Design and synthesis of silver bio-nanocomposite for photocatalytic applications.

One of the major concerns faced by humanity since Industrial Revolution is the contamination of water by the effluents released to the water bodies as a part of modernization and socio-economic development. Water purification is the most researched topic and numerous researches carried out in the past and are still going on in the present which pinpoints the seriousness of the grave issues raised by contamination of water bodies. In this work we have tried to develop a bio-nanocomposite utilizing colloidal silver nanoparticles synthesized via green protocol and arrowroot powder. Nano Silver well known for its photocatalytic properties has been incorporated into biodegradable and ecofriendly arrowroot powder for enhanced photocatalytic action and carried out degradation studies on the common contaminant dye Methylene Blue (MB). The synthesized bio nanocomposite exhibits promising photocatalytic action to be used for water purification.


INTRODUCTION:
One of the biggest challenges faced by human society in the recent decades is energy crisis and pollution of land, water and air.Water pollution is one of the serious concerns faced by human society in the past few decades.The main cause of water pollution is the chemicals released from the industrial sector and from textile dyeing in garment production.Among the different approaches to find solution to this grave problem, photocatalysis is a promising method to provide clean energy while simultaneously solving environmental issues using renewable solar energy.
Metal nanoparticles have contributed greatly in this regard because of their absorption in visible region of sunlight which corresponds to about 45% of the total sunlight.Noble metals like gold, silver in their nano regime exhibit strong absorption in the visible region which has been exploited to manipulate light to suit various Photocatalytic applications [1][2][3][4][5][6][7][8].
Green synthesis is a bottom-up strategy aiming to replace expensive, dangerous chemical reducing agent with natural materials from plants like tree leaves, plants, fruits, and bark.It is a non-toxic procedure that is friendly to the environment with less chance of contamination.
The goal of green nanoparticle synthesis is to reduce hazardous waste production and use environmentally friendly procedures.In the past decade environment friendly green synthesis of 1291 (2023) 012020 IOP Publishing doi:10.1088/1757-899X/1291/1/012020 2 silver nanoparticles has gained momentum and is being widely reported for its simple andcost effective protocol.
The noble metal nanoparticles like gold and silver owing to their surface plasmon resonance can broaden the absorption region and enhance the local electromagnetic field to facilitate visible light driven photocatalysts with high efficiency and low cost [9][10].
In this investigation we have synthesized silver nanoparticles using a green protocol using guava leaf extract at room temperature without heating and stirring.We have made an attempt to enhance the well-established photocatalytic activity of silver nanoparticles by synthesizing a novel nanocomposite using arrowroot starch.
Nanocomposites are a combination of two or more phases containing different compositions or structures where atleast one of the phases is in the nanometer range of 10-100 nm.They offer enhanced properties than conventional composites in terms of mechanical strength, thermal stability, electrical properties etc. [11].Due to its renewability, biodegradability and low cost, starch, a bio polymer has attracted interest in waste water treatment [12].Arrowroot starch consists of a mixture of two polymers of anhydroglucose units, amylase and amylopectin [13].
In this work we have synthesised bio-nanocomposite using arrowroot starch andsilver colloidal nanoparticles using simple lab technique.The synthesized silver arrowroot nanocomposite has been tested for the degradation of Methylene Blue dye under sunlight.

Synthesis of colloidal silver nanoparticles
Silver nitrate [AgNO3] was purchased from Merck.Guava leaves extract was prepared using the method employed in our previous report by boiling fresh guava leaves in distilled water which serves as both reducing and capping agent for synthesis of colloidal silver nanoparticles [14].2ml of the guava extract is added to 20 ml of freshly prepared AgNO3 solution which turned reddish brown in colour confirming the formation of silver colloidal nanoparticles (Fig. 1b).
2.2 Synthesis of Ag-arrowroot bio-nanocomposite 5 ml of Ag colloidal solution was added to1 gram of arrowroot powder and heated and stirred at 60° C till the solvent evaporated.The powder was air dried and grinded to obtain the bio-composite Figure 1(d) to be used as photocatalyst in our dye photodegradation studies.
The scheme of the synthesis of Ag arrowroot composite is illustrated in Fig 1 .In this mechanism in the presence of plant reducing agent silver ions undergo reaction to form stabilized silver nanoparticles which when heated with arrowroot powder become Ag arrowroot composite.

Measurements
The UV-Vis absorption measurements were recorded using spectrophotometer in the visible range (Ocean Optics Maya 2000).The sample morphology of was observed through Scanning Electron Microscope (Jeol).Xray diffraction (XRD) measurements of pure arrowroot powder and Agarrowroot composite were performed using X-ray diffractometer (Bruker with monochromatic Cu-K radiation (1.54A°)).The size of the synthesized nanoparticles was determined by transmission electron microscopy (TEM) Jeol/ JEM 2100 using LaB6 source operated at 200 kV.

TEM measurements
The TEM analysis of the synthesized silver nanoparticles is shown in the figure 2. It is seen that the particles are spherical in shape and in the range of around 14-25 nm.The degradation efficiency (%) is calculated using the formula E% = (1-C/C0)100, where C and C0 represent the final and initial concentrations of dye, respectively, before and after photoirradiation.It is seen that the silver nanoparticles show 23% degradation after three hours and arrowroot starch shows about 48% in three hours.It is observed that that the degradation efficiency is maximum for the nanocomposite showing about 56% degradation efficiency.Mechanical treatment by grinding refines the structure of starch, increases the amount of the amorphous phase of the starch, thereby enhancing the sample's solubility and reactivity [16] as shown by XRD, SEM and photocatalytic studies in this work.The presence of silver nanoparticles in the nanocomposite has also contributed to the improvement of photocatalytic efficiency due to its light harvesting capability in visible region.Both these factors have contributed to the enhanced photocatalytic ability of the Ag-arrowroot nanocomposite.

CONCLUSIONS
Colloidal silver nanoparticles are synthesized via sustainable green route using guava leaf extract.Further biodegradable and ecofriendly arrowroot starch has been used for the synthesis of bio nanocomposite of silver.Photocatalytic studies clearly demonstrate the superior activity of the synthesized nanocomposite over colloidal silver nanoparticles in the degradation of the Methylene blue dye.Further research in this area has the potential to engineer diverse multifunctional bio nanocomposites which can replace the non-biodegradable materials for various applications including water purification.

Figure 2
Figure 2 TEM images of different magnifications