-organic Thin Film From an Aqueous Solution

Transcription

1 Fabrication and Characterization of -organic Thin Film From an Aqueous Solution Fabrication and Characterization of TiO 2 -organic Thin Film From an Aqueous Solution W.N. Mu 1 and S.Z. Shi School of Resources and Materials, Northeastern University at Qinhuangdao Branch, Qinhuangdao , PR China Qinhuangdao ShouGang Plate Co., Ltd, Qinhuangdao , PR China Received: 9 April 2014, Accepted: 9 May 2014 Summary -organic thin films were fabricated on SAMs-coated silicon substrate based on layer-by-layer self-assembly process and chemical bath deposition method by a hydrolysis of TiCl 4 in acid aqueous solution. The influences of deposition time and solution ph value on structural properties and the thickness of films were investigated. The results of UV-vis spectrum show the absorbance of aqueous solutions at 410 nm decreases gradually with increasing deposition time and solution ph value. The thickness of -organic thin film elevates as increasing deposition time and ph value. The deposition time has significant influence on the surface morphology of films, and when the time is 2 h, a continuous, density and homogeneous film is gained from SEM micrographs. The deposited films consisting of nanoparticles are believed to be fully amorphous layers from the results of XRD. The major chemical compositions of thin films are titanium and oxygen as presented in EDS results, and O/Ti atomic ratio is close to the corresponding ratio of 2:1 for stoichiometric. The transmission infrared spectrum and UV/Vis spectrum of film indicate excellent structure and optical properties of films. Keywords: Titanium oxide; Thin film; Chemical bath deposition; Self-assembly 1. Introduction As we all know, titanium dioxide ( ) film is an important functional thin film material which could be widely applied in solar cell 1,2, microelectronics 3, photocatalysis 4,5, antifogging 6, self-cleaning coatings 7,8, transparent conducting oxides(tco) 9, gas sensing 10,11, because of its superior mechanical durability 12, excellent chemical stability 13 and high transparency. Currently, numerous attempts have been paid to the preparation technique of titanium dioxide ( ) film material in order to obtain a structurally uniform and dense thin film. These techniques including sol-gel 14,15, sputtering 16, chemical vapor deposition(cvd) 17, filtered arc deposition 18, evaporation 19, however, are limited by high cost, harsh operating conditions and intricate equipments. muwn@mail.neuq.edu.cn Smithers Information Ltd., 2014 In addition, it is hard to control the structure and surface morphology of thin films by adopting these techniques. In recent years, much attention has been focused on low-temperature preparation processes of thin film from aqueous solutions 20. Chemical bath deposition (CBD) is a typical technique, the operating temperature of which is controlled in the range of 30~100 C in aqueous solution system. This process has numerous advantages, such as less investment of equipment, low temperature and more suitability to the substrates. For example, some metal oxides can be prepared on selfassembled monolayers (SAMs) by chemical bath deposition process 21. The layer-by-layer (LBL) is a mainly self-assembly method, by which solid thin films can be prepared on a variety of substrates from solutions. The charged substrate was alternatively dipped into positively and negatively charged polymer solutions sequentially, and then organic multilayered structure was assembled on the substrates 22. The process exhibits many superiorities compared to other conventional methods, including simple operating conditions and equipment, easy to form stable films and great flexible to objects with irregular shapes and sizes 23. Our group has focused on the synthesis of -organic thin films obtained via deposition from aqueous solution on a SAMs-coated silicon substrate. The influence of deposition time and solution ph on the films thickness was investigated. The structure, chemical composition and optical properties of thin films were determined using X-ray diffractometer (XRD), energy dispersive spectrometer (EDS), scanning electron microscope (SEM), Fourier transformation infrared Polymers & Polymer Composites, Vol. 22, No. 8,

2 W.N. Mu and S.Z. Shi spectroscopy (FTIR) and UV-visible absorption spectrometer, respectively. 2 Experimental 2.1 Fabrication of SAMs The single-crystal silicon wafer cut into 2 cmx2 cm was used for the preparation of SAMs via LBL assembly. First the silicon substrate was cleaned using organic solvents and heated up to 80 C for 40 min in oxidizing piranha solution (a 3:1 volume ratio mixed solution of concentrated H 2 SO 4 and 30% H 2 ). A negatively charged hydrophilic SiOH layer was formed on the substrate surface, which is necessary for SAMs fabrication 24. Then the silicon substrate was washed with deionized water and dried spontaneously. Poly(sodium 4-styrenesulfonate) (PSS, Mw=70000 g mol -1 ), poly(allylamine hydrochloride) (PAH, Mw=70000 g mol -1 ) and poly(ethyleneimine) (PEI, Mw=70000 g mol -1 ) were purchased from Sigma-Aldrich. Three polymers were dissolved in deionized water to prepare PSS solutions of 4 g L -1, PAH solutions of 2 g L -1 and PEI solutions of 1.25 g L -1, and the solution ph was adjusted to 3.5, 3.5 and 9.0 by adding HCl, respectively. The SAMs were prepared by the alternated immersion for 20 min of the substrate into PEI solution or PAH solution and PSS solution. The operating procedures were consistent with the process described in Ref. 25. After deposition of each layer, the substrate was cleaned with deionized water in order to remove unbound polyelectrolytes and to prevent the cross-contamination of the solutions. After the second layer, only PAH and PSS were used, and this sequence was repeated four times 25. The SAMs obtained in our study are named (PEI/PSS)(PAH/PSS) 4. used. The titanium tetrachloride was dissolved in 30% hydrogen peroxide to prepare fresh precursor solutions. The titanium concentration and ph of precursor solutions were adjusted by adding deionized water and concentrated hydrochloric acid to yield a suitable condition for thin film deposition. The samples with SAMs were placed in the running solution and kept for different time at the constant temperature of 60 C. During the fabrication process of thin film, a peristaltic pump was used as a driving device to make the reaction solution cycle at the flow rate of 20 ml min -1. When the setting time was reached, the samples were ultrasonically cleaned completely by using deionized water and then dried in air. 2.3 Characterizations The solution and deposition thin films were detected using an UVvisible absorption spectrometer (Shimadzu, UV-3100) to obtain UV-vis spectrums. The surface profile measuring system (Sloan, Dektak3030) was used to analyze thin films thickness. The X-ray diffraction pattern of thin films was identified by a Rigaku-D/MAX2200 diffractometer operating with a Cu-Kα radiation by continuous scanning at a speed of 1.2 o per minute. The microstructure of thin films was observed by a scanning electron microscopy (SEM, Leo Supra 35, Germany), and the chemical compositions on these films surface were determined by energy dispersive spectrometer (EDS). Fourier transformation infrared spectrometer (8400S CE) was employed to analyze the functional groups of deposition thin films. 3 Results and discussion 3.1 Solution Chemistry of Thin Film Deposition The color of inorganic titanium salts solutions showed much variation during the deposition of thin film. The fresh aqueous solutions were orange-red, when increasing deposition time and solution ph, a rapid discoloration of solutions can be observed, and even sometimes turbidity occurred. The absorbance of solutions under different deposition time and solution ph at 410 nm was determined by the UV-visible absorption spectrometer, and the results in Figure 1 show the absorbance of aqueous solutions at 410 nm almost Figure 1. Comparison of solutions absorbance with different ph values at different deposition time 2.2 -organic Thin Film Deposition Reagent grade titanium tetrachloride, TiCl 4, hydrogen peroxide, H 2, and hydrochloric acid, HCl, were 700 Polymers & Polymer Composites, Vol. 22, No. 8, 2014

3 Fabrication and Characterization of -organic Thin Film From an Aqueous Solution linearly decreases with the increase of deposition time and solution ph. The precipitation of titanium compound takes place spontaneously because of the metastable characteristics of inorganic titanium salts solutions, and the chemical reactions occurred in the solutions can be written as follows: TiCl 4 + H 2 = Ti( )Cl 2 + 2HCl (1) Ti( )Cl 2 + 2H 2 O = + H 2 + 2HCl (2) When the deposition time increases, the precipitation rate of titanium compound increases too, leading to a remarkable decrease of titanium content in aqueous solutions, and finally resulting in the reduction of the absorbance. By changing the amount of hydrochloric acid added to the solutions, different solution ph values could be obtained. The decrease of hydrochloric acid content in solutions would render an increase of solution ph, which promotes the Figure 2. Influence of deposition time and solution ph on the thickness of thin film positive precipitation reaction taking place fast and causes an increase of precipitation rate and the decrease of solution absorbance. In addition, deposition time and solution ph value have great influences on the thickness of thin film. According to the results given in Figure 2, it can be seen that the film thickness increases significantly with increasing the deposition time and ph value. The film with a thickness of 190 nm can be obtained after 12 h when ph value is 2.2. To sum up, the increase of the deposition time and solution ph was favorable to the formation of thin films. 3.2 The Formation of - organic Thin Film The SEM images of -organic films deposited on SAMs-coated silicon substrates atr different deposition time in precursor solutions are presented in Figure 3. The deposited thin film is not continuous at 0.5 h as seen in Figure 3a. With the increase of deposition time, the particles are Figure 3. SEM micrographs of thin film deposited on SAMs-coated silicon substrate after (a) 0.5 h; (b) 1 h; (c) 2 h; (d) 4 h; (e) 6 h and (f) 8 h immersion in precursor solution Polymers & Polymer Composites, Vol. 22, No. 8,

4 W.N. Mu and S.Z. Shi gradually spread over the whole surface (Figure 3b), and finally forming an unbroken film after the immersion time of 2 h, as seen from Figure 3c. Furthermore, at this time, the film surface is dense and homogeneous. The particles on the film surface are primarily distributed from 25 nm to 35 nm. The average particle size is about 33 nm, which is mainly owing to the immersion time. When the reaction time is 2 h, particles have just started to form nucleation on the surface of SAMs-coated silicon substrate and there is no time to grow up. the thickness is about 15~20 nm, while titanium oxide layer appears as a dark region with a thickness closing to 200 nm. It is fully demonstrated that uniform layer of titanium oxide was successfully deposited on SAMs template. The result of EDS reveals that the dark region predominantly consists of titanium and oxygen. The O/Ti atomic ratio is 2.2:1 as analyzed in Figure 6, which is slightly higher than the stoichiometric ratio of O and Ti, this may be attributed to a certain amount of water contained in the grown film The Optical Properties of films The infrared spectrum between 400~4000 cm -1 of film was detected by a Fourier transformation infrared spectrometer and the results are presented in Figure 7. A strong absorption peak between 400~500 cm -1 indicates the presence of Ti-O vibrations 27. The absorption peaks in the 3500~4000 cm -1 and at about 1600 cm -1 are bending vibration of -OH on the film surface and stretching vibration of adsorbed water, respectively. However, as increasing immersion time, solution nucleation could be found on the SAMs surface, some small nuclei from solution are quickly dissolved and merged by particles on the film surface, then a number of relatively protruding particles begin to adsorb tiny particles in the solution and grow up, so the film appears big ups and downs (Figure 3d). When the reaction time is 6 h, particles on the film surface grow larger and the average particle size increase from 48 nm for 4 h to 98 nm for 6 h, as shown in Figure 3e. The film surface morphology in Figure 3f has little change comparing with immersion time of 6 h. The reason is that large particles on the film surface grow up slowly due to the particles adsorption saturation. Some little particles in the solution gradually fill the void in the middle of great particles. Figure 4. The X-ray diffraction pattern of -organic thin film Figure 5. The cross-sectional SEM image of deposited on SAMs surface The phase analysis of -organic films was performed by XRD, and the results were presented in Figure 4. It can be seen that the thin film has a structure of short-range order after comparing with the standard diffraction peaks, which presents that the film is an amorphous layer. A cross-sectional SEM image and energy dispersive spectrometer (EDS) of deposited titanium oxide film on SAMs surface is given in Figure 5. SAMs appear in the bright area, and 702 Polymers & Polymer Composites, Vol. 22, No. 8, 2014

5 Fabrication and Characterization of -organic Thin Film From an Aqueous Solution Figure 6. The content distribution of chemical elements on film surface Figure 8 is a UV/Vis spectrum of film. From the figure, we can see a high transmittance of film in the visible region. The absorption peaks occur at nm and nm is mainly attributed to an electronic transition from the valence band (- 2p orbitals) to the conduction band (Ti 4+ 3d orbitals) in film 28. There is no absorption peak above 600 nm, which indicates without Ti 2+ in film. 4. Conclusions Figure 7. Infrared spectra of thin films Figure 8. UV/Vis spectrum of film In this work, -organic thin film was fabricated based on LBL selfassembly process and CBD method by hydrolysis of titanium tetrachloride in aqueous solutions. Deposition time and solution ph have noticeable influences on the deposition and the thickness of film. The absorbance of aqueous solutions with inorganic titanium salts at 410 nm decreases gradually with increasing deposition time and solution ph. The thickness of -organic films varies between 40~190 nm as increasing the deposition time from 1 h to 12 h when ph value is 2.2. The deposition time has significant influence on the surface morphology of films. Along with the time increased from 0.5 h to 8 h, the film is formed gradually, and the film surface roughness is increased constantly. When the time is 2 h, a continuous, density and homogeneous film is gained from SEM micrographs. The thin film including nanoparticles is referred as an amorphous film. The chemical composition of the thin film is mainly titanium and oxygen, and the atomic ratio of oxygen to titanium is 2.2:1 close to the corresponding stoichiometric ratio in titanium dioxide due to the influence of some amount water on the film surface. The functional groups and the transmittance of film detected by Fourier transformation infrared spectrometer and UV-visible absorption spectrometer, respectively, indicate excellent structure and optical properties of films. Polymers & Polymer Composites, Vol. 22, No. 8,

6 W.N. Mu and S.Z. Shi Acknowledgments This work was partly supported by National Natural Science Foundation of China (No , No ) and the National Basic Research of Program of China (No. 2014CB643405). We also would like to thank the Research Fund from the Doctoral Program of Higher Education, China (No ). References 1. Khan M.A. et al., Solar Energy, 84 (2010) Nikraz S. et al., Journal of Physical Chemistry C, 116 (2012) Burns G.P., J. Appl. Phys., 65 (1989) Lin X. et al., Microporous and Mesoporous Materials, 142 (2011) Pan J.H. et al., Catalysis Science and Technology, 2 (2012) Masuda Y. and Kato K., Thin Solid Films, 516 (2008) Fateh R. et al., Journal of Physical Chemistry C, 115 (2011) Wang J. et al., Journal of Physical Chemistry C, 116 (2012) Park J.H. et al., Solar Energy Materials and Solar Cell, 95 (2011) Savage N. et al., Sens. Actuators B, 79 (2001) Seo M.H. et al., Sens. Actuators B, 137 (2009) Chong M.N. et al., Water Res., 44 (2010) Ennaoui A. et al., Sol. Energy Mater. Sol. Cells, 90 (2006) Liu J.X. et al., Thin Solid Films, 429 (2003) Yun H.S. et al., Mater. Sci. Eng., 23 (2003) Miao L. et al., Appl. Surf. Sci., (2003) Mills A. et al., J. Photochem. Photobiol. A: Chem., 151 (2002) Bendavid A. et al., Thin Solid Film, 360 (2000) Fujii T. et al., J. Mater. Res., 9 (1994) Masuda Y. et al., Solid State Ionics, 172 (2004) Gao Y.F. and Koumoto K., Crystal Growth & Design, 5 (2005) Decher G., Science, 277 (1997) De Villers M.M. et al., Advanced Drug Delivery Reviews, 63 (2011) Seu K.J. et al., Biophysical Journal, 92 (2007) Mu W.N. et al., Advanced Materials Research, (2012) Fuchs T.M. et al., Journal of Materials Chemistry, 12 (2002) Lee M.S. et al., J. Ind. Eng. Chem. 9 (2013) Hoffmann R.C. and Bill J., Thin Solid Film, 478 (2005) Polymers & Polymer Composites, Vol. 22, No. 8, 2014