Rice grain-shaped TiO2–CNT composite—A functional material with a novel morphology for dye-sensitized solar cells

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Abstract

Titanium dioxide-multiwalled carbon nanotube (denoted as TiO2–CNT) nanocomposites with a novel rice-grains nanostructure are synthesized by electrospinning and subsequent high temperature sintering. The rice grain-shaped TiO2 is single crystalline with a large surface area and the single crystallinity is retained in the TiO2–CNT composite as well. At very low CNT loadings (0.1–0.3 wt% of TiO2), the rice grain shape remains unchanged while at high CNT concentrations (8 wt%), the morphology distorts with CNTs sticking out of the rice-grain shape. The optimum concentration of CNTs in the TiO2 matrix for best performance in dye-sensitized solar cells (DSCs) is found to be 0.2 wt%, which shows a 32% enhancement in the energy conversion efficiency. The electrochemical impedance spectroscopy (EIS) and the incident photon-to-electron conversion efficiency (IPCE) measurements show that the charge transfer and collection are improved by the incorporation of CNTs into the rice grain-shaped TiO2 network. We believe that this facile one-pot method for the synthesis of the rice-grain shaped TiO2–CNT composites with high surface area and single crystallinity offers an attractive means for the mass-scale fabrication of the nanostructures for DSCs since electrospinning is a simple, cost-effective and scalable means for the commercial scale fabrication of one-dimensional nanostructures.

Highlights

► TiO2–CNT composites with the novel morphology of rice grains. ► The optimum concentration of CNTs in TiO2 matrix for best DSC performance is 0.2 wt%. ► The efficiency of DSCs is increased by ∼30% with the incorporation of 0.2 wt% CNTs.

Introduction

Since the first report by Grätzel in 1991 [1], dye-sensitized solar cells (DSCs) have gained considerable attention as the next generation solar cells because of their advantages such as simple non-vacuum fabrication, high efficiency to the tune of 11% [2], ease of large-scale production [2] and benefits comparable to that of amorphous silicon (Si) solar cells [3]. As the most extensively used semiconductor in the application of DSCs, many morphologies and fabrication steps for TiO2 have been studied in the past two decades for improving the device performance. Several new steps in DSC fabrication have been introduced such as incorporation of a compact hole-blocking layer of TiO2 on FTO (fluorine-doped tin oxide) [4], an additional scattering layer made of ∼400 nm TiO2 particles on top of the active TiO2 layer [5], TiCl4 treatment [6], and sensitization of TiO2 with a mixture of dyes having different spectral responses [7]. Doping of TiO2 with non-metals (e.g. nitrogen and silica) has also been explored [8], [9].

However, the transport of photo-injected electrons across the TiO2 network is the major limiting factor in attaining higher overall conversion efficiencies in DSCs. It is well known that the transport of photo-injected electrons across the TiO2 network occurs by diffusion and is strongly hindered by trapping and de-trapping events at grain boundaries and particle surfaces. The random path of photo-injected electrons in the TiO2 network increases the probability of their recombination with oxidized dye species or the tri-iodide electrolyte and hence a rapid transport of the electrons across the TiO2 network is desired for faster collection [10]. Therefore, one dimensional (1-D) TiO2 nanostructures have attracted the attention of scientists owing to their high surface area for dye absorption [11], enhancing the light harvesting efficiency by scattering more light at the red part of the solar spectrum [12], high intrinsic electron mobility [13], and semi-directed charge transport [14]. The use of 1-D nanostructures is supposed to shorten the electron transport pathways and enhance the accessibility of electrodes to the hole-transporting materials [15]. Several research efforts have happened/are ongoing in this direction, especially the design of one-dimensional oxide nanostructures such as nanofibers (random and aligned), nanowires, vertically oriented nanotubes and nanorods [16], [17], [18]. The transport and recombination studies on the oriented nanotubes showed an enhanced suppression of the recombination, while maintaining a relatively smooth charge transport [19].

Recently, incorporation of CNTs into TiO2 matrix has attracted the attention of scientists as a possible means to increase the efficiency of the DSC devices in view of the superior electronic properties and ease of surface functionalization of the former [20], [21]. It is demonstrated that incorporation of CNTs into the TiO2 matrix would enhance the conductivity of the TiO2 aggregates, which would facilitate faster electron transport across the TiO2 network thus minimizing charge recombination. TiO2–CNT composites have been used in DSCs by several groups [10], [20], [21], [22], [23], [24], [25], [26]. Sawatsuk et al. [24] achieved an efficiency enhancement of ∼60% by the incorporation of non-functionalized CNTs into the TiO2 particles by ultrasonication (however, no IV graph has been shown in the manuscript). Muduli et al. [23] observed that the efficiency of DSCs could be increased by 50% when a few mg of CNTs have been incorporated into the P-25 TiO2 particles by hydrothermal treatment. Enhancements to much lower levels have been reported in TiO2-coated CNTs (by sol–gel method) by Kamat et al. [10], [22], Lee et al. [20], Yen et al. [25], and Jang et al. [26], respectively. TiO2–CNT nanocomposites could be fabricated through methods such as blending [27], chemical vapor deposition [28], electrospinning [29], [30], [31], physical vapor deposition [32], sol–gel [33] and hydrothermal [23].

Recently, we have found a methodology to fabricate uniformly distributed, rice grain-shaped, single crystalline TiO2 nano/mesostructures of high surface area by electrospinning [34]. The rice grain-shaped TiO2 showed superior activity than the commercially available P-25 TiO2 in photovoltaics and photocatalysis [34], [35]. The methodology was extended further in fabricating rice grain-shaped TiO2–CNT nanocomposites with superior photocatalytic and Li-ion battery properties [36], [37]. In the present work, the electrospun TiO2-MWCNT (denoted hereafter as TiO2–CNT) nanocomposites with various wt% of CNTs were employed in the application of DSCs as photoanodes. The composite was structurally characterized by spectroscopy and microscopy. Photovoltaic characteristics were analyzed by current–voltage (IV) and incident photon-to-current conversion efficiency (IPCE) measurements. Charge transport through the TiO2–CNT network was analyzed by electrochemical impedance spectroscopy (EIS). Systematic studies with devices having various amounts of CNTs incorporated into the TiO2 network showed a 32% enhancement in the efficiency of DSCs when the CNT concentration was 0.2 wt% (in comparison to the amount of TiO2). IPCE and EIS revealed insights into the enhanced transfer of the photoexcited electrons across the TiO2 network and faster collection at the FTO. We believe that the simple fabrication of high performance TiO2–CNT nanocomposite with the interesting morphology of rice grains by electrospinning could be widely employed in DSC applications, photocatalysis, self-cleaning membranes, etc.

Section snippets

Materials

Titanium (IV) isopropoxide (TIP, 97%), polyvinyl acetate (PVAc, Mn = 500,000), N,N-dimethyl acetamide (DMAc, 99.8%), titanium(IV) chloride (TiCl4, 99%), chloroplatinic acid hexahydrate (H2PtCl4·6H2O), isopropyl alcohol, lithium iodide, iodine, 4-tert-butylpyridine, 1-propyl 2,3-dimethyl imidazolium iodide, acetonitrile, and tert-butanol were from Aldrich (Steinheim, Germany) and used as received. Multiwalled carbon nanotubes (CNTs, purity > 98%, outer diameter between 10 and 20 nm and length between

Morphological and structural characterization

Fig. 1A and C shows the smooth, continuous and randomly oriented as-spun fibers of TiO2–CNT (0.2 wt%)–PVAc and TiO2–PVAc composites, respectively, obtained by electrospinning. The average diameter of the as-spun fibers was ∼200 nm. Fig. 1B and D shows uniformly distributed rice grain-shaped nanocomposites of TiO2–CNTs and TiO2 obtained from the as-spun nanofibers by sintering at 450 °C for 3 h. Sintering results in near collapse of the continuous fiber morphology with the concomitant appearance of

Conclusions

The rice-grain shaped TiO2–CNT nanocomposites with high surface area and single crystallinity (for TiO2) were fabricated by electrospinning. The nanostructures were characterized by spectroscopy and microscopy. DSCs fabricated using the materials having systematically varied CNT concentrations in TiO2 matrix showed that the photovoltaic parameters increased with increases in CNT concentrations, reach a maximum and then decreased. It was found that the optimum concentration of CNTs in TiO2

Acknowledgments

Z.P, Y.S and N.K.E thank National University of Singapore for Ph.D fellowships. A.S.N and S.R thank National Research Foundation and M3TC (Economic Development Board), Singapore for financial support (Grant numbers: NRF 2007 EWT-CERP 01-0531, NRF-CRP4-2008-03 and R-261-501-018-414).

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