Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Effect of solvents on the extraction of natural pigments and adsorption onto TiO2 for dye-sensitized solar cell applications
Graphical abstract
Introduction
Among possible sources of clean energy in the future, the dye-sensitized solar cell (DSSC) is one of those with the highest potential [1]. It is a device that directly converts visible light into electrical energy, based on the sensitization of wide band gap semiconductors [2]. DSSC consists of a transparent conducting glass electrode with a porous layer of wide band gap semiconductor, such as TiO2 coated with a dye that serves as light sensitizer, an electrolyte layer, and a counter electrode, typically coated with graphite or platinum [3], [4]. The light sensitizer converts photons into excited electrons, which facilitate current flow. To date, sensitizers available in the market use expensive artificial dyes, such as ruthenium complexes.
The use of organic solar cells could alleviate the stated drawbacks through the use of natural pigments as sensitizing dyes for the conversion of solar energy into electrical energy [5]. These pigments are extracted from fruit, flower, leaves, seeds, and roots. Environmentally friendly natural organic dyes, which can be applied without further purification, are considered as an attractive alternative due to other significant benefits, such as simple preparation technique, low cost, easy availability, and low toxicity [6], [7]. Thus, natural pigments have been widely investigated. To date, a growing number of natural pigments are used as sensitizers in DSSCs including betalain [8], chlorophyll [9], carotenoid [10], anthocyanin [11], flavonoid [12], [13], cyanine [14], and tannin [15]. The performance of DSSC mainly depends on the dye used as sensitizer. Two important factors determine the efficiency of solar cell, namely, (i) the anchorage of the dye to the surface of semiconductor, and (ii) the absorption spectrum of the dye [16]. The structure of the pigment used as sensitizer determines the strength of its attachment onto the oxide surface of TiO2. If the structure contains carboxyl or hydroxyl functional groups, the pigment would bind strongly onto the surface of TiO2 film. The interaction between TiO2 and the dye would lead to the excited electron transfer from the dye molecules to the conduction band of the semiconductor TiO2 [17].
Most natural dyes that can be used as sensitizers for DSSCs undergo rapid photo degradation [18]. Chlorophyll, the natural photosensitizer for the photosynthesis process in green plant [19], is found in the leaves of most green plants. The most widely occurring type is chlorophyll a [20]. The application of this type of pigment has been investigated in previous studies [21]. Chlorophyll absorbs light from red, blue, and violet wavelengths and obtains its color by reflecting the green wavelength [22]. With two strong absorption peaks in the visible region located at 420 nm and 660 nm wavelengths, it is an attractive compound that can be used as natural sensitizer in the visible light range [23]. The photoelectric conversion efficiencies of DSSCs prepared by chlorophyll dyes from ipomoea, pomegranate, dried spinach, and shiso leaf extracts were 0.31% [24], 0.59% [20], 0.29% [25], and 0.59% [26], respectively. The low conversion efficiencies of these species are caused by the unavailability of bonds between the dyes and TiO2 molecules [17].
Betalains are a class of pigment present in plants of the order Caryophyllales. Betalains, which are also found in some higher fungi, replace the anthocyanins in fruits and flowers of most families of plant kingdom. Betalains are divided into two kinds, namely, the betacyanins, which include the red-violet betalain pigments, and the betaxanthins, which are yellow-orange betalain pigments [27]. Betalains absorb radiation in the visible range between 476 nm and 600 nm with similar distribution patterns for both types of betalains (betacyanins and betaxanthins), suggesting the functional similarity between these groups of natural pigments [28]. In contrast to anthocyanins that contain hydroxyl functional groups, betalains have carboxyl functional groups. The interaction between carboxyl functions present in betalains and on the surface of TiO2 film creates a stronger electron coupling bond [8]. The conversion efficiencies of DSSCs prepared from betalain dyes of Beta vulgaris, Bougainvillea spectabilis, and wild Sicilian prickly pear fruit extracts are 0.89% [6], 0.46% [29], and 1.26% [5], respectively.
The type of solvent used in the extraction process is important. Recently, different types of organic solvents have been used to extract natural dyes from different parts of the plant. The type of solvent affects the absorption spectrum of the dyes as well as the bonding between the dyes and the surface of TiO2. Earlier studies have reported that the efficiency of DSSC prepared using Lawsonia inermis extract and obtained with ethanol (η = 0.66%) is higher than that prepared with a mixture of water and ethanol (η = 0.52%) [30]. A 532 nm red shift in the absorption spectra of the red onion skin extracts was observed in the case of ethanol solution. This is the most obvious shift when compared with those of methanol (510 nm) and water (520 nm) [31]. Dumbrava et al. studied the absorption spectra of dyes extracted from red beet under the same solvent sensitive conditions. The intensity of absorption spectra is approximately equal to those of methanolic (544 nm), aqueous extracts (542 nm), and ethanolic extract (532 nm) [1].
Based on these results, three tropical plant species grown in Malaysia, namely, (i) Cordyline fruticosa, (ii) Pandannus amaryllifolius, and (iii) Hylocereus polyrhizus were selected to explore new natural dyes for DSSCs. Various solvents were used to extract pigments from the leaves of the first and second species and from the fruits of the third species. The best solvent and the optimum ratio for dye extraction were investigated via UV–Vis spectroscopy. The structure of isolated dyes was confirmed using FTIR. Finally, the morphology and composition of dyes were identified using SEM and EDX.
Section snippets
Extraction of natural dyes
The fresh leaves of C. fruticosa and P. amaryllifolius were washed with distilled water and oven-dried at 40 °C. The dried leaves were crushed into fine powder using a grinder (Mulry function disintegrator SY-04). Subsequently, 10 g samples of the powdered leaves were placed into 100 ml of nine different solvents, namely, n-hexane, ethanol, acetonitrile, chloroform, ethyl-ether, ethyl-acetate, petroleum ether, n-butyl alcohol, and methanol. The solutions were kept for one week at room temperature
Effect of solvent on extraction
Fig. 1, Fig. 2 show the effects of solvent on the absorption spectra of C. fruticosa and P. amaryllifolius leaves extracted by n-hexane, ethanol, acetonitrile, chloroform, ethyl-ether, ethyl-acetate, petroleum Ether, n-butyl alcohol, and methanol. The absorbance of C. fruticosa and P. amaryllifolius shows a broad range of wavelength frequency between 410 nm and 700 nm, which is located within the visible range, and with three main peaks located at 530, 605, and 660 nm. The absorption peaks
Conclusions
Three natural pigments, namely, betanin, betaxanthins, and chlorophyll were extracted from three plant species [C. fruticosa leaves, P. amaryllifolius (pandan leaves), and H. polyrhizus (dragon fruit)]. The dyes of betalains and chlorophyll were successfully extracted from leaves and fruits using different solvents. The dyes were confirmed using FTIR for the functional groups of C. fruticosa and P. amaryllifolius dyes. These dyes showed a similar structure with those of functional groups. The
Acknowledgments
This study was supported by the National University of Malaysia (UKM DIP-2012-02) and Solar Energy Research Institute (ERGS/1/2012/UKM/03/5). Mahmoud A.M. Al-Alwani would like to thank College of Education of Pure Sciences-Ibn Al-Haitham of the University of Baghdad.
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