Science in China Series E: Technological Sciences © 2009

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Preparation of TiO2 thin film by the LPD method on functionalized organic self-assembled monolayers HE ZhongLiang, YU ZhiWei, MIAO HongYan†, TAN GuoQiang & LIU Yan School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, China

In this paper, uniform titania (TiO2) films have been formed at 50℃ on silanol SAMs by the liquid-phase deposition (LPD) method at a temperature below 100℃. OTS (Octadecyltrichloro-Silane) self- assembled monolayers (SAMs) on glass wafers were used as substrates for the deposition of titanium dioxide thin films. This functionalized organic surface has shown to be effective for promoting the growth of films from titanic aqueous solutions by the LPD method at a low temperature below 100℃. The crystal phase composition, microstructure and topography of the as-prepared films were characterized by various techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The results indicate that the as-prepared thin films are purely crystallized anatase TiO2 constituted by nanorods after being annealed at 500℃. The pH values, concentration of reactants, and deposition temperatures play important roles in the growth of TiO2 thin films. TiO2 thin film, SAMs, LPD, anatase, nanorod

1 Introduction Titanium dioxide (TiO2) thin films are interesting as they can be applied in microelectronics[1], optical cells, solar energy conversion[2], highly efficient catalysts[3], microorganism photolysis, antifogging and self-cleaning coatings, gratings, and gate oxides in MOSFET (metaloxide-semiconductor field effect transistor)[4]. Therefore, lots of methods, such as vapor-phase-deposition methods, sol-gel method, have been developed to prepare uniform and high-quality TiO2 thin films. However, these techniques have several shortcomings, such as expensive vacuum equipment, the need to heat the substrates to crystallize the films, and the limitations of line-of-sight deposition. Sol-gel solution routes with alkoxide precursors have been successfully used to coat films on complex-shaped substrates, but post- depositional treatment at a temperature typically >400℃ is required to form thermodynamically stable phases[5]. The self-assembled monolayers (SAMs) technique is interesting as it can control the nucleation and growth of ceramic oxides by mimicking the chemistry found in

biological nucleation proteins. It has shown to be effective as templates for the control of the crystal axis orientation, polymorph specificities and morphologies of inorganic materials[6,7]. Here, uniform titania films have been formed at 50℃ on silanol SAMs by the LPD method at a low temperature below 100℃. The crystal phase composition, microstructure and topography of the as-prepared films were characterized by XRD, SEM and AFM. The high quality TiO2 film could have the potential application in microelectronics, optical cells, solar energy conversion, etc.

2 Experimental 2.1 SAM preparation The OTS-SAM was prepared by immersion of the clean glass substrate in an anhydrous toluene solution conReceived September 4, 2008; accepted September 19, 2008 doi: 10.1007/s11431-008-0332-1 † Corresponding author (email: [email protected]) Support by the National Natural Science Foundation of China (Grant No. 50672055) and National Key Technology R&D Program (Grant No. 2006BAF02A28)

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taining 1 vol% OTS for 15 min under an N2 atmosphere[8]. The substrates covered with SAMs were then baked at 120℃ for 5 min to remove residual solvent and promote chemisorption of the SAM. Finally, SAMs were exposed for 120 min to UV light (184.9 nm) (PL16-110, Nippon Sen Lights Corporation) in air. UV irradiation caused a photocleavage reaction on the exposed substrate. The exposed substrate became hydrophilic due to Si-OH group formation. 2.2 TiO2 film preparation Ammonium hexafluorotitanate ((NH4)2TiF6 (AHFT), Xi’an Sanpu Fine Chemical Plant, China) and boric acid (H3BO3, Shanghai Chemical Reagent Company, Chinese Academy of Medical Sciences) were separately dissolved in deionized water and then mixed to form a solution. An appropriate amount of HCl was added to the solution to control the pH. Water was further added to yield a final solution containing 0.05 mol/L AHFT and 0.17 mol/L H3BO3 and the pH value of the solution was controlled at 3.0. The irradiated OTS-SAM glass substrates were immersed in this solution at 50℃ for 12 h to deposit TiO2. The substrates were held vertically at the bottom of this solution to prevent particles formed in the solution from accumulation on the surface. The substrate was then sonicated for 2 min, rinsed with water and dried at room temperature. Finally the as-prepared films were annealed at 500℃ for 1 h.

mol/L AHFT and 0.17 mol/L H3BO3 at pH=3.0 for 12 h. Figure 2 shows the as-prepared TiO2 film XRD patterns of the substrates immersed in the solution at 20, 40, 50 and 60℃ after annealing at 500℃, respectively. All the samples are pure tetragonal anatase phase TiO2 and exhibit good crystallinity. The relative intensities of (101) planes and (004) planes in Figure 2 apparently increase with the increase of deposition temperature. However, the relative intensity of the pattern at 50℃ is higher than that at 60 ℃ deposition temperature. Figure 3 shows the XRD patterns of the final films deposited at different pH values. The (101) peak of the pattern is the strongest when pH=3.0. TiO2 formation is initiated in solution by the hydrolysis of Ti precursors to form solvable aquo and hydroxo species[9], which ultimately condense via elimination of water to form oxide particles. So the deposition temperature and pH value have great influences on the deposition of the anatase TiO2.

3 Results and discussion 3.1 SAM preparation

Figure 1 Water contact angles of OTS-SAMs irradiated by UV light with various time.

The octadecyl-trichloro-silane (OTS) based SAM on the hydroxylated glass substrates was utilized in the present study. The relationship between contact angle (θw) and UV irradiation time on the OTS-SAMs was verified as shown in Figure 1. Prior to irradiation, the OTS-SAMs displayed θw =110°±2° as expected to form the homogeneous film. θw decreased monotonically with the increasing UV exposure time until the surface became fully wettable (θw≤5°) after 120 min. 3.2 Film growth conditions The pH values of the solution, concentration of reactants, and deposition temperature have great effects on the crystal phase composition and crystalline of the final products. Therefore, the irradiated OTS-SAM-Glass substrates were immersed in the solution containing 0.05 138

Figure 2 tures.

XRD patterns of TiO2 thin films deposited at various tempera-

HE ZhongLiang et al. Sci China Ser E-Tech Sci | Jan. 2009 | vol. 52 | no. 1 | 137-140

Figure 3 XRD patterns of TiO2 thin films deposited from solutions of various pH values. a, pH=2.5; b, pH=3.0; c, pH=3.5.

3.3

SEM and AFM analysis

Morphology and topography of the prepared films are depicted in Figures 4 and 5. Figure 4 shows the SEM images of TiO2 thin films deposited at different pH values. The titania films were deposited at 50℃ for 12 h with 0.05 mol/L AHFT and 0.17 mol/L H3BO3. On the irradiated SAMs, a uniform film with densely packed particles was formed with pH value at 3.0 as shown in Figure 4(b). The morphology of the films did not change with a further growth. In contrast, with the pH value at

Figure 4

2.5, 3.5 and 4.0, the substrate was found uncoated in the final products. The islands would grow together as the deposition continued. All the samples were ultrasonicated to remove the loosely attached particles before being dried. The final experiments showed that the excellent films were obtained when pH=3.0. Atomic force microscopy (AFM) was used to analyze the surface topography of the titania film deposited at 50℃ for 12 h with pH=3.0. The AFM image in Figure 5 reveals a dense packing of islands with a size in the range of 20―30 nm and a height in the range of 40―65 nm. Such topography is typical for the whole specimen. The AFM image was according to the results of SEM micrograph of the same specimen.

4 Conclusions Uniform titania films have been formed at 50℃ on silanol SAMs by the LPD method after annealing at 500℃ for 1 h. The crystal phase composition, microstructure and topography of the as-prepared films were characterized by a variety of techniques, including XRD, SEM and AFM. The influences of pH values, concentration of reactants, and deposition temperatures on the growth of TiO2 thin films were investigated. The thin films were smooth, uniform and dense, with good adherence. The high quality TiO2 film could have the potential applica-

SEM images of TiO2 thin films deposited from the solutions of various pH values.( a) pH=2.5, (b) pH=3.0, (c) pH=3.5, (d) pH=4.0.

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