Biotemplated Synthesis of Titanium Oxide Nanoparticles in the Presence of Root Extract of <i>Kniphofia schemperi</i> and Its Application for Dye Sensitized Solar Cells

Bekele, Eneyew Tilahun; Zereffa, Enyew Amare; Gultom, Noto Susanto; Kuo, Dong-Hau; Gonfa, Bedasa Abdisa; Sabir, Fedlu Kedir

doi:https://doi.org/10.1155/2021/6648325

International Journal of Photoenergy

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Abstract Introduction Results and Discussion Conclusion Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2021 | Article ID 6648325 | https://doi.org/10.1155/2021/6648325

Biotemplated Synthesis of Titanium Oxide Nanoparticles in the Presence of Root Extract of Kniphofia schemperi and Its Application for Dye Sensitized Solar Cells

Eneyew Tilahun Bekele,¹Enyew Amare Zereffa,¹Noto Susanto Gultom,²Dong-Hau Kuo,²Bedasa Abdisa Gonfa,¹and Fedlu Kedir Sabir¹

Academic Editor: Kei Ohkubo

Received17 Nov 2020

Accepted08 Feb 2021

Published20 Feb 2021

Abstract

Metal oxide nanoparticles prepared by biological route using green plant parts as a template are eco-friendly as well as yield good results than the conventional methods. This present study focusses on biosynthesis and characterization of TiO₂ NPs using root extract of Kniphofia schemperi for dye-sensitized solar cells. TiO₂ NPs were synthesized using 0.25 M titanium tetra butoxide in the presence of root extract of Kniphofia schemperi with the volume ratios. The analysis result revealed that the synthesized TiO₂ NPs were thermally stable above 500°C and have spherical morphology, with the average crystalline size of 11.7, 8.3, and 8.6 nm, and band gap energy of 3.35 eV, 3.33 eV, and 3.36 eV, respectively, for the TiO₂ NPs prepared at the volume ratios of 2 : 3, 1 : 1, and 3 : 2. Biosynthesized TiO₂ NPs were used as photoanode in dye-sensitized solar cells (a device used for converting absorbed light into electricity). Solar cell devices were fabricated using roots of Kniphofia schemperi sensitizer in the presence of TiO₂ NPs biosynthesized within (2 : 3, 1 : 1, and 3 : 2) volume ratio, which showed power conversion efficiency of 0.039%, 0.117%, and 1.3%. Incident photon to current conversion efficiency (IPCE) analysis using TiO₂ (2 : 3, 1 : 1, and 3 : 2) photoelectrodes showed 6.64%, 2.66%, and 18%. Among the biosynthesized TiO₂ different volume ratios, TiO₂ (3 : 2) NPs showed relatively maximum solar cell efficiency and IPCE value due to its uniform spherical shape that enables to absorb large dye molecules on its surface, and this intern improves device efficiency.

1. Introduction

Energy exists in different forms from burning woods to obtain fire in prehistoric times to electricity productions in the modern age. Its consumption is one of the most important aspects in everyday life activities of human beings. However, the original sources of energy that peoples used to harvest have shown signs of deficiency due to the rapid growth in industrialization in many countries. As a result, increasing concerns about energy crisis, climate change, shortage in fossil fuels, and environmental issues are motivating the researcher to develop clean, sustainable, and renewable energy resources that will power the future [1].

Recent advances in solar energy conversion technologies based on organic semiconductors as light harvesting layer employ metallic oxide semiconductor (MOS) nanostructures for efficient charge extraction and transportation between the electrode and organic molecules [2]. Solar cells based on dye-sensitized and porous nanocrystalline TiO₂ photoanode with attractive performance were first reported in 1991 [3]. Dye-sensitized solar cell (DSSC) is a low-cost cell and belongs to the group of thin film solar cells. The interconnected TiO₂ NP in DSSCs has been widely used as the mesoporous electrode film layer, because it is beneficial for light adsorption with a large amount of dye molecules due to its large surface areas [4].

TiO₂ NPs have been routinely synthesized by several approaches such as hydrothermal, solvothermal, sol-gel, direct oxidation, chemical vapor deposition, electro-deposition, sonochemical, chemical bath deposition, and microwave methods [5]. However, these methods are costly, toxic, not friendly to the natural ecosystem. Nowadays, there is an increased demand to develop TiO₂ with high yielding, low cost, and nontoxicity by exploring with biological sources and eco-friendly methods. The biological method involves the use of environment-friendly materials such as plant extracts (leave, flower, bark, root, seed, and peels), bacteria, fungi, and enzymes in the synthesis of titanium dioxide nanoparticles together with the use of green solvents [6].

The species Kniphofia are herbaceous perennials growing from rhizomes. Kniphofia schemperi is also known as Kniphofia schemperi baker; its root has been traditionally applied for the treatment of various diseases such as menstrual pains, abdominal cramps, intestinal tape worm, wounds, dysmenorrheal, bacterial, and fungal infections, malaria, chest complaint, and gonorrhea, and hepatitis B [7]. Furthermore, the presence of various functional groups within the root extract of Kniphofia schemperi such as flavonoids, carbonyl groups, carboxylic acids, ketones, and phenols could be used for the synthesis of TiO₂ nanoparticles in order to avoid the use of toxic and expensive chemical species for the synthesis process. The root extract of the plant with same family name, Kniphofia foliosa, has been used for synthesis of TiO₂ NPs for antibacterial activity by Eneyew et al. [8]. And under this work, the root extract of Kniphofia schemperi was investigated as a template for the synthesis of TiO₂ NPs and also could be used as a natural sensitizer for solar cell application.

DSSCs using inorganic complex dyes and commercially available and chemically synthesized TiO₂ NPs have been explored and its corresponding performance is also measured. But titanium oxide nanoparticles are not produced using a biosynthesized and eco-friendly method for DSSCs applications as photoanode using root extract of Kniphofia schemperi [9]. To the knowledge of different researchers, green synthesis of TiO₂ NPs using indigenous medicinal plant extract from the root of Kniphofia schemperi for DSSCs application is not done yet. Therefore, the focus of the current work was biosynthesis of TiO₂ NPs in different volume ratios using titanium tetra butoxide and Kniphofia schemperi root extract as a reducing and capping agent and then to fabricate DSSCs using sensitizers obtained from root of Kniphofia schemperi. In the present findings, the root extract of Kniphofia schemperi was played two roles, in one case used as a capping and reducing agent for TiO₂ biosynthesis, and in the second case, it was used as a sensitizer during photoanode preparation to coat the prepared film.

2. Methodology

2.1. Chemicals and Reagents

Chemicals and reagents used in this work were titanium tetra butoxide (Acros organics, 98%), acetone (Sigma Aldrich), acetonitrile (Sigma Aldrich), sodium hydroxide (Sigma Aldrich), iodine crystal (Sigma Aldrich), NaI (BDH), 1-ethyl-3-methyl immidazolium iodide (EMIMM-I: Sigma Aldrich), isopropanol (Riedel-de Haen), PVP (Sigma Aldrich), (C₂H₅)₄NBF₄ (Sigma Aldrich), (ethylenedioxythiophene) (EDOT, Sigma Aldrich), absolute ethanol (Lab Tech Chemicals), triton X-100 (Sigma Aldrich), and conductive FTO glass substrate (Sigma Aldrich). All the chemicals and reagents were of analytical grade and used without further purification.

2.2. Extraction of Root of Kniphofia schemperi

Healthy and fresh roots of Kniphofia schemperi were collected and cleaned using distilled water several times and were dried in a shaded room till all the moisture content of the roots were removed. The dried Kniphofia schemperi root was ground using plant grinding machine. The extraction process was performed by mixing 15 grams of powder of the root with 250 mL of absolute ethanol in a 500 mL erlenmeyer flask. And then, the mixture was allowed to boil at 50°C for about one hour. The extracted solution was collected and stored at 4°C within a refrigerator [10]. The filtrate of the extract was used as a capping and reducing agent for the biosynthesis of TiO₂ NPs within three different volume ratios.

2.3. Biosynthesis of TiO₂ NPs Using Root of Kniphofia schemperi

TiO₂ NPs were biosynthesized by using 0.25 M of the precursor Titanium tetra butoxide, C₁₆H₃₆O₄Ti, and ethanolic root extract of Kniphofia schemperi of different volume ratios: 2 : 3 (40 mL precursor salt versus 60 mL of root extract of Kniphofia schemperi), 1 : 1 (50 mL precursor salt versus 50 mL root extract of Kniphofia schemperi), and 3 : 2 (60 mL precursor salt versus 40 root extract of Kniphofia schemperi). The prepared NPs were labeled as TiO₂ (2 : 3), TiO₂ (1 : 1), and TiO₂ (3 : 2). All solutions of different volume ratios were stirred for about five hours without heating. The precipitated was formed by drop wise addition of solution of sodium hydroxide as a precipitating agent. The solution with a precipitate was centrifuged at 1000 rpm and washed four times with distilled water and absolute ethanol, dried in oven overnight at 100°C, and calcined at 500°C for about four hours [10]. Figure 1 shows the schematic synthesis process of TiO₂ NPs using root extract of Kniphofia schemperi and from its titanium tetra butoxide precursor.

2.4. Characterizations of Biosynthesized TiO₂ NPs

Thermal gravimetric analysis was carried out using a TGA/DTA (DTG-60H Shimadzu Co., South Korea) to determine the calcination temperature of TiO₂ NPs. The infrared spectrocopy FTIR (PerkinElmer 65) was used to identify functional groups of biosynthesized TiO₂ NPs [11–13]. The crystalline structure of the biosynthesized TiO₂ NPs was investigated by X-ray diffraction using X-ray diffractometer (XRD (XRD-7000, Shimadzu Co., South Korea). The UV-Vis absorption spectrum of biosynthesized TiO₂ NPs was confirmed using UV-Vis spectrophotometer (JASCO V-670) with scan wavelength of 200 to 800 nm [14–16]. The morphology and microstructure of biosynthesized TiO₂ NPs were observed using field emission SEM-EDS (FE-SEM, JEOL-JSM 6500F, Japan) and high-resolution TEM (HRTEM, Tecnai F20 G2, Philips, Netherlands).

2.5. Extraction and Preparation of Sensitizer

Roots of Kniphofia schemperi were collected from Bale National Park, Oromia regional state, Ethiopia. Those collected samples were surface cleaned repeatedly and allowed to dry under shadow at room temperature. Then, it was ground into powder and packed in a plastic bottle by covering with aluminum foil and storing it in a dark area. The extract was obtained by taking 4 g powders of roots of Kniphofia schemperi to be socked in an erlenmeyer flask with 100 mL of absolute ethanol. The mixture solution was placed in a dark place and covered with aluminum foil to avoid photooxidation. The solution was stored for about 24 h at room temperature until the required pigment solution was extracted. Then, the solid materials were hold still overnight, and the clear solution of the dye was obtained. The clear solution to be used for sensitization purpose was separated from solid materials using glass filters [17, 18].

2.6. Fabrication Steps of DSSCs

2.6.1. Preparation of the Substrate for Deposition

The conductive fluorine-doped tin oxide (FTO) glass substrate with an area of was cleaned repeatedly with distilled water and acetone, isopropanol, and ethanol sequentially under ultrasonication for about 20 minutes in each different solvent in order to remove organic and inorganic dirts from the surface of the substrate. The substrates were thoroughly cleaned before deposition of films of biosynthesized TiO₂ (2 : 3, 1 : 1, and 3 : 2) NPs was carried out.

2.6.2. Semiconductor Oxide Material Paste Preparation and Deposition

TiO₂ paste was prepared according to the method developed by Nazeeruddin et al. [19]. As an example, 0.76 g powder from each kind of TiO₂ volume ratio was ground in a porcelain mortar with a drop-wise addition of 1 mL of H₂O containing 0.56 mL of 1 M HNO₃ to prevent reaggregation. After the powder had been dispersed by the high shear forces in the viscous paste, it was diluted by additional drop-wise of DH₂O for continuous grinding. Finally, 0.025 mL of Triton -100 was added as a dispersing agent followed by grinding well with the mixture for additional 30 minutes until the paste became homogeneous. The prepared paste was deposited near the top edge of FTO glass between the two pieces of the tape and then was coated by tape casting skill with a doctor blade [20–22]. This process was continued until the layer became uniform and smooth. The tape was removed with care without scratching the TiO₂ coating. Figure 2 shows the corresponding paste preparation procedures using biosynthesized TiO₂ NPs and its deposition on to the cleaned FTO glass substrate.

The as-prepared biosynthesized TiO₂ film was fired at 450°C in a muffle furnace for about 30 minutes to remove the organic solvents. This procedure also ensured electrical contact with good adhesion between the particle film and substrate.

2.6.3. Electrode Sensitization

Sensitizers obtained from roots of Kniphofia schemperi dye were adsorbed on top of TiO₂ photoelectrodes as follows. The TiO₂ film-coated electrode was immersed into the solutions of roots of Kniphofia schemperi (dye) until the films were fully soaked with the dye solution for about 12 hours (shown in Figure 3(a)). The TiO₂ film-coated substrates with the adsorbed molecules of dye were taken out and rinsed with the same solvent used in dye extraction (absolute ethanol) to remove the unadsorbed dyes from the surface of the TiO₂-photoanode [19].

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2.6.4. Preparation of Redox Electrolytes

The polymer gel electrolyte was prepared according to the method developed by Fan et al. [23]. This was done by taking 4 mL 1-ethylene-3-methyl immidazolium iodide (EMIM-I) of 0.9 M and 25 mL acetonitrile, a high dielectric constant organic solvent, to form a homogeneous liquid electrolyte of 5.3 g under continuous stirring. In order to obtain a better conductivity, 0.5 M of sodium iodide was dissolved in the above homogeneous liquid electrolytes and then 0.12 M iodine and 35% () of polyvinyl pyrrolidone were added. Figure 4 reveals the formation of the quasisolid-state redox electrolyte from its components. The resulting mixture was heated at 70-80°C under vigorous stirring to dissolve the added PVP followed by cooling down to room temperature. Figure 4(b) with upside-down vial containing the redox electrolyte shows that the prepared electrolyte was really quasisolid (do not flow).

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2.6.5. Coating the Counter Electrodes

The counter electrodes were prepared through electrochemical polymerization of 3, 4-ethylenedioxythiophene (EDOT) in a three-electrode cell by using an electrochemical analyzer. The electrochemical cell was consisted of FTO-spattered (conductive) glass substrate as the working electrode, a platinum foil as the counter electrode, and quasisilver/silver chloride as reference electrode. The solution/electrolyte system used for the preparation of counter electrode was performed by mixing 0.1 M EDOT and 0.1 M (C₂H₅)₄NBF₄ in acetonitrile solvent. The polymerization was carried out at a potential of +1.8 V for about 2 seconds through the bulk electrolysis with coulometer. At this potential, the electrode surface was covered with a blue-doped PEDOT film [24]. FTO glass substrates that were coated with PEDO counter electrode is shown in Figure 3(b), and then, completed device structures of the resulting DSSCs depicted in Figure 3(c) were ready for J-V and IPCE characterization.

2.6.6. Characterization of Fabricated DSSCs (J-V and IPCE)

The current-voltage characteristics of fabricated solar cells were measured using a computer controlled electrochemical analyzer (CHI630A). A 250 W xenon lamp regulated by an Oriel Power Supply (Model 68830) was used to illuminate light on the DSSC samples. The intensity of the incident light was set at 100 mW·cm^-2. The DSSC solar cell was then mounted in a sample holder inside a metal box with an area of 1 cm² opening to allow light from the source. A grating monochromator (Model 77250) was placed into the light path and used to select a wavelength between 300 and 800 nm with a 20-minute interval during measurement of incident photon to current conversion efficiency (IPCE).

3. Results and Discussion

3.1. Thermal Gravimetric and Differential Thermal Analysis (TGA-DTA)

TGA-DTA was performed as a function of temperature as it can be shown in Figure 5. The analysis was done by taking 5.94 mg of the uncalcined biosynthesized TiO₂ (1 : 1) NP sample. The observed weight loss below 150°C was due to the evacuation of physically and chemically entangled water molecules from the surface of nanomaterials. In the temperature range of 150-355°C, the weight loss was also occurred due to the process of pyrolysis and carbonization of biomass materials from the synthesized TiO₂ NPs [25].

Weight loss was again observed and continued up to 483.15°C that was associated with a strong exothermic peak, which can be attributed to the vaporization of carbonized residues present over the surface of the biosynthesized nano-TiO₂. Similar result related to the present findings was also reported [12]. After 483.15°C-900°C, no considerable weight loss was observed as observed under the spectrum; therefore, 500°C was chosen as the calcination temperature for the biosynthesized TiO₂ NPs formed within three different volume ratios.

3.2. XRD Analysis

The crystalline nature of the biosynthesized TiO₂ NPs was analyzed using X-ray diffraction measurements. Figure 6 shows the XRD crystalline nature of the biosynthesized TiO₂ NPs formed within three different volume ratios. The diffraction peaks were observed at the 2 values of ≈25.3, 38.0, 48.0, 53.2, 54.9, 62.7, 70.2, and 75.0, respectively, and along with their correspondence miller index values of (101), (004), (200), (105), (211), (204), (220), and (215), respectively. The diffraction peaks were found to be in a good agreement with JCPDS card number of 21-1272, which indicated the formation of the TiO₂ in the form of anatase. Diffraction peaks related to second phases were not observed, which confirmed that the biosynthesized TiO₂ NPs synthesized within 2 : 3, 1 : 1, and 3 : 2 ratios were pure.

The average crystalline sizes were calculated based on Debye Scherer’s equation [12], and the values were found to be 11.7, 8.3, and 8.6 nm for 2 : 3, 1 : 1, and 3 : 2 volume ratios, respectively. Similar result related to the present study was also reported [26]. As compared to XRD result of TiO₂ (3 : 2) ratio, TiO₂ (1 : 1) ratio had a relatively smaller average crystalline size of ≈8.3 nm. This difference might be due to the fact that the excess amount of root extract added and used as a template to block the grain growth of TiO₂ (1 : 1) relative to the remaining two ratios of TiO₂ NPs. As it was observed from the XRD spectrum, TiO₂ (2 : 3) NP losses their crystalline nature due to the excessive addition of the root extract of Kniphofia schemperi. The excessive root extract with the polymer chain structure is difficult to completely burn out at 500°C; therefore, those residues on TiO₂ NPs block the diffraction signals with XRD analysis.

3.3. SEM-EDS Analysis

Morphology of the formed nanoparticles within three volume ratio of precursor salt and that of Kniphofia schemperi ethanolic root extract has been studied with the help of SEM-EDS. Figure 7 depicts the SEM images of TiO₂ NPs biosynthesized in the volume ratios of (a) 2 : 3, (b) 1 : 1, and (c) 3 : 2 and the corresponding EDS analysis of TiO₂ from the selected area in SEM image. From SEM images, the morphology of TiO₂ NPs was found to be spherical in shape with distinct edges. SEM image also revealed the increase in the particle size with the corresponding increase in the amount of ethanolic root extract of Kniphofia schemperi [27]. Agglomeration of the particles in Figure 7(a) is attributed because of the presence of excess amount of root extract [28]. To confirm and gather different information, a further insight about the biosynthesized TiO₂ nanoparticles formed within a volume ratio of 2 : 3, 1 : 1, and 3 : 2, analysis of the sample using EDS techniques was performed. It is necessary to verify the presence of desired elements in the biosynthesized TiO₂ NPs formed as 2 : 3, 1 : 1, and 3 : 2. The absence of any foreign materials in all of the three ratios indicates the high purity of our synthesized TiO₂ NPs [29].

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3.4. TEM and SAED Analysis

In order to assure and get further information and insights about the feature of the TiO₂ NPs, TEM, HRTEM, and SAED analysis were carried out. Figure 8 shows TEM micrograph and the selected area electron diffraction (SAED) pattern as well as the correspondence high-resolution transmission electron microscope (HRTEM) image of TiO₂ (1 : 1) NPs. The image showed the nanoparticles of ~10 nm in size as also supported by the result of XRD analysis. The SAED pattern with circular rings for (101), (004), (200), (105), (211), (204), (220), and (215) planes indicated the polycrystalline nature of Kniphofia schemperi root mediated synthesized TiO₂, as supported by the XRD analysis results [12].

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The image in Figure 8(c) showed HRTEM of anatase TiO₂ NPs. A fringe width of 0.355 nm Figure 8(b) in particle demonstrates for the (101) plane of anatase biosynthesized TiO₂ NPs. The lattice fringes also display the nanocrystalline nature of the biosynthesized TiO₂ NP anatase phase form.

3.5. UV-Vis Absorbance and Tauc Plot Analysis

Figure 9(a) shows the UV-Vis absorption spectra of TiO₂ NPs prepared at different volume ratios of 2 : 3, 1 : 1, and 3 : 2 in terms of the 0.25 Ti precursor salt and root extract of Kniphofia schemperi. The absorbance of TiO₂ NPs showed spectral shift for TiO₂ NPs at different precursor ratios. Compared to the very low absorption of the reference TiO₂ NPs, our TiO₂ NPs biosynthesized at different volume ratios exhibited a slight shift of the absorption edge towards the visible region due to their smaller size. The energy bandgaps of the biosynthesized TiO₂ NPs were estimated using the Tauc’s plot method technique [15]. With the Tauc plot in terms of the versus hν in Figure 9(b), the energy bandgaps ()f TiO₂ NPs were determined to be 3.35, 3.33, and 3.36 eV for the different volume ratios of TiO₂ synthesized within a volume ratio of 2 : 3, 1 : 1, and 3 : 2, respectively. The slight variation in for TiO₂ NPs is due to the variation in volume ratio between Ti precursor salt and the root extract of Kniphofia schemperi that results in light absorption at different UV-Vis regions of absorption.

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3.6. Functional Group Analysis Using FTIR

Fourier transformation infrared spectroscopy analysis was done to identify and confirm the role of biomolecules that was responsible for capping and reducing of the bioreduction of TiO₂ NPs. Figure 10 displays the FTIR spectra analysis of root powder of Kniphofia schemperi (a), calcined biosynthesized TiO₂ NPs (b), and uncalcined biosynthesized TiO₂ NPs (c). Figure 10(a) shows the FTIR spectrum of root powder and the peaks located at 3409.81, 2922.63, 2840.53, 2327.02, 1620.65, 1387.52, and 1065.32 cm^-1. The bands centered at 3409.81 cm^-1 are attributed to 𝛿-H₂O bending and vibration of hydroxyl groups present in the root of Kniphofia schemperi. This peak also refers to the H-bonded alcohols and phenolic compounds. These bonds are related to the chemical composition of root of Kniphofia schemperi. The week bands found at around 2922.63 and 2840.53 cm^-1 represents H-C-H and -C-H groups associated by H-bond. The peaks appeared in the region of 1620.65 cm^-1 are ascribed to the C=O bond of aromatic rings present in the root of Kniphofia schemperi. This region also relates to O-C-O asymmetric and symmetric stretching of carboxylic acids and can be attributed to the stretching of hydroxyl and water molecules too.

The peak presented around 1620.65 cm^-1 could also represent the C=C bond of saturated hydrocarbons. The characteristic peaks of 1387.52 represents the C-H rock alkenes, while the peak observed at around 1065.32 cm^-1 indicates the presence of C-O stretching alcohols, carboxylic acids, esters, and ethers. This peak may also depict the existence of aliphatic amines [30].

Figure 10(b) showed the FTIR spectrum of calcined biosynthesized titanium oxide nanoparticles, and the peaks are observed at around 3422.97, 2327.02, 1634.60, 1380.55, and 551.81 cm^-1. The broad absorption peak observed at 3422.97 cm^-1 represents the presence of Ti-OH bonds. The characteristic peak shown at 2327.02 cm^-1 indicates the alkene group with C=H stretching. The medium absorption peak at 1634.60 cm^-1 represents the C=O stretching of primary amide or to C=C groups of aromatic rings that plays as stabilizer and reducing agents during the stage of synthesis. This band also relate to the bending mode of vibration of O-H group and flavones. The peak represented by 1380.55 cm^-1 refers to the aromatic ethers of C-O stretching and also the C-H rock like alkene groups. Peaks of C=O and C=C clearly indicates the presence of biomolecules and bioactive constituents from roots of Kniphofia schemperi extract that was used as a capping and reducing agent during the synthesis process of TiO₂ NPs within different volume ratios. The peak at 551.81 cm^-1 is indicating the presence of Ti-O stretching vibration, and this confirms the formation of metal-oxygen bonding [31]. The analysis confirms the formation of TiO₂ with many surface hydroxyl function groups. The extra peaks from the organic C and O bonds are attributed to a trace amount of extract pyrolytic residues left after a relatively lower temperature firing procedure. These results indicated that the presence of alcohols, phenols, primary amines, ketones, and aliphatic amines in the root extract of Kniphofia schemperi that have been participated in the process of synthesis of TiO₂ NPs is responsible for the bioreduction of TiO(OH)₂ to TiO₂ NPs [30].

As it can be depicted under Figure 10(c), the peaks observed at 3409.81, 2922.63, 2320.05, 1620.65 1319.36, 1038.21, and 777.97 cm^-1 represent O-H bond stretching due to adsorbed moisture, the CH, CH₂, and CH₃ functional groups linked via H- bonds, C=O bond stretching that might be due to the presence of adsorbed carbon dioxide on the surface of TiO₂, the surface water and hydroxyl groups, the C-H of alkenes, and the C-O bond of stretching of alcohols, carboxylic acids, ethers, and esters, respectively. In addition to this, the FTIR analysis indicated in Figure 9(c) shows the broad band centered at round 777.97 cm^-1 which represents a characteristic peak to be believed due to the interaction of the biosynthesized nanooxide with biomolecules of the extract. As compared to the FTIR spectra of root powder and uncalcined TiO₂ NPs, the absence of different functional groups located at around 2922.63, 2840.53, and 1038.21 cm^-1 in the calcined biosynthesized TiO₂ NPs confirms that these functional groups were eliminated during the firing process. Therefore, this characterization technique provides information as the extract plays great role and removed after calcination process was carried out [31].

3.7. Absorption Spectra of Root Extract of Kniphofia schemperi Sensitizer

Figure 11 reveals the corresponding optical absorption spectra of root of Kniphofia schemperi natural sensitizer. The roots of Kniphofia schemperi sensitizer show a strong absorption band at around 231 nm. The absorption of Kniphofia schemperi ascribes to the component, namely, anthocyanin, a group of natural phenolic compounds [32]. This result confirms that the pigment extracted from this natural dye can be used as a photosensitizer because it strongly absorbs photons in a wide UV-Vis region of electromagnetic radiation. One of the essential requirements for photosensitizer with the best performance is to have the broad and intense absorption in the UV-Vis and near-IR regions of the solar spectrum.

3.8. Current Density-Voltage Characteristics of TiO₂-Based DSSCs

The photovoltaic tests of DSSCs prepared with a natural sensitizer extracted from roots of Kniphofia schemperi were performed by measuring the current density-voltage (J-V) characteristic curves by using Oriel Class Solar Simulator under the standard light intensity of illumination at 100 mW/cm². The performance of the DSSCs prepared using different volume ratio of biosynthesized TiO₂ NPs and the roots of Kniphofia schemperi sensitizer were evaluated by different parameters such as short circuit current density (), incident photon to current efficiency also known as quantum efficiency (), open circuit voltage (), and fill factor (FF).

Figure 12 shows the J-V curves of root of Kniphofia schemperi-based DSSCs prepared using TiO₂ photoanode at different precursor ratios. The compiled data from the J-V curves were presented in Table 1. The solar cell with TiO₂ (3 : 2) biosynthesized photoelectrodes shows the highest value of 1.3%, the highest of 161 mV, the highest of , and slightly lower FF of 32.8%. The improvement in value of TiO₂ (3 : 2)-based DSSC is basically due to the improved absorption of light caused by the wider range of absorption of sensitizer adsorbed on nanoparticles, as supported with Figure 12, while the enhanced efficiency attributed due to the improved crystallinity and morphological nature of TiO₂ photoelectrodes to allow more dye adsorption on its surface, larger band gap energy, and higher interaction between TiO₂ nanocrystalline film and the root extract of the sensitizer for better charge transfer [33].

In general, the efficiencies of DSSCs not only depend on the molecular structure of the natural pigment but also depend strongly on the properties of the biosynthesized photoelectrodes such as crystallinity, morphology, and aggregation of the dye molecules [34]. It was earlier reported that the power conversion efficiency of most of the DSSCs made from natural dyes and some organic synthetic dyes suffers from the low open circuit voltage, which is also the case in the present study where the open circuit voltage ranged from 29 to 161 mV for the biosynthesized photoanode with the different precursor volume ratios.

3.9. IPCE Characteristics of Biosynthesized TiO₂-Based DSSCs

Photoaction spectra provide further insights on the photoelectrochemical performance of biosynthesized photoelectrodes with the presence of sensitizers. As it can be observed from Figure 13, relatively maximum IPCE% value was obtained using TiO₂ (3 : 2) in the presence of ethanolic root extract of Kniphofia schemperi sensitizer, and this value is 8.11%, which was located at a wavelength of 340 nm. The improvement in IPCE% value relative to the remaining photoelectrodes results from higher electron transportation at the electrolyte interfaces and good electrocatalytic activity of the phototelectrode [34]. Again, TiO₂ (1 : 1) with the presence of Kniphofia schemperi sensitizer possess an IPCE% of 2.66% which occurs at 500 nm. This action spectra response is contributed by sensitizer.

Low IPCE% value was recorded using biosynthesized phototelectrode obtained within a 2 : 3 ratio, and the value was estimated as 1.27% at a wavelength value of 800 nm. This low value of IPCE% may be attributed due to inefficient light harvesting efficiency (LHE) by the dye, that is, inefficient charge injection into the prepared TiO₂ film or inefficient collection of injected electrons [35].

For devices of DSSCs (with 2 : 3 volume ration of TiO₂ film), the IPCE values of 1~2% were recorded at 500-800 nm as shown in Figure 13, while negligibly small amount of absorption was observed for the film (Figure 9(a)). The IPCE value of DSSCs depends not only on the light-absorbing nature of semiconductor (TiO₂ NPs) but also on light absorption of sensitizer (dye), absorption and interaction of redox electrolyte, and the interaction of dye molecules with TiO₂ film.

According to previous reports [36–38], the redox couple (quasisolid-state electrolyte) involves in light absorption (in certain amount) in visible region even beyond 500 nm and has a contribution for IPCE values. The films of photoanode of TiO₂ NPs (with anatase phase structure) are highly porous. These porous structures of TiO₂ NPs provide larger surface area for anchoring of dye molecules [39, 40]. Light-absorbing dye molecules attached to the semiconductor (TiO₂ NPs) through their functional groups such as carboxylic acid, phenol, and flavonoids interact with titanium metal as ligand in the film. As of some other reports [41], the ligand-metal (push-pull) type of interaction established between dye molecule and TiO₂ NPs contributes for significant charge separation, minimizes electron-hole recombination, and minimizes series resistance between semiconductor (TiO₂ NPs) and light sensitizer that confirms the higher efficiency of photon-to-electron conversion or incident photon to current conversion efficiency (IPCE%).

4. Conclusion

In conclusion, TiO₂ NPs had been successfully biosynthesized using titanium tetra butoxide precursor salt and Kniphofia schemperi root extract at different volume ratios. The biosynthesized NPs were thermally stable above 500°C. XRD analysis proved that the average particle size of the synthesized TiO₂ NPs was found in the range of 8.3-11.7 nm. SEM-EDS revealed morphology of the biosynthesized TiO₂ NPs to be spherical and the composition with only the elements of Ti and O. The calculated bandgap energies were in the range of 3.33-3.36 eV. Functional group analysis indicated the presence of a trace amount of extract pyrolytic residues left. Among the TiO₂ NPs biosynthesized at different precursor ratios, the best solar cell conversion to electricity was obtained for Kniphofia schemperi-based dye-filled DSSC using TiO₂ (3 : 2) photoelectrodes which was estimated as 1.3%.

Data Availability

The research data used to support the findings of this study are included in the article.

Conflicts of Interest

The authors fully declare that they have no conflicts of interest.

Acknowledgments

The authors highly acknowledged Adama Science and Technology University (ASTU) for funding this research work through ASTU’s 12^th cycle research grant effectively. The authors offer their gratitude to the Department of Materials Science and Engineering at Adama Science and Technology University for allowing facilities such as thermal analyzer and XRD. Department of Chemistry at Addis Ababa University was also thanked by authors for allowing FT-IR analysis and computer controlled electrochemical analyzer (CHI630A).

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Copyright

Copyright © 2021 Eneyew Tilahun Bekele et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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International Journal of Photoenergy

Biotemplated Synthesis of Titanium Oxide Nanoparticles in the Presence of Root Extract of Kniphofia schemperi and Its Application for Dye Sensitized Solar Cells

Abstract

1. Introduction

2. Methodology

2.1. Chemicals and Reagents

2.2. Extraction of Root of Kniphofia schemperi

2.3. Biosynthesis of TiO2 NPs Using Root of Kniphofia schemperi

2.4. Characterizations of Biosynthesized TiO2 NPs

2.5. Extraction and Preparation of Sensitizer

2.6. Fabrication Steps of DSSCs

2.6.1. Preparation of the Substrate for Deposition

2.6.2. Semiconductor Oxide Material Paste Preparation and Deposition

2.6.3. Electrode Sensitization

2.6.4. Preparation of Redox Electrolytes

2.6.5. Coating the Counter Electrodes

2.6.6. Characterization of Fabricated DSSCs (J-V and IPCE)

3. Results and Discussion

3.1. Thermal Gravimetric and Differential Thermal Analysis (TGA-DTA)

3.2. XRD Analysis

3.3. SEM-EDS Analysis

3.4. TEM and SAED Analysis

3.5. UV-Vis Absorbance and Tauc Plot Analysis

3.6. Functional Group Analysis Using FTIR

3.7. Absorption Spectra of Root Extract of Kniphofia schemperi Sensitizer

3.8. Current Density-Voltage Characteristics of TiO2-Based DSSCs

3.9. IPCE Characteristics of Biosynthesized TiO2-Based DSSCs

4. Conclusion

Data Availability

Conflicts of Interest

Acknowledgments

References

Copyright

2.3. Biosynthesis of TiO₂ NPs Using Root of Kniphofia schemperi

2.4. Characterizations of Biosynthesized TiO₂ NPs

3.8. Current Density-Voltage Characteristics of TiO₂-Based DSSCs

3.9. IPCE Characteristics of Biosynthesized TiO₂-Based DSSCs