Preparation and Characterization of Pure Rutile TiO 2 Nanoparticles for Photocatalytic Study and Thin Films for Dye-Sensitized Solar Cells

Pure rutile-phase TiO2 (r-TiO2) was synthesized by a simple one pot experiment under hydrothermal condition using titanium (IV) n-butoxide as a Ti-precursor and HCl as a peptizer. The TiO2 products were characterized by XRD, TEM, ESCA, and BET surface area measurement. The r-TiO2 were rodlike in shape with average size of ∼ 61 × 32 nm at hydrothermal temperature of 220◦C for 10 h. Hydrothermal treatment at longer reaction time increased the tendency of crystal growth and also decreased the BET surface area. The degradation of methylene blue was selected as a test reaction to confer the photocatalytic activity of as-obtained r-TiO2. The results showed a strong correlation between the structure evolution, particle size, and photocatalytic performance of r-TiO2. Furthermore, the r-TiO2-based solar cell was prepared for the photovoltaic characteristics study, and the best efficiency of ∼3.16% was obtained.


Introduction
Titanium dioxide (TiO 2 ), is one of the most popular and promising materials in the field of photocatalytic applications due to its strong oxidizing power, high photostability, and redox selectivity [1].When TiO 2 is irradiated by photons with an energy higher than or equal to its band gap (∼3.2 eV), through photon absorption, the electrons can be promoted to the conduction band, generating holes in the valence band.The photogenerated electrons and holes migrate to the TiO 2 surfaces where they can induce reduction and/or oxidation of adsorbed molecules.TiO 2 is also a commonly used semiconductor for photon-electron transfer processes.In the dye-sensitized solar cells (DSSCs) invented by Grätzel et al., nanosized TiO 2 particles were used for preparing working electrodes, and the cell performance was found to be improved significantly when compared to the flat layered photoelectrodes [2].The TiO 2 crystal exists in two major forms: rutile and anatase [3,4].Anatase is thermodynamically metastable and can be transformed irreversibly to rutile phase at high temperatures [3,5].Most of the chemistry researchers have paid greater attention to anatase TiO 2 than rutile TiO 2 (r-TiO 2 ) in both photocatalytic reactions and photoelectrochemical cell because anatase phase of TiO 2 had been considered to be more active than rutile.Several excellent properties of r-TiO 2 , such as chemical inertness, superior light scattering characteristics, and lower cost [3,6], however, make it a potentially important phase in photocatalytic and photovoltaic applications.Wang et al. reported the high photocatalytic activity of r-TiO 2 for decomposition of rhodamine-B in water under artificial solar light irradiation [7].Bacsa and Kiwi found that the presence of r-TiO 2 showed enhanced catalytic activity compared to pure anatase TiO 2 during the degradation of p-coumaric acid [8].Rutile phase has also been shown to be more active than anatase in photodecomposition of H 2 S [9], and photooxidation of H 2 O with Fe 3+ [10].Park  characteristics of rutile TiO 2 -based DSSCs are comparable to those of anatase TiO 2 -based solar cells [11,12].However, due to the insufficient TiO 2 film thickness which is less than 5 μm, the electron injection current and the photon to electron conversion efficiency are limited.It has been proposed that porous film electrodes composed of one-dimensional (1D) nanomaterials provide direct electron conducting channels to the electrodes, thus the solar cell efficiency can be enhanced [13].Previously, we had successfully prepared the 1D rod-shaped r-TiO 2 nanoparticles by sol-hydrothermal procedure [14].In continuation of the previous work, we are presenting here with more detailed investigation of the influence of hydrothermal conditions, including the acid concentration and hydrothermal duration on the crystal structure, particle size, particle morphology, and photocatalytic activity of r-TiO 2 nanoparticles.The photocatalytic activity of the derived r-TiO 2 photocatalysts was tested by methylene blue degradation reactions under UV illumination.We also prepared nanocrystalline r-TiO 2 films up to 21 μm in thickness for DSSC study.The effect of r-TiO 2 film thickness and morphology on the photoelectrochemical properties was examined.The overall efficiency of r-TiO 2 -based DSSC device is rationalized in terms of r-TiO 2 film thickness and amount of dye adsorbed into the electrodes to find a meaningful property-efficiency correlation.

Hydrothermal Synthesis of Rutile TiO 2 Nanorods.
Figure 1 shows the schematic diagram for preparing r-TiO 2 nanorods.This procedure is based on the previous solhydrothermal process for synthesis of TiO 2 nanoparticles [15] with some modification.Titanium (IV) n-butoxide (Ti(O-Bu) 4 , ACROS), as a Ti precursor, was added slowly to hydrogen chloride solution (HCl) under magnetic stirring until a clear sol was formed.Then, distilled water was added dropwise into the sol and continuously stirred for three days.The white mixture formed was then transferred to a Teflon-lined autoclave, 40% filled, and heated at 220

Preparation of Rutile TiO 2 Photoanodes and DSSC Performance Measurement.
A doctor-blading technique was used to prepare the r-TiO 2 films on an FTO-(F-doped tin oxide-) coated conductive glass (2 × 3.3 × 0.3 cm 3 , Solaronix, sheet resistance 8 Ωcm −2 ) as DSSC photoanodes.Two edges of the FTO substrate were covered with Scotch tapes.The r-TiO 2 sol obtained after hydrothermal treatment for 10 h was directly applied to one of the bare edges and flattened with a home-made doctor blade by shearing across the tape-covered edges.The resulted r-TiO 2 electrodes were dried at 100 • C for 15 min followed by subsequent calcination at 450 • C for 30 min in order to remove the organic residues from the final products and to complete the crystallization.Thickness of the film was controlled by multiple coating process in which the coated substrates were subjected repeatedly to doctorblade coating, drying, and calcination steps (Figure 1).The thickness of the r-TiO 2 films was measured by a Mahr Alpha-step profiler (Perthometer S2) and confirmed by the scanning electron microscopy (SEM, Hitachi S-2400) images of cross sections.The surface morphology and crystal phase of r-TiO 2 films were investigated by SEM and XRD, respectively.
The photocurrent versus voltage (I-V) curves were measured using a computerized digital multimeter (Keithley, 2400) under the AM1.5 irradiation (1 sun), provided by a class A Thermo Oriel Xenon lamp light source (300 W).The incident power density was 100 mW cm −2 using NRELcalibrated monocrystalline Si-Solar cell (PVM134 reference cell, PV Measurement Inc.) for calibration.The efficiencies were calculated by Forter software.

Results and Discussion
3.1.Characterization of Rutile TiO 2 Nanorods.The solhydrothermal reaction employed in the present work led to the formation of nanocrystalline TiO 2 .The crystal properties depend on the peptization and hydrothermal treatment, such as acid concentration and time period.The previous study showed that using different acids in a hydrothermal reaction resulted in the formation of different TiO 2 phase [17].The product was pure rutile from an HCl medium, however, if the concentration of HCl was reduced to 1.5 M, besides rutile, anatase phase was generated as a side product.This implied that the formation of anatase by using HCl as a pepitizer was more difficult than that of rutile phase.Based on this result, we selected an HCl concentration of 3 M for the preparation of r-TiO 2 in the study presented here.Figure 2 shows a series of XRD patterns for r-TiO 2 after hydrothermal treatment at 220 • C for various duration.For convenience, the r-TiO 2 samples are hereinafter abbreviated as Ht-A-B, where A and B represent the hydrothermal temperature ( • C) and heating duration (h), respectively.It can be seen that pure rutile TiO 2 could be successfully obtained with 3 M HCl for the studied hydrothermal period from 1 to 24 hrs.This result infers that the complete formation of rutile phase could be accomplished in the solution at relatively low temperatures under appropriate conditions.In a general sol-gel process for preparing TiO 2 , the primary formed structure phase observed at low temperature is anatase which transforms to thermodynamically most stable and more condense rutile phase only upon calcinating at temperature above 500 • C [1].Note that the rutile XRD peaks became sharper as hydrothermal treatment prolonged indicating the formation of larger r-TiO 2 .The crystal sizes of r-TiO 2 obtained by analyzing the half maximum (FWHM) of (110) peak at 2θ = 27.45degree using the Scherrer equation with wavelength of the radiation of 1.5405 Å are summarized in Table 1.
The above XRD results indicate formation of r-TiO 2 crystals that can be confirmed by the ESCA measurement.Figure 3 shows the typical ESCA survey spectra of r-TiO 2 samples.The peaks appearing on the left side showed the Ti2p doublet with bonding energies of 459.4 eV for Ti2p 3/2 and 464.9 eV for Ti2p 1/2 .The right side spectrum showed the O1s peak with binding energy of 530.4 eV.Those binding energies of Ti2p and O1s from r-TiO 2 samples are all in good agreement with those in standard spectrum of TiO 2 .Compared to the individual photoelectron peaks for O1s and Ti2p from nonbonded Ti and O elements, different binding energies due to the chemical shifts were found.The binding  energy of the core level electron depends critically on the species to which it is bonded.Charge transfer from Ti to O leaves Ti (and O) with partial positive (and negative) charges, leading to a shift in core level to higher (Ti2p 3/2 , 453.7 to 459.4 eV; Ti2p 1/2 , 461.2 to 464.9 eV) and lower (O1s, 531 to 530.4 eV) binding energies associated with increased (and decreased) Coulombic attraction between core electron and the nucleus of Ti (and O) [14].
The crystal growth in different stage of hydrothermal process was traced by TEM. Figure 4 shows the TEM micrographs of r-TiO 2 samples hydrothermally treated at 220 • C for (a) 1, (b) 5, (c) 10, (d) 15, (e) 20, and (f) 24 h.As shown in Figure 4, in the initial stage of newly formed TiO 2 sol, the shape of the TiO 2 was observed as elliptical (Figure 4(a)) with average size of 26.3 × 10.8 nm.Upon increasing the autoclaving time, the r-TiO 2 crystallites build up rod-like morphology progressively.Prolonging the hydrothermal treatment also increases the average particle dimension based on the weighted-average analysis.In other words, increasing autoclaving time promotes the tendency of crystal growth under the present experimental conditions as expected from XRD analysis.In addition, a broad particle size distribution (not shown) was observed in terms of width and length, indicating that the nucleation of r-TiO 2 is much slower than its growth.The range of particle size is from 26.3 × 10.8 to 76.9 × 36.5 nm in length × width as listed in Table 1.Nevertheless, the crystals of small size from XRD indicate the incomplete crystallization for hydrothermal treatment up to 24 h.
The size variation of r-TiO 2 can also be seen from BET surface area measurement.The results listed in Table 1 show the dependence of the surface area of r-TiO 2 on the hydrothermal reaction time.It can be seen that the specific surface area shifts towards smaller values for longer heat treatment.The TiO 2 sample with one hour of hydrothermal treatment at 220 • C possesses high specific surface area (162 m 2 /g) which then decreased appreciably with a limited value around 19 m 2 /g after 24 h of hydrothermal treatment.This result confirms the observation from TEM and indicates an increase in the particle size of r-TiO 2 in increasing the reaction time.Obviously, this is due to the progressive aggregation of small crystallites into larger particles.It is known that the TiO 2 crystals grow from TiO 6 octahedra that are terminated by the surface Ti-OH groups.During the hydrothermal treatment under acidic condition, the surface hydroxyl groups can be protonated to form Ti-OH + 2 which then combines readily with another Ti-OH to form Ti-O-Ti oxygen bridge by eliminating a water molecule (dehydration) through which the crystals grow to a larger size [4].It is noted that the growth of rutile TiO 2 proceeded via oriented coalescence of the first formed TiO (Figures 4(c)-4(f)) and the simultaneous increase of width and length of r-TiO 2 from TEM analysis.

Photocatalytic Activity of Rutile TiO 2 .
In order to study the photocatalytic activity of the above-prepared r-TiO 2 , the photodecomposition of methylene blue (MB) was investigated in aqueous heterogeneous suspensions under the acidic condition.We chose pH 3.85 to study the photodegradation of MB because it decomposed scarcely in the absence of TiO 2 upon irradiation up to 7 h [16].The maximal absorption of MB solutions is at 614 and 664 nm under our experimental conditions.The photodegradation was studied by monitoring the variation of intensity at 664 nm. Figure 5 plots the relative concentration [C]/[C 0 ] of MB against irradiation time of r-TiO 2 samples prepared by hydrothermal treatment at 220 • C for different time period.With r-TiO 2 , the MB showed significant decrease in the absorbance upon irradiation.It is noted that varying the hydrothermal-treated period changes the photocatalytic efficiency of r-TiO 2 .The photocatalytic activity of r-TiO 2 increases with an increase in hydrothermal time from 1 to 15 h, but it decreases for r-TiO 2 with further hydrothermal treatment (20 and 24 h).There are two major variables that can vary during the hydrothermal process: crystallinity and surface area.Increasing the hydrothermal time increases the crystalline domain of r-TiO 2 based on XRD analysis but decreases the specific surface area based on BET measurement (Table 1).Increase in crystallinity is a positive change in photocatalytic activity since amorphous titania is known to have very low photocatalytic efficiency [1,18].Decrease in surface area, on the other hand, is a negative change in photocatalytic activity due to the reduction of surface hydroxyl groups (−OH).The photocatalysis is basically a surface phenomenon that is being very sensitive to the amount of surface OH groups which may act as the principal reactive oxidant in the photoreactions of TiO 2 [19].To derive the kinetic information, the decay of absorption due to the photodecomposition of MB was tentatively assumed to follow the first-order kinetics: where k a is the apparent rate constant for MB decomposition and [C] is the concentration of MB.To determine the reaction rate constant, curves of the variation of MB concentration as a function of illumination time were fit into this model.The rate constants for photodecomposition of MB using various r-TiO 2 samples are also listed in Table 1.At longer hydrothermaltreated period (up to 15 h), the MB decomposition rate increases which is associated with the improvement of r-TiO 2 crystallinity.Further hydrothermal treatment for more than 20 h, the photocatalysis efficiency of r-TiO 2 is deteriorated, which is believed to be due to the decrease in surface area.3.3.Application of Rutile TiO 2 to DSSCs.DSSC is a quite complicated system, and there are many factors influencing the cell efficiency [20].One of the most important parameters is the TiO 2 electrode.The crystal phase, particle shape, diameter, and surface composition of TiO 2 samples used will affect the dye adsorption, electron transport, and electrolyte diffusion in the cell as well as the light-toelectricity conversion efficiency.In this work, we chose r-TiO 2 samples obtained after 10 h hydrothermal treatment to prepare the photoanodes for DSSC study because of higher surface area with good crystallinity.As shown in Table 1, the specific surface area shifts towards smaller values for further heat treatment.The film preparation procedure and condition also play significant effect in the resultant electrode property, in particular, the film morphology and porosity.The r-TiO 2 electrodes were prepared according to the procedures described in Section 2. FTO substrate.In order to see the heat treatment effect on r-TiO 2 morphology, the TEM image was acquired from calcinated r-TiO 2 paste.As shown in Figure 7, the r-TiO 2 powders kept the rod shape morphology even with 450 • C calcination process.Contrarily, the rod-like microstructure is somehow diminished on calcinated r-TiO 2 electrodes prepared from the same pastes as shown in Figure 8 from SEM.Only irregular and shorter-rod particles with larger particle size (100 nm) were observed.Although it is not clear why the rod shape can not be kept on r-TiO 2 film, there could be one possible reason as depicted in Figure 9.The surface energy effect is beneficial to the side-(length-) by-side (length) arrangement between rod-shaped r-TiO 2 particles.As mentioned earlier, the growth of rutile TiO 2 proceeded via oriented coalescence of the first formed TiO 2 nanorods.In 3 dimensions, the r-TiO 2 nanorods are freely moved and chances are the simultaneous growth of r-TiO 2 in both length and width directions.On r-TiO 2 film, however, one of the dimensions for r-TiO 2 nanorods to move is limited and only nearby r-TiO 2 nanorods aggregate to form larger but shorter r-TiO 2 particles.Figure 10 showed the typical current-voltage (J-V) characteristics of N3-sensitized r-TiO 2 solar cells measured at 1 sun light intensity for various r-TiO 2 film thickness.Table 2 lists the photoelectric data, including photocurrent density (J sc ), open-circuit voltage (V oc ), fill factor (FF), apparent cell efficiency (η), and the dye adsorption density (Dye ads ) of the DSSCs in Figure 10.It is generally expected that the DSSC performance largely depends on the TiO 2 film thickness because changing the film thickness changes the amount of dye adsorbed on TiO 2 owing to the change of total TiO 2 surface area.For the homogeneously harvested light by the adsorbed dye molecules, one would also expect the approximately linearly dependence of photocurrent density with the film thickness [21].It can be seen from Table 2 that when the film thickness increased by 2-fold, from 5.5 to 11.3 μm, the J sc only improved by 26%, from 5.99 to 7.55 mA/cm 2 .The increase of J sc , however, is consistent with that of Dye ads which increases ∼27%, from 0.22 to 0.28 μmole/cm 2 and that of cell efficiency by ∼26% from 2.51 to 3.16.These results indicate that the photocurrent as well as cell efficiency are limited by the number of adsorbed dye molecules for film thickness up to 11.3 μm.A small decrease (5%) of V oc with film thickness may be due to the offsetting effect associated with the J sc and the number of recombination centers on surface area of the film [12].Further increase of r-TiO 2 film thickness from 11.3 to 21.0 μm, although the amount of dye adsorption increases by 43%, from 0.28 to 0.40 μmole/cm 2 , the J sc decreases by 20% from 7.55 to 6.03 mA/cm 2 as well as cell efficiency by ∼15% from 3.16 to 2.68.This is due to the increase in the numbers of recombination centers and the longer mean path of the injected electron to travel inside the cell.Moreover, when the TiO 2 films are thicker, the films become more opaque.Due to the various light absorption mechanism, the irradiation intensity decays significantly upon travelling through the thick films which is therefore detrimental to the DSSC performance [21].

Conclusions
Pure rutile phase TiO 2 nanorods have successfully been synthesized under the hydrothermal conditions.Hydrothermaltreated duration shows significant effects on the crystal domain, particle dimension, and photocatalytic activity of r-TiO 2 .Hydrothermal treatment at longer reaction time increases the tendency of crystal growth based on TEM/XRD and the BET surface area decreased as well.The photocatalytic activity of r-TiO 2 increases with an increase of hydrothermal time from 1 to 15 h due to the increase of crystal domain, but it decreases for r-TiO 2 with further hydrothermal treatment (20 and 24 h) due to the decrease in surface area.Dye-sensitized solar cells with working area of 0.25 cm 2 were fabricated from various thicknesses of electrode layers made of r-TiO 2 nanorods.The bestperforming DSSC evaluated under 1 sun condition gave current density ∼7.55 mA/cm 2 , open circuit voltage ∼0.70 V, fill factor ∼60%, and energy conversion efficiency ∼3.16%.

Figure 2 :Figure 3 :
Figure 2: A series of XRD peaks for the prepared r-TiO 2 powders after hydrothermal treatment for various duration.

Figure 5 :
Figure 5: The variation of MB intensity at the λ max = 664 nm as a function of UV irradiation time in the absence (black solid square) and presence of r-TiO 2 samples prepared for various hydrothermal time.

Figure 6 :
Figure 6: The XRD spectra for r-TiO 2 electrode and r-TiO 2 powder after calcination at 450 • C.

Figure 8 :
Figure 8: A top-view SEM image of r-TiO 2 electrodes.

Figure 9 :Figure 10 :
Figure 9: Model of the crystal enlargement of r-TiO 2 upon calcination on substrate-free (a) and substrate-limited (b) conditions.

Table 1 :
The XRD crystal domain, TEM particle size, BET surface area of r-TiO 2 samples, and their photocatalytic activities for photodecomposition of methylene blue in aqueous solution.

Table 2 :
Performance characteristics of DSSCs based on the r-TiO 2 electrode with different thickness.