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Aligned Growth of Methylene Blue Films on TiO Single Crystals Ryosuke Fukino, and Ryuzi Katoh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04676 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 25, 2017
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The Journal of Physical Chemistry
Aligned Growth of Methylene Blue Films on TiO2 Single Crystals Ryosuke Fukino and Ryuzi Katoh* Department of Chemical Biology and Applied Chemistry, College of Engineering, Nihon University, Koriyama, Fukushima 963-8642, Japan *Corresponding author:
[email protected] Abstract We found epitaxial growth of needle-like nano-sized crystals of methylene blue (MB). The nano-sized crystals grew spontaneously along the c-axis of the (110) surface of a TiO2 single crystal by using the dip-coating method from solution. This epitaxial growth of the nano-sized crystals is induced by the bridging oxygen rows align along the crystal c-axis. We also found that the alignment direction could be controlled by rubbing the surface of the substrate prior to dye-loading. The memory of the alignment direction induced by the rubbing treatment was erased by thermal annealing at 620 K. These findings suggest that the nano-structure on the TiO2 surface plays an important role in the aligned growth of MB nano-sized crystals. Introduction Aligned growth of organic films on the surface of substrates has attracted much attention not only for fundamental surface physical chemistry but also for practical applications including the preparation of devices such as organic light-emitting diodes (OLEDs)1 and organic field-effect transistors (OFETs).2 Various techniques for aligned growth have been proposed including molecular beam epitaxy (MBE), rubbing treatment, and solution shearing. In MBE, molecules are deposited on the clean surface of a substrate under ultra-high vacuum condition, and molecule alignment is induced by the anisotropy of the crystal surface caused by the crystal structure, namely epitaxial growth. In order to obtain highly aligned films, critical control of growth conditions, such as the temperature of the substrate and the deposition rate are needed.3 Although MBE has been mainly used for ultra-thin film preparation, growth of relatively thick films has been reported.4,5 Rubbing treatment is a simple technique for the preparation of large, aligned films with sufficient thickness under ambient condition, namely ultra-high vacuum is not needed. Before deposition of the molecules on the substrate, the surface of the substrate is rubbed unidirectionally with a piece of paper or a brush. Then, the linear molecules align along the direction of the rubbing on the substrate. Namely, the origin of the alignment would be due to aligned microscratches induced by the uni-directional rubbing treatment of the surface. Although a detailed mechanism that explains why rubbing treatment produces this outcome has not yet been clearly established, this technique has been widely used for the production of liquid-crystal displays.8 Recently, various techniques based on solution shearing have been proposed to achieve aligned growth of crystals from solutions, for example, dip coating by controlling the withdrawing direction of a substrate from a
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solution,6,7 and film preparation assisted by a micropillar-patterned blade.2 This method is suitable for large films under ambient condition and is relatively inexpensive, although controlling the morphology of the films remains challenging. As shown above, several principles have been operated for aligned growth of organic films. Thus, combination of such principles would be important for further development of the aligned growth method of organic films on the surface of substrates. To explore novel method for aligned growth of the organic films, we focus to the aligned films of methylene blue (MB) prepared by dip-coating from a solution, which has been extensively studied.9,10 It has been reported that optically anisotropic MB films can be prepared on the substrate after rubbing treatment of a glass, sodium chloride crystal, or metal substrate with paper or cotton. The alignment is assisted by the anisotropy of the surface structure induced by the rubbing treatment. Rubbing treatment of a film prepared by dip-coating, (i.e., rubbing treatment is performed after dip-coating) has also been reported to be effective for the preparation of aligned MB films.11 Accordingly, this would be due to aligned growth of nano-sized crystals on the surface caused by micro-scratches induced by the unidirectional rubbing treatment of the surface. Here we studied the preparation of MB films on the (110) surface of a TiO2 single crystal by using the dip-coating method with and without rubbing treatment to study the origin of the unidirectional alignment of MB molecules of the surface. On TiO2 single crystals, structural anisotropy exists because of the crystal structure of the surface. Thus, aligned growth of an MB film on TiO2 single crystals would be expected. In fact, we found that MB films prepared by using the dip-coating method without rubbing treatment showed distinct optical anisotropy and that pronounced optical absorption was observed when an electric field vector of probe light was parallel to the crystal c-axis. We also found that the aligned direction was not sensitive to the withdrawing direction of the substrate, suggesting that a solution shearing process was not operative. Accordingly, this is aligned growth of MB nano-sized crystals induced by anisotropy of the crystal surface caused by the crystal structure, namely epitaxial growth of MB micro-crystals. As mentioned above, conventional epitaxial growth based on MBE method is the alignment of molecules deposited on the clean surface of the crystals under ultra-high vacuum condition. In the present study, we could demonstrate aligned growth of MB nano-sized crystals on the surface under wet condition. This unconventional epitaxial growth would be important for further understanding of aligned growth of organic films. In addition, we found that the alignment direction of MB on the surface of the TiO2 single crystal could be controlled by the rubbing direction on the surface. The memory of the alignment direction induced by the rubbing treatment was erased by thermal annealing at 620 K, suggesting the reconstruction of micro-scratches induced by the rubbing treatment. Based on the results of our morphological observations using atomic force microscopy (AFM), we discuss the origin of the alignment of MB on the surface of TiO2 single crystals. Experimental Polished single crystals of rutile TiO2 were purchased from Furuuchi Chemical. Specimens (40 × 10 × 1 mm3) with (110) crystal faces, on which the crystal c-axis was present were used. The root-mean-
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square roughness of the face was 0.1 nm. Before preparing MB films, the specimens were rinsed with ethanol. They were then immersed in 0.1 M aqueous NaOH for 5 min, rinsed with distilled water, and annealed at 620 K for 1 h. After cooling under dry air, the crystal was immersed for 10 s in an acetonitrile solution of MB (Nacalai Tesque) and removed from the solution at a constant speed of about 1 mm s−1. The specimen was dried in air at room temperature. Rubbing treatment was carried out by manual brushing, that is, by wiping both surfaces of the crystal one hundred times in one direction with laboratory tissue paper (Crecia, Kimwipe, S-200). Surface morphology was observed with an atomic force microscope (AFM, Park Systems, XE7). Because absorbance with some MB films is weak (
