A novel BiVO₃/SnO₂ step S-scheme nano-heterojunction for an enhanced visible light photocatalytic degradation of amaranth dye and hydrogen production

Abstract

The destruction of toxic pollutants and production of hydrogen gas on the surface of semiconductors under light irradiation is the main significance of photocatalysis. Heterojunctions with matching in band gap energy are urgently required for enhancing the redox power of the charge carriers. A step S-scheme BiVO₃/SnO₂ nano-heterojunction was carefully synthesized for a successful photodegradation of amaranth dye and photocatalytic hydrogen evolution. Tetragonal SnO₂ nanoparticles of 80 m² g⁻¹ surface area and distinct mesoporous structure were fabricated by a sol–gel route in the presence of Tween-80 as the pore structure directing agent. BiVO₃ nanoparticles were deposited homogeneously on the SnO₂ surface in an ultrasonic bath of power intensity 300 W. The photocatalytic efficiency in the destruction of amaranth dye soar with increasing BiVO₃ contents of up to 10 wt%. The hydrogen evolution rate reached 8.2 mmol g⁻¹ h⁻¹, which is eight times stronger than that of pristine SnO₂. The sonicated nanocomposites were investigated by XRD, BET, FESEM, HRTEM, EDS, DRS and PL techniques. The step S-scheme heterojunction with superior oxidative and reductive power is the primary key for the exceptional photocatalytic process. The PL of terephthalic acid and the scavenger trapping experiments reveal the charge migration through the step S-scheme mechanism rather than the type (II) heterojunction mechanism.

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1. Introduction

Photocatalysis achieving water splitting, organic waste removal and the reduction of CO₂ under light irradiation is a promising route for solving the energy and environmental crisis.^1–6 The broadband light absorption and matching in the band-edge potentials of two different semiconductors is the first challenge in constructing successful heterojunctions. Various semiconductors are extensively investigated for the photodegradation of organic pollutants and photocatalytic hydrogen production.^7–14 SnO₂ with 3.5 eV band gap energy, good optical properties and definite pore structures is a promising candidate for various photocatalytic processes. However, the wide band gap energy, the lower quantum efficiency of charge carriers and the photocatalytic response in the UV region only are negative factors in optimizing its photocatalytic reactivity.^15–20 Bismuth vanadate nanoparticles possess high stability, non-toxicity, rapid formation of the electron–hole pairs by photo-excitation and negative reduction potentials.^21–29 The construction of tin oxide–bismuth vanadate heterojunctions is a hot issue in photocatalytic reactions that facilitate the charge transportation and produce charge carriers with strong redox power. Various charge migration models are postulated for charge migration between two semiconductors, such as type II heterojunction, direct Z-scheme and step S-scheme mechanisms.^30–37 The compatibility in band gap energy of SnO₂ and BiVO₃ facilitates the charge migration through the step S-scheme mechanism that involves the removal of the unnecessary electrons and holes of low potential and preserves charge carriers with strong potential for successful redox reactions. The construction of a step-scheme (S-scheme) heterojunction with superior redox ability is an efficient route to develop high-efficiency photocatalysts for various industrial and environmental applications.^38–49 Meng et al. prepared TiO₂/polydopamine nanocomposites with a considerably high efficiency in the reduction of CO₂ due to the effective charge carrier separation through the step S-scheme mechanism.⁴⁰ The photocatalytic removal of Hg⁰ under visible light is reported by Xiao et al. on the surface of the CeO₂/BiOI S-scheme heterojunction who reported that the electron migration process through the step S-scheme mechanism enhances the reactivity in the removal process.⁴¹ Li et al. incorporated Au on the surface of 2D/2D Bi₂MoO₆/g-C₃N₄ for photodegradation of rhodamine B dye through the step-scheme (S-scheme) route.⁴² Shaheer et al. indicated that photocatalytic hydrogen production on the surface of RGO supported TiO₂/In_0.5WO₃ is 12 fold higher than that on pristine TiO₂.⁴³ Wu et al. reported that oxygen vacancies activate a new S-scheme heterojunction photocatalyst built from BiOCl.⁴⁴ The novel S-scheme heterojunction photocatalysts exhibit an exceptional photocatalytic efficiency in removing aqueous diclofenac (DCF) and gaseous nitric oxide (NO) under visible light irradiation compared with pristine CuBi₂O₄ and BiOCl due to the enhanced charge separation and redox ability by the S-scheme heterojunction structure. The reduction in the electron–hole recombination through the step S-scheme mechanism is proposed for photocatalytic hydrogen production on the surface of TiO₂/SrTiO₃ synthesized by a hydrothermal process.⁴⁹ To our recent knowledge, no research was carried out on construction of step S-scheme BiVO₃/SnO₂ heterojunctions through the sonochemical process. The sonochemical route for dispersion of BiVO₃ on the SnO₂ surface is an efficient way for enhancing the chemical interaction between two semiconductors over the traditional routes such as impregnation, hydrothermal and solid state processes that weaken the modes of interaction and block the pore structure. The sonochemical route is a green way for fabricating nanocomposites due to its low cost, manipulation of definite particle size and structure, and possibility of producing large-scale products. The as-synthesized heterojunctions were characterized by XRD, HRTEM, N₂-adsorption–desorption isotherm, XPS, DRS and PL analysis. The photocatalytic efficiency of the as-synthesized step S-scheme heterojunctions was investigated by following the photodegradation of amaranth, fluorescein and rhodamine B dyes and hydrogen evolution under simulated solar radiation.

2. Materials and methods

2.1. Materials

Stannic chloride pentahydrate, Tween-80, bismuth nitrate, sodium vanadate, ammonia solution, absolute ethanol, methanol, amaranth dye, isopropanol, benzoquinone, ammonium oxalate and terephthalic acid were purchased from Sigma-Aldrich Company with purity = 99%.

2.2. Preparation of SnO₂ nanoparticles

25 ml of Tween-80 dissolved in 50 ml isopropanol and 80 ml of ethylene glycol was added to 20 g of stannic chloride dissolved in isopropanol solution followed by vigorous stirring for three hours. Sn(OH)₄ sol was formed by adding ammonia solution [1 M] with constant stirring for 3 with precise adjustment of the pH of the mixture at 8. At the last stage of preparation, the resulting sol was filtered, washed several times with distilled water to remove chloride ions, dried overnight at 100 °C and calcined for 10 hours at 460 °C.

2.3. Preparation of BiVO₃ nanoparticles

Bismuth nitrate and sodium monovanadate were dissolved in ethylene glycol and mixed together with equal concentration in an ultrasonic bath of 300 W for 45 minutes. Afterwards, the sol is filtered, washed with distilled water several times and dried at 140 °C for 24 hours.

2.4. Preparation of BiVO₃/SnO₂ nanoparticles

A definite amount of bismuth nitrate dissolved in ethylene glycol was mixed with SnO₂ nanoparticles with constant stirring for three hours in a certain ratio in an attempt to obtain (5, 10, 15 and 20 w/w%) BiVO₃/SnO₂. Then, a stoichiometric amount of sodium monovanadate dissolved in ethylene glycol was added to the above solution and placed in an ultrasonic bath with an intensity of 300 W for one hours. The above sol mixture is filtered, washed with distilled water and dried at 80 °C for 24 hours. The bare samples are denoted as SnO₂ and BiVO₃ and the samples denoted as SnBi₅, SnBi₁₀, SnBi₁₅ and SnBi₂₀ correspond to the nanocomposite containing 5, 10, 15 and 20 wt% of BiVO₃ contents.

2.5. Material characterization

A PANalytical X'PERT MPD diffractometer using (Cu [Kα₁/Kα₂]) radiation was employed for recording the crystalline parameters. The surface area and the pore structure were recorded by adsorption isotherms of N₂ at 77 K using a volumetric technique. High resolution transmission electron microscope features were estimated with an HRTEM JEOL 6340. XPS studies were carried out using a K-ALPHA (Thermo Fisher Scientific, USA) instrument with monochromatic X-ray Al Kα radiation from 10 to 1350 eV. Diffuse reflectance spectra were detected with a JASCO spectrometer (V-570). The photocatalytic power for amaranth dye destruction on the as-synthesized heterojunctions was recorded under a solar simulator. The photocatalytic degradation of pollutant dye models such as amaranth, rhodamine and fluorescein dyes was carried out over the surface of 0.1 g of the as-synthesized photocatalyst in a glass reactor under a xenon lamp of intensity 300 W. The photodegradation of dye involves mixing 0.1 g of the sample with 100 ml of dye solution of concentration 5 × 10⁻⁵ M fluorescein and amaranth dye and 2 × 10⁻⁵ with respect to rhodamine B dye, then the mixture was kept stirring for 60 min in dark to attain the adsorption–desorption equilibrium of dye on the photocatalyst surface. After equilibration, the concentration of the dye in terms of absorbance was measured using a Thermo Scientific Evolution 300 UV-vis spectrophotometer and the result was taken as the initial concentration (C₀) of the dye. Then, a xenon lamb was switched on over the suspension and at different irradiation time intervals, 5 ml of dye solution was withdrawn and analyzed using a UV-vis spectrophotometer to determine the residual concentration (C_t) of the dye. Finally, the photocatalytic activity percentage of each sample was calculated using the following equation:

Photocatalytic activity % = (C_t/C₀) × 100

The concentration of the hydroxyl groups was estimated by following the production of 2-hydroxyterephthalic acid [2-HTA] at a defined emission signal with 423 nm wavelength. The proportion of hydrogen gas evolved was detected under a 300 W xenon lamp in a glass reactor. The as-synthesized samples were suspended into 150 ml of methanol solution. After 1 h illumination, 500 μL gaseous products were taken from the reactor and then measured by gas chromatography.

3. Results and discussion

3.1. Physicochemical characterization

The diffraction pattern of pristine SnO₂ shows prevailing diffraction peaks at 2θ = 26.6, 34.2, 37.6, 51.8, 54.8, 58.4, 61.9 and 65.2° referred to the cassiterite tetragonal (rutile type) structure of SnO₂ (ICDD card no.41-1445) of space group P4₂/mnm. The XRD pattern of BiVO₃ nanoparticles shows pronounced diffraction peaks at 2θ = 12.7, 28.88, 30.38, 34.7, 35.4, 39.9, 42.6, and 53.22° marked by their miller indices [(−121), (040), (200), (002), (211) and (051)] related to monoclinic BiVO₃ crystallites. By carefully noticing the XRD spectrum, the diffraction peak at 2θ = 12.7° referred to BiVO₃ is shifted to 13.7° upon incorporation of BiVO₃ on the surface of SnO₂ nanoparticles. This change in the position of the diffraction peak of BiVO₃ can be ascribed to the strong interaction between SnO₂ and BiVO₃ that induces some change in the crystalline parameters. The crystallite size estimated by the Debye–Scherrer equation is 42, 54, 33, 25, 19 and 13 nm for SnO₂, BiVO₃, SnBi₅, SnBi₁₀, SnBi₁₅ and SnBi₂₀, respectively (Fig. 1). With incorporation of BiVO₃ nanoparticles on the SnO₂ surface, the crystallite size was drastically reduced, reflecting the favorable dispersion of BiVO₃ nanoparticles between SnO₂ crystallites decreasing their dimensions.

The porosity and the surface structure of the as-synthesized heterojunctions are verified by N₂ adsorption–desorption isotherms at 77 K on SnO₂, SnBi₅, SnBi₁₀, SnBi₁₅ and SnBi₂₀ (Fig. 2). These isotherms are referred to type IV with H1 hysteresis loop, indicating the cylindrical pore structure that facilitates the free movement of reactant molecules. The surface area determined from the BET equation is 108, 91, 78, 65 and 56 m² g⁻¹ for SnO₂ SnBi₅, SnBi₁₀, SnBi₁₅ and SnBi₂₀, respectively. With incorporation of various concentrations of BiVO₃, the surface area and hysteresis loop size are much reduced due to deposition of BiVO₃ nanoparticles on the outer surface and inside the pore structure of SnO₂ blocking a large number of active sites available for attracting N₂-molecules.

Fig. 1 (a) XRD of SnO₂, BiVO_3, SnBi₅, SnBi₁₀, SnBi₁₅ and SnBi₂₀.

Fig. 2 N₂-adsorption–desorption isotherm of SnO₂, BiVO₃, SnBi₅, SnBi₁₀, SnBi₁₅ and SnBi₂₀.

Fig. 3 illustrates the TEM images of (a) SnO₂, (b) BiVO₃ and (c) SnBi₁₀ nanoparticles. It is clearly observed that the homogeneous dispersion of BiVO₃ covers the SnO₂ matrix. Fig. 3d–f reveals the formation of a BiVO₃–SnO₂ heterojunction through production of a crystalline plane of 0.34 nm referred to the (111) plane of tetragonal SnO₂ and 0.255 referred to the (111) plane of monoclinic BiVO₃. SAED analysis shows various diffraction rings at (110), (101), (200), (211) and (220) planes belonging to tetragonal SnO₂ nanoparticles [Fig. 3g–i]. On the other hand, the different diffraction rings at (−121), (040), (200) and (002) are referred to BiVO₃ crystal. HRTEM and SAED experimental results are clear evidence for construction of a step S-scheme BiVO₃–SnO₂ heterojunction that not only enhances the separation of the charge carriers, but also improves the oxidative and reductive power of the solid sample.

Fig. 3 TEM of (a) SnO₂, (b) BiVO₃, and (c) SnBi₁₀, (d) HRTEM of (d) SnO₂, (e) BiVO₃, and (f) SnBi₁₀, and SAED of (g) SnO₂, (h) BiVO₃, and (i) SnBi₁₀.

The precise elemental composition and the oxidation state of elements in the constructed BiVO₃/SnO₂ step S-scheme heterojunction are investigated by XPS analysis. The spectrum displays two sharp peaks at 486 and 495.6 eV belonging to Sn 3d_3/2 and Sn 3d_5/2, respectively (Fig. 4). The binding energies of 516.7 and 524.0 eV are referred to V 2p_3/2 and V 2p_5/2 in BiVO₃/SnO₂ for the V⁵⁺ oxidation state. The asymmetric O 1s at a binding energy of 530 eV indicates the presence of oxygen. The binding energies of 158.5 and 164.4 eV are referred to 4f_7/2 and 4f_5/2 of Bi³⁺.

Fig. 4 XPS of Sn, O, Bi and V in SnBi₁₀.

The DRS spectrum shows an absorption band edge at 400 nm referred to SnO₂ nanoparticles; however, a prevailing band edge at 580 nm points to the absorption characteristics of BiVO₃ in the visible region [Fig. 5a]. BiVO₃ nanoparticles are beneficial for the light harvesting and the enhancement of the utilization of solar light by shifting the absorption features to higher wave length. The band gap energy [E_g] calculated from the Tauc equation is 3.20, 2.2, 3.1, 2.95 and 2.81 eV for the pristine SnO₂, BiVO₃, SnBi₅, SnBi₁₀ and SnBi₁₅, respectively [Fig. 5b].

Fig. 5 (a) DRS of SnO₂, BiVO₃, SnBi₅, SnBi₁₀, SnBi₁₅ and SnBi₂₀, (b) Tauc plot for SnO₂, BiVO₃, SnBi₅, SnBi₁₀, SnBi₁₅ and SnBi₂₀, (c) PL of SnO₂ and SnBi₁₀, (d) Photodegradation of amaranth dye over SnO₂ containing various proportions of BiVO₃.

The valence and conduction band edges at the point of zero charge of the semiconductor were estimated from the following equation:

E_VB = X − E_e + 0.5E_g

E_CB = E_VB − E_g

where X is the semiconductor electronegativity which is 6.5 and 5.7 eV for BiVO₃ and SnO₂ respectively. E_e is the energy of free electrons on the hydrogen scale (4.5 eV). Based on the above equations, SnO₂ has the conduction band potential E_CB = −0.4 eV and valence band potential E_VB = +2.8 eV; however, BiVO₃ has the conduction band potential E_CB = +0.9 eV and valence band potential E_VB = +3.1 eV. The synergetic effect between SnO₂ and BiVO₃ facilitates step S-scheme heterojunction construction through which charges' transfer as well as the positive holes and electrons in the inferior energy levels are consumed. However, a high negative and positive potential of SnO₂ conduction and valence band of BiVO₃ are preserved with strong reduction and oxidation power. The quantum efficiency of charge carrier separation is further highlighted by the photoluminescence [PL] spectrum. The reduction in the prevailing signal intensity at 435 nm of SnO₂ crystalline defects [Fig. 5c] reflects the strong quantum power of BiVO₃ in promoting the efficiency of the charge carrier separation in the novel heterojunction.

3.2. Photocatalytic degradation of amaranth dye

Amaranth dye is extensively used in industry and causes various environmental problems and health issues such as headaches, tumors, allergies, and other health problems after long periods of exposure. Amaranth dye with a complex aromatic structure is difficult to degrade by traditional methods. We select amaranth dye to compare the photocatalytic efficiency of the as-synthesized heterojunctions. The reactivity of the pristine SnO₂ and BiVO₃ is very low under solar radiation due to the wide band gap energy of SnO₂ and the rapid recombination of the charge carriers in BiVO₃ nanoparticles. The photocatalytic power soars rapidly with incorporating BiVO₃ on the SnO₂ surface and reached an optimal efficiency for the heterojunction embedded 10 wt% BiVO₃. The decrease in the photocatalytic reactivity from degrading 92% of amaranth dye to 62% on increasing BiVO₃ content from 15 to 20 wt% results from the production of several BiVO₃ crystalline layers coating the SnO₂ surface and hindering the light penetration on the photocatalyst surface. These results indicate that the amount of BiVO₃ in the nanocomposite plays a crucial role in controlling the removal of amaranth dye [Fig. 5d]. The dye degradation pseudo first order rate is 0.0023, 0.0063, 0.008, 0.0072, 0.023 and 0.0033 over SnO₂, BiVO₃, SnBi₅, SnBi₁₀, SnBi₁₅ and SnBi₂₀, respectively. The photocatalytic route is more efficient in removal of amaranth dye rather than the adsorption process that weakens the degradation process due to blocking of the active centers by dye molecules and reduces the catalyst durability. Hydroxyl radical concentration was estimated from measurements of PL of terephthalic acid that records the production of 2-hydroxy terephthalic acid monitored by the formation of a characteristic signal at 423 nm [Fig. 6a]. The reactive species nature that determines the charge migration route for removal of amaranth dye over SnBi₁₀ was elucidated by trapping scavengers such as ammonium oxalates, isopropanol, benzoquinone and silver nitrate. Silver nitrate exhibits little effect on the degradation rate, revealing that the electron conduction band plays a small role in dye degradation [Fig. 6b]. The decrease in the photocatalytic efficiency with ammonium oxalate, isopropanol and benzoquinone gives an account on the positive role of hydroxyl radicals, positive holes and superoxide radicals in dye removal. The photocatalytic stability of the prepared nanocomposite was estimated by subjecting the photocatalyst to five consecutive cycles under simulated solar irradiation. After five consecutive cycles, the photocatalyst retains 83% of its efficiency [Fig. 6c]. Fig. 6d demonstrates the influence of the photocatalyst weight on the dye degradation that indicates that the optimal value for dye degradation is 0.1 g, responsible for removal of 90% of the dye. TOC is evaluated in the revised manuscript and the TOC is reduced from 85 mg l⁻¹ to 9.2 mg l⁻¹, revealing the removal of 90% of the amaranth dye.

Fig. 6 (a) PL of terephthalic acid over SnBi₁₀, (b) effect of various scavengers over SnBi₁₀, (c) effect of regeneration of SnBi₁₀ on the removal of amaranth dye, and (d) effect of photocatalyst weight on the removal of amaranth dye.

The photocatalytic degradation of rhodamine B dye as a cationic pollutant model and fluorescein dye as another anionic pollutant model was carried out on bare SnO₂ and the optimum sample SnBi₁₀ to investigate the efficiency of the photocatalyst in degrading different pollutant models [Fig. 7]. The experimental results indicate the removal of 45% of RhB dye on bare SnO₂ compared with 78% removal over the SnBi₁₀ surface. On the other hand, removal of 16% of fluorescein dye over bare SnO₂ was observed compared with the 62% removal on the SnBi₁₀ surface. This result confirmed the positive role of BiVO₃ in improving the photocatalytic efficiency of SnO₂.

Fig. 7 Absorption spectrum for removal of rhodamine B dye over (a) SnO₂ and (b) SnBi₁₀, and absorption spectrum for removal of fluorescein dye over (c) SnO₂ and (d) SnBi₁₀.

The amount of hydrogen gas evolved was tested on the photocatalyst surface embedded different BiVO₃ proportions with methanol as the hole scavenger [Fig. 8a]. BiVO₃ nanoparticles alone exhibit weak photocatalytic activity due to the rapid recombination of the charge carriers which is the main defect for this material. The amount of hydrogen evolved on the SnO₂ surface reached 2.4 mmol h⁻¹ g⁻¹ after two hours of the photoreaction. With the incorporation of BiVO₃, the photocatalytic hydrogen evolution rate increases rapidly with increasing BiVO₃ content. Noticeably, promoted photocatalytic H₂ production activity of 8.6 mmol g⁻¹ h⁻¹ was achieved for the nanocomposite containing 10 wt% BiVO₃ content. The increase in BiVO₃ concentration reduces the photocatalytic rate, revealing that there is an optimal value for BiVO₃ content. The enhancement in the photocatalytic reactivity can be ascribed to the bifunctional role of BiVO₃; one promotes the quantum efficiency of charge separation as proved from PL analysis and another enhances the rate of hydrogen evolution. Probably, HRTEM results reveal that BiVO₃ covered SnO₂ nanoparticles by successive layers that may inhibit the migration of positive holes to react with methanol. These holes accumulated in BiVO₃ are prone to react with electrons and act as recombination centers. Fig. 8b presents the effect of photocatalyst weight on the amount of photocatalytic hydrogen production. The amount of hydrogen produced reaches a maximum value for 0.05 g of the nanocomposite. The influence of pH on the hydrogen evolution process is presented in Fig. 8c, revealing that pH = 10 is the optimal value for producing a maximum amount of hydrogen gas. The photostability of the BiVO₃/SnO₂ that retains 80% of its reactivity after five consecutive cycles suggests the reusability of these novel nanoparticles in the production of hydrogen gas [Fig. 8d].

Fig. 8 (a) The influence of amount of BiVO₃ on the photocatalytic hydrogen evolution rate (mmol g⁻¹ h⁻¹), (b) the effect of weight of the photocatalyst on the amount of hydrogen evolved (mmol g⁻¹ h⁻¹), (c) the effect of pH on the photocatalytic hydrogen evolution over SnBi₁₀, and (d) the effect of catalyst regeneration on the photocatalytic hydrogen evolution over SnBi₁₀.

Various models discuss the charge migration route between two semiconductors.^32–37 Conventional type-II heterojunctions reduced the redox ability of the photogenerated charge carriers. However, the direct Z-scheme hybrid photocatalyst system that mimics natural photosynthesis preserves both the high reductive and oxidative powers of the photogenerated charge carriers. To remove the defects of the traditional heterojunctions [type-II], and direct Z-scheme photocatalysts, a new step-scheme (S-scheme) heterojunction concept was proposed. The S-scheme heterojunction embedded two n-type semiconductor photocatalysts. The internal electric field between the two semiconductors is the driving force for the charge transfer process. As a consequence, the electrons and holes are separated in space, located in the CB of the reduction photocatalyst and the VB of the oxidation photocatalyst, respectively. The photocatalytic oxidation and reduction reactions are usually started by these holes and electrons, respectively. An efficient S-scheme heterojunction system is highly favorable, which promotes the charge dynamics and substantially sustains favorable charge potentials.

In this research, adopting the type (II) heterojunction^15,19 involves the electron transfer from the more negative to less negative potential conduction band; however, holes transfer in the opposite direction. Based on the above arguments, electrons transfer from more negative potential SnO₂ to less negative potential BiVO₃ conduction band. On the other hand, the holes transfer from BiVO₃ to SnO₂ valence band. The potential of the SnO₂ valence band is 2.8 eV which is weak to oxidize water into hydroxyl radicals with a potential of OH⁻/OH = 2.88 eV.^5,15,19 However, the potential of the BiVO₃ conduction band is 0.9 eV which is lower than the reduction potential of hydrogen H⁺/H₂. In the light of the above situation, the type II heterojunction cannot explain hydrogen evolution or dye degradation by hydroxyl radicals as it is documented from scavenger experiments. By adopting the step S-scheme mechanism,^38–49 the positive holes in the VB of SnO₂ and electrons in the CB of BiVO₃ are sacrificed. However, the positive holes in the VB of BiVO₃ with a potential of 3.1 eV easily oxidize water to produce hydroxyl radicals (OH-/OH.) and electrons in the CB of SnO₂ with a potential of −0.5 eV can reduce H⁺ to produce hydrogen gas. Fig. 9 displays the step S-scheme mechanism for electron transfer between two semiconductors. The mechanism involves electron transfer from the VB to the CB of both SnO₂ and BiVO₃, followed by removal of holes and electrons in the valence and conduction band with lower potential. On the other hand, the holes and electrons in higher valence and conduction bands are maintained with strong oxidative and reductive power. This step S-scheme mechanism is desirable for explaining our experimental data for both dye degradation and hydrogen evolution processes.

Fig. 9 A scheme for charge transfer between SnO₂ and BiVO₃ under solar light.

4. Conclusions

Herein, we have successfully fabricated a step S-scheme heterojunction composed of SnO₂ and BiVO₃ nanoparticles without using an electron mediator. Tetragonal SnO₂ nanoparticles were synthesized by a sol–gel route using Tween-80; afterwards, BiVO₃ nanoparticles were incorporated on the SnO₂ surface by a sonochemical route. In particular, there is an optimal value of BiVO₃ for enhancing the photocatalytic reactivity which seems to be 10 wt%. DRS and PL studies have demonstrated that the step S-scheme mechanism not only enhances the separation efficiency of charge carriers, but also increases the oxidizing and reducing power of the holes and electrons. The step S-scheme mechanism activates the recombination of the holes and electrons in the lower valence and conduction bands, leaving the holes and electrons in higher energy state preserved with an efficient oxidation power for amaranth dye degradation and strong reducing power for production of hydrogen gas. The controlled efficient species in amaranth dye degradation are positive holes, hydroxyl and superoxide radicals. The novel hybrid nanoparticle retains its reactivity after five consecutive cycles.

Conflicts of interest

The authors declare that there is no conflict of interest in the research work.

Acknowledgements

The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work through General Research Project under grant number (RGP.2/145/42).

References

1. M. Beneissa, N. Abbas, A. A. Saleh, N. Elboughdiri, A. Moumen, M. S. Hamdy, H. S. M. Abd-Rabboh, A. H. Galal, M. Gad Al-Metwaly and M. A. Ahmed, BiVO₃/g-C₃N₄ S-scheme heterojunction nanocomposite photocatalyst for hydrogen production and amaranth dye removal, Opt. Mater., 2021, 118, 111237.
2. M. A. Ahmed, A. Fahmy, M. G. Abo-Zaed and E. M. Hashem, Fabrication of novel AgIO₄/SnO₂ heterojunction for photocatalytic hydrogen production through direct Z-scheme mechanism, J. Photochem. Photobiol., A, 2020, 400, 112660.
3. Y. Liu, Q. Zhang, M. Xu, H. Yuan, Y. Chena, J. Zhang, K. Luo, J. Zhang and B. You, Novel and efficient synthesis of Ag-ZnO nanoparticles for the sunlight induced photocatalytic degradation, Appl. Surf. Sci., 2019, 476, 632–640.
4. M. S. Hamdy, H. S. M. Abd-Rabboh, M. Beneissa, M. Gad Al-Metwaly, A. H. Glal and M. A. Ahmed, Fabrication of novel polyaniline/ZnO heterojunction for exceptional photocatalytic hydrogen production and degradation of fluorescein dye through direct Z-scheme mechanism, Opt. Mater., 2021, 117, 111198.
5. F. A. Fouad, M. A. Ahmed, M. S. Antonious and M. F. Abdel-Messih, Synthesis of an efficient, stable and recyclable AgVO₃/ZnO nanocomposites with mixed crystalline phases for photocatalytic removal of rhodamine B dye, J. Mater. Sci.: Mater. Electron., 2020, 31, 12355–12371.
6. L. Ling, Y. Feng, H. Li, Y. Chen, J. Wen, J. Zhu and Z. Bian, Microwave induced surface enhanced pollutant adsorption and photocatalytic degradation on Ag/TiO₂, Appl. Surf. Sci., 2019, 483, 772–778.
7. El-H. Hashem, A. Fahmy, A. Abbas, M. Tarek, B. Mahran and M. A. Ahmed, Fabrication of novel AgIO₄/TiO₂ heterojunction for photocatalytic hydrogen production through direct Z-scheme mechanism, Nanotechnol. Environ. Eng., 2020, 5, 17.
8. E. M. Ezz el-regal, M. A. Ahmed, M. F. Abdel-Messih and Z. M. Abou-Gamra, Synthesis of novel ZnO nanoparticles with exceptional crystalline and photocatalytic features toward recalcitrant pollutant: Fluorescein dye, Opt. Mater., 2021, 111, 110597.
9. Z. M. Abou-Gamra, M. A. Ahmed and M. A. Hamza, Investigation of commercial PbCrO₄/TiO₂ for photodegradation of rhodamine B in aqueous solution by visible light, Nanotechnol. Environ. Eng., 2017, 2, 12.
10. M. A. Ahmed, Z. M. Abou-Gamra and M. A. ALshakhanbeh, Hesham Medien; Control synthesis of metallic gold nanoparticles homogeneously distributed on hexagonal ZnO nanoparticles for photocatalytic degradation of methylene blue dye, Environ. Nanotechnol. Monit. Manag., 2019, 12, 100217.
11. M. A. Ahmed, F. Ahmed, M. G. Abo-Zaed and E. M. Hashem, Fabrication of novel AgIO₄/SnO₂ heterojunction for photocatalytic hydrogen production through direct Z-scheme mechanism, J. Photochem. Photobiol., A, 2020, 400, 112660.
12. M. A. Hamza, Z. M. Abou-Gamra, M. A. Ahmed and H. A. A. Medien, The critical role of Tween 80 as a ‘green’ template on the physical properties and photocatalytic performance of TiO₂ nanoparticles for Rhodamine B photodegradation, J. Mater. Sci.: Mater. Electron., 2020, 31, 4650–4661.
13. H. Mao, Z. Fei, C. Bian, L. Yu, S. Chen and Y. Qian, Facile synthesis of high-performance photocatalysts based on Ag/TiO₂ composites, Ceram. Int., 2019, 45, 12586–12589.
14. M. F. Abdel Messih, A. E. Shalan, M. F. Sanad and M. A. Ahmed, Facile approach to prepare ZnO@SiO₂ nanomaterials for photocatalytic degradation of some organic pollutant models, J. Mater. Sci.: Mater. Electron., 2019, 30, 1491–1499.
15. E. E. El-Katori, M. A. Ahmed, A. A. El-Bindary and A. M. Orab, Impact of CdS/SnO₂ heterostructured nanoparticle as visible light active photocatalyst for the removal methylene blue dye, J. Photochem. Photobiol., A, 2020, 392, 112403.
16. M. F. Abdel-Messih, M. A. Ahmed and A. S. El-Sayed, Structure and Photocatalytic decolorization of Rhodamine B dye using novel mesoporous SnO₂-TiO₂ nano mixed oxides prepared by sol-gel method, J. Photochem. Photobiol., A, 2013, 260, 1–8.
17. M. A. Ahmed, M. F. Abdel Messih, E. F. El-Sherbeny, S. F. El-Hafez and A. M. M. Khalifa, Synthesis of metallic silver nanoparticles decorated mesoporous SnO₂ for removal of methylene blue dye by coupling adsorption and photocatalytic processes, J. Photochem. Photobiol., A, 2017, 346, 77–88.
18. M. A. Ahmed, M. F. Abdel-Messih and E. H. Ismail, Facile synthesis of novel microporous CdSe/SiO₂ nanocomposites selective for removal of methylene blue dye by tandem adsorption and photocatalytic process, J. Mater. Sci.: Mater. Electron., 2019, 30, 17527–17539.
19. R. Abdel-Aziz, M. A. Ahmed and M. F. Abdel-Messih, A novel UV and visible light driven photocatalyst AgIO₄/ZnO nanoparticles with highly enhanced photocatalytic performance for removal of rhodamine B and indigo carmine dye, J. Photochem. Photobiol., A, 2020, 389, 112245.
20. M. F. Abdel Messih, A. E. Shalan, M. F. Sanad and M. A. Ahmed, Facile approach to prepare ZnO@SiO₂ nanomaterials for photocatalytic degradation of some organic pollutant models, J. Mater. Sci.: Mater. Electron., 2019, 30, 1491–1499.
21. L. Chen, Q. Zhang, R. Huang, S. F. Yin, S. L. Luo and C. T. Au, Porous peanut-like Bi₂O₃–BiVO₄ composites with heterojunction: one-step synthesis and their photocatalytic properties, Dalton Trans., 2012, 41, 9513–9518.
22. H. Li, K. Yu, X. Lei, B. Guo, H. Fu and Z. Zhu, Hydrothermal synthesis of novel MoS₂/BiVO₄ hetero-nanoflowers with enhanced photocatalytic activity and a mechanism investigation, J. Phys. Chem. C, 2015, 119, 22681–22689.
23. O. Yehezkeli, A. Harguindey, D. W. Domaille, L. He and J. N. Cha, Synthesis and phase transfer of well-defined BiVO₄ nanocrystals for photocatalytic water splitting, RSC Adv., 2015, 5, 58755–58759.
24. Z. Guo, P. Li, H. Che, G. Wang, C. Wu, X. Zhang and J. Mu, One-dimensional spindle-like BiVO₄/TiO₂ nanofibers heterojunction nanocomposites with enhanced visible light photocatalytic activity, Ceram. Int., 2016, 42, 4517–4525.
25. M. F. R. Samsudin, S. Sufian, R. Bashiri, N. M. Mohamed, L. T. Siang and R. M. Ramli, Optimization of photodegradation of methylene blue over modified TiO₂/BiVO₄ photocatalysts: effects of total TiO₂ loading and different type of co-catalyst, Mater. Today: Proc., 2018, 5, 21710–21717.
26. C. Ma and M. Wei, BiVO₄-nanorod-decorated rutile/anatase TiO₂ nanofibers with enhanced photoelectrochemical performance; Matters, Lett, 2020, 299, 126849.
27. Y. R. Lv, C. J. Liu, R. K. He, X. Li and Y. H. Xu, BiVO₄/TiO₂ heterojunction with enhanced photocatalytic activities and photoelectochemistry performances under visible light illumination, Mater. Res. Bull., 2019, 117, 35–40.
28. Y. Hu, D. Li, Y. Zheng, W. Chen and G. Xiao, BiVO₄/TiO₂ nanocrystalline heterostructure: A wide spectrum responsive photocatalyst towards the highly efficient decomposition of gaseous benzene, Appl. Catal., B, 2011, 27, 30–36.
29. Z. Guo, P. Li, H. Che, G. Wang and J. Mu, One-dimensional spindle-like BiVO₄/TiO₂ nanofibers heterojunction nanocomposites with enhanced visible light photocatalytic activity, Ceram. Int., 2016, 42, 4517–4525.
30. Z. K. Heiba, M. A. Ahmed and S. I. Ahmed, Structural investigations of nano mixed oxides SnO₂–xAl₂O₃ prepared by sol–gel technique, J. Alloys Compd., 2010, 507, 253–256.
31. M. A. Ahmed, E. S. Yousef and M. F. Abdel-Messih, Preparation and characterization of nanocomposites prepared in system as SnO₂-x TiO₂ (X=25, 50 and 75 mol%), J. Sol-Gel Sci. Technol., 2011, 60, 58–65.
32. A. H. Galal, M. G. El-mahgary and M. A. Ahmed, Construction of novel AgIO₄/ZnO/graphene direct Z-scheme heterojunctions for exceptional photocatalytic hydrogen gas production, Nanotechnol. Environ. Eng., 2020, 6, 5.
33. N. Al-Zaqri, A. Alsalme, M. A. Ahmed and A. H. Galal, Construction of novel direct Z-scheme AgIO₄-g-C₃N₄ heterojunction for photocatalytic hydrogen production and photodegradation of fluorescein dye, Diam. Relat. Mater., 2020, 109, 108071.
34. M. A. Ahmed, N. Al-Zaqri, A. Ali, A. H. Glal and E. Mahmood, Rapid photocatalytic degradation of RhB dye and photocatalytic hydrogen production on novel curcumin/SnO₂ nanocomposites through direct Z-scheme mechanism, J. Mater. Sci.: Mater. Electron., 2020, 31(21), 19188–19203.
35. H. Ma, W. Ma, J. F. Chen, X.-Y. Liu, Y.-Y. Peng, Z.-Y. Yang, H. Tian and Y.-T. Long, Quantifying Visible-Light-Induced Electron Transfer Properties of Single Dye-Sensitized ZnO Entity for Water Splitting, J. Am. Chem. Soc., 2018, 140, 5272.
36. J. Chen, S. Huang, Y. Long, J. Wu, H. Li, Z. Li, Y.-J. Zeng and S. Ruan, Fabrication of ZnO/Red Phosphorus Heterostructure for Effective Photocatalytic H₂ Evolution from Water Splitting, Nanomaterials, 2018, 8, 835.
37. S. W. B. Zhu, M. Liu, L. Zhang, J. Yu and M. Zhou, Direct Z-scheme ZnO/CdS hierarchical photocatalyst for enhanced photocatalytic H2-production activity, Appl. Catal., B, 2019, 243, 19.
38. S. Wageh, A. A. Al-Ghamdi, R. J. X. Li and P. Zhang, A new heterojunction in photocatalysis: S-scheme heterojunction, Chin. J. Catal., 2021, 42, 667–669.
39. Q. Xu, L. Zhang, B. Cheng, J. Fan and J. Yu, S-Scheme Heterojunction Photocatalyst, Chem, 2020, 6, 1543–1559.
40. A. Meng, B. Cheng, H. Tan, J. Fan, C. Su and J. Yu, TiO₂/polydopamine S-scheme heterojunction photocatalyst with enhanced CO₂-reduction selectivity, Appl. Catal., B, 2021, 289, 120039.
41. Y. Xiao, Z. Ji, C. Zou, Yi. Xu, R. Wang, J. Wu, G. Liu, P. He, Q. Wang and T. Jia, Construction of CeO₂/BiOI S-scheme heterojunction for photocatalytic removal of elemental mercury, Appl. Surf. Sci., 2021, 556, 149767.
42. Q. Li, W. Zhao, Z. Zhai, K. Ren, T. Wang, H. Guan and H. Shi, 2D/2D Bi₂MoO₆/g-C₃N₄ S-scheme heterojunction photocatalyst with enhanced visible-light activity by Au loading, J. Mater. Sci. Technol., 2020, 56, 216–226.
43. A. R. M. Shaheer, V. Vinesh, S. K. Lakhera and B. Neppolian, Reduced graphene oxide as a solid-state mediator in TiO₂/In_0.5WO₃ S-scheme photocatalyst for hydrogen production, Sol. Energy, 2021, 13, 260–270.
44. S. Wu, X. Yu, J. Zhang, Y. Zhang, Y. Zhu and M. Zhu, Construction of BiOCl/CuBi₂O₄ S-scheme heterojunction with oxygen vacancy for enhanced photocatalytic diclofenac degradation and nitric oxide removal, Chem. Eng. J., 2021, 411, 128555.
45. S. Jiang, J. Cao, M. Guo, D. Cao, X. Jia, H. Lin and S. Chen, Novel S-scheme WO₃/RP composite with outstanding overall water splitting activity for H₂ and O₂ evolution under visible light, Appl. Surf. Sci., 2021, 558, 149882.
46. X. Zhang, Y. Zhang, X. Jia, N. Zhang, R. Xia, X. Zhang, Z. Wang and M. Yu, In situ fabrication of a novel S-scheme heterojunction photocatalyts Bi₂O₃/P-C₃N₄ to enhance levofloxacin removal from water, Sep. Purif. Technol., 2021, 268, 118691.
47. Q. Liu, X. He, J. Peng, X. Yu, H. Tang and J. Zhang, Hot-electron-assisted S-scheme heterojunction of tungsten oxide/graphitic carbon nitride for broad-spectrum photocatalytic H₂ generation, Chin. J. Catal., 2021, 42, 1478–1487.
48. K. Z. D. Li, Q. Tian, H. Cao, F. Orudzhev, I. A. Zvereva, J. Xu and C. Wang, Recyclable 0D/2D ZnFe₂O₄/Bi₅FeTi₃O₁₅ S-scheme heterojunction with bismuth decoration for enhanced visible-light-driven tetracycline photodegradation, Ceram. Int., 2021, 47, 17109–17119.
49. Y. Zhao, Y. Guo, J. Li and P. Li, Efficient hydrogen evolution with ZnO/SrTiO₃ S-scheme heterojunction photocatalyst sensitized by Eosin Y, Int. J. Hydrogen Energy, 2021, 46, 18922–18935.

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