Sol–Gel Synthesis of a Biotemplated Inorganic Photocatalyst: A

Laboratory Experiment pubs.acs.org/jchemeduc

Sol−Gel Synthesis of a Biotemplated Inorganic Photocatalyst: A Simple Experiment for Introducing Undergraduate Students to Materials Chemistry Vittorio Boffa,*,† Yuanzheng Yue,†,‡ and Wen He‡ †

Section of Chemistry, Aalborg University, Shongårdsholmsvej 57, 9000 DK Aalborg, Denmark Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics of Shandong Province, Shandong Polytechnic University, Jinan 250353, China

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S Supporting Information *

ABSTRACT: As part of a laboratory course, undergraduate students were asked to use baker’s yeast cells as biotemplate in preparing TiO2 powders and to test the photocatalytic activity of the resulting materials. This laboratory experience, selected because of the important environmental implications of soft chemistry and photocatalysis, provides an opportunity to teach valuable laboratory skills and to introduce students to the synthesis, isolation, and characterization of inorganic materials. This laboratory activity is adaptable to a range of educational levels and to various instrumental techniques.

KEYWORDS: Second-Year Undergraduate, Upper-Division Undergraduate, Environmental Chemistry, Inorganic Chemistry, Interdisciplinary/Multidisciplinary, Laboratory Instruction, Hands-On Learning/Manipulatives, Materials Science, Photochemistry, UV-Vis Spectroscopy

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Imprinting the shapes of biological molecules in durable inorganic materials can be easily explained to various audiences and is fascinating to both students and people outside academe. For this reason, we designed and assigned our undergraduate students an interdisciplinary experiment based on the use of baker’s yeast cells as a biotemplate in preparing TiO2 powders, which also involved testing the photocatalytic activity of the resulting materials. We selected this laboratory exercise because of the important environmental implications of photocatalysis concerning, for example, the abatement of organic pollutants from aqueous streams. This experiment, involving fuming flasks, exothermic reactions, and color changes, generated excitement among our students, so we permanently included it in the laboratory course.

ver the past decade, many scientists have been inspired by the possibility of fabricating durable inorganic materials reproducing biological shapes and architectures.1 Plant leaves and stems,2,3 cotton fibers,4 eggshell membranes,5,6 sponges,7 spider webs,8 DNA fragments,9 viruses,10 composted organic refuse,11 and many other biotemplates have been used as directing agents for the preparation of nanostructured inorganic particles, monoliths, and films. The development of biomimetic inorganic materials has been largely related to advances in sol−gel technology, which involves the phase transformation of a colloidal suspension (sol) into a nonfluid mass (gel).12−14 If this transformation occurs in the presence of a biological substance, the gel will retain the shape of the biotemplate after calcination, yielding an inorganic biomimetic material. The design of effective synthetic routes requires knowledge of sol−gel processes of nucleation, growth, and gelation and of the properties of the final oxide materials. This subject covers several scientific fields, encompassing chemical synthesis, materials science, physical chemistry, and environmental science. For this reason, sol−gel synthesis has been generally accepted as an approach to integrating various chemical subjects into a single course.15−21 The interdisciplinary nature of sol−gel experiments can be used to introduce inorganic chemistry students to a broad number of basic subjects and various instruments. © 2012 American Chemical Society and Division of Chemical Education, Inc.

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EXPERIMENT DESCRIPTION

Method

In our university, student learning is based on the cooperative solution of real-life problems, and students are trained to solve problems and perform laboratory exercises in groups of four or five starting in the first semester of their studies. This laboratory course is affiliated with lectures in the third semester and comprises various activities involving the preparation and Published: September 26, 2012 1466

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characterization of organic substances, organometallic complexes, and inorganic materials. Sixteen students divided in four groups participated in this fourth-sememster laboratory course. Before the laboratory sessions started, students were asked to discuss, within and among groups, how to perform the experiments. In this specific assignment, students had to design a synthetic route for preparing yeast-templated TiO2 powders based on experimental procedures reported in a recent pubblication by He et al.22 and to develop a method for testing the photocatalytic performance of the resulting materials. Therefore, students had to critically read the scientific paper with the help of their supervisor. The goal of this first task was to make our students more familiar with the language and the structure of scientific papers. According to He et al.,22 yeast cells can induce the formation of hierarchically organized mesoporous titania structures with high photocatalytic activity. Because students were unsure of the quantity of yeast required for the reaction, they decided, in consultation with their supervisor, to coordinate their group work to investigate the effect of yeast concentration in the reaction mixture on the photocatalytic activity of the final titania powders. According to their plan, each group had to prepare two materials, that is, a yeast-templated photocatalyst and a reference sample that was fabricated in the absence of any structural-directing agents. All groups applied the same experimental conditions except for the yeast concentration in the sol−gel mixture when preparing the biotemplated material.

where TiO2 is a solid phase dispersed in a liquid phase, that is, a colloid. However, the highly acidic pH of the concentrated HCl solution stabilized the Ti4+ ions, inhibiting the formation of TiO2 clusters so that a dark yellow solution was obtained. When TiCl4 was quickly dropped in concentrated HCl, a pale yellow precipitate was observed on top of the acidic solution, but complete dissolution of this solid was obtained in a few minutes by vigorously stirring. TiCl4 dissolution is strongly exothermic and produces white hydrochloric acid fumes and steam, so a fume hood with good ventilation and the presence of a supervisor are required for this operation (see the Hazards section). After cooling to room temperature, this solution (i.e., 10 mL of TiCl4 dissolved in 20 mL of concentrated HCl) was added dropwise to the yeast suspension. The pH of the final mixture was below 2 and TiO2 precipitation was not observed. After the mixture was stirred overnight at room temperature, concentrated ammonia solution (25%) was slowly added dropwise to the yeast−TiCl4 suspension under vigorous stirring to raise the pH and cause the nucleation and growth of TiO2 particles. At pH ∼ 5, white TiO2 flakes started forming in the suspension, and when pH ∼ 9 was reached, a thick gel was obtained. Concentrated ammonia solution, 12−15 mL, was added before observing precipitation. The formation and condensation of TiO2 clusters is a complex process that cannot be treated here. A concise explanation of sol−gel synthesis from nonsilicate precursors is given by Wright and Sommerdijk23 in a text suitable for didactical purposes; a more detailed discussion of the subject is found in the wellknown Sol−Gel Science by Brinker and Scherer.24 The TiO2 gel was suspended in 100 mL of water, poured into centrifuge tubes and centrifuged. The resulting solid was washed with 100 mL of hot water and centrifuged a second time; it was then dried overnight in a vent oven at 80 °C, calcined at 400 °C for 3 h, and finely ground in a mortar. The combustion of yeast cells during calcination yielded a highly porous material. However, at 400 °C not all the organic matter is oxidized to CO2 and carbonaceous resides remain in the material. According to He et al., these carbonaceous residues enhance the photocatalytic activity of the material, especially under visible light.22 Each group of students also prepared a reference TiO2 sample by dropping the TiCl4 solution in 30 mL of pure water and following the same synthesis procedure described above for the biotemplated samples.

Sol−Gel Synthesis

The equipment and chemicals used by each group of students are listed in Table 1. To obtain the biotemplated photocatalytic Table 1. Chemicals and Equipment Used by Each Group of Students Chemicals Commercial baker’s yeast (De Danske Gær Fabrikker) bought in a supermarket TiCl4 (≥98.0%, Fluka) HCl (32%, Sigma-Aldrich) Ammonia solution (25%, Merck) Toluidine blue (Fluka)

Equipment Fume hood Centrifuge, centrifuge tubes UV lamp (λ = 365 nm) UV−vis spectrophotometer Analytical balance Hot plate stirrer Two beakers (250 mL) 1 Conical flask (200 mL) 1 Spatula Graduated pipets or micropipets Mortar and pestle

Photocatalytic Tests

Determining the photocatalytic power of TiO2 powders is not trivial,25 so students needed to assess the advantages and limitations of the various techniques available for this analysis. On the basis of the reagents and facilities available in the laboratory, students decided to evaluate the photocatalytic activity of titania powders by the degradation of an organic dye, toluidine blue, under a 365 nm UV lamp. Toluidine blue is soluble in water and has a strong absorption peak at 630 nm, so the photodegradation process can be easily followed by measuring the dye concentration using UV−vis spectrophotometry. The items shown in Figure 1A, namely, a cardboard box, tape, a black bin liner, a cutter, and a UV lamp, were given to each group of students to fabricate a test chamber. This chamber is needed to prevent interference from external light sources, which may affect the measurement results. A simple test chamber constructed by the students is shown in Figure 1B.

powder, 0.1−2 g of commercial yeast cells was dispersed in 30 mL of deionized water. This suspension was vigorously stirred for 60 min at 40 °C, yielding a milky mixture. Meanwhile, 10 mL of TiCl4 was carefully dropped into 20 mL of concentrated HCl under a fume hood. TiCl4 is a colorless liquid with a high tendency to form TiO2 particles by reacting with air moisture or water according to the following overall reaction: TiCl4(l) + 2H 2O(l) → TiO2 (s) + 4H+(aq) + 4Cl−(aq) (1) 1467

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Figure 2. Results of the photocatalytic tests: toluidine blue abatement as a function of irradiation time for a blank sample containing a dye solution without photocatalyst, 100 mg of a TiO2 powder prepared in the absence of biotemplate, and 100 mg of a yeast-templated TiO2 powder.

Figure 1. (A) Components (listed in the text) used to make the test chamber for photocatalytic tests and (B) the student-made test chamber.

yeast cells on the morphology and photocatalytic activity of TiO2 crystals. These results were consistent with those obtained by the students and will be reported in a dedicated paper. The tests on the authors’ samples showed also that the photocatalytic performances of these powders are affected by their grain size and dispersibility.

The photocatalytic tests were performed as follows: 100 mg of TiO2 powder and 50 mL of 0.1 mg L−1 toluidine blue solution were transferred into 100 mL beakers and placed under gentle agitation on a magnetic stirrer in the student-made test chamber. The samples were then exposed to UV light for 3 h. At regular intervals, 2 mL aliquots were taken from the solutions and placed in centrifuge tubes kept in a dark container until being centrifuged. After centrifugation, samples were transferred into cuvettes to measure their absorbance at 630 nm. The dye concentration (Cdye) of these solutions was calculated using a calibration curve that the students acquired before the catalytic tests. Each student group performed the photocatalytic tests on samples containing (a) yeast-templated TiO2 powder, (b) nontemplated reference TiO2 powder, and (c) no photocatalyst. The photocatalytic tests were repeated three times for each sample. The average values and standard deviations (error bars) obtained in the catalytic tests performed by one student group are shown in Figure 2. The dye abatement percentage is defined as Cdye/C0 × 100, where C0 is the dye concentration before UV exposure. In the absence of photocatalyst, only approximately 10% of the dye was photodegraded after 180 min; in contrast, in the presence of the titania powder, nearly 100% dye abatement was achieved after the same exposure time. Figure 2 also shows a significant difference in photocatalytic activity between the TiO2 powder prepared in the presence of the yeast organic template and the reference sample prepared in the absence of the organic templates. The biotemplated sample displayed higher photolytic activity, reaching >97% dye abatement after 90 min under UV light, whereas the reference sample reached only 80 ± 6% dye abatement after the same exposure time. The results reported in Figure 2 show a yeast-templated photocatalyst prepared by adding the TiCl4 solution to 1 g of yeast cells dispersed in 30 mL of water. Similar results were obtained when twice the quantity of yeast was used. In contrast, titania powders prepared by dispersing 0.1 or 0.5 g of yeast displayed photocatalytic activity not significantly different from that of the reference nontemplated sample. These results prompted further investigation by the authors to understand the effect of

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HAZARDS Concentrated hydrochloric acid is extremely corrosive. Concentrated ammonia solution is harmful to contact with skin and eyes and can release ammonia vapors that are severely irritating to the eyes and to the respiratory tract. TiCl4 can cause severe skin burns and eye damage. For this reason, laboratory coats, gloves, and goggles should be worn and the reactions must be performed under a fume hood with adequate ventilation. Dropping TiCl4 into concentrated HCl and adding ammonia to the yeast−TiCl4 solution are highly exothermic processes, resulting in the development of toxic fumes. Laboratory tutors and teachers should carefully assist students during these two operations.

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CONCLUSIONS A simple laboratory experiment consisting of the preparation of sol−gel-derived photocatalysts is described here. In this experiment, students experienced group work and critically read scientific papers, both of which are activities relevant to their future work. They learned how to design an effective sol− gel synthetic procedure for preparing titania photocatalysts, involving the typical fabrication steps: sol preparation, gel transition, and calcination. After analyzing the possible methods for investigating the photocatalytic activity of these materials, the students decided to follow the degradation of a dye in solution using UV−vis spectroscopy, for which they designed and constructed a simple test chamber. In consultation with their supervisor, students coordinated their group work to investigate the effect of yeast concentration in the reaction mixture on the photocatalytic performance of the final consolidated material. This experiment generated enthusiasm among students, helping them learn valuable laboratory skills and enhancing their understanding of issues related to preparing and testing 1468

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inorganic materials and catalysts. This laboratory can be adapted to a range of educational levels and to various instrumental techniques, such as optical and electronic microscopy, porosimetry, X-ray diffraction, and FTIR spectroscopy, important for students’ future studies and research. The photocatalytic tests offer further didactic possibilities, for example, to introduce students to the use of high-pressure liquid chromatography or gas chromatography for the analysis of intermediate degradation products and to foster understanding of photodegradation kinetics. Considering the increasing relevance of soft chemistry and photocatalysis, undergraduate students may benefit greatly from the integration of this laboratory experiment into their experimental courses.

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ASSOCIATED CONTENT

* Supporting Information S

Instructions for students; notes for the instructors. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Hall, S. R. Proc. R. Soc. A 2009, 465, 335. (2) Valtchev, V. P.; Smaihi, M.; Faust, A. C. Chem. Mater. 2004, 16, 1350. (3) Cao, J.; Rusina, O.; Sieber, H. Ceram. Int. 2004, 30, 1971. (4) Fan, T.; Sun, B.; Gu, J.; Zhang, D.; Lau., L. W. M. Scr. Mater. 2005, 53, 893. (5) Yang, D.; Qi, L.; Ma, J. Adv. Mater. 2001, 14, 1543. (6) Maddocks, A. R.; Harris, A. T. Mater. Lett. 2009, 63, 748. (7) Zampieri, A.; Mabande, G. T. P.; Selvam, T.; Schwieger, W.; Rudolph, A.; Hermann, R.; Sieber, H.; Greil, P. Mater. Sci. Eng., C 2006, 26, 130. (8) Cao, B.; Mao, C. Langmuir 2007, 23, 10701. (9) Fujikawa, S.; Takaki, R.; Kunitake, T. Langmuir 2005, 21, 8899. (10) Fowler, C. E.; Shenton, W.; Stubbs, G.; Mann, S. Adv. Mater. 2001, 13, 1266. (11) Boffa, V.; Perrone, D. G.; Montoneri, E.; Magnacca, G.; Bertinetti, L.; Garlasco, L.; Mendichi, R. ChemSusChem 2010, 3, 445. (12) Cooper, M. M. J. Chem. Educ. 1994, 71, 307. (13) Buckley, A. M.; Greenblatt, M. J. Chem. Educ. 1994, 71, 599. (14) Cooper, M. M. J. Chem. Educ. 1995, 72, 162. (15) Ilharco, L. M.; Gaspar Martinho, J. M.; Martins, C. I. J. Chem. Educ. 1998, 75, 1466. (16) Higginbotham, C.; Pike, C. F.; Rice, J. K. J. Chem. Educ. 1998, 75, 461. (17) Laughlin, J. B.; Sarquis, J. L.; Jones, V. M.; Cox, J. A. J. Chem. Educ. 2000, 77, 77. (18) Celzard, A.; Marêché, J. F. J. Chem. Educ. 2002, 79, 854. (19) Parkin, I. P.; Manning, T. D. J. Chem. Educ. 2006, 83, 393. (20) Castle, K. J.; Rink, S. M. J. Chem. Educ. 2010, 87, 136. (21) Kulkarni, S.; Tran, V.; Ho, M. K.-M.; Phan, C.; Chin, E.; Wemmer, Z.; Sommerhalt, M. J. Chem. Educ. 2010, 87, 958. (22) He, W.; Cui, J.; Yue, Y.; Zhang, X.; Xia, X.; Liu, H.; Lui., S. J. Colloid Interface Sci. 2011, 354, 109. (23) Wright, J. D.; Sommerdijk, N. A. J. M. Sol-Gel Materials: Chemistry and Applications; Gordon & Breach Science: London, 2001. (24) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: San Diego, CA, 1990. (25) Ohtani, B. Chem. Lett. 2008, 37, 217. 1469

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