Photocatalytic activation of Ag-doped SrSnO3 nanorods under visible light for reduction of p-nitrophenol and methylene blue mineralization

The utilization of solar energy for the treatment of wastewater pollutants by photocatalysts has been considered a promising solution to address environmental problems. Herein, we have synthesized silver nanoparticle-doped strontium stannate (Ag-doped SrSnO3) nanorods by hydrothermal method followed by ultrasonic treatment. The developed nanocomposites were applied for photocatalytic reduction of p-nitrophenol (4-NP) and methylene blue (MB) mineralization under visible light illumination. The effect of hydrothermal duration time (16–25) h, Cetyltrimethylammonium bromide (CTAB) and silver nanoparticles (Ag NPs) concentration (0.5–2.5) wt% on the crystal, surface, optical, photoluminescence as well as photocatalytic activity were studied. A well-defined crystalline cubic phase of SrSnO3 was obtained. CTAB inhibits the crystal growth of SrSnO3. Reduction of 4-NP and MB mineralization were used as two-model reactions for testing the effect of Ag doping concentration on the photocatalytic activities of Ag/SrSnO3 under visible light illumination. The obtained results show that 2.0 wt% of Ag-doped SrSnO3 exhibits efficient photocatalytic reduction of 4-NP with 98.2% conversion within 5 min of reaction time. Also, 87% of the MB sample was mineralized after 1 h of visible illumination using 2.0% Ag/SrSnO3 in the presence of H2O2. Besides, we have discussed the possible photocatalytic mechanism for reduction of 4-NP and mineralization of MB using 2.0 wt% of Ag doped SrSnO3 under visible light illumination.


Introduction
Environmental pollution is one of the main global challenges of the twenty-first century. Among the various resources for environmental pollution, water pollution is regarded as one of the most negative effects for the mankind and ecosystem [1,2]. Industrial, domestic, and agricultural activities and other environmental and global changes are the main water pollution sources, which may cause serious health problems to humans [3]. Wastewater treatment and reuse for beneficial purposes is an essential requirement for a healthy lifestyle, environmental sustainability, and social equity.
Several methods have been developed for wastewater treatment. Photocatalytic degradation of pollutants is a promising technology for the efficient wastewater treatment. It is comparatively inexpensive, simple, and environment friendly [4][5][6]. In the past 30 years, various photocatalysts have been reported. Among of them, alkaline earth perovskite stannate (MSnO 3 , M = Ca 2? , Sr 2? , Ba 2? ) were used as photocatalyst in recent years [7]. Perovskite SrSnO 3 is semiconductor with wide bandgap energy (*3.8-4.2 eV) [8]. Under UV illumination, its charge carriers are easily separated. These separated charge carriers are responsible for photocatalytic activity of SrSnO 3 . The wide bandgap energy is the main disadvantages of SrSnO 3 photocatalyst. As consequences, SrSnO 3 absorbs UV light only from the solar spectrum. Another disadvantage of SrSnO 3 as a photocatalyst is high charge carriers' recombination rate [9,10].
Overcoming these challenges requires the doping of metal NPs onto perovskite. Manganese doping in CsPbCl 3 Nanocrystalline drastically enhances their optical properties due to the charge transfer of photoinduced excitons from the CsPbCl 3 host to the dopant Mn 2? centers [11]. Noble transition metals such as Ag NPs and their composites with semiconducting oxide have attracted increasing attention because they could induce Surface Plasmonic Resonance (SPR). The SPR was used to assist the efficiency of photocatalyst in the visible absorption range. When noble metal nanoparticles are deposited on the surface of a semiconductor, they display unique optical and catalytic capabilities that are not seen in bulk metal due to transferring photoexcited electrons from the Ag NPs to TiO 2 Conduction Band (CB), which facilitate charge separation. Gerisher has emphasized the importance of dioxygen reduction by CB electrons, which avoids the recombination of photogenerated charge carriers. Therefore, superoxide radical production could be the slowest stage in the reaction sequence leading to substrate oxidation [12,13] Although a wide range of alkaline earth stannate MSnO 3 (M = Sr, Ca, Ba) compounds have been synthesized and studied from a structural, magnetic, and electrical, photoluminescence, theoretical point of view [14], only a few have been attempted in studding the photocatalytic activity of pure and doped SrSnO 3 nanoparticles. For instance, Jun ploy et al. [15] applied Ag-doped SrSnO 3 composites for Methylene blue degradation under ultraviolet Radiation. Venkatesh et al. [16] prepared rGO-SrSnO 3 nanocomposites for degradation of aqueous methylene blue dye under UV light irradiation. The SrSnO 3 /g-C 3 N 4 was synthesized by a facile solid-state method and applied to visible light-mediated photocatalysis in both, wet and dry phase [17]. Subhan et al. [14] synthesized triple metal oxide from Ag/SrSnO 3 by a co-precipitation method for dye removal, antibacterial and sensing applications. However, no attempts of preparing Ag/SrSnO 3 by hybrid hydrothermal/ ultrasonic method for p-nitrophenol (4-NP) reduction and methylene blue (MB) mineralization under visible light illumination can be found among literature data.
In this report, we have focused on preparation of an efficient Ag-doped SrSnO 3 photocatalyst for degradation of common environmental pollutants in daily life as methylene blue (MB) dye and 4-nitrophenol (4-NP) under visible light illumination, and to evaluate the catalytic performance of the developed catalyst.

Preparation SrSnO 3 nanorods
Sr(NO 3 ) 2 (10 mmol), SnCl 4 Á5H 2 O (10 mmol), CTAB (1.5 mmol) and H 2 O (30 mL) were mixed together under continues stirring. Add 1 mol/L NaOH until the pH of the solution adjusted to 13, white suspension was obtained. Then, the suspension was transferred into a Teflon-lined stainless autoclave with about 80% degree of fill. The sealed vessel was then heated at 160 oC for * 18 h, and then cooled to room temperature. The obtained precipitate was filtered and washed for several times with distilled water. The obtained powder was calcinated at 700°C for 3 h [15].

Preparation Ag and Ag doped SrSnO 3
To synthesize silver nanoparticle-doped strontium stannate nanorod (Ag/SrSnO 3 ), AgNO 3 (0.5-2.5 wt%) was mixed with 1 g SrSnO 3 and 50 mL of ethylene glycol (EG, C 2 H 6 O 2 ) under magnetic stirring. The mixture was ultrasonically vibrated for 15 min to form dispersed and de-agglomerated brown suspension. Then the product was separated, washed several times with absolute ethanol, and dried in air at 70°C for 24 h [15]. Pure silver nanoparticles (Ag NP-1) were also prepared using the similar procedure using1 mmol of silver nitrate and 50 mL of ethylene glycol (EG, C 2 H 6 O 2 ). Brown product was separated by centrifugation at 12,000 rpm and washed three times by distilled water. The produced silver nanoparticles (Ag NP-2) from 2 mL of 0.001 M silver nitrate were added drop wise to 30 mL of 0.002 M sodium borohydride solution that had been chilled in an ice bath. The reaction mixture was stirred vigorously till the solution turned to pale yellow [18].

Photocatalytic reduction of 4-NP
40 lL of 4-NP (0.01 M) was added to 2.8 mL deionized water and then mixed with 80 lL of freshly prepared NaBH 4 (0.5 M). After that, 5 mg of Ag/ SrSnO 3 photocatalyst (0.0-2.5 wt%) was added to the above solution with stirring for 30 min to reach the adsorption/desorption equilibrium. The reaction mixture was illuminated using halogen lamp (500 W; k [ 420 nm). The above procedures were repeated under dark condition in the presence of 0.0-0.5 wt% Ag/SrSnO 3 . Moreover, blank experiments were done in the presence of NaBH 4 only under dark and light condition.

Photocatalytic degradation of MB
20 mg of photocatalyst (0.0-2.5) weight percent (wt%) Ag/SrSnO 3 was added to an aqueous solution of MB (in 25 mL; 10 ppm). The reaction mixture was stirred in the dark for 30 min to reach the adsorption/desorption equilibrium. Then H 2 O 2 (0.08 M) was added. Then, the halogen lamp was turned on to start the photoreaction. The decolorization of MB was followed using UV-Vis. Spectrophotometer. Also, the mineralization of MB was followed up using total organic carbon technique (TOC). The same experiment was done at three different pH values (3.0, 6.0, 11) in the presence of 2.0 wt% of Ag/SrSnO 3 under visible light illumination. A series of blank experiments for MB in the presence of H 2 O 2 , SrSnO 3 and 0.5 wt% Ag/SrSnO 3 in the dark were tested.

Characterization of the photocatalysts
The crystalline and phase structure of the materials were performed using Shimadzu 6000 X-ray diffractometer (XRD) (Cu-K radiation = 1.5406 Å ) as reported in detail elsewhere [19]. The morphology of the synthesized nanostructured were investigated by Transmission electron microscope (TEM) (200 kV, JEOL-JEM-2100, Tokyo, Japan) and scanning electron microscope (SEM) (FEI Sirion) equipped with an Energy Dispersive X-Ray (EDX) detector (S-3400 N II, Hitachi, Japan). Shimadzu UV-2450 spectrophotometer was used to measure the diffuse reflectance spectrum (UV-Vis DRS) using BaSO 4 as a reference sample. The photoluminescence (PL) spectra were characterized with (Shimadzu RF-5301PC) spectrofluorometer. Thermogravimetric analysis (TGA-50) and Differential thermal analysis (DTA-50) was tested in the range from 50 to 800 with Shimadzu thermogravimetric analyzer at a constant heating rate (10 min or 1) in nitrogen gas. Fourier-transform infrared spectroscopy (FTIR) was measured with KBr disks using JASCO FTIR-6800.

XRD
The effect of hydrothermal time at 160°C on the crystal structure of as-synthesized SrSnO 3 in the absence and presence of CTAB was studied by XRD analysis. The XRD patterns are shown in Fig. 1a (I).
The observed diffraction peaks of hexagonal phase SrSn(OH) 6 (Table 1). In addition, an increase in the lattice parameter values was observed, reflecting that heat treatment can appreciably influence SrSnO 3 crystallinity. However, on the presence of CTAB, SrSnO 3 exhibited a reverse behavior suggesting that the presence of CTAB during preparation inhibits the crystallization of SrSnO 3 . Figure 1b shows the XRD pattern of SrSnO 3 at different Ag loading content wt%. The results XRD spectral pattern exhibited XRD peaks belonging to SrSnO 3 , in addition to a minor phase of Ag NPs (2h = 38.12°and 44.2°) was observed at higher concentrations of Ag NPs (2.0 and 2.5 wt%) (JCPDS Card 04-0783), Fig. 1c shows XRD patterns of pure Ag NP-1and Ag NP-2 [22]. Doping of Ag NPs on the surface of SrSnO 3 may be due to the reduction Ag ? by EG during ultra-sonication step. As Ag NPs concentration increase, the intensity of the main diffraction peak of Ag/SrSnO 3 at (202) was gradually decreased. This phenomenon is likely due to Ag NPs can inhibit the growth of SrSnO 3 crystal ( Table 2). In addition, it was observed that the main diffraction peak was shifted to lower angle as a function of Ag NPs concentration. Also, it is observed that the lattice parameter decreased by increasing Ag NPs loading content in the Ag/SrSnO 3 , Fig. 1d. This is due to growing of small size of Ag NPs (10-40 nm) on the surface of micro scale SrSnO 3 rods, which minimize the total surface energy of Ag/SrSnO 3 system. Figure 1e shows the FTIR spectrum of the SrSn(OH) 6 precursor, SrSnO 3 and 2 wt% Ag NPsdoped SrSnO 3 NPs. SrSn(OH) 6 exhibits peak at 850 cm -1 due to bending mode of the hydroxyl group. This peak disappears after formation of SrSnO 3 NPs. SrSn(OH) 6 shows broad peaks at 3500 and 1441 cm -1 due to adsorbed water molecules [22]. However, SrSnO 3 and 2 wt% Ag/SrSnO 3 exhibit peak in the range of 521-660 cm -1 which is attributed to vibration of the O-Sn-O bridging [23]. The FTIR spectrum of 2 wt% Ag/SrSnO 3 nanocomposite shows small shift in the absorption peaks, due to the charge transfer interaction between Ag NPs and oxygen atoms in the SrSnO 3 NPs. Furthermore, the peak intensity at 1440 cm -1 was increased due to presence of the OH groups on the surface. Therefore, Ag NPs attached on the surface of SrSnO 3 can improve its surface state and generate more hydroxyl groups on the surface.
The TG-DTA analysis were performed to determine the temperature and weight change for decomposition of the precursor SrSn(OH) 6 into SrSnO 3 phase. Figure 1f displays the TGA thermogram with three different steps, which showed * 18% total weight loss on the temperature range 25-800°C. The first step from 25 to 200°C with the weight loss * 3.5% due to the evaporation of water and ethanol physically adsorbed on surfaces of the precursors. At 200-350°C, the faster weight loss (12%) corresponding to the dehydration and dihydroxylation processes of the Sr-Sn-O-H precursor. The third step 530-800°C showed complete release of H 2 O molecules from their grains, with the formation of SrSnO 3 . The observed phase change of SrSn(OH) 6 precursors into SrSnO 3 at 700°C via three endothermic peaks (37, 275, 563°C) in the DTA thermogram shows similar behavior throughout the decomposition process to produce SrSnO 3 [24]. Figure 2 shows SEM images of SrSnO 3 at different hydrothermal times in the absence and presence of CTAB. In the absence of CTAB, the irregular shape from SrSnO 3 converted to rod-like shape by increasing hydrothermal time, Fig. 2a-c. The average diameter and length of rods are 700 lm and 5 lm, respectively. Presence of CTAB during preparation allows the formation of rod shape at different hydrothermal duration times, Fig. 2d-f. The rod diameter decreases from 1-2 lm to 200-300 nm as hydrothermal time are increased from 16 to 25 h at same CTAB concentration. The rod length is also  Fig. 2j displays the presence of Sr, Sn, O and Ag elements. Figure 3a depicts the UV-Visible (UV-Vis) absorption spectra of SrSnO 3 on the absence and presence of different concentrations of Ag NPs. As shown on Fig. 3a, SrSnO 3 shows a broad absorption band at 285 nm due to intrinsic exciton absorption, [25,26]. Ag/SrSnO 3 rod shows an additional absorption band in the visible region (427 nm), Fig. 3a. The intensity of this band increases by increasing the content of Ag NPs loading, which is due to the characteristic plasmon surface resonance (SPR) phenomenon of free electron in Ag NPs conduction bands [27,28]. This confirms the presence of Ag NPs on the surface of SrSnO3, as shown in Fig. 3b. The bandgap energy values of Ag/SrSnO 3 at different Ag NPs loading concentrations were determined using the following Wood and Tauc Eq. (1) [29]: where a is the absorbance, ht is the photon energy and C is a band tailoring constant. The nature of the optical transition in the semiconductor is indicated by the exponent ''n''. The values of n are 2 and for allowed indirect and direct optical transitions, respectively [30]. Plotting (ahv) 1/2 versus hv according to relationship (1) leads to the evaluation of the bandgap energy (E g ) by extracting the linear portion of curves to (ahv) 1/2 = 0, Fig. 3c. The calculated bandgap energy values of Ag/SrSnO 3 NPs are summarized in Table 2. As can be seen from Table 2, values of E g decrease with increasing content of Ag loading, and the observed E g values are relatively lower than some reported literature values (4.1-3.9 eV) [31]. Figure 3c show a high absorption peak at 2.9 eV, which is ascribed to the surface deposition of Ag NPs. Figure 3d shows the photoluminescence (PL) emission spectra of SrSnO 3 at different Ag NPs concentrations (k ex = 275 nm). Notably, the PL emission of SrSnO 3 results from indirect transition of the excited electron from the conduction band to valance band. However, the PL of Ag/SrSnO 3 rods were quenched as Ag loading increased. This quenching is because the doped Ag NPs on the surface of SrSnO 3 acts as electron trapping under UV excitation, in which, Ag NPs can trap the SrSnO 3 excited electrons on the conduction band. We assume that Ag NPs retard the electron-hole recombination in the SrSnO 3 rods, which, increases the photocatalytic efficiency of the Ag/SrSnO 3 under UV excitation [32].
The observed 1-potential values (Fig. 3e) of SrSnO 3 , Ag/SrSnO 3 , Ag NP-1 and Ag NP-2 suspension in water are -27.4 Mv, -39 mV, -15 mV and 0 mV, respectively. These high negative charges suggesting the electrical stability of the Ag/SrSnO 3 is higher than that of SrSnO 3 and Ag NP 1-2 , i.e., resist aggregation. [33,34].The effect of pH on the zeta potential of SrSnO 3 and Ag/SrSnO 3 rods on the pH range 1-12 is shown in Fig. 3e. The estimated isoelectric points of SrSnO 3 and optimized Ag/SrSnO 3 are at pH values of 3.8 and 4.2, respectively. The observed negative zeta potential values suggesting enhanced adsorption of MB (cationic dye) on the surface of photocatalyst at these pH values [35]. This is because the photo-generated holes after irradiation   Fig. 4c, d. The obtained results show a very small change in the absorption spectrum of 4-NP in the presence of only SrSnO 3 or NaBH 4 . In all experiments, the adsorption/desorption equilibrium was performed by adding 5 mg of photocatalyst to 4-NP under stirring for 30 min in dark. as shown in Fig. 4e, weak adsorption of 4-NP was observed on the photocatalyst surface. As shown in Fig. 4, the absorption spectrum of 4-NP changed insignificantly in the presence of only Ag NP (1,2) in the dark (f, g). However, in the presence of Ag NP (1, 2) and NaBH 4 , a small change in the absorption spectrum of 4-NP was observed under visible light. This is because the negative charges of Ag NP-1 (-12 mV) and Ag-2 (-2 mV) adsorbed weakly with the negative charge of  [33]. Figure 5a shows the UV-Vis absorption spectra of blank in the dark using 0.5 wt% of Ag/SrSnO 3 and NaBH 4 . Figure 5b, f display the gradual decrease in UV-Vis absorption spectra for 4-NP as a function of visible light illumination times in the presence of Ag/ SrSnO 3 catalysts and NaBH 4 with different doping contents of Ag NPs. A new band around 300 nm was appeared due to formation of 4-aminophenol [36]. By increasing the loading content of Ag NPs, the photocatalytic activity of the Ag/SrSnO 3 increase. The higher photocatalytic reduction activity (98.2%) of 4-NP was observed within 5 min in the case of 2.0 wt Ag/SrSnO 3 photocatalyst compared with AgNP-1, 2 only.
The rate constant of the reaction can be evaluated by the pseudo first-order kinetics according to the following Eq. (2) [37]: where A o and A are the absorption intensity before illumination and at different times of illumination, respectively. k app is an apparent rate constant. The relationships between ln (A/A o ) against t over Ag/ SrSnO 3 catalysts are shown in Fig. 5g and h. The photocatalytic reduction of 4-NP follows the pseudo first-order kinetics. The degradation kinetic parameters of Ag/SrSnO 3 catalysts are summarized in Table 3. The rate constant values increase with increasing Ag NPs content, until reaches maximum at 2 wt% of Ag NPs and then decrease after adding 2.5 wt%. Compared to other reported candidates (Table 4), the as-prepared Ag-doped SrSnO 3 photocatalyst exhibit relatively higher photocatalytic reduction rate, suggesting that the prepared photocatalyst is promising material for water treatment.
The measured pH IEP value of 2.0% Ag/SrSnO 3 is 4.2, which is lower than the pk a value of 7.2 for 4-NP [38]. This indicates that under neutral condition (pH = 7.0), the surface of 2.0% Ag/SrSnO 3 and 4-NP have a negative charge, which means that in dark, the adsorption factor has less effect on the reaction rate (Figs. 3e, 4e and Table 3) [36,[39][40][41][42]. Also, the hydrolysis rate of borohydride ions increases by decreasing the pH of the medium [43,44]. However, the measured rate of reaction for blank experiment without 2.0% Ag/SrSnO 3 photocatalyst, is very low, which enhances significantly in the presence of 2.0% Ag/SrSnO 3 and NaBH 4 in visible light.

Photocatalytic degradation of methylene blue
The photodegradation efficiency of MB dye was studied using Ag/SrSnO 3 /H 2 O 2 photocatalyst. The change in UV-Vis absorption maximum at k max = 664 nm was employed to monitor the entire photodegradation process. Also, the effect of H 2 O 2 addition on the photocatalytic degradation of MB dye was also investigated in dark and under visible light illumination, as shown in Fig. 6a, b. Remarkable very slow degradation of the dye was observed in the dark under visible light illumination. However, a small change in the absorption spectrum of MB was observed in presence of SrSnO 3 and 0.5 wt% Ag/ SrSnO 3 in dark, Fig. 6c, d. Moreover, the SrSnO 3 shows a small change in the absorption spectrum of MB under visible light illumination, Fig. 6e. This is due to the high bandgap energy of SrSnO 3 (3.8 eV), which activated with UV illumination more than visible light as shown in Fig. 6e, weak adsorption of MB was observed on the photocatalyst surface under different condition, as shown in Fig. 6f. Furthermore, weak adsorption was observed in the dark, where oxidation of MB under visible light was observed in the presence and absence of Ag NP 1, 2. (g-i). As a positive-charged cationic dye, MB can reduce the adsorption of lowvalue negatively charged Ag NP-1 and AgNP-2 on the surface, lowering degradation efficiencies [33]. It is well known that MB can be present in aqueous solution as the cationic species (MB ? ) and undissociated molecules (MB 0 ). Also, the (MB ? ) is only species present at pH [ 6 [45]. At experimental pH condition, both forms are present. Although, the photocatalyst carrying a negative charge (IEP at pH = 4.2), the presence of undissociated molecules (MB 0 ) decreases the adsorption efficiency of MB on the photocatalyst surface. Ag/SrSnO 3 shows higher decolorization efficiency under visible light illumination, Fig. 7a. This due to presence of Ag NP lowering the bandgap energy and acts as a sensitizer for SrSnO 3 . To accelerate the reaction rate, H 2 O 2 was used in combination with Ag/SrSnO 3 . Figure 7b shows that the addition of H 2 O 2 enhances the decolorization efficiency of 0.5 wt% Ag/SrSnO 3 under visible illumination. Figure 7b-f show that less   Fig. 7g. The degradation rate constants were determined from the linear plot of ln(A t /A 0 ) vs time (t), (Fig. 7h) and the results are summarized in Table 5. All experiments were performed at pH = 5.5 using 0.08 mol of H 2 O 2 in the presence of 20 mg (0-2.5) wt% of Ag/SrSnO 3 .  Table 5, the decolorization of MB using Ag/SrSnO 3 is depending on pH value. Complete decolorization occurs within 26 min at pH = 11. The values of the decolorization rate constants were 0.025, 0.058 and 0.107 at pH values of 3.0, 6.0 and 11, respectively, Fig. 8d, e and in Table 5. The gradual increase in the photocatalytic activity was mainly due to the predominant cationic MB ? species at pH [ 6. Also, the negative charge of the surface of Ag/SrSnO 3 increases by increasing the pH of the solution. The Additionally, the photodegradation efficiency of MB was investigated using the highly active nanoparticles (2.0% Ag/SrSnO 3 in the presence of H 2 O 2 ) using total organic carbon before degradation (TOC 0 ) and after degradation (TOC) measurement. The obtained results show that the TOC value reduced from 100 mg/l (TOC 0 ) to 13 mg/l (TOC) after 1 h of visible light illumination. This implies  that 87% of the sample was completely decomposed after 1 h (Fig. 9a). SrSnO 3 absorption was difficult in the presence of direct light as a result of its large bandgap (3.8 eV), which greatly diminished its usage as a photocatalyst under visible light, Scheme 1 To improve the photocatalytic process, plasmonic noble metals such as Ag NPs were doped SrSnO 3 to increase visible light absorption. Degradation of organic pollutants is dominated by photo generated electron -hole pairs. As a result, the method was used to calculate the Conduction Band (CB) and VB (Valence Band) potentials for catalysts by the following Eqs. (3,4) to study the photocatalytic process [46], The potential energies of the conduction and valence bands are E CB and E VB , respectively, while E e is the energy of free electrons against a normal hydrogen electrode (4.5 eV). The resulting electronegativity of different individual semiconductors is given by the following Eq. (5) [47] v on the photocatalyst surface (Scheme 2b) [51]. Also, borohydride ions react with the Ag NPs and produce hydrogen gas as a source of an active hydrogen species on it (Ag-H). These hydrogen species are unstable and have a negative charge in the metalhydrogen structure. As consequences, it can easily attack the positively charged nitrogen in the nitro group of nitrophenols to produce nitroso group (Scheme 2c). This is followed by the reductive addition of two hydrogen atoms to form hydroxylamine (Scheme 2d). Finally, the hydroxylamine is further reduced to the 4-AP (Scheme 2e) [43,52]. Figure 9b shows trapping experiment to confirm the suggested mechanism by addition of different scavengers. We utilized p-benzoquinone PBQ (0.1 mmol), isopropanol IPA (0.1 mol L -1 ), silver nitrate AgNO 3 (0.1 mmol), and sodium oxalate Na 2-C 2 O 4 (0.1 mmol) as ÁO 2 , ÁOH, electron, and h ? scavengers, respectively. These results indicate that photogenerated electrons play a significant role in the removal of organic compound, while OH and h playing a secondary function.
From an economic point of view, the chemical stability of the photocatalyst is the main issue. Therefore, we performed the stability of Ag/SrSnO 3  Fig. 9c, d. The five cycles degradation of MB and 4-NPs were performed under the same condition in the presence of H 2 O 2 and NaBH 4 . It is apparent that the degradation efficiency of Ag/ SrSnO 3 was not significantly decreased after threetime photocatalytic cycles. The degradation efficiency of MB only decreases from 96.5 to 93.4% and reduction efficiency of 4-NPs decreased slightly from 98.2 to 95.5%. The decrease in the photocatalytic efficiency related to the decreasing of concentration Ag NPs. The ICP results showed a slight decrease of Ag NPs concentration from 2.0 to 1.88%) [35]. Whereas After 1 cycle the degradation efficiency of MB and reduction efficiency 4-NPs by Ag/SrSnO 3 in absence H 2 O 2 and NaBH 4 decreased from 96.5-10% to 98.2-5%, respectively indicating H 2 O 2 and NaBH 4 has synergistic effect with Ag/SrSnO 3 .

Conclusions
Silver-doped perovskite strontium stannate with different wt% (Ag/SrSnO 3 ) was prepared by hydrothermal method followed by ultrasonic treatment. The effect of hydrothermal time, CTAB and Ag NPs loading were studied. All samples exhibited a well-defined crystalline cubic phase of SrSnO 3 with rod like morphology. CTAB inhibits the crystal growth of SrSnO 3 .
The UV-Vis absorption measurements confirm the formation of Ag doped on the surface of SrSnO 3 via appearance of high intense surface plasmon resonance absorption band of Ag NP. The high 1-potential value of Ag/SrSnO 3 suspension (-39 mV) refers to its electrical stability. The prepared photocatalysts were applied for the reduction of 4-nitrophenol (4-NP) and degradation of MB under visible light. The obtained results show that 2 wt% of Ag/SrSnO 3 is highly active sample. 2.0 wt% Ag/SrSnO 3 photocatalyst reduces 4-NP with 100% conversion within 5 min of reaction time. Also, 87% of the MB sample was mineralized after 1 h of visible illumination using 2.0% Ag/SrSnO 3 in the presence of H 2 O 2 .

Funding
Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).

Data availability
All data generated or analyzed during this study are included in this published article.

Declarations
Conflict of interest The authors have no conflict of interest to declare that are relevant to the content of this article.
Ethical approval Not applicable.
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