Structural, optical and conductivity study of hydrothermally synthesized TiO₂ nanorods

TiO₂ nanopowder (P25) purchased by sigma Aldrich USA with high purity, sodium hydroxide (NaOH) AR grade and hydrochloric acid (HCl) AR grade was purchased from FINAR for synthesis of TiO₂ nanorods.

The TiO₂ nanopowder (P25) is used as precursor, 10N NaOH is dissolved in 40 ml of distilled water and the solution is transferred to the Teflon lined autoclave by adding 0.5 gm of P25 to the above solution and hydrothermally treated for 24 h at 130 °C then allowed to cool for 4 to 5 h at room temperature. After cooling, the collected precipitation from the autoclave is washed with deionised water for 2 to 3 times called water treatment followed by an acid treatment (HCl) to obtain the pH value of the precipitation become 7 (neutralization). Finally the powder was dried at 60 °C to produce TiO₂ nanorods. The synthesis process was clearly shown in the scheme 1.

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1.98 gm of poly vinyl alcohol (PVA) is dissolved in 30 ml of distilled water and stirred for 6 h. On the other hand the synthesized 20 and 40 mg TiO₂ nanorods were dissolved in 10 ml of distilled water, sonicated the solution for 5 min, add sonicated TiO₂ nanorods solution drop wise to the PVA solution and stirred for 3 h to get homogeneous mixture, pour the solution to the petri dish and allow solvent to evaporate at room temperature. The film peeled off is used for further characterizations.

The x-ray diffractometer (Rigaker Miniflex II) was used to study the structural properties of the samples between 0 to 60° with the scanning rate 5° per minute. The chemical composition and functional groups of the samples are examined by using Fourier transform infrared spectroscopy (FTIR, ALPHA BRUKER), spectral range between 4000 cm⁻¹ to 500 cm⁻¹ wavenumber. The surface morphology of the samples was studied with Field emission scanning electron microscope (FESEM, sigma Zeiss) with operating voltage 15 kV and at the magnification 2μm. The various thermal studies like Thermogravimetric analysis (TGA), Differential thermal analysis (DTA) and Differential scanning calorimeter (DSC) were studied by the instrument (SDT Q600 TA Instruments) with temperature range between room temperature and 800 °C at the scanning rate 10 °C per minute with a nitrogen flow rate of 20 ml per minute. The two probe work station instrument (Keithley workstation) is used to examine the current voltage (I-V) characteristics of the samples in the voltage range between −5V and +5V. The dielectric studies of the samples were carried out by using Impedance Analyzer (Agilent 4294A Precision Impedance Analyzer) in the frequency range from 40 Hz to 10 kHz.

The Fourier transform infrared spectroscopy (FTIR) analysis helps to examine the functional groups and chemical composition recorded under the wavenumber 4000–500 cm⁻¹ of TiO₂ nanorods as shown in figure 1. For the sample P25 the peak at 3311 cm⁻¹ indicates the hydroxyl group (–OH) and that band shifted to 3304 cm⁻¹ in 20 mg and to 3291 cm⁻¹ in 40 mg due to the formation of TiO₂ nanorods and the broad absorption band between 3400 and 3200 cm⁻¹ indicates stretching vibration of hydroxyl group (O–H) [8–11]. The band at 2928 cm⁻¹is assigned to the C–H stretching [9, 12] of the sample P25 and the peak slightly shifts by increasing intensity to 2926 cm⁻¹ and 2921 cm⁻¹ respectively in case of 20 mg and 40 mg samples, and the band at 1433 cm⁻¹ of P25 is shifted to 1424 cm⁻¹ and 1420 cm⁻¹ for the 20 and 40 mg respectively.

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The C–O band stretching appears at the peak 1091 cm⁻¹ is shifted to 1084 cm⁻¹ and 1071 cm⁻¹ respectively with increased peak intensity. The peaks at 1559 cm⁻¹,1433 cm⁻¹, 1375 cm⁻¹, 1562 cm⁻¹, 1424 cm⁻¹, 1372 cm⁻¹, 1571 cm⁻¹, 1426 cm⁻¹, 1370 cm⁻¹ corresponds the Ti–O–C groups of P25, 20 mg and 40 mg respectively [12]. The observed FT-IR results shifting in peak positions and variation in the peak intensity confirms the change in chemical bonds and hence forms the TiO₂ nanorods.

Figure 2 shows the FESEM images of P25, 20 mg and 40 mg TiO₂ nanorods. Figure 2(a) confirmed the presence of TiO₂ nanoparticles and figure 2(b) reveal the formation of small rod like structure at 20 mg and figure 2(c) confirmed the formation of TiO₂ nanorods perfectly.

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During the synthesis process, NaOH and HCl play the major role in changing the morphology. Due to the addition of NaOH and HCl the Ti–O–Ti bonds are broken and form an intermediate product like Ti–O–Na and Ti–OH bond and the process leads to formation of layered structure and after that Ti–O-Na and Ti–OH bonds treated with water (pH 7) which leads to the formation of TiO₂ nanorods [13, 14].

The XRD pattern of the samples P25 and TiO₂ nanorods are as shown in figure 3. The XRD spectra exhibits the characteristic peaks with corresponding (hkl) index values for P25 at 2θ = 15.69° [202], 22.64°, 25.23° [101], 35.15° [004], 43.21° [111], and 53.11° [211]. The major peaks intensity was varied and the peaks shifts for 20 mg to 2θ = 14.96°, 20.91°, 24.88°, 34.59°, 42.83° and for 40 mg to 2θ = 14.74°, 4.07°, 34.76° and 42.83°.

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There is also a broad peak at around 2θ = 13° in P25, its decrease intensity or vanish for 20 mg and 40 mg is attributed to the initiation of phase transformation of TiO₂ nanoparticles into TiO₂ nanorods. The diffraction peak at 2θ = 25.23° is confirmed the anatase phase [15] of the pure P25 and corresponding diffraction anatase phase gets stronger and intense in both the 20 and 40 mg TiO₂ nanorods. The variations in the peak positions were clearly observed and the peak intensity increased significantly with increasing the weight ratio and disappearance or vanish of a broad peak at 2θ = 13° in P25 for 20 mg and 40 mg which attributed to the initiation of phase transformation of TiO₂ nanoparticles into TiO₂ nanorods. The observed results clearly indicate that the crystallinity was increased significantly in the nanorods.

The TGA/DTA techniques are widely used to evaluate the thermal stability of materials and the TG/DT graphs of P25, TiO₂ nanorods at 20 mg and 40 mg were shown in figure 4. The three major weight loss regions are observed in TG curves [16, 17]. The first stage was observed at 30–100 °C which is attributed to the removal of water or moisture content due to the braking of carbon-hydrogen bonds in the sample. The second stage was observed between 110–230 °C which is corresponding to the loss of dopants and any acid content from the sample. The major weight loss was observed in the third stage around the 550–575 °C which is related to the destruction in the backbone of polymer chain and losses the originality of the sample.

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From the figure 4 it is noticed that the Thermogravimetric temperature of the samples P25, 20 mg and 40 mg of TiO₂ nanorods have increased upto 557 to 567 and 575 °C respectively with correspondingly increase in the decomposition temperature 319, 327 and 329 °C for P25, 20 mg and 40 mg of TiO₂ nanorods respectively, which indicates the increased thermal stability of TiO₂ nanorods.

The melting temperature (T_m) of the samples was determined by DSC technique. DSC curves of P25, 20 mg and 40 mg were shown in figure 5. The thermal stability and melting temperature of the TiO₂ nanorods were examined between room temperature and 700 °C. It is observed that the melting temperature was increased with increase in the ratio of TiO₂ nanorods and it is responsible for enhancing the crystallinity.

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The DSC curves exhibits two exothermic peaks in the temperature range from 135 to 148 °C and 528 to 549 °C. The exothermic peaks at 135 °C, 141 °C and 148 °C of P25, 20 mg and 40 mg respectively are assigned to the adsorption of water or removal of moisture content from the TiO₂ nanorods [17]. The increase in the melting temperature at the exothermic peaks at 528 °C, 542 °C and 549 °C are attributed to the phase transformation of the TiO₂ nanoparticles into TiO₂ nanorods [17] as a result the crystallinity of the TiO₂ nanorods was increased. These results are correlated with the XRD analysis.

UV- Visible absorption spectra of samples P25, 20 and40 mg TiO₂ nanorods are shown in figure 6. The information about the band structure of compounds can be studied by optical absorption spectra. An electron excited from lower to higher energy state by absorbing a photon of energy of electron inter-/intraband transition or exciting transition. The main absorption wavelength of TiO₂ nanorods corresponds to the intrinsic absorption of anatase phase of TiO₂. The absorption shows red shift towards higher wavelength from 272 nm to 276 nm and 279 nm for P25, 20 mg and 40 mg of TiO₂ nanorods respectively, it reveal the fact that the valence band shifts toward the conduction band resulting into narrowband gap. Hence the energy required for the electrons to transit from the valence band to conduction band (optical excitation) decreases due to the red shift in the absorption [18]. The obtained results suggest that, after the formation of TiO₂ nanorods the optical response was expanded and hence improves in the photo catalytic properties of TiO₂ nanorods [19].

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The optical band gaps of samples calculated using the relation between the absorption coefficient (α) and incident photon energy (hν) by the following equation,

$$\begin{eqnarray*}&&\alpha ={\displaystyle \frac{A\left({\rm{h}}v-{{\rm{E}}}_{{\rm{g}}}\right)}{{\rm{h}}v}}^{{\rm{n}}}\end{eqnarray*}$$

The direct optical band gap was simply a transfer of the electron from the top of the valence band to the bottom of the conduction band without any change in momentum. The relation between the absorption coefficient (α) for a direct transition and the photon energy (hν) was given by Fahrenbruch and Bube

$$\begin{eqnarray*}&&\alpha h\vartheta =A{\left(h\vartheta -{E}_{g}\right)}^{1/2}\end{eqnarray*}$$

The indirect band gap is a transition of electron from the valence to conduction band which is associated with a photon of the right magnitude of crystal momentum and the bottom of the conduction band does not correspond to zero crystal momentum in indirect band gap materials and the absorption coefficient dependence on the photon energy for indirect transitions [20]. The direct optical band gap values are extracted from the linear portion of (αhѵ)² versus photon energy (hν) plots [21] as shown in figure 7. The optical band gaps were decreased from 5.3 eV, to 5.2 eV and 4.9 eV for P25, 20 mg and 40 mg TiO₂ nanorods respectively.

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The dielectric permitivity of P25, TiO₂ nanorods at 20 mg and 40 mg as a function of frequency at room temperature are studied and presented the dielectric constant (ε') in figure 8 and dielectric loss (ε') in figure 9. The dielectric constant (ε') of a material is related to the dipole polarizability, which arises from electric dipoles they can change the orientation of polarization subjected to the applied electric field. The ε' of P25, TiO₂ nanorods at 20 and 40 mg is frequency dependent in lower frequency because of the dipoles are response significantly with applied electric field and the contribution ofthe space charge effect towards polarization may tend to increaseat lower frequencies as a result the high dielectric constant. The ε'of P25, TiO₂ nanorods at 20 and 40 mg is frequency independent at higher frequency region due to the dipoles are unable to respond with applied electric field and thus they behaviours like tightly bounded at high frequencies is reason to maintain the constant and low dielectric constant [22]. The dielectric constant (ε') and dielectric loss (ε') were calculated using following relations,

$$\begin{eqnarray*}&&\varepsilon ^{\prime} ={{\rm{C}}}_{{\rm{p}}}{\rm{d}}/\left({\varepsilon }_{0}{\rm{A}}\right)\,{\rm{and}}\,\varepsilon ^{\prime\prime} =\varepsilon ^{\prime} -\,\tan \,\delta \end{eqnarray*}$$

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It is observed that the dielectric constant (ε') and dielectric loss (ε') of P25, TiO₂ nanorods at 20 and 40 mg are decreased suddenly with increase in the frequency which may be because the dipole polarization failed to change the direction of orientation with applied field or the polarisation of dipoles decrease whendipole rotation cannot follow electric field changes at highfrequencies that results in the decrease the dielectricconstant. The dielectric constant (ε') and dielectric loss (ε') increased at low frequencies due to the accumulation of charges between the sample and electrode as a result there is an increase in charge carrier density due to the increased dissociation of ion aggregation andhence the occurance of the relaxation phenomenon which results in the increase of dielectric parameters at low frequency and contribute to increase the ionic conductivity [23].

I-V characteristics of the samples were examined at room temperature by using two probes Keithley workstation instrument as shown in figure 10. I-V characteristics of the samples reveal the ohmic behaviour i.e., the current increases linearly with the increase in the applied voltage in the range −5V to +5V. The current of the TiO₂ nanorods is increase in the field of induced polarization of the applied bias voltage which is attributed to enhance electrical conductivity [24]. By improving the interface and electrode contacts it is possible to obtain the good electrical characteristics with high stability which helps in high quality device applications of the TiO₂ nanorods. Due to the formation of TiO₂ nanorods the recombination of charge will decrease and increases the charge transfer rate, due to the path provided by straighten nanorods to flow easily. As a result the increase in the electron density and hence increases the conductivity.