Enhanced visible light induced photocatalytic activity on the degradation of organic pollutants by SnO nanoparticle decorated hierarchical ZnO nanostructures

Abstract

One (1D) and two-dimensional (2D) nanostructures of zinc oxide and tin oxide (ZnO/SnO) nanocomposites were synthesized by a hydrothermal method using ethylenediamine (EDA) as a capping ligand. The effect of Sn concentration on the morphology of the nanocomposites has been investigated. X-ray diffraction analysis indicated good crystallinity of samples with the presence of both ZnO and SnO phases. The morphological analysis revealed the morphological transformation from ZnO nanorods to ZnO/SnO nanosheets by adding Sn. X-ray photoelectron spectra analyses showed significant peak shift in the electronic state of Zn at the higher concentration of Sn. Elemental mapping results clearly evidenced that both ZnO and SnO moieties were uniformly distributed in the nanosheets. Photocatalytic degradation of methylene blue using as-prepared ZnO/SnO nanocomposites was nine times faster than that of pure ZnO under visible light irradiation. It could be attributed to the formation of a hetero-junction between ZnO and SnO. Our experimental results revealed that photogenerated superoxide (O₂⁻˙) radicals were the main reactive species for the degradation of MB. The maximum degradation efficiency was observed for the sample with 1 wt% of tin chloride, the MB related absorption peak completely disappeared after 6 min of irradiation. ZnO/SnO composites extended the light absorption spectra of ZnO to a visible light region and enhanced the visible light photocatalytic activity.

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1. Introduction

Treatment of organic and inorganic pollutants using oxide semiconductors as photocatalysts has been extensively studied.¹ Organic pollutants are hazardous to human health, harmful to the environment and difficult to degrade under natural environmental conditions. Titanium dioxide (TiO₂) and Zinc oxide (ZnO) are environmental-friendly photocatalyst materials owing to their high efficiency, strong oxidising power, non-toxicity and low cost.^2,3 The efficiency improvement of the ZnO photocatalytic processes using ultraviolet (UV) light and visible light is due to a wide bandgap (3.2 eV) and high rate of electron–hole recombination.⁴ Nevertheless, because of the wide band gap, ZnO photocatalysts absorb UV and very little visible light. To enhance the energy conversion efficiency during the visible photocatalytic reactions, many efforts have been made to synthesize ZnO doped with metals, nonmetals, cations, anions, and coupled with other metal oxides.⁵ Coupling of different photocatalyst semiconductors is a promising method that enhances the photocatalytic response to visible light and increases the charge separation and extends the energy range of photoexcitation.^6,7 The performance of the system depends on the energy level of a conduction band and the valence band of the two catalysts which directly influence the separation and recombination of the photogenerated electrons and holes pairs.

Several metal oxide semiconductors have sufficient band gap energy for promoting or catalyzing a broad range of photochemical reactions of environmental interest. They include TiO₂/ZnO,⁸ TiO₂/WO₃,^9,10 Fe₂O₃/ZnO,¹¹ Fe₂O₃/SnO₂,¹² ZnO/SnO₂,^13–16 and CeO₂/TiO₂.¹⁷ According to inter-particle electron transfer theory, the photoexcited electrons can be transferred between the conduction bands of coupled photocatalysts. Accordingly, the lifetime of the charge carriers is extended, and the charge separation is enhanced. For example, the photocatalytic activity of coupled ZnO/SnO₂ photocatalysts was evaluated using the decolorization of methyl orange,^18,19 and the experimental results showed that the coupled ZnO/SnO₂ photocatalysts had a higher photocatalytic activity than pure ZnO or SnO₂.

Tin oxide has two phases such as Sn(II) monoxide (SnO) (p-type) and Sn(IV) dioxide (SnO₂) (n-type). SnO₂ phase is stable under standard conditions and it has been more widely investigated for various applications.^20–23 Whereas, the preparation of SnO is more difficult than that of SnO₂ due to the oxidation of SnO. The formation of SnO₂ from SnO could be spontaneous in the ambient air based on the standard Gibbs energy of formation.²⁴ The bandgap of SnO is 2.6 eV and has a high hole mobility as a p-type semiconductor. SnO is much attracted for a variety of applications owing to the excellent optical properties. For example, Lu and co-workers synthesized ultrasonic assisted SnO using SnCl₂ and the organic solvent of ethanolamine.²⁵ Guo et al., synthesized SnO by a hydrothermal method starting from SnCl₂ and NaOH, with the mixture of SnO and SnO₂.²⁶ Hiroaki et al., controlled the morphology of SnO₂ and SnO in aqueous solution. Rutile type SnO₂ was obtained from the acidic solution at pH 0.6–3.3. On the other hand, tetragonal SnO was prepared from the alkaline solvents at pH 13.1–13.5.²⁷ Based on the reported works, the formation of SnO strongly depends on the capping agent and pH of the reaction mixture. Several researchers have reported that the formation of SnO₂ can be controlled by addition of surfactant,^28,29 varying the pH of the reaction^30–32 and the concentration of the precursor.²⁷

The growth of ZnO/SnO₂ nanocomposites and their catalytic activity using UV light irradiation has been reported in many research work. For example, Hamrouni et al.³³ prepared ZnO/SnO₂ by co-precipitation method and studied the photocatalytic activity under both the UV and visible light. The photocatalytic activity depended on the molar ratio of Zn/Sn and the calcination temperature. It is reported that the photocatalytic activity of SnO₂ was less when compared to pure and coupled ZnO/SnO₂. Xiang et al.,³⁴ synthesized ZnO/SnO₂ nanofibers with different molar ratios of Zn and Sn. Different ratios of Zn and Sn affected the phase, morphology and photocatalytic activity of ZnO/SnO₂ nanocomposites. The photodegradation ability towards waste water shows fast degradation and recycling ability. Elaheh et al.³⁵ synthesized ZnO/SnO₂ coupled oxide semiconductor. They investigated the effect of molar concentration of Zn and Sn on the phase, morphology and photocatalytic activity of the synthesized nanocomposites. From the above reports, it is concluded that the photocatalytic activity of coupled oxide photocatalyst was closely related to the ratio of the two oxides.

However, SnO₂ has the bandgap of 3.6 eV³⁶ which was higher than ZnO. Whereas the bandgap of SnO is 2.6 eV.³⁷ Therefore, SnO is a potential semiconductor material to couple with ZnO for the visible light absorption. In the present research work, we have synthesized the ZnO/SnO composites by hydrothermal method for the photocatalytic application. The effect of Sn concentration on the phase and morphology of the composite material was investigated. The functional properties of the nanocomposites ZnO/SnO have been studied by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The photocatalytic activity of the synthesized materials was characterized by quantifying the rate of methylene blue (MB) degradation in the aqueous suspension under visible light irradiation. Kinetics and reproducibility were studied.

2. Materials and methods

2.1. Chemicals and materials

Zinc acetate (Zn(CH₃COO)₂·2H₂O, 98%; Merck), tin chloride (SnCl₄·5H₂O, 99%, Merck), sodium hydroxide (NaOH, 97%; Merck), ethylenediamine (C₂H₈N₂, 99%; Merck) and methylene blue (C₁₆H₁₈N₃SCl, 99%; Merck) were used without further purification.

2.2. Synthesis of ZnO/SnO nanocomposites

In a typical synthesis, 0.2 mol L⁻¹ zinc acetate was dissolved in 50 mL of deionized water and allowed to stir for complete dissolution of the compound. Ethylenediamine (EDA) (0.8 mL) was added to the above solution as a stabilizing agent. 0.2 mol L⁻¹ of NaOH was added dropwise to the above solution to maintain pH 12 and the reaction was allowed to stir for 30 min. Finally, the mixture was transferred to a Teflon-lined autoclave and maintained at 200 °C for 4 h. The obtained precipitation was separated by centrifugation, washed with deionized water and ethanol for several times. The obtained product was dried at 80 °C for 1 h. The resulting sample was termed as ZSn0. For the synthesis of ZnO/SnO nanocomposites, the same procedure was followed with a small modification, SnCl₄·5H₂O was added in various concentrations such as 0.25, 0.50, 0.75, 1, 3, 5, 10 and 15 wt%. The samples were termed as ZSn0.25, ZSn0.50, ZSn0.75, ZSn1, ZSn3, ZSn5, ZSn10 and ZSn15, respectively.

2.3. Characterization

The obtained products were characterized by XRD using a Rigaku X-ray diffractometer (RINT 2200, Japan) with CuKα radiation and 0.02° s⁻¹ step interval. The morphology and particle size of the products were assessed by FESEM (JEOL JSM 7001F microscope) at an accelerating voltage of 15 kV and TEM (JEOL JEM 2100F microscope) at an accelerating voltage of 200 kV. Fourier transform infrared (FTIR) spectra were recorded by JASCO MFT 2000 using KBr pellet technique. UV-vis analyses were performed by a Shimadzu 3100 PC spectrophotometer (Japan). XPS were recorded by a Shimadzu ESCA 3400.

2.4. Photocatalytic studies

The photocatalytic activity of the synthesized samples (ZSn0, ZSn0.25, ZSn0.50, ZSn0.75, ZSn1, ZSn3, ZSn5, ZSn10 and ZSn15) was evaluated by examining the photo-assisted degradation of MB, as a model dye, at room temperature under a halogen lamp (500 W, 420 nm; SAIC, India) as a source of visible light irradiation. In a typical reaction, the dye concentration was fixed at 10 ppm and the solution pH was controlled at the desired level by addition of NaOH using a Kyoto Electronic AT-200 automatic titrator. The photocatalyst of a known dosage (mg L⁻¹) was added to the above pH-controlled dye solution and the suspension was stirred to achieve an absorption–desorption equilibrium state the solution which was required to keep in the dark before light irradiation.^38,39 The reaction mixture was irradiated with stirring under the halogen lamp positioned at 21 cm above the reaction mixture. The reaction vessel consisted of an external jacket for water circulation to maintain the reaction mixture at room temperature. At regular time intervals, 3 mL of the suspension was collected, centrifuged, and analyzed by a UV-vis spectrometer. The MB degradation was estimated from the decrease in the intensity of the associated characteristic band absorption at 664 nm. The photodegradation percentage of MB was calculated using the following equation:⁴⁰where C₀ and C_t are the concentrations of MB at time 0 and t (s), respectively t is the irradiation time.

3. Results and discussion

3.1. Structural analysis

Fig. 1[(a) and (b)] shows the XRD pattern of the samples. XRD pattern of pure ZnO (ZSn0) indexed to the hexagonal phase of ZnO and it was in good agreement with the standard diffraction data (JCPDS card no. 80-0075). Whereas samples ZSn0.25, ZSn0.50, ZSn0.75, ZSn1, ZSn3, ZSn5, ZSn10 and ZSn15 indicated the peaks of coupled ZnO/SnO composites. New peaks emerged at 23.18° and 60.70° in ZSn1 sample. These peaks showed the incorporation of Sn in the composite (ZSn1) and were well matched to the rhombohedral phase of ZnSnO₃ (JCPDS card no. 52-1381). As the concentration of Sn increased from 3–15 wt%, the characteristic peaks of ZnSnO₃ and ZnO gradually decreased and the characteristic peaks of SnO appeared. All the diffraction peaks indicated that the materials were well crystallized as the composites of ZnO/SnO without any other impurities. The structure of ZnO and SnO in all samples were identified as the typical hexagonal wurtzite (JCPDS card no. 05-0664) and tetragonal structure (JCPDS card no. 85-0712), respectively. No other characteristic peaks corresponded to other compounds was observed.

Fig. 1 XRD patterns of ZnO and coupled ZnO/SnO nanocomposites.

3.2. Elemental composition

The surface chemical states of ZnO and ZnO/SnO composites were investigated by XPS analysis. Fig. 2[(a)–(g)] shows the high-resolution XPS spectrum for electronic states of Zn 2p, Sn 3d and O 1s, respectively. There were two symmetric peaks in the Zn 2p region as shown in Fig. 2(a). The peaks centred at 1022.64 eV corresponded to the Zn 2p_3/2 and peak at 1045.53 eV was assigned to Zn 2p_1/2, respectively. The energy difference between two binding energies was 22.89 eV, which was in agreement with the standard value of 22.97 eV.⁴¹ The binding energies Zn 2p_3/2 of sample ZSn15 shifted from 1022.64 to 1022.21 eV when compared with that of ZnO as shown in Fig. 2(a). However, the peak position of sample ZSn0.25 to ZSn10 retained the same peak position as the ZSn0. The above samples shows the strong interaction of Sn with Zn in the composites. A possible reason is that high concentration of Sn in solution hinder the reaction with Zn²⁺ ions. Sn 3d states shown in Fig. 2(b) were symmetric and two peaks centred at 486.85 eV and 495.29 eV corresponded to the Sn 3d_5/2 and Sn 3d_3/2, respectively. The energy difference between the two peaks (8.2 eV) was very close to the value reported in the standard spectrum of Sn 3d.^42,43 The peak position of samples ZSn15 was shifted from 486.85 to 486.08 eV when compared to that of ZSn1 sample. The peaks located at 486.08 and 494.5 eV were attributed to Sn²⁺, while the peaks centred at 486.8 and 495.3 eV were related to Sn⁴⁺.⁴⁴ The XPS analysis suggested that Sn⁴⁺ species exist at the lower content of Sn. As the concentration of Sn increased, more amount of Sn²⁺ species existed in the composites, which is in good agreement with the XRD results. O 1s spectra of samples are shown in Fig. 2(c). All samples exhibited an asymmetrical shape and were deconvoluted into two peaks as shown in Fig. 2[(d)–(g)] for ZnS0, ZnS5, ZnS10 and ZnS15 sample, respectively. The main peak located at 530.21 eV was attributed to oxygen bonded with Zn in the ZnO lattices.⁴⁵ The peak located at 531.81 eV corresponded to surface adsorbed or chemisorbed oxygen in the sample.⁴⁶ The peak position was changed to 530.11 and 531.73 eV for ZSn1, 529.98 and 531.43 eV for ZSn10 and 529.66 and 531.16 eV for ZSn15, respectively.

Fig. 2 XPS spectra of (a) Zn 2p state, (b) Sn 3d state and (c) O 1s states. Magnified and deconvoluted spectra of (d) ZSn0, (e) ZSn1, (f) ZSn10 and (g) ZSn15 samples.

FTIR analysis was used to study the presence of EDA on the surface of the samples. Fig. S1 (ESI†) shows the FTIR spectra of pure ZnO and ZnO/SnO nanostructures. All the samples showed similar peaks. The absorption peak at 3421 cm⁻¹ can be attributed to the stretching vibrations of structural hydroxyl groups. The peak at 2368 cm⁻¹ corresponded to the C–O stretch which resulted from the atmospheric CO₂. The peak at 1628 cm⁻¹ were due to the N–H bending vibration which confirmed the presence of amine molecules existing in the nanostructures. The strong peak at 559 cm⁻¹ was attributed to the stretching modes of Zn–O. The ZnO/SnO samples show that the peak appeared in between 484 and 523 cm⁻¹ was assigned as Sn–O stretching vibration.⁴⁷ As the Sn content increased the peak broadening increased as shown in samples ZSn10 and ZSn15. Similar observation was reported in the literature.⁴⁸ Based on these results, it can be clearly confirmed the presence of EDA and SnO on the ZnO nanorods.

3.3. Morphological analysis

Fig. 3 shows the FESEM images of as-prepared ZnO and ZnO/SnO nanocomposites. The primary ZnO nanocrystals made self-assemble each other by oriented attachment mechanism. Thus the nanorods randomly oriented to form a flower-like morphology as shown in Fig. 3(a). When 0.25–1% Sn was incorporated in ZnO, the Sn nanoparticles were coated on the surface of ZnO nanorods (Fig. 3(b)–(e)). Whereas the weight percentage of Sn increased as 3, 5, 10 and 15%, the morphology changed from nanorods to nanosheets. Moreover, the diameter of the nanoparticles on the surface of nanorods and nanosheets increased with an increasing of Sn. Fig. S2 (ESI†) shows the elemental mapping of the Zn, O, Sn and C in the ZSn15 sample. It is clearly evident that both Zn and Sn elements were distributed uniformly in the nanosheets of ZSn15.

Fig. 3 FESEM images of sample (a) ZSn0, (b) ZSn0.25, (c) ZSn0.50, (d) ZSn0.75, (e) ZSn1, (f) ZSn3, (g) ZSn5, (h) ZSn10 and (i) ZSn15.

The morphology of as-prepared nanorods and nanosheets were further analyzed by TEM as shown in Fig. 4. Fig. 4(a) shows the TEM image of sample ZSn0 in which the surface of the nanorods was smooth without any defects. It was well accordant with the FESEM image. Sample ZSn0.25–ZSn1 (Fig. 4(b)–(e)) shows that the surface of nanorods was decorated with SnO nanoparticles with the size of about 8–10 nm. When the Sn weight percentage increased as 10 and 15%, the ZnO nanorods transformed to nanosheets. The size of the nanoparticles is increased with the increase in the weight percentage of Sn as shown in Fig. 4. This is due to the hydrolysis of SnCl₂ in the composition.⁴⁹

Fig. 4 TEM images of sample (a) ZSn0, (b) ZSn0.25, (c) ZSn0.50, (d) ZSn0.75, (e) ZSn1, (f) ZSn10 and (g) ZSn15.

Fig. 5 shows the UV-vis diffuse reflectance spectroscopy of as-prepared ZnO and ZnO/SnO nanocomposites. We found that the samples exhibited two absorption onset for the ZnO/SnO composites. The onset value at 378 nm corresponds to ZnO, the onset values at 450 nm (ZSn3), 460 nm (ZSn5), 471 nm (ZSn10) and 475 nm (ZSn15) corresponds to SnO respectively. The onset at 398 nm corresponds to sample ZSn1 conforms the formation of ZnSnO₃. From the onset absorption data, it was clearly evident that the obtain material could efficiently absorb UV and visible light that could contribute to degradation of organic compounds.

Fig. 5 UV absorption spectra of ZnO and ZnO/SnO nanocomposites.

3.4. Photocatalytic activity

The photocatalytic activity of the samples with different Sn concentration for the degradations of MB are illustrated in Fig. 6. When the ZnO/SnO catalyst was irradiated by visible light using xenon lamp, electrons (e⁻) in the valence band can be excited to the conduction band with simultaneous generation of the same amount of holes (h⁺) in the valence band. The photogenerated electrons and holes are separated under the influence of the electrostatic field induced by different work functions. Thus, electrons move to SnO side and holes to ZnO side. The photogenerated electrons and holes in the ZnO/SnO hetero-junction catalyst could inject into a reaction medium and participate in chemical reactions.⁵⁰ The hole in the valence band can react with H₂O or hydroxyl ions adsorbed at the particle surface to produce hydroxyl radicals (OH⁻), while the electron in the conduction band can reduce O₂ to produce superoxide radicals (O₂⁻) and subsequently other reactive oxygen species (i.e., H₂O₂ and OH⁻). Both, holes and OH⁻ are extremely reactive toward organic compounds.^51,52 The time-dependent UV-vis absorption spectra of MB under visible light irradiation with sample ZSn0, ZSn0.25, ZSn0.50, ZSn0.75, ZSn1, ZSn3, ZSn5, ZSn10 and ZSn15 are presented in Fig. 6(a)–(i). Fig. 6(e) shows a rapid decrease in the initial absorbance of the peak that disappeared completely after 6 min of irradiation. As the concentration of the Sn increased to 0.25, 0.50 and 0.75% (22, 14 and 10 min) the degradation time decreased. Further increase in the concentration of Sn to 3, 5, 10 and 15% (14, 28, 28 and 48 min), photo degradation time increased as shown in Fig. 6(b)–(d) and (f)–(i). Fig. 6(a) shows the time dependent UV absorption spectra of pure ZnO catalyst. MB was completely decomposed with the irradiation time of 48 min which was very less degradation of MB solution when compared to ZnO/SnO nanocomposites. ZnO/SnO composite showed higher photocatalytic activity compared to pure ZnO (ZSn0).

Fig. 6 UV absorption spectra of MB degradation of samples ZSn0, ZSn0.25, ZSn0.50, ZSn0.75, ZSn1, ZSn3, ZSn5, ZSn10 and ZSn15.

Fig. 7(a) illustrates the effect of MB degradation on pure ZnO and ZnO/SnO nanocomposites. The photocatalytic reaction is mainly due to the surface atomic arrangement and the interface between the catalyst surface and organic pollutants.⁴⁰ The optimum content of Sn is the important factors to affect the photocatalytic activity of the ZnO/SnO photocatalyst. The order of photocatalytic performance was 1% of ZnO/SnO (hexagonal nanorods) > 0.75% of ZnO/SnO (nanorods) > 3% of ZnO/SnO (nanorods) > 0.50% of ZnO/SnO (nanorods) > 0.25% of ZnO/SnO (hexagonal nanorods) > 5% of ZnO/SnO (nanorods) > 10% of ZnO/SnO (nanorods and nanosheets) > 15% of ZnO/SnO (nanosheets) > ZnO (nanoflowers). It suggested that coupling of SnO improves the photocatalytic activity of ZnO. The degradation rate of coupled ZnO/SnO catalysts decreased with increase in Sn contents from 1 to 15%. Sample ZnS1 shows the highest activity among all samples. The obtained photocatalyst is stable for the reuse and the Sn²⁺ are not oxidized to Sn⁴⁺ as shown in the XPS spectra (ESI Fig. S3†). The photocatalytic enhancement of ZnO/SnO composites is due to the formation of hetero-junctions between ZnO/SnO nanocomposites which may be responsible for inhibiting electron/hole pair recombination.⁵³ Therefore, the optimum concentration of SnO is important to achieve high photocatalytic activity.

Fig. 7 Effect of dye degradation efficiency (a) time (min) vs. dye degradation (%) and b) MB over ZnO/SnO in the presence of various scavengers under visible light irradiation.

Fig. S4 (ESI†) shows the photocatalytic performance of sample ZSn1 under UV light (365 nm). UV absorption spectra at various intervals under UV light irradiation of sample ZSn1 for the degradation of MB are shown in Fig. S4(a).† Under UV light irradiation, the sample ZSn1 showed the less degradation of MB. The degradation percentage of MB was about 61% after 40 min of irradiation (Fig. S4(b)†). Whereas, the same sample exhibited the degradation percentage of 90% in 6 min under visible light irradiation. It is clear that the visible light drove photocatalyst (ZnS1) shows the best enhancement in the photodecomposition of MB under visible light irradiation.

Hydroxyl radical (˙OH) and superoxide anions (O₂⁻˙) are the possible active species in the photodegradation of organic pollutants. To detect the active species during the photocatalytic reaction, nitrogen (N₂) and disodium ethylenediaminetetraacetate (EDTA-2Na)⁵⁴ were used as scavengers. The obtained results are shown in Fig. 7(b). In the presence of EDTA as a ˙OH radical scavenger, only 17% degradation was decreased compared to the scavenger free photocatalytic system of MB degradation as shown in Fig. 7(b). This conforms that the ˙OH radical is not the main active species in the photocatalysis process. To further determine the degradation mechanism, another experiment was performed under N₂ atmosphere as shown in Fig. 7(b). A high purity N₂ gas was continuously purged throughout the reaction process under ambient condition, which eliminates the dissolved oxygen content of the reaction solution and thereby prevents the formation of O₂⁻˙.^55–57 As a result, only 47% in MB degradation was observed after 6 min of visible light irradiation instead of 90% in a normal atmospheric condition. The photocatalytic activity was significantly suppressed in the presence of N₂ gas, indicating that O₂⁻˙ radicals play a major role in the photo degradation process.

Fig. S5 (ESI†) shows the reusability of ZSn1 photocatalyst for the degradation of MB examined over three cycles under visible light irradiation. After the photocatalysis experiments, the catalyst was separated from the reaction mixture by centrifugation and the concentration of the dye solution was adjusted to its initial value. Photocatalysts were reused for three cycles, and the obtained degradation values were 90.90, 90.64 and 89.92% for first, second and third cycles, respectively. It clearly demonstrated that the synthesized sample could be used for multiple cycles of photocatalysis process. In addition to that, we have investigated the structural property of the used catalyst (after 3rd cycle) by XRD and XPS analysis as shown in Fig. S6 (ESI†). The obtained XRD pattern and XPS spectra exhibited the same peak positions as before the photocatalysis process which confirmed the stability of the synthesized samples.

3.5. Kinetic study

To understand the reaction kinetics of MB degradation, we applied the Langmuir–Hinshelwood model, which is well established for organic compound degradation in the presence of heterogeneous photocatalysts⁵⁸ as expressed bywhere dC/dt is the rate of dye degradation (mg L⁻¹ min), C₀ and C_t (mg L⁻¹) are the dye concentrations at time 0 and t, respectively, k is the rate constant (min⁻¹), K_dye is the adsorption coefficient of the dye on the photocatalyst particle (L mg⁻¹), K_app is the calculated apparent rate constant, and t is the reaction time (min).

To investigate whether the process obeyed pseudo-first-order kinetics, plots of ln(C₀/C_t) versus irradiation time for the adsorption and degradation of MB on ZnO/SnO nanocomposites were examined. Fig. 8 shows the linear relationship for ln(C₀/C_t) plotted against irradiation time. K value increased with the addition of Sn concentration from 0.25 to 1% (0.130 min⁻¹ to 0.409 min⁻¹). When the Sn concentration was increased, further the K value decreased from 0.103 min⁻¹ to 0.054 min⁻¹. The kinetic data obtained by the pseudo-first order model such as apparent rate constants (K_app), corresponding correlation coefficients (R²) and maximum dye degradation in the presence of ZnO nanostructures are presented in Table 1. The introduction of Sn into the ZnO lattice increased the photocatalytic activity by increasing the lifetime of charge carrier.¹⁹ The reason is that the surface decorated Sn nanoparticles is for high photocatalytic activity and suppress the recombination of photogenerated electron/hole pairs.

Fig. 8 Plots of ln(C₀/C_t) as a function of time (min) for the photodegradation of MB over the ZnO/SnO nanocomposites.

Table 1 Observed pseudo-first order rate constants, R² values, maximum degradation (%) and time required for maximum degradation of ZnO/SnO nanostructures

Sample	Kapp (ZnO)	R^2	Maximum degradation (%)	Time taken for maximum degradation (min)
ZSn0	0.053	0.9769	92.74	48
ZSn0.25	0.1301	0.9834	93.55	22
ZSn0.50	0.1866	0.9927	92.01	14
ZSn0.75	0.2536	0.9909	91.05	10
ZSn1	0.409	0.9942	90.90	06
ZSn3	0.212	0.9921	94.69	14
ZSn5	0.103	0.998	92.46	28
ZSn10	0.086	0.9906	90.81	28
ZSn15	0.054	0.9939	91.49	48

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The enhanced photocataytic activity for the ZnO/SnO heterostructure (Fig. 9) can be explained as follows: when the ZnO/SnO heterostructure was excited by visible light with a photon energy higher or equal to the band gap of ZnO and SnO, electrons and hole pairs are generated. From the energy band diagram of the ZnO/SnO heterojunction, it could be found that the photogenerated electrons transfer occurred from the CB (E_CB = 2.9 eV) of SnO to the CB (E_CB = 4.3 eV) of ZnO. Meanwhile, the photogenerated hole transfer could take place from the VB (E_VB = 7.7 eV) of ZnO to the VB (E_VB = 5.8 eV) of SnO, suggesting that the photogenerated electrons and holes were efficiently separated.^59,60 These electrons or holes that transfer to the SnO or ZnO active surface of the nanostructures immediately join in the redox reactions, in which electrons reduce dissolved molecular oxygen to produce superoxide radical anions (˙O₂⁻), while holes oxidize H₂O molecular to yield hydroxyl radicals (HO˙). Organic dye pollutants (MB) are eventually oxidized by these highly active species to CO₂ and H₂O products.^59,61

Fig. 9 Photocatalytic mechanism of ZnO/SnO nanocomposites.

4. Conclusion

ZnO/SnO composites were successfully synthesized by the hydrothermal process. In the process, SnO nanoparticles were decorated on the surface of ZnO nanorods and nanosheets. The investigation of photocatalytic activity indicated that the ZnO/SnO nanorods possessed a higher photocatalytic activity compared to ZnO/SnO nanosheets and pure ZnO for the degradation of MB under visible light irradiation. The increase in Sn content led to decrease in the photocatalytic activity of the ZnO/SnO photocatalyst because of the very low photocatalytic activity of SnO. The results indicate that the sample ZSn1 showed the highest photocatalytic activity for the MB photodegradation. Photogenerated superoxide (˙O₂⁻) radicals over ZnO/SnO supported photocatalyst which was the main reactive species for the degradation of MB. The degradation efficiency of MB under visible light irradiation due to the formation of ZnO/SnO hetero-junction improved the separation of photogenerated electrons and holes.

Acknowledgements

This work was financially supported by Grant-in-Aid for Scientific Research (B) (25289087), Grant-in-Aid for JSPS Fellows (24-12363, 25-13360) from the Ministry of Education, Culture, Sports, Science and Technology of Japan, and the cooperative research projects of the Research Institute of Electronics, Shizuoka University. The authors would like to thank Center for Instrumental Analysis, Shizuoka University, Hamamatsu, Japan for the characterization techniques.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19824d

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