Chapter 3: Controlled Growth of TiO 2 Nanostructures
3.6. Growth mechanism
etc. The diameters of the different nanostructures measured from the FESEM and TEM images are shown in Table 3.1.
When anatase TiO2 powder precursor reacts with concentrated alkaline NaOH solution, Ti-O-Ti bonds are broken and the Na+ ions intercalated within the TiO6 octahedral units and then recrystallized into layered Na2Ti5O11⋅H2O nanosheets. After ultrasonic HCl treatment and washing, hydrated hydrogen pentatitanate (H2Ti5O11⋅H2O) is formed by the ion exchange of Na+ with H+ and the by-product NaCl is formed. The final anatase TiO2 products are obtained by dehydration process during the calcinations at 500 °C for 5 h. For simplification, the overall chemical processes of the as-prepared samples are as follows.
4ܱܶ݅ଶ +2ܱܰܽܪ→ܰܽଶܶ݅ସܱଽ+ܪଶ ܱ (3.1) 5ܰܽଶܶ݅ସܱଽ+10ܪܥ݈ →4ܪଶܶ݅ହܱଵଵ∙ ܪଶܱ +10ܰܽܥ݈ (3.2) ܪଶܶ݅ହܱଵଵ∙ ܪଶܱ →5ܱܶ݅ଶ +2ܪଶܱ (3.3)
Thus, the final anatase TiO2 nanostructured products are obtained by the three steps reaction process: (i) the reaction of TiO2 precursor with alkaline NaOH solution, (ii) the ion exchange between Na+ and H+ and (iii) the final calcination. The early growth stage of 1D nanostructure at 130 °C in alkaline water solvent shows the coexistence of nanosheets and partially developed NTs, indicating that NTs are formed from the layered titanate nanosheets.
At comparatively higher temperature (155 °C) and pressure, the nanosheets completely roll up to form a bunch of NTs well oriented in a particular directions as shown in Fig. 3.2(b).
The driving force for the rolling may be asymmetric due to the hydrogen deficiency in surface layers23 together with unsymmetrical surface forces due to locally high surface energy. Another reason for the rolling of nanosheets at high temperature and pressure may be due to the mechanical tension that arises during the process of dissolution/crystallization in nanosheets.8 When structurally similar surfaces approach, atoms of opposing surfaces may combine chemically with each other. The chemical coordination results in NTs bundles bunching along identical crystal faces.14, 25 Interestingly, note that NRs are also observed in the same product shown in Fig. 3.2(c) and 3.5(b). This reveals that some of the NTs detached from the bundles of NTs and transformed into NRs. At comparatively higher temperature (180 °C), we observed only NRs instead of any intermediate NTs. This is expected because during the stirring, the bunch of NTs may split and simultaneous shrinkage and growth may occur due to high temperature and pressure to form condensed NRs. The main advantage of the stirring is the splitting of the NTs and nanosheets. At the same time, the constant motion due to stirring prevents sedimentation and forces the intimate mixing of
the system. After splitting of the bunch of NTs, the NTs are detached from the bundles and the pressure acts on the individual nanotube and hence the effect is more as compared to the pressure acting on the bunch of the NTs. That is why the NTs transform to condensed NRs.
The observed pressure is 8 bar showing in manometer which is in-built with the high pressure reactor set up, when the temperature is 180 °C under stirring condition for the alkaline water solvent. In addition, constant motion fields created by internal stirring are probably effective in continuously breaking up freshly formed nanotube assemblies, so bundling is prevented again. The thickening and splitting nature of the nanosheets are clearly shown in Fig. 3.3(a) for the early growth stage at 130 °C in alkaline ethanol cosolvent.
However, no such NTs are observed at 155 °C as observed in case of alkaline water solvent, when ethanol was used as a cosolvent under similar conditions.
Note that the length of the NRs is more than that of the NRs grown in alkaline water solvent at the same reaction temperature, indicating the fast growth rate of the samples in ethanol cosolvent. The rapid growth rate in alkaline ethanol mixed solvent accelerates the formation of thick nanosheets and further splitting of the thick nanosheets results in the formation of NRs at higher temperature and pressure. When the reaction temperature increased to 180 °C, only NRs and NRs/NWs are observed in alkaline water solvent and alkaline ethanol cosolvents, respectively. The possible reasons are (i) the thickening of nanosheets unable to fold and split into NRs/NWs. The high temperature as well as stirring may help to accelerate the reaction for the nucleation of thicker nanosheets and subsequently splitting of the thick nanosheets; (ii) the very fast growth rate and stirring promoted detachment of the NTs from the bunch of NTs which results in rapid shrinkage of the NTs to ultimately form NRs at higher temperature and pressure. The observed pressures at 180 °C reaction temperature under stirring condition are 8 and 16 bar for the alkaline water solvent and alkaline ethanol cosolvent, respectively. In case of ethylene glycol cosolvent, the TiO2 nanoparticles react with NaOH to form thick layered nanosheets at low growth temperature (130 °C) and then the nanosheets are splitted to form small NRs at comparatively high temperature (155 °C) and further growth of small nanorods turns to NRbs at 180 °C.
Note that a quite unique morphology is formed at 180 °C in ethylene glycol cosolvent, i.e., uniform nanoporous TiO2 NRbs which are observed from the TEM images as shown in Fig. 3.5(g) and (h). At low growth temperature, the growth/ reaction rate is low
compared to alkaline water solvent and ethanol cosolvent by comparing the FESEM images.
However, at 180 °C, the growth rate is more in case of ethylene glycol cosolvent compared to water solvent and ethanol cosolvent. These results indicate that the solvent plays an important role to control the morphology of the products. A possible reason for this type of morphology may be due to the polarity and coordinating ability of a cosolvent have a strong effect on the solubility, reactivity and diffusion behavior of the reactants, thus ultimately influencing the structure and morphological features of the resulting products.26 The parent TiO6 octahedra of TiO2 materials may coordinate with glycol to form chain-like structure,27 whereas the NaOH may form titanate nanosheets by sharing the vertices edges of the octahedra and resulting NRbs with nanopores are formed at high temperature and pressure.
The growth mechanism and evolution of different 1D nanostructures are better elucidated in the schematic diagram shown in Fig. 3.6. The crossed dashed-line in the diagram indicates that the NRs/NTs are not formed directly from the TiO2 NPs and it essentially involves the formation of the nanosheets followed by splitting and/or rolling to form NRs/ NTs of TiO2.
Fig. 3.6. Schematic of the growth mechanism of different 1D nanostructures of TiO2 formed at different temperatures using various solvents.