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 transformat[r]
(1)Original Article
Preparation and characterization of SnO2 doped TiO2 nanoparticles:
Effect of phase changes on the photocatalytic and catalytic activity
Shawky M Hassana,*, Awad I Ahmeda, Mohammed A Mannaab,**
aChemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt bChemistry Department, Faculty of Science, Amran University, Sa'dah, Yemen
a r t i c l e i n f o
Article history:
Received 28 February 2019 Received in revised form 17 June 2019
Accepted 23 June 2019 Available online xxx Keywords:
SnO2/TiO2nanoparticle
Calcination temperature Photodegradation Xanthene Methylene blue Rhodamine B Phenol
a b s t r a c t
The effects of phase changes on the photocatalytic and catalytic activities of SnO2/TiO2nanoparticles
prepared via surfactant-assisted sol-gel method were investigated The as-prepared SnO2/TiO2 was
calcined at 400, 500, 600, and 700C 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 400and 600C, and then the anatase completely transformed to rutile phase at 700C 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/TiO2nanoparticles calcined at 500C 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/)
1 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 mi-croorganisms, 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 semi-conductors photocatalysts, TiO2has considerable attention due to
its special optoelectronic properties, physicochemical stability and nontoxicity[19e23] TiO2has 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 TiO2 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 TiO2powders The most popular method depends
on doping TiO2with metal and nonmetal elements[29,30],
semi-conductor coupling[31], dye sensitization[32]… etc Coupling TiO2
with other semiconductors can enhance the photoactivity of TiO2
due to the reducing of the recombination rate of e/h pairs
[31,33e35] Coupling SnO2and TiO2is 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 TiO2with SnO2can reduce e/h pairs recombination rate which
in-creases the photocatalytic activity of TiO2[36]
In addition, the calcination temperature can affect the structure, morphology, crystal phase, the crystal size of the TiO2doped SnO2
which in turn affects the photoactivity, and catalytic activity of the SnO2/TiO2nanoparticle[37e39] However, few studies have been
carried out on the effects of calcination temperatures on structural, photocatalytic, biological and catalytic properties of SnO2/TiO2
* Corresponding author ** Corresponding author
E-mail addresses:smhassan@mans.edu.eg(S.M Hassan),mnnaam@yahoo.com
(M.A Mannaa)
Peer review under responsibility of Vietnam National University, Hanoi
Contents lists available atScienceDirect
Journal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
https://doi.org/10.1016/j.jsamd.2019.06.004
(2)nanoparticles Sato et al and Zhang et al showed that calcination of samples leads to release of lattice oxygen from TiO2 which
en-hances the photocatalytic activity[40,41]
The present study aims to study the effect of phase changes on the photocatalytic and catalytic properties of the SnO2/TiO2
nano-particles The catalytic activity of SnO2/TiO2 nanoparticles was
investigated by photodegradation of MB, RhB and phenol as well as the synthesis of 14-phenyl-14H-dibenzo [a,j] xanthene
2 Experimental
2.1 Preparation of SnO2/TiO2nanoparticles
A conventional sol-gel method was employed to prepare SnO2/
TiO2nanoparticles from titanium (IV) isopropoxide (Aldrich, 97%)
as a Ti-precursor and SnCl4.xH2O as a Sn-precursor CTAB was used
as template and ethanol as solvent The synthetic procedure was carried out as follows[19,42]: g of CTAB was dissolved in 50 ml of ethanol and stirred for 30 min; then 11.7 ml of titanium (IV) iso-propoxide was added under continuously stirred conditions 0.70 g of SnCl4.xH2O was dissolved in ethanol and added to the mixture
under vigorous stirring for h with 1:9 mol% ratio of SnO2:TiO2
Then, 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 100C for 24 h Finally, the powder was calcined at 400, 500, 600and 700C for h
2.2 Characterization
XRD patterns were conducted on a Philips PW 1830 diffrac-tometer 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 cm1using the KBr disk technique The UV-vis diffuse reflectance spectra (DRS) of the samples were examined by a PerkinElmer Lambda 950 instru-ment to estimate the bandgap energy of the prepared photo-catalysts Photoluminescence (PL) spectra were measured on an FP-6500fluorescence spectrophotometer with the excitation wave-length of 315 nm
2.3 Catalytic activity measurements 2.3.1 Photocatalytic activity evaluation
The photocatalytic activity of the SnO2/TiO2nanoparticles was
measured by the photodegradation of MB, RhB and phenol solu-tions under UV-vis irradiation The examination of the photo-catalytic reactions was occurred using a cooling-water-cycle system keeping the reaction temperature constant The source of light was Halogen lamp (400 W) whichfixed 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 L1) The reaction was initially stirred for 30 in the dark to achieve the adsorption-desorption equilibrium of dye on the surface of the catalyst After that, ml of the solution was taken atfixed intervals; centrifuged and ml of the supernatant was diluted in a 10 mlflask for analysis on a Shimadzu, MPC-2200 UV-vis spectrophotometer atlmax666 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]:
D%¼
Co Ct
Co
100
(3)where Coand Ctare 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 Na2EDTA, isopropanol (IPA), carbon
tetra-chloride (CCl4), and benzoquinone (BQ) as scavengers of Hỵ, $OH,
eeand $O
¡, respectively, at concentration of mM[44] The COD
was determined using HACH DR2800 photometer The minerali-zation (%COD) of MB and RhB solutions after photodegradation were calculated from the equation:
%COD¼
CODInitial CODFinal
CODInitial
100
2.3.2 Synthesis of 14-phenyl-14H-dibenzo [a,j] xanthene
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 120C for h) in an oil bath at 125C under stirring for the appropriate time The reaction completion was examined by TLC The catalyst was separated from the product by simplefiltrationwhere the solid product was dissolved in chloroform Chloroform was evapo-rated 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:
Yieldwt%ị ẳ Obtained weight of product Theoretical weight of product 100
Table
Structural and catalytic properties and %yield for SnO2/TiO2nanoparticles calcined at different temperatures
Temperature (oC) D (nm) E
g(eV) SBET(m2/g) Vp(cm3/g) DP(nm) %Xanthene
400 6.9 3.07 33.5 0.13 9.3 89.12
500 9.2 2.95 29.6 0.11 10.6 93.50
600 18.4 2.94 25.7 0.09 16.2 78.58
700 23.5 2.91 19.7 0.07 25.3 68.62
(4)3 Results and discussion 3.1 XRD analysis
XRD patterns of the SnO2/TiO2 nanoparticles calcined at
different temperature are shown inFig It can be seen that all the samples were composed of anatase (2q¼ 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 400C, the transformation of anatase to rutile phase is small and increased with increasing the tempera-ture to 600C and at 700C the anatase peak disappeared These results indicate that the rutile phase is more stable at the high calcination temperatures The peaks associated with the corre-sponding SnO2are not detected in the XRD patterns for samples
calcined at 400 and 500 C, which indicate that SnO2 is well
dispersed on the TiO2surface At 700C, new peaks appeared at
2Ɵ ¼ 26.7, 32.32and 33.9which indicating the aggregation of
SnO2crystals on TiO2surface[49] The crystallite size of SnO2/TiO2
nanoparticles was calculated and listed inTable It is clearly shown, with increasing the calcination temperature, the crystal-lite size increased gradually This because of increasing the par-ticles 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]
3.2 TEM analysis
The morphology and particles size of SnO2/TiO2 calcined at
different temperatures were analyzed by TEM and HRTEM.Fig
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 tem-perature to 700C, only 0.32 interplanar spaces appeared This con-firms 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
3.3 SEM analysis
Fig illustrates the surface morphology of SnO2/TiO2
nano-particles calcined at different temperatures The images show that the increasing in the calcination temperature was accompanied by in-creases in the protrusion and aggregation of SnO2on the surface of
TiO2due to the densification of the TiO2morphology[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 ag-gregation of particles at high calcination temperature[6]
(5)3.4 Surface areas and pore size distribution
Fig 4a shows nitrogen adsorption-desorption isotherms of SnO2/TiO2calcined at 400, 500, 600, and 700C The samples
exhibited typical type IV adsorption isotherms, indicating the characteristics of mesoporous materials[39] With increasing the calcination temperature from 400to 700C, the specific surface area and pore volume decrease, whereas the mean pore size in-creases (Table 1) Moreover, with increasing the calcination tem-perature, 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
The pore size distribution was calculated from the desorption branch of the isotherm and presented inFig 4b It can be seen that the calcination temperature influenced the pore size distribution of the SnO2/TiO2nanoparticles With increasing the calcination
tem-perature, 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
(6)3.5 FTIR measurements
Fig illustrates the FTIR spectra of SnO2/TiO2 nanoparticles
calcined at 400, 500, 600, and 700 C The spectra display broadband centered at 3410 cm1which assigned to the stretching vibration of eOH and/or physically adsorbed water on the SnO2/
TiO2 surface [22,53] Another band appeared at 1625 cm1 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 400C is suf fi-cient to remove the template The broadband in the region below 800 cm1is 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 cm1assigned to the hetero TieOeSn bond[42] At 700C, the intensity of the bands at 1625 cm1decreased This is due to the release of hydroxyl groups on the surface of SnO2/TiO2
nano-particles when calcined at 700C[55]
Fig FTIR spectra of the SnO2/TiO2nanoparticles calcined at (a) 400(b) 500(c) 600(d) 700C
(7)3.6 UVevis diffuse reflectance
UVevis spectra of the SnO2/TiO2nanoparticles calcined at 400,
500, 600,and 700 C are shown inFig All samples show a strong absorption below 450 nm due to the interband electronic
transitions[6,43] It's reported that the coupling of TiO2with SnO2
can improve the photocatalytic activity This may be due to created additional energy levels by Sn ions in the bandgap of TiO2[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
Fig The PL spectra of SnO2/TiO2calcined at different temperatures
(8)oxygen vacancies [58,59] The bandgap energy (Eg) can be
esti-mated according to the relation[60,61]:
ahv ¼ Aðhv Eg
n
whereais the absorbance coefficient, h is the Planck constant, v is the wavenumber, A is a constant and Egis the bandgap energy in
which n¼ 1/2 for direct bandgap materials and n ¼ for indirect bandgap[62]
The bandgap energy values of SnO2/TiO2 nanoparticles
calcined at 400, 500, 600, and 700C were estimated from the plot of (ahn)2 versus photon energy in electron volts (Fig inset) The obtained Eg are shown in Table The
re-sults show the Eg 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]
3.7 Photoluminescence spectra
Photoluminescence spectra of the SnO2/TiO2 calcined at
different temperature were conducted in the wavelength range of 350e600 nm As presented inFig 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 TiO2, respectively
[6,37] The PL emission peaks at about 470 nm are possibly
Fig Photodegradation of (a) MB and (b) RhB over SnO2/TiO2calcined at 500C in the absence and presence of different scavengers under similar reaction conditions
(9)attributed to defect states in the band gaps resulting from oxygen vacancies at different depths[20]
Moreover, the PL intensity decreased with the increasing calci-nation temperature from 400 to 500C and then enhanced sharply at 600 and 700C The weak PL intensity of SnO2/TiO2calcined at
500C suggested a low recombination efficiency of the photoin-duced e/h pairs and consequently a longer lifetime of the photo-induced electrons [37] Increasing PL intensity of the SnO2/TiO2
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 8shows the photodegradation of aqueous solutions of MB, RhB and phenol over SnO2/TiO2 nanoparticles calcined at 400,
500, 600, and 700C The photocatalytic activity of the SnO2/TiO2
increases with increasing the calcination temperature to reach a maximum at 500C and then decreases with the further increase in the calcination temperature These results indicate that at 500C the interaction between mixed phases is the strongest which makes the sample more active than that calcined at 400 C and above 500C Also at 500C, the samples show good crystallization
Fig 10 % COD removal and photodegradation of MB and RhB dyes vs time
0 20 40 60 80 100
0 1 2 3 4 5
ln
(
Co /Ct
)
Time ( )
400 C 500 C 600 C 700 C
( a )
0 20 40 60 80 100
0 1 2 3 4 5
ln (C
o
/Ct
)
Time ( )
400 C 500 C 600 C 700 C
( b )
Fig 11 The pseudo-first-order kinetics of degradation of (a) MB (b) RhB over SnO2/TiO2nanoparticles at different calcination temperature
Table
Correlation coefficients and rate constants for MB and RhB photodegradation Calcination temperature MB RhB
K1 R2 K1 R2
400 0.02863 0.99090 0.02529 0.98893
500 0.03871 0.98749 0.03033 0.99290
600 0.02347 0.98832 0.01837 0.98555
(10)and low surface defects, which in turn enhanced the photocatalytic activity[6,64] Also, the samples that calcined blew 500C show weak photocatalytic activity than that calcined at 500C due to low crystallization of anatase phase[31] As the temperature increases above 500C, 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 photo-degradation of MB, RhB, and phenol because the photocatalytic activity of rutile phase is lower than that of the anatase phase
[36,66]
Fig 9shows the effects of the addition of radicals scavengers on the photodegradation of MB and RhB over SnO2/TiO2 calcined at
500 C The results showed slightly retardation of MB and RhB degradation after additions of Na2EDTA and BQ indicating small
effects of Hỵand $O2¡species in the photodegrading of MB and RhB,
while the additions of CCl4 and IPA were accompanied with
remarkably decrease in the photodegradation of MB and RhB indicating that eeand $OH played the main role in the degradation
process Scheme illustrates the suggested photodegradation mechanism of MB, RhB and phenol over SnO2/TiO2
Fig 10 shows the %COD removal of MB and RhB solutions after photodegradation for 180 of irradiation The results illustrate that the SnO2/TiO2that calcined at 500C showed the
highest photodegradation and %COD removal values of MB and RhB, indicating that the calcination at 500C is the appropriate temperature The difference in the values of both photo-degradation 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 SnO2/TiO2nanoparticles calcined at 400,
500, 600, and 700C by LangmuireHinshelwood kinetic model This model belongs to the first-order kinetics according to the following formula[67]:
In
Co
Ct
¼ kt
where Co and Ct are concentrations of dye at initial and after
irradiation time t (min) and k is the rate constant of dyes photo-degradation Fig 11a, b show the kinetic curves of photo-degradation of MB and RhB over SnO2/TiO2 nanoparticles,
respectively The rate constants (k) and the correlation coefficients (R2) were calculated and listed inTable The linear relationship between ln (Co/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 500C and then decreases as the calcination temperature increases
3.8.2 Synthesis of 14-phenyl-14H-dibenzo [a,j] xanthene
OH +
H O
O
SnO2/TiO2
2
(11)Effect of calcination temperature of SnO2/TiO2nanoparticles on
the formation of xanthene is shown inFig 12andTable The re-sults illustrate that the % yield increased with increasing the calcination temperature to 500 C and then decreased as the calcination temperature increased The calcination at 500C is the optimum temperature of SnO2/TiO2nanoparticles where the
cata-lyst 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 NbCl4 [71]
indicate that the SnO2/TiO2acted as an efficacious catalyst
The reusability of SnO2/TiO2 nanoparticles calcined at 500 C
was checked using the recovered catalyst The catalyst was recov-ered by dissolving the product in chloroform and separated by simplefiltration, washed with chloroform and dried at 100C for
1 h The results showed that no significant loss in the catalytic ac-tivity of the SnO2/TiO2nanoparticles with increasing the number of
reuse times of the catalyst as shown inFig 13
4 Conclusion
SnO2/TiO2nanoparticles 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 700C The optimum calcination temperature of the SnO2/TiO2
catalyst is 500C 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 h Increasing the calci-nation 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 physico-chemical properties of the SnO2/TiO2 nanoparticles strongly
depend on the calcination temperature
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