Structural Aspects in the Study of Nanocrystalline Materials

Principle difference between nanocrystalline and bulk solid materials is based on the great distinction of the surface-to-volume ratio. Indeed, Snc/Vnc Ssc/Vsc = SpC/VPC, which are realized very often in essential changes of adsorptive, electrochemical, catalytic and photocatalytic properties of nanosized and massive particles. In rather many works the specific surface area S and/or the diameter of the particles 2R were determined and compared with the features [38, 50, 109-121]. S-value is usually estimated by means of the BET method at adsorption of small molecules onto the surface (N2 [110, 112, 115-117], Ar [38, 121], CH3OH [49]) from the gas phase. 2R-values are calculated from X-ray diffraction (XRD) [38, 109-114, 120, 121] or TEM [112, 113, 117-120] data. The XRD was also used for controlling the phase state (A or R) of the Ti02 material. Existence of noticeable amounts of the brookite phase (7-18% in the range of 70-400°C) was observed in [114]. 

Thermal treatment of titanium dioxide precipitates at temperatures between 200°C and 600°C produced powders of agglomerated crystallites as determined by XRD. The average diameter 2R of the crystallites estimated from the half-widths of the diffraction peaks is shown as a function of the calcination temperature Tca]c in Fig.8.3, where we collected the experimental results for Ti02 from several publications. An increase in the Tcak of the powders leads to an increase in the particle size 2R especially in the case of rutile [38]. Unfortunately, in this basic work M. Anpo et al. did not present TcaJc values at which they prepared their anatase samples for 2R and S measurements. 

Tca]c dependences reported in [109, 111, 113] are very similar but differ strongly from those published in [112] (Fig. 8.3). Probably, this is a result of the applied alkoxide method of Ti02 synthesis (Ti[OCH(CH3)2]4 was a starting compound in [112]), and not the conventional sol-gel method. Indeed, as it was shown in [111], a crystallite 

Dependences of 2R on Tcaic for several pure or mixed semiconductor oxides are presented in Fig. 8.4. Iron doped titania photocatalysts with different iron contents at Tcalc below 400°C had iron ions uniformly distributed in the anatase-Ti02 phase [114]. At Tcalc 400°C, 1 wt % Fe samples performed the same behaviour of 2R as without iron, and at Tcalo 600°C in the samples with 10 wt % Fe content, the formation of hematite phases interacted with the titania phases was observed in XRD experiments. The crystalline structure of Ti02 phases was distorted at high Tcalc which also resulted in 5-fold decrease of 2R as compared to 1 wt % Fe case (Fig. 8.4). 

It was noticed that the preparation method also affects the particle size 2R increased from 4 to 47 nm and from 7 to 40 nm in the range of 350 Tcaic 770 K for particles synthesized by sol-gel or gel method correspondingly [117]. Serious difference between 2R vj. Tcaic curves observed for ln203 particles in [109] and [120] could be also explained by variations in synthetic procedures. 

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