Introduction

Our environment has been exposed to numerous hazardous organic pollutants which in accordance is expelled from varied chemical industries, pharmaceutical companies, dye factories and refineries, etc. Drastic changes have to be categorized and executed since these pollutants have their own effects which could tremendously inhibit several causes and perturb human and other living species (Lam et al. 2018). Numerous techniques are being queued in list like physical methods and biological and chemical methods with several sub techniques available. But, drawbacks associated with these are unbalanced in removal of all pollutants, maintenance of pH, temperature in a perfect way, and pollutant transfer without decomposition stands as an additional risk factor (Siva et al. 2020). Thus, all these have stopped their usage and looking towards effective, cost-efficient, and abundant means of technique, and photocatalytic degradation (photocatalysis) shines out with exhaustible significances. Being one of the easiest techniques to reduce water contamination, photocatalytic degradation of organic pollutants for water treatment has attracted wide attention due to its effectiveness, easy operation, and ideally producing non-toxic end products (Saravanan et al. 2011). Photocatalysis is normally based on the absorption of light by semiconductor nano-photocatalyst thereby exciting the electron from the lower energy band to higher energy band that leads to formation of electron-hole pair (Lv et al. 2011). These photocatalysts in absorbance of sunlight (photons) further accelerate the reactions to occur. Researchers have proved that the wide bandgap semiconductor photocatalyst can degrade various organic pollutants under UV and visible light irradiations which offers great abilities for complete elimination of toxic chemicals. A key restriction in accomplishing high photocatalytic efficiency is quick recombination of photogenerated charge carriers. Recombination has faster kinetics than surface redox reactions and greatly reduces the quantum efficiency of photocatalysis (Lv et al. 2012; Sun et al. 2011; Barrientos et al. 2018). The semiconductor nanoparticles known as semiconductor clusters, quantum particles, or quantum dots form a class of new materials by which size-dependent properties are observed. In semiconductors, size-dependent properties occur when the size of particles is comparable or smaller than Bohr diameter of exciton for that material. In most of the semiconductors, the Bohr radius of exciton is few nanometers (Murugadoss and Ramasamy 2012). ZnO is one of the semiconductor materials in group of II–VI with a wide bandgap of 3.26 eV (Devi et al. 2015) and large excitation binding energy of about 60 eV (Ma et al. 2013). ZnO nanoparticle has the high potential being used in solar cells, photocatalysis, gas sensors, and electrical and optical device (Medhi et al. 2020; Bindhu et al. 2020). Hence, the synthesis and morphology modifications of ZnO nanoparticle attracted more attention (Hong et al. 2006; Chen et al. 2017). We can also modify their properties by reducing the size of the particle to the nanoscale range (Sirelkhatim et al. 2015). Doping of non-metallic ion with ZnO improves the efficiency of photocatalysis (Ertis and Boz 2017; Sankar Ganesh et al. 2017). Sn tends to hold numerous factors for doping with ZnO nanoparticle to increase the efficiency of photocatalysis (Yang et al. 2010) and to tuning the bandgap which control the recombination of photocatalysis. Coupling with other conductors and semiconductors, such as Ga, In, has been extensively reported (Bae et al. 2005). The ZnO and doped ZnO have been synthesized by various techniques that are classified as physical and chemical methods (Rekha et al. 2010; Murugadoss 2012). In this study, ZnO and Sn-doped ZnO nanoparticles were prepared by co-precipitation method through PVP as a capping agent (Murugadoss 2012).

Clean water is essential to life as it is a fundamental human need. The unrestrained discharge of pollutants from industries adversely affects countless life forms on earth. Many people are dying due to water-related diseases, which mean that safe drinking water is of great importance. Various types of dyes are being manufactured for using in the textile industries particularly in developing countries. Azo colors, the most well-known dyes in textile industries, cause general health issues. Furthermore, these dyes are stable and non-biodegradable. These dyes are extremely unsafe because of their cancer-causing nature and toxicity. Therefore, the improvement of filtration techniques for treatment of these dyes in wastewater is of high need. Methylene blue (MB) is the most common cationic dye used in textiles, wood silk, and dying cotton. Cationic dyes are basic and carrying a positive charge in their molecule. Furthermore, these dyes are water soluble and yield colored cations in solution (Christie 2002) which causes many diseases for humans and animals (Rafatullah et al. 2010; Ramasamy et al. 2013).

Some reports have been addressed on the studies of photocatalytic degradation in utilization of Sn being doped with zinc oxide. Jian-Hui Sun et al. have reported a work over photocatalytic performance of Sn-doped ZnO for methylene blue via microwave heating method. On the other hand, Muhammad Arshad et al. have described the influence of varying solvents towards the photocatalytic activity of Sn-doped ZnO. Herein, we have investigated the photocatalytic degradation of MB by pure ZnO and different concentrations (1, 2, 3, 4, and 5 %) of Sn-doped ZnO. Among the concentrations, 4% of Sn was chosen as best doping level by enhancement of degradation efficiency. The photocatalytic enhancement is ascribed to the morphological variations with active sites of absorbing the organic pollutants. And thus, this work would be of greater significance with required surface area providing material for the absorption in a manner of cost-effectiveness (material) which utilizes sunlight of abundance resource and a simple co-precipitation method contributing higher crystalline material (Arshad et al. 2018).

Materials and methods

Materials

Zinc acetate (Zn(CH3CO2)2) and tin (IV) chloride pentahydrate (SnCl4·5H2O) were purchased from Nice chemicals and VETECH, respectively. Polyvinylpyrrolidone ((C6H9NO)n, PVP), sodium hydroxide (NaOH), and ethanol were obtained from SDFCL. Double-distilled water was used throughout this experiment.

Synthesis of pure zinc oxide (ZnO) and tin-doped zinc oxide (Sn/ZnO) nanoparticles

Zinc acetate 4.6 g (0.025 M) was dissolved in 50 mL of distilled water and 0.5 g of PVP dissolved in 50 mL of distilled water separately under constant stirring followed by addition of PVP solution into the zinc acetate solution. The mixture was left on stirring for 30 min. The PVP was employed as a surfactant which could help in regulating crystal growth, less surface tension, and controlled nanoparticle synthesis whereas no other additional treatments are involved. Typically, different concentrations of tin (IV) chloride pentahydrate was dissolved in 25 mL of ethanol (different weight ratios as 1%, 2%, 3%, 4%, and 5%) and then added separately to the above reaction mixture and stirred for another 10 min at room temperature. Moreover, 1.5 g of NaOH was introduced in 50 mL of distilled water in dropwise manner into the above reaction mixture and allowed to stir for 2 h. A white precipitate was obtained which was frequently washed with distilled water and ethanol for several times and dried in an oven at 50 °C overnight. The obtained powder was utilized for further conformational and photocatalytic studies. Different weight ratios of Sn- (1, 2, 3, 4, and 5%) doped ZnO were prepared successfully and named as pure ZnO, 1-Sn/ZnO, 2-Sn/ZnO, 3-Sn/ZnO, 4-Sn/ZnO, and 5-Sn/ZnO, respectively.

Photocatalytic degradation of methylene blue

The MB was used as a model pollutant to evaluate the photocatalytic activity of pure ZnO and Sn-doped ZnO. Twenty-five milligrams of the catalyst was added into the 25 mL of dye solution (20 mg/L). Then, the dye solution is irradiated with sunlight for 3 h. During irradiation, 3 mL of dye solutions was taken out from the system at regular time interval. The absorbance maximum for MB is observed at 663 nm. The degradation of MB was determined by UV-Visible spectrophotometer.

Characterization

Crystalline structure and phase identification of synthesized Sn-doped ZnO samples were performed by X-ray diffraction (XRD) analysis (Rigaku–Ultima IV). The morphology and structure were identified by field emission scanning electron microscope (FESEM; FEI Quanta-250 FEG microscope). The oxidation state and chemical composition of the samples were studied by X-ray photoelectron spectroscopy (XPS). The optical studies were carried out using a UV-Visible spectrophotometer (JASCO V-650 spectrophotometer). Vibrational phonon modes of the sample were analyzed by Raman spectroscopy (Horiba jobin-LabRam-HR) and emission energy of the material is identified by fluroscence spectroscopy (Spectrofluorometer (Fluoro Max-4L).

Results and discussion

The structural properties of ZnO and Sn-doped ZnO nanoparticles were evaluated by means of XRD measurement. Figure 1a depicts the XRD patterns of pure ZnO and Sn-doped ZnO nanoparticles. The peak broadening of ZnO nanoparticles indicates that particles are in very small size. The average crystalline size of ZnO and doped nanoparticle are calculated using the Scherrer formula,

$$ \mathrm{D}=\left(\mathrm{K}\uplambda \right)/\left(\upbeta\ \mathrm{cos}\uptheta \right) $$
(1)

where D is the mean size of the grain, K is dimensionless shape factor (0.94), λ is wavelength of X-ray, β is full width half maximum (FWHM), and θ is Bragg’s angle. The diffraction peaks of pure ZnO phases observed at 2θ values of 31.64°, 34.39°, 36.26°, 47.60°, 54.46°, 62.80°, 67.89°, and 69° were assigned to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (2 0 1), and (2 0 2) planes in accordance with hexagonal wurtzite crystal structure and match well with the JCPDS card no 36-1451 (Ameen et al. 2012). The characteristic peaks for Sn-doped ZnO nanoparticles appear on 22.80° of 2θ and small decrease in the lattice parameter of unit cell has been observed by increasing the Sn concentration. This may be due to different ionic radii of Zn and Sn. Figure 1b shows shifting of peak positions of pure ZnO and Sn-doped ZnO. Furthermore, the average grain size of pure ZnO and Sn-doped ZnO nanoparticles are 26 nm and 35 nm, respectively (Verma et al. 2015; Ganesh et al. 2017).

Fig. 1
figure 1

a XRD pattern of pure ZnO and Sn-doped ZnO nanoparticles. b XRD peak of magnified (1 0 1) plane of pure ZnO and Sn-doped ZnO nanoparticles

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The surface morphology of samples was performed by field emission scanning electron microscope (FESEM), which provided surface information of pure ZnO and Sn-doped ZnO nanoparticles (Fig. 2). The images clearly display the hexagonal plate-like structure with irregular arrangement, and there are no distinguishable changes observed when doping Sn with ZnO (Fig. 2b, c) but thickness of the hexagonal plate of pure ZnO was reduced for doped ZnO. It could be helpful to increase the surface area and high photocatalytic behavior.

Fig. 2
figure 2

FESEM image of a pure ZnO and b, c 4-Sn/ZnO nanoparticles

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EDAX spectra reveal the qualitative and quantitative information of the elemental composition of the sample. Fig. S1(a-f) shows the EDAX spectra of pure ZnO and Sn-doped ZnO nanoparticles. From the EDAX spectra, we can conclude that there are no other materials, such as impurities or adducts, present in the samples. The intensity of spectra correlates with the amount of elements present in the synthesized samples. The EDAX spectrum of pure ZnO shows clear peaks of Zn and O with weight percentages of 76.87 and 23.13, respectively, and Sn-doped ZnO shows clear peaks for Zn, O, and Sn. Table S1 shows the weight percentages of elemental composition of pure and Sn-doped ZnO nanoparticles.

XPS is the proper technique to study the composition and electronic states of zinc, tin, and oxygen (Pan and Zhou 2020). The XPS analysis revealed that the samples consisted of C, Zn, O, and Zn, and no impurities were found. Figures 3a and 4a depict the wide scan XPS survey spectrum of pure ZnO and Sn-doped ZnO samples, respectively. Figure 3b shows the results of Zn 2p with splitting of 2p3/2 and 2p1/2 peaks at 1020.31 eV and 1043.26 eV, respectively, which is perfectly matched with divalent oxidation state of Zn2+. Furthermore, Fig. 3c displays results of O1s spectrum, which shows two different forms of oxygen located at 529.31 eV and 531.20 eV. The lower binding energy (529.31 eV) is assigned to oxygen in the Zn–O bonding and the other peak at 531.20 eV attributed to OH group absorbed on the surface of ZnO nanoparticles. Figure 4b shows results of Sn 3d spectrum, which is located at 485.5 eV and 494.6 eV corresponding to Sn 3d5/2 and Sn 3d3/2, respectively. The Sn 3d3/2 signal was intense because of the Auger Zn L3M45M45 transition. Similarly, Fig. 4c–e displays clearly the Zn 2p, O1s, and C1s regions and the obtained result is in good agreement with the previous report (Al-Gaashani et al. 2013; Sankar Ganesh et al. 2017; Murugadoss et al. 2015; Liu et al. 2015).

Fig. 3
figure 3

XPS spectra of ZnO nanoparticles. a Survey spectrum, high-resolution signals of b Zn 2p, c O 1s, and d C 1s

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Fig. 4
figure 4

XPS spectra of Sn-doped ZnO nanoparticles. a Survey spectrum, high-resolution signals of b Sn 3d, c Zn 2p, d O 1s, and e C 1s

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To further investigate on structural information of ZnO and Sn-doped ZnO nanoparticle, Raman analysis was performed. The Raman spectra in Fig. 5 show three inherent Raman bands at about 97 cm−1, 437 cm−1, and 932 cm−1, assigned to E2 (low), E2 (high), and 2TO (transverse-optical) for hexagonal ZnO suggesting wurtzite ZnO structure. The Sn-doped ZnO displayed three distinctive Raman vibration modes centered at 276 cm−1, 437 cm−1, and 932 cm−1, respectively. Raman bands at 97 cm−1, 437 cm−1, and 930 cm−1 are characteristic modes of E2 (low), E2 (high), and 2TO (transverse-optical). The other band at 276 cm−1 could be attributed to the electric field–induced silent B1 low mode. It was believed that ion implantation process introduced disorder-activated Raman scatterings contributing to the emergence of silent B1 low mode.

Fig. 5
figure 5

Raman spectra of pure ZnO and Sn-doped ZnO nanoparticles

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The diffuse reflectance spectra of ZnO and Sn-doped ZnO nanoparticles were recorded using UV-Vis spectrometer as a function of wavelength in the range 200–800 nm as shown in Fig. 6. The bandgap of semiconductors was influenced by various factors: temperature, pressure, electric and magnetic fields, impurities. Therefore, bandgap is responsive to the structural perfection of the material. The diffuse reflectance, R, is related to the Kubelka-Munk function F(R) by the relation

$$ \mathrm{F}\left(\mathrm{R}\right)={\left(1-\mathrm{R}\right)}^2/2\mathrm{R} $$
(2)
Fig. 6
figure 6

Diffuse reflectance spectra of pure ZnO and Sn-doped ZnO nanoparticles

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The bandgap of ZnO and Sn-doped ZnO nanoparticle was calculated by F(R)2 vs hν (Kumar et al. 2016). Thus, the bandgap of ZnO is 3.26 eV and the same for 1-Sn/ZnO, 2-Sn/ZnO, 3-Sn/ZnO, 4-Sn/ZnO, 5-Sn/ZnO are 3.26 eV, 3.25 eV, 3.27 eV, 3.28 eV, and 3.29 eV, respectively. The slight change in the bandgap value is mainly attributed to the variation of the particle size (Surya et al. 2017).

Photoluminescence spectroscopy (PL) is an important technique for studying the purity of crystalline quality, separation, and transfer behaviors of photogenerated carriers of semiconducting catalysts. The PL spectra of pure ZnO and Sn-doped ZnO nanoparticles obtained in the range of 360–600 nm at RT under the excitation wavelength of 360 nm is shown in Fig. 7. Particularly, the weaker PL intensity imitates a lower recombination probability. As shown in Fig. 7, the Sn-doped ZnO exhibited a lower PL intensity than pure ZnO. The weaker intensity level of Sn/ZnO suggests lower recombination rate of charge carriers (Murugadoss et al. 2016). The doping of Sn could influence the conduction and valence bands. Principally, doping of impurity element (Sn) is helpful for trapping excitons (electron and hole), which could lead to boost the charge separation and inhibiting the recombination. PL study clearly reveals the possibilities of enhanced photocatalytic activity of Sn-doped ZnO catalysts (Bedia et al. 2015; Yurddaskal et al. 2018; Saleh and Taufik 2019).

Fig. 7
figure 7

PL spectra of pure ZnO and Sn-doped ZnO nanoparticles

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The photocatalytic activity of a catalyst was determined by the following factors such as crystal structure, size of catalytic particle, morphology, and dopant concentration. The photocatalytic activity of pure ZnO and Sn-doped ZnO catalysts was studied using methylene blue as a model dye under natural sunlight irradiation. The photocatalytic degradation was determined by UV-visible spectra. Figure 8a, b depicts the UV-visible spectra for the MB dye treated for different times under visible light in the presence of pure ZnO and Sn-doped ZnO photocatalysts, respectively. The intensity is reduced with increasing irradiation time and also the absorption peak (maxima) shifted slightly due to degradation of chromophore to form intermediate products. The degradation efficiency was calculated by

$$ \mathrm{Degradation}\%=\left\{\left({\mathrm{C}}_0--\mathrm{C}\right)/{\mathrm{C}}_0\right\}\times 100 $$
(3)

where C0 and C are the initial and final concentration of dye.

Fig. 8
figure 8

a–f Photocatalytic activity of pure ZnO and Sn-doped ZnO nanoparticles

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The photocatalytic study was carried out with different catalysts such as pure ZnO, 1-Sn/ZnO, 2-Sn/ZnO, 3-Sn/ZnO, 4-Sn/ZnO, and 5-Sn/ZnO. Figure 8a, b illustrates pure and 4% Sn-doped ZnO catalyst and the MB dye degradation rate. A maximum performance was obtained with 4-Sn/ZnO. In the absence of catalysts, degradation of MB was negligible. In addition, pure ZnO nanoparticle exhibited very limited photocatalytic performance for sunlight irradiation. At 180 min, the sample gets degraded about 55.75% only. For Sn-doped ZnO nanoparticle, the degradation was carried by the same method but the degradation rate was dramatically increased compared to pure ZnO nanoparticles. A remarkable boost of photocatalytic process was achieved by doping of Sn into ZnO. The notable enhancement of dye degradation might be due to the fact that sub-energy level acts as traps for electrons and holes. Accordingly, photogenerated electrons in valence band of ZnO move towards the impurity energy levels or the conduction band and produce a large number of superoxide radicals (.O2−) under the irradiation of visible light.

For different ratio Sn-doped ZnO catalysts, the degradation rates were found to be 55.79 to 95.5% at 5 to 180 min (Fig. S2). The electron-hole pair of Sn-doped ZnO nanoparticles and pure ZnO nanoparticle is slightly different. The rate of reaction gradually increases from 30 to 180 min for pure and doped samples. Among the different concentrations, 4-Sn/ZnO gives an efficiency of about 94.5%. The photocatalytic process occurs as follows: under the sunlight, the catalyst that absorbs light energy subsequently generates electron-hole charge carrier. Then, hydroxyl radicals were formed by reacting hole with water molecules and hydroxyl ion. This hydroxyl radical absorbs the OH−, which is a primary oxidant for the MB dye degradation. The final products from the degradation of methylene blue are CO2 and H2O. Table S2 shows the degradation efficiency of pure and Sn-doped ZnO nanoparticles. The generation and separation of photon-induced electron-hole pair are main factors for the degradation of dye. Figure 8c shows dye degradation efficiency of pure and different concentrations of Sn doped ZnO nanoparticles. Figure 8d indicates the relation plot between Lagergren rate constant value k and time, and it can be analyzedby simple linear regression R2 value. Thus, the rate constant of samples is linear due to the surface of the catalyst so degradation process does follow pseudo-first-order kinetic model for MB dye. The hexagonal plate structure of the doped ZnO offered more surface area with increasing defects on the surface. Due to the effective light absorption of the hexagonal plates, the electrons and holes can be rapidly separated under direct sunlight irradiation resulted in increasing photocatalytic efficiency achieved (Beura et al. 2018).

Figure 8e depicts the efficiency of with and without scavenger studies. The addition of methanol and p-BQ only shows notable changes in photodegradation of azo dyes. Furthermore, the optical studies reveal that sub-level acts as traps for electrons and holes generated after the contact of pure ZnO to solar light. Hence, the photogenerated electrons in the VB of pure ZnO move to the impurity energy levels (Sn) or the CB and produce a huge amount of superoxide radicals (.O2−) under the irradiation of visible light. For a moment, the photo-excited holes react with water to form a highly oxidative hydroxyl radical species (.OH) (Kumar et al. 2019; Song et al. 2019). The mechanism of MB degradation is described as follows,

$$ \mathrm{ZnO},\mathrm{Sn}/\mathrm{ZnO}+\mathrm{h}\upnu \longrightarrow {\mathrm{h}}^{+}\left(\mathrm{VB}\right)+{\mathrm{e}}^{--}\left(\mathrm{CB}\right) $$
(4)
$$ {\mathrm{OH}}^{--}+{\mathrm{h}}^{+}\longrightarrow \mathrm{OH} $$
(5)
$$ {\mathrm{H}}_2\mathrm{O}+{\mathrm{h}}^{+}\longrightarrow \mathrm{OH}+{\mathrm{H}}^{+} $$
(6)
$$ {\mathrm{O}}_2+{\mathrm{e}}^{--}\longrightarrow {\mathrm{O}}_2^{--} $$
(7)
$$ \mathrm{Dye}+\mathrm{OH}+{\mathrm{O}}_2^{--}\longrightarrow \mathrm{Degradation}\kern.5em \mathrm{products} $$
(8)
Fig. 9
figure 9

Schematic diagram illustrating the mechanism of charge separation and photocatalytic activity of Sn-doped ZnO photocatalyst under the sunlight irradiation

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Table 1 Photocatalytic activity studies of pure and Sn-doped ZnO nanoparticles
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Figure 8f depicts the reusability of photocatalysts for photocatalytic degradation of MB by recycling the catalyst for five times. Mostly, the regeneration of powder-like catalyst ischallenging but the heterogeneous and doped catalysts overcome the loss through regeneration. The catalyst recycling iscarried out by centrifuge, water wash, and ethanol treatment followed by gentle drying. The results reveal the stability ofcatalyst and no significant loss even after five cycles of photocatalytic process. It clearly shows that the Sn-doped ZnOcatalyst could prominently support for practical application. The Lagergren rate equation is most widely used for degradation of adsorbate from aqueous solution. The Lagergren first-order model can be represented as

$$ \mathrm{K}=-\ln \kern.5em \mathrm{C}/{\mathrm{C}}_0 $$
(9)

where C and C0 are the amounts of dye degradation (mg L−1) at equilibrium and at time t (min), respectively (Karthick et al. 2014). The k is the Lagergren rate constant of first-order reaction (min−1). Table 1 shows the results obtained from pseudo-first-order kinetic models beside the experimental k values. The calculated k values of first-order kinetics are generally increased with increasing interval of all samples for degradation of MB. The photocatalytic reaction generally includes photoexcitation, charge separation and migration, and surfaceoxidation-reduction reactions (Saravanan et al. 2013). Thephotocatalytic mechanism of ZnO and Sn-doped ZnO isshown in Fig. 9. After Sn doping, the absorbance wavelengthof Sn/ZnO moves slightly to blue shift, which is confirmed byUV spectra. As shown in Table 1, it is clear that the opticalbandgap slightly increased (3.26–3.29 eV) or shifted to higherenergy (blue shift) with increasing Sn doping concentration.This blue-shift action could explain the variation of bandstructure. The bandgap changes were attributed to the Burstein-Moss bandgap widening and bandgap narrowingdue to electron-electron and electron-impurity scattering.Generally, the active catalyst generates h+, OH−, and O2− and it is necessary to detect which reactive species plays agreat role during photodegradation. It is helpful to understandthe mechanism of the photodegradation of dyes over Sn doped ZnO. Hence, h+, OH−, and O2− are eliminated byadding EDTA (h+ scavenger) (Sun et al. 2014), methanol(·OH scavenger) (Shao et al. 2013), and p-BQ (·O2− scavenger) (Zhang and Zeng 2012) into reaction solution. Table 2 shows the comparison of ZnO and Sn-doped ZnO of present work with previously reported photocatalysts.

Table 2 Comparison of current and reported studies of various ZnO-based nanoparticles
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Conclusion

In summary, pure ZnO and Sn-doped ZnO nanoparticles were synthesized by simple co-precipitation method. The nanoparticles showed high crystalline, attractive morphology, and enhanced optical absorption as confirmed by XRD, SEM, EDAX, Raman spectroscopy, UV-Vis, and photoluminescence spectrophotometer. The composition of the elements of the pure ZnO and Sn-doped ZnO samples were confirmed by the XPS study. The Sn-doped ZnO photocatalyst showed an enhanced photocatalytic degradation efficiency of MB (azo dye) under sunlight irradiation. Particularly, 4-Sn/ZnO photocatalyst achieved the highest efficiency compared to other samples. Due to the effective light absorption of the hexagonal plates and efficient electrons and holes, separation under direct sunlight irradiation supported for the enhancement of the photocatalytic efficiency. The rate constant value of 4-Sn/ZnO (0.03636 k min−1) was approximately four times higher than that of pure ZnO (0.00928 k min−1) for degradation of MB. The kinetics and the mechanism for enhanced catalytic activity were explored. In the meantime, PL studies reveal the influence of Sn dopant and confirm the modification of ZnO energy level. The lower PL intensity explained the suppression of recombination rate to enhance the photocatalytic activity. The best photocatalyst, 4-Sn/ZnO, exhibits 94.5% efficiency and good stability compared to other samples.