TiO2 nanocrystals

Low-Temperature Synthesis of HighlyCrystalline TiO2 Nanocrystals and theirApplication to Photocatalysis**

Sangjin Han, Sang-Hyun Choi, Seok-Soon Kim,Min Cho, Byungchul Jang, Dong-Yu Kim,Jeyong Yoon, and Taeghwan Hyeon*

Nanocrystals of transition metal oxides have attracted agreat deal of attention from researchers in various fieldsdue to their numerous technological applications.[1] Amongthem, titania (TiO2) nanocrystals have been the most inten-sively studied owing to their versatile applications, which in-clude solar cells,[2] photocatalysts,[3] and photochromic devi-ces.[4] Many synthetic methods have been reported for thepreparation of TiO2 nanocrystals, including sol–gel reac-tions,[5] hydrothermal reactions,[6] nonhydrolytic sol–gel reac-tions,[7] template methods,[8] and reactions in reverse mi-celles.[9] TiO2 nanocrystals with various morphologies andshapes, such as nanorods,[10] nanotubes,[11] nanowires,[12] andnanospheres[13] can be produced depending on the syntheticmethod used. Due to the high reactivity of titanium precur-sors such as TiCl4 and titanium alkoxides, the control of thereaction rate is a key factor in obtaining TiO2 nanocrystalswith the desired crystalline structure and/or shapes. Chem-seddine and co-workers have reported the synthesis of uni-form-sized TiO2 nanocrystals whose shapes varied depend-ing on the ratio of Me4NOH to titanium alkoxide.[14] How-ever, the synthesis was performed at a very low concentra-tion and produced only a small quantity of the nanocrystals.Weller and co-workers have reported the controlled growthof TiO2 nanocrystals by modulation of the hydrolysis rate,using oleic acid as a stabilizing surfactant at 80 8C,[15] andJun et al. have reported the surfactant-mediated shape evo-lution of anatase nanocrystals in nonaqueous media at300 8C.[16] Although many of these high-quality TiO2 nano-

[*] Dr. S. Han, S.-H. Choi, B. Jang, Prof. T. HyeonNational Creative Research Initiative Center for OxideNanocrystalline MaterialsSchool of Chemical and Biological EngineeringSeoul National University, Seoul 151-744 (Korea)Fax: (+82)2-886-8457E-mail: thyeon@plaza.snu.ac.kr

S.-S. Kim, Prof. D.-Y. KimDepartment of Materials Science and EngineeringGwangju Institute of Science and Technology (GIST), Gwangju500-712 (Korea)

M. Cho, Prof. J. YoonSchool of Chemical and Biological EngineeringSeoul National University, Seoul 151-744 (Korea)

[**] T.H. would like to thank the National Creative Research InitiativeProgram of the Korean Ministry of Science and Technology forfinancial support.

Supporting information for this article is available on the WWWunder http://www.small-journal.com or from the author.

812 A 2005 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim DOI: 10.1002/smll.200400142 small 2005, 1, No. 8-9, 812 –816

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crystals were synthesized in organic media, they cannot bereadily used for many applications because they are stabi-lized with organic surfactants. Herein, we report on theroom temperature synthesis of highly crystalline TiO2 nano-crystals in aqueous media without the need for further hy-drothermal treatment.

Figure 1 shows low- and high-resolution TEM images ofthe TiO2 nanocrystals synthesized under various reaction

conditions. In the case where 1m NaCl was used, sphericalanatase nanocrystals with a particle size of 6 nm were pro-duced (Figure 1a). The HRTEM image (Figure 1b) of thenanocrystal clearly shows two {101} lattice planes and one{001} lattice plane. When 1m CH3COOH was employed inthe synthesis, short anatase nanorods with an average sizeof 4=20 nm were produced (Figure 1c). The HRTEMimage (Figure 1d) of these nanorods exhibits zigzag surface

features with the [001] direction ofthe anatase being parallel to thelong axis of the nanorods, which isvery similar to the characteristicsof the anatase nanorods reportedpreviously.[15,16] In the case where1m HCl was used, long rutile nano-rods with an average size of 6=50 nm were generated (Figure 1e).The HRTEM image (Figure 1 f) ofthe rutile nanorods shows the pres-ence of a smooth surface feature,with the [110] direction of therutile being vertical to the longaxis of the nanorods.

The BET surface areas of thespherical anatase nanocrystals,anatase nanorods, and rutile nano-rods were found to be 212, 191,and 30 m2g�1, respectively. It iswell known that hydrothermaltreatment or refluxing at high tem-perature is normally required tosynthesize crystalline TiO2 nano-crystals in aqueous media, whichresults in the formation of materi-als with a low surface area. How-ever, as shown above, we wereable to synthesize highly crystallineTiO2 nanocrystals with variousshapes and crystal structures inaqueous media at room tempera-ture. In the absence of the P-123polymer, TiO2 nanocrystals withmixed crystal structures and vari-ous shapes were generated (seeSupporting Information). In theabsence of salt or acid, extensivelyagglomerated and poorly crystal-line TiO2 nanoparticles wereformed. These results demonstratethat the presence of salt or acid isessential to synthesize the TiO2

nanocrystals with unique shapesand crystal structures, and that theP-123 controls the hydrolysis andcondensation of the titanium tet-raisopropoxide (TTIP) precursor.

The X-ray diffraction (XRD)patterns (Figure 2) of the TiO2

nanocrystals show that the anatase

Figure 1. TEM (a, c, and e) and HRTEM (b, d, and f) images of TiO2 nanocrystals: (a and b) anatasenanocrystals synthesized with NaCl, (c and d) short anatase nanorods synthesized with CH3COOH,and (e and f) long rutile nanorods synthesized with HCl. The arrows indicate the direction of thelattice plane, and the rectangular boxes were enlarged to the HRTEM images.

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structure is formed in the presence of NaCl or CH3COOH,whereas the rutile structure is formed with HCl, which is invery good agreement with the HRTEM results. The XRDpattern of the anatase nanorods exhibits a sharper (004)peak than that of the spherical anatase nanocrystals, therebydemonstrating that the anisotropic growth of the nanorodsproceeds along the c-axis of the anatase. Drying at 85 8C toremove ethanol and water increases the crystallinity of theproducts and simultaneously decreases the surface area. Forexample, the crystallinity of the anatase nanorods producedin acetic acid solution is significantly improved in the dryingprocess but the surface area decreases from 302 to189 m2g�1 (see Supporting Information).

Because nanostructured TiO2 materials are well-knownphotocatalysts, we evaluated the photocatalytic activity ofour TiO2 nanocrystals by conducting photooxidation experi-ments with Eosin Y. For comparison, we conducted thesame photocatalytic reactions with Degussa P-25, which isthe most commonly used TiO2-based photocatalyst. We con-ducted several control experiments, including photoreactionwith Eosin Y only (without using any TiO2 species). The re-sults are shown in Figure 3. A small degree of photodegra-dation occurred when Eosin Y (without using TiO2) was il-luminated with UV light. However, our TiO2 nanocrystals

exhibited much higher photocatalytic activities than Degus-sa P-25 or Eosin Y only. The order of photocatalytic activityof TiO2 is as follows: anatase nanorods synthesized with1m CH3COOH> spherical anatase nanocrystals synthesizedwith 1m NaCl> rutile nanorods synthesized with 1m HCl>Degussa P-25>Eosin Y only. The photocatalytic activity ofthe anatase nanorods is two times higher than that of De-gussa P-25. Ying and co-workers have reported that smallerTiO2 nanoparticles exhibit lower photocatalytic activity thantheir larger counterparts due to rapid surface hole–electronpair recombination.[17] The surface-to-volume ratio of nano-rods is higher than that of spherical nanocrystals, which re-sults in a higher density of active sites available for surfacereactions as well as a higher interfacial charge-carrier trans-fer rate. In addition, the increased delocalization of carriersin nanorods is considered to reduce hole–electron pair re-combinations.[15,18] Our finding that the anatase nanorodsexhibit higher photocatalytic activity than the spherical ana-tase nanocrystals is consistent with these previous re-ports.[15,16] Moreover, the anatase nanorods exhibit betterphotocatalytic activity than the rutile nanorods, probablydue to the higher surface area of spherical anatase nano-crystals than that of rutile nanorods.

We also performed photocatalytic degradation of phenolusing various TiO2 catalysts. The results are shown inFigure 4. The order of photocatalytic activity of TiO2 is as

follows: anatase nanorods synthesized with 1m

CH3COOH> spherical anatase nanocrystals synthesizedwith 1m NaCl>Degussa P-25. The overall trend of the ac-tivity is very similar to that of Eosin Y photodegradation. Inthe case of anatase nanorods, the superior catalytic activityseems to be due to the higher surface-to-volume ratio andthe proposed mechanism of the separation or recombinationrate of electron and holes.[19]

We also tested the photocatalytic inactivation of E. coliusing our TiO2 nanocrystals and Degussa P-25. As shown inFigure 5, significant inactivation of E. coli was observedwith the spherical anatase nanocrystals and the anatasenanorods. Furthermore, the photocatalytic ability of TiO2 isas follows: anatase nanorods synthesized with 1m

CH3COOH> spherical anatase nanocrystals synthesized

Figure 2. Powder XRD patterns of TiO2 nanocrystals synthesized withHCl (rutile nanorods, top), CH3COOH (anatase nanorods, middle),and NaCl (anatase nanocrystals, bottom).

Figure 3. Photocatalytic activity of various TiO2 nanocrystals for thephotooxidation of Eosin Y.

Figure 4. Photocatalytic activity of various TiO2 nanocrystals for thephotooxidation of phenol.

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with 1m NaCl>Degussa P-25. These different inactivationrates in the photocatalytic kinetics of E. coli inactivationcan be understood on the basis of the fact that each type ofTiO2 nanocrystals has a different ability and mechanism togenerate hydroxyl radicals, which are the main species re-sponsible for E. coli inactivation.[20a] In addition, the electro-static repulsion between the TiO2 particles and E. coli dueto their negatively charged surfaces might affect the photo-catalytic activity for microbial inactivation.[20b]

In conclusion, we synthesized highly crystalline TiO2

nanocrystals with various shapes and crystal structures fromcontrolled aqueous sol–gel reactions at room temperature.These anatase TiO2 nanocrystals show superior photocata-lytic activities for Eosin Y photooxidation, photocatalyticoxidation of phenol, and E. coli inactivation to those of De-gussa P-25. Due to the simple and mild reaction conditions,the current synthetic method can be readily applied tolarge-scale production of TiO2 nanocrystals.

Experimental Section

In a typical synthesis, 20 mL of titanium tetraisopropoxide(TTIP) was slowly added to 100 mL of an aqueous 1m NaCl, 1m

CH3COOH, or 1m HCl solution containing 4 g of Pluronic P-123(PEO20PPO70PEO20) triblock copolymer at room temperature. Inthe case where 1m HCl was used, the reaction mixture was stir-red for 48 h and the product was collected by centrifugation. Inthe other cases, the resulting reaction mixture was stirred for24 h and the product was retrieved by filtration. To remove theexcess P-123 polymer, the powdery product was washed twicewith an excess of ethanol and then dried at 85 8C for 2 h. Ele-mental analysis showed that most of the polymer was success-fully removed.

To evaluate the photocatalytic activity, we conducted photo-oxidation experiments of Eosin Y in the presence of our TiO2

nanocrystals and Degussa P-25, which is the most popularlyused TiO2-based photocatalyst.[21] For an accurate comparison ofphotocatalytic activity, 50 mg of each type of TiO2 nanocrystalwas added to 50 mL of an aqueous 2910�5m Eosin Y solutionwith vigorous stirring. To maximize the adsorption of the dyeonto the TiO2 surface, the resulting mixture was kept in the dark

for 30 min. The Eosin Y solution was then collected by centrifuga-tion and C0 was measured by UV/Vis absorption spectroscopyusing the Beer–Lambert law. The Eosin Y solution including TiO2

was stirred whilst illuminating it with a Xe lamp equipped withan AM 1.5 filter. To plot Ct/C0 against illumination time, the sus-pension was collected and the concentration of Eosin Y wasmeasured by UV/Vis absorption spectroscopy at 20 min inter-vals.

TiO2 nanocrystals (0.2 g) were mixed with a phenol solution(0.2 g in 1 L of deionized water) and stirred. Using the same pro-cess for the photooxidation experiment of Eosin Y, the change ofphenol concentration in the solution was plotted using the peakof phenol at around 270 nm in the UV/Vis spectra.[19]

The experiments for the photocatalytic inactivation of E. coliwere conducted for three different TiO2 nanocrystals: Degussa P-25, anatase nanorods with dimensions of 4920 nm, and 6 nmspherical anatase nanocrystals. The TiO2 particles (1 gL�1) weresonicated for 30 min in order to disperse the particles uniformly.The disinfection experiments were carried out in a 60 mL Pyrexreactor (UV cut-off<300 nm) with 50 mL of solution; a schemeshowing the experimental apparatus is given in the SupportingInformation (Figure S4). The slurry of TiO2 and E. coli was inten-sively mixed with a magnetic stirrer (EYELA Co., RC-2, Japan) at aspeed of 1100 rpm to allow complete mixing. Illumination wasprovided by four black light blue lamps (BLB 18 W, Philips Co.,Netherlands), which emit in the range 300–420 nm. The lampswere positioned inside the reactor, with one being placed oneach side. The emission spectrum of the BLB lamp was mea-sured with an Acton Research Detection system (Spectrapro-500,USA). The light intensity, which was measured by ferrioxalate ac-tinometry, was 7.9910�6 EinsteinL�1s�1. The reaction tempera-ture was maintained at 20 8C with a thermostatic chamber (JeioTech Co., Korea). The pH was adjusted to 7.1 with a phosphatebuffer (KH2PO4/NaOH). The resulting phosphate buffer solutionswere maintained at approximately 20 mm. In general, five sam-ples were collected after 30–120 min to measure the populationof E. coli. Viable concentration of E. coli was measured withoutseparating the microorganism from the TiO2 nanocrystals, as de-scribed by Cho et al.[20] In a control test, no inactivation of E. coliwas observed in the absence of either BLB radiation or TiO2 par-ticles within the present experimental timescale. All microbial in-activation experiments were repeated three times to confirmtheir reproducibility and their average values with statistical de-viation were used for the data analysis.

Supporting Information: XRD patterns and TEM images ofTiO2 nanocrystals synthesized under various reaction conditionsand a scheme showing the experimental apparatus for the pho-tocatalytic reactions.

Keywords:nanocrystalline materials · photocatalysis ·sol–gel processes · titania

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Figure 5. Photocatalytic inactivation of E. coli using various TiO2

nanocrystals.

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Received: November 30, 2004Revised: April 20, 2005Published online on June 29, 2005

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