Environmentally benign synthesis of TiO2-ZnO nanocomposite for efficient dye-sensitized solar cell

This work presents betanin dye-loaded TiO2-ZnO thin film solar cells for solar energy harvesting. The TiO2-ZnO nanocomposite was prepared by the one-step microwave-assisted technique. Structural studies exhibit mixed phases of rutile and wurtzite structure of TiO2 and ZnO respectively. The morphological investigations of deposited TiO2-ZnO nanocomposite showed interconnected many-fold nanoflakes morphology. EDS confirms the formation of stoichiometric TiO2-ZnO nanocomposite films. An optical study demonstrates electronic transition with a bandgap energy range of 2.72 to 2.94 eV. Photovoltaic performance shows photocurrent from 1.62 to 2.73 mA cm–2 with the photovoltage of 659–795 mV in the range with a 3.25% photo-conversion efficiency for the sample of TZO3 dye-sensitized solar cell (DSSCs).


Introduction
Our current civilization is rich in complex and advanced technical pieces of equipment, but still, the technologies require additional invention to work quicker and better. The gadgets should consume lower energy, be non-polluting and cope with future needs. A search for an energy-generating, storing, and finest energy-saving gadget using photochemistry and photovoltaic might essentially help tackle the energy crisis. Solar energy is regarded as the most long-term and dependable renewable energy source. The rationale for this is that solar energy is plentiful and could produce 7500 times more electricity than the present world energy usage [1]. Many studies have been conducted to see how novel technologies like photovoltaic cells and thermoelectric cells have been used earlier [2,3]. Till-date, photovoltaic cells have been proven to be the most promising for harnessing solar energy. DSSCs have become the focus of interest because of the reasons like exceptional manufacturing simplicity, cost-effectiveness, and high conversion efficiency. The strategies adopted to improve DSSCs performance are mostly the dye and the semiconductor centric and need to optimize for the maximum light absorbance, better electron transportation, and the electronic communication between the dye and the semiconductors [4,5]. The growth related to clean and renewable energy technologies is critical in the situation of global warming, energy demands, and the diminution of fossil fuels. Solar cells [6,7] are one of the most practical methods for converting solar energy directly into electricity. Photo-sensitive solar cells are an alternative to typical silicon-based solar cells because of their speedy manufacturing process, low costs, and environmental advantages. The photo-sensitized solar cell has the potential to replace silicon-based solar cells.
Solar cells based on silicon have been used in commercial applications for a long time [8,9]. However, producing silicon and building a gadget from it takes more energy, time, and money. Michael Grätzel and his coworkers revealed a Dye-Sensitized Solar Cell (DSSCs) in 1991 that parallels photosynthesis at a minimal cost, time, and power [10]. With the use of nanotechnology, DSSCs have progressed from 3% to 13% efficiency since their discovery [11]. Electron transport material (ETM) (Photocathode) and the light-absorbing material (LAM) are the two essential electrodes whose performance in DSSCs is crucial in achieving greater efficiency. In general, the dye used as a sensitizer in DSSCs has a substantial impact on light absorption and subsequent electrical conversion [12]. The ruthenium complex as the dye has emerged as the most efficient, with a photoconversion efficiency of up to 14% [13]. However, ruthenium's high cost, toxicity, and scarcity, as well as multistep and difficult preparation techniques and purification, are the key roadblocks to commercialization. Following Gratzel and Kay's pioneering work on the utilization of natural chlorophyll and anthocyanin dyes in DSSCs [14], hopefully, the hunt for stable, low-cost, easily available/synthesizable natural/synthetic dyes continues. The photoanodes used in DSSCs are generally made up of employing transparent fluorine-doped tin oxide (FTO) as a base.
Because of the great expectations and vast prospects, this study has become the most fascinating and vital topic [15][16][17]. In recent years, semiconductor oxides like dyes can also be tuned to attain the required characteristics for specific use [18][19][20][21]. The most popular method involves doping with other elements and alloying/composite making. The latter can be evolved into nanostructures, for example, ZnO has a variety of morphologies like nano-needles, nano-combs, and nanostructures [22][23][24]. The addition of the second metal oxide can improve the light harvest spectrum and the number of electron pairs being harvested. The composite semiconductor oxides were also helping to create composite networks with better characteristics. Transition metals including nickel, copper, lanthanum, and vanadium have been doped into TiO 2 to increase its photovoltaic performance and catalysis [25][26][27][28][29]. Nevertheless, substantial research has been done on the combination of TiO 2 and ZnO in the past. A wide bandgap of ZnO and TiO 2 semiconductors absorbs UV light exclusively in the UV region [30][31][32]. A composite can absorb visible light with a wavelength of 400 nm and so increase photoreaction. Increased charge separation in the composite can also help to avoid electron-hole recombination. Because ZnO and TiO 2 have comparable bonding strength, similar energy gaps, and almost identical energy levels, the combination of the two oxides can generate a nanocomposite with superior physicochemical characteristics [33][34][35][36]. In this context, utilizing metal oxide nano-composites as n-type metal oxide semiconductors would have a large impact on the process of charge separation and injection. Nanocomposite materials provide a variety of benefits, including ease in the manufacture of varieties of shapes, safety, and reduced fabrication costs [37].
Considering the important role of the semiconductor, we developed and characterized a TiO 2 -ZnO nanocomposite semiconductor thin film over FTO, using microwave-assisted techniques. The photoanodes were prepared by coating semiconductor film with a stable, most commonly available, reluctant to degradation, betanin dye as a photo-sensitizer [38]. Betanin is photo-sensitive, capable of absorbing photons, presents a simple structure, and has few degradation paths in conventional iodide-tri-iodide redox couples appear to be ideal as simple, true energy-generating solar cells [39]. Additionally, Many researchers have been utilized costly and toxic dye (ruthenium base dye, perovskite-containing pb etc). In this work, we implemented the stable and cost effective natural dye betanin as a sensitizer. We observed ∼3.25% PCE without using any Ru based dyes, the addition of any enhancers, stabilizers, and charge transport facilitator agents so as to make the DSSC for solar energy harvesting [40] , Sodium hydroxide (NaOH, 98%) was procured from Thomas Baker. Titanium tetra-isopropoxide and zinc nitrate were utilized as a source of titanium and zinc respectively. Hydrochloric acid was used to adjust the pH of the solution.
For the synthesis of TiO 2 -ZnO thin films, typically 10 ml (0.3 M) Titanium tetraisopropoxide solution was taken in a 100 ml beaker and homogenized with doubly distilled water using a magnetic stirrer. A 10 ml (0.3 M) zinc nitrate solution was added to the above solution under continuous stirring for over half an hour. The relative quantities of both the precursors were taken in different proportions to get the desired composites as shown in table 1.
The reaction bath was stirred for 2 min and the bath was kept in a microwave oven. The bath was irradiated with a microwave power of 180 W to provide homogeneous heating. The microwave power is adjusted to reduce overheating above 180 W the precipitation was observed. The TiO 2 -ZnO nanocomposite powder was formed after 2 h of microwave irradiation. This powder was subjected to annealing in a muffle furnace at 450°C for 2 h to improve the crystal growth of the sample. Similarly, pure samples of TiO 2 (TO) and ZnO (ZO) were synthesized and annealed at 450°C for 2 h to improve the crystal growth of the sample.
The TiO 2 -ZnO nanocomposite thin film deposited on the conducting base fluorine-doped tin oxide (FTO) glass substrate. The FTO was cleaned ultrasonically successively in isopropyl alcohol, acetone, and doubly distilled water thoroughly. The Pure TiO 2 , ZnO, and TiO 2 -ZnO nanocomposite thin films were deposited over precleaned slides using a screen printing technique. The photo-anode was prepared by dipping the as-prepared pure TiO 2 , pure ZnO, and TiO 2 -ZnO films in the aqueous 1% betanin dye solution and allowed to soak overnight in dark. A betanin dye-loaded pure TiO 2 , pure ZnO, and TiO 2 -ZnO film appears as a reddish-orange color. In the typical fabrication of a photoelectrochemical cell, the deposited TiO 2 -ZnO nanocomposite films are used as the working electrode, Graphite is used as the counter electrode, and polyiodide solution is used as the redox electrolyte. The PEC cell configuration was equipped as FTO|TiO 2 -ZnO/Betanin dye| Polyiodide solution| Graphite. A flow chart diagram of the methodology adopted to prepare dye-loaded TiO 2 -ZnO-based film is depicted in figure 1. The TiO 2 :ZnO composition having the ratio of 1:0, 1:3, 1:1, 3:1, and 0:1 have been synthesized and designated as TO, TZO1 TZO2, TZO3, and ZO respectively.

Characterization techniques
The properties of synthesized pure TiO 2 and pure ZnO and TiO 2 -ZnO nanocomposite were determined using various characterization tools. The structural study and phase identification was carried out by using x-ray diffraction Crystal structure of the thin films were analyzed by using an x-ray diffractometer (XRD) (Brucker AXS D 8) using Cu-Kα radiation (λ = 1.5406 Å) in the 20°-80°at 2θ range. The morphology and the composition of Titanium, Zinc, and Oxygen were illustrated by using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) (JEOL-JSM Model-6360A). The crystalline nature and particle size of

Result and discussion
3.1. UV-Visible spectroscopy of betanin dye Betacyninis are found in the Beta vulgaris plant. It is extremely effective in absorbing visible light. Beta vulgaris was used to extract the Betanin coloring. Betanin is an excellent antioxidant with a great propensity for forming complexes with metal ions. Alteration of light absorption properties can be achieved by Co-pigments of Betanins. The carboxylic group (-COOH) in Betanin aids in the binding of dye molecules to the surface of pure TiO 2 , pure ZnO, and TiO 2 -ZnO nanocomposite thin films. The two hydroxyl groups on the ortho locations of the betanin's aromatic ring can additionally link to semiconductors and execute forward and reverse electron transport pathways, resulting in the enhanced electronic coupling of the dye with metal oxide [41]. The maximum absorption of Betanin has been observed at 545 nm as shown in figure 2(a).
The betanin structure comprises a glyconebetanidin, which is coupled to the -C-S glucose unit because of its low cost, ease of processing, non-toxic nature, and natural pigment, conversion of light energy to electricity can be achieved by natural pigment because it can act as sensitizers.   [42]. The investigations on the frontiers of the betanin have shown that the utmost empty molecular orbital was found on the benzene ring [40]. Benzene ring can lose electron density during absorption while the Dihydropyridine (DHP) ring gains charge, resulting in a partially excited state electron transition [43].
The XRD pattern was recorded with a 2θ range between 20-80°. TiO 2 exhibits a rutile phase (JCPDS file No. 21-1276), while ZnO occurs in the hexagonal wurtzite phase (JCPDS file No.36-1451). In addition, mixed major prominent peaks of TiO 2 and ZnO were seen with the rutile and hexagonal wurtzite crystal structure of TiO 2 and ZnO respectively in the annealed sample of TZO1, TZO2, and TZO3. It is confirmed that the composite samples show the presence of the combined phases of TiO 2 and ZnO. The peaks of TiO 2 and ZnO were separately indexed in XRD with the prefix @ band # respectively. The composite showed an increase in peak intensity of the (002) plane of ZnO with a change in compositions from TZO1 toward TZO3 confirming the TiO 2 -ZnO nanocomposite thin film formation.
Where 'D' stands for the crystallite size, 'λ' stands for the wavelength of source-target (CuKα = 1.5406 Å) and 'β' stand for the full width at half maxima, and 'θ' stands for diffraction angle. The obtained crystallite sizes are shown in table 2.

UV-Visible spectroscopy
The optical properties of the materials were examined by using a UV-Visible spectrophotometer (Perkin-Elmer Lambda 45) within the 190-1100 nm wavelength range. The optical band gap energy of the films was calculated by extrapolation of the linear part of the (αhν) 2 versus hν plot, as shown in figure 7.
The obtained gap energy values were 2.72, 3.1, 2.94, 2.92, and 2.75 eV for the TO, ZO, TZO1, TZO2, and TZO3 samples respectively. A blue shift in bandgap energy is observed from sample TO to ZO.

XPS study
XPS characterization was carried out to affirm the elemental composition as well as confirm the valence state of elements in TiO 2 -ZnO nanocomposite.
A full scan spectra of the TiO 2 -ZnO of sample TZO3 is shown in figure 8(a), with the C1s peak situated at 284.7 eV [45]. The two peaks in figure 8(b), which correspond to Ti 2p1/2 and Ti 2p3/2, were found to be consistent with the values ascribed to the chemical element state of Ti4+ in a TiO lattice [46]. Figure 8 shows the Zn 2p1/2 and Zn 2p3/2 spectrum derived from the TiO 2 -ZnO nanocomposite of sample TZO3 (c). The distinctive peaks are located at 1045.41 and 1022.15 eV respectively [47]. The scan spectra of the O 1s area are shown in figure 8(d). There are two fitting curves, with the higher energy peak representing oxygen in an oxygendeficient region and the lower energy peak possibly representing a defective oxide component [48].

Photoelectrochemical performance of TiO 2 /ZnO DSSCs
The interconnected many-fold nanoflakes morphology is seen for the TiO 2 -ZnO nanocomposite in the SEM data. Such type of morphology is useful for application in dye-sensitized TiO 2 -ZnO based solar cells. The orange-colored dye-sensitized TiO 2 -ZnO nanocomposite deposited over FTO glass served as the photoelectrode, and bare graphite rod was used as the counter electrode, which played the vital role of a promoter facilitating the reduction of I 3 − ions.
The I − / I 3 − redox couple provides a better charge transfer between the I − and I 3− and the contribution to the total charge transport current was found to dominate when the concentration of the reduced and the oxidized counterparts of the redox couple was equal and high [49]. With 30 W cm -2 light irradiation from a tungsten filament lamp, the current density-voltage characteristics of dye-sensitized TiO 2 -ZnO nanocomposite photoanode were recorded using a potentiostat. Figure 9 depicts the schematic mechanism of the betanin dyesensitized TiO 2 -ZnO thin film. A PEC cell was made out of a working electrode, a graphite rod served as a counter electrode, while the equimolar mixture of 0.1 --/ M I I 3 -was used as a redox mediator. The constructed DSSCs cell, in the absence of light, exhibits rectifying qualities similar to a diode. Figure 10 shows  visible light. The important DSSCs parameter fills factor was obtained by using equation (2) and conversion efficiency was obtained by equation (3).
Where, J max denoted for the maximum current, V max denoted for maximum voltage, η denoted for the conversion efficiency, J sc denoted for the short circuit current density, V oc is denoted for the open-circuit voltage, and 'P in ' is denoted for the intensity of incident light. For TO, TZO1, TZO2, TZO3, and ZO samples of TiO 2 -ZnO nanocomposite photoanode, photocurrent increased from 1.62 mA cm −2 to 2.73 mA cm −2 with photovoltage increasing from 659 to 795 mV. The sample ZO exhibits a lower conversion efficiency (1.77%) while the sample TO exhibits a conversion efficiency (1.81%). The defect states and series resistance besides a lower bandgap caused by the small-sized nanorod structures with a reduced surface area could have exhibited a high grain boundary resistance leading to the lower performance. The conversion efficiency of the TZO1 and TZO2 samples showed a comparatively higher response of 2.39 and 3.13% respectively. It could be related to a change in the length of nanoflakes and a systematic decrease in the bandgap than TO. This effectively encompasses more near-UV spectral portions improving the electrochemical behavior. The photoconversion efficiency of the TZO3 was increased to 3.25%. The interconnected nanoflakes microstructure absorbs a lot of light and produces more electron-hole pairs across. Effective absorption of dye by the TZO3 sample of TiO 2 -ZnO nanocomposite thin films together with the lowest bandgap was mostly responsible for the increase in the PEC of DSSCs. Nanoflacks provide a scattering layer for light absorption with the fewest grain boundaries. The TZO3 sample showed a higher surface area leading to higher absorption of the betanin leading to increased conversion efficiency [38]. Figure 11 shows the bar graph of DSSCs conversion efficiency for the sample TO, ZO, TZO1, TZO2, and TZO3 respectively, the same data is also presented in table 4.
The conversion efficiency was enhanced from 1.77 to 3.25% by changing the surface shape from nanospheres to interconnected nanoflakes with increasing crystallinity. The interconnected many-fold nanoflakes morphology could provide a large surface area for light absorption [50]. Because of the uniform, compact nature of sample TZO3, degenerate electrons accumulate at the FTO-TiO 2 -ZnO nanocomposite-dye interphase, eventually potential shifts to a higher value. The creation of nanoflakes-like morphology in TiO 2 -ZnO nanocomposite thin films aided by TZO3 exhibits substantial light scattering and the highest photoconversion efficiency [51,52]. The Comparative study of betanin to the other sensitizers reported in the literature are shown in table 5.

Electrochemical impedance spectroscopy of TiO 2 -ZnO DSSCs
The charge transfer resistance (R ct ) of the dye-sensitized TiO 2 -ZnO solar cell may be precisely measured using electrochemical impedance spectroscopy (DSSCs). The electrochemical characteristics of TO, TZO1, TZO2,  TZO3, and ZO were assessed using the EIS method. The produced dye-sensitized TiO 2 -ZnO thin films were employed as a working electrode, Ag/AgCl as a reference electrode, and Pt wire as a counter electrode in a unique three-electrode cell assembly. In the I/I 3̄r edox pair, electrochemical processes were carried out. The Nyquist plots for the TO, TZO1, TZO2, TZO3, and ZO samples are shown in figure 12. It is divided into three sections that correspond to the high, middle, and low-frequency regions. The photocathode's series resistance (Rs) is coupled to the initial impedance. The Inset of figure 12 shows a typical Randles circuit connected with Warburg resistance (sample TO). The Rs is the ohmic series resistance of the electrode configuration that participates in the photoelectrodeelectrolyte electrical interaction. At the TiO2-ZnO dye-sensitized photoanode-electrolyte interface, the middle frequency region is consistent with charge transfer resistance (Rct) and double-layer capacitance (C dl ). The system's Warburg resistance (Zw) was linked to the low-frequency area. The low-frequency portion showed   Warburg impedance (Zw), which was caused by ion diffusion in the electrolyte. The magnitudes of R s , R ct , C dl , and Z w associated with DSSCs are analyzed EIS data by suitable in the Randles circuit and tabularized in table 6.
In comparison to TO, TZO1, TZO2, and ZO DSSCSS, the lowered values of Rs and Rsh were attributed to a decline in charge transfer at the FTO/TiO 2 -ZnO-dye crossing point for TZO3. The lower semicircle in a Nyquist plot is known to have a lower Rct. The TZO3 sample has a narrower semicircle than the TO, TZO1, TZO2, and ZO samples. The development of a smaller semicircle confirms a reduction in the DSSCs system's charge transfer resistance, with reduced Rct for TZO3 attributed to improvements in crystallinity and the construction of nanoflakes structures that govern the diffusion channel length. The electrical conductivity improves when Rct decreases, resulting in increased photo-conversion efficiency. The decrease in charge transfer resistance is due to the nanoflakes-like morphology of the TiO 2 -ZnO-dye photocathode, which offers surface compactness [40,56,57].

Conclusion
In this report, Betanin dye-sensitized TiO 2 -ZnO nanocomposite was deposited successfully by using a microwave-assisted technique. The structural studies exhibited mixed phases of rutile and hexagonal wurtzite structure of titanium oxide and zinc oxide respectively. The morphological study reveals that nanograins to nanoflakes surface structures. The EDS data reveals the presence of Ti, Zn, and O elements and exhibits the formation of stoichiometric TiO 2 -ZnO nanocomposite films. TEM confirms the interconnected many-fold nanoflakes morphology of sample TZO3. The composition and oxidation state of the components in the TiO 2 -ZnO nanocomposite is confirmed using XPS analysis. An optical absorption study proves electronic transition with bandgap energy of 2.72 to 2.94 eV. A blue shift in bandgap energy is observed from sample TZO1 to TZO3 The TiO 2 -ZnO nanocomposite interconnected nanoflakes growth causes an improvement in conversion efficiency, as compared to tiny nanocrystal form. The representative TZO3 sample of DSSCs shows the highest photo-conversion efficiency of 3.25%. The lowered charge transfer resistance is obtained with to the nanoflakes-like morphology of the TiO 2 -ZnO-dye photocathode, which is responsible to offers surface compactness and results in increased conversion efficiency. The use of a Betanin dye in TiO 2 -ZnO-based DSSCs exhibited a simple, cost-effective way to harvest solar energy.

Acknowledgments
For the lab infrastructure used in this work, the author AGT gratefully acknowledges Miraj Mahavidyalaya Miraj and Rajaram College, Kolhapur.

Data availability statement
The data that support the findings of this study are available upon reasonable request from the authors. The data that support the findings of this study are available upon reasonable request from the authors.