Structural and Optical Properties of AgO Nanoparticles Synthesized using Solid State Combustion

Silver oxide nanoparticles (AgO NPs) were synthesized by solid state combustion technique with AgNO3 as a precursor and cow dung cake and cow urine as fuel. XRD results exhibit prominent peaks at 28°, 32°, 46° respectively indicating the AgO nanoparticles in cubic phase. Furthermore, structural elucidation through Rietveld refinement has confirmed the existence Ag2O3 nanoparticles, thereby indicating the formation of silver oxide nanoparticles. UV-Vis-NIR results are indicative that that the silver nanoparticles have exhibited a strong SPR peak at 435 nm, indicating the existence of AgNPs and it is interesting to note that solid state combustion of AgO NPs with cow dung cake has yielded to size of Ago Nps around 4.44 nm and AgO Nps with cow dung cake and cow urine has yielded to size of AgO Nps about 6.31 nm.

Thermal decomposition had been an effective method to synthesis metallic nanoparticles (MNPs) and it has gained much attention because of their ease of accessibility to tailor the shape and size of the particles through temperature or choice of an appropriate fuel cells [1].These nanoparticles exhibit unique physical, chemical and biological properties, as their relatively larger surface area to the volume ratio is tuned, there is a possibility of either increased reactivity or stability in a chemical process, enhanced mechanical strength, etc., which enables them to venture into various applications.The electronic structure of nanoparticles reveals either semiconducting or metallic, or insulating character [2].MNPs play a vital role in research domains in the field of materials, chemistry, medicine, agriculture, information technology, biomedical, optical, electronics, catalysis, environment etc., MNPs in particular have potential application in the biomedical field due to its anticancer properties, enhancement in radiotherapy, drug delivery, thermal ablation, antibacterial properties and so on.Nanoparticles provide effective carriers for biomolecules such as DNA, RNA, or proteins, protecting these materials from degradation and transporting them across the cell-membrane barrier.The size of nanoparticles used for the drug delivery was greater than 100 nm.As the size of nanoparticles is less they can penetrate through the cell membrane and deliver the drug to the target [3].
Collective oscillation of conduction electrons at a resonant frequency lead to a phenomenon called as surface plasmon resonance (SPR).The material absorbs light at the SPR resonant wavelength, the free electrons in the conduction band start to vibrate and dissipate energy.This process usually occurs at the surface of the material.The size of the nanoparticle increases when the plasmon resonance wavelength moves towards longer wavelength (red shift).These properties find potential applications in biosensors as they tend to cover water window regions [4].Silica-Au Nano shells are tuned to perform SPR at near-infrared regions [5].
There is also a trend in enhancement of luminescence behaviour in the AgO nanoparticles leads towards development of surface enhanced Raman scattering (SERS) technologies.These nanoparticles have been widely used for plasmonic surface enhancement and also amplify the optical response of an electrochemical sensors.The electrochemical mechanism is based on the SPR which is influences the plasmonic metal nanoparticles with desired size and shape [6].
In general, cancer therapy is based on the destruction of cancer cells by lasers.When the highintensity laser is incident on cancer cells the neighbouring cells might get affected.To control this, nanoparticles are used such that damage to the healthy cell is controlled [3].
This work discusses the synthesis of AgO nanoparticles from cow urine and cow dung cakes as fuels in the thermal decomposition process.As part of the Indian knowledge, traditionally agnihotri's use cowdung cakes for the sacrifices (havan's).This cow dung cake as fuel has been an inspiration to perform this research work in understanding the structure-property relationship of AgO nanoparticles.The prepared samples were subjected towards XRD and UV-Vis-NIR studies.

Synthesis of Ag2O3 nano-composites:
Silver chloride of 0.01mol% was transferred into a agate mortar pestle and grinded to a fine homogenous powder mixed with the appropriate amount of cow dung cake and cow urine (Bos Indicus) and the later mixture was transferred into a porcelain crucible.These samples were subjected to heat treatment in the muffle furnace at 600 ℃ and held at the operating temperature for four hours.During this process, as part of the thermal decomposition process, as the operating temperature is higher than the boiling point of the precursor, they tend to fragment from bulk particles and thereby result in formation of nanoparticles and thus it is indicative to see, that these particles have relatively lesser crystallite size.Thus conventional top-down method may cause significant crystallographic damage in the system [7].The obtained sample was crushed again using agate mortar pestle.These powdered samples were stored in an airtight vial and sent for further characterizations.The sample prepared from cow dung cake is coded as A1, silver chloride with cow dung cake is coded as A2, silver chloride, cow dung cake and cow urine is coded as A3.The schematic process for synthesising Ag Nps with Bos Indicus and cow dung cake as fuel is shown in Figure 1.

XRD
The peaks of AgO particles indicates the Miller planes viz., (102), (024), (134), (22-6), ( 230), (26-1) (1-8-1) of the crystal lattice, which indicates that they have a simple cubic structure [8].Existence of sharp peak around 22°, in sample A1 which is heat treatment of the fuel i.e., cow dung cake indicates that there could be an existence of carbon nanotubes [Ref] with its crystallite size around 28 nm, which is calculated using Debye-Scherrer equation [9].It is also clear that, the Ag-Nps play a dominant role in the synthesis of nanoparticles as in case of samples A2 and A3, the featured peak at 22° is missing.To elucidate its crystal structure, these samples were subjected to Rietveld refinement using expo2014 software on the raw XRD data [10].It was found that the raw XRD data fitted the standard model with the least error.Williamson-Hall analysis was performed on the XRD data of the samples and their crystallite size and the strain were calculated using the formula C = 0.9  m = ε.Where 'C' is the y-intercept obtained from the graph plotted of 4sin θ versus βcos θ and m is the slope of the graph which were depicted in figure 2 and values were recorded in table 1.The slope gives the strain of the nanoparticle.We can notice that the size of nanoparticles was found to be 77.9 nm and 75.5 nm for A2 and A3 respectively.The strain was found to be 0.0017 and 0.00183 for A2 and A3 respectively.77.9922×10 -9 75.5025×10 -9 χ 2  1.08×10 -6 2.86×10 -7 R 2 0.844972267 0.948868412

UV-Visible Spectroscopy
The UV-Vis absorption spectrum of the sample A1 shows absorption peak at 220 nm is assigned to the π-plasmon of the graphitic structure, and 259 nm originates from the absorption features of CNT.The absorption spectrum obtained in our experiment provides evidence that the nanocarbons in the collision products comprise many defective carbon nanostructures instead of fine carbon nanostructures [11].The synthesised AgO nanoparticles (A2 and A3) has shown higher absorption peak at 251 to 271 nm respectively which indicates the role of organic bond from the fuels and the existence of silver oxide is reflected through the characteristic SPR peak around 431-440 nm [1].The position and shape of the surface plasmon absorption of noble metal nanoclusters are strongly dependent on the particle size, dielectric medium and surface adsorbed species, and may shift due to various factors during the production process.AgO nanoparticles have free electrons, which give SPR absorption band due to the combined vibration of electrons of AgO nanoparticles as well as from the result it can be tuned with the synthesis process and also with change in composition of the reducing agents [12], [13].Additionally the energy bandgap values of the prepared CNT is 2.73eV and AgO nanoparticles (A2 and A3) is 3.08 and 2.91eV [14].The size of NPs estimated theoretically by applying Mie theory of scattering as mentioned in the reference [15]; Where, Vf is the Fermi velocity, c is speed of light in vacuum, Δλ is FWHM, λ 2 SPR is the surface plasmon resonance wavelength.
The size of the nanoparticle was found to be 4.41 nm and 6.31 nm for A2 and A3 samples respectively.The reduction in bandgap also indicates the red shift in its energy which correlates with higher wavelength shift in its SPR positions and the results correlate with the increase in the size of nanoparticle for sample A3.

Conclusion
The Silver nanoparticles were synthesised through thermal decomposition method using gomutra and cow dung cake as fuel agents.XRD analysis of the sample A1 indicates the formation CNT, A2 and A3 dominates with the formation of Ag2O3 nanoparticles.The size of the nanoparticles was found to vary due to the fuel agents.The UV-Vis characteristic peaks of CNT were found at 220 and 259 nm, the bandgap is at 2.73eV and AgO nanoparticles were found at 251 and 271 nm and 431 and440 nm, and the bandgap of the nanoparticles were found to be 2.91 and 3.08eV and the red shift in the SPR positions are clearly indicative that that the size of nanoparticle is slightly larger with combination of cow dung cake and gomutra together than cow dung cake alone.

Figure 1 .
Figure 1.Schematic diagram of thermal decomposition method

Figure 2 .
Figure 2. XRD patterns of the CNTs and nanosized Ag2O3 samples

Table 1 .
Results of W-H Plot.