2009 Ordered hierarchical mesoporous anatase TiO2 from yeast biotemplates

Colloids and Surfaces B:Biointerfaces 74 (2009) 274–278

Contents lists available at ScienceDirect

Colloids and Surfaces B:

Biointerfaces

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /c o l s u r f

b

Ordered hierarchical mesoporous anatase TiO 2from yeast biotemplates

Jingjie Cui a ,Wen He b ,?,Hongtao Liu c ,Shijun Liao a ,?,Yuanzheng Yue b ,d

a

School of Chemistry and Chemical Engineering,South China University of Technology,Guangzhou,510640,PR China b

Department of Materials,Shandong Institute of Light Industry,Jinan 250100,PR China c

College of Chemistry &Chemical Engineering,Central South University,Changsha 410083PR China d

Section of Chemistry,Aalborg University,DK-9000Aalborg,Denmark

a r t i c l e i n f o Article history:

Received 29June 2009

Received in revised form 18July 2009Accepted 22July 2009

Available online 30 July 2009Keywords:

Mesoporous titanium dioxide Yeast cells

Biomimetic synthesis Electrocatalytic activity

Oxygen reduction reaction (ORR)

a b s t r a c t

A novel approach is proposed to prepare ordered hierarchical mesoporous TiO 2by using yeast cells as the template via biomimetic mineralization.The structure and the mophology were characterized by atomic force microscopy (AFM),X-ray diffraction (XRD),energy-dispersive X-ray analysis (EDX),Raman spectra,high-resolution transmission electron microscopy (HRTEM),and N 2adsorption–desorption isotherms (NADI).The bio-templated TiO 2has a hierarchical pore structure mainly distributed at 4.7nm and 11.3nm.An air electrode fabricated using this hierarchical mesoporous TiO 2exhibited remarkable electrocatalytic activity for oxygen reduction reaction (ORR).Compared to the electrolytic manganese dioxide (EMD)air electrode employed commercially,a 90%higher catalytic reduction current was achieved for this mesoporous TiO 2electrode.It is supposed that the hierarchically mesoporous structure and ordered pore distribution play important roles in signi?cantly reducing electrochemical polarization and improving mass transport of the air diffusion electrode.This simple and ef?cient approach could spread as a general way to prepare advanced mesoporous materials in mild condition.

Crown Copyright ? 2009 Published by Elsevier B.V. All rights reserved.

1.Introduction

Porous materials have attracted great interest due to their special structures and excellent properties for many potential appli-cations in catalysis,biosensors,and functional electronic devices [1,2].Over the recent years,enormous mesoporous materials with various enhanced functions have been prepared via chemical meth-ods,including manganese oxides [3],iron oxide [4],titanosilicate [5],TiO 2[6,7],and carbon related materials [8,9].A key issue in fabricating functional porous materials is the control of size and morphology [10,11].Recently,hierarchical mesoporous materials,with three-dimensional structures patterned over multiple length scales,attracted lots of attention due to their excellent properties.For example,hierarchical mesoporous zeolites possessed strong acidity and higher catalytic activity than conventional zeolites [12],and hierarchical mesoporous TiO 2exhibit better photocatalytic activity [13,14].

Biomineralization is the process of mineralization in living system,currently ?nds increasing research interest [15–18].The advantage of biomineralization processes is that they not only yield advanced synthetic materials in an environment benign sys-

?Corresponding authors.Tel.:+8653189631018;fax:+8653189631518.

E-mail addresses:hewen1960@https://www.wendangku.net/doc/8714819690.html, (W.He),chsjliao@https://www.wendangku.net/doc/8714819690.html, (S.Liao).

tem,but also enable control over the phase,size,morphology and crystallographic orientation of inorganic crystals.In addition,biomineralization is a typical self-organized formation of hierarchi-cal structures at ambient atmosphere [18].

As the most well-known multifunctional materials,titanium oxides are widely investigated in various ?elds [19–27].Currently,it becomes popular to synthesize TiO 2via biomimetic method,e.g.TiO 2was synthesized through a biocatalytic route [28],and the hier-archical pores were controlled using biotemplates including green leaves,bamboo inner membrane,and eggshell membrane [29–31].Protamine–titiania hybrid microcapsules were newly reported by Jiang et al.[32]that using a composite policy by combining layer-by-layer assembly with biomimetic mineralization to form hierarchical structures.

In this work,based on the biomineralization assembly concept,we demonstrate a facile,controllable,and universal biomimetic synthesis of hierarchical ordered mesoporous TiO 2by using low-cost microbe yeast cells as templates.The as-prepared hier-archical mesoporous TiO 2was ?rst employed as the electrocatalyst and exhibited remarkable electrocatalytic activity for the oxygen reduction reaction (ORR)in alkaline solution.This hierarchical mesoporous TiO 2is considered to be very promising for application in biosensor,fuel cell,and metal–air battery ?elds.Additionally,this simple and environmentally-friendly method can be eas-ily scaled up for industrial production of advanced mesoporous materials.

0927-7765/$–see front matter.Crown Copyright ? 2009 Published by Elsevier B.V. All rights reserved.doi:10.1016/j.colsurfb.2009.07.030

J.Cui et al./Colloids and Surfaces B:Biointerfaces74 (2009) 274–278275

2.Experimental

2.1.Synthesis of bio-templated TiO2

The quantitative yeast cells were cultivated in a30mL of glucose aqueous solution at room temperature.After30min stirring,a uni-form bioemulsion was formed.The critical micelle concentration (CMC)of the bioemulsion is0.0067g mL?1.The solution of10mL of TiCl4( 99.0%)dissolved into20mL of HCl(36–38%)was added into the above bioemulsion dropwise and kept stirring for24h.The ammonia hydroxide solution(25–28%)was then added dropwise to adjust the pH value9–10.The formed white precipitate was collected and carefully washed with distilled water and absolute ethanol,and dried at80?C in air.The dried samples were heat-treated at400?C for1h to obtain the?nal products.All reagents were of analytical reagent grade.

2.2.Structure characterization

X-ray diffraction(XRD)measurements were performed at a PANalytical X’Pert PRO X-ray diffractometer with Cu K?( =0.15418nm)and Co K?( =0.17900nm)incident radiation.The diffraction patterns were collected at room temperature in the ranges of1.1and10?(Co K?),as well as10and70?(Cu K?)of2?, respectively.

Energy dispersive X-ray analysis(EDX)was performed to quali-tatively determine elemental composition of the samples.

The Raman spectra were recorded on a HJY LabRAM Aramis spectrometer(HORIBA Jobin Yvon,France)with a slit width of 200?m,using a green laser at532nm.The output power of the laser was3mW.Spectra were collected three times with a resolution of 0.15cm?1.

High-resolution transmission electron microscopy(HRTEM) was carried out on a Philips Tecnai20U-TWIN microscope,working at300kV.A trace amount of sample was dispersed in ethanol solu-tion by sonication,and then deposited on a carbon-coated copper grid,which was used as a TEM specimen.

The N2adsorption–desorption isotherms(NADI)were car-ried out at77K using a computer controlled sorption analyzer (Micromeritics,Gemini V2.0)operating in the continuous mode. The samples were degassed at200?C10h prior to the measure-ment.The pore size distribution was calculated from desorption branch of the isotherm by the Barret–Joyner–Halenda(BJH)model.

Atomic force microscopy(IIIa AFM)was used to observe the morphologies of the synthesized materials.The samples for AFM measurements were prepared by dropping the ethanol suspension with the dispersed particles dropwise onto a freshly cleaved mica piece,and air-dried.

2.3.Preparation of air electrode and electrochemical

measurement

The catalyst,acetylene black(AB),activated charcoal,and poly-tetra?uoroethylene(PTFE)suspension(60wt%in H2O)with weight ratios of4:1:2:2were uniformly mixed and dispersed in excess of ethanol and got dough-like paste.The paste was rolled into a sheet of0.3mm thickness.Another sheet containing only AB and PTFE with weight ratios of1:1was prepared by the same process.The two sheets were then rolled together and pressed into a piece of nickel foam.The foam loaded with active materials was heat-treated at 200?C for2h,and thus got0.5mm thick air electrode.

The electrochemical measurements were performed in a three-electrode con?guration.A large-area Pt disk and an Hg/HgO/OH?(35wt%)setup were used as the counter electrode and the ref-erence electrode,respectively.A1cm2active air electrode was served as the work electrode.Aqueous potassium hydroxide solu-tion(35wt%)was used as the electrolyte in all experiments. The polarization curves were acquired under sweeping rate of 10mV/s on a CHI660electrochemical work station.The electro-chemical impedance spectra were recorded in the frequency range of100mHz to100kHz on an Autolab electrochemical work sta-tion.

3.Results and discussion

3.1.Biomimetic synthesis of ordered hierarchical mesoporous TiO2

As illustrated in Scheme1,an approach with biomimetic mineralization was developed to prepare ordered hierarchical mesoporous TiO2.The yeast cells were used as biotemplates for the synthesis of ordered hierarchical mesoporous TiO2.Hence,yeast cells play a key role in forming an ordered hierarchical mesoporous structure.Yeasts are ubiquitous unicellular microorganisms[33]. During the cultivation processes of yeast cells(fermentation),a bioemulsi?er with some hydrophilic anion groups including car-boxyl(COO?)and–OPO32?were produced on the surface of cells [34,35].It should be mentioned that the hydrophilic anion groups mainly played two roles:provided oriented nucleation sites for target cations[36–38],and accumulated more negative charges on the bio-template surface[39,40].Positively charged titanium cations were combined with the negatively charged COO?and –OPO32?groups,and were self-assembled to the yeast cells sur-face by electrostatic interaction when the TiCl4aqueous solution and the yeast were mixed and stirred at room temperature.Next, when ammonia hydroxide solution was added to the system,the yeast/Ti(OH)4core-shell spheres were formed due to in situ hydrol-ysis and condensation of titanium cations.Finally,after calcination, the yeast/Ti(OH)4core-shell spheres converted to mesoporous TiO2 and the products retained the pro?le of the yeast cell walls.The ordered hierarchical mesoporous morphology was proved by the atomic force microscope(AFM)images of the yeast cells and the results for characterization of the bio-templated TiO2were shown in Fig.1.Periodic spiculate clusters,and“skeletal”mesh-like struc-tures can be observed at the surface of yeast cells(Fig.1left). The bio-templated TiO2exhibited?at microcapsule like cell shape (Fig.1right).The ordered hierarchical structures of yeast cells were replicated during TiO2formation processess.It is clear in the selected area in Fig.1that there are two types of mesoporous

net-Scheme1.Illustration of ordered hierarchical mesoporous TiO2preparation process.

276J.Cui et al./Colloids and Surfaces B:Biointerfaces

74 (2009) 274–278

Fig.1.AFM images of yeast cells and the bio-templated mesoporous TiO 2.

work structures on the surface of bio-templated TiO 2(region A and B).

3.2.Structure and composition characterizations of bio-templated mesoporous TiO 2

Powder X-ray diffraction patterns were taken from the meso-pore materials at low angles to assess the mesopore structure and at higher angles to evaluate the crystalline structure [41].And the diffraction peak observed in the X-ray pattern at low angles corre-lated to the mesopore size [42].Fig.2a showed the small angle XRD pattern (SAXRD)of bio-templated TiO 2.A broad scattering peak appeared at 2?between 2and 10?indicated wide pore size distri-bution.The strongest diffractive intensity at 3.22?is corresponding to a d -spacing of 3.5nm.In the course of the biomimetic miner-alization process,the mesoporous TiO 2accurately replicated the textures of yeast cells (see Fig.1).The broad peak is assigned to exis-tence of ordered worm-like textural pores in the bio-templated TiO 2[42].The results of wide angle X-ray diffraction (WAXRD)(shown in Fig.2b)and energy-dispersive X-ray analysis (EDX,see Fig.3)show that the sample obtained via calcination at 400?C consists of major anatase phase (87.42wt%)and slight carbon (11.44wt%)from microbial cells.Based on Scherrer formula,the size of anatase tita-nia particles was calculated about 10nm.Fig.4shows the Raman spectra of bio-templated mesoporous TiO 2.The Raman peaks at 395,515and 638cm ?1were assigned to B 1g,A 1g and Eg,respec-tively,which disclose the typical anatase TiO 2structure.This is consistent with the WAXRD result.

High-resolution transmission electron microscopy (HRTEM)examination (Fig.5)provides more intuitionistic information about the morphology and microstructure of the bio-templated TiO 2.Worm-like pores with different pore size were clearly observed in Fig.5.The pore diameters estimated from HRTEM image are about 3–15nm.The anatase lattice fringes of the bio-templated meso-porous TiO 2are easily seen.Distance of the aligned lattice fringe spacing was measured to be 0.35nm,which is consistent with the d 101spacing of anatase TiO 2.The anatase phase is also con?rmed by analyzing the electron diffraction (ED)pattern (Fig.5,inset).The mesoporosity was further characterized by nitrogen-adsorption measurements.Fig.6shows the adsorption–desorption isotherm and Barret–Joyner–Halenda (BJH)pore-size-distribution curve of the synthesized TiO 2sample.According to International

Union of Pure and Applied Chemistry (IUPAC)regulation [43],the shape of the isotherm (Fig.6a)of the bio-templated TiO 2is a typical type-IV pattern that indicates mesoporous characteristics.The bio-templated mesoporous TiO 2shows a wide and bimodal pore-size distribution centered on 4.7and 11.3nm (Fig.6b).Two hysteresis loops (in Fig.6a)respectively corresponded to opening-type

struc-

Fig.2.XRD patterns of the bio-templated mesoporous TiO 2:(a)Small angle X-ray diffraction (SAXRD)pattern;(b)Wide angle XRD pattern.

J.Cui et al./Colloids and Surfaces B:Biointerfaces 74 (2009) 274–278

277

Fig.3.EDX of bio-templated mesoporous TiO 2

.

Fig.4.Raman spectrum of bio-templated mesoporous TiO 2.

ture (region 1)and plugged pores structure (region 2)pores [44].The intensity ratio of pore volumes (see Fig.6b)is 7:1,indicating the opening-type structure is predominant.

In comparison with generally used chemical methods to gen-erate mesoporous TiO 2,the advantages of our method primarily include:(i)the used templates are low-cost and non-toxic,(ii)hier-archical mesoporous structures are easily controlled.

Compared

Fig.5.HRTEM image of the bio-templated mesoporous TiO 2.

with other biotemplates,the yeast cells are much cheaper and more easily scaled up for industrial applications.

3.3.Electrocatalytic activity for ORR of bio-templated mesoporous TiO 2

Oxygen reduction is one of the most important electrochem-ical reactions since it has multiple applications in a variety of research ?elds,ranging from energy conversion to corrosion science [45].Herein,electrocatalytic activity for ORR of the bio-templated mesoporous TiO 2was investigated in 35wt%KOH aqueous alka-line solution.Fig.7a shows comparison of the electrochemical polarization curves for ORR in the alkaline solution.The normal nano-anatase titanium dioxide exhibits only relatively poor elec-trocatalytic activity as an oxygen-reduction catalyst (curve C).The electrochemical polarization curves showed that the bio-templated mesoporous anatase TiO 2have excellent electrocatalytic activity for oxygen reduction reaction (ORR)(curve A).Compared with EMD (non-porous material)commercially available air electrode,the potential of air electrode with bio-templated mesoporous TiO 2catalyst (curve A)was more positive by 64mV than that of EMD (curve B)in the low current density region (?40mA/cm 2),indicat-ing bio-templated mesoporous TiO 2catalyst has the higher catalytic activity for the ORR;and a 90%higher catalytic current was achieved for this mesoporous TiO 2electrode at ?0.6V.The excellent perfor-mance for the oxygen reduction of the bio-templated mesoporous TiO 2is closely related to its higher speci?c surface and ordered mesoporous structure.Obviously,the ordered mesoporous and tex-tural structures with hierarchical pore size distribution

provided

Fig.6.N 2adsorption–desorption isotherm (a)and pore size distribution (b)of the bio-templated mesoporous TiO 2.

278J.Cui et al./Colloids and Surfaces B:Biointerfaces

74 (2009) 274–278

https://www.wendangku.net/doc/8714819690.html,parison of Polarization curves(a)and Nyquist plots(b)of the ORR with (A)bio-templated TiO2;(B)EMD;(C)normal nano-anatase TiO2.

larger3-phase interfacial area and shorter diffusion path,thus facil-itating mass transfer[14,42,46].

Fig.7b shows the electrochemical impedance spectra of the bio-templated mesoporous anatase TiO2and EMD air electrodes.It can be seen that all spectra yielded two loops including one low fre-quency loop(1)and one high frequency loop(2)in the frequency range.The high frequency loop is attributed to the serial Ohmic resistances inside the catalyst layer,while the low frequency arc is attributed to the electrochemical resistance for ORR(charge trans-fer resistance)reaction that correlated to the morphology of the electrodes(e.g.diffusion processes)[47].It is obvious that the bio-templated mesoporous TiO2electrode has lower electrochemical resistance for ORR,implying an easier diffusion process for oxygen molecules and electrolyte ions(i.e.OH?).This result is consistent with that from the polarization experiments.

4.Conclusions

A mesoporous TiO2has been successfully prepared using yeast cells as templates.The results of WXRD and EDX show that the synthesized sample consists of major anatase phase and slight conductive carbon.SAXRD,AFM and BJH analysis con?rmed the ordered hierarchical mesoporous structure,and the pore size dis-tribution centered at4.7and11.3nm.The bio-templated mesopores TiO2air electrode exhibits excellent electrocatalytic activity for oxy-gen reduction reaction(ORR)in alkaline solution.The enhanced catalytic performance is ascribed to texture-templated ordered mesoporous structures that effectively speedup mass transport and relieve polarization of the electrode.The proposed method actually provides a facile,controllable,and universal way for biomimetic synthesis of advanced mesoporous materials in mild condition.Acknowledgement

We acknowledge Shandong Institute of Light Industry,China, for their technological support.This work was supported by Natural Science Foundation of China(Grant No.50732003, 9732005CB623902,20476034,20673040,20876062, 2006A50102003).

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成人本科学位英语统一考试真题及答案

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