Int. J. Electrochem. Sci., 8 (2013) 336 - 346
International Journal of
ELECTROCHEMICAL
SCIENCE
www.electrochemsci.org
N-doped Mesoporous Titania as a Photoelectrochemical
Working Electrode for Dye-Sensitized Solar Cells
Shou-Heng Liu*, Jhe-Wei Syu
Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 80778, Taiwan
*E-mail: [email protected]
Received: 1 October 2012 / Accepted: 29 November 2012 / Published: 1 January 2013
Nanocrystalline titania (Non-Meso TiO2), mesoporous titania (Meso TiO2) and nitrogen-doped mesoporous titania (N-doped Meso TiO2) were synthesized via a simple evaporation induced self-assembly (EISA) process. The obtained materials were thoroughly characterized by various spectroscopic and analytical techniques, including small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-visible and X-ray photoelectron spectroscopies (XPS) analysis. Among all prepared photoanodes for dye-sensitized solar cells (DSSCs), the N-doped Meso TiO2 photoanodes showed the maximum conversion efficiency with an open circuit voltage (Voc) of 0.75 V, a short circuit density (Jsc) of 11.07 mA cm-2, a fill factor (FF) of 63%, and an efficiency of 5.3%. This may be attributed to the formation of O-Ti-N linkage in the N-doped Meso TiO2 electrode, retarding the recombination reaction at the interface of TiO2 photoelectrode and electrolyte as compared to the Non-Meso TiO2 and Meso TiO2 DSSCs.
Keywords: Mesoporous titania, N-doping, Dye-sensitized solar cells, Photoelectrochemistry.
1. INTRODUCTION
Dye-sensitized solar cells (DSSCs) have been considered as one of the next-generation power sources due to their advantages of low-cost and high-efficiency solar energy conversion [1-9]. The dye sensitizers are photoexcited and sequentially fast injection of electrons into the TiO2 conduction band is happened during operation. However, the injected electrons of conduction band may recombine with oxidized dye molecules or react with redox species in electrolyte, resulting in decreasing overall conversion efficiency. To unravel this problem, various methods have been investigated for retarding the charge recombination of DSSCs, such as the utilization of novel dyes [10,11], core-shell nanomaterials [12-14], and doping nonmetal [15-19]. Among the nonmetal doping systems, N-doping has been demonstrated to be a good candidate due to its unique properties of high thermal stability,
low carrier-recombination centers [20,21] and perfecting the oxygen deficiency and decreasing the back reaction mentioned above. Since the study by Asahi et al. in 2001 [22], many methods have been reported to prepare N-doping of TiO2, such as thermal oxidation, anodization, sputtering and ion implantation methods [23-25]. Nonetheless, these methods need to be performed under relatively severe conditions and hence hinder the large-scale implementation.
Mesoporous materials, which possess hierarchical and tunable pore architecture, have received enormous interest because of their potential applications in many fields such as catalysis, energy storage and conversion, and separation technology [26-34]. Among them, mesoporous TiO2 with high specific surface area can facilitate a high degree of photosensitive dye loading as well as an efficient light harvesting. Therefore, a variety of potential applications in photocatalytic and photovoltaic applications was widely found in the recent years [35-40]. Generally, most of the approaches for the preparation of mesoporous TiO2 use soft-templating and hard-templating routes [41,42]. Even though above-mentioned methods are well established for preparing mesoporous TiO2, the synthesis routes invoked in those materials were still restricted by the ineffectiveness in material cost and preparation time, which further hinder their industrial applications. Furthermore, the synthesis of TiO2 by integrating mesoporosity and N-doping via a simple route is very scarce.
In this paper, a facile procedure to fabricate nanocrystalline titania (denoted as Non-Meso TiO2), mesoporous titania (denoted as Meso TiO2) and doped mesoporous TiO2 (denoted as N-doped Meso TiO2) by using evaporation induced self-assembly (EISA) approach was reported. The nanomaterials so fabricated were found to possess N-doped and mesoporous structural TiO2 which had superior photovoltaic properties, rendering potential applications in solar-energy related areas, for example, as photoanodes for DSSCs.
2.EXPERIMENTAL
For preparation of titania without mesoporosity (denoted as Non-Meso TiO2), ca. 2 g pluronic F127 tri-block copolymer (EO106PO70EO106, MW = 12600, Sigma-Aldrich) was dissolved in a mixture solution containing 40 mL of absolute ethanol with 3.3 g (35 wt%) of hydrochloric acid by constantly stirring. Then, 1.3 g of citric acid was added in 4 mL water and then introduced into the above solution under vigorous stirring for 2 h at room temperature. About 5.7 g titanium (IV) isopropoxide (Acros) and x g of urea (x = 0) dissolved in y mL (y = 160) of absolute ethanol was added slowly to the above solution. The mixture solution was stirred at 343 K for 2 days to obtain the solid product by the evaporation induced self-assembly (EISA) technique. Finally, the sample was increased the temperature to 773 K with a heating rate of 1 K min-1 and eventually maintained at the same temperature for 5 h in the presence of air to remove template. Similar procedures were adopted for the syntheses of titania with mesoporosity (denoted as Meso TiO2), except that the absolute ethanol (y) was added with 40 mL during EISA process. For preparation of N-doped mesoporous titania (denoted as N-doped Meso TiO2), a similar procedure was also carried out except that x and y values mentioned above were 2.5 and 40, respectively.
The mesostructure of samples was investigated by small-angle X-ray scattering (SAXS) using a Nanostar U system (Bruker, AXS Gmbh). The crystalline properties of all samples were analyzed by large-angle X-ray diffraction (XRD) with a PANalytical (X’Pert PRO) instrument using Cu-Kα radiation (λ = 0.1541 nm). Nitrogen adsorption-desorption isotherms were measured at 77 K on a Micromeritics ASAP 2020 analyzer. The Brunauer-Emmett-Teller (BET) approach was used to evaluate specific surface area from nitrogen adsorption data in the relative pressure (P/P0) range from 0.05 to 0.2. Pore size distribution curves were analyzed by the BJH method from the adsorption branch. The total pore volume was estimated from the amount adsorbed at the P/P0 of 0.99. The high-resolution transmission electron microscopy (TEM) was done at room temperature using an electron microscope (JEOL TEM-3010) operating at an electron acceleration voltage of 200 kV. The electronic structure and N-doping amount of samples were determined by X-ray photoelectron spectra (XPS) which obtained on a spectrometer (Kratos Axis Ultra DLD) with a constant pass energy of 20 eV followed by irradiating a sample pellet (6mm in diameter) with amonochromatic Al-Ka (1486.6 eV) X-ray under ultra-high vacuum condition (10-10 Torr). Absorbance spectra of samples were recorded in the diffuse reflectance mode using JASCO V-670 UV-visible spectrometer equipped with an integrating sphere setup.
The TiO2 and modified TiO2 photoanodes were prepared by doctor blading the TiO2 paste on the glass FTO (fluorine-doped tin oxide). The adsorption of dye on the TiO2 surface was carried out by immersing the prepared photoanodes in the ethanol solution containing N719 dye for 16 h at room temperature. The dye-coated photoanode and a Pt-coated FTO glass cathode were assembled like a sandwich via a 60 µm spacer of hot-melt thermal foil (Solaronix SA). An electrolyte containing 0.1 M of LiI (Aldrich), 0.05 M of I2 (Riedel-de Haen), 0.6 M of DMPII (Solaronix SA), and 0.5 M of 4-tert-butylpyridine (Aldrich) in acetonitrile was then infiltrated between the two electrodes of the DSSC and sealed with AB epoxy. The photocurrent-voltage characteristics of DSSCs were measured with a Keithley model 2400 digital source meter under one-sun illumination (AM1.5, 100 mW cm-2).
3. RESULTS AND DISCUSSION
Figure 1 shows the small-angle powder X-ray scattering patterns of various samples. No feature was observed for Non-Meso TiO2 sample, indicating the lack of a long-range mesostructural ordering. However, one broad peak centering at 2 0.3o was observed for the Meso TiO2 sample, suggesting that TiO2 nanoparticles assembled with a well-ordered array of mesopores. However, a decrease in peak intensity of N-doped Meso TiO2 sample due to the reduced mesoporous ordering was found upon the addition of urea during EISA, implying that presence of urea in synthetic mixture perturb the self-assembly process. The mesoporous structures of various samples are also confirmed by N2 adsorption/desorption isotherms. Meso TiO2 and N-doped Meso TiO2 samples showed typical signatures for mesoporosity, namely type-IV isotherms with well-defined hysteresis loops (not shown). The textural properties of various samples including BET surface area, average pore diameter, and pore volume derived from N2 adsorption/desorption isotherms and pore size distributions are listed in Table 1. Meso TiO2 and N-doped Meso TiO2 samples were found to possess higher BET surface areas
photoanode exhibited a surpassing photovoltaic performance, which possessed mesoporous structures with N-doping could successfully retard the charge recombination between TiO2 photoelectrode and electrolyte interface and enhanced electron lifetime, which may be due to the fact that incorporation of nitrogen replaced the oxygen deficiency to form O-Ti-N in the titania crystal lattice. Thus, the N-doped Meso TiO2 materials so fabricated should render future practical and cost-effective applications in solar-energy related areas, for instance, as photoanodes for DSSCs.
ACKNOWLEDGEMENTS
The financial support of the Taiwan National Science Council (NSC 101-2628-E-151-003-MY3) is gratefully acknowledged.
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