Synthesis of Single Crystalline Anatase TiO 2 (001) Tetragonal Nanosheet-Array Films on Fluorine-Doped Tin Oxide Substrate Shuanglong Feng, Junyou Yang, w Hu Zhu, Ming Liu, Jiansheng Zhang, Jin Wu, and Jiayu Wan State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China Single crystalline anatase TiO 2 tetragonal nanosheet-array film on a transparent conductive fluorine-doped tin oxide (FTO) sub- strate was successfully prepared by hydrothermal method. The products were characterized with XRD, SEM, and transmission electron microscopy measurements. The formation mechanism of the tetragonal (001) TiO 2 nanosheets is discussed and the effect of growth parameters, such as acidity, growth time, growth tem- perature, initial reactant concentration, and additives, are inves- tigated. This work demonstrates a clean and effective route to the morphology-controlled preparation of large area single-crystal- line TiO 2 nanostructured array films on FTO glass. I. Introduction O WING to its high photovoltaic and photochemical activity, the preparation of nanostructured anatase TiO 2 with spe- cial morphology and orientation is of great interest of scientists in recent years. 1–4 For anatase TiO 2 nanocrystals, the minority (001) facets are more reactive. 5 Furthermore, single-crystalline TiO 2 nanostructure films are more preferred over polycrystalline ones for the ordered bulk heterojunction application. Therefore, anatase TiO 2 films with a high percentage of reactive (001) facets are more favorable for applications in solar cells, photonic and optoelectronic devices, sensors, and photocatalysis. However, most of anatase TiO 2 nanocrystals are mainly dominated by the thermodynamically stable (101) facets, rather than the much more reactive (001) facets. 6 As a result, the information of the crystal facet effect on its characteristics is limited, and the ana- tase TiO 2 single crystals with controlled special well-faceted sur- faces are seldom reported in the published literature. 7 Recently, an important breakthrough in the preparation of anatase TiO 2 crystals with exposed (001) facets was achieved by Yang et al. 8,9 They reported that the fluorine-terminated (001) plane is energetically stabler than the (101) plane, and high- purity truncated anatase bipyramids TiO 2 single crystals with a high percentage of reactive (001) facets were obtained. A similar strategy was adopted by Xie and colleagues, and anatase TiO 2 nanosheets with 89% exposed (001) facets and excellent photo- catalytic efficiency was synthesized. 10 Using a digesting electro- spun nanofibers consisting of amorphous TiO 2 and poly(vinyl pyrrolidone) in an aqueous acetic acid solution and followed by hydrothermal treatment, 11 Dai and colleagues also prepared anatase TiO 2 nanocrystals with exposed and chemically active (00- 1) facets. However, no single-crystal anatase TiO 2 film with a large percentage of reactive (001) facets has been reported yet. In the present work, a single-crystal anatase TiO 2 film with a large percentage of reactive (001) facets on a transparent con- ducting substrate has been successfully prepared via a simple hydrothermal process at a relatively low temperature without the presence of any surfactant. The formation mechanism of this single-crystal anatase TiO 2 film and the effect of various syn- thesis parameters on its morphology are discussed. II. Experimental Section TiO 2 tetragonal nanosheet-array films were synthesized with a method similar to that described by Liu and Aydil. 12 A mixture of deionized water and concentrated hydrochloric acid (36.5%– 38% by weight) with total volume of 60 mL was stirred for at ambient conditions for 5 min in a Teflon-lined stainless-steel autoclave (100 mL volume), and 1 mL of tetrabutyl titanate (97% Aldrich, Sigma-Aldrich Corporation, MO) and 0.5 g am- monium hexafluorotitanate ((NH 4 ) 2 TiF 6 ) were added into the solution and stirred for another 5 min, then two pieces of fluo- rine-doped tin oxide (FTO) substrates (F: SnO 2 , Tec 15, 10 O/&), ultrasonically cleaned for 60 min in a mixed solution of deionized water, acetone and ethanol with a volume ratio of 1:1:1, were placed at the bottom of the Teflon-liner with the conductive side facing up. The hydrothermal synthesis was conducted at 1001– 1801C for 1–24 h in an electric oven. After synthesis, the autoclave was cooled down, and the FTO substrates were taken out and rinsed with deionized water thoroughly and then dried for further analysis. The XRD patterns of the as-prepared films were recorded in a Philip X’pert X-ray diffractometer (PANalytical, Almelo, the Netherlands) (CuKa irradiation, l 5 0.15418 nm) from 101 to 801 at a scanning speed of 2.41/min. Morphology and structure information of the products were examined with a Sirion 200 field emission scanning electron microscope (FE-SEM) and a trans- mission electron microscopy (TEM, FEI Tecnai G230, Hillsboro, OR), respectively. In a typical FE-SEM measurement, a small piece of sample film was adhered onto a copper stub using double- sided carbon tape. The specimens for TEM imaging were pre- pared by suspending solid samples in acetone. About 1–2 mg of white sample detached from the FTO substrate was added to 5 mL of acetone in a small glass vial, followed by sonication for 30 min. A few drops of the sonicated suspension were dropped onto a carbon-coated 200 mesh copper grid and dried under ambient conditions before imaging. III. Results and Discussion Figure 1(a) shows the SEM image of a tetragonal nanosheet- array film synthesized by hydrothermal treatment at 1501C for 12 h in a 60 mL hydrochloric acid/water (1:1 by v/v) mixture solution with addition of 1 mL tetrabutyl titanate and 0.5 g (NH 4 ) 2 TiF 6 . It can be seen that the FTO substrate is covered with a large number of tetragonal nanosheets R. Parra—contributing editor This work was financially supported by the National Natural Science Foundation of China (Grant No. 50827204, No.51072062) and the Fundamental Research Funds for the Central Universities (2010ZD014). The technical assistance from the Analytical and Testing Center of HUST is also gratefully acknowledged. w Author to whom correspondence should be addressed. e-mail: jyyang@mail.hust. edu.cn Manuscript No. 28525. Received September 3, 2010; approved October 18, 2010. J ournal J. Am. Ceram. Soc., 94 [2] 310–315 (2011) DOI: 10.1111/j.1551-2916.2010.04266.x r 2010 The American Ceramic Society 310