Microwave assisted biosynthesis of rice shaped ZnO nanoparticles using Amorphophallus konjac tuber extract and its application in dye sensitized solar cells

ZnO is one of photoconductive n-type semiconductors with band gap energy of 3.37 eV with good transparency [1], high electron mobility [2] and strong room temperature luminescence [3]. Hence, it is highly preferred for variety of applications like sensors, memory devices UV-light emitting diodes, piezoelectric devices, photodiodes and photodetectors [4]. Apart from these applications, ZnO nanoparticles possess the potential to be used as photoelectrodes for energy conversion of solar energy to electricity. The dye sensitized solar cells are now becoming most efficient third generation solar cells. Remarkable conversion efficiencies have been reported with ZnO nanoparticles as photoelectrodes using different nanostructures and combinations [5–7]. In spite of these achievements cost and eco-friendly nature of these cells could be improved if the natural products were involved in construction of these cells. Cost effective eco-friendly dyes and electrolyte have been utilized in these solar cells and were successful in achieving better conversion efficiencies [8–13]. This paper presents a green approach to fabricate dye sensitized solar cells via biosynthesis of photoelectrode material.

ZnO nanoparticles are synthesized by different methods such as wet chemical method [14], vapor phase method [15], hydrothermal method [16], precipitation method [17], atomic layer deposition [18] and sonochemical method [19]. These methods involve toxic reagents, expensive equipment with tedious and also time consuming processes.

Biosynthesis method is considered to be an alternative to the conventional methods which involve the use of plant extracts [20], fungi [21], viruses [22] and bacteria [23] in the nanoparticle synthesis process. Among the above mentioned, green synthesis process using plant extracts is relatively simple. Indeed, it does not require any special equipment and complex procedures. The phytoconstituents present in the plant extracts act as reducing and capping agents in the synthesis process, further they also act as a template which modifies and controls the shape of the nanoparticles [24]. The nanoparticles synthesized by biosynthesis method result in different morphologies such as spherical shaped [25], fibril shaped [26], hexagonal shaped [27], cubical shaped [28] and triangular shaped [29] nanoparticles. Plant extracts from Terminalia cuneata is used to synthesize silver nanoparticles [30], Hibiscus sabdariffa is used to synthesis of gold nanoparticles [31], Magnolia kobus leaf extracts are used to synthesis of copper nanoparticles [32], peels of Punica granatum are used for synthesizing ZnO nanoparticles [33]. Owing to the above said, the biosynthesis of metal oxide nanoparticles is considered in the literature as a reliable method.

In this study a novel synthesis method of ZnO nanoparticles using Amorphophallus konjac tubers extracts has been presented. Amorphophallus konjac tubers containing glucomannan, glucose, fructose, and sucrose has been reported. Every 100 g of konjac tuber contains 1.64 g protein, 0.004 g fat, 57 mg phosphorus, 4.06 mg iron, 123 mg zinc, 0.2 mg manganese, 0.25 mg chromium, 0.08 mg copper and 79.37 mg glucomannan [34]. The air dried 1 g powder of Amorphophallus konjac tubers contains almost 46.33 mg of flavonoid content, 12.67 mg of phenolic content [35].

The synthesis of ZnO nanoparticles involves microwave assisted drying process. Microwave drying helps in rapid and homogenous heating of the reaction mixture to the desired temperature, which saves time and energy. Various morphologies, like rod shaped, spherical shaped, flower shaped, can be obtained by microwave irradiation technique [36–38]. The phytochemicals present in the Amorphophallus konjac tuber extracts show good vibration response to the microwave irradiation [39]. Prepared ZnO nanoparticles were used for making a dye sensitized solar cell. Photoelectrodes for the dye sensitized solar cell were prepared by coating a thick film of prepared ZnO nanoparticles over the conductive side of FTO substrate by doctor blade method. The thick film provides more dye absorption over the nanoparticles compared with thin films and results in good photon to electron conversion efficiency [40]. The prepared photoelectrodes were sensitized using natural dyes extracted from fruits of Terminalia catappa, flowers of Callistemon citrinus and leaves of Euphorbia pulcherrima. The characteristics of synthesized ZnO nanoparticles and fabricated dye sensitized solar cells have been studied and analyzed.

Zinc acetate dehydrate Zn(CH₃COO)₂·2H₂O (99 %) was purchased from Sigma-Aldrich and was used as received. Fresh Amorphophallus konjac tubers were purchased from local market in Coimbatore, Tamilnadu.

The XRD patterns of the synthesized ZnO nanoparticles are shown in the Fig. 1a. The results show that there are nine peaks at 31.93°, 34.71°, 36.51°, 47.76°, 56.83°, 63.06°, 68.16°, 69.28°, 77.31° corresponding to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 1), (1 1 2), (2 0 2) planes, clearly indicating that the prepared sample is of wurtzite type. No other peak related to impurities has been observed in the diffraction pattern. The interplanar spacing of synthesized ZnO nanoparticles was calculated using Debye-Scherrer equation: D=0.9λ/(βcosθ)$$D = 0.9\lambda /(\beta \cos \theta )$$(1) where D is the crystallite size, λ is the wavelength of X-ray used, θ is Bragg’s angle and β is the full width at half maximum. The average crystallite size was determined to be 17.9 nm.

Fig. 1b shows the FESEM image of biosynthesized ZnO nanoparticles. The ZnO nanoparticles are in the form of rice-like structures with the length of 237 nm and diameter of 76 nm. The inset shows the image of rice which is similar to the morphology of the biosynthesized ZnO nanoparticles. Fig. 1c shows the EDAX spectrum of the biosynthesized nanoparticles which confirms the only presence of elemental zinc and oxygen. The weight percentage of elemental zinc and oxygen was found to be 70.44 % and 29.56 %, respectively.

The TEM image of biosynthesized ZnO nanoparticles is shown in Fig. 1d. The image confirms the rice shaped morphology of the synthesized ZnO nanoparticles. The selected area electron diffraction (SAED) pattern of the biosynthesized ZnO nanoparticles is shown in Fig. 2a. The pattern shows a regular polycrystalline ring with diffraction spots produced due to superposition of several single crystal orientations. The image also shows some weak diffraction spots. Fig. 2b shows the HRTEM image of the biosynthesized ZnO nanoparticles. The image shows the presence of lattice fringes with d-spacing of 5.86 Å.

The absorption spectra were investigated in solution state for both dye and dye sensitized nanoparticles. Fig. 3a shows the UV-Vis absorption spectra of as-synthesized ZnO nanoparticles. The absorption peak is observed at 372 nm. Fig. 3b shows the Tauc’s plot of the biosynthesized ZnO nanoparticles. The graph presents (αhν)² as a function of photon energy hν, where α is the absorption coefficient. The linear portion of the graph shows the value of band gap as 3.11 eV which is lower than that of the pure ZnO (3.3 eV). This reduced band gap energy may be due to some plant chemicals which are substituted into the lattice. This is in agreement with the previous studies on green synthesis of ZnO nanoparticles using Plectranthus amboinicus [41].

The absorption spectra of flower extract of Callistemon citrinus and ZnO photoelectrode sensitized by flower extracts of Callistemon citrinus in ethanolic solution is shown in Fig. 4a. The absorption spectra indicate that the light absorbance of the photoelectrode has been increased due to sensitization. The flower extracts of Callistemon citrinus contains cyanidin-3,5-diglucoside which provides red color to the flower. Cyanidin-3,5-diglucoside belongs to the anthocyanins which are a class of natural chemicals belonging to the flavonoids. The maximum absorption wavelength of cyanidin-3,5-diglucoside lies in the range of 520 nm to 540 nm, depending on its concentration and pH. The chemical structure of cyanidin-3,5-diglucoside is shown in Fig. 5a.

The leaves of Euphorbia pulcherrima are rich in germanicol which is used in sunscreen lotions since it absorbs light to give red color to its leaves. Fig. 4b shows the absorption spectra of the extracts from leaves Euphorbia pulcherrima and ZnO photoelectrode sensitized with Euphorbia pulcherrima leaf extracts in ethanolic solution. This spectrum also shows that the absorption peak increase due to sensitization. The chemical structure of germanicol is shown in the Fig. 5b.

Fig. 4c shows the absorbance spectra of fruit extracts of Terminalia catappa and ZnO photoelectrode sensitized by fruit extracts of Terminalia catappa in ethanolic solution. The absorbance spectra of sensitized ZnO electrodes show the light absorbance which has been increased due to natural sensitizers. Callistemon citrinus absorbs in the wide wavelength range of 460 nm to 570 nm with an absorption peak at 536 nm. The light absorbance of ZnO photoelectrode has been improved by the sensitization of fruit extracts of Terminalia catappa. 100 g of the fruits of Terminalia catappa contains β-carotene (2.1 mg), ascorbic acid (138.6 mg) and vitamin E (7.25 mg) as the major constituents [42]. The chemical structure of β-carotene is shown in Fig. 5c. The improvement in light absorption of the sensitized photoelectrode is mainly due to the presence of β-carotene in the dye [43].

Fig. 6 shows the recorded J-V characteristics of prepared solar cells and the calculated solar cell parameters, such as open circuit voltage, short circuit current density, fill factor, and efficiency. The performance of the dye sensitized solar cell depends on the morphology of the photoelectrode [44]. The parameters of the dye sensitized solar cells prepared using biosynthesized, rice shaped ZnO nanoparticles are given in Table 1. ZnO photoelectrodes sensitized with leaf extracts of Euphorbia pulcherrima have a power conversion efficiency of 1.66 % followed by the ZnO photoelectrode sensitized by Terminalia catappa fruit extracts with an efficiency of 1.63 %. The conversion efficiency of Callistemon citrinus flower extract sensitized photoelectrode shows a relatively lower conversion efficiency of 1.43 %. The devices remained stable for 14 day after which there was a decrease in J_sc values.

The result substantiates that the natural dye can be used as an effective sensitizer giving some modification of morphology of photoanode for better efficiency of the solar cell. The rice shaped nanoparticles absorb some amount of dye on their surface due to biosynthesis process involved. ZnO nanoparticles have some organic compounds attached to their surface which bind the dye to the surface. The results show that the biosynthesized nanoparticles could improve the efficiency of dye sensitized solar cells, when sensitized with natural dyes.

Rice shaped ZnO nanoparticles have been synthesized by microwave assisted biosynthesis method for the first time. The rice shaped morphology was obtained due to the phytochemicals present in Amorphophallus konjac tuber extracts. The X-ray diffraction studies and EDAX spectra confirmed that the prepared sample contain ZnO nanoparticles. The band gap energy determined from UV-Vis spectroscopy studies is 3.11 eV. The prepared ZnO nanoparticles were used to prepare photoelectrodes for dye sensitized solar cell application. ZnO photoelectrodes were sensitized using natural dyes extracted from fruits of Terminalia catappa, flowers of Callistemon citrinus and leaves of Euphorbia pulcherrima. The solar cell fabricated using ZnO sensitized with Euphorbia pulcherrima exhibited the best efficiency of 1.66 %.