Titanium oxide(TiO₂) has three polymorphs, namely, rutile, anatase and brookite, each of them crystallizing in various forms in nature[1]. How are the morphology determined depending upon growth environments? Especially in the case of anatase, control of the crystal form has practical importance because its photocatalytic activity depends on the orientation of the surfaces. In order to control the crystal shapes, we need to know stabilities of crystal faces in various environments.

Stabilities of crystal faces can be compared by observing polyhedral micro-etch pits at crystal faces with atomic force microscope(AFM). One of the authors employed the method in studying relative stabilities of crystal surfaces of less water-soluble aragonite(CaCO₃)[2-4] and anhydrite(CaSO₄)[5] in aqueous solutions. Similar method can be applied to the studies of oxide crystals if appropriate etchants are obtained. In this study, three low-index faces of anatase were etched by heating with KHSO₄, which produces SO₃. The polyhedral etch pits formed were observed with AFM ex situ. The space group and lattice constants of anatase are tetragonal I4₁/amd, a=b=378.67, and c=951.49 pm[6]. The crystal faces studied are c(001), p(101) and m(100), where the alphabets were added to show the relationship with the classical nomenclature frequently used in mineralogy books[1]. The c- and p-faces used were naturally grown ones, while the m-face was sliced out of a natural crystal.

The AFM images shown in Figure 1 were observed after the c(001) surface was etched. With the smaller etch pit in (a), u{1,1,10} facets consisting of e{112} steps and c(001) terraces were stabilized. With the larger pit in (b), on the other hand, z{103} facets were observed in addition to x{116}=e{112}+4·c(001) facets. The e(112) and z(103) faces are nearly as stable as the c(001) face, since Ti atoms at the three faces are all 5-coordinated.

Figure 1. AFM images of TiO₂(001) surface after etching, where {11n} and {103} facets are stabilized.

  Figure 2. AFM images of (a)TiO₂(101) and (b)(100) surfaces after etching. Stable facets forming sidewalls of etch pits contain (100), (101) and (112) components.

The p(101) is considered to be the most stable surface since equal numbers of 6- and 5-coordinated Ti atoms are densely packed on it. In the etching of the p-face, trapezoidal etch pits were formed as the one shown in Figure 2(a). The bases are formed by m(100) and k(102)=(101)+(001) facets. The m(100) face is considered to be nearly as stable as the p-face, because equal numbers of 6- and 5-coordinated Ti atoms make the surface. Two other sidewalls of the pit are combination of e{112} ledges and p(101) terraces.

When the m(100) face was etched, triangular pits formed by p(101) and two {n13} facets were observed as shown in Figure 2(b). The (n13) facet is a combination of τ(213) ledges and m(100) terraces.

In the oxidative environment employed, relative stabilities of the crystal faces are determined by coordination numbers of Ti at the surfaces. The most stable p(101) and m(100) faces have 5- and 6-coordinated Ti atoms. Slightly less stable c(001), e(112), z(103) and k(102) faces have 5-coordinated Ti atoms on it. Other faces containing 4-cordinated Ti atoms, such as u(1,1,10) and x(116), appear only temporarily in the etching process. A natural crystal form of truncated tetragonal bipyramid having p-, m-, e-, c-,and u-faces is reported in literature[1]. The crystal was most probably formed in an oxidative condition like the one we used.

Acknowlegement

This work was supported by JSPS KAKENHI 24510144 and 20510097, and The Institute of Science and Engineering, Chuo University.　　　　　

References 

[1] E. S. Dana, The System of Mineralogy –Descriptive Mineralogy, Wiley, New York, 6^th ed. 1900, pp.237-243.

[2] M. Kwak and H. Shindo, J. Cryst. Growth, 275 (2005) e1655-1659.

[3] H. Shindo and M. Kwak, Phys. Chem. Chem. Phys., 7 (2005) 691-696.

[4] Y. Shirota, K. Niki and H. Shindo, J. Cryst. Growth, 324 (2011) 190-195.

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