Atomically flat oxide single crystals, some of which have now become commercially available, are getting more and more important today. It is because they can provide a useful platform not only in high-quality oxide epitaxy and its device applications, but also in fundamental investigations of oxide surface science. In this review, I summarize some of the wet processes that have made a breakthrough in the preparation of such an atomically flat oxide single crystal surface, focusing on the chemistry involved in these wet processes. It includes those of TiO2 and MgO(001) oxide single crystals, and the benefits we can get from the use of these atomically flat oxide single crystals are also displayed.
Essentially atomically smooth(100) and (110) n-TiO2 (rutile) surfaces were prepared by wet-method. The success in preparation of the atomically smooth and stable n-TiO2 surfaces enabled us to reveal clear crystalface dependences of the surface band edges, hole reactivity and various kinds of photo-induced reactions in aqueous solutions. This paper reviews our research of photoelectrochemical properties of single crystal TiO2 surfaces.
We developed an ultraprecise surface preparation method named catalyst referred etching (CARE) in which a reference plate is employed to be copied on to the work surface during chemical etching. The reference plate has a catalytic nature to enhance etching reaction just on the surface. Single crystalline SiC (0001), GaN (0001), and ZnO (0001) surfaces were processed by CARE, and evaluated to be nearly atomically flat and crystallographically highly ordered.
Since metal oxide surfaces are chemically stable in aqueous solutions, their atomic structures and chemical properties can be utilized in nanofabrication and nano-biotechnology in aqueous environment. This article describes phenomena related to water layers on single-crystalline metal oxide surfaces. On sapphire (0001) surfaces, domain structures with different hydrophilicity and charged states are spontaneously formed during thermal annealing. This surface exhibits domain-selective adsorption of protein molecules. Phase separation on metal oxide surfaces is classified into structural and chemical ones. On the chemically phase-separated surfaces, terraces with different surface chemistry coexist. Surface chemistry in aqueous environment can be characterized by adsorption force measurement of a tip to the substrate surface using atomic force microscopy (AFM). Water layers on a metal oxide surface can be observed by AFM by attaching graphene films to the oxide surface. Chemical phase separation on titania (100) surfaces is also demonstrated.
First-principles studies on TiO2/H2O interfaces, representative photocatalytic systems, are reviewed. First I show our recent first-principles molecular dynamics study on anatase TiO2/H2O interfaces, with emphasis on the difference of calculation techniques and results from those in the previous studies. Then, arguments on water adsorption mode (molecular or dissociative etc.) on TiO2 rutile (110) surface are discussed. Finally, with our recent results, I introduce the difference of surface nanostructure stability between vacuum and aqueous conditions, which is relevant to the difference between dry and wet processes.
A chemical mechanical polishing (CMP) mechanism of a glass surface by a CeO2 abrasive grain under water environment has not been elucidated because the CMP process is complicated combination of chemical reactions and mechanical polishing. In this review, we introduce our successful clarification of the CMP mechanism by computational simulation methods. First, we revealed that the oxygen defects in the CeO2 abrasive grains lead to the generation of exposed Ce3+ atoms on the CeO2 surface and then the chemical reactions of the exposed Ce3+ atom and the glass surface make the elongation of Si-O bonds. We also clarified that H2O molecules react with the above elongated Si-O bonds and then the Si-O bonds are dissociated. These chemical reactions are suggested to soften the glass surface and enhance the mechanical polishing. Furthermore, according to the above clarified CMP mechanism, we succeeded to propose the design principles for the alternative materials of CeO2 abrasive grains.
We have developed a low-temperature, high-magnetic-field scanning tunneling microscope combined with pulsedlaser deposition, and studied atomic-scale growth of SrTiO3 (STO) and LaAlO3 thin films. A (√13×√13 )-R33.7o STO(001) substrate, prepared in oxygen atmosphere, is atomically resolved and plays a crucial role in elucidating the initialgrowth process. When homoepitaxial STO films were grown on the (√13×√13 ) substrate surfaces, additional TiOx layer of the substrate was transferred to the film surfaces. Atomic-scale microscopic/spectroscopic approach opens up awayto atom-by-atom controlled oxide epitaxial films and heterostructures.