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0King Saud University Arabian Journal of Chemistry
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SPECIAL ISSUE: ENVIRONMENTAL CHEMISTRY
Photocatalytic decomposition of dyes using ZnO doped SnO2 nanoparticles prepared by solvothermal method
Mohamed M. Rashad a, Adel A. Ismail a'*, I. Osama a, I.A. Ibrahim a, Abdel-Hakim T. Kandil b
a Central Metallurgical Research & Development Institute (CMRDI), P.O. Box 87, Helwan 11421, Egypt b Department of Chemistry, Faculty of Science, Helwan University, Cairo, Egypt
Received 4 July 2013; accepted 16 August 2013 Available online 25 August 2013
KEYWORDS
Doping;
Stannic oxide;
Solvothermal method;
Photocatalysis
Abstract ZnO doped SnO2 has been successfully synthesized by the solvothermal method using methanol as organic solvent. The effect of ZnO/SnO2 molar ratios on the crystal structure, microstructure, optical and photocatalytic properties has been investigated. The synthesized samples are characterized by X-ray diffraction, transmission electron microscopy, N2 physical adsorption, FT-IR spectroscopy and UV-Vis spectroscopy. XRD results revealed that all diffraction peaks positions agree well with the reflection of a tetragonal rutile structure of SnO2 phase without extra peaks at 0.1ZnO:0.9SnO2 and 0.2ZnO:0.8SnO2 molar ratios. However, the secondary phase of ZnO at 0.3ZnO:0.7SnO2 molar ratio was investigated. TEM images revealed that the shape of SnO2 particles was spherical and the particle sizes of SnO2 and 0.3ZnO:0.7SnO2 molar ratio were 6.2 and 16.4 nm, respectively. The newly prepared samples have been tested by the determination of photocatalytic degradation of methylene blue (MB). The results indicated that Zn2+ doping at 0.3ZnO:0.7 SnO2 molar ratio showed the highest photocatalytic activity for the MB photodegradation. The heightened photocatalytic activity of ZnO/SnO2 could be ascribed to the enhanced charge separation derived from the coupling of ZnO with SnO2 due to the potential energy differences between SnO2 and ZnO. The recycling tests demonstrated that 0.3ZnO:0.7 SnO2 photocatalysts were quite stable during that liquid-solid heterogeneous photocatalysis since no decrease in activity in the first four cycles was observed.
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* Corresponding author. Tel./fax: +20 2 25010643. E-mail address: aismail@cmrdi.sci.eg (A.A. Ismail). Peer review under responsibility of King Saud University.
1. Introduction
There has been extensive interest in developing semiconductor photocatalysts with high activities for environmental applications such as air purification, water disinfection, hazardous waste remediation, and water purification (Hoffmann et al., 1995; Fujishima et al., 2000; Kormann et al., 1991; Zhang et al., 2005; Ismail, 2008; Kostedt et al., 2008). The
1878-5352 © 2013 Production and hosting by Elsevier B.V. on behalf of King Saud University. http://dx.doi.Org/10.1016/j.arabjc.2013.08.016
photochemistry of semiconductor nanoparticles and nanopar-ticulate materials is a fast growing area, both in terms of research and commercial activity (Brokken-Zijp et al., 2011; Mills and Lee, 2002). A photocatalytic process is based on the generation of electron-hole pairs by means of band-gap radiation that can give rise to redox reactions with the species adsorbed on the surface of the photocatalysts (Wang et al., 2005; Yuan and Xu, 2010). However, the high degree of recombination of photogenerated electrons and holes in semiconductors decreased greatly their photocatalytic efficiency and impeded the practical application of the photocatalytic technique in the degradation of contaminants in water and air. Thus, a major challenge in photocatalysis is the need to increase the charge separation efficiency of the photocatalyst and its photocatalytic efficiency. Fortunately, the doping semiconductor particles with difference in band-gap energy could increase the charge separation and extend the energy range of photoexcitation, so doping semiconductor photocatalysts exhibited higher photocatalytic activity for both gas- and liquid-phase reactions than single semiconductor photocatalysts (Pal et al., 2001; Yu et al., 1998; Ismail and Bouzid, 2013; Mahamoud et al., 2012; Lin and Chiang, 2012). ZnO, TiO2 and SnO2 have been recognized to be preferable materials for photocatalytic processes due to their high photo-sensitivity, low cost and chemical stability. However, the band gap energy of TiO2 (3.2 eV), ZnO (3.3 eV) and SnO2 (3.6 eV) limits the absorption ability of the high-energy portion (UV) of the sunlight, which results in relatively small efficiency (Fouad et al., 2006; Ismail et al., 2010; Vinodgopal et al., 1996). SnO2 is particularly interesting because it has outstanding optical, electrical and mechanical properties, is a versatile material and is widely used as the most attractive material for gas sensor applications, as a catalyst during the oxidation of organic compounds, as a key component in rechargeable Li batteries, and as a master element in opto-electronic devices (Borges et al., 2010; Zheng et al., 2007). A number of preparations of SnO2 nanoparticles by methods such as co-precipitation (Rumyants-eva et al., 2005), sol-gel method (Broussous et al., 2002; Gu et al., 2003; Kotsikau et al., 2004), solvothermal (Sathyaseelan et al., 2010), chemical vapor deposition (Suh et al., 1999; Huh et al., 1999), VLS method (He et al., 2006), laser abla-tion(Monredon et al., 2002) and hydrothermal method (Firooz et al., 2008) have been developed. Among these methods, solvothermal method is well suited for the production of nano-structure materials, because of its relatively low processing cost, being much milder and the ability to control the grain size (Anandan and Rajendran, 2010). In this study, ZnO doped SnO2 at different molar ratios has been successfully synthesized by the solvothermal method using methanol as organic solvent. The newly prepared samples have been tested by the determination of photocatalytic degradation of MB. The latter compound was used because it exhibits a high molar absorptivity (or molar extinction coefficient) allowing the rate of bleaching to be readily evaluated.
2. Experimental section
2.1. Chemicals and materials
Chemically grade SnCl4-5H2O, Zn(CH3COO)2 and methanol were purchased from Sigma-Aldrich and used as the starting
materials for SnO2 and ZnO, whereas NaOH was purchased from Fluka.
2.2. Procedure
2.2.1. Preparation of ZnO/SnO2 nanoparticles A stoichiometric amount of SnCl4-5H2O and Zn(CH3COO)2 was dissolved in 300 ml methanol. 5 M sodium hydroxide solution was obtained by dissolving it in methanol then added to the above solution. It was employed as a base to attain the pH = 7 for complete precipitation of Zn. The resulting solutions were poured into a stainless Telfon-lined 100 ml capacity autoclave. The autoclave was sealed and heated to maintain at 150 0C for 6 h, and then air-cooled to room temperature. The precipitates were repeatedly washed with deionized water and absolute methanol then dried at 80 0C for 24 h. The formed powder was annealed at 800 0C for 30 min to get crystalline powders using a tube furnace in static atmosphere.
2.3. Physical characterization
The crystallite phases of different annealed samples identified by X-ray diffraction (XRD) using a Brucker axis D8 diffrac-tometer with crystallographic data software Topas 2 using Cu-Ka radiation. The crystallite size of undoped and doped stannic oxide nanoparticles was quantitatively determined based on Scherrer's formula, further most intense peak of the SnO2 phase. The particles morphology was observed using Transmission electron microscopy (TEM) recorded with a JEOL-JEM-1230 microscope. The UV-Vis absorption spectrum was recorded by a UV-Vis-NIR scanning spectropho-tometer (Jasco-V-570 Spectrophotometer, Japan) using a 1 cm path length quartz cell. UV-Vis spectra were detected in the diffuse reflectance mode (R) and transformed to the Kubelka-Munk function F(R) to separate the extent of light absorption from scattering. Furthermore the band gap values were obtained from the plot of the modified Kubelka-Munk function (F(R) E)2 versus the energy of the absorbed light E (Tauc et al., 1966; Ismail et al., 2012).The band gap energy was determined from the intersection of tangent through the point of inflection in the absorption band and the photon energy axis. Fourier transforms infrared spectrometer (FT-IR; JASCO, Model 6300) spectrum was recorded in KBr dispersion in the range of 400-4000 cm-1.
2.4. Photocatalytic activity tests
The photocatalytic tests were performed in an aqueous solution using MB (C16H18ClN3S) (Aldrich, kmax = 664 nm) as the probe molecule. A quartz photoreactor was filled with 250 cm3 aqueous solution of MB at a concentration of 10~5 M (3.198 ppm) and mixed with 0.1 g of newly prepared photocatalysts. UV irradiation was performed by a 150 W medium pressure xenon lamp (Osram) immersed in a quartz jacket and equipped with a cooling tube. This lamp provides a relatively continuous light output from 250 to 700 nm, with a number of sharp lines occurring near 450 nm and above 800 nm. The lamp was switched on 30 min prior to the start of the reaction to stabilize the power of its emission and the reactor was cooled by circulation of H2O. The temperature of the cooling water was stabilized to perform the reactions
at 25 0C. The concentration of the substrate after equilibration was measured and taken as the initial concentration (C0) to discount the adsorption in the dark. MB samples were withdrawn at regular intervals (C) from the upper part of the reactor with the catalyst being removed from the liquid phase by filtration through nylon syringe filters (pore size: 0.45 im). The photodegradation rate was determined by measuring the MB remaining in the aqueous solution through 3 h of illumination. Measurements were carried out using JASCO V-570 UV-Vis spectrophotometer.
3. Results and discussion
3.1. Structural studies
Fig. 1 represents the XRD patterns of undoped SnO2 and 0.1ZnO:0.9SnO2, 0.2ZnO:0.8SnO2 and 0.3ZnO:0.7SnO2 molar ratios. It is clear that peak positions agree well with the reflection of tetragonal rutile structure of the SnO2 phase (JCPDS#72-1147). Furthermore, there are no extra peaks of ZnO at 0.1ZnO:0.9SnO2 and 0.2ZnO:0.8SnO2 molar ratios where the diffraction peaks arise at (1 10), (101), (200), (1 11), (211), (220) and (002) without any other phases detected indicating that Zn ions were embedded into the crystal lattice of SnO2. Also, the results indicate that Zn ions get substituted at the Sn site without changing the cassiterite structure. However, secondary phase of hexagonal ZnO was detected at 0.3ZnO:0.7SnO2 molar ratio (Ismail et al., 2005) (JCPDS#79-2205). It is observed that the peak position shifts to higher 2h as the ZnO molar ratio increases. Table 1 summarize the variation of the crystallite size and the change in lattice parameters calculated from the (110) and (101) peaks of un-doped SnO2 and Zn doped SnO2. The results indicated that the lattice parameters, unit cell volume and the crystallite size were decreased with increasing Zn substitution ratio (Table 1).
Fig. 2 shows the TEM images of undoped SnO2 (Fig. 2a) and 0.3ZnO:0.7SnO2 (Fig. 2b) molar ratio. It revealed that the shape of SnO2 particles were spherical and agglomerated (Fig. 2a). Zn substituted SnO2 had reduced the agglomeration degree of the undoped sample and the shape of Zn substituted SnO2 particles was nearly spherical and cubical indicating the
presence of a second phase (Fig. 2b). These micrographs show that the particle sizes of SnO2 and 0.3ZnO:0.7SnO2 molar ratio samples were 6.2 and 16.4 nm, respectively which was in agreement with the crystallite size calculated XRD analysis as you see in Table 1.
Fig. 3 showed FTIR spectra of undoped SnO2 and 0.3ZnO:0.7SnO2 molar ratio samples within 400-4000 cm-1. The results revealed that the peak at 3438 cm-1 comes from the stretching mode vibrations of OH, while the peak at 1636 cm-1 is attributed to the bending vibrations of adsorbed H2O molecules. The small peak at 1364 cm-1 is due to the bending vibrations of C-H in the methyl which maybe comes from the residues of preparation processes. It indicates that there may be a very small amount of organic residues in the SnO2 and ZnO nanoparticles. Also, the strong peak at 615 cm-1 can be ascribed to the vibration of Sn-O bond in SnO2 as shown in the literature (Dutta and De, 2007; Zhong and Liu, 1999; Monredon et al., 2002).
UV-Vis spectroscopy was performed on all samples (un-doped SnO2 and 0.1ZnO:0.9SnO2, 0.2ZnO:0.8SnO2 and 0.3ZnO:0.7SnO2 molar ratios) to observe the change in optical properties of the formed SnO2 as a function of Zn substitution. The transmittance edge of ZnO/SnO2 samples varies as the content of Zn ions in SnO2 nanoparticles varies. They show a slight red shift in the band transition from the normal value with adding the ZnO dopant at 0.3ZnO:0.7SnO2 molar ratio (Fig. 4). The optical band gap is calculated from plotting (aht)n versus ht and a direct transition is found for undoped and substituted SnO2 nanoparticles. The data are summarized in Table 1. The calculated band gap of undoped SnO2 nanopar-ticles was found to be 3.75 eV, which is higher than the reported value of the bulk SnO2 i.e. 3.6 eV (Sharma et al., 2011; Fang et al., 2008). The band gaps of 0.1ZnO:0.9SnO2, 0.2ZnO:0.8SnO2 and 0.3ZnO:0.7SnO2 photocatalysts estimated from the tangent lines are 3.61, 3.56, and 3.33 eV, respectively. These values are all in the range of experimental error (±0.1 eV) (Table 1). The appearance of such larger band gap energy is expected in SnO2 nanoparticles because of their small size 6 nm.
3.2. Photocatalytic properties
26 degree
Figure 1 XRD patterns of undoped SnO2 and 0.1ZnO:0.9SnO2, 0.2ZnO:0.8SnO2 and 0.3ZnO:0.7SnO2 molar ratios.
When the ZnO/SnO2 catalyst is radiated by UV light with photon energy higher or equal to the band gaps of SnO2 and ZnO, electrons (e-) in the VB can be excited to the CB with simultaneous generation of the same amount of holes (h+) in the VB. The photogenerated electrons and holes are separated under the influence of the electrostatic field induced by different work functions. Thus, electrons move to the SnO2 side and holes to the ZnO side. The photogenerated electrons and holes in the ZnO/SnO2 heterojunction nanocatalyst could inject into a reaction medium and participate in chemical reactions (Zheng et al., 2009). The hole in the valence band can react with H2O or hydroxide ions adsorbed at the particle surface to produce hy-droxyl radicals (OH.), while the electron in the conduction band can reduce O2 to produce superoxide radicals (O2.-) and subsequently other reactive oxygen species (i.e., H2O2 and OH.). Both, holes and OH. are extremely reactive toward organic compounds (Hoffmann et al., 1995; Ismail and Bahnemann, 2011). The time-dependent UV-Vis spectra of MB during the irradiation under UV light with different catalytic materials
Table 1 Variation of crystal size, lattice parameters, cell volume and band gap energy of undoped SnO2 and ZnO:SnO2 at different
molar ratios.
Sample Cs (nm) a = b (A) c (A) Cell volume BG (eV)
SnO2 5.5 4.75 3.19 71.565 3.75
0.1ZnO:0.9SnO2 13.7 4.73 3.18 71.154 3.61
0.2ZnO:0.8SnO2 13.9 4.74 3.18 71.394 3.56
0.3ZnO:0.7SnO2 18.7 4.74 3.19 71.115 3.33
Cs, crystallite size, BG, bandgap energy.
Figure 2 TEM images of (a) undoped SnO2 and (b) 0.3ZnO:0.7SnO2 molar ratio.
3438.14
- Undoped SnO2
- 0.3Zn0:0.7Sn0_
3500 3000 2500 2000 1500 Wavenumber/ cm"1
Figure 3 FTIR spectra for undoped SnO2 and 0.3ZnO:0.7SnO2 molar ratio.
are shown in Fig. 5. It can be observed that the maximum absor-bance of 664 nm decreases gradually according to the photocat-alysts after 240 min. The photocatalytic degradation of MB was carried out using the newly prepared photocatalysts, undoped SnO2 and 0.3ZnO:0.7SnO2 molar ratio (Fig. 5a and b). It is clearly seen that, the strong absorption bands of MB those that are located at k = 290 nm and k = 664 nm steadily decrease upon increasing irradiation times. The photocatalytic activity improved significantly in the presence of Zn2+ doping showing the highest photocatalytic activity for the MB decoloration
Figure 4 Plot of transferred Kubelka-Munk versus energy of the light absorbed of the undoped SnO2 and 0.3ZnO:0.7SnO2 molar ratio.
where the absorbance at kmax = 664 nm decreased from 0.75 to 0.15 after nearly 240 min of irradiation using 0.3ZnO:0.7S-nO2 sample (Fig. 5b). The enhancement in the photocatalytic activity may come from the hetero-junctions of the ZnO/SnO2 photocatalyst, its surface area and its dispersion in solution. The photocatalytic activities of the undoped either SnO2 or ZnO and ZnO/SnO2 samples at different molar ratios for the degradations of MB were illustrated in Fig. 6. The photocata-
u £ ra
(5 0.3
-Dark 1
30 min 1 1
-60 min /1
-90 min r\
-120min
-150 min J .
-180 min fj ;
-210 min / !
240 min ' il
400 500 600
Wavelength/ nm
60 90 120
Time/min
0.8 0.70.68 0.5 -I n a
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Figure 5 The absorbance spectra changes of MB solution in undoped SnO2 (a) and 0.3ZnO:0.7SnO2 molar ratio (b).
Figure 6 Photocatalytic decolorization kinetics of MB using either undoped SnO2 or ZnO, 0.1ZnO:0.9SnO2, 0.2ZnO:0.8SnO2 and 0.3ZnO:0.7SnO2 molar ratios. (MB concentration [10~5M], volume 250 ml.)
0.3ZnO:0.7SnO2 > ZnO:0.8SnO2 > 0.1ZnO:0.9SnO2, which suggested that the Zn2+ doping enhanced the photocatalytic activity of SnO2. The 0.3ZnO:0.7SnO2 photocatalyst exhibited the highest photocatalytic efficiency with the MB concentration reduced as much as 100% in 240 min. Fig. 7 shows that the degradation kinetics of MB over the 0.3ZnO:0.7SnO2 photocata-lysts annealed at 400, 600, 700 and 800 0C. It shows that the photocatalytic activity of the 0.3ZnO:0.7SnO2 photocatalyst increased when the annealed temperature was increased from 400 to 800 0C. To explore the advantage of 0.3ZnO:0.7SnO2molar ratio and its applicability, reuse cycles of new photocatalysts were tested for photodegradation of MB (Fig. 8). The recycling tests demonstrated that a photocatalyst with 0.3ZnO:0.7SnO2 molar ratio was quite stable during that liquid-solid
lytic activities of the Zn doped SnO2 photocatalysts change with the Zn contents, photocatalysts with 0.1ZnO:0.9SnO2, 0.2ZnO:0.8SnO2 and 0.3ZnO:0.7SnO2 molar ratios show higher photocatalytic activities than undoped SnO2 (Fig. 6). Also, the decrease of SnO2 content led to increase in the photocatalytic activity of the doped photocatalyst because of the very low photocatalytic activity of SnO2. On the other hand, the photoactiv-ity of undoped ZnO photocatalysts is higher than undoped SnO2, however, photocatalyst with 0.3ZnO:0.7SnO2 molar ratio is much better than either undoped SnO2 or ZnO. This may be due to the formation of hetero-junctions between ZnO, SnO2 and Zn2SnO4 (Lin and Chiang, 2012; Sathyaseelan et al., 2010; Firooz et al., 2008; Tennakone and Bandara, 2001). The heightened photocatalytic activity of ZnO/SnO2 could be further ascribed to the enhanced charge separation derived from the coupling of ZnO with SnO2 due to the potential energy differences between SnO2 and ZnO, which would promote interfacial charge-transfer relative to that of charge carrier recombination (Dodd et al., 2006; Zheng et al., 2009) thus enhancing the photocatalytic activity. The order of photocata-lytic activity of Zn doped SnO2 at 210 min was as follows:
-n- 400 C -C- 600oC -A- 700oC -V- 800oC
—I—
~~1— 60
—r~ 90
120 150 Time/min
Figure 7 Photocatalytic decolorization kinetics of MB using 0.3ZnO:0.7SnO2 molar ratio photocatalysts annealed at different temperatures 400, 600, 700 and 800 oc. (MB concentration [10~5 M], volume 250 ml.)
IS 60-
1 2 3 4 5
Recycling time
Figure 8 Repeated cycles up to 5 times of photodegradation of MB over 0.3ZnO:0.7SnO2 molar ratio photocatalysts under UV lights. (MB concentration [10-5M], volume 250 ml.)
heterogeneous photocatalysis since no decrease in activity in the first three cycles was observed. However, the photocatalytic activity was decreased to 90% after five cycles. In general, ZnO/SnO2 nanocomposites are stable and can be recycled with a slight loss of their photochemical activity.
4. Conclusion
ZnO doped SnO2 have been successfully synthesized by the solvothermal method using methanol as organic solvent. The lattice parameter, crystallite size, and unit cell volume were increased with increasing of the Zn substitution molar ratios. However, the band gap was decreased with increasing of the Zn substitution. The highest band energy was 3.75 eV for undoped SnO2 whereas the lowest band energy was 3.33 eV at 0.3ZnO:0.7SnO2 molar ratio. The results indicated that Zn2+ doping at 0.3ZnO:0.7SnO2 molar ratio showed the highest photocatalytic activity for the MB photodegradation. The recycling tests demonstrated that a 0.3ZnO:0.7SnO2 molar ratio was quite stable during liquid-solid heterogeneous photocatalysis since no decrease in activity in the first three cycles was observed.
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