Textural Enhancement of Hydrothermally Grown TiO2 Nanoparticles and Bilayer-Nanorods

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Textural Enhancement of Hydrothermally Grown TiO2 Nanoparticles and Bilayer-Nanorods for Better Optical Transport 1

J. Sahaya Selva Mary1, V. Chandrakala1, Neena Bachan1, P. Naveen Kumar1, K. Pugazhendhi1, J. Merline Shyla1,a 1 – Department of Physics, Energy NanoTechnology Centre (ENTeC), Loyola Institute of Frontier Energy (LIFE), Loyola College, Chennai, India a – jmshyla@gmail.com DOI 10.2412/mmse.9.99.459 provided by Seo4U.link

Keywords: bilayer-nanorods, nanoparticles, hydrothermal, photoconductivity.

ABSTRACT. TiO2 nanostructures have been studied as photoanode materials via improvement of their textural and electronic properties for Dye Sensitized Solar Cells (DSSCs). They have exhibited appreciable photovoltaic performance owing to their excellent electron transport and high specific surface area. We report herein, the comparative analysis of TiO2 nanoparticles (TPs) and Bilayer-TiO2 nanorods (B-TRs) prepared by hydrothermal method at 200 ºC for 12 h and 120 ºC for 12 h respectively using an autoclave unit. The as-synthesized samples were characterized using High Resolution Scanning Electron Microscopy (HR-SEM), Energy Dispersive X-ray (EDAX), Fourier Transform Infrared (FT-IR) spectroscopy, Ultra Violet -Visible Spectroscopy (UV-Vis) and Photoconductivity techniques. The morphological results showed that the TPs are spherical in shape with diameter in the range of 18-29 nm and the B-TRs revealed the formation of hierarchical nanostructures on top of aligned nanorod trunks possessing porous nature and dimensions of ~ 262 nm diameter and ~ 660 nm length. FTIR spectra confirmed the presence of Ti-O-Ti vibrations in both the cases. The optical properties of TPs and B-TRs showed a strong absorption edge in the UV region.

Photoconductivity techniques revealed the ohmic nature of the samples with a linear increase in both dark and photocurrent with corresponding increase in the applied field. However, in B-TRs there is a significant increase in photocurrent than TPs which suggests a strong capability of absorbing light. Thus we can conclude that the bilayer nanostructure with better photoresponse, can be used as a promising photo anode material for DSSCs.

Introduction. In the past decade, extensive research has been done in the development of technology for efficient utilizing of renewable energy. Among them, photovoltaic is considered as the most promising technology due to its availability, sustainability and reliability [1-2]. Although photovoltaic devices built on silicon or compound semiconductors have achieved high efficiency for practical use, they still require major breakthrough to meet the long-term goal of very-low cost production [3]. Among the various semiconducting metal oxides, TiO2 has attracted considerable attention in the field of energy conversion and environmental protection [4] due to its cost effectiveness, non-toxic nature, accessibility, stability [5] and unique photoelectric conversion capability [6]. Functional properties of TiO2 are influenced by many factors such as crystallinity, particle size, surface area, and synthesis techniques [7]. TiO2 nanostructures have been studied as photoanode materials for DSSCs and they have exhibited appreciable photovoltaic performance owing to their excellent electron transport and high specific surface area [8]. Synthesis methods such as hydrothermal, solvothermal, sol-gel, direct oxidation, chemical vapour deposition (CVD), electro deposition, and microwave methods have been used for the synthesis of TiO2 nanostructures [9].

Among these, hydrothermal technique is the most important and promising fabrication method for nanoscale materials [10]. In this study, a comparative analysis of TiO2 nanostructures (TPs and BTRs), prepared by hydrothermal method using Teflon-lined stainless steel autoclave was investigated.

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© 2017 The Authors. Published by Magnolithe GmbH. This is an open access article under the CC BY-NC-ND license http://creativecommons.org/licenses/by-nc-nd/4.0/

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Experimental Procedure. Synthesis of TPs. The TiO2 nanoparticles were obtained by the hydrothermal reaction of Titanium (IV) butoxide, ethanol, acetic acid, and deionized water in autoclave at 200 °C for 12h. The obtained sample was dried at 100 °C for 2h and finally calcined at 550 °C for 1.30h.

Synthesis of B-TRs. Titanium (IV) butoxide (TBOT) was mixed with deionized water and hydrochloric acid. The mixture solution was stirred well for 30 mins at room temperature. Then the cleaned FTO substrates were placed into an autoclave filled with the solution and kept inside the furnace. The hydrothermal treatment was done at 120°C for 12h. The obtained product was washed thoroughly several times with deionized water. The resultant samples were calcined at 450 °C for 2h.

Characterization Techniques. The morphology and microstructure of the samples were examined by a High Resolution Scanning Electron Microscope (HITACHI S-4800) with Energy Dispersive X-ray spectrometry (EDX). Fourier transformed infrared (FTIR) spectra of the samples were recorded using a Perkin Elmer R×1 spectrometer ranging from 4000 to 400 cm-1, respectively. The optical absorption properties were measured in the range 200-600 nm using CARY 5E UV–Vis–NIR spectrophotometer.

The field dependent dark and photo conductivity tests were recorded by using a Kiethley Picoammeter 6485 and the constant voltage source.

Results and Discussion

Textural Properties. The surface morphology was analyzed using HR-SEM. Fig.1a show the HRSEM image of the as-synthesized TPs. The TPs exhibit good spherical morphology [11] with diameter ranging from 18-29 nm. HR-SEM image Fig. 1b which clearly shows the arrangement of vertically oriented dense B-TRs, with flower bunch at the top of nanorods surface [12]. Diameters of the nanorods are found to be ~262 nm and the length around 660 nm. It is observed that the nanorods were formed in a hierarchical order and are highly porous in nature. The morphology and porous nature of the TiO2 layer could possess high internal surface area for efficient dye adsorption which plays an important role in the improved photoelectric conversion efficiency of DSSCs [6]. Hence we could conclude that for enhancing electron transport in DSSCs it is important to supplant the straight nanorods or nanoparticles by bilayer nanorods which give an immediate conduction pathway for fast gathering of photogenerated electrons [13] thereby reducing charge recombination [14].

Fig. 1. HR-SEM image (a) TPs and (b) B-TRs. Compositional analysis. Energy Dispersive X-ray spectrometry (EDX) analysis of B-TRs and TPs shows the presence of Titanium and Oxygen elements as shown in Fig. 2 a)-b). Sn and F elements were originated from FTO substrate Fig. 2. (b) [15].

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Fig. 2. EDAX image of (a) TPs and (b) B-TRs. Spectroscopic Analysis. Fig. 3 a)-b) represent the Fourier Transform Infrared (FTIR) spectrum of B-TRs and TPs. The broad spectrum shows the asymmetrical and symmetrical stretching vibration of hydroxyl group (-OH) at 3365 cm-1 (3350–3450 cm-1) and 1625 cm-1 (1620–1630 cm-1) [16]. The band centred at around 450, 563, 645, 704, 761 cm-1 (450–800 cm-1) is characteristic of a Ti-O stretching and Ti–O–Ti distortion vibration [17].

Fig. 3. FTIR image of (a) TPs and (b) B-TRs. Optical analysis. The optical properties of TPs and B-TRs were studied by UV–Vis diffuse reflectance spectroscopy, which is shown in Fig. 4 a)-b).The absorption spectra of the TPs and B-TRs show an enhanced absorption in the UV region (353 nm and 370 nm). When compared to TPs a slight blue shift is observed in the case of B-TRs, which could be due to the hierarchical nanostructure [18].

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Fig. 4. UV–Vis diffuse absorption spectra image of (a) TPs and (b) B-TRs. The optical band gap of the sample was calculated by Kubelka-Munk function [F(R) hν]2 versus photon energy (hν) as shown in Fig. 5 a)-b) [28]. From the optical absorption edge, the band gap of TPs and B-TRs nanomaterials have been found to be 2.4 eV and 3.3 eV respectively suggesting an enhanced surface area in the latter.

Fig. 5. K-M plot image of (a) TPs and (b) B-TRs. Electro-optical analysis. The field-dependent dark and photoconductive behaviour of TPs and BTRs are depicted in Fig. 6 a)-b). The plots indicate a linear increase of current in the dark and visible light-illuminated TPs and B-TRs samples with increase in the applied field [19]. It is observed that the photocurrent (IP) is significantly greater than the dark current (ID) in B-TRs. This is due to the hierarchical structure of B-TRs, which has a strong capability of absorbing light in the near visible region [20]. Thus, we could conclude from the results that B-TRs, which revealed better photo response than TPs qualify as appropriate candidates for photovoltaic applications.

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Fig. 6. Electro-optical image of (a) TPs and (b) B-TRs. Summary. Semiconductor TiO2 with different nanostructures, such as bilayer nanorods and nanoparticles were synthesized via hydrothermal process. The morphological results showed that the TPs are spherical in shape and the B-TRs display bilayer nanorods formation with flower bunch on the top of aligned nanorod trunks. The as-synthesized B-TRs which were composed of longer nanorods could effectively increase electron recombination lifetime and direct electron transport rate than that of TPs. FTIR spectra confirmed the presence of Ti-O-Ti vibrations in both the cases. The optical properties of TPs and B-TRs showed a strong absorption in the UV region. The band gap calculated from KM-plot was found to be 3.3 eV for B-TRs and 2.4 eV for TPs. The field-dependent dark and photoconductivity behaviour of B-TRs and TPs are observed, that the photocurrent (IP) is found to be greater than the dark current (ID). But in B-TRs there is a significant increase in photocurrent than TPs suggesting strong light absorbing capability consequential to the increase in surface area. Thus we can conclude the bilayer nanostructure with better photo response is a promising photo anode material for DSSCs. Acknowledgement. The funding extended by the Loyola College - Times of India Research Grants (6LCTOI1421F002) towards this work is gratefully acknowledged.

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