Enhanced Refractive Index Sensing Performance Using Hydrothermally Prepared Tricomposite Nanoower Structure of Ta 2 O 5 :Si:Graphite

We describe a tricomposite nanoower structure based method to measure the trace amount of refractive index in aqueous solutions. It utilizes tantalum (v) oxide, silicon and graphite to fabricate the tricomposite nanostructures. The tricomposite nanoower structures were prepared using hydrothermal method where the concentration of (x) of Si in Ta 2 O 5 was varied while the concentration of graphite was kept constant. The concentration of Si in Ta 2 O 5 was measured by Maxwell-Garnett model using volume lling factor 'f' (0 ≤ f ≤ 1) of Si in Ta 2 O 5. The fabricated Ta 2 O 5 :Si:Graphite tricomposite nanoower structures were characterized by SEM, XRD, UV-Vis, PL, FTIR characterization techniques. Then aqueous solution of varying refractive indices were prepared in the range 1.33-1.39 in the already prepared tricomposite nanoower structure solution. The refractive index measurement were probed by measuring absorption spectra corresponding to each tricomposite nanoower structures. The performance of the sensor was explored in terms of shift in peak absorption spectra, sensitivity and moreover the limit of detection. The sensor shows sensitivity and limit of detection of (156-260) nm/RIU and 5.14x10 -3 RIU respectively. A linear declining of sensitivity was observed within the refractive index range. The sensor possesses a distinguished feature of using tricomposite nanoower structure which is an ecient method for refractive index measurement. The systematic characterizations of all the fabricated samples of MWCNT: Si x TiO 2(1−x) were performed. The investigations were performed so as to obtain the structural/morphological evolution and the electronic and optical properties of MWCNT: Si x TiO 2(1−x) tricomposite nanostructures. The overall measurement of structure/morphology of MWCNT: Si x TiO 2(1−x) tricomposite nanostructures was performed using SEM and TEM. Finally the optical properties were examined using UV-Vis, FTIR and photoluminescence (PL) spectroscopic techniques. The composition of TiO 2 , Si and MWCNT was evaluated using X-ray diffraction, XPS spectroscopic techniques. Finally, photo response and capacitive response of the MWCNT: Si x TiO 2(1−x) tricomposite nanostructures was investigated using V-I characteristics and cyclic voltmetery respectively. of 'f'. This process was repeated for all the values of 'f'.


Introduction
Refractive index (R.I.) is a prominent optical property of any material. It possess a very important role in optics as by using refractive index variation, one can easily investigate the optical behavior of any materials. Since it is an important parameter, therefore for designing optical systems, measurement of refractive index should be an done by simple and accurate method. By measuring the refractive index, can also measure several other parameters. Hence, several chemicals and biochemical molecules can be identi ed/measured using refractive index measurements [1] Sometimes the purity of the chemical can also be checked by measurement of their refractive indices. If we consider, the measurement of any chemical then an estimation of various parameters such as its concentration, pH and detection limit can be measured by quantifying their refractive indices [2,3]. The obtained optical response corresponding to the analyte of varying refractive indices can be evaluated and thus the performance of a sensor can be analyzed. Further, in various drug industries, refractive index variations are widely used to calculate even trace amount of drug concentration. Moreover, water purity can also be checked by monitoring the refractive index of water which is being treated [4][5][6]. In addition to this, refractive index measurements proves to be very useful industrial processes. Since there is limited availability of refractive index function of several materials and there is strong requirement to measure the refractive index functions of newly indenti ed combinational nanomaterials therefore, it is very necessary to develop a highly sensitive accurate, simple and cheap refractive index sensor. The developed sensor should be such that it can signi cantly measure the extended range of refractive index values so as to satisfy the demand of various sensing applications. Reports have been found on various techniques, experimental procedures and instrumentation for refractive index measurements which are based on diverse physical and chemical principles. Several researchers have developed signi cant sensing structures for refractive index measurement. The most common refractive index sensors are those which are based optical ber technique. Although to enhance the performance of sensor evolution have been done on the existing ones. Some researchers have used geometric arrangements to enhance the sensitivity. To further improve the performance ber gratings were employed. Interferometry is yet another method to improve the sensing performances. Several researchers have used surface plasmon resonance (SPR) technique to measure the refractive indices [7][8][9][10]. Then they employed localized surface plasmon resonance method.
Various materials such as semiconductor oxide were also used to measure/improve the refractive index.
Microstructured bers and resonators have also been used [11] These sensors offer reasonably good sensing performances but also possesses several limitations like they employ complex experimental set up, expensive fabrication, variation in output due to uctuations in light source, temperature. Some method possesses poor mechanical strength, tedious/prolonged sample preparation. Some technique gives non-linear output response with a very limited refractive index sensing range etc. Due to these shortcomings of the existing sensors, the implementation of such sensor would be a great challenge and it provoke the further improvements in designing of such sensors for commercial\operation, highly sensitive/accurate and cheap. Hydrothermal method is an immensely simple, powerful, easy, cheap and method of fabrication. Using this method, the fabricated nanostructures are found to be pure exact.
Single nanoparticles as well as composite nanostructures can be easily fabricated. The type of materials which can be fabricated using hydrothermal method may semiconductors, ceramics and metal or metal oxides etc. Also, the obtained product of nanostructures which are fabricated using hydrothermal method are found to be free from contamination because this method does not require additional preservative or precursors during fabrication process. Therefore, hydrothermal method is categories as "green technique". The nal obtained nanostructures are generally found to be highly stable and pure and are nely dispersed in solution. The agglomeration (if any) can be removed using ultra sonication. In hydrothermal process, generally ower-like or petal-like nanstructures can be easily fabricated. The process involves ''Hydrothermal autoclave'' instrument which is based on the principle of ''pressure cooker'' where temperature and pressure is given to the dispersion solution of any materials. Before doing the hydrothermal autoclave, we have to rst prepare the dispersion solution of materials which we want to make nanostructures. The transparent dispersion can be prepared in deionised water by ultrasonication of the dispersion before and after the hydrothermal process. The operating parameters in the hydrothermal process are the value of temperature and pressure within the autoclave. These two parameters should appropriately chosen so as to fabricate ne nanostructures with desired morphology and tailored size distribution. Moreover, the hydrothermally prepared nanostructures can be used without any further heat treatment as the nanostructures are synthesized in states of high temperature and pressure. In addition to nanostructures, hydrothermal process can easily prepare alloyed of different kind of materials, bimetallic, nanocomposite or combinational nanostructures of materials with different mechanical optical and thermal properties.
Lately, nano ower structures have emerged as pioneering nanostructures which is widely used in scienti c and commercial applications. Functionally, nano ower structures integrate multiple individual petals like structures into a comnibed form which proves to be very powerful and bene cial nanostructures with enhanced physical and chemical properties which may be otherwise unavailable. Liquid phase hydrothermal method is widely employed to fabricate nano ower structures. In this process, we can prepare nano ower of single as well as multiple materials. The distribution of several petals of different materials in a single ower can be used for various applications simultaneously. Nano ower structures fabricated using sequential hydrothermal method are obtained to be highly pure with controllable dimensionality and size distribution. The obtained nano ower structure are able to furnish productive interface. [12]. The idea of fabricating nano ower which carries petals of different types materials (with different properties) can be employed to develop a device with tailored properties of all the materials. The tailored properties of each materials provide/enhance the overall performance of the device. The different material which we can choose may be a metals, metal oxides, carbon materials, or semiconductors. Thus, combinational nano ower structures if fabricated would be of huge importance with unparallel performances.
This work presents the fabrication of nano ower structures consisting of graphite surrounded by petals of silicon and tantalum (v) oxide (Ta 2 O 5 ) in liquid medium using hydrothermal process. Since graphite is naturally occurring form of crystalline carbon, it has emerged as highly promising conducting material. It is very soft in nature with very low speci c gravity. Graphite possess unusual physical and chemical properties which proves to be very useful for electronics, optical and sensing applications. Combinational compounds of graphite with other materials, such as metal or semiconductor oxides further increase their application. Published reports support the applicability of graphite as suitable materials for sensing applications. Since few decades, silicon is widely used in electronic and optical application because of its good semiconducting properties, thermal and chemical stability. Therefore, to further improve the performance of the Si based devices, many improvement have been done on modifying Si. One of the e cient ways to modify the properties of Si is to combine Si with oxides materials like Ta 2 O 5 because oxide materials possesses huge defects levels. Ta 2 O 5 is a high refractive index metal oxide which is extensively used for the fabrication and characterization of highly sensitive plasmonic ber-optic biosensors due to a rich enhancement of evanescent eld at sensor-analyte interface. This eld enhancement helps in achieving an appreciable shift in the peak absorption wavelength values which consequently improves the sensitivity of the sensor to a considerable extent. Further, the usage of Ta 2 O 5 in the form of surrounding shell builds up a temperature insensitive surface around the central core in the core-shell geometry which improves the stability of the nanostructures. The implementation of Ta 2 O 5 can be conveniently carried out in the form of bulk nanolayers or nanostructures of a variety of shapes to suit a particular sensing application. Moreover, Ta 2 O 5 exhibits excellent biocompatibility which furnishes a productive scaffold for functionalization of biomolecules, and thus, projects e cient prospects for biosensing applications. Due to these attributes, Ta 2 O 5 manifests superior opto-electronic properties, and therefore, it is extensively used to fabricate highly sensitive plasmonic ber-optic sensors. The Maxwell-Garnett model [13]. The effective dielectric function of Si (x) Ta 2 O 5(1−x) (0≤x≤1) consider the volume lling factor 'f', which is obtained using the following relation: In eq. (2) density of Si is 2.33 g/cm 3 and that of Ta

Experimental set up
The schematic diagram of experimental setup for the characterization of Ta 2 O 5 :Si:Graphite tricomposite nano ower structure for refractive index sensing is depicted in Fig. 2. The setup consists of the halogen lamp to produce the white light. Then to focus the white light, lens was xed so that the focused beam reaches to the monochromator. To launch all the light beam inside the glass cuvette, we chose the numerical aperture (NA) of the lens large. Then, the glass cuvette having the samples of varying refractive was xed to ensure the incidence of focused light beam to the solution. The glass cuvette was designed in such a way that it can be sealed while measurements. The output light beam from the cuvette reaches to the detector. The light beam coming from the detector may be weak so we aligned an ampli er to improve the output light intensity. The ampli er was then connected to the personal computer to record the absorption spectrum. The measurements were repeated for all the varying refractive index solution and for all the fabricated Ta 2 O 5 :Si:Graphite tricomposite nano ower structure. The glass cuvette was evacuated time to time so as to remove the residual and undesirable liquids from it. The samples of varying refractive index were prepared and their absorption spectra were recorded at room temperature and pressure. The peak absorption wavelengths corresponding to different refractive indices were determined from these absorption spectra.

Results And Discussion
First of all, for the morphological study of the fabricated Ta 2 O 5 :Si:Graphite tricomposite nano ower structure, we carried out scanning electron microscopy (SEM) at various magni cation i.e, for 5 µm, 0.5 µm, 200 nm, and 100 nm. The SEM images depicted in gure 3 (a-d) are Ta 2 O 5 :Si:Graphite tricomposite nano ower structure for f=0.5 with above mentioned magni cations. In Fig. 1 (a-d), it may be noticeable that for lower magni cation values i.e. from 5 µm, 0.5 µm, the bunches of nano ower were clearly visible which con rms the presence of ower-like structures. As the magni cation is increased i.e. for 100 nm shown in Fig. 3 (c), clear image of the petals of ower can be seen. In Fig. 3 (  and shown in gure 6 (a). It may be noticeable from the gure that for f=0, PL intensity is also lowest. As the value of 'f' increases from f=0.2 to f=0.4 PL intensity also increases but as the value of 'f' further slightly increased from 0.4 to 0.5, PL intensity starts decreasing. On further increasing the value of 'f' from 0.5 to 0.8, PL intensity becomes constant. From the trend of the PL intensity for f=0 to 0.8, it is noticeable that maximum PL intensity is observed when f=0.4. The pictorial representation given in Fig. 6 (b) shows the reason of the obtained trend of the PL intensity and the variation of defects levels for varying values of 'f' is also explained. The photoluminescence is de ned by the number of emitted photons due to excitations of light and the associated electrons which are coming back from conduction band to valence band during recombination. In Fig. 6 (b) it is de ned that when f=0, PLI is lowest because the number of defects level (metastable states) would also be low. But for higher values of 'f' PLI is more so there would be more defects level so electrons reaches to more number of metastable states and therefore it takes more time in returning to valence band from the conduction band consequently emits more number of photons before recombination. The highest PL intensity is obtained when f=0.4 therefore it may be inferred that maximum defects levels are found at f=0.4. Similarly as PL intensity decreases the defects levels get altered in the same fashion.
Now, experiments were performed for the Ta 2 O 5 :Si:Graphite tricomposite nanostructure for all the values of 'f' towards refractive index sensing applications. The absorption spectra of the prepared solution for f=0.4 in refractive index range of 1.33 to 1.38 is shown in Fig. 7 (a). The corresponding variation of peak absorption of Fig. 7 (a) is plotted in Fig. 7 (b). In accordance with the absorption spectra of Fig. 7(a), peak absorption wavelength is observed to increase with an increase in the refractive index of the solution i.e., a red-shift in the peak absorption wavelength is obtained which is clearly shown in Fig. 7 (b). This is termed as the calibration curve for the refractive index sensor which governs the non-linear trend of peak absorption wavelength with refractive index. It is worthwhile to mention that the amount of wavelength shift in the absorption spectrum is very sensitive to the surrounding refractive index. For medium with lower refractive index, more energy is required to collectively excite the surface electrons to generate LSPR signal. Thus, the peak absorption wavelength values con ne towards lower wavelength regime. As the refractive index of the medium is increased, comparatively smaller amounts of energy are needed to generate LSPR signal consequently, the peak absorption wavelength shifts towards higher wavelength values. Now, from the calibration curve, sensitivity of the refractive index sensor can be evaluated. Sensitivity is a measure of the shift attained in peak absorption wavelength with respect to the change in refractive index. Mathematically, sensitivity is calculated from the slope of the calibration curve. For each refractive index value, the value of sensitivity is measured and is plotted as a function of the refractive index in inset of Fig. 7 (b). As is evident from gure, the sensitivity curve pursue a declination trend with an increase in the refractive index. The maximum value sensitivity is determined to be ~(156-260) nm/RIU in (1.33-1.38) refractive index range. Since the mathematical equation narrating the calibration curve is a polynomial of second degree, its differentiation yields a linear equation, and thus, the sensitivity is noticed to follow a linear trend with refractive index.
Limit of detection (LOD) is yet another parameter to characterize a sensor. A further investigation has been made to measure LOD of the present refractive index sensor. For LOD, refractive index resolution should be measured. LOD is used to estimate the minimum possible value of refractive index that can be measured with the sensing layout.
Interpreted mathematically, refractive index resolution provides the minimum possible change in the refractive index wavelength attainable with the sensing framework. In this aspect, refractive index resolution of present sensor is evaluated. Resolution is evaluated from the below mentioned formula [15] (3) Here, Δλ S.D. denotes the standard deviation calculated in the measurement of peak absorption wavelengths for different refractive index solutions and S (RI: 1.33) represents the sensitivity attained at the minimum refractive index point (1.33). From Fig. 3(b), Δλ S.D. is estimated to be 0.4148 nm. Also, the sensitivity at refractive index value 1.33 is 155.56 nm/RIU. Using these values in equation (5) Fig. 8. It may be noticeable from gure that as the value of 'f ' increases from 0 to 0.4, shift in peak absorbance wavelength increase. But on further increasing the value of 'f' from 0.5 to 1, shift in peak absorbance wavelength starts decreasing and become minimum at f=1. This trend is found to be in reasonable agreement with PL intensity shown and discussed previously in Fig. 6 (a). The maximum peak in PL intensity is found to be at f=0.4. Therefore it may be justi ed that the amount of defects level is the reason for absorption of light and hence for the shift in peak absorbance wavelength. It is worthwhile to mention here peak absorbance is observed due to the interaction of photons with that electron cloud results in the generation LSPR signal. When there is difference in refractive indices of the surrounding medium then the shift in peak absorbance wavelength is observed. This phenomenon is depicted well in Fig. 9.

Conclusion
This work elucidates on the fabrication and characterization of Ta 2 O 5 :Si:Graphite tricomposite nano ower structure using hydrothermal method and depicted their utility in refractive index sensors. The absorption spectra obtained for Ta 2 O 5 :Si:Graphite tricomposite nano ower structure prepared in varying refractive index solutions were studied to probe their refractive index sensing characteristics. A shift equal in the peak absorption wavelength is observed for varying the refractive index of the surrounding medium from 1.33 to 1.38. The operational parameters of the sensor are volume lling factor of Si in Ta   Opto uidic plasmon-based sensor using immobilized ensembles of Au nanospheres on exposed core of microstructured ber         Schematic of reason for shift in absorbance for varying refractive index.