Optical absorption and photoluminescence properties of Cucurbita maxima dye adsorption on TiO2 nanoparticles

Dye-sensitised solar cells (DSSCs) are 3rd generation photovoltaic device that imitate photosynthesis in plants. The fundamental concept of a DSSCs is that the photoanode is covered by the dye as a sensitiser. Natural dyes from plant-based extracts have gained attention as alternatives to toxic and expensive commercial dye sensitisers. Various studies have been conducted on the use of natural plant dye extracts for DSSCs . However, more fundamental studies on their adsorption on TiO2 photoanode nanoparticles are still not well understood. In this study, we investigated the crystal structure, optical absorption, and photoluminescence properties of TiO2, Cucurbita maxima, and Cucurbita maxima dyes adsorbed on TiO2 nanoparticles as potential materials for DSSCs. Raman spectra confirmed the anatase phase of the TiO2 nanoparticles. The particle size of 12 ± 2 nm was confirmed through the transmission electron microscope. The optical absorption properties of Cucurbita maxima show two distinct absorption bands: blue visible (450–500 nm) and red visible (635–674 nm). The photoluminescence spectra of the dye extract and its adsorption onto the TiO2 nanoparticles showed two prominent peaks in the blue and red regions of the electromagnetic spectrum. No significant peak is observed in the green region of the electromagnetic spectrum. These studies shed more light on the fundamental properties of chlorophyll adsorption on TiO2 nanoparticles and their optical and photoluminescence properties for applications as sensitisers in DSSCs.


Introduction
Dye-sensitised solar cells are 3rd generation solid-state photovoltaic devices that imitate photosynthesis in plants. The fundamental concept of DSSCs is that the photoanode is covered by the dye as a sensitiser. Dyesensitised solar cells are preferred over other photovoltaics because they can be processed cheaply, and there is an abundance of materials that can be dispensed as portable devices and used in indoor facilities, such as chargers, solar keyboards, and solar bags [1]. The main components of DSSC are porous crystalline wide-semiconductor bandgap electrodes, dye sensitisers, electrolytes, and counter electrodes [2]. The sensitising dye is a crucial parameter, and the most efficient dyes are ruthenium (II) and osmium (II) complexes with interesting features such as good photon absorption in the visible range of the electromagnetic spectrum, with a long excited lifetime and high-efficiency metal-to-ligand charge transfer [3]. However, they are expensive, contain heavy metals which cause deleterious environmental effects, and require sophisticated preparation techniques [4]. Natural dye sensitisers are an attractive area of research because of the abundance, low cost, nontoxicity, and biodegradability of materials. Natural dye extracts from the leaves, roots, tree bark, and flowers have been studied and applied as sensitisers with acceptable levels of efficiency [5][6][7][8][9][10]. However, few experimental studies provide optical absorption and photoluminescence properties on the interaction of dye extracts with wide-band-gap semiconductor materials; therefore, more fundamental studies on natural dye sensitisers are still needed. In this study, we investigated the optical and photoluminescence properties of Cucurbita maxima dye extract and its adsorption onto TiO 2 nanoparticles for potential applications in DSSCs. Raman spectra confirmed the anatase phase of the TiO 2 nanoparticles. The particle size of 12 ± 2 nm was confirmed through the transmission electron microscope. The optical absorption properties showed two distinct absorption bands in the blue visible (450-500 nm) and red visible (635-674 nm) regions of the electromagnetic spectrum. The photoluminescence spectra of the dye extract and its adsorption onto the TiO 2 nanoparticles showed two peaks in the blue and red regions of the electromagnetic spectrum with no green luminescence. These studies shed light on the fundamental properties of chlorophyll adsorption on TiO 2 nanoparticles and their optical and photoluminescence properties for applications as sensitisers in photovoltaic devices such as DSSCs.

Materials and methods
TiO 2 powder with a particle size < 25 nm (anatase phase) obtained from Sigma Aldrich was used for the preparation of TiO 2 thin films. The soda-lime glass substrate (SLG) was cleaned thoroughly using deionised (DI) water followed by ethanol. The mixture was sonicated for 3 h in a mixture of DI water and ethanol. One gram of TiO 2 nano-powder was dissolved in 300 mg of polyethylene glycol containing a mixture of 6 ml of glacial acetic acid and 6 ml of double-distilled water. The mixture was placed in an ultrasonic bath for three hours. Then, 200 μl was drop-casted on a soda lime glass substrate twice and left to dry at room temperature (30°C). The thin films were annealed at 450°C for 30 min in a tubular furnace in air. The furnace was naturally cooled to room temperature. The annealed TiO 2 nanoparticles on SLG were soaked in the dye for 24 h and then removed and kept in the dark at room temperature for 30 days before further characterisation.

Chlorophyll extraction and adsorption
Pumpkin (Cucurbita maxima) leaves were collected from Ugandan flora and washed with tap water, followed by distilled water, and dried in an oven for 24 h at 60°C. The dried leaves were pounded into a powder form using a mortar and pestle. Pumpkin leaf powder (10 g) was dissolved in 20 ml of ethanol in amber bottles placed in the dark for 36 h. The dye was then filtered through Whatman filter paper number 3 and stored in amber bottles for further investigation.

Characterization methods
The structural properties of the TiO 2 thin films were investigated using Raman spectroscopy with a 532 nm laser. The optical absorption properties of the TiO 2 thin films and pumpkin dye extracts were investigated using a UV-visible-NIR spectrophotometre in the 200-1100 nm range. The photoluminescence (PL) properties of the TiO 2 thin films, pumpkin leaf powder, and pumpkin dye adsorption on TiO 2 nanoparticles were investigated using an FL spectrometer instrument with a 350-nm laser source. High-resolution transmission electron microscopy (HRTEM) with an accelerating voltage of 200 kV was used to investigate the size of the TiO 2 nanoparticles. Scanning electron microscopy (SEM) was used to investigate the surface morphology of TiO 2 nanoparticles. The elemental composition of the TiO 2 nanoparticles was investigated using energy-dispersive x-ray spectroscopy (EDX). The details of the SEM, TEM, PL, and UV-vis-NIR instruments were the same as those reported elsewhere [11].

Structural and surface morphology
The Raman spectra of the pristine and dye-sensitised TiO 2 nanoparticles are shown in figure 1(a). The spectra show vibrational bands at 141 (E g ), 193 (E g ), 393 (B 1g ), 514 (B 1g ), and 636 (E g ) cm −1, which are characteristics of the TiO 2 anatase phase of space group I4 1 /amd [12][13][14]. It is noted that the Raman spectra of dye-sensitised TiO 2 nanoparticles have broader and more intense peaks than those of pristine one, which could be attributed to the reduction in defect states when the dye was adsorbed onto the TiO 2 nanoparticles. The surface morphologies of the pristine and dye-sensitised TiO 2 nanoparticles are shown in figures 1(b) and (c), respectively. The surface consists of agglomerated, rough, and nano-sized randomly distributed particles for the pristine TiO 2 nanoparticles, which is advantageous for dye adsorption. For dye-sensitised TiO 2 nanoparticles, fairly uniform, well-connected, and distributed nanoparticles are observed, which may result in improved photon-to-electric conversion efficiency [15]. The EDX spectra of the pristine and sensitised TiO 2 nanoparticles are shown in the insets of figures 1(b) and (c), respectively. The average elemental composition shows the presence of Ti (∼ 56.6%), O (∼ 43.4%) for pristine and O (∼ 61.2%), Ti (∼ 31.8%) for dye-sensitized TiO 2 nanoparticles. This elemental composition is within the expected stoichiometry. The slight change in the elemental concentration is attributed to the change in the bond length in TiO 2 owing to the adsorption of the dye on the TiO 2 nanoparticles [15]. The TEM image shown in figure 1(d) confirms that the TiO 2 nanoparticles with a normal distribution were fitted using ImageJ software [16]. The TiO 2 nanoparticles showed a normal distribution, with a mean particle size of 12 ± 2 nm, as shown in figure 1(e).

Optical properties of pumpkin dye extracts
The optical absorption properties of Cucurbita maxima leaf dye extract at different concentrations are shown in figure 2(a). Five broad absorption bands were observed in the ranges of <400, 450-500, 521-551, 596-630, and 635-674 nm for all concentrations, with maximum peaks located at 333, 421, 462, 533, 613, and 662 nm, respectively, as shown in figures 2(a) and (b) respectively. A peak at approximately 333 nm represents the existence of C=O or C=C functional groups that are responsible for the n-π * electronic transition that can initiate electrical circuits in DSSCs and can facilitate their adsorption onto TiO 2 surfaces [17,18]. Two prominent absorption bands in the blue visible (450-500 nm) and red visible (635-674 nm) ranges have been observed in other related studies and are attributed to the presence of chlorophyll b and chlorophyll a [19,20]. From figure 1(a), it is observed that the absorbance increases with dye concentration and can follow the Beer-Lambert Law represented by equation (1).
Where A absorbance, ε is the molar absorption coefficient, C is the molar concentration, and l is the optical path length [21]. Broadening of the bands in the range 635-674 nm was observed for higher concentrations, indicating chlorophyll-chlorophyll interactions, as has been observed in similar studies [19]. The bandgap energy of the material in the solid-state form is the difference between the conduction and valence band energies.
In organic molecules, the valence band is represented by the highest occupied molecular orbitals (HOMOs), and the conduction band is represented by the lowest unoccupied molecular orbitals (LUMOs) [22]. The value of the band gap for organic dyes can easily be obtained from the maximum absorption peak in the UV-Visible spectra and computed using the relation where λ max is the maximum wavelength in the absorption spectrum. The calculated band gap using this approach were 3.72, 2.95, 2.68, 2.33, 2.02, and 1.87 eV for λ max corresponding to absorption peaks located at wavelengths of 333, 421, 462, 533, 613, and 662 nm respectively. To ascertain the accuracy of these energy values, the band gap of the Cucurbita maxima dye extract was estimated using the Tauc plot, as shown in figure 2(c) for direct optical transitions. The Tauc plots were based on equation (3).
where α is the material-dependent absorption coefficient, hυ is the photon energy, A is a constant, E g is the optical bandgap, and n is the type of electronic transition, which is 2 for the direct band gap and ½ for the indirect band gap [23][24][25][26]. The direct allowed band gaps were calculated as 1.81 and 2.52 eV. These two band gaps are consistent with the energies corresponding to the maximum absorption peaks at 662 (1.87 eV) and 462 nm (2.68 eV), respectively. These energies represent two optical windows for light absorption by the chlorophyll. Blue visible region and near-red visible region of the electromagnetic spectrum. The values of the bandgap in the two optical absorption windows in the visible range show that the extracted dye can be an effective sensitiser for photovoltaic applications. The Tauc plot method is known to provide approximate values of the band gap because it assumes an ideal parabolic band structure and depends on the type of transition [27]. This method has been adopted to calculate the band gap for chlorophyll a and anthocyanin pigments in related studies with acceptable levels of accuracy [17,19,20,28,29]. The general molecular structure of chlorophyll is shown in figure 2(d).
3.3. Optical absorption properties of TiO 2 3.3.1. Thin film and TiO 2 /Dye adsorption We investigated the optical properties of nanocrystalline TiO 2 thin films on soda-lime glass substrate (SLG) and Cucurbita maxima dye adsorption on TiO 2 nanoparticles using diffuse reflectance spectra (DRS), as shown in figure 3(a). The DRS was then converted to an equivalent absorption spectrum using the Kubelka-Munk (KM) function described in detail elsewhere [24,33]. This function is represented by equation (4). ( ) These terms have their usual meaning, as described elsewhere [24]. The band gap was computed by making a Tauc plot related to equation (5) The terms in equation (5) have their usual meanings, as explained in previous related studies [11,34]. The optical bandgap of the TiO 2 sample was estimated from the Tauc plot, as shown in the inset of figure 3(b), that is, a plot of (αhν) 2 against hν for the direct bandgap material and calculated from the linear fit as 3.33 eV. The absorbance of Cucurbita adsorbed on TiO 2 nanoparticles is shown in figure 3(c), which shows three distinct regions, which correspond to the optical bands for TiO 2 in the Uv region: Cucurbita maxima in the blue visible (450-500 nm) and red visible (635-674 nm) ranges which imply the presence of chlorophyll b and chlorophyll a, respectively, that have been adsorbed on the TiO 2 nanoparticles [19,20]. The inset of figure 3(c) shows the enlarged region of the expected region where the adsorption of Cucurbita maxima dye on TiO 2 nanoparticles is expected. The bandgap was estimated using a Tauc plot, as shown in figure 3(d). The inset of figure 3(d) shows an enlarged region of Cucurbita maxima dye adsorption. Three band gaps were estimated as 3.29 eV corresponding to TiO 2 region, 1.88 and 2.55 eV corresponding to the absorption region of Cucurbita maxima adsorbed on TiO 2 nanoparticles. These studies confirm that Cucurbita maxima can be used as an effective sensitiser for applications in DSSCs.

Photoluminescence properties of TiO 2 thin film and TiO 2 -Pumpkin dye adsorption
Photoluminescence of dye pigments is important for establishing a link between photoemission and the conversion to photoelectricity from natural plant-based dye sensitisers for DSSCs applications [35]. Figures 4(a) -(c) show the photoluminescence emission spectra of TiO 2 nanoparticles, Cucurbita maxima powder dye, and Cucurbita maxima dye adsorption on TiO 2 nanoparticles, respectively. For the TiO 2 nanoparticles, it was found that the maximum peak intensity was approximately 435 nm. Three peaks were observed for C. maxima. A broad peak was observed in the blue region with a maximum intensity of 462 nm, a sharp peak at 675 nm in the red region, and an arm at approximately 726 nm. These peak intensities correspond to energy values of 2.85 eV for TiO 2 and 2.68, 1.83, and 1.71 eV for Cucurbita maxima. Two peaks were identified for the Cucurbita dye extract adsorbed on TiO 2 nanoparticles, two peaks are identified. They are positioned in the blue region at 451 nm, corresponding to 2.75 eV, and red region at 677 nm to 1.83 eV. The energy values were consistent with those obtained by UV-vis spectroscopy. No green emission intensity is observed in the spectra. Similar peaks were observed in other studies [36]. Figure 4(c) shows that the photoluminescence peak at 726 nm disappeared. The disappearance of this photoluminescence peak may improve the optoelectronic performance of the solar cells [37][38][39].

Conclusion and way forward
We provided the crystal structural, optical, and photoluminescence properties of TiO 2 , Cucurbita maxima, and Cucurbita maxima dye adsorbed on TiO 2 nanoparticles for potential application in DSSCs. The optical absorption properties of Cucurbita maxima show two prominent absorption bands: blue visible (450-500 nm) and red visible (635-674 nm). The photoluminescence spectra of the dye extract and its adsorption on the TiO 2 nanoparticles showed two prominent peaks in the blue and red regions of the electromagnetic spectrum, and no significant peak was observed in the green region of the electromagnetic spectrum. This study provides a more fundamental understanding of the applications of chlorophyll as a sensitiser in DSSCs. For future applications in DSSCs, and for effective optical absorption across the entire UV-visible-near-infrared range electromagnetic spectrum, it is recommended to make a composite of Cucurbita maxima dye extract with another pigment that absorbs in the green region to enhance the photo-absorption and improve the photo-to-electric conversion efficiency in DSSCs.