Natural sensitizers-mesoporous TiO2 hybrid nanomaterial for future optoelectronic applications

Abstract Optoelectronics deals with the design and development of electronic devices including photodetector (PD), solar cells and LEDs for light detection, generation and application for a variety of purposes. It includes X-rays, Gamma rays, Infrared, Ultraviolet and Visible light. In the current work, we developed a self-powered and efficient UV–Visible PD by sensitizing mesoporous TiO2 powder with a natural sensitizer Ficus Benghalensis (Banyan) and Rubia Cordifolia (Manjishtha). Prominent enhancement of visible light absorption was noted due to sensitizers as compared to pure TiO2 with the decrease in band gap from 3.13 eV to 3.01 eV. TiO2 photoanodes fabricated with and without dye loading were characterized using XRD, FESEM and UV–Visible and FTIR spectroscopy and used to fabricate a PD device with an active area of 0. 25 cm2. At zero bias, the Banyan-loaded TiO2 PD (B-TiO2) demonstrates enhanced photo response by nearly three times than Manjishtha-loaded PD (M-TiO2). At zero bias voltage, the PD (B-TiO2) displayed very high photosensitivity (8665), Dark current density (126 nA), Photocurrent density (158 µA), Photoresponsivity (1.88 mA/W), Rise time (0.31S) and Decay time (0.35S), respectively. Therefore, the use of novel dye for electricity generation in this study opens new routes to design future optoelectronics devices.


Introduction
The photodetector is a light sensor that transforms light energy into electric signals and has a wide range of applications, including photoelectrochemical cells [1], image recognition [2], optical communications, biological chemical sensors, light-wave communications, flame detection, ozone monitoring and missile detection.Traditionally, photodetectors have been made of silicon and a photomultiplier tube (PMT) for UV radiation.But, these materials have to be replaced because PMTs require high voltage and vacuum environments that also need optical filters to block the infrared and visible regions of the electromagnetic spectrum and need high cost [3].The metal oxides such as Ga 2 O 3 [4][5][6], ZnO [7], SnO 2 [8] and TiO 2 [9,10] have drawn a lot of interest as potential replacements for UV photodetectors that operate on their own due to their wide range of bandgap and also explored in visible light photocatalysis that is energy harvesting and hydrogen generation that is energy generation and environment cleaning as well [11][12][13].Highly stable and sustainable photodetectors with self-powered features are currently receiving a lot of interest due to the worldwide energy crisis.However, photodetectors with high sensitivity, high responsivity and tunability in broadband regions have been particularly sought-after in a variety of scientific and industrial applications.It is observed that the high ultraviolet (UV) responsiveness of photodetectors is essential for many applications, including artificial intelligence [14] chemical and biological investigation [15], flame sensors and space-optical communication [16] but in addition to this visible light responsivity is also important in security, plasmonic detection of nanoparticles and smart sensors [17,18].For such interesting applications, ideal metal-oxide photodetectors are sturdy, low-cost and highly susceptible to both UV and visible light [19].Among the numerous metal oxides TiO 2 is a promising material due to its nontoxic nature, wide direct band gap with ~3 eV for rutile and ~3.2 eV for anatase phases, chemical resistance, physical steadiness, high photoelectrochemical competency and photo-stability [20].Nanosized TiO 2 that is nanoparticles [21], nanowires [22] and nanofibers [23] have been widely used as self-powered UV photodetector.Prior to this, the development of nanosized thin-film dye-sensitized solar cells (DSSCs) led to a major development in photo-electrochemical photodetector technology [24].Photoanode modification based on TiO 2 was reported with the use of graphene previously [25].There are several reports available on mesoporous TiO 2 nanoparticles where first time, the facile synthesis of mesoporous black TiO 2 with an ordered mesostructure, high surface areas and highly crystalline anatase pore walls.A thermally stable and high-surface-area mesoporous TiO 2 (OMT) was first fabricated via an evaporation-induced self-assembly (EISA) method [26].Efforts have been taken for the surface tunability of mesoporous TiO 2 nanoparticles that led to the enhanced photocurrent by sensitizing it with water-soluble CdSe Quantum Dots (QDs) [19].Furthermore, CdS nanoparticles can be employed as visible light photosensitizers in TiO 2 heterojunctions.However, they are hampered by material instability and slow interfacial kinetics in the hydrogen evolution reaction [27].Moreover, the toxicity of cadmium has been a major concern for the sustainable environment as well as mankind.To overcome this problem, the production of low-cost, nontoxic and naturally derived organic dye-sensitized optoelectronic devices with wide-gap TiO 2 semiconductors is trending and is of great interest [20].The natural dyes employed for sensitizing DSSC have been the subject of numerous investigations since they are the main source of the photo-generated current with the advantage of natural availability and nontoxicity [28].Several naturally available dyes were efficiently used across the world for getting satisfactory results of photocurrents and thereby preventing the recombination of electrons [29].In the present work, green synthesized TiO 2 nanoparticles were used as anode material of UV-Vis photodetector which was sensitized with natural dyes viz.Rubia Cordifolia (Manjishtha) and Ficus Benghalensis (Banyan).The physicochemical properties and photodetector characteristics of pure and dye-loaded TiO 2 nanoparticles were examined.Comparably, dye-sensitized TiO 2 photodetectors have a greater potential for self-powered broadband photo-sensing applications.

Materials
A glass substrate coated with Fluorine-doped tin oxide (FTO) of 8 Ω/sq.sheet resistance, Iodolyte AN-50 as an electrolyte, Meltonix 1170-60 sealing film and Pt paste were purchased from Solaronix, Switzerland, 3 M scotch tape from Amazon, India.Titanium (IV) isopropoxide, ethanol, methanol and ammonia (NH 3 ), ethyl cellulose, acetylacetone were purchased from Ultra-Pure Lab Chem Industries LLP (Mumbai) and terpineol anhydrous from Sigma Aldrich.The Fresh leaves of F.Religiosa (Peepal) and Ficus Benghalensis (Banyan) were collected from the campus of Symbiosis International (Deemed) University, Lavale, while the roots of Rubia Cordifolia (Manjishtha) were collected from University of Pune campus.

Extraction of natural dyes
Three plants viz.Manjishtha, Peepal and Banyan were used to extract natural dyes for this study.The leaves were then washed with running and deionized water to remove dust and dirt particles before being dried in an oven at 60 °C for 15-20 min.The dried leaves were cut into small pieces; 10 g leaves of Peepal and Banyan were weighed and the extraction process is carried out by using 200 mL of methanol in Soxhlet apparatus upon three cycles of 40 min each at a temperature of 60 °C.Similarly, the 10 g roots of Manjishtha were weighed, and the same extraction process is carried out by using 200 mL of ethanol.These three extracts were filtered using Whatman filter paper no. 1.The filtrate of Peepal leaves was used for the synthesizing of TiO 2 nanoparticles, while the remaining two, Banyan and Manjishtha were used as sensitizers for DSSC.

Synthesis of photoanode (TiO 2 )
TiO 2 nanoparticles were synthesized using the green synthesis method previously reported with slight modifications [30].Here, Titanium Tetra Isopropoxide (TTIP) was used as a precursor and Peepal leaves extract was used as an environmental friendly and low-cost reducing agent.In the reaction, 13.5 g TTIP was added to the 40 mL ethanol and stirred continuously at 600-700 rpm for 3 h at room temperature.Then, 13.5 mL extract and 10 mL ethanol were added dropwise to the reaction with an increased temperature of 36 °C and further stirred for 15 min.In the next step, 10 mL Ammonia solution (NH 3 ) was added dropwise with continuous stirring for hydrolysis maintaining the pH= 11.In the end, the white-colored product was obtained which was further washed with ethanol and dried.Finally, it was calcined at 450 °C for 2 h using a programmed muffle furnace to obtain the desired nanoparticles.

Fabrication of TiO 2 photodetector
In view of the fabrication of the photodetector, initially, the TiO 2 paste was prepared by blending the 3.5 g green synthesized TiO 2 powder, with 9 mL ethanol using a homogenizer with the subsequent addition of 3.5 g ethyl cellulose and 1.75 mL terpineol anhydrous and 1.75 mL acetyl acetone.The gummy slurry is sonicated for 4 h to form a homogeneous paste.Further, three FTO substrates were cleaned using soap solution, distilled water, acetone, and finally, with ethanol and sonicated for 10 min before being air dried at room temperature.The contours of each substrate were covered with scotch tape to prepare the 0.25 cm 2 active area film.Then, for each substrate, TiO 2 paste was dropped cast in the center point of the active area and evenly spread with a doctor blade.Further, these films were sintered at 450 °C for 1 h.Deposited films were immersed in each of the 10 mL solutions of dyes viz.M-TiO 2 and B-TiO 2 and kept overnight for sensitization required for further study.

Mechanism of B-TiO 2 photodetector
As shown in Figure 1, a band diagram of the B-TiO 2 loaded photodetector is constructed considering the sandwich structure of two glass substrates coated with fluorine-doped conducting oxide (FTO).On the one side, chlorophyll sensitized TiO 2 semiconductor is pasted called photoanode (working electrode), and the other side of the device is called counter electrode where FTO is coated with Platinum (Pt).Electrolyte, i.e., I 3-/I -filled between two electrodes acts as a conductor or mediator.The work functions of fermi energy of photoanode and electrolyte play a very important role in photocurrent generation and are required for the generation of open circuit voltage (V OC ) in the photodetector.The required band positions (HOMO/LUMO) of FTO, TiO 2 , chlorophyll as a sensitizer and work function of I 3-/I -are referred from previous reports [31][32][33][34].Initially, the chlorophyll from the Banyan sensitizer molecule absorbs the energy (hυ) and gets excited to the higher energy levels (first excited state) leaving behind the hole.The excited state electron in the conduction band of sensitizer then jump to the LUMO having a high electron affinity of TiO 2 .The electron further travels through the TiO 2 nanoparticles and reaches FTO.Reached electron travel through the outer circuit to the counter electrode Pt and hence the current gets generated.The difference between the energy levels or conduction band position of chlorophyll and TiO 2 is 1.22 eV and is responsible for open circuit voltage V OC .Further the mediator I 3-collects the electron from Pt electron and gets reduced to I -.The dye regeneration process starts again due to the recombination of electron of I -and hole generated in dye during excitation and the circuit continues to electron flow and current generation again.

Characterizations
For probing the structural properties of TiO 2 , XRD patterns were recorded using Bruker AXS D8, Cu Kα λ = 1.54 A°.The optical properties of pure and dye-loaded TiO 2 nanoparticles were measured using JASCO V − 750, UV-Visible spectrophotometer.The morphology of TiO 2 powder was determined by using FEI Nova Nano SEM 450.FTIR spectra of Pure and Dye loaded TiO 2 were measured using 'Shimadzu IR affinity diamond ATR spectrophotometer' .The PDs characteristics were measured using an electrometer-Keithley 2450 source meter.The photoresponse of the samples were measured at an intensity of 100 mW/cm 2 .

Structural studies
X-ray diffraction (XRD) study of the green synthesized and dye loaded TiO 2 powder was carried out to examine the structural properties that is crystal structure, phase fraction and crystallite size.The XRD patterns are shown in Figure 2a-c confirm the mixed phase comprising the Rutile (R) and Anatase (A) phases of TiO 2 powder.The crystal structures were drawn using VESTA (open source) software shown in Figure 2d,e denotes the Rutile and Anatase phases, respectively.The dye loading on TiO 2 also can change the phase as reported in the previous study, that is melanin loading has changed the Rutile TiO 2 to Mixed phase TiO 2 [35].In our case, there was no phase change observed in all three TiO 2 that is pure, M-TiO 2 and B-TiO 2 .The anatase phase was confirmed by matching with the JCPDS card no.894921 with space group I4 1 /and(141) and lattice constants a = 3.785 Aͦ and c = 9.514 Aͦ and Rutile phase was matched with the literature [36].
Crystallite size for both phases was calculated using the Scherrer formula [37].Anatase and rutile mass fractions of the TiO 2 and B-TiO 2 are calculated using Spur's formulae (1) and ( 2 ( where f a and f r are the anatase and rutile fractions where Ia and Ir denote the integrated intensities of the most intense anatase (101) and rutile (110) peak, respectively.The results are tabulated in the table in Figure 2. From the calculations, we obtained the Rutile phase as the more dominant phase of TiO 2 which directly contributes to more photo absorption properties than photocatalysis properties [38].Rutile is generally recognized to be the most thermodynamically stable phase of TiO 2 .That is, rutile is the ultimate phase of TiO 2 after high-temperature calcination [39].TiO 2 in its single phase is proven to be superhydrophobic and in its mixed phase, it is hydrophobic which is comparatively beneficial for adsorption-loaded dyes [40].Moreover, the occurrence of two different phases in TiO 2 powder allows for finding their photo-absorption in the visible range while it absorbs the UV region for pure Anatase or Rutile phase [41].

Optical absorption studies
UV-Visible absorption spectra of both the natural dyes are shown in Figure 3a. Figure 3a shows that the Manjishtha dye absorbs the visible wavelength that is 414 nm [42] and Banyan dye shows a prominent absorbance peak at 280 nm assigned to the absorption of light by aromatic amino acids like tyrosine, tryptophan or phenylalanine residues in the proteins.The presence of chlorophyll in the leaf extract of Banyan gives the absorbance at 419 nm.As shown in Figure 3b the absorption wavelength of pure TiO 2 is in the UV region (~300nm) and gives the direct band gap of 3.13 eV.The absorbance of dyes plays a very important role in the present study whereas loading of natural dyes on pure TiO 2 has significantly enhanced absorption of light into the visible region.Direct and indirect band gaps were evaluated using the Kubelca-Munk function from the DRS spectra with direct band gap of 3.13 eV, 3.04 eV and 3.01 eV of TiO 2 , M-TiO 2 and B-TiO 2 , respectively, and indirect band gaps of 1.55 eV and 1.75 eV for B-TiO 2 and M-TiO 2 , respectively.The decrease in the indirect band gap values after the dye loading results in the increased in number of electron-hole pair generation and hence an increase in photocurrent.

Stability of natural sensitizers
Many reports are available on the stability of natural sensitizers over the greater time provided with proper storage conditions [32].The optical absorption stability after 30 days for both the sensitizers kept at 3 °C was studied using UV-Visible spectroscopy as shown in Figure 3a which indicates that the absorbance reduces with negligible values after 30 days.The absorbance of both the dyes recorded on first day and after 30 days are the same with no changes in the position of absorption peaks and intensity.

Morphological studies
The morphology of TiO 2 powder is shown in Figure 4 and the histogram shown in the inset was calculated for the determination of average particle size.We obtained porous spherical clusters of average particle size of 20-30 nm.Also, an uneven distribution of nanosized particles was observed.The mesoporous nature of nanoparticles provided a larger adsorption surface for dye molecules and could enhance the ability of photo absorption by TiO 2 nanoparticles [41].

Chemical interaction (FTIR) studies
FTIR spectroscopy study was done for the determination of functional groups in the pure TiO 2 and dye-loaded TiO 2 .Figure 5 shows the feature at 434 cm −1 exhibits the presence of a Ti-O-Ti bond.Plant organs like roots, stem and leaf consists of various light-absorbing phytochemicals and are responsible for photocurrent generation [29].The characteristic features reflected in FTIR spectra of Pure and Dyes loaded TiO 2 are summarized in Table 1

Assembling of DSSCs and measurements
Meltonix 1170-60, the thermoplastic sealing film made of Surlyn® of thickness 60 µm was used as spacers between TiO 2 electrode as a photodetector with an active area of 6 × 6 mm and a counter electrode made of platinum coated FTO facing each other according to the  active area.Place the assembly on the hot plate and set the temperature at about 110 °C.Apply even pressure all over the gasket so that within ten seconds, the hot melt material glues the electrodes together.The liquid electrolyte, Iodolyte AN-50 was used to fill the electrode gap.Just before testing, add a few drops of electrolyte till no air bubble is present.The schematic design of the photodetector setup is shown in Figure 6.The structure of the self-powered UV-Visible TiO 2 photodetector schematically shown in Figure 6a is similar to that of a dye-sensitized solar cell.In this, the transparent conductive FTO substrate with dye-sensitized titanium dioxide (TiO 2 ) film acts as an anode, and the FTO substrate with platinum (Pt) catalyst as a cathode is sandwiched together with a spacer.The gap is filled with iodide/triiodide (I -/I 3 -) redox couple electrolytes.After the illumination with light intensity, the photodetector characteristics were measured using a Keithley electrometer connected to a computer interface.

Performance of self-powered UV-Vis photodetector
Figure 7a-d demonstrates the I-V characteristics of these three samples under dark and light intensity 100 mW/cm 2 on an active area of 0.25 cm 2 .The photocurrent changes   substantially when the light switch is switched on, and photocurrent responses do not diminish as the illuminated time increases, showing strong stability [43].It shows that with increasing illumination intensity, the photocurrent rises linearly and eventually tends to reach saturation.This can be accounted for by the increase in electron and hole recombination under high light intensity caused by a reduction in carrier recombination time [44].Figure 7b shows the enlarged view of Figure 7a where pure TiO 2 absorbs the light of the UV-region as well as in dark and under illumination conditions, photocurrents coincide with negligible difference of 0.01 mA.
The fabricated devices give a photocurrent of −10 mA and 4.5 mA for TiO 2 , −9.4 mA and 9.4 mA for M-TiO 2 and −9.8 mA and 9.9 mA for B-TiO 2 at nearly zero bias voltage for an active area of 0.25 cm 2 .The results revealed that the photocurrent is enhanced by two orders of magnitude using B-TiO 2 as compared to TiO 2 .Hence, the maximum photonic energy is absorbed by dye molecules which results in an increase in photocurrent with dye-loaded TiO 2 which might be due to the light absorption by the chlorophyll present in the dye which is lacking in the case of melanin [35].The photodetector's response time is another crucial factor that significantly affects application prospects.The single-cycle photoresponse was carried out at zero bias voltage.Figure 8 shows the time-dependent photoresponse data of six cycles for the photodetectors with anode materials TiO 2 and dye-loaded TiO 2 .It is observed that the TiO 2 -based photodetector shows a photocurrent density of 59 µA/cm 2 whereas the M-TiO 2 and B-TiO 2 based photodetector shows a photocurrent density of 104 µA/cm 2 and 158 µA/cm 2 , respectively.The B-TiO 2 -based photodetector enhances the magnitude of photocurrent density by three times that of the TiO 2 -based photodetector.A photodetector's rise time is the amount of time taken by it to go from the dark current value to 90% of its highest photocurrent value.Similar to the decay rate, decay time refers to the amount of time it takes for the photocurrent to decrease to 10% of its greatest value.
Figure 9 shows the single-cycle photo response for the measurement of the rise and decay time of the prepared photodetector.It is observed that all the prepared photode-  photodetector shows decreased rise and decay time of 0.31 s and 0.35 s, respectively.It concludes that the loading of Manjishtha (M-TiO 2 ) and Banyan (B-TiO 2 ) on TiO 2 improves the sensitivity of the photodetector.The performance of a photodetector is determined by several factors, including photoresponsivity, photosensitivity and response time.One of the most crucial factors in determining the sensitivity of an optoelectronic device is the photoresponsivity (R λ ), which measures the amount of photoexcited current produced per unit power of illuminated light on a photodetector's active area [45,46] and is calculated by,

R I P A
where pλ is the incident light's intensity (100 mW/cm 2 ), ΔI = I photo -I dark , is the change in photocurrent caused by the impact of incident light on the photo-sensing area, and a is an active area of the film (0.25 cm 2 ). it was observed that the responsivity of fabricated photodetectors increases with the loading of sensitizers viz.manjishtha and Banyan on tio 2 .
Another key component of the photodetector is the photosensitivity parameter, which measures how current changes in relation to the dark current.The photosensitivity (ξ) is the difference in current (ΔI) when it is normalized to the dark current.The formula for the Photosensitivity (ξ) calculation is expressed as [47] Due to the increase in photocurrent, the photosensitivity increases with the loading of Manjishtha and Banyan dye on TiO 2 .The increase in photocurrent in M-TiO 2 and B-TiO 2 may be due to (i) Because of its high sensitivity, narrow bandgap and high absorption coefficient, B-TiO 2 is a superior material for photodetection.(ii) The B-TiO 2 film's high crystalline quality minimizes grain boundary recombination and defect density which allows electrons to move through the film more freely.The values of photocurrent, rise and decay time, photoresponsivity and photosensitivity are listed in Table 2. Further Table 3 shows the comparative analysis of photocurrent densities with previously reported data.Hence, natural dye obtained from Banyan leaves might be effective in solar cell applications.

Conclusion
In summary, self-powered broadband photodetection was studied using TiO 2 and dye-loaded TiO 2 nanoparticles coated on FTO glass substrates.The structural, optical and

Figure 2 .
Figure 2. powder xrD patterns for tio 2 , B-tio 2 and m-tio 2 in (a-c) showing crystalline nature of tio 2 , (d) and (e)showing crystal structure of rutile (r) and anatase (a) phase of tio 2 and table in figure1showing crystallite size and phase fraction of tio 2 powder.

Figure 3 .
Figure 3. shows uV-Visible absorption spectra of (a) two natural dyes, i.e., manjishtha and Banyan for first day and after 30 days showing their absorption stability (b) tio 2 (c) manjishtha loaded tio 2 that is m-tio 2 (d) Banyan loaded tio 2 that is B-tio 2 and (c and d) shows their respective tauc plots for direct and indirect band gap estimation, respectively.

Figure 4 .
Figure 4. fEsEm micrograph showing morphology of tio 2 consisting of clusters of nanoparticles and inset showing histogram with average particle size of tio 2 powder.

Figure 7 .
Figure 7. (a, c, d) Evolution of the photocurrent vs. voltage during continuous illumination and in absence of light of tio 2 , manjishtha loaded tio 2 (m-tio 2 ) and Banyan loaded tio 2 (B-tio 2 ), respectively (b) enlarged view of photoresponse of tio 2 in the dark and under illumination.
tector gives a fast response to light.The rise time and decay time of the TiO 2 -based photodetector were 0.46 s and 0.50 s, respectively.Whereas the M-TiO 2 -based photodetector gives a rise time of 0.34 s and a decay time of 0.46 s.The B-TiO 2 -based

Table 1 .
characteristic features of pure and dye loaded tio 2 observed in ftir spectra.

Table 2 .
performance of tio 2 based self-powered photodetector.

Table 3 .
reported photocurrent densities obtained using different natural dyes.characterizations of pure and dye-sensitized TiO 2 have been studied in detail.The device demonstrated broadband photo sensing under zero bias voltage.The major photodetector parameters such as dark current density (I dark ), photocurrent density (J ph ), rise time (τ rise ), decay time (τ decay ), photoresponsivity (Rλ) and photosensitivity (ξ) were measured without any external bias.Banyan loaded, a B-TiO 2 photodetector showed excellent results with photoresponsivity of 1.88 mA/W, photosensitivity of 8665 and rise, and decay time of 0.31 and 0.35 s, respectively.Our study indicated the potential use of Banyan dye sensitized TiO 2 for future self-powered optoelectronics devices and applications. morphological