UV-Light Mediated Biosynthesis of Silver Nanowires; Characterization, Dye Degradation Potential and Kinetic Studies

: Herrin, a simple and eco-friendly method for the synthesis of silver nanowires (Ag-NWs) has been reported. Silver nanowires were synthesized using Psidium guajava seed extract that acted as a reducing agent as well as a stabilizing agent for silver nitrate solution. Synthesis was carried out at 50 ◦ C temperature under continuous UV-irradiation. Silver nanowires were initially characterized by a UV-visible and FTIR spectrophotometer. In addition, morphology and particle size of synthesized Ag-NWs were determined using Field Emission Scanning Electron Microscopy and X-ray diffraction (XRD) techniques. Nanowires were found to have 12.8 µ m length and 200–500 nm diameter and cubic phase morphology. Furthermore, the catalytic potential of Ag-NWs for the degradation of methyl orange dye (MO) was determined. The selected dye was degraded successfully that conﬁrmed the catalytic potential of Ag-NWs. The authors concluded that Ag-NWs can be synthesized using plant extract having excellent morphological features as well as impressive catalytic potential.


Introduction
Synthetic organic dyes have been reported to cause countless serious threats to the ecosystem and human health as a huge amount of these chemicals is being used on daily basis. The tremendous growth of textile industries has increased the use of dyes as well as pigments [1][2][3]. The effluents from such industries are usually discharged into the environment without treatment, which alters the composition of surface and ground water and risks the health of living beings. With the passage of time, dyes in an aqueous medium undergo chemical degradation followed by transformation to toxic chemical entities [4]. The degradation products of dyes indirectly or directly enter the food web and originate unadorned toxic impacts on living beings.

Preparation of Psidium Guajava Seeds Extract
Psidium guajava seeds were collected washed and dried in an oven around 50 • C till 48 h. Psidium guajava seeds were powdered using a grinder and powder of 0.40 micron was collected using specific sieves. Psidium guajava seeds (0.1 g) were taken in a flask and 100 mL distilled water was added followed by constant stirring at 40 • C temperature for 50 min [39]. After filtration, the plant extract was collected and concentrated using a rotary evaporator [40].

Synthesis of Silver Nanowires
In order to prepare Ag-NWs, Psidium guajava seeds extract (5 mL) was added into the aqueous solution of AgNO 3 (1 Mm/10 mL) under standard conditions. The mixture was constantly stirred at 50 • C with continuous irradiation of UV-light of 265 nm for four hours. The color change from milky white to yellow and finally orange indicated the formation of the silver nanowires [41]. Ag-nanowires were collected after filtration followed by oven drying at 50 • C for 30 min ( Figure 1A,B).

Synthesis of Silver Nanowires
In order to prepare Ag-NWs, Psidium guajava seeds extract (5 mL) was added into the aqueous solution of AgNO3 (1 Mm/10 mL) under standard conditions. The mixture was constantly stirred at 50 °C with continuous irradiation of UV-light of 265 nm for four hours. The color change from milky white to yellow and finally orange indicated the formation of the silver nanowires [41]. Ag-nanowires were collected after filtration followed by oven drying at 50 °C for 30 min ( Figure 1A Step-wise representation for the formation of Silver-Nanowires (Ag-NWs) by using Psidium guajava seeds extract under UV-irradiation. (B). Extract + AgNO3 solution (a), Orange colored Ag-NWs (b), Ag-NWs powder (c).

Characterization of Ag Nanowire
The fabrication of Ag-NWs 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. 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 × π/180), λ = 1.5406 × 10 −10 .and cos θ = Bragg's 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 the USA (Hillsboro, ORE, USA). The functional group deter-mination was carried out by utilizing Alpha-II FTIR-ATR by BRUKERS Internationals (Urbandale, IA, USA).

Catalytic Potential of Ag-NWs
In order to evaluate the catalytic potential of Ag-NWs, degradation of methyl orange (MO) dye was conducted in the presence of Ag-NWs. Sodium borohydride (0.6 mL of 17.6 Step-wise representation for the formation of Silver-Nanowires (Ag-NWs) by using Psidium guajava seeds extract under UV-irradiation. (B). Extract + AgNO 3 solution (a), Orange colored Ag-NWs (b), Ag-NWs powder (c).

Characterization of Ag Nanowire
The fabrication of Ag-NWs 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. 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 × π/180), λ = 1.5406 × 10 −10 .and cos θ = Bragg's 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 the USA (Hillsboro, ORE, USA). The functional group deter-mination was carried out by utilizing Alpha-II FTIR-ATR by BRUKERS Internationals (Urbandale, IA, USA).

Catalytic Potential of Ag-NWs
In order to evaluate the catalytic potential of Ag-NWs, degradation of methyl orange (MO) dye was conducted in the presence of Ag-NWs. Sodium borohydride (0.6 mL of 17.6 mM) along with Ag-NWs as catalyst (0.4 mL of 0.64 to 3.84 mg/mL) was used for dye degradation (1.6 mL of 0.062 to 0.102 mM). Reaction mixtures with and without Ag-NWs were monitored using UV/Vis spectra from 300-800 nm every 2 min.

UV-Visible Analysis of Ag-NWs
UV-visible spectroscopy is a convenient and preliminary method for the characterization of nanomaterials [42]. The Ag-nanowires bands show a strong absorption band and produce certain colors due to the surface plasma resonance appearing at 435.0 nm with a progressive increase in absorbance for 30 min (Figure 2) [43]. The characteristic absorption peak for Ag-nanowires synthesized by any method is reported from 300-440 nm ranges which is due to the oscillation of electrons in the conduction band [44,45]. Moreover, the trapped electron can also be involved in intra-center transitions between Ag 0 /Ag + , due to which the absorption band could appear in the 440-550 nm range [46]. It is clear from spectra that there is no peak in Psidium guajava seed extract solution which illustrates the absence of Ag-NWs. But after mixing AgNO 3 solution with Guajava extract, a change in color to yellowish-brown represented the reduction of silver ion and a peak appeared at around 435.0 nm due to surface plasma resonance that confirmed the synthesis of nanowires [47]. mM) along with Ag-NWs as catalyst (0.4 mL of 0.64 to 3.84 mg/mL) was used for dye degradation (1.6 mL of 0.062 to 0.102 mM). Reaction mixtures with and without Ag-NWs were monitored using UV/Vis spectra from 300-800 nm every 2 min.

UV-Visible Analysis of Ag-NWs
UV-visible spectroscopy is a convenient and preliminary method for the characterization of nanomaterials [42]. The Ag-nanowires bands show a strong absorption band and produce certain colors due to the surface plasma resonance appearing at 435.0 nm with a progressive increase in absorbance for 30 min (Figure 2) [43]. The characteristic absorption peak for Ag-nanowires synthesized by any method is reported from 300-440 nm ranges which is due to the oscillation of electrons in the conduction band [44,45]. Moreover, the trapped electron can also be involved in intra-center transitions between Ag 0 /Ag + , due to which the absorption band could appear in the 440-550 nm range [46]. It is clear from spectra that there is no peak in Psidium Guajava seed extract solution which illustrates the absence of Ag-NWs. But after mixing AgNO3 solution with Guajava extract, a change in color to yellowish-brown represented the reduction of silver ion and a peak appeared at around 435.0 nm due to surface plasma resonance that confirmed the synthesis of nanowires [47].
The increase in absorption from 300 nm ( Figure 2) corresponded to the development of different sizes of silver nanowires.

FTIR Studies of Ag NWs
FTIR analysis of P. Guajava extract and Ag-NWs was conducted to find out the functional groups accountable for the reduction and capping of silver ions ( Figure 3). The existence of peaks at 3400-3300 cm −1 and 1600 cm −1 may perhaps be due to -OH stretching of the alcohol and carboxylic acids, respectively [48,49]. The peak 3400-3300 cm −1 was shifted towards a shorter wavelength after reacting with silver, maybe due to the interaction of Ag with the carboxylic acid -OH group present in the extract [50,51]. The peaks around 2197 cm −1 and 1619 cm −1 affirm the presence of α, β-substituted unsaturated carbonyl group-containing entities. Two peaks observed at 2161 cm −1 and 2009 cm −1 confirm the iso-cyanate and thio-cyanate functionalities, respectively [52,53]. The peaks at 1735 The increase in absorption from 300 nm ( Figure 2) corresponded to the development of different sizes of silver nanowires.

FTIR Studies of Ag NWs
FTIR analysis of P. guajava extract and Ag-NWs was conducted to find out the functional groups accountable for the reduction and capping of silver ions (Figure 3). The existence of peaks at 3400-3300 cm −1 and 1600 cm −1 may perhaps be due to -OH stretching of the alcohol and carboxylic acids, respectively [48,49]. The peak 3400-3300 cm −1 was shifted towards a shorter wavelength after reacting with silver, maybe due to the interaction of Ag with the carboxylic acid -OH group present in the extract [50,51]. The peaks around 2197 cm −1 and 1619 cm −1 affirm the presence of α, β-substituted unsaturated carbonyl group-containing entities. Two peaks observed at 2161 cm −1 and 2009 cm −1 confirm the iso-cyanate and thio-cyanate functionalities, respectively [52,53]. The peaks at 1735 cm −1 and 1647 cm −1 confirm the presence of ortho-substituted six-membered lactone. The -OH bending of α, β-unsaturated carboxylic was recorded at 1430-1400 cm −1 and it becomes stronger in the case of Ag-NWs [53][54][55]. The hydrogen bonding between -OH group of nanostructures and α, β-unsaturated carboxylic acts to reduce silver nitrate to silver ions and also works as a capping agent and stabilizes the Ag-NWs [49] (Table 1).

SEM analysis of Ag-NWs
The morphology and surface properties of the product were determined by field emission scanning electron microscopy (FE-SEM) (Figure 4a-d). It is evident that the product has a wire-like morphology mainly formed and stabilized due to the action of unsaturated linear ketones. The well-separated wires have ~12.8 μm length and ~200-500 nm diameter. This unique morphology results in an excellent surface area expansion, resulting in brilliant catalytic removal of the organic dyes. The performance of a catalyst is chiefly dependent on the surface area, stability, surface charge and shape of the molecules of a catalyst. Amongst them, surface area and shape have been shown to have a more powerful role in the adsorption and reduction of the adsorbed entities in photocatalysis phenomena as mentioned in previous works [56].

SEM Analysis of Ag-NWs
The morphology and surface properties of the product were determined by field emission scanning electron microscopy (FE-SEM) (Figure 4a-d). It is evident that the product has a wire-like morphology mainly formed and stabilized due to the action of unsaturated linear ketones. The well-separated wires have~12.8 µm length and~200-500 nm diameter. This unique morphology results in an excellent surface area expansion, resulting in brilliant catalytic removal of the organic dyes. The performance of a catalyst is chiefly dependent on the surface area, stability, surface charge and shape of the molecules of a catalyst. Amongst them, surface area and shape have been shown to have a more powerful role in the adsorption and reduction of the adsorbed entities in photocatalysis phenomena as mentioned in previous works [56].

XRD Analysis of Ag-NWs
Silver nanowires were also characterized using the X-ray diffraction method to elucidate the structure ( Figure 5). The fingerprint pattern has four typical diffraction features corresponding to (111), (200), (020), (220) and (131) planes, and all the four peaks might be indexed to the phase-centered cubic structure of silver (JCPDS-04-0783) [57,58]. The final product owes 100% purity as there was no peak detected for reflection, which mainly corresponds to nitrate ions of precursor solution and other impurities. The peak intensity profile was individual of the face-centered cuboidal structure of Ag-nanowires. The crystallite size is not entirely clear in FESEM due to the large nanowire diameter [59]. However, the particle size is calculated by the Debye-Sherrer Equation. (1) where Davg is the average crystallite size, constant K is the shape factor (0.9), λ is the wavelength (0.154 nm), β corresponds to the peak width at half maximum intensity while θ is the peak position. The Davg Ag-NWs is 24.83 nm, which is quite close to the reported one (30-35 nm) [59,60].

XRD Analysis of Ag-NWs
Silver nanowires were also characterized using the X-ray diffraction method to elucidate the structure ( Figure 5). The fingerprint pattern has four typical diffraction features corresponding to (111), (200), (020), (220) and (131) planes, and all the four peaks might be indexed to the phase-centered cubic structure of silver (JCPDS-04-0783) [57,58]. The final product owes 100% purity as there was no peak detected for reflection, which mainly corresponds to nitrate ions of precursor solution and other impurities. The peak intensity profile was individual of the face-centered cuboidal structure of Ag-nanowires. The crystallite size is not entirely clear in FESEM due to the large nanowire diameter [59]. However, the particle size is calculated by the Debye-Sherrer Equation.
where D avg is the average crystallite size, constant K is the shape factor (0.9), λ is the wavelength (0.154 nm), β corresponds to the peak width at half maximum intensity while θ is the peak position. The D avg Ag-NWs is 24.83 nm, which is quite close to the reported one (30-35 nm) [59,60].

Catalytic Potential of Ag-NWs
Methyl Orange (MO) is extensively used as a textile dye (azo-dye) for the dying of textile fabrics [61]. The reduction of MO was achieved by using freshly prepared Ag-NWs with excessive NaBH4. The rate of reduction of methyl orange without catalyst in the presence of NaBH4 is very slow (Figure 6a). This poor performance is due to the presence of a high energy barrier of mutually repulsive interactions between the borohydride anion and methyl orange ion, which should be overcome only by a catalyst [62]. Moreover, in the presence of a catalyst only no reduction occurs due to the same interactions as mentioned above (Figure 6b). However, in the presence of a catalyst and NaBH4, reduction of azodye takes place (Model reaction). Initially, NaBH4 and catalyst will adsorb on the surface of the dye and then a reduction reaction will proceed at a 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 min (Figure 6c).

Catalytic Potential of Ag-NWs
Methyl Orange (MO) is extensively used as a textile dye (azo-dye) for the dying of textile fabrics [61]. The reduction of MO was achieved by using freshly prepared Ag-NWs with excessive NaBH 4 . The rate of reduction of methyl orange without catalyst in the presence of NaBH 4 is very slow (Figure 6a). This poor performance is due to the presence of a high energy barrier of mutually repulsive interactions between the borohydride anion and methyl orange ion, which should be overcome only by a catalyst [62]. Moreover, in the presence of a catalyst only no reduction occurs due to the same interactions as mentioned above (Figure 6b). However, in the presence of a 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 a reduction reaction will proceed at a 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 min (Figure 6c).

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 added to the reaction mixture. It is evident from Figure 7a, that from 0 to 5 min, the reaction started with a very slow speed as there are molecules moving towards the surface of Ag-NWs and speed up from 6 to 20 min due to their interaction at the surface of the catalyst, then ultimately reaches to completion after 20-25 min. The inset graph shows the negative slopes used for the determination of k obs [63].

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 added to the reaction mixture. It is evident from Figure 7a, that from 0 to 5 min, the reaction started with a very slow speed as there are molecules moving towards the surface of Ag-NWs and speed up from 6 to 20 min due to their interaction at the surface of the catalyst, then ultimately reaches to completion after 20-25 min. The inset graph shows the negative slopes used for the determination of kobs [63].
It is evident from Figure 7b, the rate of reaction increases fasts from 0.60-1.28 mg/mL because the degradation takes place exponentially at the start of the reaction due to the presence of active sites. Later on, the reaction rate becomes slow from 2.56-3.84 mg/mL due to occupied active sites [64]. The most effective degradation out of all the adsorbents' concentrations was monitored at 1.28 mg/mL with the help of kobs graph plotted between kobs vs concentration of Ag-NWs while keeping the NaBH4 concentration (17.6 mM) and MO-dye (0.082 mM) constant. The half-life (min) of the observed rate constant (min −1 ) calculated for these pseudo-first-order reactions is mentioned in Table 1. It is evident from Figure 7b, the rate of reaction increases fasts from 0.60-1.28 mg/mL because the degradation takes place exponentially at the start of the reaction due to the presence of active sites. Later on, the reaction rate becomes slow from 2.56-3.84 mg/mL due to occupied active sites [64]. The most effective degradation out of all the adsorbents' concentrations was monitored at 1.28 mg/mL with 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. The half-life (min) of the observed rate constant (min −1 ) calculated for these pseudo-first-order reactions is mentioned in Table 1.

Effect of MO Dye
MO dye's concentrations were taken, ranging from 0.072 to 0.102 mM with 0.010 mM difference. Figure 8a shows the induction, reaction and completion times of the catalytic reaction between MO-dye and NaBH4 on the surface of Ag-NWs. As the concentration of the MO-dye increases, while keeping the concentrations of Ag-NWs and NaBH4 constant, the time of degradation increases [65]. The reason behind this factor is the constant concentration of Ag-NWs, which illustrates the limitation of surface-active sites for incoming reactants [66]. The figure inset (6) shows the negative slopes for finding the apparent rate constant for Pseudo first-order reaction following the Langmuir-Hinshelwood mechanism (LHM) [67,68]. Figure 8b, illustrates the graph between kobs on ordinate and concentration of MOdye on abscissa while keeping catalyst dosage (1.28 mg/mL) and NaBH4 (17.6 mM) constant.

Effect of MO Dye
MO dye's concentrations were taken, ranging from 0.072 to 0.102 mM with 0.010 mM difference. Figure 8a shows the induction, reaction and completion times of the catalytic reaction between MO-dye and NaBH 4 on the surface of Ag-NWs. As the concentration of the MO-dye increases, while keeping the concentrations of Ag-NWs and NaBH 4 constant, the time of degradation increases [65]. The reason behind this factor is the constant concentration of Ag-NWs, which illustrates the limitation of surface-active sites for incoming reactants [66]. The figure inset (6) shows the negative slopes for finding the apparent rate constant for Pseudo first-order reaction following the Langmuir-Hinshelwood mechanism (LHM) [67,68]. sorption of the incoming MO-dye molecules on the surface catalytically enhanced by Ag-NWs. This point is fruitful in the optimization of reaction. After this concentration, the decrease in kobs value is due to the excessive amount of MO-dye molecules on the surface of limited Ag-NWs. Here, the double-layer adsorption occurs on the surface of Ag-NWs, which restricts the effective adsorption. Half-life (min) and kobs values were calculated and incorporated in Table 2.

Conclusions
The Ag-NWs were successfully fabricated using AgNO3 as a precursor and Psidium guajava seed extract under continuous irradiation of UV light. The extract strongly acted as a reducing as well as a stabilizing agent. UV-visible spectra confirm the formation of the Ag-NWs. The average crystallite size of Ag-NWs was 196.4 nm, and the morphology was cubic face having 12.8 μm length and internal diameter of 200-500 nm. Synthesized  This graph explains that initially, the k obs value remains constant while, at 0.082 mM its concentration increases to the maximum k obs value. This is due to the maximum adsorption of the incoming MO-dye molecules on the surface catalytically enhanced by Ag-NWs. This point is fruitful in the optimization of reaction. After this concentration, the decrease in k obs value is due to the excessive amount of MO-dye molecules on the surface of limited Ag-NWs. Here, the double-layer adsorption occurs on the surface of Ag-NWs, which restricts the effective adsorption. Half-life (min) and k obs values were calculated and incorporated in Table 2.

Conclusions
The Ag-NWs were successfully fabricated using AgNO 3 as a precursor and Psidium guajava seed extract under continuous irradiation of UV light. The extract strongly acted as a reducing as well as a stabilizing agent. UV-visible spectra confirm the formation of the Ag-NWs. The average crystallite size of Ag-NWs was 196.4 nm, and the morphology was cubic face having 12.8 µm length and internal diameter of 200-500 nm. Synthesized Ag-NWs was then used for the catalytic degradation MO-dye, NaBH 4 and Ag-NWs. This study proposed reaction completion merely in 25 min and kinetic studies of the data confirmed pseudo-first-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 the degradation of azo-dyes that can be a good tool for the treatment of water from the textile industry.

Highlights
• UV-light-mediated green synthesis of Ag-NWs using Psidium guajava seed extract.

•
Dimensions of Ag-NWs were 12-8 µm length and 200-500 nm diameter. • Morphology of Ag-NWs was confirmed by SEM and XRD. • Methyl orange dye degradation was achieved using Ag-NWs. • Kinetics of degradation reaction was studied.