Indian Journal of Pure & Applied Physics Vol. 56, September 2018, pp. 703-707

Study of effect of annealing on morphology of hydrothermally synthesized potassium titanate fibers

Shailendra Rawata, Jyotsna Sharmaa,b & Shatendra Sharmac*

a School of Basic and Applied Sciences, K R Mangalam University, Gurugram 122 001, India bAmity School of Applied Sciences, Amity University, Gurugram 122 413, India

cUniversity Science Instrumentation Centre, Jawaharlal Nehru University, New Delhi 110 067, India

Received 30 October 2017; accepted 27 March 2018

The potassium titanate fibers have been synthesized using TiO2 and KOH as startup materials in one step hydrothermal method. The thickness of synthesized fibres ranges from 20 to 100 nm whereas the length varies from 200 nm to 200 microns. The effect of annealing at various temperatures on the synthesized nanofibers has also been investigated by characterization using XRD, SEM, TEM, Raman and FT Raman. The annealing at 800 °C has been found to yield a pure K2Ti6O13 phase when its XRD peaks have been compared with the standard XRD data. The growth of nanowires is along the (010) direction. However, when annealed at 400 °C, 600 °C, 800 °C and 1000 °C the samples show the transformation into several other structural phases. However, annealing above 800 °C causes all the structures of nanowires to transform into K2Ti6O13. The TEM image shows that the product formed contains a large quantity of fibres with almost uniform thicknesses but with varied lengths.

Keywords: Potassium titanate, Hydrothermal synthesis, Characterization

1 Introduction Titanates belong to the class of inorganic

compounds composed of titanium oxides. These materials are insoluble in water and possess high melting point and show diamagnetic nature1-4. The unique properties of the material such as high chemical stability and corrosion resistance make these superior from the other metal oxide materials5-8. In the past decade one dimensional nanostructures have attracted extraordinary attention for their novel physical properties9-10 and potential applications in constructing nanoscale electronic and optoelectronic devices11,12. The wide range applications of potassium titanate in different fields are due to its controllable particle size, morphology and crystalline structure13-16. Titanate nanotubes and nanowires are particularly interesting because they have large specific surfaces and may enhance the photolytic activity17-19, leading to the higher potential applications in whisker-reinforced plastics and metals20.

Various techniques have been used to prepare potassium titanate such as chemical vapour deposition, sputtering, hydrothermal and sol-gel

processes21-23. Potassium titanate belongs to a wide family of crystals with different morphologies that are of great interest owing to their tunnel or layered type crystalline structures24-28. Hydrothermal synthesis is a promising low cost method for potassium titanate synthesis due to the combined effects of solvent, temperature and pressure on the ionic reaction equilibrium that can stabilize desirable product while inhibiting the formation of undersirable compounds18,19. Moreover, hydrothermal synthesis yields highly pure, homogeneous whiskers at a considerably lower temperature compared to the solid state reaction. Hydrothermal growth is a one-step, environment friendly and inexpensive way in which the morphology, size and purity of the product can be controlled under moderate conditions. It is also possible to have high production efficiency in the formation of nanosized particles or nanowires. In present work synthesis, characterization and annealing of potassium titanate nanofibers is discussed. 2 Experimental

The samples are prepared by hydrothermal method using TiO2 (99% pure anatase phase) as the Ti source and KOH as potassium source. Both materials used

———————— *Corresponding author (E-mail: shatendra@gmail.com)

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for the synthesis, TiO2 and KOH are purchased from Fischer scientific. These chemicals are used without any further modification or purification. The 10.08 g KOH and 1.76 g anatase TiO2 were mixed in 100 mL of deionized water using a magnetic stirrer for 1 h. The homogeneous mixture obtained is then transferred to a Teflon lined steel autoclave and placed in an oven at 200 °C for 48 h. The sample is allowed to cool to the room temperature and washed with DI water several times through vacuum filtration unit to remove the traces of residual KOH. The sample is then dried at 110 °C in the oven for 24 h. The complete process flow chart is shown in Fig. 1.

The synthesized dried powder sample is divided into five portions named as To (orginal), T1, T2, T3 and T4. The portion of the sample named T1 is kept in the oven at 400 °C for a 24 h. After 24 h and normal cooling, the sample T1 is washed again with de-ionized water and filtered. The sample T1 is

dried an oven at 110 °C for 2-3 h. Similarly other samples named as T2, T3 and T4 are annealed at 600 °C, 800 °C, 1000 °C, respectively, for 24 h.

All samples T0, T1 ,T2, T3 and T4 are then characterized to study structural change using the techniques dicussed below. 3 Results and Discussion

3.1 X-ray diffraction(XRD) XRD measurements are performed using an X-ray

diffractometer (Model Panlytical XpertPro-8) at a wavelength of Cu (λ=1.54 Å). The XRD spectra in Figs 2-5 show the effect of annealing on the synthesized nanofibers. The growth of nanowires is along the (010) direction. The sample annealed at 800 °C shows pure K2Ti6O13 phase. However, annealing above 800 °C causes all structures of nanowires to convert into K2Ti6O13. The K2O and K2Ti4O9 peaks are also observed in the samples. The XRD results show that some other phases of the potassium titanate such as K2Ti2O5, K2Ti3O7, K2Ti4O9 and K2Ti8O17 may also be present at different stages

Fig. 2– XRD of the un-annealed potassium titanate sample (To).

Fig. 3 – XRD of the annealed potassium titanate sample (T2) at 600 oC.

Fig. 1 – The flow chart showing the process of synthesis ofpotassium titanate.

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of annealing besides K2Ti6O13. The transformation of such structures has been reported earlier in calcination method at temperatures higher than 600 C to attain temperature stability29-31.

There are large variations of intensity and peak positions observed in the peaks of the samples as seen from the above figures which indicates that annealing of the sample causes reasonable change in the phases of the material. 3.2 Scanning electron microscopy (SEM)

The morphology and structure of the fibers is observed under scanning electron microscope (Model: Zeiss EVO40). The SEM images (Figs 6 and 7) show the uniform thickness fibers of potassium titanate with thickness of about 20 to 100 nm. The annealing of fibres at 800 oC improves the structure of fibers (Fig. 8)

Fig. 6 – SEM of sample To (original sample).

Fig. 7 – SEM of sample To (original sample).

Fig. 8 – SEM of sample T3 (annealed at 800 oC). 3.3 Transmission electron microscopy (TEM)

Transmission electron microscopy (TEM) imaging is performed using a JEOL 2100F transmission electron microscope operating at 200 kV. TEM specimens are prepared by dispersing the sample in

Fig. 4 – XRD of annealed sample T4 at 800 oC.

Fig. 5 – XRD of annealed sample T5 at 1000 oC .

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alcohol. The sample solution is then dropped onto a carbon film supported on a copper grid and dried in air. The TEM images also reveal that the structure of the fibers also modifies with anneling (Figs 9 and 10). Figure 11 shows the electron diffraction pattern of the sample (with d=0.291 nm). Figure 12 shows

the SAED pattern with HRTEM at 200KV showing the well defined crystal structure of the sample (annealed at 800 oC). 3.4 Raman and FT Raman spectra

The raman spectra are recorded by both a simple table top Raman and with an FT Raman system (Model WiTec Alpha-300 ). Figures 13 and 14 show the Raman spectra of unannealed and annealed

Fig. 9 – TEM of sample T1 (annealed at 400 oC).

Fig. 10 – TEM of sample T2 (annealed at 600 oC).

Fig. 11 – Electron diffraction pattern of the sample (d= 0.291 nm.)

Fig. 12 – SAED pattern of the sample with HRTEM at 200 kV.

Fig. 13 – Raman spectrum of un-annealed sample T4 (UA).

Fig. 14 – Raman spectrum of annealed sample T4 (A) at 800 oC.

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samples with simple raman spectrometer that indicate the crystallanity improves with annealing as the observed peakes get well defined and sharp after annealing. The FT raman spectrum of annealed sample T4 at 1000 oC shows very sharp peaks but the intensity of peaks at 396 nm and 634 nm are drastically reduced (Fig. 15). 4 Conclusions

The potassium titanate nanofibers are successfully synthesized by the hydrothermal method with a good commercially viable yield. The thickness of these fibres ranges from 20 to 100 nm whereas the lengths vary from 200 nm to 200 microns. Characterization of samples is carried out by using XRD, SEM, TEM and Raman spectroscopy in order to determine the structure of the nanofibres. The synthesized potassium titanate fibers are annealed in a furnace at 400 oC, 600 oC, 800 oC and 1000 oC. The annealing of the samples at various temperatures shows a transformation of phases of the material. The XRD patterns indicate that the phases of the samples annealed at 400 oC, 600 oC and 1000 oC are mixture of many phases but the sample annealed at 800 oC show presence of K2Ti6O13. The TEM image showing that the product contains a large quantity of fibres with almost uniform internal structure. Imaging by SEM and TEM as well as Raman spectra show that there is a structural change in the material phases due to annealing at various temperatures. The hydrothermal method of synthesis gives a good yield of potassium titanate fibres but the thickness and length of fibres are not precisely controllable. Acknowledgement

The financial assistance under UPOE-II project by Jawaharlal Nehru University, New Delhi 110 067, India is gratefully acknowledged. The help from

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Fig. 15 – FT Raman spectrum of annealed sample T4 (A) at 1000 oC.