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Sonochemical Synthesis of Mesoporous Tin Oxide D. N. Srivastava, S. Chappel, O. Palchik, A. Zaban,* and A. Gedanken* Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel Received December 23, 2001. In Final Form: March 11, 2002 Nanosize mesoporous tin oxide was prepared by a sonochemical approach, using tin ethoxide as the inorganic precursor and cetyltrimethylammonium bromide as the organic structure-directing agent. The formation of mesoporous SnO2 was confirmed by comparing its wide-angle X-ray diffractograms and IR spectra with previously reported data. The pore sizes measured from transmission electron microscope photographs were on the order of 3-5 nm. The Brunauer-Emmett-Teller surface area is reported after calcination at various temperatures and solvent extraction. The N2 adsorption-desorption plot followed the H2 type hysteresis showing less defined pore size and pore distribution. The thermogravimetric analysis data show that most of the weight loss occurs in the temperature range of 40-400 °C in the as-prepared sample, indicating the removal of the surfactant. Approximately 32% of the sample weight is lost up to 400 °C, and the remaining tin oxide is stable. The porous tin oxide prepared in this way was used in dye-sensitized solar cells.
Introduction Tin oxide has multifaceted technological applications, both in physical and life sciences.1 These applications include various types of sensors, electrochemical cells, optical devices, and catalysts. The sensor applications of tin oxide date back to 1991.2 Various types of tin oxide based sensors of humidity,3 CO, CCl4,4 liquefied petroleum gas,5 and organic hydrocarbons6 were reported. The concept of “electronic nose” also utilizes tin oxide in two arrays of eight quartz microbalance sensors.7 Tin oxide is also used for the fabrication of optical devices such as electrochromic windows8 and dye-based photoelectrochemical solar cells.9 Owing to such a large range of applications, various methods have been applied for the synthesis of tin oxides. These methods include sol-gel,10,11 chemical vapor deposition,12 magnetron sputtering,13 evaporation of elemental tin in an oxygen atmosphere,14 and sonochemistry.15 Since most of the applications of tin oxide require high surface * To whom correspondence should be addressed. Fax: +972-35351250. E-mail:
[email protected] (A. Gedanken), zabana@ mail.biu.ac.il (A. Zaban). (1) Kossovsky, N.; Gelman, A.; Sponsler, E.; Millett, D. J. Appl. Biomater. 1991, 2, 251. (2) Gardner, J. W.; Shurmer, H. V.; Corcoran, P. Sens. Actuators, B 1991, 4, 117. (3) Ansari, S. G.; Ansari, Z. A.; Kadam, M. R.; Karekar, R. N.; Aiyer, R. C. Sens. Actuators, B 1994, 21, 159. (4) Chaturvedi, A.; Mishra, V. N.; Dwivedi, R; Srivastava, S. K. Microelectron. J. 1999, 30, 259. (5) Chung, W. Y.; Shim, C. H.; Choi, S. D.; Lee, D. D. Sens. Actuators, B 1994, 20, 139. (6) Gotz, A.; Gracia, I.; Cane, C.; Lora-Tamayo, E.; Horrillo, M. C.; Getino, J.; Garcia, C.; Gutierrez, J. Sens. Actuators, B 1997, 44, 483. (7) Baby, R. E.; Cabezas, M; deReca, E. N. W. Sens. Actuators, B 2000, 69, 214. (8) Cummins, D.; Boschloo, G.; Ryan, M.; Corr, D.; Rao, S. N.; Fitzmaurice, D. J. Phys. Chem. B 2000 104, 11449. (9) Ferrere, S.; Zaban, A.; Gregg, B. A. J. Phys. Chem. B 1997, 101, 4490. (10) Wang, D; Wen, G.; Chen, J.; Zhang, S; Li, F. Phys. Rev. B 1994, 49, 14282. (11) Maddalena, A.; Del Maschio, R; Dire, S.; Raccanelli, A. J. NonCryst. Solids 1990 121, 365. (12) Tarey, R. D.; Raju, T. A. Thin Solid Films 1995, 128, 11. (13) Minami, T.; Nanto, H.; Takata, S. Jpn. J. Appl. Phys. 1988, 27, L287. (14) Schlosser, V.; Wind, G. In Proceedings of the 8th EC Photo Voltaic Solar Energy Conference, P-998, Florence, Italy, 1988; Kluwer: Boston, 1988.
area, attempts were made to fabricate mesoporous tin oxides, typically using supramolecular templates. However, there are only a few reports of mesoporous structured tin oxides that are stable after the removal of the surfactants.16,17 Rao and Ulagappan hydrolyzed SnCl4 in the presence of dioctylsulfosuccinate and obtained a mesoporous material with an average pore size of 3.3 nm.18 However, the removal of the surfactant either by calcination at 400 °C or by solvent extraction resulted in the collapse of the mesoporous structure. Qi et al. tried the same procedure in the presence of sodium dodecylsulfonate and found a pore size of 4.1 nm. But again, the mesostructure was not stable after calcination at 400 °C.19 Among the stable mesostructures, Pinnavaia’s synthesis resulted in a structure with a pore size of 5.6 nm, which was stable up to 350 °C. Calcination at 400 °C spoiled the structure.16 The electrochemical approach of Chen and Liu resulted in a relatively stable structure.17 They used a tetradecylamine as a structure-directing agent and tin isopropoxide as a precursor. Alternatively, cetyltrimethylammonium bromide (CTAB) was used with [Sn(OH)6]2as an inorganic precursor. After calcination at 500 °C for 2 h, the Brunauer-Emmett-Teller (BET) surface areas of the mesoporous SnO2 made by using the amine and the CTAB templating were found to be 107 and 143 m2/g, respectively. In the current work, we extended our sonochemical approach of preparing mesoporous metal oxides to tin oxide.20 Tin ethoxide was used as the inorganic precursor, and CTAB as the structure-directing agent. The mesoporous tin oxide synthesized in this work was characterized by IR spectroscopy, wide- and small-angle X-ray diffraction (XRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and surface area measurements. The surface area was found to be comparable with data from earlier reports in which different methods were utilized, except for one case wherein a 2-fold (15) Zhu, J.; Lu, Z.; Aruna, T.; Aurbach, D.; Gedanken, A. Chem. Mater. 2000, 12, 2557. (16) Severin, K. G.; Abdel-Fattah, T. M.; Pinnavaia, T. J. Chem. Commun. 1998, 1471. (17) Chen, F.; Liu, M. Chem. Commun. 1999, 1829. (18) Ulagappan, N.; Rao, C. N. R. Chem. Commun. 1996, 1685. (19) Qi, L.; Ma. J.; Cheng, M.; Zhao, Z. Langmuir 1998, 14, 2579. (20) Wang, Y.; Tang, X.; Yin, L.; Huang, W.; Hacohen, Y. R.; Gedanken, A. Adv. Mater. 2000, 12, 1183.
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higher surface area was reported.17 The main advantage of our method relates to the synthesis rate. The amount of material that we make within 3 h requires 48-72 h in the other methods. Experimental Section Chemicals. Tin ethoxide (Alfa-Aesar) and CTAB (Sigma) were used as received. All solvents were of AR grade and used as received. Water was used after double distillation. Instruments. IR spectra were recorded on a Nicolet Impact 410 infrared spectrophotometer in KBr medium. Wide- and smallangle XRD files were recorded using a Bruker D8 diffractometer (Cu KR radiation). TEM micrographs were obtained using a JEOL-JEM 100 SX microscope. The samples were prepared by a pickup from a suspension of dried powder in absolute ethanol. Surface area was measured by a Micromeritics (Gemini 2375) analyzer. The nitrogen adsorption and desorption isotherms were measured at 77 K after preheating the sample to 150 °C for 2 h. The TGA and differential scanning calorimetry (DSC) data were collected on a Mettler TGA/SDTA 851 and a Mettler DSC 30, respectively. The heating and cooling rate in the TGA experiments was 10 °C/min. Synthesis. Twenty milliliters of a solution of 0.05 M CTAB in ethanol was added to 0.005 mol tin ethoxide in a specially designed round-bottom sonication flask. Then 50 mL of water was added followed by 40 mL of NH4OH. The solution pH was adjusted to 10. The sonication flask was fixed in a water bath at room temperature (20-25 °C), and the system was irradiated in air by a high-intensity ultrasonic horn (Ti-horn, 20 kHz, 100 W/cm2 at 65% efficiency) for 3 h. After the sonication, the milky white precipitate was centrifuged at 9000 rpm and washed repeatedly with distilled water till the NH3 was removed. The precipitate was dried under vacuum, calcined at various temperatures for various periods of time, and finally characterized. Some of the dried samples were placed in absolute ethanol for 6, 12, and 24 h to extract the CTAB. The residue following the solvent extraction was centrifuged and dried as reported above, before characterization. Preparation of SnO2 Electrodes and Measurements of the Solar Cells. Mesoporous SnO2 electrodes were prepared by coating a conducting glass support (Libby Owens Ford, 8 Ω/square, F-doped SnO2) with a suspension of the SnO2 particles described above. The conducting glass was cleaned with soap, rinsed with deionized water, and then dried in an air stream. Partial evaporation of the SnO2 suspension resulted in a paste which was spread on the conducting substrates with a glass rod, using adhesive tape as spacers. After drying in air, the films were annealed at 250, 350 , and 450 °C for 60 min in air. The thickness of the obtained SnO2 films was between 0.5 and 1 µm, as measured with a profilometer (Mitutoyo, Surftest SV 500). The thickness measurement revealed a high roughness of the films. The dye [cis-di(isothiocyanato)-bis(4,4′-dicarboxy-2,2′bipyridine) ruthenium(II)] (N3, Solaronix SA Co.) used to sensitize the electrodes was adsorbed by immersing the electrodes in a 5 × 10-4 M solution of dye in absolute ethanol, overnight. To avoid water contamination in the dye solution, the films were heated to 80 °C before the immersion. A sandwich-type configuration was employed to measure the performance of the dye-sensitized solar cell, using a Pt-coated F-doped SnO2 film as a counter electrode and 0.5 M Li/0.05 M I2 in 1:1 acetonitrile-NMO (3-methyl-2-oxazolidinone) as the electrolyte. The cell was illuminated with a calibrated Xe lamp and direct sunlight. An Eco-Chemie potentiostat was used to measure the photocurrent and photovoltage. Fluorescence emission spectra were measured with an Aminco Bowman AB2 spectroflourometer, using Ruthenium 470 dye (Solaronix SA) as the sensitizer.
Results & Discussion Fourier Transform Infrared (FT-IR) Spectroscopy. The IR spectra of the mesoporous SnO2 as prepared, after calcination at various temperatures and after solvent extraction, are given in Figure 1. The as-prepared sample exhibited an intense, broad peak ranging from ∼3600 to
Figure 1. FT-IR spectra of SnO2: (a) as-prepared sample, (b) calcined at 250 °C for 2 h, (c) calcined at 300 °C for 2 h, (d) calcined at 350 °C for 2 h, (e) after extraction for 6 h, (f) after extraction for 12 h, (g) after extraction for 24 h.
∼2500 cm-1 with two broad maxima at ∼3429 and ∼3153 cm-1 and two sharp peaks at ∼2911 and ∼2844 cm-1. The N-H band observed around 3153 cm-1 is probably the result of NH3 traces left after washing. The peak at 3429 cm-1 is believed to be caused by adsorbed water. The intensities of both peaks decrease after calcination or solvent extraction. Two sharp peaks at ∼2911 and ∼2844 cm-1 in the asprepared spectrum are attributed to C-H symmetric and asymmetric vibration of -CH2- in CTAB. These peaks disappear after calcination but are observed with reduced intensity in the solvent-extracted samples. The peaks around 2374 cm-1 belong to residual CO2 traces in the atmosphere of the spectrometer. The intensities of the CO2 peaks do not change after calcination or solvent extraction. The bands centered at ∼1629 and ∼1481 cm-1 are also assigned to water and ammonia, with a sharp decrease in intensity after calcination or solvent extraction. The bands observed in the range of 850-1350
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Figure 3. TGA curves: (a) as-prepared sample, (b) calcined at 350 °C for 2 h. The inset shows the DSC curve of the asprepared sample.
Figure 2. X-ray diffraction patterns: (a) as-prepared sample, (b) calcined at 250 °C for 2 h, (c) small-angle diffraction pattern for as-prepared sample (inset).
cm-1 are assigned to the bending mode of different types of surface hydroxyl groups. The intensity of the IR bands at 1259 and 970 cm-1 reduced with increasing temperature. The peaks related to SnO2 appear at a lower frequency. The peaks at ∼660 and ∼550 cm-1 are attributed to the Sn-O-Sn antisymmetric vibrations, whereas the peak at ∼470 cm-1 is attributed to the symmetric vibration. X-ray Diffraction Studies. Figure 2 shows the wideand small-angle XRD patterns of the as-prepared and calcined SnO2. The wide-angle XRD pattern of the asprepared sample shows very broad peaks (Figure 2a). The peak positions fit well with reported values for the cassiterite phase (JCPDS 41-1445). The XRD peaks are found to be sharper after calcination at 250 °C, indicating the increase in crystalline nature, that is, destruction of the mesostructure (Figure 2b). The low-angle X-ray diffraction pattern of the asprepared samples is shown as an inset in Figure 2. The spectrum shows an increase of the XRD counts at around 2θ ) 10°, followed by a plateau from 7° to 2° 2θ value. The sharp increase in the counts measured below 2θ ) 1° cannot be resolved. The reason for the absence of a distinct peak is probably that long-range ordering is lacking in the mesostructure, or at least that the order is limited to small regions. The lack of long-range ordering may be attributed to the larger size of the tin ions in comparison with silicon (in the case of MCM-41), leading to a less effective electrostatic interaction between the inorganic species and the organic surfactant. Alternatively, the lack of long-range order may result from the high rate of the reaction. Previous reports show that a slow rate of hydrolysis improves the ordering of the mesostructure.21 In our case, the reaction rate is high which may be a reason for less ordered structure. This type of porous material is often called “wormhole”-like porous material. Thermal Analysis. The TGA plots for the as-prepared and calcined samples are given in Figure 3. In the temperature range of 40-400 °C, the as-prepared sample loses approximately 32% of the weight. No weight loss was observed after 400 °C up to 500 °C. The initial loss of 5-8% up to 170 °C is attributed to the loss of adsorbed (21) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56.
Table 1. BET Surface Area of the SnO2 Mesoporous Materials after Calcination at Various Temperatures and after Solvent Extraction (As-Prepared Sample, 0.40 m2/g) calcined surface area (m2/g)
solvent extracted time (h)
surface area (m2/g)
temp (°C)
1h
2h
4h
6 12 24
24 92 3
250 300 350 450
136 146 102 39
134 156 91 43
122 56 433
water and NH3. Our previous report on SnO2 showed about 15% weight loss up to 400 °C.15 The additional 17% weight loss is attributed to the loss of CTAB in the temperature range 175-225 °C. This weight loss is reduced to only 16% for the sample that was calcined at 350 °C. DSC Studies. The DSC curve for the as-prepared sample is given as an inset in Figure 3. The DSC has been done in two heating and cooling cycles. The first heating cycle shows an exothermic band ranging from 230 to 500 °C, which is not present in the second heating cycle. This observation shows an irreversible amorphous to crystalline phase change of the SnO2. The initial endothermic peak is attributed to the loss of water and ammonia and is in accordance with the TGA results. TEM Studies. The TEM photographs of the asprepared sample, the calcined sample (at 300 °C for 4 h), and the sample after solvent extraction of 24 h are given in Figure 4a-c. It is clear from these photographs that the as-prepared sample has a good porous structure with many small pores of 2-5 nm. After removal of the surfactant by any method, the structure is spoiled. The surfactant removal leads to breaking, enlarging, and scattering of the particles, which can be easily seen in Figure 4. Consequently, the surface area of the SnO2 is reduced, as verified by the surface area measurements. Surface Area Measurements. The BET surface area results are given in Table 1. The surface area of the asprepared sample is very small, 0.4 m2/g, increasing considerably after calcination at 300 °C for 2 h to 156 m2/g. Further heating to 350 °C destroys the structure, and the area is reduced to 43 m2/g. Calcination of the sample for 4 h even at 300 °C results in a destruction of the mesostructure. Similarly, extraction of CTAB by ethanol for 24 h also resulted in a drastic decrease of the surface area, but some of the structure is maintained with extractions of shorter duration. The BET surface area is found to be 24 and 92 m2/g after extractions of 6 h and 12 h, respectively. The improved surface area after solvent
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Figure 4. TEM photographs: (a) as-prepared sample, (b) calcined at 300 °C for 4 h, (c) after solvent extraction for 24 h.
Figure 5. Adsorption-desorption isotherm for SnO2 mesoporous structure after calcination at 300 °C for 2 h.
extraction is due to the gradual removal of the surfactant from the pores of the tin oxide, which is also evident from the gradual decrease of the intensity of the -CH2vibration peak of the relevant IR spectra (Figure 1e,f). After full removal of the surfactant, the mesostructure collapses, reducing the surface area to 3 m2/g (Figure 1g). Figure 5 shows the N2 adsorption-desorption isotherm for the calcined sample at 300 °C for 2 h, that is, the sample having the highest surface area. The curve shows a typical H2 type of hysteresis (according to IUPAC classification), which is characteristic of inorganic porous oxides, with less defined pore distribution and shape.22 The pore size distributions were obtained according to the BarrettJoyner-Halenda (BJH) method using the Halsey equation for multilayer thickness.22 This is shown in Figure 6. This plot shows that the dominant peaks are in a mesoporous range with a narrow peak at around 3.2 nm. Application in Dye-Sensitized Solar Cells. The high conversion efficiency of dye-sensitized solar cells (DSSCs) necessitates a high surface area of the wide band gap (22) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pietotti, R. A.; Rouquerol, J.; Siemienieska, T. Pure Appl. Chem. 1985, 57, 603.
Figure 6. Pore size distribution for SnO2 mesoporous structure after calcination at 300 °C for 2 h.
semiconductor electrodes. The nanoporous TiO2 electrode is, so far, the most studied wide band gap semiconductor yielding the highest solar-to-energy conversion efficiency.23 However, in some cases SnO2 has advantages over TiO2, despite having a lower efficiency. These cases mostly relate to dyes that cannot inject into TiO2 due to an energetic mismatch. The porous SnO2 electrodes were made using the mesoporous SnO2 particles produced by the sonochemical method. These electrodes were incorporated in DSSCs without any modification, after sintering at 250, 350, and 450 °C. The performance of these DSSCs is summarized in Table 2. The results show that the photocurrent generated in these cells is highly dependent on the sintering process. Only after sintering at 450 °C did the cell generate a photocurrent (5.0 mA/cm2) that resembles the current achieved with standard SnO2-based DSSCs.24 (23) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737-740.
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Table 2. Influence of Different Sintering Times on the Solar Cell Performance amount of conversion adsorbed dye Jsc/dye sintering Jsc Voc efficiency per substrate amount at [mA/cm2] [mV] [%] area (au) (au) no 250 °C 350 °C 450 °C
0.09 0.24 2.94 5.27
276 369 354 369
0.013 0.051 0.600 0.676
1.00 0.46 0.34 0.53
0.01 0.05 0.87 1.00
The photocurrent threshold with respect to the sintering temperature is 350 °C. At this temperature, the surfactant is totally removed as measured by TGA (see Thermal Analysis above). Thus, it seems that the low current observed using the electrodes sintered at low temperatures could be attributed to the inability of the dye to adsorb directly onto the SnO2 surface. The electron injection across an organic mediator is known to be very low. This conclusion may be supported by the open circuit voltage values of the cells, that is, the high value obtained for the cell consisting of the film sintered at 250 °C. It is possible that the adsorbed surfactant slows the recombination process while allowing limited injection.25 The results presented in Table 2 show that the best performance is achieved using the electrode that was sintered at 450 °C, despite the fact that the surface area of this electrode is the lowest. This behavior is presented in Table 2 in terms of photocurrent per the amount of dye adsorbed onto the illuminated electrode. The performance improvement after sintering at 450 °C could be related to (24) Chappel, S.; Zaban, A. Sol. Energy Mater. Sol. Cells, submitted for publication. (25) Gregg, B. A.; Pichot, F.; Ferrere, S.; Fields, C. L. J. Phys. Chem. B 2001, 105, 1422.
the removal of an organic layer that was not detected by the TGA. However, steady-state fluorescence measurements of dyed films showed that the injection efficiency does not change with the 450 °C sintering. Another possibility for the increase of the DSSC efficiency at 450 °C relates to structural issues that can increase the electron collection efficiency, either by the extension of the electron diffusion length or by a decrease of the SnO2support interfacial resistance. Conclusion We have reported a new, rapid method for the preparation of mesoporous tin oxide. The structure is found to be stable up to 250 °C, with a BET surface area comparable with those in earlier reports. The advantage of our method is that it takes only 3 h for such a synthesis whereas the methods used in all earlier reports require 48-72 h. Sonochemistry does not affect the mechanism of the formation of mesostructures as such, but it enhances the rate of the reaction by increasing the molecular mobility and hence the number of molecular collisions. The reason for the improved mobility is the increase in temperature and pressure following the cavitation phenomenon.26 Acknowledgment. D.N.S. is thankful to the Council of Higher Education of Israel, Jerusalem, for providing financial support in the form of a Postdoctoral Fellowship. S. Chappel thanks the Israeli Ministry of Science (MOS). We also thank Ms. Louise Braverman for editorial assistance. A. Gedanken acknowledges the support of the German Ministry of Science (BMBF) through the DeutschIsraelische Projektpartnerschaft (DIP). LA015761+ (26) Sonochemistry: the uses of ultrasound in chemistry; Mason, T. J., Ed.; Royal Society of Chemistry: Cambridge, 1990.
