Preparation and characterization of SnO2 doped TiO2 nanoparticles: Effect of phase changes on the photocatalytic and catalytic activity

The effects of phase changes on the photocatalytic and catalytic activities of SnO2/TiO2 nanoparticles prepared via surfactant-assisted sol-gel method were investigated. The as-prepared SnO2/TiO2 was calcined at 400 , 500 , 600 , and 700 C. The prepared samples were studied by XRD, TEM, SEM, FTIR, BET, UV-vis diffuse reflection spectroscopy (DRS) and Photoluminescence (PL) spectra. The results showed that the crystallite size and anatase-to-rutile phase transformation increase greatly with increasing the calcination temperature. The transformation of anatase to rutile phase was found to be between 400 and 600 C, and then the anatase completely transformed to rutile phase at 700 C. Also, the specific surface area and pore volume decrease, whereas the mean pore size increases with increasing the calcination temperature. The effect of calcination temperature on the catalytic activity of the samples was tested by different applications: photodegradation of Methylene Blue (MB), Rhodamine B (RhB) dyes and phenol and synthesis of xanthene (14-phenyl-14H-dibenzo [a,j]xanthene). The mineralization of MB and RhB has been confirmed by chemical oxygen demand (COD) measurements. The results showed that the SnO2/TiO2 nanoparticles calcined at 500 C exhibit the highest photocatalytic and catalytic activities. © 2019 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).


Introduction
Metal oxides play an important role in heterogeneous catalysis as solid catalysts in the industry and many synthetic conversions [1,2]. In recent years, metal oxide semiconductors were used as photocatalysts for environmental protection from pollutants that resulted from industrial waste products such as dyes, organic and inorganic pollutants which caused considerable problems to microorganisms, aquatic environments, and human beings [3e11]. Photodegradation method is one of the most popular methods in wastewater treatment due to its effectiveness, operational simplicity, and low cost [12e18]. Among various oxides semiconductors photocatalysts, TiO 2 has considerable attention due to its special optoelectronic properties, physicochemical stability and nontoxicity [19e23]. TiO 2 has a wide bandgap (3.2 eV) and the fast recombination of the photogenerated charge carriers (electron/ hole, e/h, pairs) still hinders the application of this technique [24,25]. The photocatalytic activity of TiO 2 can be improved by morphological modifications [26] and chemical modifications [27], or a combination of morphological and chemical modifications [28]. Different methods have been developed for enhancing the efficiency of the TiO 2 powders. The most popular method depends on doping TiO 2 with metal and nonmetal elements [29,30], semiconductor coupling [31], dye sensitization [32] … etc. Coupling TiO 2 with other semiconductors can enhance the photoactivity of TiO 2 due to the reducing of the recombination rate of e/h pairs [31,33e35]. Coupling SnO 2 and TiO 2 is one of the effective methods to lower e/h pair's recombination [3], which increases the quantum efficiency and enhances the photocatalytic activity. Hence, coupling TiO 2 with SnO 2 can reduce e/h pairs recombination rate which increases the photocatalytic activity of TiO 2 [36].
In addition, the calcination temperature can affect the structure, morphology, crystal phase, the crystal size of the TiO 2 doped SnO 2 which in turn affects the photoactivity, and catalytic activity of the SnO 2 /TiO 2 nanoparticle [37e39]. However, few studies have been carried out on the effects of calcination temperatures on structural, photocatalytic, biological and catalytic properties of SnO 2 /TiO 2 nanoparticles. Sato et al. and Zhang et al. showed that calcination of samples leads to release of lattice oxygen from TiO 2 which enhances the photocatalytic activity [40,41].
The present study aims to study the effect of phase changes on the photocatalytic and catalytic properties of the SnO 2 /TiO 2 nanoparticles. The catalytic activity of SnO 2 /TiO 2 nanoparticles was investigated by photodegradation of MB, RhB and phenol as well as the synthesis of 14-phenyl-14H-dibenzo [a,j] xanthene.

Preparation of SnO 2 /TiO 2 nanoparticles
A conventional sol-gel method was employed to prepare SnO 2 / TiO 2 nanoparticles from titanium (IV) isopropoxide (Aldrich, 97%) as a Ti-precursor and SnCl 4 .xH 2 O as a Sn-precursor. CTAB was used as template and ethanol as solvent. The synthetic procedure was carried out as follows [19,42]: 2 g of CTAB was dissolved in 50 ml of ethanol and stirred for 30 min; then 11.7 ml of titanium (IV) isopropoxide was added under continuously stirred conditions. 0.70 g of SnCl 4 .xH 2 O was dissolved in ethanol and added to the mixture under vigorous stirring for 3 h with 1:9 mol% ratio of SnO 2 :TiO 2 . Then, 5 ml of ammonia (32%) was added dropwise to the mixture. The mixture was left in air for 24 h to complete the reaction. After that, the gel was filtrated and washed with de-ionized water several times until the ammonia and all chloride ions were removed (chloride ions tested by silver nitrate solution) and then dried in an oven at 100 C for 24 h. Finally, the powder was calcined at 400 , 500 , 600 and 700 C for 3 h.

Characterization
XRD patterns were conducted on a Philips PW 1830 diffractometer with Cu Ka radiation operated at 40 kV (2q range of 10e80 ) and the crystallite size (D) was calculated from the Scherrer equation [36]. Transmission electron microscopy (TEM) was performed using a JEOL 2000FX operated at 120 kV. The SEM micrographs were obtained using SEM: JEOL JSM-5800LV. Surface area measurements were conducted on a Quantachrome Autosorb 3B using nitrogen as the adsorbent. The surface area was calculated using the BrunauereEmmetteTeller (BET) equation from the adsorption branch. The pore size distribution was calculated by analyzing the adsorption branch of the nitrogen sorption isotherm using BarreteJoynereHalenda (BJH) method. Fourier transform infrared (FTIR) spectra were performed using Shimadzu FTIR. The spectra were recorded in the range of 400e4000 cm À1 using the KBr disk technique. The UV-vis diffuse reflectance spectra (DRS) of the samples were examined by a PerkinElmer Lambda 950 instrument to estimate the bandgap energy of the prepared photocatalysts. Photoluminescence (PL) spectra were measured on an FP-6500 fluorescence spectrophotometer with the excitation wavelength of 315 nm.

Photocatalytic activity evaluation
The photocatalytic activity of the SnO 2 /TiO 2 nanoparticles was measured by the photodegradation of MB, RhB and phenol solutions under UV-vis irradiation. The examination of the photocatalytic reactions was occurred using a cooling-water-cycle system keeping the reaction temperature constant. The source of light was Halogen lamp (400 W) which fixed at a distance of 30 cm from the reactor. The mixture of 0.05 g of the catalyst was dispersed in 50 ml of dye (10 mg. L À1 ). The reaction was initially stirred for 30 min in the dark to achieve the adsorption-desorption equilibrium of dye on the surface of the catalyst. After that, 2 ml of the solution was taken at fixed intervals; centrifuged and 1 ml of the supernatant was diluted in a 10 ml flask for analysis on a Shimadzu, MPC-2200 UV-vis spectrophotometer at l max 666 nm for MB and 554 nm for RhB and 276 nm for phenol. The photocatalytic degradation rate (D %) has been calculated according to the following formula [43]: where C o and C t are the concentration of dye solution at initial and after irradiation time (t). Also, for exploring the reactive species might produce in the photocatalytic reaction, we used different scavengers including Na 2 EDTA, isopropanol (IPA), carbon tetrachloride (CCl 4 ), and benzoquinone (BQ) as scavengers of H þ , $OH, e e and $O 2 ¡ , respectively, at concentration of 1 mM [44]. The COD was determined using HACH DR2800 photometer. The mineralization (%COD) of MB and RhB solutions after photodegradation were calculated from the equation: The reaction was carried out using a mixture of the benzaldehyde (1 mmol) and b-naphthol (2 mmol) with 0.10 g of the activated catalyst (at 120 C for 2 h) in an oil bath at 125 C under stirring for the appropriate time. The reaction completion was examined by TLC. The catalyst was separated from the product by simple filtration where the solid product was dissolved in chloroform. Chloroform was evaporated and the product was recrystallized using aqueous ethanol (15%) for two times [45,46]. The product was identified by m.p. and FTIR spectra. The %yield of xanthene was calculated as follows: Yield ðwt%Þ ¼ Obtained weight of product Theoretical weight of product Â 100 Table 1 Structural and catalytic properties and %yield for SnO 2 /TiO 2 nanoparticles calcined at different temperatures.

XRD analysis
XRD patterns of the SnO 2 /TiO 2 nanoparticles calcined at different temperature are shown in Fig. 1. It can be seen that all the samples were composed of anatase (2q ¼ 25.28 ) and rutile (2q ¼ 27.5 ) phases [47,48]. The intensity of the peaks that attributed to the anatase phase decreased with increasing the calcination temperature, while the rutile phase increased and became more preferential, indicating the improvement of rutile phase crystallization. At 400 C, the transformation of anatase to rutile phase is small and increased with increasing the temperature to 600 C and at 700 C the anatase peak disappeared. These results indicate that the rutile phase is more stable at the high calcination temperatures. The peaks associated with the corresponding SnO 2 are not detected in the XRD patterns for samples calcined at 400 and 500 C, which indicate that SnO 2 is well dispersed on the TiO 2 surface. At 700 C, new peaks appeared at 2Ɵ ¼ 26.7 , 32.32 and 33.9 which indicating the aggregation of SnO 2 crystals on TiO 2 surface [49]. The crystallite size of SnO 2 /TiO 2 nanoparticles was calculated and listed in Table 1. It is clearly shown, with increasing the calcination temperature, the crystallite size increased gradually. This because of increasing the particles aggregation accelerate the growth of crystallite sizes [43]. According to the kinetics studies, the transformation from anatase-to-rutile phase needs high activation energy to overcome both strain energy for the oxygen ions and break the TieO bonds as the titanium ions redistribute [50].

TEM analysis
The morphology and particles size of SnO 2 /TiO 2 calcined at different temperatures were analyzed by TEM and HRTEM. Fig. 2 shows that the average particle size increased with increasing the calcination temperature. This resulted due to fuse the particles together and forming larger agglomerates [51]. Both samples showed an almost spherical shape with different average particle sizes. HRTEM images exhibit lattice fringes with interplanar spacing 0.34 nm and 0.32 nm which corresponding to (101) anatase and (110) rutile planes, respectively [43]. With increasing the calcination temperature to 700 C, only 0.32 interplanar spaces appeared. This confirms the transformation of anatase to rutile with increasing calcination temperature. These results showed that the rutile phase is more stable at high calcination temperatures compared with the anatase phase. Fig. 3 illustrates the surface morphology of SnO 2 /TiO 2 nanoparticles calcined at different temperatures. The images show that the increasing in the calcination temperature was accompanied by increases in the protrusion and aggregation of SnO 2 on the surface of TiO 2 due to the densification of the TiO 2 morphology [52]. Also, the average size of aggregated particles increased with increasing the calcination temperature. The increase in the particle size resulted due to the primary crystallite size of anatase and rutile increases during the heat treatment and another reason is due to the increasing aggregation of particles at high calcination temperature [6].  Fig. 4a shows nitrogen adsorption-desorption isotherms of SnO 2 /TiO 2 calcined at 400 , 500 , 600 , and 700 C. The samples exhibited typical type IV adsorption isotherms, indicating the characteristics of mesoporous materials [39]. With increasing the calcination temperature from 400 to 700 C, the specific surface area and pore volume decrease, whereas the mean pore size increases (Table 1). Moreover, with increasing the calcination temperature, the hysteresis loops shift to higher relative pressure range and the areas of the hysteresis loops decrease indicating that some pores collapse during the calcination [41]. This indicated that the average pore size increased and the volume of pore decreased with increasing calcination temperature.

Surface areas and pore size distribution
The pore size distribution was calculated from the desorption branch of the isotherm and presented in Fig. 4b. It can be seen that the calcination temperature influenced the pore size distribution of the SnO 2 /TiO 2 nanoparticles. With increasing the calcination temperature, the BJH pore size distribution of samples exhibited a systematic shift toward larger mesopores which can be associated with the severe collapse of the initial porous structure occurred for the calcination temperature increases.  Fig. 5 illustrates the FTIR spectra of SnO 2 /TiO 2 nanoparticles calcined at 400 , 500 , 600 , and 700 C. The spectra display broadband centered at 3410 cm À1 which assigned to the stretching vibration of eOH and/or physically adsorbed water on the SnO 2 / TiO 2 surface [22,53]. Another band appeared at 1625 cm À1 is related to the bending vibration of hydroxyl groups on the surface of the oxides [22,54]. No bands correspond to the organic template, CTAB, indicating that the calcination treatment at 400 C is sufficient to remove the template. The broadband in the region below 800 cm À1 is associated with the stretching mode of vibrations of bridged SneOeSn, TieOeTi and TieOeSn bonds of titanium and tin oxides [3,53]. The small bands that notice at 1350 and 1030 cm À1 assigned to the hetero TieOeSn bond [42]. At 700 C, the intensity of the bands at 1625 cm À1 decreased. This is due to the release of hydroxyl groups on the surface of SnO 2 /TiO 2 nanoparticles when calcined at 700 C [55].

UVevis diffuse reflectance
UVevis spectra of the SnO 2 /TiO 2 nanoparticles calcined at 400 , 500 , 600 , and 700 C are shown in Fig. 6. All samples show a strong absorption below 450 nm due to the interband electronic transitions [6,43]. It's reported that the coupling of TiO 2 with SnO 2 can improve the photocatalytic activity. This may be due to created additional energy levels by Sn ions in the bandgap of TiO 2 [56,57], which facilitates the transition of electrons from VB to the CB. The small absorption edges in the visible region are mainly caused by  oxygen vacancies [58,59]. The bandgap energy (E g ) can be estimated according to the relation [60,61]: where a is the absorbance coefficient, h is the Planck constant, v is the wavenumber, A is a constant and E g is the bandgap energy in which n ¼ 1/2 for direct bandgap materials and n ¼ 2 for indirect bandgap [62]. The bandgap energy values of SnO 2 /TiO 2 nanoparticles calcined at 400 , 500 , 600 , and 700 C were estimated from the plot of (ahn) 2 versus photon energy in electron volts (Fig. 6 inset). The obtained E g are shown in Table 1. The results show the E g became narrower with increasing the calcination temperature. This may due to two reasons: the first, as the calcination temperature increased, the crystallite size increased and led to a decrease in the bandgap energy, and the second reason, due to the phase transformation increased with increasing the calcination temperature where the bandgap of the rutile phase is smaller than that of anatase phase [6,37,41,43].

Photoluminescence spectra
Photoluminescence spectra of the SnO 2 /TiO 2 calcined at different temperature were conducted in the wavelength range of 350e600 nm. As presented in Fig. 7, the shape of the PL spectra for all samples were similar. The PL signals at about 385 and 405 nm were ascribed to the band-band PL emission which was generated by the incident light with energy approximately equal to that of the band gaps of the anatase and the rutile phases of TiO 2 , respectively [6,37]. The PL emission peaks at about 470 nm are possibly attributed to defect states in the band gaps resulting from oxygen vacancies at different depths [20].
Moreover, the PL intensity decreased with the increasing calcination temperature from 400 to 500 C and then enhanced sharply at 600 and 700 C. The weak PL intensity of SnO 2 /TiO 2 calcined at 500 C suggested a low recombination efficiency of the photoinduced e/h pairs and consequently a longer lifetime of the photoinduced electrons [37]. Increasing PL intensity of the SnO 2 /TiO 2 with increasing the calcination temperature could be ascribed to the excessive rutile phase and the destruction of the surface microstructure [63].
3.8. Catalytic activity measurements 3.8.1. Photocatalytic measurements Fig. 8 shows the photodegradation of aqueous solutions of MB, RhB and phenol over SnO 2 /TiO 2 nanoparticles calcined at 400 , 500 , 600 , and 700 C. The photocatalytic activity of the SnO 2 /TiO 2 increases with increasing the calcination temperature to reach a maximum at 500 C and then decreases with the further increase in the calcination temperature. These results indicate that at 500 C the interaction between mixed phases is the strongest which makes the sample more active than that calcined at 400 C and above 500 C. Also at 500 C, the samples show good crystallization  and low surface defects, which in turn enhanced the photocatalytic activity [6,64]. Also, the samples that calcined blew 500 C show weak photocatalytic activity than that calcined at 500 C due to low crystallization of anatase phase [31]. As the temperature increases above 500 C, the photoactivity decreases due to the increases in phase transformation [65]. Increasing the amount of rutile phase compared to that of the anatase phase led to decrease the photodegradation of MB, RhB, and phenol because the photocatalytic activity of rutile phase is lower than that of the anatase phase [36,66]. Fig. 9 shows the effects of the addition of radicals scavengers on the photodegradation of MB and RhB over SnO 2 /TiO 2 calcined at 500 C. The results showed slightly retardation of MB and RhB degradation after additions of Na 2 EDTA and BQ indicating small effects of H þ and $O 2 ¡ species in the photodegrading of MB and RhB, while the additions of CCl 4 and IPA were accompanied with remarkably decrease in the photodegradation of MB and RhB indicating that e e and $OH played the main role in the degradation process. Scheme 1 illustrates the suggested photodegradation mechanism of MB, RhB and phenol over SnO 2 /TiO 2 . Fig. 10 shows the %COD removal of MB and RhB solutions after photodegradation for 180 min of irradiation. The results illustrate that the SnO 2 /TiO 2 that calcined at 500 C showed the highest photodegradation and %COD removal values of MB and RhB, indicating that the calcination at 500 C is the appropriate temperature. The difference in the values of both photodegradation of MB and RhB and %COD refers to the presence of small amounts of colorless intermediates that not degraded. The significant COD removal values confirm the mineralization of MB and RhB.
The kinetic study of the photocatalytic degradation of MB and RhB was investigated for SnO 2 /TiO 2 nanoparticles calcined at 400 , 500 , 600 , and 700 C by LangmuireHinshelwood kinetic model. This model belongs to the first-order kinetics according to the following formula [67]: where C o and C t are concentrations of dye at initial and after irradiation time t (min) and k is the rate constant of dyes photodegradation. Fig. 11a, b show the kinetic curves of photodegradation of MB and RhB over SnO 2 /TiO 2 nanoparticles, respectively. The rate constants (k) and the correlation coefficients (R 2 ) were calculated and listed in Table 2. The linear relationship between ln (C o /C) and t indicates that the degradation of MB and RhB obey the pseudo-first-order reaction. The value of k increases with increasing the calcination temperature to reach a maximum at 500 C and then decreases as the calcination temperature increases. Effect of calcination temperature of SnO 2 /TiO 2 nanoparticles on the formation of xanthene is shown in Fig. 12 and Table 1. The results illustrate that the % yield increased with increasing the calcination temperature to 500 C and then decreased as the calcination temperature increased. The calcination at 500 C is the optimum temperature of SnO 2 /TiO 2 nanoparticles where the catalyst showed the highest catalytic activity. Compared with other results obtained over other catalysts as sulfamic acid/Cr-MIL-101 [68], modified SBA-15, MCM-41 [69], ZnO [70] and NbCl 4 [71] indicate that the SnO 2 /TiO 2 acted as an efficacious catalyst.
The reusability of SnO 2 /TiO 2 nanoparticles calcined at 500 C was checked using the recovered catalyst. The catalyst was recovered by dissolving the product in chloroform and separated by simple filtration, washed with chloroform and dried at 100 C for 1 h. The results showed that no significant loss in the catalytic activity of the SnO 2 /TiO 2 nanoparticles with increasing the number of reuse times of the catalyst as shown in Fig. 13.

Conclusion
SnO 2 /TiO 2 nanoparticles have been successfully synthesized via surfactant-assisted sol-gel method. The results showed that the anatase to rutile phase transformation and the crystallite size increased with increasing the calcination temperature. The anatase to rutile phase transformation increased with increasing the calcination temperature and the anatase phase disappeared at 700 C. The optimum calcination temperature of the SnO 2 /TiO 2 catalyst is 500 C. At this temperature, the % yield of xanthene was 93.5% whereas the photodegradation percentage of MB and RhB dyes was 100% and 90% for phenol after 2 h. Increasing the calcination temperature over 500 C led to a sharp decrease in the catalytic activity. The presence of anatase and rutile phases together showed higher activity compared with anatase or rutile alone. These results show that the catalytic activities and physicochemical properties of the SnO 2 /TiO 2 nanoparticles strongly depend on the calcination temperature.