Characterization and Photocatalytic Performance of Tin Oxide

Ind. Eng. Chem. Res. 2009, 48, 1249–1252

1249

Characterization and Photocatalytic Performance of Tin Oxide D. Solı´s-Casados,* E. Vigueras-Santiago, S. Herna´ndez-Lo´pez, and M. A. Camacho-Lo´pez Facultad de Quı´mica, UAEM, Paseo Tollocan esq. Paseo Colo´n s/n C.P. 50, 000 Toluca, Estado de Me´xico, Me´xico

Crystalline tin oxide was prepared by using the method of thermal decomposition of stannic chloride pentahydrate. Decomposition process was studied by thermogravimetric analysis and differential scanning calorimetry. This precursor salt was thermally treated at 800 °C during 4 hours to obtain a crystallite size similar to the reference sample. The crystallite size obtained for the as-prepared sample was 25 nm. For comparison purposes Titania P25 from Degussa Co. (crystallite size of 30 nm) was used as the reference sample. The as-obtained sample was characterized by the X-ray powder diffraction technique and infrared spectroscopy. The results indicate that SnO2 was obtained in the crystalline cassiterite phase. N2 physisorption measurements were carried out to determine the textural properties of the obtained SnO2. Methylene blue photodegradation in the presence of the SnO2 catalyst was evaluated in a preliminary way monitoring the absorbance spectrum after the UV irradiation experiments were done (lamp emitting at 254 nm). Introduction Dyes are organic compounds that need to be photodegradated. Since most dyes have been considered as nonbiodegradable materials, they represent an environmental problem contributing to water pollution.1 Commercial dyes are used in paper and textile industries, and their products contaminate surface water sources. Among the large number of inorganic materials used for this purpose, titania in its anatase crystalline phase has been the most extensively used as commercial catalyst to solve this problem owing to its high photocatalytic activity under ultraviolet irradiation, suitable band gap energy, and so on.2 However, because there is so much to be studied to obtain a material which performs close to or better than titania not only under UV-irradiation but also under visibleirradiation, the photocatalytic performance of new materials such as tin oxide has been researched. Also, new natural photodegradation processes are being developed.3,4 Tin oxide is one of the most attractive metal oxides because of its unique optical and electrical properties. It is an n-type transparent semiconductor with a rutile structure. It has found many practical applications, mainly as gas sensor material, electrode dye-sensitized in solar cells, catalysts, and electrodes for lithium batteries.5 It is worth noting that pure tin oxide has not been intensively studied as a photocatalyst. However, photocatalytic studies on TiO2-SnO2 mixture have been reported.6,7 Some authors have found that the SnO2TiO2 enhances photocatalytic activity. The aim of this work is to prove that pure tin oxide obtained by a simple method has similar photocatalyst properties than commercial titanium oxide (Degussa P25) in the degradation of MB for reaction times in the range 0-1 h. The synthesis, characterization, and photocatalytic performance of SnO2 powders are presented. Experimental Details Thermal Decomposition of SnCl4 · 5H2O. Thermogravimetric analysis (TGA/DSC) was done on SnCl4 · 5H2O precursor salt to study the temperature of thermal decomposition to obtain * To whom correspondence should be addressed. E-mail: [email protected], [email protected]. Tel.: + 52 722 1806321.

SnO2 and to observe if there is any change on sample during thermal treatment. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) experiments were simultaneously recorded by using the calorimeter SDT Q600 of TA instruments. This technique enabled us to determine the temperature to obtain SnO2 from SnCl4 · 5H2O, and to estimate the adequate temperature for thermal treatment where decomposition take place. SnO2 was obtained from thermal decomposition of the precursor salt, tin(IV) chloride pentahydrate (SnCl4 · 5H2O). A 1 g portion of precursor salt (Aldrich, purity g98%) was thermally treated in air using a programmable temperature oven (Lindberg, model 3040). The sample was heated from room temperature to 800 °C at a heating rate of 5 °C/min. The samples were kept at 800 °C for 4 h to get the salt decomposition and the crystallite size similar to titania (C-2). The titania P25 was used as a reference sample (C-1). Characterization of Catalysts. The C-1 and C-2 samples were characterized by X-ray powder diffraction and infrared spectroscopies and also by the N2 physisorption measurements. The X-ray diffractograms were obtained by means of an Advance D-8 diffractometer by Bruker by using the Cu KR radiation line (λ ) 1.5406 Å). The analysis was done to identify the crystalline phases present in the samples. The diffractograms were recorded in the range of 2θ from 2 to 80° and were analyzed with the Fullprof program in order to perform the Rietveld structure refinement and determine crystallite sizes. The infrared spectra were carried out in an Avatar model 360 by Nicolet. SnO2 pressed disks or TiO2 samples diluted with KBr were analyzed by FTIR spectroscopy. The spectra were recorded at room temperature, in the 400-4000 cm-1 range, 300 scans, and a 4 cm-1 resolution. The N2 physisorption measurements were recorded using an ASAP 2000 Micromeritics equipment to evaluate the textural properties (SBET, mean pore diameter and total pore volume), prior to the physisorption measurements, both samples were outgassed for 3 h at 100 °C. The specific surface areas were calculated by the BET method (SBET). The N2 adsorption/ desorption isotherms were measured at liquid N2 temperature (-196 °C) taken from P/Po from 0 to 0.99. The pore volume (Vp) was determined by N2 adsorption at a relative pressure of 0.99. The pore size distributions were obtained

10.1021/ie800604u CCC: $40.75  2009 American Chemical Society Published on Web 01/08/2009

1250 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009

Figure 1. Thermogravimetric profile of the C-2 sample.

from the isotherm desorption branch from the Barret-JoynerHalenda method (BJH) through Kelvin’s equation. In general, the errors found in repeated measurements of the specific surface area determinations were within 2-3% of the total surface area. Catalytic Activities. Methylene blue (MB) was chosen to simulate the pollutant in the printing and dyeing industry. The initial concentration of MB solution was of 10 µmol/L, 0.1 g of catalyst was added to 10 mL MB solution. The MB solution with catalyst was stirred for 0.5 h in the darkroom to reach the equilibrium of surface adsorption. An ultraviolet lamp (central emission at 254 nm) was used to irradiate the catalytic system. The energy density of irradiation was kept constant at 100 mJ/ cm2. An aliquot was taken and analyzed each 0.5 h; samples at 0.5, 1, 1.5, and 2 h were analyzed. The concentrations of MB at different irradiation times were followed by UV-vis spectroscopy. The visible absorption spectrum was used to study the reaction rate in the photocatalytic experiments. The absorption spectra of the MB solution were recorded in a spectrophotometer Cary 5000 by Varian. The spectrum of MB solution, before irradiation, was taken as a blank (C-0). The MB concentration was followed by the decrease in the intensity of the visible-band located at 662 nm, which was analyzed as a function of the irradiation time. The photocatalytic activity of both the C-1 and C-2 samples were also compared by calculating the decomposition conversion percent of MB through the reaction time for each catalyst. The decomposition conversion was calculated by the following equation x ) 100(Ao - At/Ao), where x is the conversion percent; Ao, the initial concentration of MB; and At, the concentration at t reaction time. The decomposition of MB was followed through 2 h of reaction using a TiO2 (P25) from Degussa Company as a catalyst, which was followed in order to compare at the same reaction conditions the catalytic performance between the SnO2 and TiO2 nanoparticles. Results and Discussion The thermal analysis was done to elucidate the decomposition temperature from the tin chloride to tin oxide. Figure 1 shows the thermogravimetric analysis profile; it can be observed that water is eliminated by thermal treatment during calcination at 100 °C. The highest weight loss was observed in this period, which was assigned to the sample dehydration. From the thermogravimetric analysis profile, an inflection at 148 °C that

Figure 2. DRX profiles of the photocatalysts (a) C-2 and (b) C-1, the remarked crystalline phases are the cassiterite (/) and anatase (().

Figure 3. Infrared spectroscopy of photocatalysts (a) tin oxide (C-2) and (b) titania P25 (C-1).

finished at 175 °C was observed; this could be assigned to the salt decomposition obtaining SnO2. Thermal treatment was chosen as 800 °C, enough to make sure that SnO2 in its crystalline form and crystallite sizes similar to the P25 reference sample were obtained. The differential scanning calorimetry indicates some heat changes which means that the changes on the samples produce heat adsorption when the decomposition takes place (148 °C), and also when the SnO2 crystallization takes place (400 °C), complementing the thermogravimetric information. Figure 2 shows the X-ray diffractograms for tin oxide (pattern a) and titanium dioxide (pattern b). From the X-ray diffraction patterns, the crystallographic phases were identified for both samples tin oxide (C-2) and titanium dioxide (C-1). The cassiterite crystalline phase was found in the C-2 sample, while a mixture of anatase and rutile phases was identified for titanium dioxide. The diffraction lines corresponding to planes of the anatase polycrystalline phase are labeled with an asterisk (/), and a black diamond ([) indicates the rutile one (diffractogram b). Diffraction peaks were indexed with the standard from the Joint Committee of Powder Diffraction Standards (JCPDS 41-1445 (cassiterite), 21-1272 (anatase), and 21-1276 (rutile). Figure 3 shows the infrared spectra of the SnO2 sample (C-2) and P25 (C-1) in the range 400-4000 cm-1. The

Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1251

Figure 4. N2-Physisorption isotherms for (a) C-2 and (b) C-1 samples. Table 1. Textural Properties of the P25 (C-1) and SnO2 (C-2) Samples SBET interparticle voids total pore volume, crystallite size sample (m2/g) Dp,(nm) Vt (cc/g) (nm) C-1 C-2

51 29

77 37

0.97 0.26

30 25

spectrum 3a corresponding to the sample (C-2) presents bands at 510, 620, 1056, 1630, and 3417 cm-1. The bands located at 510 and 620 cm-1 could be attributed to the SnO2,9 while the bands at 1630 and 3417 can be assigned to the physisorbed water vibrations. The band at 1056 cm-1 could be attributed to the hydroxyl-tin bonds10 from the hydroxyl groups in tin oxide surface. The spectrum 3b corresponds to the sample C-1; the presence of bands at 643, 804, 1630 and 3390 cm-1 can be seen. In this case the bands located at 643 and 804 are attributed to the anatase and rutile of TiO2, respectively. Additionally, the N2-physisorption isotherms (Figure 4) show that the C-1 sample has a nonporous structure, where it can be assumed that the sample is conformed by crystallites; the C-2 sample is conformed by crystallites as well, however, they are in a closer arrangement, this can be assumed from the N2physisorption isotherms, where smaller interparticle voids are observed than those in the C-1 sample. Table 1 shows the differences between the textural properties in both samples. The crystallite sizes are similar, around 25 and 30 nm for C-2 and C-1, respectively; however, their textural properties are different. Crystallite sizes were determined by the Rietveld structure refinement method from the DRX results and included on this table. The samples have an acceptable specific surface area; the C-2 sample, obtained by thermal decomposition has 29 m2/g, whereas the commercial titania P25 from Degussa (C-1) has a specific surface area of 51 m2/g. Metal oxides obtained by the sol-gel or surfactant-assisted techniques give rise to homogeneous nanometric crystallite sizes which in general confers improved physicochemical properties.8,11 So, tin oxide could enhance its catalytic performance with the decrease and homogenization of its crystallite size (5-7 nm). The inset in Figure 5 shows the absorption spectrum of the methylene blue dye in the wavelength range 400-800 nm. To calculate the conversion percent, the intensity of the band located at 662 nm is measured for each irradiation time. Figure 5 shows the conversion percent in the range 0-2 h of reaction time. A close catalytic performance was observed for the tin oxide (C-2), which shows a similar catalytic performance at shorter times (0-1 h reaction time), to that observed for the analogous material titania P25, which is the most remarkable result in this work. It seems that SnO2 has

Figure 5. Decomposition percent as a function of the reaction time: (a) C-1, (b) C-2 and (c) C-0. The inset is the absorption spectrum of MB.

a lower catalytic activity due to its surface area (29 m2/g), which is minor with respect to that of TiO2 (51 m2/g). Another important factor that can affect the catalytic performance is the recombination time of the electron-hole pairs for each catalyst. In this sense, some researchers have published results on the SnO2-TiO2(P25) system.6,7 K. Vinodgopal et al. have found that the incident photon to photocurrent efficiency (IPCE) is minor for SnO2 and TiO2 (P25) for wavelength incident radiation in the range 300-450 nm.7 They showed that the SnO2-TiO2(P25) system enhances the catalytic performance to degrade the textile Azo dye. It can be noted that band gap energy (Ebg) for SnO2 (3.6 eV) and TiO2 (3.2 eV) do not affect the catalytic performance of each catalyst, because of the energy of the radiation to generate electron-hole pairs is well above the 4.8 eV (254 nm) Ebg of the two metal oxides. Conclusions Nanometric tin oxide in the cassiterite phase was obtained by using the simple method of thermal decomposition of stannic chloride pentahydrate. Photocatalytic results show that tin oxide is very similar to the commercial Titania P25 at short reaction times (0-1.0 h) in the degradation MB; for larger reaction times titania is better. This work shows that tin oxide in its most basic form has potential application in the photocatalysis area. Acknowledgment We want to thank the financial support given by both the UAEM/2349, 2638 and SEP-PROMEP/103.5/07/2572 Projects; D. Solis thanks CONACyT and the SIEA-UAEM for support, and also thanks M en C. Manuel Aguilar for the X-ray powder diffraction analysis. Literature Cited (1) Bhatkhande, Dhananjay S.; Pangakar, Vishwas G; Beenackers, A. C. M. J. Chem. Technol. Biotechnol. 2001, 77, 102–106. (2) Seok, N. W.; Young, H. G. Characterization and photocatalytic performance of nanosize TiO2 powders prepared by the solvothermal method. Korean J. Chem. Eng. 2003, 20-6, 1149–1153. (3) Medina-Valtierra, J.; Garcı´a-Servin, J.; Frausto-Reyes, C.; Calixto, S. Encapsulamiento de anatasa comercial en pelı´culas delgadas de TiOx depositadas sobre micro-rodillos de vidrio para la fotodegradacio´n del fenol. ReV. Mex. Ing. Quı´m. 2005, 4, 191–201. (4) Uddin, M. J.; Cesano, F.; Bonino, F.; Bordiga, S.; Spoto, G.; Scarano, D.; Zecchina, A. Photoactive TiO2 films on cellulose fibres: synthesis and characterization. J. Photochem. Photobiol., A 2007, 189, 286–294.

1252 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 (5) Batzill, M.; Diebold, U. The surface and materials science of tin oxide. Prog. Surf. Sci. 2005, 79, 47–154. (6) Zhou, M; Yu, J; Liu, S; Zhai, P; Jiang, L. Effects of calcination temperatures on photocatalytic activity of SnO2/TiO2 composite films prepared by EPD method. J. Hazard. Mater. 2008, 154, 1141. (7) Vinodgopal, K; Bedja, I; Kamat, V. Nanostructured semiconductor films for photocatalysis. Photochemical behaviour of SnO2/TiO2 composite systems and its role in photocatalytic degradation of a textile Azo dye. Chem. Mater. 1996, 8, 2180. (8) Kansal, S. K.; Singh, M; Sud, D. Studies on photodegradation of two commercial dyes in aqueous phase using different photocatalysts. J. Hazard. Mater. 2007, 141, 581–590. (9) Dieguez, A.; Romano-Rodriguez, A.; Vila`, A.; Morante, J. R. J. Appl. Phys. 2001, 90, 1550.

(10) Zhang, J.; Gao, L. Synthesis and characterization of nanocrystalline tin oxide by sol-gel method. J. Solid State Chem. 2004, 177, 1425– 1430. (11) Solı´s, D.; Vigueras-Santiago, E.; Herna´ndez-Lo´pez, S.; Go´mezCorte´s, A.; Aguilar, M.; Camacho-Lo´pez, M. A.; Textural, structural and electrical properties of TiO2 nanoparticles using the Brij 35 and B123 surfactants. Sci. Technol. AdV. Mater. 2008, 9, 025003 (6pp).

ReceiVed for reView April 15, 2008 ReVised manuscript receiVed November 14, 2008 Accepted November 24, 2008 IE800604U