Accepted Manuscript Performance of Caesalpinia Sappan heartwood extract as photo sensitizer for dye sensitized solar cells S. Ananth, P. Vivek, G. Saravana Kumar, P. Murugakoothan PII: DOI: Reference:
S1386-1425(14)01274-8 http://dx.doi.org/10.1016/j.saa.2014.08.083 SAA 12604
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
13 May 2014 18 August 2014 24 August 2014
Please cite this article as: S. Ananth, P. Vivek, G. Saravana Kumar, P. Murugakoothan, Performance of Caesalpinia Sappan heartwood extract as photo sensitizer for dye sensitized solar cells, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.08.083
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Performance of Caesalpinia Sappan heartwood extract as photo sensitizer for dye sensitized solar cells S. Ananth, P. Vivek, G. Saravana Kumar, P. Murugakoothan* MRDL, PG & Research Department of Physics, Pachaiyappa’s College, Chennai 600 030, India.
*Corresponding author mobile: +91 9444 447 586. E-mail address: [email protected] (P. Murugakoothan). Abstract A natural dye extracted from Caesalpinia Sappan heartwood was used as photo sensitizer for the first time to fabricate titanium dioxide (TiO2) nanoparticles based dye sensitized solar cells. Brazilin and brazilein are the major pigments present in the natural dye and their optimized molecular structure were calculated using Density functional theory (DFT) at 6 – 31G (d) level. The HOMO – LUMO were performed to reveal the energy gap using optimized structure. Pure TiO2 nanoparticles in anatase phase were synthesized by sol – gel technique. The pure and natural dye sensitized TiO2 nanoparticles were subjected to structural, optical, spectral and morphological studies. Low cost and environment friendly dye sensitized solar cells were fabricated using natural dye sensitized TiO2 based photo anodes. The solar light to electron conversion efficiency of Caesalpinia Sappan heartwood extract sensitized dye sensitized solar cell is 1.1%. Keywords: Dye Sensitized Solar Cell, Titanium dioxide, Sol – gel, Natural dye, density functional theory, Caesalpinia Sappan heartwood. Introduction The ever increasing demand for more energy requires our immediate action for proper utilization of natural sources like solar energy wind energy, tidal energy etc. Solar energy is an important renewable energy source to harvest clean energy. Solar cells convert light photons from solar light energy into electrons by the
principle of photoelectric effect. Among the various solar cells available, dye sensitized solar cell (DSSC) is an attempt to replicate nature’s photosynthesis process which has merits, like less expensive, simple fabrication, choice of color, design, environment friendly etc. [1, 2]. The DSSC consists of a metal oxide semiconductor as photo anode, a dye sensitizer, an electrolyte and a counter electrode [3]. In DSSC, the dye sensitizer adsorbed on semiconductor captures the incident photon and generates electron / hole [4]. The excited electrons are injected into the conduction band of the semiconductor and transported to counter electrode. The dye molecule is regenerated by the redox system, which itself is regenerated at the counter electrode by electrons passed through load [5]. This is in contrast to conventional silicon based solar cells, where the semiconductor performs both task of light absorption and charge separation. In addition, DSSC does not need high purity and other advanced fabrication requirements unlike silicon based solar cells. Hence, it can be fabricated at low cost to harvest green energy from abundantly available sunlight. The efficiency of DSSC depends on its major components, design and fabrication process [1]. Hence, optimizing each component is essential to achieve maximum efficiency. Titanium dioxide (TiO2) is the most successful photo anode material used in DSSC due to its low cost, abundance, non – toxic, high stability, biocompatible and environment friendly etc. [6]. The sol – gel, a versatile technique was used to synthesis pure TiO2 nanoparticles. Among photo sensitizers, the ruthenium polypyridyl complex inorganic dyes are most successful and showing high efficiencies [7]. But, alarming factors, like high cost, heavy metal presence and complicated synthesis processes etc. have attracted the focus towards natural dyes as photo sensitizers [8]. Natural dyes are environment friendly, very cheap, easily extractable using cheap organic solvents and abundantly available in plant parts, like flowers, seeds, barks, leaves, stem etc. [9]. So, natural dyes can be used as an effective alternative
to toxic and expensive inorganic dye sensitizers. Natural dye pigments, like chlorophyll, betanins, carotenoids, anthocyanins and tannins are successfully used as sensitizers in DSSC [10, 11]. The solar light to electron conversion efficiency of DSSC prepared by chlorophyll dye from pomegranate leaf extract is 0.597% and that of anthocyanin dye from mulberry extract is 0.548% [12]. The natural dye extracts of red cabbage and blue pea sensitized DSSC showed the efficiencies of 0.73% and 0.67% respectively [13]. The anthocyanin dye extracted from Hibiscus sabdariffa L. flower and the chlorophyll dye from Undaria pinnatifida showed the efficiencies of 0.27% [14] and 0.178% [4] respectively. The natural dye extracted from Kerria japoni (Caotenoid) and Rosa Chinensis (Anthocyanin) as photo sensitizer to DSSC showed the efficiencies of 0.22% and 0.29% respectively [7]. The ivy gourd fruit extract and red frangipani flower extract showed the efficiencies of 0.076% and 0.301% [11]. The DSSCs sensitized by the fruit extract of Melastoma Malabathricum L. and Rose Bengal dye showed the efficiencies of 1.37% [9] and 2.09% [15] respectively. In general, the efficiency of the natural dye sensitized DSSCs are lesser than that of synthetic dye sensitized DSSCs. Attempts were made to improve the efficiency of natural dye sensitized DSSCs by using dye mixtures, increased dye concentration and extraction in various solvents etc. To achieve higher efficiency, the natural dyes must bind strongly to the TiO 2 surface by means of their anchoring group to ensure efficient electron injection into the conduction band of TiO2. In this work, a new natural dye extracted from Caesalpinia Sappan heartwood was used as dye sensitizer for the fabrication of TiO2 nanoparticles based DSSCs. The Caesalpinia Sappan heartwood extract sensitized DSSC showed a promising solar light to electron conversion efficiency of 1.1%.
Materials and methods Materials The titanium isopropoxide is purchased from Sigma – Aldrich and used as titanium precursor. The isopropanol and nitric acid are purchased from Merck. The Caesalpinia
sappan
heartwood
was
purchased
from
herbal
shop,
Thiruvananthapuram, Kerala, India. Apparatus The powder X – ray diffraction (PXRD) study was carried out using ISO DEBYEFLEX 2000 diffractometer employing CuKα (λ = 1.5418 Å) radiation. The UV–vis absorption spectrum was recorded in the wavelength range from 300 to 800 nm using Shimadzu Model 1601 spectrophotometer. The Perkin Elmer Spectrum1 FT – IR instrument with a resolution of 1.0 cm-1 was used to identify the functional groups present in the samples. F E I Quanta FEG 200 – High Resolution Scanning Electron Microscope with a resolution of 1.2 nm was used to study the morphological properties. The I – V response of the fabricated solar cells were studied using Keithley 2400 sourcemeter with a Xenon lamp of 100 mWcm-2 as solar simulator at standard AM 1.5 illumination. Preparation of Pure TiO2 The pure TiO2 nanoparticles were synthesized using sol – gel method by taking titanium isopropoxide as titanium precursor and an alcohol with distilled water as hydrolysis medium. The 15 mL isopropyl alcohol was mixed thoroughly with 250 mL of distilled water and the initial pH was noted as 8.75. The pH has strong influence on the formation and size distribution of TiO2 nanoparticles. When the pH is higher than 2, a white suspension of rough precipitants was formed immediately. On the other hand, when the pH is 2, a homogeneous suspension of fine particles was formed. Hence, the pH value was adjusted to 2 by adding nitric acid drop wise. This solution was stirred vigorously and 5 mL of titanium
isopropoxide solution was added drop wise to result a white precipitation. After hydrolysis process, the turbid solution containing TiO2 precipitation was heated up to 80°C for 3 hours. Then, the mixture was kept at room temperature for an hour for aging. This yields a white suspension which is washed in distilled water first and then in ethanol to remove byproduct impurities. The prepared white precipitate was dried at 150 °C for 15 hours to obtain fine particles of pure TiO2. Extraction of Natural Dye Caesalpinia Sappan Linn is a species of flowering tree in legume family, namely fabaceae. It is native to south India, Southeast Asia and the Malay Archipelago. Caesalpinia Sappan, a small thorny tree shown in fig. 1(a) grows 6 to 9 meter height. The aqueous extract was prepared the simple procedure [16] by soaking 150 gram of cleaned and dried Caesalpinia sappan heartwood in fig. 1(b) to 100 mL of distilled water and kept for 12 hours. The mixture was filtered and dark orange red extract was collected. The extract was concentrated and used as photo sensitizer to TiO2 nanoparticles for the fabrication of DSSCs. Brazilin, a reddish dye, is the major pigment present in the extract which is used for dyeing fabric, making red paints and inks. Brazilin on oxidation results more strong red pigment called brazilein. Both pigments are tetra cyclic with two aromatic rings, one pyrone and one five membered ring [17]. The structures of brazilin and brazilein pigment are shown in fig. 1(c) and 1(d). Computational Studies on Natural Dye The optimized molecular geometry of brazilin and brazilein was computed by employing the DFT methods using Gaussian 03 program package [18]. The Becke’s three parameter (local, non – local, HF) with Lee – Yang – Parr hybrid correlation functional (B3LYP) has been used with split valence basis set 6 – 31G augmented by ‘d’ polarization functions added [19, 20]. The optimized geometry of brazilin and brazilein using B3LYP methods are shown in fig. 1(c) and 1(d)
gives the minimum energy values as –992.99 a.u. and –992.86 a.u. respectively. The optimized structure was used to calculate HOMO-LUMO energy gap. The electronic transition from ground state to first excited state is mainly explained by one electron excitation from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO). The HOMO represents the ability to donate an electron, LUMO as an electron acceptor, represents the ability to obtain an electron. The energy of HOMO is directly related to ionization potential and LUMO energy is directly related to electron affinity. These orbitals determine the way in which the molecule interacts with other species. The frontier molecular orbital energy gap helps to characterize the chemical reactivity and kinetic stability of the molecule. A molecule with a smaller energy gap between HOMO – LUMO is more polarizable, with high chemical reactivity and low kinetic stability. The 3D frontier molecular orbital (FMOs) plots of brazilin and brazilein pigments in fig. 2 and 3 are localized on almost the whole molecule in which the positive and negative phases are represented by red and green color respectively. The energy gap for both pigments is similar. Hence, even if brazilin pigment gets oxidized to brazilein, it doesn’t alter the reactivity with TiO2 nanoparticles. Fabrication of DSSC The pure TiO2 obtained by conventional sol – gel technique was made into a paste using titanium isopropoxide. A thin film was coated using doctor blade technique on the FTO glass plate. This coating process was repeated twice to form a thick layer of TiO2 on the FTO glass plate. The dried TiO2 coated glass plate was sintered at 450ºC for 30 minutes to improve the electronic contact between the TiO2 nanoparticles and to eliminate internal gas and voids. The photo anode was soaked in Caesalpinia Sappan heartwood extract for 10 hours to adsorb dye onto the TiO2 surface. A platinum coated FTO glass plate was used as the counter
electrode. The two electrodes were joined together with dye sensitized TiO 2 at the middle without creating air bubbles. The liquid electrolyte (I - / I3-) was placed through the fine holes in the two electrodes carefully and sealed to prepare TiO2 DSSC. Results and Discussion POWDER XRD analysis The pure TiO2 nanoparticles synthesized by sol – gel method are calcined at 250º C for 2 hours. The Powder X – ray diffraction pattern of TiO2 nanoparticles after calcination, shown in fig. 4, gives the structural, crystalline quality and crystalline size information. The nano crystalline anatase structure of natural dyed TiO2 is confirmed by the presence of (101), (0 0 4), (2 0 0), (2 1 1), (2 0 4), (1 1 6) and (2 1 5) diffraction peaks. The diffraction peaks belonging to other polymorphs of TiO2 namely, rutile and brookite are not present. The broadened characteristic diffraction peaks indicates the smaller size of the natural dyed TiO2 nanoparticles.
The
grain
size
was
obtained
by
Scherrer’s
equation,
D = Kλ / (β cosθ), where, ‘D’ is grain size, ‘λ’ is wavelength of the CuKα X – ray radiation (λ = 1.5418 Å),‘K’ is shape factor – a dimensionless constant (0.9 in case of spherical shaped particles) and ‘β’ is the full width at half – maximum height (FWHM) of the respective diffraction peaks [21]. The average grain size of TiO2 nanoparticles in anatase phase is approximately 23 nm. UV–vis absorption spectral analysis The UV – vis absorption spectrum of pure TiO2 nanoparticles is shown in fig. 5(a) shows that the TiO2 nanoparticles absorb light photons in UV and nearby regions only. Hence, we need a separate dye sensitizer attached with TiO2 for making it suitable for solar cell applications. From the absorption spectrum of pure TiO2 nanoparticles, the cutoff wavelength (λ) found to occur at 356 nm. The band gap energy can be calculated by the relation Eg = 1239.8 / λ eV (λ in nm) [22]. The
band gap of TiO2 nanoparticles was found to be 3.48 eV. The band gap of normal or bulk TiO2 is 3.2 eV and this variation is due to the change in particle size [23]. The absorption spectrum of natural dye sensitized TiO2 in fig. 5 (b) shows enhanced light photon absorption and extension of absorption region than in pure TiO2. In addition, it shows a sharp absorption peak at 349 nm and a broad absorption peak at 658 nm. This improvement in absorbance is due to more natural dye molecules adsorbed on TiO2 surface which leads to increase in photo current and in total conversion efficiency [24]. And, the charge injected into the conduction band of the TiO2 is also affected by the type of attraction between sensitizer and its anchoring group with TiO2 [14]. The dye structure of brazilein dye possesses several C = O and – OH groups which are capable of anchoring to the Ti sites on the TiO2 surface. The cutoff wavelength of natural dye sensitized TiO2 is 426 nm and the band gap is found to be 2.91 eV. The band gap of TiO2 is related to the wavelength range absorbed and the band gap decreases with increasing absorption wavelength [22]. The absorption study reveals that decrease in band gap results in enhancement of photo sensitization behavior of TiO2 nanoparticles. FTIR Spectral Analysis The Fourier transform infra red (FTIR) spectra of pure and natural dye sensitized TiO2 nanoparticles are shown in fig. 6(a) and 6(b) respectively. Both spectra has the characteristic vibration of Ti – O bond at 646 cm-1 (in pure) and 644 cm-1, 747 cm-1 (in natural dye sensitized) which normally occur in between the standard range of 450 – 1000 cm-1[25]. The peaks present at 1029 cm-1 and 1054 cm-1 in colored TiO2 are due to ring stretching. The peaks present at 1170 cm-1, 1254 cm-1 and 1289 cm-1 in natural dye sensitized TiO2 are assigned to C – C – H bending, C – C stretching and C – O stretching vibrations respectively [17]. The peak present at 1318 cm-1 in colored TiO2 is assigned to bending mode of
O – C – C stretching vibration. The peaks present at 1046 cm-1 in pure TiO2 are assigned to O – H plane bending vibration mode [9]. The absorption peaks at 1623 cm-1 in pure and 1622 cm-1 in natural dye sensitized TiO2 is assigned to O – H bending modes [25]. The absorption peak at 1384 cm-1 in both TiO2 is due to nitrate ions from the nitric acid which is added to adjust the pH during synthesis [26]. The absorption peaks at 3234 cm-1 in pure and 3185 cm-1 in natural dye sensitized TiO2 is assigned to O – H stretching vibration mode of water molecules. Scanning Electron Microscope studies The scanning electron microscope (SEM) image of pure TiO2 nanoparticles synthesized by sol – gel method is shown in fig. 7 (a). The pure TiO2 nanoparticles have agglomerated together to form nano clusters. These nano clusters will affect the photo electric behavior of TiO2. Hence, to minimize this agglomeration, capping agent is needed. The SEM image of natural dye sensitized TiO2 nanoparticles in fig.7(b) shows improvement in morphology as nearly spherical particles. The dye aggregation on nano crystalline film produces absorptivity that may block the physical contact between the electrolyte and TiO 2. Hence, the reduction in efficiency will occur [27]. And, the uniform adsorption of natural dye on TiO2 surface reduces the possibility of dye aggregation also. Energy Dispersive X – Ray spectrum analysis The energy dispersive X – ray (E – DAX) spectrum of TiO2 nanoparticles is given in fig. 7 (c). The spectrum has prominent peaks of Ti and O. From the peaks, it is confirmed that the nanoparticles synthesized by sol – gel technique belongs to pure TiO2. The weight contributions are 33.83 and 61.45 percentage for Oxygen and Titanium respectively which is in agreement with 2:1 stoichiometry of TiO2. Both the elements together contribute 95.28 percentage of the total weight. This indicates the purity of TiO2 as there is no impurity material presence. The E – DAX spectrum of natural dye sensitized TiO2 nanoparticles is shown in fig. 7 (d). The
weight contributions are 36.01, 33.13 and 30.85 percentages for Oxygen, Titanium and Carbon respectively. The change in weight contributions of natural dye sensitized TiO2 is attributed to O – Ti – O bond changed into O – Ti – OOC due to adsorption of natural dye on TiO2 surface. Efficiency Studies The current – voltage (I – V) characteristics of Caesalpinia Sappan heartwood extract sensitized photo anodes prepared by TiO2 nanoparticles based DSSC under the AM 1.5 G illumination at 100 mW/cm2 is shown in fig. 8. The light photon to electron conversion efficiency was studied. The fill factor (FF) was found using the relation as given below, FF = (Imax × Vmax) / (Isc × Voc) Where, Imax and Vmax denote the maximum output value of current and voltage respectively, and Isc and Voc denote the short – circuit current and open – circuit voltage respectively. The natural dye sensitized TiO2 based DSSC exhibits the observed values of Isc = 3.3 mA cm-2, Voc = 0.49 V and the calculated value of FF is 67.26%. The total energy conversion efficiency was calculated using the relation as given below, η = (Jsc × Voc × FF) / Pin where, Pin denotes the energy of incident photon. The efficiency of Caesalpinia Sappan heartwood extract sensitized TiO2 based DSSC was found as 1.1%. Conclusion The natural dye extracted from Caesalpinia Sappan heartwood was successfully used as dye sensitizer for first time to pure nanoparticles synthesized using sol – gel techniques. The optimized molecular structure of brazilin and brazilein pigments was performed and their HOMO, LUMO were studied. The PXRD analysis confirms the anatase phase of TiO2 nanoparticles calcined at 250ºC.
The UV – vis absorption studies confirm the more dye absorbance in natural dye sensitized TiO2 and FTIR spectra confirms the formation of TiO2 in sol – gel process. The SEM images confirm the improved morphology and reduced agglomeration in natural dye sensitized TiO2. DSSCs were fabricated using TiO2 sensitized by brazilein pigment rich natural dye. The light to electron conversion efficiency of natural dye sensitized TiO2 based DSSC is 1.1%. The natural dye extracts of Caesalpinia Sappan heartwood can be used as an effective photo sensitizer for making natural dye based DSSCs with good efficiency. References [1]
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Figure Captions Fig. 1(a) Caesalpinia Sappan Linn. Tree Fig. 1(b) Caesalpinia Sappan heartwood Fig. 1(c) Optimized molecular structure of brazilin pigment Fig. 1(d) Optimized molecular structure of brazilein pigment Fig.2 HOMO – LUMO of brazilin pigment Fig.3 HOMO – LUMO of brazilein pigment Fig. 4 PXRD pattern of TiO2 after calcination at 250°C. Fig.5 (a) Absorption spectrum of pure TiO2 Fig.5 (b) Absorption spectrum of natural dye sensitized TiO2 Fig.6 (a) FTIR spectrum of pure TiO2 Fig.6 (b) FTIR spectrum of natural dye sensitized TiO2 Fig.7 (a) SEM Image of pure TiO2 Fig.7 (b) SEM Image of natural dye sensitized TiO2 Fig.7 (c) E – DAX spectrum of pure TiO2 Fig.7 (d) E – DAX spectrum of natural dye sensitized TiO2 Fig.8 I – V characteristics of natural dye sensitized TiO2 based DSSC
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Caesalpinia Sappan heartwood extract as new dye sensitizer for DSSC. Brazilin and brazilein are the major pigments. Optimized molecular structure was calculated. HOMO – LUMO were studied. Efficiency is 1.1% for natural dye sensitized DSSC.
S1386-1425(14)01274-8 http://dx.doi.org/10.1016/j.saa.2014.08.083 SAA 12604
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
13 May 2014 18 August 2014 24 August 2014
Please cite this article as: S. Ananth, P. Vivek, G. Saravana Kumar, P. Murugakoothan, Performance of Caesalpinia Sappan heartwood extract as photo sensitizer for dye sensitized solar cells, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.08.083
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Performance of Caesalpinia Sappan heartwood extract as photo sensitizer for dye sensitized solar cells S. Ananth, P. Vivek, G. Saravana Kumar, P. Murugakoothan* MRDL, PG & Research Department of Physics, Pachaiyappa’s College, Chennai 600 030, India.
*Corresponding author mobile: +91 9444 447 586. E-mail address: [email protected] (P. Murugakoothan). Abstract A natural dye extracted from Caesalpinia Sappan heartwood was used as photo sensitizer for the first time to fabricate titanium dioxide (TiO2) nanoparticles based dye sensitized solar cells. Brazilin and brazilein are the major pigments present in the natural dye and their optimized molecular structure were calculated using Density functional theory (DFT) at 6 – 31G (d) level. The HOMO – LUMO were performed to reveal the energy gap using optimized structure. Pure TiO2 nanoparticles in anatase phase were synthesized by sol – gel technique. The pure and natural dye sensitized TiO2 nanoparticles were subjected to structural, optical, spectral and morphological studies. Low cost and environment friendly dye sensitized solar cells were fabricated using natural dye sensitized TiO2 based photo anodes. The solar light to electron conversion efficiency of Caesalpinia Sappan heartwood extract sensitized dye sensitized solar cell is 1.1%. Keywords: Dye Sensitized Solar Cell, Titanium dioxide, Sol – gel, Natural dye, density functional theory, Caesalpinia Sappan heartwood. Introduction The ever increasing demand for more energy requires our immediate action for proper utilization of natural sources like solar energy wind energy, tidal energy etc. Solar energy is an important renewable energy source to harvest clean energy. Solar cells convert light photons from solar light energy into electrons by the
principle of photoelectric effect. Among the various solar cells available, dye sensitized solar cell (DSSC) is an attempt to replicate nature’s photosynthesis process which has merits, like less expensive, simple fabrication, choice of color, design, environment friendly etc. [1, 2]. The DSSC consists of a metal oxide semiconductor as photo anode, a dye sensitizer, an electrolyte and a counter electrode [3]. In DSSC, the dye sensitizer adsorbed on semiconductor captures the incident photon and generates electron / hole [4]. The excited electrons are injected into the conduction band of the semiconductor and transported to counter electrode. The dye molecule is regenerated by the redox system, which itself is regenerated at the counter electrode by electrons passed through load [5]. This is in contrast to conventional silicon based solar cells, where the semiconductor performs both task of light absorption and charge separation. In addition, DSSC does not need high purity and other advanced fabrication requirements unlike silicon based solar cells. Hence, it can be fabricated at low cost to harvest green energy from abundantly available sunlight. The efficiency of DSSC depends on its major components, design and fabrication process [1]. Hence, optimizing each component is essential to achieve maximum efficiency. Titanium dioxide (TiO2) is the most successful photo anode material used in DSSC due to its low cost, abundance, non – toxic, high stability, biocompatible and environment friendly etc. [6]. The sol – gel, a versatile technique was used to synthesis pure TiO2 nanoparticles. Among photo sensitizers, the ruthenium polypyridyl complex inorganic dyes are most successful and showing high efficiencies [7]. But, alarming factors, like high cost, heavy metal presence and complicated synthesis processes etc. have attracted the focus towards natural dyes as photo sensitizers [8]. Natural dyes are environment friendly, very cheap, easily extractable using cheap organic solvents and abundantly available in plant parts, like flowers, seeds, barks, leaves, stem etc. [9]. So, natural dyes can be used as an effective alternative
to toxic and expensive inorganic dye sensitizers. Natural dye pigments, like chlorophyll, betanins, carotenoids, anthocyanins and tannins are successfully used as sensitizers in DSSC [10, 11]. The solar light to electron conversion efficiency of DSSC prepared by chlorophyll dye from pomegranate leaf extract is 0.597% and that of anthocyanin dye from mulberry extract is 0.548% [12]. The natural dye extracts of red cabbage and blue pea sensitized DSSC showed the efficiencies of 0.73% and 0.67% respectively [13]. The anthocyanin dye extracted from Hibiscus sabdariffa L. flower and the chlorophyll dye from Undaria pinnatifida showed the efficiencies of 0.27% [14] and 0.178% [4] respectively. The natural dye extracted from Kerria japoni (Caotenoid) and Rosa Chinensis (Anthocyanin) as photo sensitizer to DSSC showed the efficiencies of 0.22% and 0.29% respectively [7]. The ivy gourd fruit extract and red frangipani flower extract showed the efficiencies of 0.076% and 0.301% [11]. The DSSCs sensitized by the fruit extract of Melastoma Malabathricum L. and Rose Bengal dye showed the efficiencies of 1.37% [9] and 2.09% [15] respectively. In general, the efficiency of the natural dye sensitized DSSCs are lesser than that of synthetic dye sensitized DSSCs. Attempts were made to improve the efficiency of natural dye sensitized DSSCs by using dye mixtures, increased dye concentration and extraction in various solvents etc. To achieve higher efficiency, the natural dyes must bind strongly to the TiO 2 surface by means of their anchoring group to ensure efficient electron injection into the conduction band of TiO2. In this work, a new natural dye extracted from Caesalpinia Sappan heartwood was used as dye sensitizer for the fabrication of TiO2 nanoparticles based DSSCs. The Caesalpinia Sappan heartwood extract sensitized DSSC showed a promising solar light to electron conversion efficiency of 1.1%.
Materials and methods Materials The titanium isopropoxide is purchased from Sigma – Aldrich and used as titanium precursor. The isopropanol and nitric acid are purchased from Merck. The Caesalpinia
sappan
heartwood
was
purchased
from
herbal
shop,
Thiruvananthapuram, Kerala, India. Apparatus The powder X – ray diffraction (PXRD) study was carried out using ISO DEBYEFLEX 2000 diffractometer employing CuKα (λ = 1.5418 Å) radiation. The UV–vis absorption spectrum was recorded in the wavelength range from 300 to 800 nm using Shimadzu Model 1601 spectrophotometer. The Perkin Elmer Spectrum1 FT – IR instrument with a resolution of 1.0 cm-1 was used to identify the functional groups present in the samples. F E I Quanta FEG 200 – High Resolution Scanning Electron Microscope with a resolution of 1.2 nm was used to study the morphological properties. The I – V response of the fabricated solar cells were studied using Keithley 2400 sourcemeter with a Xenon lamp of 100 mWcm-2 as solar simulator at standard AM 1.5 illumination. Preparation of Pure TiO2 The pure TiO2 nanoparticles were synthesized using sol – gel method by taking titanium isopropoxide as titanium precursor and an alcohol with distilled water as hydrolysis medium. The 15 mL isopropyl alcohol was mixed thoroughly with 250 mL of distilled water and the initial pH was noted as 8.75. The pH has strong influence on the formation and size distribution of TiO2 nanoparticles. When the pH is higher than 2, a white suspension of rough precipitants was formed immediately. On the other hand, when the pH is 2, a homogeneous suspension of fine particles was formed. Hence, the pH value was adjusted to 2 by adding nitric acid drop wise. This solution was stirred vigorously and 5 mL of titanium
isopropoxide solution was added drop wise to result a white precipitation. After hydrolysis process, the turbid solution containing TiO2 precipitation was heated up to 80°C for 3 hours. Then, the mixture was kept at room temperature for an hour for aging. This yields a white suspension which is washed in distilled water first and then in ethanol to remove byproduct impurities. The prepared white precipitate was dried at 150 °C for 15 hours to obtain fine particles of pure TiO2. Extraction of Natural Dye Caesalpinia Sappan Linn is a species of flowering tree in legume family, namely fabaceae. It is native to south India, Southeast Asia and the Malay Archipelago. Caesalpinia Sappan, a small thorny tree shown in fig. 1(a) grows 6 to 9 meter height. The aqueous extract was prepared the simple procedure [16] by soaking 150 gram of cleaned and dried Caesalpinia sappan heartwood in fig. 1(b) to 100 mL of distilled water and kept for 12 hours. The mixture was filtered and dark orange red extract was collected. The extract was concentrated and used as photo sensitizer to TiO2 nanoparticles for the fabrication of DSSCs. Brazilin, a reddish dye, is the major pigment present in the extract which is used for dyeing fabric, making red paints and inks. Brazilin on oxidation results more strong red pigment called brazilein. Both pigments are tetra cyclic with two aromatic rings, one pyrone and one five membered ring [17]. The structures of brazilin and brazilein pigment are shown in fig. 1(c) and 1(d). Computational Studies on Natural Dye The optimized molecular geometry of brazilin and brazilein was computed by employing the DFT methods using Gaussian 03 program package [18]. The Becke’s three parameter (local, non – local, HF) with Lee – Yang – Parr hybrid correlation functional (B3LYP) has been used with split valence basis set 6 – 31G augmented by ‘d’ polarization functions added [19, 20]. The optimized geometry of brazilin and brazilein using B3LYP methods are shown in fig. 1(c) and 1(d)
gives the minimum energy values as –992.99 a.u. and –992.86 a.u. respectively. The optimized structure was used to calculate HOMO-LUMO energy gap. The electronic transition from ground state to first excited state is mainly explained by one electron excitation from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO). The HOMO represents the ability to donate an electron, LUMO as an electron acceptor, represents the ability to obtain an electron. The energy of HOMO is directly related to ionization potential and LUMO energy is directly related to electron affinity. These orbitals determine the way in which the molecule interacts with other species. The frontier molecular orbital energy gap helps to characterize the chemical reactivity and kinetic stability of the molecule. A molecule with a smaller energy gap between HOMO – LUMO is more polarizable, with high chemical reactivity and low kinetic stability. The 3D frontier molecular orbital (FMOs) plots of brazilin and brazilein pigments in fig. 2 and 3 are localized on almost the whole molecule in which the positive and negative phases are represented by red and green color respectively. The energy gap for both pigments is similar. Hence, even if brazilin pigment gets oxidized to brazilein, it doesn’t alter the reactivity with TiO2 nanoparticles. Fabrication of DSSC The pure TiO2 obtained by conventional sol – gel technique was made into a paste using titanium isopropoxide. A thin film was coated using doctor blade technique on the FTO glass plate. This coating process was repeated twice to form a thick layer of TiO2 on the FTO glass plate. The dried TiO2 coated glass plate was sintered at 450ºC for 30 minutes to improve the electronic contact between the TiO2 nanoparticles and to eliminate internal gas and voids. The photo anode was soaked in Caesalpinia Sappan heartwood extract for 10 hours to adsorb dye onto the TiO2 surface. A platinum coated FTO glass plate was used as the counter
electrode. The two electrodes were joined together with dye sensitized TiO 2 at the middle without creating air bubbles. The liquid electrolyte (I - / I3-) was placed through the fine holes in the two electrodes carefully and sealed to prepare TiO2 DSSC. Results and Discussion POWDER XRD analysis The pure TiO2 nanoparticles synthesized by sol – gel method are calcined at 250º C for 2 hours. The Powder X – ray diffraction pattern of TiO2 nanoparticles after calcination, shown in fig. 4, gives the structural, crystalline quality and crystalline size information. The nano crystalline anatase structure of natural dyed TiO2 is confirmed by the presence of (101), (0 0 4), (2 0 0), (2 1 1), (2 0 4), (1 1 6) and (2 1 5) diffraction peaks. The diffraction peaks belonging to other polymorphs of TiO2 namely, rutile and brookite are not present. The broadened characteristic diffraction peaks indicates the smaller size of the natural dyed TiO2 nanoparticles.
The
grain
size
was
obtained
by
Scherrer’s
equation,
D = Kλ / (β cosθ), where, ‘D’ is grain size, ‘λ’ is wavelength of the CuKα X – ray radiation (λ = 1.5418 Å),‘K’ is shape factor – a dimensionless constant (0.9 in case of spherical shaped particles) and ‘β’ is the full width at half – maximum height (FWHM) of the respective diffraction peaks [21]. The average grain size of TiO2 nanoparticles in anatase phase is approximately 23 nm. UV–vis absorption spectral analysis The UV – vis absorption spectrum of pure TiO2 nanoparticles is shown in fig. 5(a) shows that the TiO2 nanoparticles absorb light photons in UV and nearby regions only. Hence, we need a separate dye sensitizer attached with TiO2 for making it suitable for solar cell applications. From the absorption spectrum of pure TiO2 nanoparticles, the cutoff wavelength (λ) found to occur at 356 nm. The band gap energy can be calculated by the relation Eg = 1239.8 / λ eV (λ in nm) [22]. The
band gap of TiO2 nanoparticles was found to be 3.48 eV. The band gap of normal or bulk TiO2 is 3.2 eV and this variation is due to the change in particle size [23]. The absorption spectrum of natural dye sensitized TiO2 in fig. 5 (b) shows enhanced light photon absorption and extension of absorption region than in pure TiO2. In addition, it shows a sharp absorption peak at 349 nm and a broad absorption peak at 658 nm. This improvement in absorbance is due to more natural dye molecules adsorbed on TiO2 surface which leads to increase in photo current and in total conversion efficiency [24]. And, the charge injected into the conduction band of the TiO2 is also affected by the type of attraction between sensitizer and its anchoring group with TiO2 [14]. The dye structure of brazilein dye possesses several C = O and – OH groups which are capable of anchoring to the Ti sites on the TiO2 surface. The cutoff wavelength of natural dye sensitized TiO2 is 426 nm and the band gap is found to be 2.91 eV. The band gap of TiO2 is related to the wavelength range absorbed and the band gap decreases with increasing absorption wavelength [22]. The absorption study reveals that decrease in band gap results in enhancement of photo sensitization behavior of TiO2 nanoparticles. FTIR Spectral Analysis The Fourier transform infra red (FTIR) spectra of pure and natural dye sensitized TiO2 nanoparticles are shown in fig. 6(a) and 6(b) respectively. Both spectra has the characteristic vibration of Ti – O bond at 646 cm-1 (in pure) and 644 cm-1, 747 cm-1 (in natural dye sensitized) which normally occur in between the standard range of 450 – 1000 cm-1[25]. The peaks present at 1029 cm-1 and 1054 cm-1 in colored TiO2 are due to ring stretching. The peaks present at 1170 cm-1, 1254 cm-1 and 1289 cm-1 in natural dye sensitized TiO2 are assigned to C – C – H bending, C – C stretching and C – O stretching vibrations respectively [17]. The peak present at 1318 cm-1 in colored TiO2 is assigned to bending mode of
O – C – C stretching vibration. The peaks present at 1046 cm-1 in pure TiO2 are assigned to O – H plane bending vibration mode [9]. The absorption peaks at 1623 cm-1 in pure and 1622 cm-1 in natural dye sensitized TiO2 is assigned to O – H bending modes [25]. The absorption peak at 1384 cm-1 in both TiO2 is due to nitrate ions from the nitric acid which is added to adjust the pH during synthesis [26]. The absorption peaks at 3234 cm-1 in pure and 3185 cm-1 in natural dye sensitized TiO2 is assigned to O – H stretching vibration mode of water molecules. Scanning Electron Microscope studies The scanning electron microscope (SEM) image of pure TiO2 nanoparticles synthesized by sol – gel method is shown in fig. 7 (a). The pure TiO2 nanoparticles have agglomerated together to form nano clusters. These nano clusters will affect the photo electric behavior of TiO2. Hence, to minimize this agglomeration, capping agent is needed. The SEM image of natural dye sensitized TiO2 nanoparticles in fig.7(b) shows improvement in morphology as nearly spherical particles. The dye aggregation on nano crystalline film produces absorptivity that may block the physical contact between the electrolyte and TiO 2. Hence, the reduction in efficiency will occur [27]. And, the uniform adsorption of natural dye on TiO2 surface reduces the possibility of dye aggregation also. Energy Dispersive X – Ray spectrum analysis The energy dispersive X – ray (E – DAX) spectrum of TiO2 nanoparticles is given in fig. 7 (c). The spectrum has prominent peaks of Ti and O. From the peaks, it is confirmed that the nanoparticles synthesized by sol – gel technique belongs to pure TiO2. The weight contributions are 33.83 and 61.45 percentage for Oxygen and Titanium respectively which is in agreement with 2:1 stoichiometry of TiO2. Both the elements together contribute 95.28 percentage of the total weight. This indicates the purity of TiO2 as there is no impurity material presence. The E – DAX spectrum of natural dye sensitized TiO2 nanoparticles is shown in fig. 7 (d). The
weight contributions are 36.01, 33.13 and 30.85 percentages for Oxygen, Titanium and Carbon respectively. The change in weight contributions of natural dye sensitized TiO2 is attributed to O – Ti – O bond changed into O – Ti – OOC due to adsorption of natural dye on TiO2 surface. Efficiency Studies The current – voltage (I – V) characteristics of Caesalpinia Sappan heartwood extract sensitized photo anodes prepared by TiO2 nanoparticles based DSSC under the AM 1.5 G illumination at 100 mW/cm2 is shown in fig. 8. The light photon to electron conversion efficiency was studied. The fill factor (FF) was found using the relation as given below, FF = (Imax × Vmax) / (Isc × Voc) Where, Imax and Vmax denote the maximum output value of current and voltage respectively, and Isc and Voc denote the short – circuit current and open – circuit voltage respectively. The natural dye sensitized TiO2 based DSSC exhibits the observed values of Isc = 3.3 mA cm-2, Voc = 0.49 V and the calculated value of FF is 67.26%. The total energy conversion efficiency was calculated using the relation as given below, η = (Jsc × Voc × FF) / Pin where, Pin denotes the energy of incident photon. The efficiency of Caesalpinia Sappan heartwood extract sensitized TiO2 based DSSC was found as 1.1%. Conclusion The natural dye extracted from Caesalpinia Sappan heartwood was successfully used as dye sensitizer for first time to pure nanoparticles synthesized using sol – gel techniques. The optimized molecular structure of brazilin and brazilein pigments was performed and their HOMO, LUMO were studied. The PXRD analysis confirms the anatase phase of TiO2 nanoparticles calcined at 250ºC.
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Figure Captions Fig. 1(a) Caesalpinia Sappan Linn. Tree Fig. 1(b) Caesalpinia Sappan heartwood Fig. 1(c) Optimized molecular structure of brazilin pigment Fig. 1(d) Optimized molecular structure of brazilein pigment Fig.2 HOMO – LUMO of brazilin pigment Fig.3 HOMO – LUMO of brazilein pigment Fig. 4 PXRD pattern of TiO2 after calcination at 250°C. Fig.5 (a) Absorption spectrum of pure TiO2 Fig.5 (b) Absorption spectrum of natural dye sensitized TiO2 Fig.6 (a) FTIR spectrum of pure TiO2 Fig.6 (b) FTIR spectrum of natural dye sensitized TiO2 Fig.7 (a) SEM Image of pure TiO2 Fig.7 (b) SEM Image of natural dye sensitized TiO2 Fig.7 (c) E – DAX spectrum of pure TiO2 Fig.7 (d) E – DAX spectrum of natural dye sensitized TiO2 Fig.8 I – V characteristics of natural dye sensitized TiO2 based DSSC
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Caesalpinia Sappan heartwood extract as new dye sensitizer for DSSC. Brazilin and brazilein are the major pigments. Optimized molecular structure was calculated. HOMO – LUMO were studied. Efficiency is 1.1% for natural dye sensitized DSSC.
