Testing of photocatalytic potential of silver nanoparticles produced through nonthermal plasma reduction reaction and stabilized with saccharides

Abstract

In this study, silver nanoparticles (AgNPs) were produced through an atmospheric pressure plasma reduction reaction and tested for photodegradation of methyl blue (MB) under sunlight exposure. The argon plasma born reactive species were used to reduce silver ions to AgNPs in the solution. Glucose, fructose and sucrose were also added in the solution to stabilize the growth process. The glucose stabilized reaction produced the smallest nanoparticles of 12 nm, while sucrose stabilized reaction produced relatively larger nanoparticles (14 nm). The nanoparticles exhibited rough morphology and narrow diameter distribution regardless of stabilizer type. The narrow diameter distribution and small band gap helped activating majority of nanoparticles at a single wavelength of light spectrum. The band gap energy of AgNPs varied from 2.22 eV to 2.41 eV, depending on the saccharide type. The photoluminescence spectroscopy of AgNPs produced emission peaks at 413 nm, 415 nm, and 418 nm. The photocatalytic potential of AgNP samples was checked by degrading MB dye under sunlight. The degradation reaction reached a saturation level of 98% after 60 min of light exposure.

Keywords

Silver nanoparticles argon plasma jet saccharides photodegradation methyl blue

1 Introduction

The applications of nanotechnology are growing every day in almost all fields of science and engineering, including biomedicines, sensors, chemical engineering, photocatalysis, drug delivery, cell, organelle labeling, imaging, smart devices, etc. High surface area to volume ratio is the key characteristic of nanomaterials, which distinguish them from their bulk counterparts [1]. This characteristic is important for antiviral, antibacterial, catalytic [2], antifungal [3], remedial [4], diagnostic [5], conductive and plasmon applications of nanomaterials [6, 7]. Different routes can be used to synthesize metal nanoparticles for practical applications. Most of these methods remain either extremely costly or leave adverse impact on environment and human health [8, 9]. Usually, these methods are time consuming and require toxic reducing agents [10]. The management of process born waste is also a matter of concern.

The synthesis of nanomaterials using plasma reduction method (PRM) in open air is relatively a new research endeavor [11, 12]. PRM has the potential of production nanomaterials from metal salts without requiring hazardous chemicals, impacting the environment. Since plasma reactive species work as reducing agents, no additional reducing chemicals are needed to reduce the metal ions into nanoparticles [12]. The plasma species are produced during breakup of air or other gases into a discharge at liquid-plasma interface. These species are environmental friendly and have short life time [13]. After reducing the metal salts into nanoparticles, unassimilated plasma species get neutralized by interacting with other radicals. In open air, the possible plasma chemistry includes reactive oxygen species (O, O₂, O₃, OH) and nitrogen species (NO, NO₂) along with UVA and UVB radiations. These species are also effective in disinfecting the pathogens. The plasma emitted UV radiations weaken the bacteria cell wall, while oxidizing species oxidize the cell membrane. The electrostatic forces, developed on the cell wall due to charging of membrane, weakens the tensile strength of the cell. Effective plasma parameters to influence the uniformity, dispersity and morphology of nanomaterials are operating voltage, concentration of metal precursor, stabilizing agent, discharge time and plasma precursor [14, 15].

In PRM, saccharides like glucose [16], fructose [17] and sucrose [14] are used to minimize the agglomeration and stabilize the growth of nanoparticles. Previously, AgNPs have been produced using microplasma technique by adjusting the molar concentration of the stabilizer and salt. However, clear conclusion has never been drawn on the effect of stabilizers on AgNPs production through liquid-plasma interaction method. In the presented work, glucose, fructose and sucrose were tested for their role in stabilizing and shaping AgNPs. These nanoparticles were considered for photodegradation of MB dye under sunlight exposure.

Synthetic dyes and pigments are commonly used by industries such as pharmacy, fruit, paint, cosmetics, paper, textiles, dyeing, and plastics. These dyes can cause various environmental and health problems. Different conventional treatments are used for dye removal, such as filtration, coagulation, reverse osmosis and adsorption. Dyes are more difficult to remove from wastewater because they contain complex molecular structures and high stability [18]. The photocatalytic degradation of synthetic dyes in wastewater is in limelight in recent years [19]. The photoactive metal oxides like titanium dioxide (TiO₂) [20], zinc oxide (ZnO) [21], silver oxide, etc. in their pure form and composited with other materials are being extensively for photodegradation of organic dyes. TiO₂ with band gap of 3.2 eV is the most effective photocatalyst used to derive the dye degradation activity under visible light exposure [22]. However, due to relatively large band gap, it is effective under UV-light, which contributes only 5–10% to the total solar radiations. The visible part of sunlight is more than 50% of total radiations [22]. The nanoparticles of certain transition metals, such as silver, can be good alternative of TiO₂ for photodegradation of organic dyes. AgNPs can be employed as a visible light active photocatalyst for solar energy transformation, hydrogen production and degradation of organic pollutants.

2 Experimental section

2.1 Materials

Silver nanoparticles were synthesized using silver nitrate (AgNO₃), glucose (C₆H₁₂O₆), fructose (C₆H₁₂O₆) and sucrose (C₁₂H₂₂O₁₁). All chemicals were AR grade, which were used without further purification. Deionized water was used to prepare aqueous solutions. The photodegradation experiments were conducted on methyl blue dye, which was purchased from a local chemical store.

2.2 Plasma reduction method setup

Figure 1 shows schematic of plasma jet configuration used for preparation of AgNPs. A stainless-steel needle cathode was used as a plasma jet. The inner diameter of the jet was 0.7 mm and length was 25 mm. The cathode was covered with heat resistive coating. The jet was positioned at 15 mm above the silver salt solution and connected to the negative terminal of high-tension DC supply through a resistance box. The resistance was set at 100 kΩ to avoid breakdown and short circuiting. The positive terminal of battery was connected to the silver anode immersed in the solution. The argon plasma jet was sustained between solution and needle cathode by flowing argon gas through the needle at a fixed flowrate. The argon flow turned into plasma jet by developing a conducting path between positively charged solution and cathode when DC voltage was set at 10 kV. In each experiment, PRM process was carried out for 30 min to produce AgNPs.

Fig. 1

Schematic of plasma reduction setup used for production of AgNPs with different stabilizing agents.

2.3 Synthesis of silver nanoparticles

In a typical one-step synthesis process, 100 ml solution was obtained by adding 5 mM of AgNO₃ and 2 mM of different saccharides (glucose, fructose and sucrose). The effect of saccharides on the synthesis of AgNPs was studied by changing the parameters given in Table 1. The AgNO₃ electrolyte solution was put under plasma exposure and dissociated into Ag⁺ cations and NO₃^– anions (AgNO₃ ⟶ Ag⁺ + NO₃^–). On plasma ignition, the active radicals/compounds and hydrated electrons, such as hydrogen (H), atomic oxygen (O), hydrogen peroxide (H₂O₂) and hydroxyl radical (OH) from plasma reduced Ag⁺ cations to AgNPs (Ag⁺ + e^– ⟶ Ag). The electrons and H radicals irradiation from plasma played key role in the formation of nanoparticles due to fast coefficient rate [23]. On plasma exposure, the solution changes its color from transparent to black with the passage of time. Change in color is an indication of formation of nanoparticles. At the end of the process, AgNPs were separated from the solution through centrifugation and washed with distilled water several times. The particles were dried in a vacuum oven at 80°C for 2 hours. The final product was characterized using UV-vis spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy.

Table 1
Summary of the parameters set for synthesis of AgNPs

Stabilizing agent Molar Conc. Discharge current (mA) DC voltage Constant (kV) Resistance (kΩ) Flow rate (sccm) Discharge Discharge Time

Glucose 2 mM 15 10 100 1200 30 min

Fructose 2 mM 15 10 100 1200 30 min

Sucrose 2 mM 15 10 100 1200 30 min

Stabilizing agent	Molar Conc.	Discharge current (mA)	DC voltage Constant (kV)	Resistance (kΩ)	Flow rate (sccm)	Discharge Discharge Time
Glucose	2 mM	15	10	100	1200	30 min
Fructose	2 mM	15	10	100	1200	30 min
Sucrose	2 mM	15	10	100	1200	30 min

2.4 Catalytic degradation of methyl blue

The synthesized AgNPs were used for the photodegradation of MB dye under visible light irradiation. In this experiment, 0.1 g of AgNPs was added to 100 ml of 10^–8 M dye. The aqueous solution was magnetically stirred for balancing the equilibrium condition and then placed in dark for 12 hours to attain adsorption-desorption equilibrium. The dye solution was irradiated with visible light. A sample of degraded dye was taken after every 10 min to record degradation by using UV-Vis spectroscopy. The volume of all the dye solutions was kept constant by using double distilled water. The photodegradation efficiency of MB was calculated using the equation: $Degradation (%) = \frac{(Ady e_{o} - Ady e_{t})}{Ady e_{o}} \times 100$ (1)

Where Adye₀ is the initial absorbance of MB and Adye_t is the absorbance of MB after certain time t. Stability is also an important indicator for the industrial applications of the photocatalysts. The photocatalytic stability of as-prepared nanoparticles was studied by performing multiple degradation cycles with the same sample of the nanoparticles. The stability of the photocatalyst was determined by performing five successive cycles with the same catalyst sample.

3 Results and discussions

3.1 UV–visible analysis

Figure 2 shows UV spectra of the sucrose, fructose and glucose stabilized AgNPs. The most intense absorption peak was observed near around 300.47 nm. The type of stabilizer did not affect UV adsorption range of the nanoparticles. The maximum optical band gap of AgNPs was measured about 2.41 eV using a sucrose stabilizer. The Band gap energies of AgNPs were calculated using Tauc Plot method (a graph between (αhv)² and hv) according to the following equation: $ah υ = B {(h υ - E_{g})}^{n}$ (2)

Fig. 2

UV absorption spectra of sucrose, fructose and glucose stabilized AgNPs.

Figure 3 shows the Tauc Plots of all three samples. The calculated band gap energies match well with the values reported by Shuaib et al. [13]. As nanoparticles are composed of small number of atoms compared to bulk materials which is the main reason of larger band-gap energy of AgNPs as compared to bulk silver. Thus, the overlapping of energy levels decreases which in turn decreases the bandwidth and increases the gap between valance and conduction band. Conversely, bulk contains a large number of atoms due to which overlapping of energy levels increases which in return increases the band size and decreases the energy gap between valance and conduction band [24].

Fig. 3

Tauc-plots for calculation of band gap energy of glucose stabilized, fructose stabilized and sucrose stabilized AgNPs.

3.2 Morphology and elemental composition of nanoparticles

SEM is important technique to study the morphology of particles at nanoscale. The synthesized AgNPs were encapsulated by saccharides. The type of stabilizer did not affect the shape of the nanoparticles synthesized under liquid-plasma interaction method. It was observed from Fig. 4 that the prepared AgNPs exhibit nearly spherical shape with an unavoidable aggregation due to stabilizing agent and elemental composition of 69.81% weight of pure Ag compared with 16.48% of oxygen. Further, it also shows a higher atomic percentage of 24.45% of Ag and 38.91% of oxygen, as shown in Table 2, and somehow Si and C in very less percentages were also present. The sucrose stabilized nanoparticles showed a strong peak of Ag along with weak carbon, oxygen, potassium, and aluminum peaks, which may originate from the stabilizer that was bound to the surface of AgNPs as shown in Fig. 5.

Fig. 4

SEM images of AgNPs stabilized with (a) glucose, (b) fructose and (c) sucrose.

Table 2

Elemental composition of silver nanoparticles

Element	Apparent Concentration	k Ratio	Wt.%	Standard Label
C	4.93	0.04927	13.25	C Vit
O	5.11	0.0172	15.75	SiO₂
Al	0.82	0.00588	1.4	Al₂O₃
Cl	1.41	0.0123	1.98	NaCl
K	4.17	0.03532	5.59	KBr
Ag	34.16	0.34158	62.03	Ag
Total:			100

Fig. 5

EDX spectra of AgNPs stabilized with (a) glucose (b) fructose and (c) sucrose.

3.3 Structural analysis of nanoparticles

X-ray diffraction (XRD) analysis was used to investigate the crystalline structure of AgNPs. XRD spectra of synthesized AgNPs are shown in Fig. 6. The most intense peak obtained from XRD data is reported at 2θ= 26.404 corresponding to (121) plane. The average particle size of AgNPs was estimated from the well-known Scherrer’s formula [25]. $D = \frac{K λ}{β \cos θ}$ (3)

Fig. 6

XRD spectra of AgNPs stabilized with glucose, fructose and sucrose.

Where, D is the particle size, k is the constant (0.94), λ is the wavelength of X-ray (0.1541 nm) derived from Bragg’s equation (2dsin = nλ), β is the full width at half maximum (FWHM), and θ is the angle of diffraction.

The diffraction peaks in XRD patterns confirmed the crystalline nature of prepared AgNPs. The diffraction peaks for sucrose stabilized AgNPs are relatively wider and smoother compared to those for glucose and fructose stabilized nanoparticles. The observed peaks broadening and noise were probably due to the presence of macromolecules which may be responsible for the reduction of Ag⁺ ions [26]. The average particle sizes were measured about 10, 12, and 14 nm for glucose, fructose and sucrose stabilized AgNPs. The observed XRD results were in good agreement with the structural analysis of AgNPs prepared by biosynthesis method and analyzed by Ibrahim et al. [27].

3.4 Photoluminescence spectroscopy analysis

In UV-Vis spectra, the synthesized AgNPs showed strong absorbance around 300 nm in the visible region (400–800 nm). Photoluminescence (PL) is a process which involves photoexcitation of electrons from lower energy states to higher energy states. The excitations cause fluorescence emissions [24]. The previous literature reveals that the shape and the size of nanoparticles depend on the emission and absorption spectra of nanoparticles in addition to size effect [28, 29]. The glucose, fructose and sucrose stabilized AgNPs showed emission peaks around 413 nm, 418 nm, and 415 nm, respectively, as shown in Fig. 7. The PL spectra of different stabilized AgNPs were recorded under identical conditions with the excitation photon energy of 2.98 eV (325 nm laser line) under the Xenon lamp source. The peak intensity and broadening of PL spectra of prepared AgNPs depended on the size and shape of nanoparticles. The glucose and fructose stabilized AgNPs showed emission peaks at 413 nm and 415 nm, respectively, while sucrose stabilized AgNPs showed wide emission up to 418 nm of excitation wavelength, and the emission band showed blue-shift. Because of the complexing effect between the stabilizer and the AgNPs, the substitution of sucrose stabilizer improved the photoluminescence property of AgNPs.

Fig. 7

Photoluminescence spectra of AgNPs stabilized with glucose, fructose and sucrose.

3.5 Catalytic degradation of organic dyes

Methyl blue (MB) is a solid brown dye colorant that belongs to a phenothiazine family with a molecular weight of 319.85 g/mole and chemical formula C₃₇H₂₇N₃Na₂O₉S₃. It is commonly used as a colorant in a variety of applications. It is also used to study the trace levels of sulfide ions in aquatic samples [18]. A strong interaction between cationic MB and negatively charged catalyst surface at pH 9 can explain the maximum MB degradation [30]. When excessive hydroxide ions (OH^-) are present in a basic medium, photo-oxidation of OH^* by hole pair formation on the catalyst surface produces hydroxyl free radicals (HO^*) (h + VB + OH^- ⟶ ^*OH) [37]. The key oxidizing species involved in the degradation of MB were ^*OH radicals [31]. In oxidized form, the aqueous solution MB is bright blue. Without addition of a nano-catalyst, MB shows no change in color of aqueous solution, as shown in Fig. 8(a).

Fig. 8

(a) Photocatalytic degradation of methyl blue under sunlight using silver nanoparticles. (b) Change in degradation percentage of methyl bule with light exposure time.

The degradation experiments were conducted with and without add AgNPs to the dye solution. No degradation of dye was observed in the absence of nanoparticles or degradation occurred at a very slow rate which was difficult to detect from color and UV-visible analysis. However, the addition of AgNPs significantly increased the rate of degradation over time. The degradation experiment was carried out for 60 minutes. Maximum degradation percentage was achieved after the said time period, which indicates good catalytic effect of the as-synthesized AgNPs. Figure 8(b) reveals change in degradation percentage of MB with light exposure time. This was clear from the faint and absolute bleaching of MB blue color and a decrease in λ_max intensity (Fig. 8a). Table 3 shows a change in degradation percentage and pH of the dye solution with irradiation time. The proposed dye degradation mechanism using AgNPs is illustrated in Fig. 9. The photocatalytic mechanism of AgNPs can be summarized as follows: $Ag + hv \to {Ag}^{+} + e^{-}$ (4) $e^{-} + O_{2} \to^{*} {O_{2}}^{-}$ (5) $A g^{+} \to Ag + h^{+}$ (6) $h^{+} + H_{2} O / - O H^{-} \to^{*} OH$ (7) ${MB + OH}^{*} \to Degradation of MB$ (8)

Table 3

Time dependent degradation of methyl blue under sunlight using silver nanoparticles

Sample	Irradiation time	pH	% Degradation
T₀	0 min	5.8	η₀ = 0%
T₁	10 min	6.1	η₁ = 42%
T₂	20 min	6.4	η₂ = 50 %
T₃	30 min	5.9	η₃ = 58.3%
T₄	40 min	5.6	η₄ = 76.4%
T₅	50 min	5.7	η₅ = 82.2%
T₆	60 min	5.6	η₆ = 95.4%

Fig. 9

Mechanism of photocatalytic activity of silver nanoparticles for degradation of methyl blue dye.

Table 4 provides a comparison of dye degradation using pure and mixed metal catalysts. The reported catalysts were prepared using different methods under different conditions. This comparison is based on time taken to achieve maximum degradation percentage. This work shows relatively faster degradation rate as compared to most of the photocatalysts reported in the literature. The high degradation rate might be due to high purity and small particle size of the tested AgNPs.

Table 4

Comparison of photocatalytic activity of this study with the previous studies

Sr. No	Catalyst	Synthesis Method	Dye/Conc.	Degrade Time	Light Source	Ref.
1	AgNPs	Liquid plasma interaction method	MB/	60 min	Visible light (Halogen Lamp)	Present study
2	Ag₂O/TiO₂	Wet impregnation method	MB/ 5–20 ppm	Visible light	[32]
3	AgNPs	Biosynthesis method	MB/95.3±0.30	72 h	Sun light	[33]
4	ZnNPs	Green synthesis method	methylene blue and eosin yellow	180 min	UV illumination	[34]
5	FeNPs	Bio synthesis method	AYR dye	42 h	Solar light Irradiation	[35]
6	AuNPs	Green synthesis	methylene blue and Congo red	120 min	UV-Vis light	[36]
7	Ag/WO₃ NPs	Hydrothermal method	MO	90 min	Visible light	[37]
8	Fe/TiO₂	Ball milling process	Rh-B	120 min	UV irradiation	[38]
9	AgNPs	Green synthesis method	MO	12 h	Visible light	[39]
10	AgNPs	Green route method	Coomassie Brilliant Blue	30,60,90 min	UV light	[40]

3.6 Stability of the photocatalyst

Stability is the most important factor for the industrial applications of the photocatalysts. The photocatalytic stability of as-prepared AgNPs during photocatalytic degradation of dye in water was studied under identical experimental conditions. The stability of the photocatalyst was determined for five successive cycles. It is evident from Fig. 10 that all samples showed strong stability for consecutive five cycles of dye degradation. Only 4% decrease in photocatalytic activity was observed after 5 cycles. After first cycle, the stability was measured about 98%, which reduced to 94% after 5 cycles. The small reduction in the activity can be attributed to the saturation of the catalytic sites with intermediate products. Overall, the synthesized photocatalysts did not show any significant decrease in the activity.

Fig. 10

Stability analysis of photocatalytic activity of AgNPs over 5 consecutive cycles.

4 Conclusions

In this work, silver nanoparticles were synthesized via a liquid-plasma interaction technique. The silver ions were reduced to nanoparticles by using the plasma born reactive species. Since plasma reduction is a contactless process, relatively pure silver nanoparticles of well-defined band gap energy were obtained. The maximum optical band gap of nanoparticles was measured about 2.41 eV. The average particle size was measured about 10, 12, and 14 nm for glucose, fructose and sucrose stabilized nanoparticles. The glucose, fructose and sucrose stabilized AgNPs showed emission peaks around 413 nm, 418 nm, and 415 nm, respectively. Maximum degradation percentage was achieved after 60 min of light exposure, which indicates good catalytic effect of the as-synthesized silver nanoparticles. This work shows relatively faster degradation rate as compared to most of the photocatalysts reported in the literature. The high degradation rate might be due to high purity and small particle size of the tested nanoparticles.

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