A study of the structural, morphological, and optical properties of shock treated SnO2 nanoparticles: removal of Victoria blue dye.

In this work, Tin Oxide (SnO2) nanoparticles (NPs) were prepared by green microwave followed by hydrothermal methods, using tea extract as a reducing agent. To verify the stability of physical and chemical properties of SnO2 NPs, samples were subjected to shock impulsion experimentation. Different characterization techniques were employed to analyze the crystallinity, molecular structure, and optical parameters of the control SnO2 and shock wave exposed SnO2 NPs. Powder X-ray diffraction (PXRD) revealed no significant change in crystal structure. Williamson - Hall analysis demonstrates that the stress and strain between Sn-O changes during the impulsion of shocks. Rietveld analysis reveals change in the bond length between Sn-O. The molecular structure is not affected during shock loading, but the optical properties do change. From the photocatalytic experiment, we find that the parameters such as stress, strain, and bond length make an enormous impact in photocatalytic application.

Introduction

One of the major drawbacks observed in the textile and color industry is the removal of colors (dyes) from wastewater. There are different types of dyes, including organic, inorganic, and azo. The color and other properties of dye are determined by chemical structure, and divided into anionic and cationic dyes [1, 2, 3]. Among the cationic dyes, Victoria blue (VB) is widely utilized as a bright coloring agent in various industries, including wool, silk, nylon, and acrylics with a molecular weight of 506.08. It has the same functional group as onium groups and N+ ions, and exhibits excitation wavelengths of 610 nm [4]. VB dye contains cationic functional groups that can dissociate into positively charged ions in a water solution. The use of metal oxides to break chemical bonds in the VB dye has stimulated significant research in recent years. There are several metal oxides used as catalysts. Among the several catalysts, Tin oxide (SnO2) is considered to be the most effective for eliminating hazardous dye from the wastewater via photon energy [5, 6, 7, 8, 9, 10]. It is also used for other applications such as photocatalytic, electrochemical, and sensing. Tin oxide is widely used for photocatalytic applications because of its ability to remove the cationic dye through electrostatic attraction between VB dyes with SnO2 NPs [11]. Tin (iv) oxide appears as white or off-white crystalline solid or powder, with a melting point of 1127 °C, and density of 6.95 g/cm3 at room temperature. SnO is a versatile intermediate to metallic Sn, Sn3O4, and SnO2 the thermodynamically unstable nature of Sn (IV). Therefore it is relatively difficult to grow SnO single-crystals [12, 13]. Generally, SnO2 possesses excellent electrical, optical, catalytic, and thermal properties. Numerous methods to synthesize SnO2 NPs, including hydrothermal [14, 15], microwave oven [16, 17], sol-gel [18, 19, 20], co-precipitation [21, 22], solid-state [23, 24], and green methods [25, 26, 27, 28, 29] have been used to obtain SnO2 NPs. Each method has both advantages and disadvantages. In the present study, the SnO2 NPS were synthesized by microwave radiation followed by a hydrothermal method using tea extract as a reducing agent. The entire reaction took place in a compressed liquid environment. In the microwave method, microwave radiation is used as a heat source. The advantage of this method is that it is eco-friendly, cost effective and easy to synthesize. Moreover, there is no need for any chemical reducing agents because tea extract itself acts as a reducing agent. The combination of these methods effectively produces NPs with controlled shape and particle size [30, 31, 32]. Stability of the material's property is also crucial for the device fabrication and photocatalytic application. For that, extreme pressure is applied to the material in the form of shockwaves. During the shock treatment, the material changes such physical properties as its grain size, phase, electronic state, morphology and oxidation state [33, 34]. Research reports are readily available on shock treated metal oxides such as ZnO [35], TiO2 [36], AgO [37], NiO [38]. Based on the recent reports [39, 40] bulk SnO2 is rutile in structure, tetragonal symmetry of P4/mnm space group at ambient pressure and temperature. At the pressure of 11.8 GPa, the rutile structure becomes CaCl2 type structure of orthorhombic symmetry of the Pnnm space group. Material changes its phase into α-PbO2, orthorhombic symmetry of the Pbcn space group at 12 GPa (Pnnm to Pbcn). Pyrite FeS2 structure cubic symmetry of Pa-3 space group transformation occurs at the pressure of 25 Gpa. For SnO2, tetragonal structure remains stable up to 18 GPa for 8 nm grain size nanoparticles. Above that pressure, the phase changes from tetragonal to cubic with the discontinuity in the volume.

Therefore, we undertook an investigation of SnO2 NPs prepared by a green microwave-assisted hydrothermal method. We examined the prepared SnO2 NPs exposed to different numbers of shock pulses (50, 100, and 150) and scrutinized its structural, molecular, and optical stabilities. The control SnO2 and shock exposed SnO2 NPs were also explored for photocatalytic application under UV light for the removal of VB dye as a model pollutant.

Experimental details

Materials used

For the synthesis of SnO2 nanoparticles, SnCl2. 2H2O (Stannous chloride) was used as a precursor material, analytical grade purchased from Merck (India). The green tea bags (camellia sinensis) were procured from a commercial tea vendor (Tajmahal, India). For all experiments, double deionized water was used.

Synthesis of SnO2 nanoparticles

SnO2 NPs were synthesized by microwave heating followed by a hydrothermal route. To accomplish the synthesis, after the 60 ml DI water reached the boiling temperature, 2 ​g of tea were boiled for 2 min. The resultant 50 ml dark brown color solution was cooled to room temperature and filtered through filter paper (pore size of 10 ​μm), and the filtered tea extract was used for SnO2 NPs preparation. 0.1 M of SnCl2. 2H2O was mixed with 50 ml of green tea extract and sonicated in the solution for 30 min (power 50 W, frequency 40 kHz) at room temperature. Once the solution was sufficiently mixed, the mixture was irradiated in a domestic microwave for 3 min at the power of 700 W at the frequency of 2450 Hz. Subsequently, the colloidal mixture was transferred into a stainless steel autoclave. The hydrothermal reaction was executed at 180 °C for 24 h. The powder was collected, and then triple cleansed of excess chloride ions by means of ethanol mixed in DI water. The sample was annealed at 300 °C for 4 h. During the annealing process, SnO2 NPs were formed through the oxidation of Sn0.

SnO2 NPs formation mechanism

In SnO2 NPs formation, tea extract functions both as capping and a reducing agent. Tea extract contains the potassium metal present in the large amount of 92–151 mg/l and other metals such as sodium (35–69 mg/l), calcium (1.9–3.5 mg/l), fluoride (0.8–2.0 mg/l), aluminum (1.0–2.2 mg/l), manganese (0.52–1.9 mg/l), and iron (0.020–0.128 mg/l). Tea extract contains polyphenols which also acts as an antioxidant. This phenolic group contains four flavonoid groups, including epicatechin gallate (ECG) (203–471 mg/l), epigallocatechin gallate (EGCG) (117–442 mg/l), epicatechin (EC) (25–81 mg/l), and epigallocatechin (EGC) (16.9–150 mg/l) [41]. In the first step, SnCl2. 2H2O is converted into Sn0 in sonication (Eqs. (1), (2)), and microwave process by means of epigallocatechin (EGC) in the phenolic group. –OH is the major group in epigallocatechin (EGC) in the reduction process [42]. During the hydrothermal process, the reduced Sn particles develop into a well-defined structure.

The second step is the phase changing process. By means of aerial oxidation (Eq. (3)) during the annealing process, Sn transforms into SnO2 NPs.

Photo degradation experiment

The photo degradation process of control SnO2 and shock exposed SnO2 NPs were examined under UV illumination. Victoria blue (VB) dye was employed as a model pollutant. To achieve an adsorption-desorption process between the SnO2 NPs and the VB dye, a solution of 5 ppm (5 mg/L) VB dye and a substantial quantity of photocatalyst were magnetically stirred in a Pyrex glass vessel at dark conditions for 20 min. The UV light was placed above the solution. The distance between the UV light and the solution was approximately 10 cm. The specification of UV light was 8 W, 365 nm, UV light. The temperature maintained throughout the entire experiment was between 28-32 °C. At the conclusion of the adsorption process, the UV light was illuminated. At appropriate intervals, .5 ml of aliquots were collected. The collected aliquots were centrifuged to extract the particles. Collected aliquots concentrations were measured via UV-Visible absorbance spectrophotometer at the maximum absorption λ = 610 nm. The degradation efficiency of VB dye was defined as follows (Eq. (4)),where C VB dye initial concentration and C is the concentration at different reaction times (min).

Kinetics study

For the determination of the rate of VB dye degradation, first and second-order kinetics were applied. The rate of decolorization of control SnO2 and shock treated individual SnO2 NPs was discerned by applying pseudo-first and second-order kinetic models.

First order kinetic model is expressed as (Eq. (5)) [37, 43],

Second order kinetic model is expressed as (Eq. (6)),where C0 is the initial absorbance or concentration value of VB dye, Ct concentration at a given time't’, rate constants for first and second-order are k1 and k2. The correlation coefficient (r2) is obtained by the plot of first and second-order reaction graphs. For the first order, the graph plots between ln(C0/Ct) vs time, for the second-order 1/Ct-1/C0 vs time.

Shock wave loading

The powder samples were placed in a 10 × 10 × 2 mm3 polyethylene sample cell. A table top pressure driven Reddy tube generated shock waves at 1.7 Mach number with a transient pressure, and temperature of 0.857 MPa, and 594 K, respectively. Subsequently, shock pulses (50, 100, 150 counts) were exposed sequentially at timed intervals of 5 s.

Material characterization

Powder X-ray diffraction (PXRD) analysis was done with an Ultima III max (Rigaku) X-ray diffractometer with Cu-Ka (λ = 1.5418 Å) radiation in the range 20º-80° at grazing angle 1°, and step size 0.01 deg. Fourier transform infrared spectrophotometer (Jasco FT/IR-4600) identified the functional groups of the samples. KBr was used to prepare the sample in the pellet form for FTIR analysis. Microscopic images were captured by Field emission scanning electron microscope (FESEM), using a SIGMA HV – Carl Zeiss apparatus. Photoluminescence (PL) properties of the samples were recorded by a spectrofluorometer (JASCO FP-8300). UV-vis diffuse reflectance spectroscopy (SHIMADZU/UV2600) was employed to obtain data on the synthesized samples, such as absorption spectrum and band gap energy.

Results and discussion

XRD

Structural stability, purity, and grain size of the control SnO2 and shock loaded SnO2 NPs were identified by using a powder X-ray diffractometer (PXRD). Figure 1 Shows the XRD pattern of control SnO2 and shocked SnO2 NPs. All the XRD patterns accurately correspond to JCPDS card 72–1147 with a space group of P42/mnm. Moreover, the observed intensity peaks belong to the tetragonal structure. The intensity peaks of SnO2 at the 2θ range are 26.9, 33.9, 52.2, and 65 belong to the hkl planes of (110), (101), (211), and (112). The grain size of the control SnO2 and shock exposed SnO2 NPs was calculated via Debye-Scherer's formula.

Figure 1

XRD pattern of control SnO2 and shock exposed SnO2 NPs.

The control SnO2 NPs reveals an average grain size of 11.71 nm. Shock loaded samples were compared with control SnO2 samples to find the structural stability. When the shock wave was applied to the samples, no new peaks appeared nor did any change occur in the existing peaks. This result clearly demonstrates that SnO2 did not alter or change phase when the samples were impacted by an impulsion of 150 shocks. The intensity of the peak did not change, which demonstrates stability in nature. When the number of shocks (50,100 and 150) increases, the grain size is also found to increase (14.92, 14.36, and 18.22 nm). This may be due to the dynamic recrystallization that occurs during the explosion of shock treatment, i.e fusion of grains [38, 44]. As per the report [35], the NPs with lower grain sizes have better stability than those possessing higher grain sizes.

The Rietveld refinement was performed using the FULLProf-suite software for the measured powder X-ray diffraction data of control SnO2 and shock loaded samples. The samples exhibited the same tetragonal structure even at shocking conditions, shown in Figure 2. The stability of the material is primarily based on the orientation of the crystal, atomic position, grains, and the bond length between Sn–O. During shock treatment the minute change in the bond length between the Sn–O occurs but the orientation of the crystal does not alter. As per the literature report, only beyond 18 GPa the 8 nm SnO2 sample changes its tetragonal symmetry to cubic [39]. This provides clear evidence for the high mechanical stability and high shockwave resistance nature of SnO2 compared with TiO2 metal oxide nanoparticles which undergo phase transition while shock loaded condition [36]. By using pseudo-Voigt profile function, the Le Bail refinement parameters, such as the correct fit, are denoted by χ2, R factors (RP = profile factor, RB = Bragg factor, and RF = crystallographic factor). Occupancy gives the details about the number of atoms in a unit cell of Sn and O, atomic coordinates of Sn and O, the volume of the cell (V), and lattice constant parameters (a, b, c).All four samples were calculated, and obtained the particular profile refinement parameters listed in Table (1).

Figure 2

The Rietveld refinement of (a) control SnO2 (b) 50, (c) 100 and (d) 150 shock exposed SnO2 NPs.

Table 1

Profile refinement parameters of control SnO2 and shock exposed SnO2 NPs.

samples	control SnO2	50 Shock	100 Shock	150 Shock
Crystal system	Tetragonal	Tetragonal	Tetragonal	Tetragonal
Space group	P 42/mnm(136)	P 42/mnm(136)	P 42/mnm(136)	P 42/mnm(136)
Lattice parameter(Ǻ)
a = b≠c	4.73700 and 3.18500	4.73700 and 3.18500	4.73700 and 3.18500	4.73700 and 3.18500
α = β = γ	90.0000	90.0000	90.0000	90.0000
Unit cell volume(Ǻ)	72.844	71.707	71.791	72.131
Atomic coordinates
Sn
X	0.0000	0.0000	0.0000	0.0000
Y	0.0000	0.0000	0.0000	0.0000
Z	0.0000	0.0000	0.0000	0.0000
Occupancy	2.10290	0.63608	0.53304	3.71841
O
X	0.30278	0.28659	0.29856	0.29619
Y	0.30278	0.28659	0.29856	0.29619
Z	0.0000	0.0000	0.0000	0.0000
Occupancy	0.89800	11.3799	1.10646	0.49639
Bond length (Sn– O)Å	2.21	2.27	2.23	2.24
Refinement parameters
RP	25.5	32.9	20.2	32.1
RWP	32.8	39.9	26.4	39.1
REXP	19.6	26.9	19.4	22.6
χ2	4.39	2.19	1.85	3.000
RBragg	16.5	14.8	7.33	11.1
RF	7.56	14.8	4.49	10.4
X-ray density(g/cm3)	7.00	7.00	7.00	7.00

Williamson –Hall method

Uniform deformation model (UDM)

Defect and distortion in the crystalline powder create the stain in the NPs structure. Any crystalline change that occurs during the impact of shock can be analyzed in the Uniform deformation model. Strain-induced line broadening β. Crystallite size from Scherer's formula also yields information about line broadening (1/cosθ). The total line broadening is obtained when adding Strain-induced line broadening with crystallite size [45]. The Uniform deformation model (UDM) is isotropic. Therefore, the strain in the crystal is uniform in all crystallographic directions. Figure 3 gives the lot the graph between 4sinθ vs βcosθ. 4sinθ and βcosθ. The Y intercept of the plot gives the crystallite size and the slope of the plot gives strain value.

Figure 3

UDM plot of (a) control SnO2 (b) 50, (c) 100 and (d) 150 shock exposed SnO2 NPs.

Uniform stress deformation model (USDM)

Particle elasticity and the crystal have a very low strain, making it impossible to calculate the crystal parameters directly. However, it can be calculated by using Hooke's law. Generally, Hooke's law reveals the relationship between stress (σ) and strain (ϵ), where E is the young's modulus or modulus elasticity. Young's modulus is different for different structures, for tetragonal structure young's modulus equation gives in [45]. Sij are elastic compliance for the tetragonal structure S11 = 7.426, S12 = −4.408, S13 = 1.0438, S33 = 2.946, S44 = 9.7 and S66 = 4.8216. In the USDM model stress is anisotropic. By plotting the graph between 4sinθ/E (X-axis) versus βcosθ (Y-axis) shown in Figure 4, the slope of the straight line gives the stress value (σ) and the Y-intercept gives the crystallite size.

Figure 4

USDM plot of (a) control SnO2 (b) 50, (c) 100 and (d) 150 shock exposed SnO2 NPs.

Uniform deformation energy density model (UDEDM)

The UDEDM model also discloses the anisotropic behavior of the crystal; however, in some cases, the presumption is not confirmed. Therefore, the energy density is calculated based on Hooke's law. Figure 5 depicts the graph plotted between 4sinθ (2/E) 1/2 and βcosθ. The slope of the straight line gives the stress value (σ) and the Y-intercept gives the crystallite size. Negative slope designates compression of lattice [46, 47]. In the literature report, negative slope most often occurs with particles of smaller crystal size, whereas the positive slope is caused by the lattice expansion or tensile strain of the NPs [46, 47]. The summary of the W–H plot results tabulated in Table (2).

Figure 5

UDEDM plot of (a) control SnO2 (b) 50, (c) 100 and (d) 150 shock exposed SnO2 NPs.

Table 2

W–H report of control SnO2 and shock exposed SnO2 NPs.

Sr.No	Samples	Scherrer method D(nm)	Williamson–Hall method
			UDM		USDM			UDEDM
			ϵ × 10−3	D (nm)	σ (GPa)	ϵ × 10−3	D (nm)	U (GPa)	σ (GPa)	ϵ × 10−3	D (nm)
1.	control SnO2	11.71	1.49	13.25	12.32	1.6555	13.51	12.27	38.67	1.342	13.75
2.	50- Shock	14.92	0.9597	12.87	8.31	1.1446	13.13	9.71	25.65	1.193	13.8
3.	100- Shock	13.36	1.41	13.49	11.41	1.5933	13.76	10.54	38.62	1.2962	13.96
4.	150- Shock	18.22	0.9358	12.85	8.16	1.1102	13.44	8.94	24.78	1.074	13.78

FTIR

The presence of functional groups, identification of unknown samples, and chemical bonding of the samples were analyzed by using FTIR analysis. Figure (6a) shows the FTIR spectrum of tea extract. The graph shows a wide absorption band at 3310 cm−1 due to the OH functional group in alcohols and phenolic compounds. Another intense peak at the 1628 cm−1 range indicates C=O polyphenol stretching and C=C stretching in the aromatic ring [48]. The control SnO2 and 50, 100, 150 shocks treated SnO2 NPs is shown in Figure (6b). The FTIR spectrum reveals absorption peaks in the same region for all three samples (50, 100, 150 shock), and no peak shifting or vanishing, nor emerging of new peaks, even after the impulsion of 150 shocks. The broad absorption band at 3431 cm−1 and 1637 cm−1 is due to the vibration of absorbed water molecules on the surface of the sample. The most important peak for SnO2 NPs in the range of 632 cm−1 is the vibration of the asymmetric stretching of Sn–O [49, 50, 51]. This peak, which was exhibited in all the samples, indicates SnO2 has higher molecular stability. For control SnO2 NPs shows one intense peak at the range of 1160 cm−1 in the FTIR spectra may be assigned to a vibration of hydroxyl–tin bonds (Sn–OH) of different organic types of surface hydroxyl groups [50]. This peak disappeared for the shock exposed nanoparticles. Vanishing of peak (1160 cm−1) and decreasing intensity peak (3431 cm−1) indicates that hydroxyl radical was greatly affected due to the explosion of shock wave.

Figure 6

Shows the FTIR spectrum of (a) tea extract and (b) control SnO2 and shock exposed SnO2 NPs.

FESEM

Figure 7 depicts shock exposed and controlled SnO2 NPs show the same structure with nearly the same particle size. All the NPs exhibit a small granular structure, above that small ball like SnO2 nano crystal-loaded it. The control SnO2 NPs show a grain size of 30 nm. From the figure it is clear that there is no change in the morphology or size.

Figure 7

FESEM image of (a) control SnO2 and (b) 50, (c) 100, and (d) 150 shock exposed SnO2 NPs.

UV-vis spectroscopy

The optical property of shock-loaded and controlled SnO2 NPs was characterized by using UV-Visible spectroscopy. Figure (8a) shows the UV absorption and (8b) Tauc plot of the control SnO2 and shock loaded SnO2 NPs. The control SnO2 NPs shows the high absorption and broad peak at the region of 370 nm, which appropriately corresponds to previously reported values [51, 52, 53, 54]. The broad and high absorption peak is due to the distribution and low crystallite size of the SnO2 NPs. At the same time 50, 100, 150 shocks treated SnO2 NPs shows the absorption in the same wavelength region (370–371 nm) only the change in the absorption intensity and the peak will sharpen. This is due to the defects created in the crystal structure during the impact of shocks. These defects and increasing crystal size diminish them and sharpen the absorption peaks [55]. There is no shift in the peaks; either red shift or blue shift occurs. All the peaks appear within the region of 370–371 nm. The sharp and broad absorption edge affects the Bandgap of the material.

Figure 8

(a) UV absorption spectrum and (b) Tauc plot of control SnO2 and shock exposed SnO2 NPs.

To understand more about the in-depth quantitative behavior of the SnO2 NPs, the Bandgap energy of the control SnO2 and shock exposed NPs were calculated from their absorption spectrum according to Eq. (7),where α is the absorption coefficient, hν is the energy of the photon, Eg is Band Gap energy, a is constant, and n is the nature of the semiconductor whether it is a direct transition (n = 1/2) or indirect transition (n = 2). Generally SnO2 direct Bandgap cassiterite (ore of tin) semiconducting material. The control SnO2 NPs show the Bandgap of 3.40 eV. Whereas the shock treated SnO2 NPs show the Bandgap of 3.59 eV, 3.66 eV, and 3.48 eV for 50 shocks, 100 shocks, and 150 shocks, shown in Figure (8b). The variation of band gap may be due to the defects created during the propagation of the shock waves. In addition to that, the change in particle size plays a vital role in the band gap. The size of the test samples are 30 nm, 26 ​nm, 33 nm and 30 nm for control, 50, 100 and 150 shocked conditions, respectively, which clearly reflects on the band gap value changes. These results clearly reveal that the band gap energy can be tuned precisely, which is why it is used as a semiconducting material for many applications.

Photoluminescence study

Figure (9) Shows the PL spectra for the SnO2 NPs; the emission peak at 330 nm is due to the self-internal recombination of the charged carriers. The broad peak in the region from 590-615 nm indicates the defects in the SnO2 NPs [56]. Control SnO2 and shock treated NPs show the intensity peak in the same region. Among these, 50 shocks show higher intensity peaks due to the small internal stress value between Sn–O. Therefore, the recombination rate will increase. Generally, higher bond length between Sn–O creates a higher intensity peak. 100,150 shocks SnO2 NPs disclose lower intensity peaks, demonstrating that the bond length is affected during the impulsion of shocks. Thus, the oxygen vacancies are created in the structure [57]. Reduction of intensity peak can also occur between surface area and concentration volume of oxygen vacancies.

Figure 9

PL spectra for the control SnO2 and shock exposed SnO2 NPs.

Photocatalytic activity and degradation mechanism of SnO2 NPs

The control SnO2 and shock exposed samples demonstrate different Bandgap and PL intensity. Based on varying intensities, the catalytic efficiency of the SnO2 NPs varies. The control SnO2 NPs exhibit the highest efficiency of 96% within 90 min. This is due to the defect-free structure of SnO2 NPs at control conditions. The 50 shock, 100 shocks, 150 shocks exposed SnO2 NPs display the lowest efficiency of 74%, 92%, 86% compared to control SnO2 NPs within 90 min, as shown in Figure (10).The low efficiency due to the particle interaction during the shock treatment creates structural defects; these defects act as the recombination center for e- and h+. When the shock impulsion is applied, structural deformation like stress and strain occurs and influences the photocatalytic property. From the USDM studies, the stain value calculated 1.1446 (50 shocks), 1.5933 (100 shocks), and 0.7602 (150 shocks). Increasing strain value causes a blue shift in the absorption peak; accordingly, the Bandgap value also changes [58, 59].

Figure 10

(a) control SnO2, (b) 50 shocks, (c) 100 shocks, and (d) 150 shocks VB dye degradation.

Figure (11) Shows the efficiency and kinetics graphs of SnO2 NPs. Kinetics study also reveals that correlation coefficient value (r2) for control SnO2, 50 shocks, 100 shocks, and 150 shocks SnO2 NPs exhibits the pseudo-first-order kinetics values of 0.9779, 0.9377, 0.9724, and 0.9904 while the second-order kinetic values of 0.6472, 0.7689, 0.7648, and 0.8728. This will clearly show the control SnO2 and shock exposed NPs follows the first-order reaction, shown in Table (3).

Figure 11

(a) & (b) Efficiency graphs, (c) Pseudo first, and (d) second order kinetics graphs.

Table 3

Efficiency and kinetics values of control SnO2 and shock exposed SnO2 NPs.

Sr.No	Samples	Degradation efficiency (%)	Victoria blue (VB)
			First order model		Second order model
			Rate constant (min−1)	r2	Rate constant (min−1)	r2
1.	control SnO2	96	0.0322	0.9779	0.2226	0.6472
2.	50 shock	74	0.0144	0.9377	0.1273	0.7689
3.	100 shock	92	0.0284	0.9724	0.1523	0.7648
4.	150 shock	86	0.0214	0.9904	0.0772	0.8728

The photocatalytic activity test was carried under UV light due to the wide band of the SnO2 NPs. Under UV light (376 nm) irradiation the following reaction takes place in SnO2 NPs, Mechanism of SnO2 NPs VB dye degradation shown in Figure (12).

Figure 12

Mechanism of SnO2 NPs VB dye degradation.

When the UV light penetrates the photocatalyst the electrons (e-) get excited from the valence band to the conduction band due to the higher photon energy of UV light compared to SnO2 band gap energy (Eq. (8)).

Conduction band reaction: Excited electrons will react with oxygen molecules (O2) to form superoxide anions (O2-) (-0.33 vs NHE). These superoxide ions react with dye molecules to form intermediate products with water (Eqs. (9), (10)).

Valence band reaction: Holes will react with water molecules (H2O) to form a hydroxyl radical (OH−) (+2.33 vs NHE). These hydroxyls have enough power to break the dye molecules' chemical structure to form an intermediate with water (Eqs. (11), (12)).

Conclusion

SnO2 nanoparticles were successfully synthesized by a toxic free green microwave-assisted hydrothermal method and then the green synthesized nanoparticles were treated with 50, 100, and 150 shock pulses. The powder XRD characterization confirmed that SnO2 nanoparticles belong to a tetragonal structure and confirmed its highly crystalline and stabilized structure under the impulsion of shock conditions. From Rietveld refinement profile analysis study, it is evident that the photocatalytic efficiency in degrading VB dye under UV light is higher in the control SnO2 NPs than the shock treated samples. This may be due to the changes in the bond length and other parameters like stress, strain, etc., which is confirmed by the W–H plot. FTIR exhibits high molecular stabilized SnO2 nanoparticles. FESEM images are clearly evident for the morphology of SnO2 nanoparticles did not change, only the particle size was changed. Control, 50, 100, and 150 shocks treated SnO2 nanoparticles show a stable optical property.

Declarations

Author contribution statement

M. Jarvin: Conceived and designed the experiments; Performed the experiments; Wrote the paper.

S. S. R. Inbanathan, S. A. Martin Britto Dhas: Analyzed and interpreted the data; Wrote the paper.

D. Rani Rosaline: Contributed reagents, materials, analysis tools or data.

A. Josephine Prabha: Performed the experiments.

Funding statement

This work was supported by UGC-DAE-CSR, Indore center, India and by the Consortium for Scientific Research (CSR) Indore center (grant:CSR IC -236/2017-18/1317, Dt 31st March-2018).

Data availability statement

Data will be made available on request.

Declaration of interests statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.