Ind. Eng. Chem. Res. 1998, 37, 1317-1323
1317
Structural Investigation of Hydrous TiO2 Precipitates and Their Aging Products by X-ray Diffraction, Atomic Force Microscopy, and Transmission Electron Microscopy Juho-Pertti Jalava* Kemira Pigments Oy, FIN-28840 Pori, Finland
Lauri Heikkila1 ,† Oskari Hovi, and Reino Laiho Wihuri Physical Laboratory, University of Turku, FIN-20014 Turku, Finland
Erkki Hiltunen Department of Medical Physics and Chemistry, Institute of Biomedicine, University of Turku, P.O. Box 123, FIN-20521 Turku, Finland
Arvi Hakanen Department of Physics, University of Turku, FIN-20014 Turku, Finland
Harri Ha1 rma1 ‡ Department of Chemistry and Biochemistry, University of Turku, FIN-20014 Turku, Finland
The structure of TiO2 hydrates is studied by X-ray diffraction, atomic force microscopy (AFM), and transmission electron microscopy (TEM). The samples prepared by NH4OH precipitation from an aqueous TiCl4 solution are amorphous or partially crystalline. They consist of a mixture of distorted anatase and brookite crystallites, each having a diameter of 20 nm. The sample prepared by thermal hydrolysis from a titanium oxide sulfate solution was totally crystalline, composed only of distorted anatase crystallites with a diameter of 15 nm. Primary particles can be oriented in chainlike structures. The crystallinity and the crystal shape reveal the sample history. The applicability of the AFM and TEM measurements for structural investigations of the powder specimens concerned is briefly discussed. 1. Introduction Surface structures of the precipitates prepared by hydrolyzing titanium oxide sulfate or titanium tetrachloride in acidic water solutions are not well known, although these compounds are very important intermediate products in the manufacture of titanium dioxide pigments by the sulfate process (Jalava, 1992). Investigated by X-ray diffraction methods, these precipitates, depending on the preparation technique, appear to be amorphous or to consist of very small rutile or anatase crystallites with a diameter less than 10 nm (Weiser and Milligan, 1934; Ooi et al., 1988). Jalava et al. (1998) have observed by small-angle X-ray scattering (SAXS) studies that the X-ray amorphous structures contain mass fractalic aggregated particles with a particle diameter of approximately 1.5 nm. Some of the aggregates have a tendency to crystallize slowly to particles exhibiting porous surface fractal character and a crystalline (anatase or brookite) kernel. Atomic force microscopy (AFM) has been used, for example, to study titanium dioxide thin films (Ritala * To whom all correspondence should be addressed. Email:
[email protected]. † Present address: Department of Applied Physics, University of Turku, FIN-20014 Turku, Finland. ‡ Present address: Department of Biotechnology, University of Turku, Tykisto¨nkatu 6A, FIN-20520 Turku, Finland.
et al., 1993). In addition to AFM, scanning tunneling microscopy (STM) or scanning tunneling spectroscopy (STS) has been used to investigate electrically conducting TiO2 rutile single crystals (Fan and Bard, 1990; Clark and Kesmodel, 1992; Rohrer et al., 1992; Zhong et al., 1992), but to our knowledge no corresponding results for powder samples are available. This is not surprising because the number of STM studies on powder specimens, in general, is very limited (Friedbacher et al., 1991; Kwetkus and Sattler, 1992; Zhang et al., 1992). To develop further the sulfate process, better structural knowledge of intermediate products in the process is important in order to understand and simulate the kinetics of the precipitation of the TiO2 hydrates. The goal of this research is to measure material structures in different scales and to compare results obtained with different methods. Atomic force microscopy is used to investigate nanometer-size details of the surface structures of TiO2 hydrate precipitates. X-ray diffraction and transmission electron microscopy are used to characterize the particle size and crystallinity. The samples are hydrolyzed from water solutions of titanium tetrachloride or titanium oxide sulfate. We are interested in their structure and in the aging products of the former. The same sample material was earlier investigated with the SAXS method (Jalava et al., 1998).
S0888-5885(97)00469-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/18/1998
1318 Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 Table 1. DI File of the C1a Sample peak no. 1 2 3 4 5 6 7 8 9 10 11 12
phase
hkl
anatase + brookite brookite brookite brookite brookite anatase anatase brookite brookite brookite anatase brookite
101 120 121 200 012 201 004 112 022 221 032 200 132
2θ (deg)
D-spacing (nm)
25.45
3.497
30.83 33.00 36.27 37.32 37.92 38.18 40.15 42.33 46.07 48.02 49.17
2.898 2.712 2.475 2.408 2.371 2.355 2.244 2.134 1.969 1.893 1.852
I/Imax (%) 100 27 0.2 12 18 25 26 0.7 6 10 31 0.8
half-width (deg) 0.72 0.48 0.48 0.36 0.24 0.20 0.32 0.12 0.28 0.24 0.16 0.32
Table 2. DI File of the S Sample peak no. 1 2 3 4
phase
2θ D-spacing I/Imax half-width hkl (deg) (nm) (%) (deg)
anatase 220 25.40 anatase 112 38.36 copper (sample 43.35 holder) anatase 200 47.96
3.503 2.344 2.086
100 10 0.7
0.88 0.72 0.48
1.895
38
0.72
2. Experimental Methods 2.1. Preparation of the Samples. TiO2 hydrates were precipitated either by NH4OH neutralization from an aqueous titanium tetrachloride solution at
