Controlled synthesis and photoelectrochemical properties of highly ordered TiO₂ nanorods

Abstract

Highly ordered TiO₂ nanorods (NRs) were grown directly on fluorine doped tin oxide (FTO) substrate by a single step hydrothermal synthesis at 150 °C. The morphology of TiO₂ was significantly altered by changing the solvents of ethanol/deionized (DI) water as the precursor solutions. With ethanol/DI water (80 : 20 v/v) as solvent, the highly ordered tetragonal TiO₂ NRs were achieved on FTO substrates with the average lengths of ∼2–4 μm and diameters ∼50–70 nm respectively. While, the round headed and distorted hexagonal TiO₂ NRs were formed with ethanol/DI water ratios of 50 : 50 v/v and 0 : 100 v/v as the precursor solutions respectively. The grown TiO₂ NRs possessed the anatase phase with typical tetragonal crystal structures. XPS studies evidenced that the grown TiO₂ NRs exhibited the O : Ti stoichiometric ratio of 2 : 1 with lower impurities of the carbon species. The grown TiO₂ NRs thin film substrates were applied as photoanodes for efficient dye-sensitized solar cells (DSSCs). A solar-to-electricity conversion efficiency of ∼3.2% was achieved by DSSC, fabricated with highly ordered tetragonal TiO₂ NRs photoanode, whereas DSSC with the distorted hexagonal TiO₂ NRs photoanode showed inferior overall conversion efficiency (∼1.08%). The improved photovoltaic performance was credited to the highly ordered morphology of the TiO₂ NRs, which executed the high charge collection and the transfer of electrons at the interfaces of the TiO₂ NRs photoanode and the electrolyte layer.

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Introduction

Due to versatile and the exotic properties, titania (TiO₂) nanomaterials are so far used in many technological applications as a photocatalyst, photovoltaic material, gas sensor, optical coating, structural ceramic, electrical circuit varistor, biocompatible material for bone implants, and spacer material for magnetic spin valve systems etc.^1–9 The dimensionality of TiO₂ at the nanoscale level is the crucial characteristic for determining the physiological and electrical properties. In recent years, one dimensional (1D) TiO₂ nanomaterials like nanorods (NRs), nanowires (NWs) and nanotubes (NTs) display significantly larger surface areas as compared to bulk materials, which deliver unique chemical and the physical properties¹⁰ and ¹¹ contribute towards the electrical and photoelectrochemical applications.^12,13 The 1D TiO₂ such as NRs and NTs have shown the reduced recombination rate for the excited electron-hole pair and display unique optical and the electric properties.^14,15 Particularly, the vertically grown TiO₂ NRs allow shorter and the uninterrupted electrical pathways for the photogenerated carriers and improves the charge separation and charge transport properties in many photoelectrochemical devices like dye sensitized solar cells (DSSCs).¹⁶ As compared to TiO₂ nanoparticles,^17,18 it is expected that the highly oriented TiO₂ NRs could be the potential electrode and photocatalyst material for several photoelectrochemical applications.

So far, several techniques are available for synthesizing the oriented or disoriented 1D TiO₂ nanostructures on nontransparent or nonconductive substrates.^19–21 The hydrothermal method is an effective, easily controllable and low cost technique for the growth of highly oriented TiO₂ NRs on various substrates.²² Until now, little work has been reported for the growth of crystalline TiO₂ NRs on the transparent conducting oxide (TCO) by the hydrothermal method.²³ The randomly assembled and the connected dense TiO₂ NR arrays were grown on the TCO substrates by the hydrothermal method.²⁴ Recently, the direct growth of rutile TiO₂ NR arrays on FTO substrates was investigated by Liu et al.,²⁵ which demonstrated the photovoltaic properties of DSSCs. In this paper, the highly ordered TiO₂ NRs are directly grown on the FTO substrate by a single step hydrothermal synthesis with ethanol/deionized (DI) water and DI water in the precursor solutions at 150 °C. The highly ordered tetragonal TiO₂ NRs were obtained on FTO substrates with ethanol/DI water (80 : 20 v/v) as solvent, however, the round headed and distorted hexagonal TiO₂ NRs were formed with ethanol/DI water ratios of 50 : 50 v/v and 0 : 100 v/v respectively. An improved solar-to-electric conversion efficiency of ∼3.2% is achieved by the highly ordered tetragonal TiO₂ NRs under 100 mW cm⁻² (1.5 AM).

Materials and methods

a) Synthesis of TiO₂ NRs on FTO substrate

For highly ordered tetragonal TiO₂ NRs, the precursor solution was obtained by dissolving 50 ml of concentrated hydrochloric acid (HCl, Daejung Chemicals, 35%) in 50 ml of a mixed solution of absolute ethanol/DI water (80 : 20 v/v), and the mixture was stirred at ambient conditions for 30 min. Here, the HCl was added to optimize the control of oxide precipitation and the solution stability. Thereafter, 1.5 ml titanium butoxide (Ti(OB)₄, Aldrich-sigma, 98% assay) was added drop by drop into the precursor solution under constant stirring for 1 h. The pH of the reaction mixture was in the range of ∼1.5–2. The FTO (FTO glass, Hartford Glass Co., 8Ω sq⁻¹, 80% transmittance in visible spectrum) glass substrates were degreased and cleaned sequentially by sonicating in acetone, ethanol and DI for about 30 min and were finally dried in an air. The whole reaction mixture solution was transferred into a Teflon beaker and the pieces of cleaned FTO substrates were placed in the Teflon beaker with the conducting side facing downward. The Teflon beaker was sealed into a stainless steel autoclave and maintained at 150 °C for 10 h. After completion of the reaction, the autoclave was cooled under flowing water for about 15 min. The substrates were taken out and rinsed with DI water and methanol several times and allowed to dry in ambient air. On the other hand, the round headed and distorted hexagonal TiO₂ NRs were synthesized in a similar way, using 50 ml of a solution of ethanol/DI water (50 : 50, v/v) and (0 : 100, v/v) with 50 ml concentrated HCl and 1.5 ml Ti(OB)₄ respectively.

b) Fabrication of aligned TiO₂ NR based DSSCs

The fabrication of DSSC was performed accordingly as reported elsewhere.²⁶ In brief, the prepared TiO₂ NR thin film FTO substrates were immersed in an ethanolic solution of ruthenium(II) 535 bis-TBA dye (0.3 mM, N-719, Solaronix) for 20 h at room temperature under dark conditions. The dye adsorbed TiO₂ NR thin film electrodes were rinsed with absolute ethanol and dried under a nitrogen stream at 40 °C. The platinum (Pt) deposited FTO substrates were used as a counter electrode and was placed over the dye immobilized TiO₂ NR thin film electrode. The edges of the cell were sealed with a 60 μm thick Surlyn sheet (SX 1170-60, Solaronix) by hot-pressing the two electrodes at 80 °C. An electrolyte of specified composition (0.5 M LiI, 0.05 mM I₂, and 0.2 M tert-butyl pyridine in acetonitrile) was introduced through the holes of the counter electrode on the dye immobilized TiO₂ NR thin film photoanode and the holes were sealed with a piece of small microscopic glass and a Surlyn sheet.

c) Characterization

The TiO₂ NRs thin films were characterized by the field emission scanning electron microscopy (FESEM, Hitachi S-4700) and transmission electron microscopy (TEM, JEM-2010-JEOL, Japan). Crystalline structures of the TiO₂ NRs thin film were studied by X-ray powder diffraction (XRD, Rigaku, Cu-Kα, λ = 1.54178 A°) in the Bragg angle ranging between 20°. The Raman spectrum (Raman microscope, Renishaw) and the UV-visible absorbance (2550 Shimadzu, Japan) were used to characterize the optical properties and the absorption spectra. X-rays Photoelectron Spectroscopy (XPS) was performed by using AXIS-NOVA CJ109, Kratos Inc., ranges 0–800 eV to investigate the bonding of TiO₂. A current density (J)-voltage (V) curve was measured using a computerized digital multimeter (model 2000, Keithley) with a variable load. A 1000 W metal halide lamp was used as a light source and its light intensity was adjusted to simulate 1.5 AM at 100 mW cm⁻² with a Si photo detector fitted with a Ka-5 filter as a reference, calibrated at NREL, USA. A black tape mask was placed on top of the cell during the J-V measurements. The operating temperature in the range of 20–30 °C was maintained by the small cooling fan. The incident photon-to-current conversion efficiency (IPCE) was carried out by a specially designed IPCE system for DSSC by PV measurements, Inc., USA. Before performing the IPCE measurement, the system was calibrated with a silicon photodiode, which was calibrated using the NIST-calibrated photodiode G425 as a standard. IPCE results of DSSCs were collected as a function of wavelength from 400 to 800 nm using a 75 W Xe lamp as a light source for generating monochromatic beam at a low chopping frequency.

Results and discussion

Fig. 1 shows the typical surface FESEM images of the TiO₂ NR thin films deposited on FTO substrates by the hydrothermal process with variations of the ethanol/DI water precursor solution. The distorted hexagonal TiO₂ NRs (Fig. 1(a, b)) of average diameter ∼100–200 nm and length of ∼3.0 μm are obtained with an ethanol/DI water ratio of 0 : 100 v/v as the precursor solution whereas, Fig. 1(c, d) exhibits the round headed TiO₂ NRs with ethanol/DI water (50 : 50 v/v) as the precursor solution. Sequentially, the highly ordered tetragonal TiO₂ NRs have grown on the FTO substrate with the precursor solution of ethanol/DI water (80 : 20 v/v). The grown TiO₂ NRs possess the average lengths of ∼2–4 μm and diameters of ∼50–70 nm respectively, as shown in Fig. 1(e, f). The high amount of ethanol in the precursor solution is crucial to achieve the highly ordered nanorods. Thus, it could be seen that the morphology of the TiO₂ NRs changes with the solvents such as ethanol/DI water and DI water as the precursor solutions.

Fig. 1 Low and high resolution FESEM images of the TiO₂ NR thin films obtained with the precursor solutions of ethanol/DI water with ratios of (a, b) 0 : 100 v/v, (c, d) 50 : 50 v/v and (e, f) 80 : 20 v/v.

Fig. 2 shows the transmission electron microscopy (TEM), high resolution (HR) TEM and the selected area electron patterns (SAED) of the grown TiO₂ NR coated FTO substrate. Similar to theFESEM results, the highly ordered tetrgonal TiO₂ NRs from the precursor solution of ethanol/DI water (80 : 20 v/v) solvent comprises the average length of ∼2–4 μm and the diameter of ∼50–70 nm, as shown in Fig. 2 (a). Each NR is made of a bundle of the densely packed nanofibers (NFs) with an average fibril's diameter of ∼5 nm. The corresponding SAED pattern (Fig. 2(b)) displays the clear phases, suggesting the high crystal quality with the single crystalline fibrils derived from TiO₂ NRs along the [001] direction. However, the HRTEM image (Fig. 2(c)) shows the well-resolved lattice fringes of the grown TiO₂ NRs and estimates an average interplanar distance of ∼0.35 nm between the two fringes, which reveals the typical interplanar distance of anatase TiO₂.²⁷ On the other side, the width and length of distorted hexagonal TiO₂ NRs are respectively observed as ∼200 nm and ∼3.2 μm, as seen in Fig. 2 (d).

Fig. 2 TEM image of (a) highly ordered tetragonal TiO₂ NRs, (b) SAED patterns, (c) HRTEM image and (d) TEM image of grown hexagonal distorted TiO₂ NRs.

Fig. 3 displays the XRD patterns of highly ordered tetragonal and distorted hexagonal TiO₂ NRs grown on the FTO substrate. The grown TiO₂ NRs from both precursor solutions exhibit the anatase phase with the peaks at 25.1°, 37.9°, 48.1°, 53.8° and 55.1°, which correspond to typical anatase TiO₂ materials and indexes at JCPDS no. 89-4203. However, the diffraction peaks of the FTO substrate are also observed at 33.8°, 35.7° and 52.8° (JCPDS no. 88-0287). On comparison with the distorted hexagonal TiO₂ NRs, the intensities of XRD diffraction peaks have minutely changed, which might indicate the high crystalline nature of the highly ordered tetragonal TiO₂ NRs.

Fig. 3 XRD patterns of (a) the distorted hexagonal TiO₂ NRs and (b) the highly ordered tetragonal TiO₂ NR thin film.

To understand the absorption and optical properties, UV-Vis spectroscopy of the TiO₂ NR thin film substrates are examined. As shown in Fig. 4, both the grown TiO₂ NRs exhibit a broad absorption in the ultraviolet region (<400 nm), corresponding to the typical anatase TiO₂ absorption edge in the UV region.²⁸ Furthermore, the band gap energies are calculated as ∼3.26 eV and ∼3.21 eV for the highly ordered tetragonal and distorted hexagonal TiO₂ NRs respectively, which are very close to the anatase TiO₂ in bulk. The low band gap energy and the absorption intensity of the distorted hexagonal TiO₂ might display the low optical properties of TiO₂ materials.

Fig. 4 UV-Vis spectroscopy of (a) the distorted hexagonal TiO₂ NRs and (b) the highly ordered tetragonal TiO₂ NR thin film.

Fig. 5 shows the Raman scattering spectra of the grown highly ordered tetragonal and distorted hexagonal TiO₂ NRs. The grown TiO₂ NRs display the Raman band in the range of 200–800 cm⁻¹, representing the characteristic peaks of O–Ti–O in TiO₂ nanomaterials.²⁹ In both grown TiO₂ NRs, the three Raman bands are observed at ∼396, ∼514.4 and ∼638.1 cm⁻¹ in the spectrum, which corresponds to the phonon modes of the anatase phase.^30,31 The Raman band at ∼ 638.1 cm⁻¹ has been assigned to the symmetric Ti–O vibration from the A_g symmetric modes of the TiO₆ octahedra,³² whereas those at ∼396 and ∼514.4 cm⁻¹ refer to the splitting of the degenerate mode of the TiO₆ octahedra in the TiO₂ nanostructures.³³ Importantly, the appearance of the strong and broad band peak at ∼396 cm⁻¹ is the main peak, which indicates the formation of the TiO₂ nanostructures.³⁴ Compared to the distorted hexagonal TiO₂ NRs, the high intensity Raman bands of highly ordered tetragonal TiO₂ NRs might ascribe to its better optical and crystalline properties.

Fig. 5 Raman scattering spectroscopy of (a) the distorted hexagonal TiO₂ NRs and (b) the highly ordered tetragonal TiO₂ NR thin film.

X-Ray photoelectron spectroscopy (XPS) has been carried out to investigate the chemical state of the elements in the grown TiO₂ NRs (Fig. 6). As shown in Fig. 6(a), Ti 2p spectrum is the doublet binding energy peaks at ∼459.2 eV and ∼464.9 eV, corresponding to Ti 2p_3/2 and Ti 2p_1/2 respectively.^35,36 The observed binding energy of Ti 2p accomplishes the +4 oxidation state of Ti, i.e., the Ti⁺⁴ state. The peak separation of ∼5.7 eV is noticed between the Ti 2p_1/2 and Ti 2p _3/2 binding energies, which is in excellent agreement with the reported pure TiO₂ values.³⁷ The deconvoluted O 1s XPS spectra (Fig. 6(b)) displays four fitted peaks at ∼530.4, ∼531.3, ∼532.1, and ∼532.6 eV. The main central binding at ∼530.4 eV represents the oxygen attached to the Ti atom in the manner of Ti–O–Ti (lattice O), however, the other three O 1s peaks appear at ∼531.3 eV, 532.1 eV and 532.6 eV, ascribed to C=O, the hydroxyl group attached with Ti, and C–OH (and C–O–C) species respectively.^38,39 Similarly, the C 1s XPS (Fig. 6(c)) shows the two binding energies at ∼285.4 eV and 289.1 eV, assigning the C–C and C–O species, which usually occur by the presence of impurities from the metal alkoxide precursor during the hydrothermal process.^40,41 Thus, from Ti 2p and O 1s results, the grown TiO₂ NRs might form in the nearly stoichiometric ratios of Ti=O (2 : 1) and contain much less impurities of the C species.

Fig. 6 Ti 2p (a), O 1s (b) and C 1s (c) XPS spectra of the grown highly ordered tetragonal TiO₂ NRs.

The current density–voltage (J–V) characteristics (Fig. 7(a)) have been performed to elucidate the performance of the DSSCs fabricated with the photoanodes of grown TiO₂ NRs and were measured under a light intensity of 100 mW cm⁻² (1.5 AM). DSSCs fabricated with the photoanode of distorted hexagonal TiO₂ NRs executes a relatively low solar efficiency of ∼1.08%, with a low short circuit current (J_SC) of ∼4.48 mA cm⁻², open circuit voltage (V_OC) ∼0.571 V and fill factor (FF) of ∼0.42. However, DSSCs fabricated with the photoanode of highly ordered tetragonal TiO₂ NRs shows an appreciably improved overall conversion efficiency of ∼3.2% with a high J_SC of ∼8.7 mA cm⁻², V_OC of ∼0.67 V, and FF of ∼0.54. Noticeably, the photovoltaic performance is significantly enhanced by about three times as compared to the photoanode of distorted hexagonal TiO₂ NRs based DSSC. Moreover, the DSSC with the photoanode of highly ordered tetragonal TiO₂ NRs shows increased J_SC, V_OC and FF by ∼48%, ∼15% and ∼22% respectively as compared with the photoanode of distorted hexagonal TiO₂ NRs. It is seen that the size of the NRs also plays an important role for achieving the high photocurrent density and performance of the device. It is known that the high photovoltaic performance and photocurrent density are related to high light harvesting through the highly uniform and high surface to volume ratio of the photoanode materials.⁴² Moreover, in general, the TiO₂ thin film electrodes with larger particles have the smaller surface area and produce moderate contact points between nanoparticles at the interface of the sintered nanoparticles and the conducting substrate, leading to the lower availability of the active surface for dye adsorption, which perhaps decreases the amount of light absorbed and generates the large number of electrons and holes.⁴³ Whereas, the TiO₂ thin film with smaller particles acquires the larger surface area and higher number of contact points of the sintered colloidal particles present at the interface of the nanoparticles and the conducting substrate, which gives rise to larger dye adsorption and higher light harvesting efficiency.⁴³ In this case, the distorted hexagonal TiO₂ NRs consist of larger diameters and lengths as compared to the highly ordered TiO₂ NRs, as shown in the FESEM images. It is believed that the smaller diameters of the NRs might generate the high light harvesting efficiency, which might lead to the high photocurrent density and the conversion efficiency. It is found that the photoanode of highly ordered tetragonal TiO₂ NRs attains the higher dye loading than the photoanode of the distorted hexagonal TiO₂ NRs, as shown in the UV-Vis spectra (Fig. 7(b)) of the dye desorption from dye absorbed TiO₂ NRs photoanodes in NaOH solution. Herein, the enhanced photovoltaic performance and J_SC are related to the highly ordered NRs morphology, high dye loading and improved light harvesting efficiency through the high surface area of the film. Besides these, the unique ordered morphology of the NRs might retard the recombination rate and contribute to longer electron lifetimes,⁴⁴ resulting in the increased V_OC and FF of device. While, the low efficiency of DSSC with distorted hexagonal TiO₂ NRs might due to a nonuniform surface and the existence of large voids between the two NRs, which might increase the recombination rate between the electrolyte layer and the FTO substrate.

Fig. 7 (A) J-V curve of the DSSC fabricated with (a) distorted hexagonal TiO₂ NRs and (b) highly ordered tetragonal TiO₂ NRs. (B) UV-Vis spectroscopy of desorbed dye from (a) distorted hexagonal TiO₂ NRs and (b) highly ordered tetragonal TiO₂ NRs.

Fig. 8 illustrates the IPCE of DSSCs fabricated with the photoanodes of highly ordered tetragonal TiO₂ NRs and distorted hexagonal TiO₂ NRs. The broad IPCE curves are obtained by both DSSCs in the absorption edge of visible spectrum from 400–800 nm. The photoanode of highly ordered tetragonal TiO₂ NRs based DSSC exhibits the maximum IPCE of ∼31.5% at the highest absorption edge of 528 nm, whereas ∼17.9% IPCE at 528 nm is achieved by the photoanode of distorted hexagonal TiO₂ NRs based DSSC. It is noticed that DSSC with photoanode of highly ordered tetragonal TiO₂ NRs has considerably improved IPCE by approximately two times as compared to DSSC with the photoanode of distorted hexagonal TiO₂ NRs. In general, the high dye loading of the working electrode results in the high light harvesting efficiency which leads to the electron injection from dye to CB of TiO₂. Thus, the enhanced dye loading, light harvesting and IPCE results in increased J_SC, V_OC and the photovoltaic performance for DSSC with the photoanode of highly ordered tetragonal TiO₂ NRs.

Fig. 8 IPCE curve of the DSSC fabricated with (a) the distorted hexagonal TiO₂ NR photoanode and (b) the highly ordered tetragonal TiO₂ NR photoanode.

Conclusions

TiO₂ NRs are grown directly on FTO substrates by a single step hydrothermal synthesis at 150 °C with a ethanol/DI water mixture as the precursor solution. The highly ordered tetragonal TiO₂ NRs have been obtained on FTO substrates with ethanol/DI water (80 : 20 v/v) as solvent. Whereas, the round headed and distorted hexagonal TiO₂ NRs are synthesized with ethanol/DI water ratios of 50 : 50 v/v and 0 : 100 v/v as the precursor solutions respectively. The highly ordered tetragonal TiO₂ NRs possess the anatase phase and tetragonal crystal structure with improved optical properties. XPS studies provide evidence that the grown TiO₂ NRs exhibit O=Ti stoichiometric ratios of 2 : 1 with much less impurities of the carbon species. A DSSC fabricated with the highly ordered tetragonal TiO₂ NR photoanode achieves a solar-to-electricity conversion efficiency of ∼3.2%, whereas the inferior overall conversion efficiency (∼1.08%) is observed for the DSSC fabricated with the distorted hexagonal TiO₂ NR photoanode. The improved photovoltaic performances are credited to the highly ordered morphology, which executes the high charge collection and the transfer of electrons at the interface of the ordered tetragonal TiO₂ NR photoanode and the electrolyte layer.

Acknowledgements

Dr. Sadia Ameen gratefully acknowledges the post doctoral fellowship granted by National Research Foundation (NRF 2011) for foreign researchers. This work is fully supported by NRF Project # 2011-0029527. This work was also supported by Research Funds of Chonbuk National University in 2009. We would like to thank Mr. Kang Jong-Gyun, Center for University-Wide Research Facilities, Chonbuk National University for his cooperation in TEM images. We also acknowledge the Korea Basic Science Institute, Jeonju branch, for utilizing their FESEM facility.

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Footnote

† Authors contributed equally to this work

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