UV-light Mediated Green Synthesis of Silver Nanowires and Their Catalytic Degradation Potential Against Methyl Orange

In the present study, a simple and eco-friendly method for the synthesis of silver nanowires (Ag-NWs) has been reported. Psidium guajava seed extract was used as a reducing agent for silver nitrate solution at 70 °C temperature under continues UV-irradiation, for the production of Ag-NWs. Silver nanowires were initially characterized by using UV-visible and FTIR spectrophotometer. Synthesis of nanowires and involvement of functional groups was conrmed by FT-IR spectra. The morphology and particle size of synthesized Ag-NWs was determined using Field Emission Scanning Electron Microscopy (FESEM) and X-ray diffraction (XRD). XRD results revealed cubic phase morphology of Ag-NWs. Nanowires were found having 12-8 μm length and 300-500 nm diameter. In addition, catalytic potential of the sample for degradation of methyl orange dye (MO) was tracked spectrophotometrically. The results exposed that; bio-synthesized silver nanowires were found having excellent morphological features as well as impressive catalytic potential.


Introduction
Heavy metals, organic and un-natural dyes cause countless serious complications to human health and eco-system, however, manipulation of such chemicals are unescapable due to their widespread applications in diverse meadows of science and technology. With the hasty growth of industries for instance ore mining, textile industries, paper and paint industry, fertilizers and pesticide industries etc., an extensive aggregate of dyes, heavy metals and pigments are predisposed afterward the industrial progressions (Mance, 2012). The e uents commencing from such industries are discharged-off into natural-surfaces and ground-water resources. These toxic-e uents amends the composition of surfacewater and consequently sound effects to the healthiness of living-beings. With the passageway of time, pigments and dyes available in water submit yourself to degradation chemically and transformed to supplementary hazardous toxic-chemical entities (Chang & Chen, 2009). Finally, these heavy metals and dyes possibly will indirectly or directly enter the food-web and originate unadorned toxic-impacts on surroundings. Thus, it is lethal to become aware of and reduce such engineering-wastes from water.
However, biological synthesis is the hot choice with advantages over physical and chemical methods as it is quick, eco-friendly, highly stable, and cost-effective. Biological synthesis does not acquire any culture growth and does not produce toxic residues to contaminate the atmosphere (Kulkarni & Muddapur, 2014).
Catalytic reduction mainly depends upon the morphology (Sun, 2010)  Herein, we are reporting a facile and green approach to synthesize the Ag-Nanowires (Ag-NW) by using Psidium guajava seed extract to stabilize the nanowires, without any external stabilizers like ITO-glass electrode, electrostatic charge which was the classical approach to synthesize the nanowires. The plant seed extract by dissolving 0.1 g of the dry and nely divided form in 100 mL water at 60°C for 50 minutes followed by the vacuum ltration and used as a precursor for the reduction of silver ions. Then freshly prepared 0.1 mM solution of AgNO 3 was taken in conical ask. The addition of 5 mL of Psidium guajava seed extract was done to AgNO 3 under continues stirring and UV-irradiation of 265 nm wavelength white light, and stirred the reaction mixture for 4 hours. This is rst effort to synthesize Ag-NW via a green, ecofriendly and template free pathway, with greater stability of Ag-NW.

Materials
Silver nitrate was acquired from Merck, Germany, sodium hydroxide (NaOH) and sodium borohydride (NaBH 4 ) was obtained from Sigma Aldrich, UK. The methyl orange dye was purchased from Fisher, Pakistan Ltd. All chemicals were in analytical grade and used without further puri cation. Double deionized water was used throughout the analysis.

Preparation of Psidium Guajava Seeds Extract
Psidium guajava seeds were collected washed and dried in oven around 60°C till 48 hours. Ground the Psidium guajava seeds in grinder and ltered with mesh sieved of 0.40 micron and stored in Eppendorf tube. The 0.1 g of the Psidium guajava seeds in 100 mL distilled water were heated in a conical ask with constant stirring at 60°C temperature for 50 minutes (Sharma et al., 2021). The ltration was carried out to separate the plant extract with vacuum ltration apparatus. Freshly prepared plant extract was used for the further experiments (Sathiyavimal et al., 2021).

Synthesis of Silver Nanowires
A solution of AgNO 3 of 0.1 mM concentration was prepared by dissolving AgNO 3 in deionized water. In a classic green-synthesis practice, Psidium guajava seeds extract was used to reduce and cap the Ag ions. 5 mL of Psidium guajava seeds extract was added into the aqueous solution of AgNO 3 under standard conditions. The mixture was constantly stirred at 70°C in a homemade UV-lamp containing a single wavelength of 265 nm of white light for four hours by using magnetic stirrer. The color change from milky white to yellow and nally orange was showing the formation of the silver nanowires. Ag-nanowires were obtained on ltration and followed by calcination at 50°C for 30 minutes (Scheme 1 and Fig. 1).

Catalytic activity of MO dye
Catalytic-degradation of MO dye was supervised over UV/Vis spectrophotometer using Psidium Guajava seed extract's stabilized Ag-NWs. The degradation was done by time dependent kinetic studies with constant 3 minutes' time interval. 0.4 mL quartz cuvettes were utilized during all reactions. 0.6 mL of 17.6 mM constant concentration of NaBH 4 was used for the optimized model reaction as well as for both factors study. For catalyst Ag-NWs dosage factor, 0.4 mL of different concentrations of Ag-NWs (0.64 to 3.84 mg/mL) were used. Similarly, 1.6 mL of MO dye having concentration ranges from 0.062 to 0.102 mM were used for MO dye's factor study. All the reactions follow Pseudo rst order reaction with Langmuir-Hinshelwood mechanism (LHM) as the 200% more NaBH 4 was used in contrast with MO dye.
Reaction mixture without Ag-NWs was observed over UV/Vis spectra ranges from 300-800 nm as reference to compare with reaction mixture containing Ag-NW's UV/Vis spectra for the con rmation of Ag-NWs.

Characterization Of Ag-nws
The fabrication of AgNWs was preliminarily established by recording the absorbance in UV/Vis spectra at a range of 300-800 nm. The change in Surface Plasmon Resonance (SPR) of nanoparticles in the dispersion was recorded using UV/Vis spectrophotometer by CECIL-Aquarius 7400 ce UK. The XRD patterns of Ag-nanowires were collected on Bruker AXS-D8 Advanced X-Ray diffractometer with Cu Kα radiations of λ = 1.5406 Å and scanning angle 2θ over the range of 10-80˚. Crystallite size was calculated by using Scherer Equation CS = Kλ/β cos θ, where CS is the crystallite size, constant K = 0.94, β is the full width at half maximum (FWHM), (β = FWHM x π/180), λ = 1.5406×10-10 and cos θ = Braggs angle. Fourier Transformation Infrared Spectroscopy (FTIR) was used to characterize the nanoparticles using the powder sample by ATR in the range of 400-4000 cm − 1 . Scanning electron microscopy (SEM) images were recorded using FEI-NOVA-450 Nano-SEM (FE-SEM) by USA. The functional group determination was carried out by utilizing Alpha-II FTIR-ATR by BRUKERS Internationals (USA).

FTIR spectra of Ag NWs
FTIR spectrum of as-prepared Ag-NWs was conceded to pinpoint the biomolecules accountable for the reduction and capping of silver ions (Fig. 3). The existence of peaks at 3341.8, 2927.4 and 1234 cm − 1 may perhaps be due to -N-H stretching of the secondary amine and its salts and primary amine, respectively. The peak 3341.8 cm − 1 shrink in case of silver due to interaction -OH group of nanowires with amines. The peaks around 2197.4 cm − 1 and 1619 cm − 1 a rms the presence of α, β-substituted unsaturated ketones. Two peaks observed at 2161.3 cm − 1 and 2009.4 cm − 1 con rms the iso-cyanate and thio-cyanate functionalities, respectively. The peaks at 1735 cm − 1 and 1647 cm − 1 con rm the presence of ortho-substituted six-membered lactone. The -OH bending of α, β-unsaturated carboxylic was recorded at 1419.6 cm − 1 . Secondary amine its salts and α, β-unsaturated carboxylic acts as reduced silver nitrate to silver ions also work as a capping agent and stabilizes the Ag-NWs (Le et al., 2021).

Scanning Electron Microscopy
Morphology and surface properties of the product was determined by eld emission scanning electron microscopy (FE-SEM) (Fig. 4). It is evident that the product has wires i.e., morphology mainly formed and stabilized due to the action of unsaturated linear ketones. The well separated wires have ~ 12 − 8 µm length and ~ 500 − 300 nm diameter. This unique morphology results an excellent surface area expansion, mainly responsible for the brilliant catalytic reduction of the organic dyes. The performance of a catalyst chie y dependent on surface area, stability, surface charge, and shape of the molecules of a catalyst. Amongst them surface area and shape have shown more powerful role for the adsorption and reduction of the adsorbed entities in photocatalysis phenomena as mentioned in previous works (Satsangi, 2020

XRD analysis of Ag-NWs
As-obtained Ag-NWs were further characterized by X-ray diffraction method to elucidate the crystal structure and crystallite size (Fig. 5). The nger-print pattern has four typical diffraction features corresponds to (111), (200), (020), (220), and (131) planes, and all the four peaks might be indexed to standard cubic phase of silver (JCPDS-870720) (Nagasundari, Muthu, Kaviyarasu, Al Farraj, & Alkufeidy, 2021). The nal product owes 100% purity as there was no peak detected for re ection, mainly corresponds to nitrate ions and other impurities. The peak intensity pro le was individual of simple cubic construction of Ag-nanowires. The size of Ag-NWs was determined by using Debye-Scherrer equation-1 (Lim, Marks, & Rowles, 2020). (eq-1) is the crystalline size, k is the wavelength of x-ray used, b is the full width at half maximum light of the maximum intensity peak and h is the Bragg's angle. The crystallite size of the synthesized Ag-nanowires is estimated ~ 19.63 nm.

Catalytic Reduction Of Methyl Orange (Mo)
Methyl Orange (MO) is widely used as an indicator as well as a textile dye (azo-dye) for dying of textile fabrics (Carolin, Kumar, & Joshiba, 2021). The reduction of MO was calculated by using freshly prepared Ag-NWs in excessive NaBH 4 . The rate of reduction of methyl orange without catalyst in the presence of NaBH 4 is very slow (Fig. 6-a). This poor performance is due to the presence of high energy barrier of mutually repulsive interactions between the borohydride anion and methyl orange ion, which should be overcome only by catalyst (Wu et al., 2020). Moreover, in the presence of catalyst only no reduction occurs due to the same interactions as mentioned above (Fig. 6-b). However, in the presence of catalyst and NaBH 4 , reduction of azo-dye takes place (Model reaction). Initially, NaBH 4 and catalyst will adsorb on the surface of the dye and then reduction reaction will proceed at faster rate. The characteristic peak of MO solution was recorded ~ 458 nm and catalytic reduction was observed by a sharp decline in intensity merely in 25 minutes (Fig. 6-c).

Effect of Catalyst dosage
The effect of concentration of catalyst-dosage was determined by changing the amount of Ag-NWs from 0.60-3.84 mg/mL (Fig. 7-a). The rate of reaction increases fasts from 0.60-1.28 mg/mL because the adsorption takes place exponentially in the start of reaction due to the presence of active sites. Later on the reaction rate becomes slow from 2.56-3.84 mg/mL due to the occupation of the available active sites (Rodwihok et al., 2020). The effective degradation out of all the adsorbent's concentration was monitored at 1.28 mg/mL by the help of k obs graph plotted between k obs vs concentration of Ag-NWs, while keeping the NaBH 4 concentration (17.6 mM) and MO-dye (0.082 mM) constant.
It is evident from the Fig. 7-b, that from 0-5 minutes' reaction was started with a very slow speed as there are molecules moving towards the surface of Ag-NWs and speed-up from 6-20 minutes due to their interaction at the surface of catalyst, then ultimately reaches to completion after 20-25 minutes. The inset graph shows the negative slopes which used for the determination of k obs (Shan et al., 2020).

Conclusion
The Ag-NWs were successfully fabricated using AgNO 3 as precursor and Psidium guajava seed extract under continuous irradiation of UV light. The extract strongly acted as reducing as well as stabilizing agent. UV-visible spectra con rm the formation of the Ag-NWs. The average crystallite size of Ag-NWs was 19.63 nm and morphology were cubic face having 12-8 μm length and internal diameter of 300-500 nm. Synthesized Ag-NWs was then used for catalytic degradation MO-dye, NaBH 4 and Ag-NWs. The reaction completed in 25 minutes and kinetic studies of the data con rmed pseudo rst order reaction. Hence, Psidium guajava seed extract can be used for the synthesis of Ag-NWs. The authors recommended that Ag-NWs can be exploited for degradation of azo-dyes that can be a good tool for treatment of water from textile industry.

Declarations Ethical Approval
All the data and information in this manuscript original and not been published in any other Journal before this.

Con ict of Interest
Author declares no con ict of interest with anyone.

Funding
There is no funding for this project.

Availability of data and materials
All original data and materials are available on demand.