1 Introduction

Paper, dyeing, plastic and textile industries use colour for staining their products and consuming large amount of water to clean, it can lead formation of waste water with dye and it can harmful to the nature [1,2,3,4]. Currently, 100,000 various types of dyes produced with an annual production rate of 7 × 105. Among them, textile industries consume dye about 36,000 ton/year, 10–20% of which release dye content waste water in to the environment can lead to pollute the marine life. Hence it is a very important task to reduce releasing of dye and organic pollutant in to the aquatic life.

Presently, the photocatalysis has attracted special attention as an economic and environmentally safe option for solving energy and pollution problems [5].

From the literature survey, the synthesized nanoparticles (Nps) by greener approach methods are useful for pharmaceutical applications and waste water treatment [6]. The advantages of using plant extracts in Ag NPs fabrication is non-hazardous simplicity in scaling up synthesis [7]. In the current work, ZnO nanoparticles are prepared by a novel, simple, efficient, environment friendly and green route using Azadirachta indicagum extract, which plays as a fuel. It is also reported that using leaf extract, it is possible to control the shape and size of the particles. Being biotemplates, it also hinders agglomeration of nanoparticles [8].

ZnO is an important and distinctive inorganic substance, because of its distinctive characteristics and novel applications in various fields of science and technology [9]. ZnO has capacity to exhibit piezoelectric, pyroelectric, optoelectronic, catalysis and semiconducting properties [10]. Due to this, ZnO is a multifunctional compound which is used in the field of light emitting diodes, biosensors, diluted and ferromagnetic materials for spintronics solar cells, photocatalysis, transistors and also acts as antioxidants and antibacterial agent [11,12,13]. ZnO is the most prominent n-type wide band gap metal oxide semiconductor ( ~ 3.37 eV) with high excitation binding energy (60 m eV) [14,15,16,17]. Various methods were discovered for synthesize of ZnO Nps such as hydrothermal, direct precipitation, sol–gel and solvothermal methods [18,19,20,21,22,23]. These methods need costliest starting material, lengthy procedures, sophisticated apparatus and more time required. But the neem gum assisted combustion method was constructed as it is an easier, less energy and time consuming method among all the methods. The major advantages of green synthesis was reduce pollution to the globe and useful for synthesis materials easily, eco-friendly and reasonable quantities.

Neem Gum, a distinctive plant gum that originates from the neem tree (Melia Azidirachta, maliaceae) is a complex polysaccharide acid salt. It has been medicinally used in India for many centuries [24]. Neem gum on hydrolysis yields D-galactose, D-glucuronic acid, L-fucose and L-arabinose (Fig. 1). The aldobiuronic acid is the ingredient of neem gum derived from graded hydrolysis is shown to be 4-O-(D-glucopyranosyluronic acid)-D-galactopyranose.

Fig. 1
figure 1

Major compounds present in neem gum

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The present work reports the synthesis of ZnO and Ag–ZnO NPs using neem gum as fuel. The prepared NPs were characterized using powder X-ray diffraction (PXRD), Fourier transform infrared analysis (FTIR), UV-diffused reflectant spectrum (UV-DRS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of the ZnO NPs.

2 Experimental

2.1 Preparation of ZnO and Ag–ZnO Nps

Preparation of ZnO and Ag–ZnO Nps was employed by combustion method. Zinc nitrate and Silver nitrate were used as precursors and Azadirachta indica gum was used as a fuel.

The stoichiometric ratios of Zn(NO3)2·6H2O and Ag(NO3) were taken in a beaker with varying concentrations Azadirachta indica gum (120, 240, 360, 480, 600 mg) with 15 mL of H2O. The solution is stirred until to become homogeneous. This mixture is taken into a preheated muffle furnace maintained at 500 ºC, the smoldering type of the combustion reaction occurs and nanocrystalline ZnO was formed within 5 min. The schematic representation for the synthesis Ag–ZnO was shown in Fig. 2.

Fig. 2
figure 2

Graphical representation for the synthesis of ZnO and Ag–ZnO nanomaterials by combustion method

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2.2 Characterization

XRD data were recorded in Rigaku Smart Lab. FTIR analysis was recorded using Bruker Alpha-p spectrometer. The morphology of the synthesized nanoparticles was observed using SEM (Hitachi-7000 Table top). Shape and size of the nanoparticles were determined by TEM (JEOL 3010). Photoluminescence (PL) studies were recorded using fluorescence spectrophotometer (Agilent technology Cary Eclipse).

2.3 Photocatalytic degradation of dye

Prepared ZnO and Ag doped ZnO nanoparticles were taken for degradation of methylene blue (MB) using 120 W light for source of radiation. 5, 10, 15, and 20 mg of photocatalyst were added to 100 mL of various concentrations of MB solutions (5, 10, 15, and 20 ppm) in a HEBER photo reactor [25]. The solution was continuously mixed by passing air in dark for 30 min to confirm adsorption–desorption equilibrium between MB and photocatalyst before irradiation. Suspension (2 mL) was taken from the solution mixture at intervals of 30 min. The dispersed ZnO/Ag–ZnO photocatalyst was removed using a spin win microcentrifuge. The degradation % can be calculated using the formula.

$$\% \;{\text{of}}\;{\text{degradation}} = \left( {\text{C}}_{\rm i} - {\text{C}}_{\rm f} \right)/{\text{C}}_{\rm i} \times 100$$
(1)

where Ci and Cf are the initial and final concentration of the dye.

3 Results and discussion

3.1 XRD study

The XRD patterns of ZnO and Ag–ZnO Nps are shown in Fig. 3. The pure ZnO (Fig. 1a) displays XRD pattern peaks, that can at 2θ = 31.91°, 34.58°, 36.37° and 47.52° were indexed with the planes (101) (002) (101) and (102) for the resultant particles with Hexagonal phase. The structure of the resultant data is according to the JCPDS card number 3-888. The XRD pattern of Ag–ZnO Nps (Fig. 1b) shows an additional peaks at 38.1° (111) and 44.1° (200) compared to undoped ZnO Nps corresponding to cubic phase silver (JCPDS number 2-109) [26]. The average crystallite size was calculated using Debye–Scherrer equation and it was found to be 13.34 and 13.33 nm for ZnO and Ag–ZnO Nps respectively.

Fig. 3
figure 3

Powder XRD patterns of the ZnO nanopowders prepared by (a) 120 mg, (b), a1 240 mg, (c) 360 mg, (d) 480 mg, (e) 600 mg of neem gum. (b1) 3% (c1) 5% and (d1) 7% Ag–ZnO synthesized using 240 mg of neem gum

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$${\text{D}} = {\text{K}}\uplambda /\upbeta\;\cos\uptheta$$
(2)

where, K is constant (0.94), λ is wavelength, β is the full width at half maxima and D is the crystallite size.

To study the structural changes in the different catalysts were examined with the help of Rietveld refinement studies. To get exact lattice parameters, Rietveld refinement analysis (Fig. 4) was performed by using FULLPROOF software assuming P63mc (186) space group for hexagonal structure. Pseudo-voigt function was utilized to fit the various parameters to the data point. Diamond software was utilized for extracting the possible packing diagram (Fig. 5), from which we cannot see the presence of Ag atoms in a ZnO crystals this might be less concentration of dopant. The refined parameters such as occupancy, atomic functional positions are displayed in Table 1. The fitting parameters (Rp, Rwp and χ2) indicate a good agreement between the refined and observed PXRD patterns for the hexagonal ZnO phase.

Fig. 4
figure 4

Rietveld refinement of ZnO and Ag(0–7%)−ZnO PXRD

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Fig. 5
figure 5

ZnO packing aiagram a Pure b Ag doped (3 mol%), c 5 mol% and d 7 mol%

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Table 1 Rietveld refined structural parameters for ZnO
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As the doping concentration increases there will be an increase of density and decrease of cell volume this is due to the addition of Ag ions in the crystal of ZnO. The χ2 value is increases from pure ZnO to 7% Ag doped ZnO. This indicates that there is a less fitting with standard graph due to interfering Ag ions in the crystallites of ZnO.

3.2 FTIR study

The FTIR spectra of prepared ZnO and Ag–ZnO nanoparticles were recorded from 350 to 4000 cm−1, represented in Fig. 6. In this, significant vibrational band observed in the range of 400–500 cm−1 is corresponds characteristic stretching mode of Zn–O bond. In the IR spectra of Ag–ZnO Nps, intensity of the peak is reduced because of the formation of Ag nanoparticles on the surface of ZnO [27]. A broad peak at 3431 cm−1 (stretching) and 1330–1670 cm−1 (bending) indicates the presence of adsorbed atmospheric moisture [28, 29].

Fig. 6
figure 6

FTIR spectra of ZnO and Ag–ZnO NPs synthesized by Azadirachta indica gum

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3.3 UV-DRS studies

DRS studies of the synthesized materials were displayed in Fig. 7a. The spectrum displays strong band in between the wavelength region of 372–390 nm was attributed due to absorption of the host lattice [30]. Figure 7b shows the bandgap of synthesized materials calculated from Kubelka–Munk (KM) function. The obtained energy bandgap (Eg) for synthesized material were tabulated in inset of Fig. 7b.

Fig. 7
figure 7

UV–DRS spectrum of Ag–ZnO NPs synthesized by Azadirachta indica gum

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3.4 SEM studies

SEM images of Ag–ZnO nanomaterials are shown in Fig. 8. From the SEM images it is clearly confirmed that the material synthesized by combustion method is highly porous in nature. The pores were clearly visible in Fig. 8d and the pore diameter is varying from 70 to 500 nm. These pores were created during the synthesis due to the escaping of gases at higher temperatures. The pores were uniformly formed and looks like a honeycomb like structures [31,32,33].

Fig. 8
figure 8

SEM images of porous Ag–ZnO nanomaterials with different magnification view a 50 μm, b 10 μm, c 5 μm and d 2 μm

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3.5 TEM studies

Figure 9 shows the TEM images, HR-TEM.images and SAED pattern of synthesized Ag–ZnO porous nanomaterials using Azardichta indica gum as a fuel by combustion method. These particles are well dispersed, which acquired the size around 15 nm diameter and these particles are looks like the spherical shape in nature. HR-TEM images give the clear information about the d-spacing values which are different for ZnO and Ag–ZnO. For ZnO the d-spacing value is 0.236 nm, which belongs to the (101) plane and for Ag–ZnO, the d-spacing value is 0.24 nm, which belongs to (111) plane. Presence of Ag on the ZnO material can be easily marked as a dark spots, because the atomic weight of Ag is more than Zn. From the SAED pattern, it is confirmed as a polycrystalline in nature due to the diffracted spots are combined together to form a fringes of circles and these d-spacing values are well matched with the d-spacing values from HR-TEM and also calculated from XRD data.

Fig. 9
figure 9

TEM images (a, b, c), HR-TEM images (d, e) and SAED pattern (f) of Ag–ZnO nanomaterials

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4 Photocatalytic degradation of ZnO and Ag–ZnO NPs

Under UV irradiation, semiconductor absorbs photon energy with equal to or higher than the bandgap of the semiconductors, it generates electrons and holes on the surface of the photocatalyst. If the charge carriers do not recombine, they can migrate on the surface where free electrons form a reduction of oxygen and form peroxide and superoxide radicals, and the generated holes oxidize water and form OH·. This species is highly reactive and unstable. It ultimately acts on organic compounds and oxidizes carcinogenic dyes to CO2, water and inorganic acids. The photocatalytic action of the dye is enhanced by various factors of the photocatalyst, namely, particle size, phase composition, shape, crystallinity, size distribution, surface area, surface hydroxyl group density and band gap [34]. Synthetic ZnO NPs were used as photocatalysts to investigate the degradation of methylene blue under UV sources. An aliquot of 2 mL was withdrawn from the UV–Vis reactor for every 30 min, centrifuged and the absorption of the remaining dye was recorded.

Figure 10 displays the photocatalytic degradation activity of bare ZnO and 3, 5, 7% Ag doped ZnO Nps over methylene blue dye. In this 5% Ag doped ZnO Nps exhibits good photocatalytic degradation activity when compared with bare ZnO, 3%, 7% Ag doped ZnO Nps. As the increases of Ag dopant from 3 to 5%, the rate of dye degradation also increases and again increase the Ag dopant from 5 to 7%, the rate of dye degradation was decreased. From this, it is clear that 5% Ag dopant was found to be optimum concentration. When the % of Ag higher than the optimal value, the rate of dye degradation was decreased due to metallic Ag act as recombination centers which is caused by the electrostatic attraction of negatively charged Ag and positively charged holes. Furthermore excessive Ag reduced the number of photons absorbed by the photocatalyst because of light-filtered effect [35].

Fig. 10
figure 10

Photocatalytic degradation of methylene blue with different catalysts

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4.1 Effect of dye concentration

Photocatalytic activity of ZnO Nps mainly depends on the concentration of the dye. To know the maximum concentration of dye for efficient oxidation, the experiment was carried out by constant catalytic loading (10 mg) and changed the concentration of dye from 5 to 20 ppm with the presence of UV light (Fig. 11). As the dye concentration increases from 5 to 20 ppm, the photocatalytic degradation is reduced from 80 to 20% and 10 ppm is the most effective concentration for dye degradation. Generally, as the concentration of dye increases, rate of degradation was decreases [36].

Fig. 11
figure 11

Degradation of MB with varying concentration of dye and fixed weight of catalyst (10 mg)

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4.2 Effect of catalyst load

Figure 12 displays the degradation of MB dye with varying catalyst load (5–20 mg) by keeping the dye concentration constant (5 ppm). It is clearly reveals that as the catalyst load increases, the rate of dye degradation increases from 90 to 100%. This is because more active sites are available due to increased catalyst load [37].

Fig. 12
figure 12

Degradation of methylene blue with varying catalyst load and constant dye concentration

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4.3 Effect of pH

In order to evaluate the optimal pH for the degradation of MB dye, experiments were conducted at different pH (2 to 12) by maintaining the constant catalyst load (10 mg) and concentration of dye (5 ppm), the data was represented in Fig. 13. It reveals that the degradation of MB was excellent in the basic condition [38, 39] with the highest rate of degradation observed at pH 10. Above this pH the degradation decreases, and can be explained on the basis of zero potential charge (ZPC). For ZnO, ZPC is 9.0 ± 0.3, and if it exceeds this value, the adsorbed OH− ions form the negatively charged surface. Due to the presence of many OH− ions on the surface of catalyst, OH˙ radical generation is lowered and which act as primary oxidizing agents and are responsible for lowering the degradation of MB dye [40,41,42,43].

Fig. 13
figure 13

Degradation of MB with varying pH of the solution and keeping the catalyst load and constant dye constant

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4.4 Catalyst recycling

To evaluate the stability of the photocatalyst, a recycling experiment was carried out to degradation the methylene blue dye (Fig. 14). The experiment was done with catalyst (10 mg) and 5 ppm dye (100 mL). The stability of the catalyst checked by removing the catalyst by filtration, washed with ethanol and dried at 120 oC used for new cycles. Degradation of MB process was repeated for five cycles. In each cycle nearly 98–95% of MB were degraded with visible light irradiation which indicates that there is no loss of activity of catalyst [44]. The same experiment was also carried out with pure ZnO shows poor degradation of MB dye. The generation of excitons in pure ZnO was not easy due to the large bandgap and insufficient wavelength of visible light [45]. The efficiency of degradation of MB was almost the same for 5 cycles. It clearly shows the reduction of efficiency about nearly 8% for five cycles catalyst recycling.

Fig. 14
figure 14

Recycling of 10 mg of catalyst with 100 mL of 5 ppm dye

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4.5 Mechanism

Scheme A schematic representation of the degradation of MB using Ag–ZnO is shown below (Scheme 1).

Scheme 1
scheme 1

Graphical representation for the degradation of MB with Ag–ZnO under the irradiation of UV light

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ZnO + hν → ZnO (h+vb + e−cb)

OH−ads + h+vb → OH·ads (in basic medium)

MB + OH·ads → dye degradation

4.6 Detection of OH· Radicals

OH· (hydroxyl radical) is an unstable and reactive chemical species that is more important for the degradation of organic dyes using Coumarin as a probe molecule. This is a simple and sensitive method for OH radical detection. Coumarin reacts with OH radicals to form the 7-hydroxyl Coumarin, a luminous material that exhibits a PL peak at 456 nm.

In this process, 130 mg of ZnO was dispersed in 50 mL of 1 mM coumarin aqueous solution irradiated with UV light of 120 W. 2 mL of Aliquots of sample collected for every 10 min and the PL spectra was measured using Cary eclipse spectroflorimeter (Fig. 15). It shows that the formation of OH· increases with an increase of time. This OH· is responsible for the degradation of organic dyes [46].

Fig. 15
figure 15

PL spectrum of coumarin/hydroxyl coumarin under different time of irradiation

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5 Photoluminescence study

The PL study is a useful method for determining the efficiency of charge carrier separation in semiconductors [47]. Figure 16b shows the PL emission spectrum of ZnO and Ag–ZnO Nps taken at room temperature with an excitation wavelength of 380 nm. The pure ZnO nanoparticles exhibit a strong UV emission peak at 525 nm and also a weak emission peak at 488 nm. In Ag–ZnO nanoparticles, there is a decrease of intensity in PL spectrum which is in acceptable criteria with the Stern–Volmer quenching process, this indicates a higher charge carrier separation efficiency for Ag–ZnO Nps. The emission peaks obtained are in UV and visible region and could be attributed to bound excitons and defect states located on the surface of nanostructured pure and doped ZnO Nps. Figure 16a shows a PL excitation spectrum with emission at 542 nm [48, 49].

Fig. 16
figure 16

PL spectrum of ZnO and Ag–ZnO a excitation spectra, b emission spectra, c CIE diagram

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The color coordinates are used to find out the emission color of the material. It can be estimated by the Commission's international De I'Eclairage system (CIE). Figure 16c represents the CIE diagram of ZnO and Ag–ZnO nanoparticles and it clearly reveals that both the materials are emitting the green colored light.

6 Conclusion

ZnO and Ag–ZnO Nps are successfully synthesized by solution combustion method using Azadirachta indica gum as a fuel. The NPs are confirmed by XRD and FTIR studies. The TEM analysis showed spherical shape with 15 nm particle size. Ag-doped ZnO acts as a good photocatalyst for the degradation of methylene blue due to the presence of Ag, which reduced the electron–hole recombination on the surface of the catalyst. Hence synthesized material is measured as a potent aspirant for the degradation of MB dye. In addition, it also shows the PL spectrum and it will be using for the green emitting LED applications.