Chapter 5: Strongly Enhanced Visible Light Photoelectrocatalytic Hydrogen Evolution
6.3. Results and Discussion
6.3.2. FETEM Analysis
sample T20, after vacuum RTA the OV density increases dramatically and reaches the highest value among all the samples. This effect is found to be less in the sample T20AR (air RTA, oxygen-rich environment). In the case of furnace annealing, the OV density increases from that of the untreated sample, but not as high as in RTA treated sample. Thus, it is clear that the RTA treatment is superior over the normal furnace annealing to create controlled defects in the TiO2
system.
Fig. 6.4. (a) FETEM image of T20 showing a clearer view of the self-grown NCs on TiO2 NF. (b) HRTEM lattice fringe pattern of TiO2 NCs on TiO2 NF. The arbitrary shape of the NCs is shown with dotted boundary lines. (c) SAED pattern of the self-grown NCs on the TiO2 NFs showing single crystal pattern. (d) FETEM images of T20VR showing the presence of spherical TiO2 QDs on the TiO2 NF. (e) HRTEM lattice fringe pattern of a crystalline TiO2
QD in T20VR. (f) SAED pattern of the TiO2 QD. (g) FETEM image of self-grown NCs in T80, (h) a magnified view of the NCs showing the arbitrary shape of the NCs on the surface of TiO2 NF. (i) HRTEM lattice image of TiO2 NCs. (j, k) FETEM images of T80VR at lower and higher magnification showing TiO2 NPs on the TiO2 NF, and (l) HRTEM lattice fringe pattern of TiO2 NPs.
The lattice d-spacing calculated here for all the cases is smaller than the standard value, which indicates a compressive strain in the TiO2 lattice, and this is consistent with the Raman and XPS analyses discussed later.
Fig. 6.5. The size distribution of F-TiO2 NCs grown on (a) T20, (c) T60 and (e) T80. (b, d, f) The respective size distribution after vacuum RTA treatment. The average diameter (D) with lognormal fitting and width of the distribution (W) are shown as an inset.
Spherical TiO2 QDs are formed here from the arbitrary shaped TiO2 NCs by the TRA treatment, which involves an extremely fast change in the thermal environment (20 °C/sec) of the sample.
During the RTA treatment under vacuum, TiO2 NCs with average size 5.0 nm (for T20) are observed to be transformed into spherical QDs with average diameter 4.2 nm, which falls in the weak quantum confinement regime10, and thus these ultrasmall spherical NPs are referred as QDs. For T60 and T80, the NCs with average size are 8.4 nm and 11.4 nm, respectively, and these are transformed into spherical NPs with sizes 9.7 nm and 12.8 nm, respectively after vacuum RTA, see Fig. 6.5. STEM elemental mapping analysis of T20 and T20VR also confirms the presence of Ti, O and F in the system, as shown in Fig. 6.6. After RTA treatment, the concentration of F reduces greatly, though it does not vanish completely, which may be due to the elimination of surface adsorbed F but not the F doped in the TiO2 crystal lattice. It appears that F doped in the TiO2 lattice has high thermal stability.
Now we attempt to explain the shape evolution of TiO2 NCs from arbitrary shape into spherical NPs/QDs during the RTA treatment. In the classical nucleation theory11, 12, interface/boundary of the NCs plays an important role in the evolution of the NCs to a critical size. Classical nucleation theory predicts that to reduce the total free energy, the size of the NCs may reduce/increase to arrive at the critical radius and spherical shape assumes a minimal surface area. Due to the thermal stress during the RTA process, the interfacial contact angle for
the attached NCs increases when the shape evolves to spherical type, and this helps in achieving the minimum free energy.
Fig. 6.6. STEM images of mesoporous TiO2 NFs and its elemental mapping in T20: (a-d) as-grown, and (e-h) after vacuum RTA.
In the present case, after 3 min vacuum RTA, we observe a remarkable shape evolution of self- grown TiO2 NCs to perfect spherical shape in each case. This may be explained as follows. As compared to its bulk form, the melting point of the TiO2 NCs reduces substantially due to the formation of very small NCs. With the thrust of the thermal energy during RTA, the NCs tend to make a higher contact angle with the TiO2 NFs interface to reduce the surface energy. Thus, the arbitrary shaped TiO2 NCs transform into the spherical QDs having higher contact angle.
Additionally, during the vacuum RTA, the concentration of OV defects in the sample enhances dramatically, which induces compressive strain in the lattice that in turn squeeze the crystallite size of the TiO2 QDs. Note that in case of normal furnace annealing, such a shape evolution to spherical QDs was not observed, implying the critical role of thermal stress during the RTA process in achieving the spherical NPs/QDs along with high defect density. Further, it has been reported that fluorine has an important role in controlling the faceted growth of TiO2
nanostructures. Yang et al.13 reported that in the absence of fluorine, no crystal facet control was observed for TiO2 and only spherical polycrystalline anatase particles were formed. In this case, we noticed that after RTA, surface adsorbed fluorine is completely removed from the TiO2 NCs, as revealed from XPS results (see Section 6.3.3.3) and this helps partly in achieving the growth of spherical TiO2 NPs/QDs after annealing.
6.3.3. Structural Analysis