Capacitive and Bioenzymevoltaic Characteristics of Photosynthetic Chromolena Odorata – Nanostructured Zinc Oxide System

Zinc Oxide nanoparticles (ZnO) have been successfully synthesized by a chemical method at a low temperature of 60C and dried at 80C. The matching of the observed 2 value with the JCPDS value of the XRD pattern showed that ZnO was synthesized. The pattern obtained showed high peaks as an indication of crystallinity at low temperature without annealing. The average grain size of the crystal obtained using Debye scherrer formular is 18.6 nm. The crystal structure lattice system is hexagonal wurzite structure. The SEM images obtained showed that the material is made up of agglomerated grains resulting in nanostructured islands. The direct optical band gap was obtained using Tauc’s relationship to be 2.4 eV. A bioenzymevoltaic cell was developed in form of a Dye Sensitized Solar Cell but with a photosynthetic material: Chromolaena Odorata acting as both the dye and the electrolyte. The short circuit current of 1.5 A and open circuit voltage of 170 mV were obtained for the continuous and residual energy on the system. We found that the device exhibited a bio-capacitive effect, as the photosynthetic material generates electron and the energy is being stored in the system as much as 39 A with a corresponding voltage of 196 mV.


INTRODUCTION
Renewable source of energy has attracted much attention from researchers throughout the world because of rapid diminishing in fossil fuel. Consequently, there is need for alternative source of energy. There is also climate change due to alteration in natural carbon cycle and increase in carbon dioxide in the atmosphere due to human activities that emits carbon dioxide through combustion of fossil fuels (coal, natural gas and oil) for energy, transportation and industrial use. This heat trapping gases leads to global warming. There is therefore need for a clean energy generation from sources such as solar energy, wind energy In addition to the photosensitizer component of the dye sensitized solar cell is an electrolyte, containing redox mediator in an organic solvent affecting dye-regeneration by providing ionic conductivity in the bulk of the solution and sets the potential barrier necessary for energy conversion. 6 The common electrolyte being used in fabricating a DSSC is the iodide triiodide electrolyte solution which facilitate the transport of charge between the working and counter electrode (Potassium iodide mixed with iodine in waterfree ethylene glycol).
In this work, we report the development of a photosynthetic Chromolaena Odorata -Nanostructured Zinc Oxide system similar to that of a dye sensitized solar cell. The shift from the traditional DSSC configuration is the use of the encapsulated isolated photosynthetic material as both the sensitizer and an electrolyte. The sensitizer in this case is referred to as enzyme-sensitizer in which the electron is generated biologically by enzymes and injected into the semiconductor. Enzymatic processes of dark reaction photosynthesis follow immediately after the light catalytic photosynthesis process. The carbon fixation is therefore powered by energy released during redox reaction by ATP and by the reducing agent NADPH which are biochemical fuel present in the prepared photosynthetic material. The continuous energy release by the encapsulated material is therefore responsible for the charge regeneration process in the system. The carbohydrate formed undergoes oxidative degradation through the process of anaerobic metabolism, which also involves the flow of electrons.
The photosynthetic material serves as our redox electrolyte due to its capacitive feature of storing the continuously biologically self-generated electrons. This characteristic is preserved by method of encapsulation of Chromolaena Odorata because of the tendency of degradation of the material, by this, enzymatic process is temporarily sustained. This system can therefore generate energy even in the dark, since its taking advantage of the dark reaction photosynthesis, which was initiated by solar energy.
We also report the successful synthesis of Zinc oxide nanoparticles at a low temperature of 60 0 C. With peaks corresponding to hexagonal structure of zinc oxide. This suggests that zinc oxide can be prepared on non-conventional substrate such as plastic, at that temperature.

EXPERIMENTAL SETUP
The materials used for the formation of the Chromolena Odorata -Nanostructured Characterization of the synthesized material and the system were carried out using X-ray diffractometer, UV spectrophotometer and scanning electron microscope. A magnetic stirrer with electric heater were also used to stir the sample, heat and dry the sample.

Experimental Procedure for Synthesizing ZnO nanoparticles at low temperature
The precursor was prepared by adding 25 ml of distilled water to 0.65 g of Zinc Acetate Dihydrate [Zn(CH3COO)2.2H2O] at room temperature and 1.83 ml of Ammonium Hydroxide was then added with continuous stirring for 15 minutes to get a homogeneous solution. Thereafter, 11.65 ml of Sodium Hydroxide was added to the homogenous solution and it was stirred for 1 hour at room temperature. 9 The overall solution was heated at 60 0 C for 3 hours in an oven, thereafter; it was filtered and dried at 80 0 C for 5 hours in a furnace.
For a third-generation solar cells namely Dye-Sensitized Solar Cells (DSSCs), Zinc oxide (ZnO) has been widely studied as a promising alternative photoanode material. Since the inception of research on TiO2-based DSSCs, ZnO has also been studied, since TiO2 and ZnO have similar electron affinities and almost the same band gap energies, 3.2 and 3.3 eV respectively. ZnO, however, has much higher electron diffusivity than TiO2. 10, 11

X-Ray Diffraction (XRD)
XRD is the conventional method for exploration of crystalline structure of nanomaterials. Crystalline phase and details of the working electrode semiconductor film lattice structure are determined by XRD. XRD spectrum presents intensity versus 2 (angle of X-ray to the sample). In the X-ray diffraction, an incident beam of X-ray is scattered from the surface and diffraction pattern is formed as a consequence of the scattering of short wavelength X-ray photons by electronic cloud of highly ordered positive ion centers within the material. Regularity and ordering of the ions forming a lattice in the sample can be detected as well. 16 The Bragg formula dsinθ = nλ is used for determination of lattice spacing in which d is the distance between special crystal planes, θ is the angle of the X-ray beam to the sample, n is an integer number and λ is the wavelength of X-ray beam. Peak broadening can be related to small crystal size, which analysed using the Scherrer formulae, τ = Kλ/ βcosθ, in which θ is considered for a specific XRD spectra peak, K is dimensionless shape factor close to 1, β is full width at half maximum of the peak and τ is the mean crystallite size for the specific crystal plane of the peak.

Scanning Electron Microscope
Scattering of high energy electron beams from the surface of nanomaterials is the concept of scanning electron microscopy (SEM) to study the surface morphology of nanostructures. 17 SEM is the conventional and standard method for microscopy studies of nanostructure film morphologies such as DSSC working and counter electrodes. 18,19 Thicknesses of the layers in a DSSC can be measured by the cross sectional SEM which is the standard method to estimate the thickness of solid state device layers. 20

Preparation of Chromolaena Odorata
The leaves of the plant was plucked at Ilorin, Nigeria, with geographic coordinates of 8°30′N 4°33′E. The leaf is locally known as "Ewe-Akintola", it is greenish in color and soft in texture. A good quantity was cut into pieces without adding water and then crushed and ground into a green paste.
It is a photosynthetic material which converts solar radiation to chemical energy and this chemical energy is more functional in DSSC. This is achieved by fixing carbon dioxide from the atmosphere. Because of the conversion of carbon dioxide to metabolites, such as starches, sugars and carbohydrates, oxygen is produced as a by-product. 12 The material is abundantly available in the western part of Africa.

Fabrication of the Bioenzymevoltaic System
The system was fabricated as shown in Figure 1.  The photosynthetic enzymatic material acting as the dye and an electrolyte was then attached to the Zinc Oxide inside the encapsulator and a copper plate was used as the back contact.
The device is of 10cm in height and of 5cm wide.

X-ray Diffractometer (XRD) Analysis
The structural properties of the Zinc Oxide prepared at low temperature 60 0 C was characterized with X-ray diffractometer with 2θ values range between 20-80 o . Figure 2 shows the XRD pattern for the ZnO nanostructured particles. with lattice constant plane oriented in the direction (101). Data obtained at each peak was used to calculate the grain size as shown in Table 1 for ZnO.
where λ is the X-ray wavelength (0.1541nm), β is the full width at half maximum (FWHM) of a diffraction peak, θ is the diffraction angle, and K is the Scherrer's constant. 13 According to the diffraction peak positions and the width at half-maximum, the mean size of ZnO can be estimated to be approximately 18.2 nm. This confirms that the material (ZnO) synthesized is nanostructured scaled material.
The nanocrystalline Zinc Oxide was successfully synthesized at a low temperature 60°C. The route of preparation is a unique and of great advantage because there is no need for annealing the as-prepared material at high temperature before achieving a crystal structure as illustrated in the XRD pattern shown above.

Scanning Electron Microscope (SEM) analysis
Scanning electron microscopy is a convenient technique to study the microstructure of ZnO nanoparticle samples. This was employed to characterize the surface morphology.
Unlike bulk materials, properties of nanomaterials are strongly correlated to shape. This shape is attained during growth through a self-assembling process dictated by the interplay of size and molecular interactions. Deviations from bulk properties become prominent as the

UV-Visible Spectroscopy
The absorption spectrum of synthesized zinc oxide nanoparticle is as shown in Figure 4, showing a high absorption within the visible region. A graph of ( ℎ ) 2 versus ℎ is shown in Figure 5. By extrapolating the straight-line portion of the graph to energy axis, the band gap of the samples is obtained. The band gap is found to be2.4 .

Current generated with Time
The quantitative values of the current generated with time in minutes was measured and illustrated by the graph in Figure 6. The current regenerated (indicated by the arrow pointing upwards) after each discharge is observed to be increasing with time from about 8 μA after 30 minutes to about 40 μA after 390 minutes. The discharge occurs when the system is short circuited but does not reduce to zero referred to, here, as residual current, due to a continuous regeneration of charges biologically through a redox reaction, that are injected into zinc oxide.
The increasing regenerated accumulated charges in the photosynthetic material with time as shown in Figure 6, injected in the zinc oxide are stored in the system until they are discharged during measurement of the short circuit current. This illustrates the capacitive feature of the system. The rate of regeneration of the charges were obtained using the equation below. .

Peak regenerated Current per period Initial Residual Current for the same period Rate
Period of Current Regeneration   Each regeneration period is represented by the time taken for the current to get to the peak of the regenerated current from the initial residual value of the current. The graph of the rate of charge regeneration with time is as shown in Figure 7.

Current -Voltage Measurement of the System
Charge generation is through two main processes. These are Enzymatic reduction of Carbon dioxide during which chemical fuel such as ATP and NADP + which are charge carriers releases electron by redox reaction as seen in Figure 9. The other stage of charge generation is the oxidative degradation of carbohydrate as seen in Figure 10. The electrons generated are injected into the large band gap semiconductor ZnO.

CONCLUSION
A system with charge storing capacity has been developed in the dye sensitized solar cell configuration. It exhibits a bioenzymevoltaic effect by sensitizing the large band gap zinc oxide with electron generated enzymatically. The conventional electrolyte iodide/triiodide is here replaced with encapsulated photosynthetic material with selfgenerated charges by the enzyme (RUBISCO) through dark reaction photosynthesis. The capacitive effect is shown by the increase in the short circuit current Isc with time. The regenerative effect is illustrated with restoration increase in the charge on the system. The large band gap sensitized nanostructured zinc oxide was synthesized using a wet chemistry at low temperature of about 60  C. The XRD pattern shows a nanocrystalline property of the material, cubic structure crystal phase. The direct optical band gap obtained is 2.4 eV. A short circuit current of 1.5 μA was obtained with a corresponding OCV of 170 mV for the bioemymevoltaic capacity of the system and highest short circuit current for the capacitive effect is 39 μA and the corresponding OCV is 196 mV.

ACKNOWLEDGEMENT
Authors gratefully acknowledge the support of Physics Department, University of Ilorin in carrying out this work. OAR appreciate CIMAV for his doctoral program.