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SolGel Synthesis of a Biotemplated Inorganic Photocatalyst: ASimple Experiment for Introducing Undergraduate Students toMaterials Chemistry

Vittorio Boffa,*, Yuanzheng Yue,, and Wen He

Section of Chemistry, Aalborg University, Shongardsholmsvej 57, 9000 DK Aalborg, DenmarkKey Laboratory of Processing and Testing Technology of Glass & Functional Ceramics of Shandong Province, ShandongPolytechnic University, Jinan 250353, China

*S Supporting Information

ABSTRACT: As part of a laboratory course, undergraduate students were askedto use bakers yeast cells as biotemplate in preparing TiO2powders and to testthe photocatalytic activity of the resulting materials. This laboratory experience,

selected because of the important environmental implications of soft chemistryand photocatalysis, provides an opportunity to teach valuable laboratory skillsand to introduce students to the synthesis, isolation, and characterization ofinorganic materials. This laboratory activity is adaptable to a range of educationallevels 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

Over the past decade, many scientists have been inspiredby the possibili ty of fabricating durable inorganicmaterials reproducing biological shapes and architectures.1

Plant leaves and stems,2,3 cottonbers,4 eggshellmembranes,5,6

sponges,7 spiderwebs,8 DNA fragments,9 viruses,10 compostedorganic refuse,11 and many other biotemplates have been usedas directing agents for the preparation of nanostructuredinorganic particles, monoliths, and lms.

The development of biomimetic inorganic materials has beenlargely related to advances in solgel technology, whichinvolves the phase transformation of a colloidal suspension(sol) into a nonuid mass (gel).1214 If this transformationoccurs in the presence of a biological substance, the gel will

retain the shape of the biotemplate after calcination, yielding aninorganic biomimetic material. The design of effective syntheticroutes requires knowledge of solgel processes of nucleation,growth, and gelation and of the properties of the nal oxidematerials. This subject covers several scientic elds,encompassing chemical synthesis, materials science, physicalchemistry, and environmental science. For this reason, solgelsynthesis has been generally accepted as an approach tointegrating various chemical subjects into a single course.1521

The interdisciplinary nature of solgel experiments can be usedto introduce inorganic chemistry students to a broad number of

basic subjects and various instruments.

Imprinting the shapes of biological molecules in durableinorganic materials can be easily explained to various audiencesand is fascinating to both students and people outside academe.For this reason, we designed and assigned our undergraduatestudents an interdisciplinary experiment based on the use of

bakers yeast cells as a biotemplate in preparing TiO 2powders,which also involved testing the photocatalytic activity of theresulting materials. We selected this laboratory exercise becauseof the important environmental implications of photocatalysisconcerning, for example, the abatement of organic pollutantsfrom aqueous streams. This experiment, involving fumingasks, exothermic reactions, and color changes, generatedexcitement among our students, so we permanently included itin the laboratory course.

EXPERIMENT DESCRIPTION

Method

In our university, student learning is based on the cooperativesolution of real-life problems, and students are trained to solveproblems and perform laboratory exercises in groups of four orve starting in the rst semester of their studies. This laboratorycourse is affiliated with lectures in the third semester andcomprises various activities involving the preparation and

Published: September 26, 2012

Laboratory Experiment

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characterization of organic substances, organometallic com-plexes, and inorganic materials. Sixteen students divided in fourgroups participated in this fourth-sememster laboratory course.Before the laboratory sessions started, students were asked todiscuss, within and among groups, how to perform theexperiments. In this specic assignment, students had to designa synthetic route for preparing yeast-templated TiO2 powders

based on experimental procedures reported in a recentpubblication by He et al.22 and to develop a method fortesting the photocatalytic performance of the resultingmaterials. Therefore, students had to critically read the scienticpaper with the help of their supervisor. The goal of this rsttask was to make our students more familiar with the languageand the structure of scientic papers.

According to He et al.,22yeast cells can induce the formationof hierarchically organized mesoporous titania structures withhigh photocatalytic activity. Because students were unsure ofthe quantity of yeast required for the reaction, they decided, inconsultation with their supervisor, to coordinate their group

work to investigate the effect of yeast concentration in thereaction mixture on the photocatalytic activity of the naltitania powders. According to their plan, each group had to

prepare two materials, that is, a yeast-templated photocatalystand a reference sample that was fabricated in the absence of anystructural-directing agents. All groups applied the sameexperimental conditions except for the yeast concentration inthe solgel mixture when preparing the biotemplated material.

SolGel Synthesis

The equipment and chemicals used by each group of studentsare listed in Table1. To obtain the biotemplated photocatalytic

powder, 0.12 g of commercial yeast cells was dispersed in 30mL of deionized water. This suspension was vigorously stirredfor 60 min at 40 C, yielding a milky mixture. Meanwhile, 10mL of TiCl4was carefully dropped into 20 mL of concentratedHCl under a fume hood. TiCl4is a colorless liquid with a hightendency to form TiO2particles by reacting with air moistureor water according to the following overall reaction:

+ + ++ TiCl (l) 2H O(l) TiO (s) 4H (aq) 4Cl (aq)4 2 2

(1)

where TiO2is a solid phase dispersed in a liquid phase, that is, acolloid. However, the highly acidic pH of the concentrated HClsolution stabilized the Ti4+ ions, inhibiting the formation ofTiO2clusters so that a dark yellow solution was obtained.

When TiCl4 was quickly dropped in concentrated HCl, apale yellow precipitate was observed on top of the acidicsolution, but complete dissolution of this solid was obtained ina few minutes by vigorously stirring. TiCl

4 dissolution is

strongly exothermic and produces white hydrochloric acidfumes and steam, so a fume hood with good ventilation and thepresence of a supervisor are required for this operation (see theHazards section). After cooling to room temperature, thissolution (i.e., 10 mL of TiCl4 dissolved in 20 mL ofconcentrated HCl) was added dropwise to the yeastsuspension. The pH of the nal mixture was below 2 andTiO2 precipitation was not observed. After the mixture wasstirred overnight at room temperature, concentrated ammoniasolution (25%) was slowly added dropwise to the yeastTiCl4suspension under vigorous stirring to raise the pH and causethe nucleation and growth of TiO2particles. At pH 5, whiteTiO2 akes started forming in the suspension, and when pH 9 was reached, a thick gel was obtained. Concentrated ammoniasolution, 1215 mL, was added before observing precipitation.The formation and condensation of TiO2clusters is a complexprocess that cannot be treated here. A concise explanation ofsolgel synthesisfrom nonsilicate precursors is given by Wrightand Sommerdijk23 in a text suitable for didactical purposes; amore detailed discussion of the subject is found in the well-knownSolGel Scienceby Brinker and Scherer.24

The TiO2gel was suspended in 100 mL of water, poured intocentrifuge tubes and centrifuged. The resulting solid was

washed with 100 mL of hot water and centrifuged a secondtime; it was then dried overnight in a vent oven at 80 C,calcined at 400 C for 3 h, and nely ground in a mortar. Thecombustion of yeast cells during calcination yielded a highlyporous material. However, at 400 C not all the organic matteris oxidized to CO2 and carbonaceous resides remain in thematerial. According to He et al., these carbonaceous residuesenhance the photocatalytic activity of the material, especiallyunder visible light.22

Each group of students also prepared a reference TiO2sample by dropping the TiCl4 solution in 30 mL of pure

water and following the same synthesis procedure describedabove for the biotemplated samples.

Photocatalytic Tests

Determining the photocatalytic power of TiO2powders is nottrivial,25 so students needed to assess the advantages andlimitations 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 photocatalyticactivity of titania powders by the degradation of an organic dye,toluidine blue, under a 365 nm UV lamp. Toluidine blue issoluble in water and has a strong absorption peak at 630 nm, sothe photodegradation process can be easily followed bymeasuring the dye concentration using UVvis spectropho-tometry. The items shown in Figure 1A, namely, a cardboard

box, tape, a black bin liner, a cutter, and a UV lamp, were givento each group of students to fabricate a test chamber. Thischamber is needed to prevent interference from external lightsources, which may affect the measurement results. A simpletest chamber constructed by the students is shown in Figure1B.

Table 1. Chemicals and Equipment Used by Each Group ofStudents

Chemicals Equipment

Commercial bakers yeast (De Danske Gr

Fabrikker) bought in a supermarket

Fume hood

TiCl4 (98.0%, Fluka) Centrifuge, centrifugetubes

HCl (32%, Sigma-Aldrich) UV lamp(= 365 nm)

Ammonia solution (25%, Merck) UVvisspectrophotometer

Toluidine blue (Fluka) Analytical balance

Hot plate stirrer

Two beakers(250 mL)

1 Conical ask(200 mL)

1 Spatula

Graduated pipets ormicropipets

Mortar and pestle

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The photocatalytic tests were performed as follows: 100 mgof TiO2 powder and 50 mL of 0.1 mg L

1 toluidine bluesolution were transferred into 100 mL beakers and placedunder gentle agitation on a magnetic stirrer in the student-madetest chamber. The samples were then exposed to UV light for 3h. At regular intervals, 2 mL aliquots were taken from thesolutions and placed in centrifuge tubes kept in a dark containeruntil being centrifuged. After centrifugation, samples weretransferred into cuvettes to measure their absorbance at 630nm. The dye concentration (Cdye) of these solutions wascalculated 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 photo-catalyst. The photocatalytic tests were repeated three times foreach sample. The average values and standard deviations (error

bars) obtained in the catalytic tests performed by one studentgroup are shown in Figure2. The dye abatement percentage isdened as Cdye/C0 100, where C0 is the dye concentration

before UV exposure. In the absence of photocatalyst, onlyapproximately 10% of the dye was photodegraded after 180min; in contrast, in the presence of the titania powder, nearly100% dye abatement was achieved after the same exposuretime. Figure 2 also shows a signicant difference in photo-catalytic activity between the TiO2 powder prepared in thepresence of the yeast organic template and the reference sample

prepared in the absence of the organic templates. Thebiotemplated sample displayed higher photolytic activity,reaching >97% dye abatement after 90 min under UV light,

whereas the reference sample reached only 80 6% dyeabatement after the same exposure time. The results reportedin Figure2 show a yeast-templated photocatalyst prepared byadding the TiCl4solution to 1 g of yeast cells dispersed in 30mL of water. Similar results were obtained when twice thequantity of yeast was used. In contrast, titania powdersprepared by dispersing 0.1 or 0.5 g of yeast displayedphotocatalytic activity not signicantly different from that ofthe reference nontemplated sample. These results promptedfurther investigation by the authors to understand the effect of

yeast cells on the morphology and photocatalytic activity ofTiO2 crystals. These results were consistent with thoseobtained by the students and will be reported in a dedicatedpaper. The tests on the authors samples showed also that thephotocatalytic performances of these powders are affected bytheir grain size and dispersibility.

HAZARDSConcentrated hydrochloric acid is extremely corrosive.Concentrated ammonia solution is harmful to contact withskin and eyes and can release ammonia vapors that are severelyirritating to the eyes and to the respiratory tract. TiCl4 cancause 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 adequateventilation. Dropping TiCl4into concentrated HCl and addingammonia to the yeastTiCl4 solution are highly exothermicprocesses, resulting in the development of toxic fumes.Laboratory tutors and teachers should carefully assist studentsduring these two operations.

CONCLUSIONSA simple laboratory experiment consisting of the preparation ofsolgel-derived photocatalysts is described here. In thisexperiment, students experienced group work and criticallyread scientic papers, both of which are activities relevant totheir future work. They learned how to design an effective solgel synthetic procedure for preparing titania photocatalysts,

involving the typical fabrication steps: sol preparation, geltransition, and calcination. After analyzing the possible methodsfor investigating the photocatalytic activity of these materials,the students decided to follow the degradation of a dye insolution using UVvis spectroscopy, for which they designedand constructed a simple test chamber. In consultation withtheir supervisor, students coordinated their group work toinvestigate the effect of yeast concentration in the reactionmixture on the photocatalytic performance of the nalconsolidated material.

This experiment generated enthusiasm among students,helping them learn valuable laboratory skills and enhancingtheir understanding of issues related to preparing and testing

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

Figure 2.Results of the photocatalytic tests: toluidine blue abatementas a function of irradiation time for a blank sample containing a dyesolution without photocatalyst, 100 mg of a TiO2powder prepared inthe absence of biotemplate, and 100 mg of a yeast-templated TiO2powder.




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inorganic materials and catalysts. This laboratory can beadapted to a range of educational levels and to variousinstrumental techniques, such as optical and electronicmicroscopy, porosimetry, X-ray diffraction, and FTIR spectros-copy, important for studentsfuture studies and research. Thephotocatalytic tests offer further didactic possibilities, forexample, to introduce students to the use of high-pressureliquid chromatography or gas chromatography for the analysisof intermediate degradation products and to foster under-standing of photodegradation kinetics. Considering theincreasing relevance of soft chemistry and photocatalysis,undergraduate students may benet greatly from theintegration of this laboratory experiment into their exper-imental courses.

ASSOCIATED CONTENT*S Supporting Information

Instructions for students; notes for the instructors. Thismaterial is available via the Internet athttp://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

Notes

The authors declare no competing nancial interest.

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