirred for 24 h. Finally, the precipitates were collected by centrifugation and washed with anhydrous ethanol for several times.

Details of the characterization of synthesized nanohybrids are pro­ vided in the supplementary materials [44,45]. 2.12. Photocatalytic experiments Crystal violet (CV) and methyl orange (MO) were chosen as organic dyes and pollutants to test the photocatalytic performances of the pre­ pared samples under visible light. A Xe arc 300 W Oriel with maximum radiation was used as source of the visible light. The filters of 10 cm IR water and UV cutoff (λ > 400 nm) were employed for the light beam in order to ensure that the samples are irradiated by visible light only. To avoid overheating, it was focused onto a cylindrical Pyrex photoreactor containing magnetic stirrer and air conditioner system. All the powders (3 mg) were dispersed in a 150 mL solution of CV and MO (0.02 g/L) and placed under visible light irradiation. The pH of initial dye solutions was adjusted between 5 and 10 by a 0.01 M buffer solution of ammo­ nium acetate and phosphate. The solutions were stirred in the darkness for 30 min at room temperature in order to achieve adsorptiondesorption equilibrium. Photodegradation of crystal violet (CV) and methyl orange (MO) were monitored using UV–vis spectroscopy using their absorption bands at 588 and 464 nm respectively. At certain times, 2.5 mL solution of the reaction including suspended photocatalyst was taken and centrifuged in order to remove the photocatalyst before the UV–Vis spectroscopy. Eventually, photocatalytic degradation percent­ age was measured using the following equation:

2.5. Synthesis of TiO2/Ceramic 0.2 g of ceramic was dispersed in a mixture of 70 mL hexane and 0.3 mL deionized water, followed by the addition of Ti(OC2H5)4 (0.6 mL) under ultrasonication for 1 h. The mixture was transferred into a Teflonlined autoclave and kept at 100 � C for 3 h. The precipitates were collected and washed with hexane for two times and then dried at room temperature. Finally, the dried powders were calcined at 800 � C for 3 h. 2.6. Synthesis of TiO2/Nb2O5 At first, 3 mL solution of Ti(OC2H5)4 and 0.3 mL of Nb2(OC2H5)10 were added to a beaker containing 14 mL absolute ethanol. Then, the solution was added dropwise to a mixed solution of 16 mL deionized water and 16 mL absolute ethanol under vigorous stirring at room temperature. Then, the gel suspension was directly transferred into a 70 mL Teflon-sealed autoclave and subjected to crystallization at 80 � C for 24 h. The suspension was then centrifuged for separation of the powder. The residue was washed with ethanol for three times in order to remove the hydrogen bonds and consequently minimize the aggregation. Finally, the obtained pale blue powders were dried at room temperature.

%R ¼ (C0-Ct)/C0 � 100 Where %R is photodegradation percentage, C0 and Ct are respectively the initial concentration and concentration at certain times [46].

2.7. Synthesis of TiO2/Ceramic/SnO2

3. Results and discussions

0.2 gr of TiO2/Ceramic were dispersed in a mixed solution of NH4OH (25 wt% solution, 15 mL) and distilled deionized water (40 mL). After homogenization, the solution of 0.124 gr of SnCl4.5H2O in 25 mL deionized water was added to the above solution. After 24 h stirring, the resulting precipitate was filtered, washed with water several times to remove the residue and impurities and dried at room temperature. Finally, the obtained powder was calcined at 500 � C for 1 h.

3.1. FT-IR studies Fourier Transmission Infrared Spectroscopy (FTIR) spectra were recorded to identify functional groups of the new nanohybrids (Fig. 1). Fig. 1A, B and C (curve a), being related to the pure TiO2 nanoparticles, show a broad band centered at 3473 cm 1 which is assigned to the stretching vibration of the hydroxyl group on the surface [47]. The band at 1644 cm 1 is attributed to the OH bending mode of adsorbed water. Moreover, the broad peak at 701 cm 1 is corresponded to the vibration Ti–O–Ti bands, indicating that the formation of metal oxygen bonds [48]. For the TiO2/Ceramic nanohybrid (Fig. 1A and C (curve b)), the bands at 772 cm 1 and 1100 cm 1 are respectively attributed to the symmetric and asymmetric stretching vibrations of Si–O–Si. However, the broad band at 953 cm 1 belongs to the Si–O–Ti vibration, indicating that the coating of TiO2 was successfully performed on the ceramic surface [49]. Some new bonds have been established between Ti, O, and Si elements in the nanohybrids [50]. Furthermore, the band at 680 cm 1 can be attributed to the Ti–O–Ti vibration [51,52]. The presence of water is confirmed by the appearance of its bending and stretching bands at 1641 cm 1 and 3408 cm 1, respectively. The characteristic peaks of TiO2/Nb2O5 (Fig. 1A and B (curve c)) were detected in the FTIR spectrum. The band at 695 cm 1 belongs to the Ti – O – Ti vibrations in TiO2 lattice [53,54]. Additionally, the band centered at 940 cm 1 is attributed to the Nb–O stretching vibrations of Nb-doped TiO2 [55]. Besides, the band at 766 cm 1, being present in the Nb-doped TiO2 samples spectra, can be corresponded to the Nb–O–Ti vibrations [55]. The band observed at 1635 cm 1 is attributed to the bending frequency of H–O–H of adsorbed water on TiO2 surface [56]. In the FT-IR spectrum of the TiO2/Ceramic/SnO2 sample (Fig. 1A and C (curve d)), the appearance of a band at 961 cm 1 confirms the presence of Ti–O–Si stretching band in the spectra [57,58]. Also, the stretching vibration at

2.8. Synthesis of TiO2/Nb2O5/SnO2 TiO2/Nb2O5/SnO2 was prepared similar to section 2.7; however, TiO2/Ceramic should be replaced by TiO2/Nb2O5. 2.9. Synthesis of TiO2/Ceramic/SnO2/RGO At first, graphene oxide (GO) was synthesized by the modified Hummers’ method [41]. 2 mg of GO was ultrasonically dissolved in a mixture of distilled water (20 mL) and absolute ethanol (10 mL) for 1 h. Then, 0.2 g TiO2/Ceramic was introduced to the GO solution and stirred for 2 h to obtain a homogeneous suspension. The suspension was then placed in a 70 mL teflon-sealed autoclave at 120 � C for 3 h in order to reduce the GO and deposit the TiO2/Ceramic onto the RGO substrate. The resulting precipitates were collected and washed with water several times to remove the residue and impurities. The obtained gray powder was dried at room temperature. 2.10. Synthesis of TiO2/Nb2O5/SnO2/RGO TiO2/Nb2O5/SnO2/RGO was prepared similar to the section of 2.9; however, TiO2/Ceramic/SnO2 should be replaced by TiO2/Nb2O5/SnO2. 3

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Journal of Physics and Chemistry of Solids 140 (2020) 109271

Fig. 1a. FT-IR spectra of the samples a) TiO2, b) TiO2/Ceramic, c) TiO2/Nb2O5, d) TiO2/Ceramic/SnO2, e) TiO2/Nb2O5/SnO2, f) TiO2/Ceramic/SnO2/RGO and g) TiO2/Nb2O5/SnO2/RGO.

Fig. 1b. FT-IR spectra of the samples a) TiO2, c) TiO2/Nb2O5, e) TiO2/Nb2O5/SnO2 and g) TiO2/Nb2O5/SnO2/RGO.

675 cm 1 represents the Ti–O–Sn bond in the nanohybrid [59]. Besides, the presence of a band appeared at 565 cm 1 can be related to the Ti–O–Ti bond [60,61]. The observed displacement for the bands at 701 cm 1 in pure TiO2 and 565 cm 1 in TiO2/Ceramic/SnO2 reveals that the

TiO2/SnO2 layer has been successfully inserted on the ceramic surface. In the FT-IR spectrum of TiO2/Nb2O5/SnO2 (Fig. 1A and B (curve e), the stretching vibrations between 400 cm 1 and 700 cm 1 demonstrate Sn–O–Sn, Ti–O–Ti and Ti–O–Sn bonds in the nanohybrid [62,63]. 4

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Journal of Physics and Chemistry of Solids 140 (2020) 109271

Fig. 1c. FT-IR spectra of the samples a) TiO2, b) TiO2/Ceramic, d) TiO2/Ceramic/SnO2 and f) TiO2/Ceramic/SnO2/RGO.

Moreover, in the FT-IR spectrum of TiO2/Nb2O5/SnO2, the band observed at 690 cm 1 is related to the Nb–O–Ti vibrations [64]. The characteristic peaks at1730 and 1220 cm 1 are corresponded to the – O groups on GO sheets [65]. The disappearance of such bands in C– TiO2/Ceramic/SnO2/RGO (Fig. 1A and C (curve f)) indicates that the GO is efficiently converted to RGO. Compared with the pure TiO2, the typical bands at 559 and 670 cm 1 are respectively attributed to the symmetric and asymmetric stretching modes of O–Sn–O. The band at 960 cm 1 is related to the stretching mode of Si–O–Ti. It shows the effective combination of the components i.e. TiO2, Ceramic, SnO2 and RGO [66–68]. Furthermore, pure TiO2 nanoparticles have a low fre­ quency band at 730 cm 1 which belongs to the vibration of Ti–O–Ti bonds [69]. All the mentioned bands can be found in the TiO2/N­ b2O5/SnO2/RGO spectrum (Fig. 1A and B (curve g)). Besides, the band for Nb-doped TiO2 is observed at 729 cm 1 in the FT-IR spectrum of the TiO2/Nb2O5/SnO2/RGO sample [70]. These results suggested that the incorporation of TiO2/Ceramic/SnO2 and TiO2/Nb2O5/SnO2 nano­ hybrids onto the reduced graphene oxide (RGO) sheets has successfully been performed. 3.2. XRD analysis

Fig. 2. XRD patterns of the samples a) TiO2, b) TiO2/Ceramic, c) TiO2/Nb2O5, d) TiO2/Ceramic/SnO2, e) TiO2/Nb2O5/SnO2, f) TiO2/Ceramic/SnO2/RGO and g) TiO2/Nb2O5/SnO2/RGO.

The crystal structures of the samples were analyzed by X-ray diffraction (XRD) method. The XRD pattern of TiO2, TiO2/Ceramic, TiO2/Nb2O5, TiO2/Ceramic/SnO2, TiO2/Nb2O5/SnO2, TiO2/Ceramic/ SnO2/RGO and TiO2/Nb2O5/SnO2/RGO nanohybrids are provided in Fig. 2. The TiO2, TiO2/Nb2O5, TiO2/Nb2O5/SnO2, TiO2/Nb2O5/SnO2/ RGO samples exhibit diffraction peaks at 25.40� , 38.01� , 48.01� , 55.2� and 62.7� which are respectively corresponded to the (101), (004), (200), (211) and (204) planes of anatase TiO2 with tetragonal structure (JCPDS 086–1157) [71]. This phase is the best phase of TiO2 for pho­ tocatalytic applications [72]. No characteristic peaks of rutile or brookite phases are observed in the samples which are indicative of a high degree of purity, low calcination temperature and small particle size for the samples [73]. For the TiO2/Ceramic, TiO2/Ceramic/SnO2, TiO2/Ceramic/SnO2/RGO nanohybrids, the peaks at 2θ ¼ 27.18� , 35.77� , 41.33� , 43.2� , 54.40� , 57.27� , 63.05� and 68.54� are

respectively attributed to the (110), (101),(111),(210),(211),(220), (002) and (301) planes of rutile TiO2 (JCPDS 001–7812) [74]. Fig. 2a shows XRD pattern of TiO2 nanoparticles. In the XRD pattern of the TiO2/Ceramic nanohybrid (Fig. 2b), excluding the diffraction peaks of the rutile phase, no peaks for SiO2 were observed because of its amor­ phous structure. Also, no Fe3O4 phase was detected in the pattern, suggesting that the Fe3O4 is good encapsulated by SiO2 and no inter­ action is detected between the Fe3O4 and TiO2 [75]. The XRD pattern of the TiO2/Nb2O5 nanohybrid is shown in Fig. 2c. No diffraction peaks related to the niobium species can be observed, which can be due to the reasons like high dispersion of niobium, amorphous structure and low degree of doping [76]. According to the literature [77], the introduction 5

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Journal of Physics and Chemistry of Solids 140 (2020) 109271

of Nb inhibits the anatase-to-rutile phase transformation in TiO2 which might be corresponded to the formation of strong Nb–O–Ti bonds. These bonds prevent the motion of Ti atoms required for initiation of the phase transformation. According to the XRD patterns shown in Fig. 2d, the diffraction peaks of TiO2/Ceramic/SnO2 were similar to those of TiO2/Ceramic and no SnO2 phase was detected. It suggests that the Sn species have highly been dispersed in the mixture of photocatalysts and the Sn4þ ions have been incorporated into the TiO2/Ceramic lattice. Another probable reason is that the bands related to SnO2 are over­ lapped with the crystalline plane of TiO2 [78]. In addition to the anatase phase, the XRD patterns of TiO2/Nb2O5/SnO2 (Fig. 2e) contain some peaks at 26.57� , 33.77� , 37.76� and 51.75� which are respectively related to the reflection planes of (110), (101), (200) and (211) in tetragonal tin oxide (SnO2) phase (JCPDS No.072–1147) [79]. It is worthy to note that TiO2 and SnO2 patterns both have joint peaks at 26.6� , 54.75� and 73.2� which are reasonably not distinguishable. The XRD patterns for the TiO2/Ceramic/SnO2/RGO (Fig. 2f) and TiO2/N­ b2O5/SnO2/RGO (Fig. 2g) nanohybrids confirm the formation of rutile and anatase phases, respectively. The photocatalyst containing anatase phase of TiO2 gives a higher photocatalytic activity rather than that of rutile phase [80]. The reason is that TiO2/Ceramic/SnO2/RGO nano­ hybrid has been prepared at high calcination temperature. At high temperatures, the rutile phase is dominated which is due that the anatase to rutile transformation occurs at high temperatures. As a matter of fact, the rutile phase decreases the photocatalytic efficiency of the TiO2/Ceramic/SnO2/RGO nanohybrids. It should be noted that the XRD pattern of RGO includes a charac­ teristic peak at 24.6� for the (002) reflection plane of graphite [81]. This peak is absent in the patterns of TiO2/Ceramic/SnO2/RGO and TiO2/Nb2O5/SnO2/RGO nanohybrids. The reason is probably due to the degradation of RGO or its amorphous structure after preparation of the nanohybrid. It confirms that the RGO sheets are uniformly dispersed and completely covered by the nanohybrids [82]. Moreover, TiO2/Cer­ amic/SnO2/RGO and TiO2/Nb2O5/SnO2/RGO nanohybrids are able to interact with the RGO surface through the processes like physisorption, charge transfer interactions and electrostatic binding [83]. When the nanohybrids are incorporated to the RGO, the undesirable agglomera­ tion in RGO can be minimized or inhibited, preserving high surface area and other essential physical or chemical properties of graphene [84]. Using the Scherrer equation (D ¼ 0.89λ/βcosθ), the crystallite sizes of TiO2 nanoparticles were calculated for all the samples. In this equa­ tion, λ is the wavelength for the Cu Kα radiation (1.54056 Å), β is the full width half maximum (FWHM) in radian and θ is Bragg’s angle [85]. The average crystallite sizes of the TiO2, TiO2/Ceramic, TiO2/Nb2O5,

TiO2/Ceramic/SnO2, TiO2/Nb2O5/SnO2, TiO2/Ceramic/SnO2/RGO and TiO2/Nb2O5/SnO2/RGO samples were calculated to be 37, 16.7, 14.8, 13.8, 12.5, 10.4 and 3.5 nm, respectively. 3.3. Morphological analysis As a matter of fact, the particles have a high tendency to aggregate which is due to their high surface energy. Then, it decreases the pho­ tocatalytic yield of the TiO2 nanoparticles which is definitely related to decreasing the surface area. Accordingly, it makes the morphological investigation to be necessarily performed for the particles. Fig. 3 dem­ onstrates the SEM images of the pure TiO2, TiO2/Ceramic, TiO2/Nb2O5, TiO2/Ceramic/SnO2, TiO2/Nb2O5/SnO2, TiO2/Ceramic/SnO2/RGO, TiO2/Nb2O5/SnO2/RGO samples. The pure TiO2 particles show an irregular and spherical-shaped morphology having particles with large size, arising from the agglomeration of primary particles (Fig. 3a and e). Whilst, the TiO2/Ceramic (Fig. 3b) nanohybrid shows smaller particles compared with those of pure TiO2, having relatively porous and spherical-shaped particles. The particles of TiO2/Nb2O5 (Fig. 3c), having spherical shape, have shown to be smaller and less aggregated in com­ parison to those of TiO2/Ceramic and pure TiO2. The TiO2/Ceramic/ SnO2 sample (Fig. 3d) contains protuberant but agglomerated particles. The particle size of this nanohybrid is smaller than TiO2/Ceramic and pure TiO2 nanoparticles. For the TiO2/Nb2O5/SnO2 nanohybrid (Fig. 3f), it can be seen that Nb2O5/SnO2 nanohybrid is uniformly dispersed in the TiO2 matrix having the smaller size rather than TiO2/ Ceramic/SnO2, TiO2/Nb2O5, TiO2/Ceramic and TiO2 nanoparticles. The TiO2/Ceramic/SnO2/RGO nanohybrid (Fig. 3g) displays porous and spherical-shaped particles, being smaller and less aggregated rather than pure TiO2. It can be concluded that the concurrent presence of Ceramic, SnO2 and RGO provides more catalytically active sites, high capability of oxygen adsorption, high surface area and consequently high photo­ catalytic performance. Fig. 3h displays the representative SEM image of TiO2/Nb2O5/SnO2/RGO nanohybrid. The TiO2/Nb2O5/SnO2 nano­ hybrid with small size is uniformly and homogeneously dispersed on the surface of RGO sheets. The combination of RGO sheets and TiO2/Nb2O5/ SnO2 nanohybrid prevents the particles from agglomeration and en­ hances the stability of the product. It should be due that RGO inhibits the agglomeration of TiO2/Nb2O5/SnO2 particles, making a larger surface area which ultimately facilitates the electron transfer process [86]. These results are similarly observed for the TiO2/Ceramic/SnO2/RGO nanohybrid. It should be noted that the formation of hetero-structured junctions (two or three semiconductors) using carbon-based materials like graphene oxide results in the efficient separation and transportation

Fig. 3. SEM images of the samples a, e) TiO2, b) TiO2/Ceramic, c) TiO2/Nb2O5, d) TiO2/Ceramic/SnO2, f) TiO2/Nb2O5/SnO2, g) TiO2/Ceramic/SnO2/RGO and h) TiO2/Nb2O5/SnO2/RGO. 6

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Journal of Physics and Chemistry of Solids 140 (2020) 109271

of the electron–hole pairs [87]. The coupling of Nb2O5 with other semiconductors and also reduced graphene oxide leads to more uniform and smaller particles, more homogeneous surface and better dispersity rather than the other samples (TiO2/Ceramic/SnO2/RGO, TiO2/N­ b2O5/SnO2, TiO2/Ceramic/SnO2, TiO2/Nb2O5, TiO2/Ceramic and TiO2). This coupling is also beneficial for increasing the photocatalytic activity.

3.6. TEM studies of pure TiO2 nanoparticles, TiO2/ceramic/SnO2/RGO and TiO2/Nb2O5/Sno2/RGO nanohybrids Fig. 4 demonstrates the TEM images of pristine TiO2 nanoparticles (a), TiO2/Ceramic/SnO2/RGO (b) and TiO2/Nb2O5/SnO2/RGO (c) nanohybrids. Fig. 4a proves that the degree of aggregation is high for the nanoparticles of pristine TiO2. The TiO2 nanoparticles without carrier tend to agglomerate which is related to their strong physical interaction [94]. The TEM image of TiO2/Ceramic/SnO2/RGO (Fig. 4b) exhibits uniform distribution of TiO2/Ceramic/SnO2 nanohybrid on the RGO surface with much lower aggregation rather than TiO2 nanoparticles. However, some aggregated spherical nanoparticles can also be observed. Fig. 4c and d reveal the TEM images of TiO2/Nb2O5/S­ nO2/RGO nanohybrid, confirming the spherical shape with a grainy structure and small particle size. The TiO2/Nb2O5/SnO2 nanohybrids are homogeneously and uniformly dispersed on the surface of the reduced graphene sheets. In this sample, the aggregation is efficiently prohibited by the presence of polar oxygenated functional groups on the GO sheets, serving as anchoring sites for the TiO2/Nb2O5/SnO2 nano­ hybrid [95]. Noteworthy, the reduction of GO and formation of the TiO2/Nb2O5/SnO2/RGO nanohybrid simultaneously occurs under ul­ trasonic conditions. The small average particle size can increase the surface area and accordingly enhance the capacity for the adsorption of pollutants. On the other hand, the large surface area of the TiO2/N­ b2O5/SnO2/RGO nanohybrid can promote the photogenerated electron transfer from TiO2/Nb2O5/SnO2 to RGO so, resulting in the charge separation and photocatalytic activity. HR-TEM analysis (Fig. 4e and f) confirm that the TiO2/Nb2O5/SnO2/RGO nanohybrid is appropriately formed with interfacial interaction between TiO2, Nb2O5 and SnO2 with RGO. Also, the components are uniformly distributed by the self-assembly process. Moreover, mono-dispersed particles smaller than 4 nm are well dispersed. Therefore, modification of TiO2 by Nb2O5, SnO2 (semiconductors) and RGO minimizes aggregation and also makes the smallest particle size. In such conditions, the photocatalytic efficiency for the TiO2/Nb2O5/SnO2/RGO can be improved in comparison to the TiO2/Ceramic/SnO2/RGO, TiO2/Nb2O5/SnO2, TiO2/Ceramic/SnO2, TiO2/Nb2O5, TiO2/Ceramic and pure TiO2 samples. Eventually, the average particle sizes of 37 nm, 10 nm and 3.26 nm were respectively obtained for the samples pure TiO2, TiO2/Ceramic/SnO2/RGO and TiO2/Nb2O5/SnO2/RGO. These values are relatively close to the values calculated by Scherrer formula.

3.4. Energy dispersive X-ray (EDX) analysis EDX spectra of the TiO2/Ceramic, TiO2/Nb2O5, TiO2/Ceramic/ SnO2, TiO2/Nb2O5/SnO2, TiO2/Ceramic/SnO2/RGO, TiO2/Nb2O5/ SnO2/RGO nanohybrids and details are provided in the supplementary content (Figs. S1a, b, c, d, e and f). Moreover, the numerical EDX results are listed in Table S1. 3.5. BET studies Table S2 summarizes the obtained average pore size and also the textural properties of the samples pristine TiO2, TiO2/Ceramic, TiO2/ Ceramic/SnO2, TiO2/Ceramic/SnO2/RGO, TiO2/Nb2O5, TiO2/Nb2O5/ SnO2 and TiO2/Nb2O5/SnO2/RGO. Among all the samples, TiO2 nano­ particles exhibits the lowest BET surface, Langmuir surface, pore size and volume. It is indicated that the synthesis process in liquid medium results in the easier aggregation of TiO2 nanoparticles which decreases the BET surface area the materials [88]. The following order was ob­ tained for the BET surface values: TiO2/Nb2O5/SnO2/RGO > TiO2/Ceramic/SnO2/RGO > TiO2/ Nb2O5/SnO2> TiO2/Ceramic/SnO2> TiO2/Nb2O5> TiO2/Ceramic > TiO2 Among all the nanohybrids, TiO2/Nb2O5/SnO2/RGO has the largest surface area which is justified by the presence of the Nb2O5 in the TiO2 matrix. In fact, Nb2O5 is able to play the role of a deagglomerator for the TiO2 nanoparticles which is related to its electrostatic repulsion effect [78,89]. Compared with TiO2/Nb2O5/SnO2, TiO2/Ceramic/SnO2, TiO2/Nb2O5, TiO2/Ceramic, TiO2 samples, the TiO2/Nb2O5/SnO2/RGO nanohybrid has a larger BET surface area which demonstrates that the physical adsorptivity is vividly enhanced for the nanohybrid. It should be noted that the lower specific surface area of the TiO2/Ceramic, TiO2/Ceramic/SnO2 and TiO2/Ceramic/SnO2/RGO rather than the other samples is due to their high calcination temperatures. With increasing the calcination temperature the specific surface area and pore volume decrease [90]. Therefore, the pore structure parameters of these nanohybrids depend on the calcination temperature [34]. The prepa­ ration of the TiO2/Nb2O5/SnO2/RGO nanohybrid by hydrothermal method and the large BET area make high adsorption ability for the photocatalyst toward the organic pollutants [91]. Furthermore, homo­ geneous dispersion of TiO2/Nb2O5/SnO2 on the surface of RGO sheets inhibited agglomeration of this nanohybrid. On the other hand, the increased adsorptivity should be largely assigned to the selective adsorption of the aromatic dyes on the photocatalyst. Specifically, we proposed the adsorption was noncovalent and driven by the π-π stacking between CV, MB and aromatic regions of the reduced graphene oxide, which was similar to the conjugation between aromatic molecules and CNTs [92]. Thus, there would be a synergetic effect between adsorp­ tivity and photoreactivity, and a coupling between adsorption and photocatalytical reaction could be obtained in a single process, resulting in a considerable improvement in photodegradation of CV and MO for TiO2/Nb2O5/SnO2/RGO nanohybrid compared to other samples. The large BET surface area enhance the photogenerated electrons and holes to participate in photocatalytic efficiency and improve degradation of organic dyes [93].

3.7. Photocatalytic behavior of TiO2/Ceramic, TiO2/Nb2O5, TiO2/ Ceramic/SnO2, TiO2/Nb2O5/SnO2, TiO2/Ceramic/SnO2/RGO and TiO2/ Nb2O5/SnO2/RGO nanohybrids Using the solutions of CV (crystal violet) and MO (methyl orange) organic dyes in water, the photodegradation capabilities of the TiO2 nanoparticles, TiO2/Nb2O5, TiO2/Nb2O5/PANI and TiO2/Nb2O5/RGO were examined under visible light irradiation at room temperature. The photodegradation process was monitored by UV–Vis spectroscopy using decrease in the bands at 558 nm and 464 nm for CV and MO dyes, respectively. It finally results in the estimation of the photodegradation rate for which the following trend is observed: TiO2/Nb2O5/SnO2/RGO > TiO2/Ceramic/SnO2/RGO > TiO2/ Nb2O5/SnO2> TiO2/Ceramic/SnO2> TiO2/Nb2O5> TiO2/Ceramic > TiO2 Under dark conditions, the photocatalytic degradations have been performed using the TiO2/Nb2O5/SnO2/RGO. For the CV dye, 3 mL of reaction solution containing photocatalyst in neutral pH value was taken and then centrifuged for separation of photocatalyst particles from the solution and consequently was analyzed by UV–Vis spectroscopy. In fact, no photocatalytic degradation was observed under dark conditions. Furthermore, the SEM image of dried powder of the photocatalyst after the first photocatalytic cycle indicates that there is no considerable change in the shape of the samples (seen Fig. 5a and b). 7

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Journal of Physics and Chemistry of Solids 140 (2020) 109271

Fig. 4. TEM images of the samples a) TiO2 and b) TiO2/Ceramic/SnO2/RGO and c, d) TiO2/Nb2O5/SnO2/RGO. Include HRTEM images of the e, f) TiO2/Nb2O5/ SnO2/RGO.

Fig. 5. The SEM images of nanocomposite TiO2/Nb2O5/SnO2/RGO before (a) and after (b) one cycle of photocatalytic degradation.

The graph of photodegradation rate versus time (120 min) of all the samples for CV and MO organic dyes is depicted respectively in Figure (6a, b). The numerical results for the photodegradatdion of CV and MO by the prepared samples were summarized in Table 1. It can be observed that, at all the certain times, TiO2/Nb2O5/SnO2/RGO nanohybrid ex­ hibits the best photodegradation efficiency compared with the other

samples. With regard to the offered results, it can be concluded that the presence of Nb2O5, SnO2 as two semiconductors with reduced graphene oxide and the forming of multicomponent heterojunctions (TiO2/ Nb2O5/SnO2/RGO) for increasing the utilization of sunlight and improving the separation/transportation of the electron–hole pairs. This progress in this field indicates that forming heterojunctions affords a very promising strategy to increase the photocatalytic activities of 8

Journal of Physics and Chemistry of Solids 140 (2020) 109271

S. Zarrin and F. Heshmatpour

Fig. 6. Photocatalytic activities of all the samples toward a) CV and b) MO in 120 min of exposure (1 � 10 temperature ¼ 30–35 � C, concentration of CV and MO in water solution ¼ 20 mg/dm3.

Photocatalyst

Dye

Time (min)

Dye conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

TiO2/Nb2O5/SnO2/RGO TiO2/Ceramic/SnO2/RGO TiO2/Nb2O5/SnO2 TiO2/Ceramic/SnO2 TiO2/Nb2O5 TiO2/Ceramic TiO2 TiO2/Nb2O5/SnO2/RGO TiO2/Ceramic/SnO2/RGO TiO2/Nb2O5/SnO2 TiO2/Ceramic/SnO2 TiO2/Nb2O5 TiO2/Ceramic TiO2

CV CV CV CV CV CV CV MO MO MO MO MO MO MO

120 120 120 120 120 120 120 120 120 120 120 120 120 120

98 93 91 85 78 63 38 95 91 87 81 65 54 23

mmol of photocatalyst, light intensity ¼ 300 w, pH ¼ 7,

photocatalytic semiconductors by enhanced charge carrier separation and thus reduced recombination [87]. Nb2O5 semiconductor is also a promising material for the photocatalytic degradation of organic dyes and contaminants having the advantageous properties such as of easy removal, excellent chemical stability, high active surface area and small particles. Then, it is able to overcome the limitations in the traditional photocatalysts such as TiO2 and SnO2 [96,97]. For these reasons TiO2/Nb2O5/SnO2/RGO affords a higher photocatalytic activity compared with TiO2/Ceramic/SnO2/RGO. The RGO in TiO2/Nb2O5/S­ nO2/RGO and TiO2/Ceramic/SnO2/RGO samples makes more active photocatalytic sites relative to TiO2/Nb2O5/SnO2, TiO2/Ceramic/SnO2, TiO2/Nb2O5, TiO2/Ceramic and TiO2 samples. It is due to the small particles, superior electron conductivity, great adsorptivity of dyes and contaminants, uniformity of the particles and enhanced charge separa­ tion and transportation properties. Besides, the TiO2/Nb2O5/SnO2 nanohybrid has higher photocatalytic efficiency rather than TiO2/Cer­ amic/SnO2, TiO2/Nb2O5, TiO2/Ceramic and pure TiO2. It is related to formation of a heterojunction (TiO2/Nb2O5/SnO2), being able to improve dispersity, light adsorption properties, charge separation. It also prevents the recombination of electron–hole pairs and accordingly enhances the photocatalytic activity.

Table 1 The numerical results of photodegradatdion of CV and MO by the prepared samples. Entry

4

9

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Journal of Physics and Chemistry of Solids 140 (2020) 109271

3.8. Effect of pH

acetate and phosphate. The results for CV and MO are depicted in Fig. 7a and b. The highest photodegradation values were obtained at pH ¼ 10 and 4 for CV (100%) and MO (98%), respectively. The surface of a photo­ catalyst possesses positive and negative charges respectively in acidic and basic solutions. Moreover, in the low pH values (acidic conditions), the concentration of hydroxide ions is diminished due to the presence of Hþ ions. In such conditions, concentration of hydroxyl radicals is reduced and accordingly the efficiency of photodegradation is signifi­ cantly decreased [100]. In basic conditions, the reaction of hydroxide ions with positive pores of photocatalyst results in hydroxyl radicals which have a crucial role in the photodegradation process [101]. As observed above, the generation of hydroxyl radicals is favored in basic conditions, being related to the large number of hydroxide ions. At higher pH values, the adsorption of cationic CV dye is favored on the negatively-charged surface of the photocatalyst. The remarkable photocatalytic degradation of CV dye in basic pH can be attributed to the efficient conversion of hydroxide ions to hydroxyl radicals by TiO2 [102]. Also, the number of positive charges on the TiO2 surface de­ creases with increasing the pH value. Therefore, the photodegradation efficiency increases as the pH increases up to 10. These conditions are useful for CV adsorption by the photocatalyst whereas in acidic solution (pH ¼ 4), the adsorption will be restrained. In photo-degradation of CV in basic solution, the formation of more hydroxyl radicals coming from the oxidation of adsorbed water or adsorbed hydroxyl anion results in more primary oxidants. Hence, the recombination of hole-electron pairs can be prevented in the presence of oxygen. On the contrary, since the

The pH is an effective parameter in the photocatalytic reactions taking place on particulate surfaces. The surface charge of the photo­ catalyst and therefore the adsorption behavior of the dyes strongly depend on the pH value [98]. Therefore, pH plays an essential role both in the reaction mechanisms and in the characteristics of dyes. It is able to contribute to dye degradation, hydroxyl radical attack, direct oxidation by the positive hole and direct reduction by the electron in the con­ ducting band. The effects of pH on the photocatalytic operations are hardly inter­ preted due to its multiple roles; for example the electrostatic interactions between the factors like photocatalyst surface, solvent molecules, sub­ strate and charged radicals formed during the reaction process. The surface of the photocatalyst can respectively be protonated and depro­ tonated under acidic and alkaline conditions. (see the following equa­ tions) [98]. TiOH þ Hþ → TiOH2þ

(1)

TiOH þ OH → TiO þH2O

(2)

The point of zero charge (pzc) of the TiO2 [99] is at the pH ¼ 6.9. Thus, the TiO2 surface is positively charged in acidic media (pH < 6.9), while it is negatively charged under alkaline conditions (pH > 6.9). The influence of pH of the initial dye solution on the photocatalytic activity was studied while the other variables were constant. The pH was tuned in the region 4–10 employing the buffer solutions of ammonium

Fig. 7. The effect of various values of pH on photodegradation of a) CV and b) MO. 10

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Journal of Physics and Chemistry of Solids 140 (2020) 109271

MO (methyl orange) dye has an anionic form in the solution, it exhibits a higher degradation in acidic conditions. This may be corresponded to the electrostatic interactions between the positive photocatalyst surface and dye anions leading to strong adsorption of MO dye on the surface of the photocatalyst. Similar findings have also been previously reported for the photocatalytic activity of TiO2 for degradation of the other azo dyes [103]. Furthermore, the positive holes are formed as the major oxidation types at low pH values which are reacted with hydroxide ions, forming hydroxyl radicals and accordingly increasing the activity of process. Conversely, in basic media, a columbic repulsion is created between the photocatalyst surface with negative charge and the hy­ droxide anions, inhibiting the generation of hydroxyl radicals which results in the reduction of the photodegradation efficiency [104]. After the photodegradation reaction is completed, the suggested products are CO2 and H2O [105]: The photocatalytic degradation of CV and MO ended up to the sug­ gested products in the presence of TiO2/Nb2O5/SnO2/RGO. In basic and acidic conditions, the products are respectively as follow: Products for CV: NO3 þ NH4 þ Cl þ CO2 þ H2O

(3)

Products for MO: NO3 þ Naþþ SO24 þ Hþ þ CO2 þ H2O

(4)

conditions. Under visible light irradiation, the highest photodegradation per­ centage in optimized conditions is corresponded to the sample TiO2/ Nb2O5/SnO2/RGO. In the present investigation, the concentrations of CV and MO are considered to be 20 mg/L at pH value of 7. The obtained data are shown in Fig. 10a and b. It can be observed that the photo­ degradation percentage of the pure TiO2 are respectively obtained to be 38% and 23% for CV and MO dyes after 120 min of light exposure. However, the photodegradation ability of the TiO2/Nb2O5/SnO2/RGO is respectively equal to 100% and 98% for CV and MO dyes. Accordingly, the results appropriately indicate the advantages of the nanohybrid modified by the factors like Nb2O5, SnO2 and RGO. For CV and MO dyes, the graphs of –LnC/C0 versus time were drawn, being respectively depicted in Fig. 11a and b. The graphs can be fitted into the pseudo-first order rate equation (Langmuir-Hinshelwood) i.e. –Ln(Ct/C0) ¼ kappt in which C0 is the initial concentration, Ct is the concentration at the certain times (t) and kapp is the calculated apparent rate constant [106]. Additionally, the calculated k and R2 values are summarized in Table 2 which respectively the kinetic constant and linear regression coefficient. Applying the sample TiO2/Nb2O5/SnO2/RGO, the rate constants were obtained to be 0.0524 and 0.0335 cm 1 for degradation of CV and MO, respectively. The obtained values are almost 5 and 3 folds greater than those of pristine TiO2, confirming the beneficial presence of the

Figs. 8 and 9 respectively depict the UV–Vis spectra for the degra­ dation of CV and MO by TiO2/Nb2O5/SnO2/RGO in acidic and basic

Fig. 8. The shape of absorption spectra of the TiO2/Nb2O5/SnO2/RGO nanocomposite in (a) basic solution and (b) acidic solution in the degradation of CV. 11

Journal of Physics and Chemistry of Solids 140 (2020) 109271

S. Zarrin and F. Heshmatpour

Fig. 9. The shape of absorption spectra of the TiO2/Nb2O5/SnO2/RGO nanocomposite in (a) basic solution and (b) acidic solution in the degradation of MO.

Nb2O5, SnO2 and RGO species.

adsorption efficiency of organic pollutants. There is a slight drop in photocatalytic efficiency at the ninth and tenth cycle which might be related to the loss of photocatalyst during washing or filtration. It can be concluded that TiO2/Nb2O5/SnO2/RGO exhibits the highest adsorption capability and also is a stable photocatalyst for the degradation of CV and MO dyes. In fact, there is a strong interaction between the additives (Nb2O5, SnO2 and RGO) and TiO2 matrix which preserves the efficiency of photodegradation process.

3.9. Stability and reusability of TiO2/Nb2O5/SnO2/RGO In order to examine the stability and reusability of the photocatalyst, ten consecutive cycles were tested for the sample TiO2/Nb2O5/SnO2/ RGO the CV and MO dyes. At the end of each run, the photocatalyst (TiO2/Nb2O5/SnO2/RGO) was recovered by a simple filtration and immediately washed with water, acetone and ethanol. Afterwards, after drying it was exposed by visible light for 30 min before the next run. Fig. 12 contains the obtained results of both CV and MO dyes. It is predicted that, even after ten consecutive runs, MB and MO dyes were adsorbed on the surface of TiO2/Nb2O5/SnO2/RGO photocatalyst. It obviously indicates that reduced graphene oxide, niobium oxide and tin oxide exhibit high adsorption abilities for both of the dyes. It is sug­ gested that the high stability and reusability of TiO2/Nb2O5/SnO2/RGO could be related the inhibition of electron-hole recombination, its high surface area and the presence of a heterojunction system on RGO. It is worthy to note that, Nb2O5 exhibits the excellent chemical stability rather than other semiconductors [107]. The mentioned properties of the TiO2/Nb2O5/SnO2/RGO nanohybrid are able to enhance the light

3.10. The influence of particle size The smaller particles have larger surface area per volume unite and thus more active sites. Narrower particle distributions improve the dispersion and homogeneity parameters. Numerous studies reveal that both particle size and distribution parameters have substantial effects on the stability of dispersion and photocatalytic performance [108]. The effect of particle size is also important on dispersion of particles in so­ lution, in order to obtain a uniform and stable suspension [109]. Moreover, when the particle size of the semiconductor is very small, the electrons transfer across the semiconductor interface will be signifi­ cantly accelerated. Therefore, it prevents the recombination of 12

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Journal of Physics and Chemistry of Solids 140 (2020) 109271

Fig. 10. Comparison of the maximum degradation for a) CV and b) MO by different photocatalysts.

Photodegradation efficiency ¼ CODInitial - CODFinal/ CODInitial � 100

electron-hole pairs [110]. In this work, the best photocatalytic efficiency is observed for TiO2/Nb2O5/SnO2/RGO, which exhibits the lowest particle size among the other samples for degradation of CV and MO.

(5)

3.12. UV–vis diffuse reflectance spectral analysis (DRS)

3.11. Evaluation of chemical oxygen demand (COD)

For all the samples, diffuse reflectance spectroscopy was employed to determine the absorption shifts and the bandgap energy. The Kubel­ ka–Munk formula was also applied for the analysis of diffuse reflectance spectra [111]. Fig. 13 reveals the UV–Vis diffuse reflectance spectra (DRS) of the samples pure TiO2, TiO2/Ceramic, TiO2/Nb2O5, TiO2/­ Ceramic/SnO2, TiO2/Nb2O5/SnO2, TiO2/Ceramic/SnO2/RGO and TiO2/Nb2O5/SnO2/RGO. In the UV region, all the samples show typical absorption bands, being related to the intrinsic band gap of TiO2. This band belongs to the electron transitions from VB to CB (O2p→Ti3d) [112]. In this way, in the visible light region, the shifts of absorption shoulders can be observed for the modified samples. All the corre­ sponding bands are red-shifted compared with pure TiO2 nanoparticles

To further study the photocatalytic efficiency of the fabricated nanosamples, the COD analysis was carried out. The organic wastes are measured by COD analysis in terms of total oxygen quantity required for the oxidation of organic dyes to CO2 and H2O species. The numerical data of the COD values for the CV and MB solutions are respectively summarized in Tables 3 and 4. The photodegradation of the dyes is confirmed by the COD analysis. For both CV and MO, the Best COD values are attributed to the sample TiO2/Nb2O5/SnO2/RGO. Besides, the following equation is employed for the calculation of degradation effi­ ciency for the dyes:

13

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Journal of Physics and Chemistry of Solids 140 (2020) 109271

Fig. 11. Comparison of the linear kinetic curves for degradation of a) CV and b) MO by different photocatalysts.

which are indicative of decrease in the TiO2 band gap. The absorption edge for TiO2/Nb2O5/SnO2/RGO is calculated in the visible region which has a shift compared with those of the samples TiO2/Cer­ TiO2/Nb2O5/SnO2, TiO2/Ceramic/SnO2, amic/SnO2/RGO, TiO2/Nb2O5, TiO2/Ceramic and TiO2. The observed red shift can be attributed to the chemical bonds between the TiO2/Nb2O5/SnO2 and RGO species. The Ti–O–C bond formation resembles that for the carbon-doped TiO2/Nb2O5/SnO2 [113]. However, the amount of the observed red shift is hard to be determined which is due to that the absorption background (400–800 nm) is increased when the RGO embedded into the TiO2/Nb2O5/SnO2 matrix. The transformed Kubel­ ka–Munk functions are plotted in Figs. 14 and 15. The band gaps were roughly evaluated to be 3.22, 3.1, 2.98, 2.7, 2.6, 2.5 and 2.1 eV for the TiO2, TiO2/Ceramic, TiO2/Nb2O5, TiO2/Ceramic/SnO2, TiO2/N­ b2O5/SnO2, TiO2/Ceramic/SnO2/RGO and TiO2/Nb2O5/SnO2/RGO samples, respectively. Moreover, the lower band gap of the

TiO2/Nb2O5/SnO2/RGO rather than TiO2/Ceramic/SnO2/RGO is obvi­ ously due to the presence of three semiconductors with reduced gra­ phene oxide. The strong absorption band for the sample TiO2/Nb2O5/SnO2/RGO confirms its high photocatalytic efficiency. The energies of the band gap are calculated by the following equa­ tion i.e Eq. (6): Eg ¼

hc 1240 ¼ λ λ

(6)

Where Eg stands for the band gap energy (eV) which is measured by the graph of (Abs � E (eV))m vs E (eV) with the direct method (m ¼ 2) [114]. Additionally, the ECB and EVB parameters can be calculated via Eqs. (7) and (8) [115,116]. ECB ¼ χ- Ee

14

0.5Eg

(7)

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Journal of Physics and Chemistry of Solids 140 (2020) 109271

Table 2 k and R2 values for the samples TiO2, TiO2/Ceramic, TiO2/Nb2O5, TiO2/Ceramic/SnO2, TiO2/Nb2O5/SnO2, TiO2/Ceramic/SnO2/RGO and TiO2/Nb2O5/SnO2/RGO for photodegradation of CV and MO dyes under visible light. Kinetic constant (k, min

Degradation of CV TiO2/Nb2O5/SnO2/RGO TiO2/Ceramic/SnO2/RGO TiO2/Nb2O5/SnO2 TiO2/Ceramic/SnO2 TiO2/Nb2O5 TiO2/Ceramic TiO2

0.0524 0.0513 0.0479 0.04125 0.03874 0.02845 0.01048

EVB ¼ χ- Ee þ 0.5Eg

1

)

R2

Degradation of MO

Kinetic constant (k, min

0.9993 0.9984 0.9979 0.9957 0.9917 0.9874 0.9642

TiO2/Nb2O5/SnO2/RGO TiO2/Ceramic/SnO2/RGO TiO2/Nb2O5/SnO2 TiO2/Ceramic/SnO2 TiO2/Nb2O5 TiO2/Ceramic TiO2

0.0335 0.03021 0.02874 0.02178 0.01983 0.019463 0.01116

(8)

3.13. Photocatalytic mechanism For the UV light-induced redox reactions, TiO2 acts as a strong semiconductor which is due to its electronic construction including a filled VB and an empty CB. When sufficient photochemical energy is applied, electrons will be transferred from valence band into the con­ duction band of the semiconductor. It creates a hole (hþ) in the valence bond (VB) and consequently generates an electron-hole pair. As a matter of fact, the energy of UV light should be equal or larger than that of the band gap energy (>3.2 eV for TiO2 semiconductor). These charge car­ riers (e /hþ) can be transferred to the surface of the photocatalyst, where they are obtainable to undergo redox reactions with substrates [117]. The electron-hole pairs are able to treat with an electron donor or acceptor in the mentioned excited state which produce the species like � OH, O2 � , H2O2, etc. The generated radicals at the surface of the photocatalyst are intensely reactive, being able to oxidize or reduce dye molecules to the end-products [118]. The chemical reactions governing the photocatalytic degradation of organic dyes are given by the following equations [10].

O2 þ e H 2O þ

CB

CB

þ hþVB

→ �O2

hþVB

þ

→ H þ �OH

(9) (10) (11)

�OH þ organic dyes → degradation products

(12)

�O2 þ organic dyes → degradation products

(13)

)

R2 0.9998 0.9974 0.9968 0.9965 0.9879 0.9871 0.9578

gives an excellent ability to nanohybrid for photo-excitation of the electron–hole (e /hþ) pairs in the active sites [120]. Furthermore, the special band structure of TiO2/Nb2O5/SnO2/RGO heterojunctions plays an essential role in the enhancement of photocatalytic efficiency for the degradation of CV and MO dyes. Consequently, a tentative mechanism for the band configuration at the contacted interface of TiO2/N­ b2O5/SnO2/RGO nanohybrid can be suggested (see graphical abstract and Fig. 16). The schematic diagram of the energy band gap and the charge transfer in the TiO2/Nb2O5/SnO2/RGO composite semi­ conductor system has been exhibited in the graphical abstract. When the TiO2/Nb2O5/SnO2 surface is sensitized by RGO (TiO2/Nb2O5/S­ nO2/RGO), the photocatalytic efficiency increases in comparison to those of the TiO2/Ceramic/SnO2/RGO, TiO2/Nb2O5/SnO2, TiO2/Cer­ amic/SnO2, TiO2/Nb2O5, TiO2/Ceramic and pure TiO2. This finding should be corresponded to better separation of electron-hole pairs at the interface between RGO and TiO2/Nb2O5/SnO2 rather than TiO2/Cer­ amic/SnO2. Actually, charge carriers are able to exhibit superior mobility in graphene oxide so that GO shows a comparatively excellent optical transparency [121]. Since the band gap values of SnO2, Nb2O5 and TiO2 are respectively equal to 3.8 eV, 3.4 eV and 3.2 eV, it can be proposed that these semiconductors have different electron affinity and band configuration. The TiO2 conduction band (CB) edge is located lower than that of Nb2O5 [122] and higher than that of SnO2. Therefore, a “staggered” type II heterojunction at the interface of the TiO2/N­ b2O5/SnO2/RGO nanohybrid is formed. As mentioned above, by visible light excitation, electrons in the valence band (VB) of TiO2 are trans­ ferred to conduction band (CB). Whilst, the hole generates in valence band (VB), and due to the Fermi energy level of TiO2 is higher than that of SnO2, the electrons in conduction band (CB) were driven by the po­ tential energy to transfer to the conduction band (CB) of SnO2. On the contrary, the photogenerated hole can be transferred from valence band (VB) of SnO2 to valence band of TiO2 [123]. It can be observed that in this reaction, the electrons and holes are efficiently separated at the SnO2/TiO2 interfaces, resulting in a large number of charges. The charges can participate in degradation of dyes and prevent electron-hole pairs from recombination enhancing the quantum efficiency. In the same way, when the electron–hole pairs are created on the surface of Nb2O5, electrons can be separated from holes by transferring to TiO2 and then SnO2 along the potential gradient (see graphical abstract) [124]. In contrast, RGO can act as an electron-trap so that the photogenerated electrons can be moved to the RGO [125]. The trapped electrons on reduced graphene oxide are able to react with the dissolved oxygen to form reactive oxygen species such as O2 � , �OH, etc [126]. Conse­ quently, the electron hole recombination rate significantly decreases. Furthermore, the photogenerated electrons generated on the TiO2/N­ b2O5/SnO2 surface can be trapped by the oxygen molecules in the so­ lution, directly, in order to synthesize the new reactive species like superoxide anion-radicals. The produced anion-radicals are able to treat with water molecules, resulting in creation of hydroxyl radicals which in turn can degrade the organic dyes. In contrast, the holes on VB of Nb2O5 react with hydroxyls or absorbed water molecules to make the �OH radicals on the surface. Therefore, the holes are able to oxidize the dye

Where χ stands for the absolute semiconductor electronegativity which is equal to 5.81 for TiO2 [76], Ee is equal to the free electrons energy on the NHE scale (�4.5 eV) and Eg stands for the semiconductor band gap. In the edge potential of the samples TiO2, TiO2/Ceramic, TiO2/Nb2O5, TiO2/Ceramic/SnO2, TiO2/Nb2O5/SnO2, TiO2/Ceramic/SnO2/RGO and TiO2/Nb2O5/SnO2/RGO, the ECB values were calculated to be 0.3, 0.24, 0.18, 0.04, 0.01, 0.06 and 0.26 eV respectively. Additionally, the corresponding EVB values were evaluated to be 2.92, 2.86, 2.8, 2.66, 2.61, 2.56 and 2.36 eV respectively, in the edge potential of the mentioned samples. Therefore, the ECB and EVB values for the sample TiO2/Nb2O5/SnO2/RGO have down-shifted compared with those of TiO2, TiO2/Ceramic, TiO2/Nb2O5, TiO2/Ceramic/SnO2, TiO2/N­ b2O5/SnO2, TiO2/Ceramic/SnO2/RGO. Besides, their corresponding energy bands are appropriately coordinated suggesting that an efficient heterojunction is formed (three semiconductors with RGO) which makes an increase in charge separation.

TiO2 þ hʋ → e

1

In this work, the visible light-induced photocatalytic activity of the TiO2/Nb2O5/SnO2/RGO semiconductor is enhanced. As previously re­ ported [119], numerous parameters exist for the enhancement of pho­ tocatalytic activity of the composites. First of all, the high specific surface area of the TiO2/Nb2O5/SnO2/RGO makes strong adsorption ability for the nanohybrid surface toward the target molecules. It also 15

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Journal of Physics and Chemistry of Solids 140 (2020) 109271

Fig. 12. Stability of the TiO2/Nb2O5/SnO2/RGO nanocomposite for photodegradation of a) MB and b) MO in water solution under visible light.

molecules, directly [127]. The mechanism is schematically depicted in Fig. 16. The higher specific surface area of TiO2/Nb2O5/SnO2/RGO hetero­ junction would provide stronger adsorption ability of photocatalyst surface toward target molecules and better ability to photoexcite the electron-hole pairs in the active sites [128]. Moreover, the special band structure of TiO2/Nb2O5/SnO2 heterojunction plays vital role in the enhancement of photocatalytic activity for the discoloration of CV and MO solution. This progress in this field indicates that forming hetero­ junctions affords a very promising strategy to increase the photo­ catalytic activities of photocatalytic semiconductors by enhanced charge carrier separation and thus reduced recombination [87]. SnO2 act as co-catalyst and trap the electrons. Therefore, the lifetime of the electron-hole pairs is increased [129]. SnO2 is an n-type semiconductor

that has a varied range of potential applications in catalysts, gas sensors, transistors, batteries and transparent electrodes [130,131]. The struc­ ture, band gap, and chemical stability of SnO2 are similar to those of TiO2, which is an extensively used photocatalyst. Furthermore, tin (IV) oxide has no adverse health effects and is weakly absorbed by the human body when inhaled or injected [132]. Therefore, this semiconductor is an ideal photocatalyst. As mentioned above, semiconductor photo­ catalytic processes are based on the production of electron–hole pairs by means of band gap radiation [133,134], thus, tin (IV) oxide nano­ particles are expected to display high photocatalytic efficiency due to their large surface areas. Nb2O5 semiconductor is also a promising ma­ terial for the photocatalytic degradation of organic dyes and contami­ nants having the advantageous properties such as of easy removal, excellent chemical stability, high active surface area and small particles. 16

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Journal of Physics and Chemistry of Solids 140 (2020) 109271

Table 3 Chemical Oxygen Demand (COD) removal of CV (20 mg/L, pH at 7,(6 � 10 mmol of photocatalyst), temperature 30–35 � C. Catal

Initial COD mg/L

Final COD mg/L

Photodegradation efficiency, %

TiO2 TiO2/Ceramic TiO2/Nb2O5 TiO2/Ceramic/ SnO2 TiO2/Nb2O5/SnO2 TiO2/Ceramic/ SnO2/RGO TiO2/Nb2O5/SnO2/ RGO

35.13 37.2 39.02 40.05

21.78 14.13 8.584 6.0075

38 62 78 85

42.3 44.5

2.96 1.78

93 96

46.8

0.468

99

Table 4 Chemical Oxygen Demand (COD) removal of MO (20 mg/L, pH at 7, (6 � 10 mmol of photocatalyst), temperature 30–35 � C. Catal

Initial COD mg/L

Final COD mg/L

Photodegradation efficiency, %

TiO2 TiO2/Ceramic TiO2/Nb2O5 TiO2/Ceramic/ SnO2 TiO2/Nb2O5/SnO2 TiO2/Ceramic/ SnO2/RGO TiO2/Nb2O5/SnO2/ RGO

39.21 40.2 41.8 43.06

30.191 18.49 14.63 8.181

23 54 65 81

47.45 48.07

5.219 3.364

89 93

49.84

0.996

98

oxide to the CB of the semiconductor, resulting in the separation of electrons and holes, inhibiting their recombination and leading to the visible-light responsive photoactivity [135–137]. A good photosensi­ tizer should exhibit high optical absorption at an extensive range of wavelengths, particularly in the visible light region, and a sufficiently long lifetime of the excited state to react with the semiconductors. The possible second mechanism explains that reduced graphene oxide, in different kinds of semiconductor nanohybrid, can be causing band gap narrowed and increased visible light photocatalytic activity [138,139]. In the third mechanism, reduced graphene oxide is trapping of photo­ generated electrons from CB of the semiconductor as an electron acceptor, leading to the slow recombination of photogenerated charge carriers and enhance in the photocatalytic performance [140]. In this state, the semiconductor must inherently have a band structure where the conduction band position of reduced graphene oxide is lower than the main photocatalyst. For these reasons suggesting that an efficient multi-material nanohybride system is formed (three semiconductors including TiO2, Nb2O5 and SnO2 with reduced graphene oxide) which enhanced the photocatalytic activity. On the basis of the above analysis, the combination of RGO and these three semiconductors (with different energy levels) is a promising choice for enhancing the photocatalytic activity of TiO2 nanoparticles under visible light irradiation. Consequently, surface modification of TiO2 nanoparticles by RGO and Nb2O5/SnO2 as two semiconductors can be a straightforward route for enhancing the photocatalytic activity of the TiO2 nanoparticles. The products have shown the properties like small crystallite size, low degree of agglomeration, large specific surface area, active surface area, reduced recombination and increased charge carrier lifetime. These properties help the TiO2/Nb2O5/SnO2/RGO nanohybrid exhibiting a higher photocatalytic efficiency rather than TiO2/Ceramic/SnO2/RGO, TiO2/Nb2O5/SnO2, TiO2/Ceramic/SnO2, TiO2/Nb2O5, TiO2/Ceramic and TiO2. Additionally, the TiO2 modified by RGO and Nb2O5/SnO2 displays an outstanding photostability and high photodegradation efficiency for the presence of Nb2O5.

4

4

The enhancement of photocatalytic performance of reduced graphene oxide-semiconductor nanohybrids can be explained by numerous pho­ tocatalytic mechanisms. It is still an excessive challenge for nanohybrids containing RGO to study the accurate mechanism of photocatalytic ac­ tivity enhancement. Generally, the possible mechanisms can be happened through three different pathways. In the first mechanism, the new role of reduced graphene oxide as a macromolecular photosensi­ tizer studied where RGO behaves as an organic dye-like macromolecule photosensitizer for wide band gap semiconductors [135]. In this mechanism, the semiconductor cannot generate the electron and hole, but in this technically possible that reduced graphene oxide is photo­ excited under visible light. In other words, RGO acts as semiconductor. The photoexcited electrons transfer from the CB of reduced graphene

3.14. Advantages of this research Titanium dioxide nanocomposite structures can make and tune other properties such as mid-band-gap electronic states which can alter charge migration or create a red shift in the absorption spectrum. Moreover, formation of heterojunctions between TiO2, semiconductors and other materials can yield visible light absorption by the added material with charge separation facilitated by the TiO2. The one main polymorphs of

Fig. 13. UV–vis diffuse reflectance spectra (DRS) of all the samples. 17

S. Zarrin and F. Heshmatpour

Journal of Physics and Chemistry of Solids 140 (2020) 109271

Fig. 14. The plot of transformed Kubelka–Munk function versus the energy of light for a) TiO2, b) TiO2/Ceramic, c) TiO2/Nb2O5, d) TiO2/Ceramic/SnO2, e) TiO2/ Nb2O5/SnO2, f) TiO2/Ceramic/SnO2/RGO and g) TiO2/Nb2O5/SnO2/RGO.

TiO2 which show the highest photoactivity are the anatase phases. This phase of TiO2 is typically considered more favorable as it has a higher reduction potential and a slower rate of recombination of electron hole pairs. When light is absorbed by anatase phase of titanium dioxide,

electrons and holes are produced, charges migrate to the surface, and redox reactions occur. This ideal case assumes easy charge migration to the surfaces and low charge recombination [141]. The advantages of ceramics are discussed in detail in the introduction. 18

S. Zarrin and F. Heshmatpour

Journal of Physics and Chemistry of Solids 140 (2020) 109271

Fig. 15. The plot of transformed Kubelka–Munk function versus the energy of light for all the samples.

Fig. 16. Schematic band diagrams of TiO2/Nb2O5/SnO2/RGO nanohybride showing the charge transportation processes leading to the photocatalytic degradation of organic dyes.

Nb2O5 is an n-type and wide bandgap semiconductor, which has been widely investigated in recent years for photocatalytic activity, electrochemical and energy storage applications [142]. Its light ab­ sorption can be impressively shifted into the visible region by synthe­ sizing nanocomposites with small bandgap materials. Remarkably, heterojunctions prepared with small amounts of Nb2O5 have presented a significant improvement in the photocatalytic efficiency of TiO2 [142]. Nb2O5, like TiO2, displays good chemical stability, commercial avail­ ability and nontoxicity. Thus, Nb2O5/TiO2 system can be expected as promising nanocomposite photocatalyst for application in industrial photocatalysis. Nb2O5-doping can stabilize the anatase phase of TiO2 and inhibit the phase transition to rutile. This delaying can be attributed to the formation of strong Nb–O–Ti bonds, which prevent the movement of surface Ti atoms required to initiate the phase transformation [143]. The mobility of oxygen vacancies also influences the phase trans­ formation rate, since they act as nucleation sites for the anatase to rutile phase transition [144]. The results show that Nb2O5 decreases the op­ tical indirect band gap of TiO2. These results propose that Nb2O5 extend the absorbance of TiO2 to the visible light. Therefore, the photocatalytic efficiency is expected to increase under visible light irradiation as the band gap is reduced and generate more electron–hole pairs under it. The

literature presents some controversies about the effect of Nb2O5 on the band gap. Yang et al. [145] found a reduced band gap for Nb-doped TiO2 porous microspheres. Those divergences can be assigned to the particle morphology and/or dispersion of Nb2O5 on TiO2 surfaces. We chose SnO2, as another semiconductor which is physically and chemically stable and has a medium refractive index, as an overcoat material for building a heterojunction with TiO2 and improving the photocatalytic activity of it [146]. In this paper, we have been discussed about the photocatalytic heterojunction system based on the results on photocatalytic efficiency The band gap, structure and chemical stability of SnO2 are similar to those of titanium dioxide, which is an extensively used photocatalyst. Furthermore, SnO2 has no adverse health influences and is weakly absorbed by the human body when injected or inhaled [130–132]. Therefore, SnO2 is potentially an ideal photocatalyst and semiconductor. As mentioned before semiconductor photocatalytic processes are based on the production of electron–hole pairs by means of band gap radiation [133,134], so SnO2 nanoparticles are expected to display high photocatalytic efficiency due to their large surface areas. SnO2 has commonly been used as a component of nanocomposite pho­ tocatalysts [147]. The construction of heterostructure between these two or three semiconductors seems to be beneficial for photocatalysis 19

S. Zarrin and F. Heshmatpour

Journal of Physics and Chemistry of Solids 140 (2020) 109271

process. The valence band of SnO2 (3.5 V) is more positive than that of TiO2 anatase phase (2.91 V), thus being obtained a type II hetero­ structure [148]. In this system, the photogenerated electrons have been transferred to the conduction band of SnO2 and the holes remained in the valence band of TiO2 so, the recombination of electron and hole pairs is inhibited, leading to an increased photocatalytic efficiency [149]. More recently, reduced graphene oxide (RGO) used in this experi­ ment received wider attention because of its good mechanical strength, chemical stability, high electron mobility, high surface area and optical properties [150–154]. The surface of the relatively inert reduced gra­ phene oxide becomes very active due to the introduction of a large number of oxygen-containing functional groups such as epoxy group, hydroxyl group, carbonyl group and a carboxyl group [155,156]. Thus, the surface of RGO can also be connected to specific functions such as inorganic particles, biomolecules and polymers [157]. The photo­ catalytic improvement of RGO/TiO2 with other semiconductors can be assigned to three aspects: first, a large number of π–π conjugated double bonds are present on the reduced graphene oxide surface, so the organic molecules in the solution can be well enriched into RGO [158,159]. Hence, the hydroxyl radicals and photogenerated holes produced by TiO2 supported on the surface of RGO under ultraviolet light have a good degradation effect on organic dyes [160,161]. Second, under the visible light illumination, the photogenerated electrons and holes generated by anatase phase of TiO2 can be impressively separated, thereby preventing carrier recombination [162–164]. Third, composite systems solves the problem of large bandgap energy [165]. In view of the advantages mentioned above, the formation of these heterojunctions (TiO2/Nb2O5/SnO2/RGO, TiO2/Ceramic/SnO2/RGO, TiO2/Nb2O5/SnO2 and TiO2/Ceramic/SnO2 nanohybrids as new pho­ tocatalysts) has been performed and compared their photocatalytic ac­ tivity with each other and with those of previously reported TiO2/ Nb2O5, TiO2/Ceramic and TiO2 samples. Among these heterojunctions TiO2/Nb2O5/SnO2/RGO nanohybrid being able to improve dispersity, light adsorption properties, charge separation. It also prevents the recombination of electron–hole pairs and accordingly enhances the photocatalytic activity.

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4. Conclusion The TiO2/Nb2O5/SnO2/RGO, TiO2/Ceramic/SnO2/RGO, TiO2/ Nb2O5/SnO2 and TiO2/Ceramic/SnO2 as new heterojunctions were successfully synthesized by combining the facile hydrothermal and solgel methods. The synthesized heterojunctions were employed as cata­ lysts for photodegradation of CV and MO dyes in aqueous solution under visible light exposure. Using the characterization techniques such as FTIR, XRD, SEM, EDX, BET and TEM the formation of TiO2/Nb2O5/SnO2 and TiO2/Ceramic/SnO2 samples was confirmed. Also, using the mentioned techniques, it has been verified that the TiO2/Nb2O5/SnO2 and TiO2/Ceramic/SnO2 nanohybrids were successfully deposited on RGO surface to give the TiO2/Nb2O5/SnO2/RGO and TiO2/Ceramic/ SnO2/RGO products, respectively. The obtained results indicated that TiO2/Nb2O5/SnO2/RGO nanohybrid possesses the highest photo­ catalytic efficiency in relation to the TiO2/Ceramic/SnO2/RGO, TiO2/ Nb2O5/SnO2, TiO2/Ceramic/SnO2, TiO2/Nb2O5, TiO2/Ceramic and TiO2 samples. Consequently, due to the presence of Nb2O5 and SnO2 along with RGO, this modification is considered to be a facile, suitable and efficient method for increasing the photocatalytic activity of TiO2 nanoparticles. The presence of Nb2O5 prevents the anatase-to-rutile transformation in TiO2 and gives an excellent chemical stability. Therefore, modified TiO2 samples are promising materials for the pho­ tocatalytic degradation of organic dyes due to their low degree of agglomeration, large surface area, easy removal, active surface area and small particle size.

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