Preparation of Dual Z-Scheme Bi2MoO6/ZnSnO3/ZnO Heterostructure Photocatalyst for Ecient Visible Light Degradation of Organic Pollutants

： An important means of achieving efficient charge separation and improving photocatalytic activity is the construction of heterostructures. In this study, the Bi 2 MoO 6 /ZnSnO 3 /ZnO heterostructure photocatalyst was synthesized by the hydrothermal method. The synthesized samples were carefully examined by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscope (SEM), high-resolution transmission electron microscopy (HR-TEM), photoluminescence (PL), and other analytical techniques. Meanwhile, the photocatalytic performance was further evaluated by multi-mode photocatalytic degradation with crystal violet (CV). The results show that the composite material has a relatively homogeneous cubic structure in size and shape. In the cubic structure, a heterogeneous structure exists between Bi 2 MoO 6 , ZnSnO 3 and ZnO. Simultaneously, the dramatic changes in physical morphology, such as the specific surface area and particle size of the composites, led to a unique set of properties, such as a significant climb in light absorption properties and superior photocatalytic activity. In addition, the Bi 2 MoO 6 /ZnSnO 3 /ZnO composite material shows lower photoluminescence intensity, smaller arc radius, and stronger photocurrent response compared to ZnO, Bi 2 MoO 6 and ZnSnO 3 /ZnO. Meanwhile, Bi 2 MoO 6 /ZnSnO 3 /ZnO shows higher photocatalytic efficiency for crystal violet (CV) and tetracycline hydrochloride (TC) the degradation of crystal violet by dual Z-scheme composites are proposed. In conclusion, this study provides a feasible strategy for the photocatalytic degradation of organic pollutants by introducing ZnSnO 3 and Bi 2 MoO 6 to successfully construct composite catalysts with dual Z-scheme heterostructures.


Introduction
In the last decades, rapid industrial development, population explosion, and the misuse of chemical disposal methods have resulted in environmental pollution and caused serious health problems to humans [1,2]. Among different wastewaters, the wastewater from the dye industry is one of the main causes of environmental pollution, especially water pollution [3][4][5]. Approximately 650,000 tons of different dyes are used in the production of textiles every year [6,7]. Various dyes are widely used in silk, wool, cotton dyeing, and other textile industries [8,9]. Since these dyes do not have biodegradable and carcinogenic properties, they must be pretreated before emission [10].
Semiconductor photocatalysts have attracted extensive attention due to their wide application scopes in the direct elimination of harmful environmental pollutants, such as chromium, dyes, drugs, and other heavy metals using solar energy [11,12]. Among the various photocatalysts currently studied, zinc oxide (ZnO) has been widely recognized as a photocatalytic material with great promise due to its unique physicochemical properties, photochemical stability, and nontoxicity [13,14]. There are two main reasons for the low photocatalytic activity of pure zinc oxide. First, ZnO has a wide band gap value and therefore can only absorb ultraviolet (UV) light in the solar spectrum, which severely limits its photocatalytic applications [15]. Second, the rapid light-induced recombination rate of electron-hole pairs in ZnO severely reduces its photocatalytic efficiency [16].
Therefore, an effective strategy needs to be developed to extend the photoresponse range of ZnO in the UV region to the visible light part and keep it photoactive for a longer time.
Constructing semiconductor heterostructures by utilizing visible light active materials is a simple and effective way to enhance the photocatalytic performance of ZnO [17]. The reason for using this way is that the heterojunction interface in this heterostructure not only helps to expand its light absorption range, allowing it to absorb visible light, but also extends the carrier lifetime 3 simultaneously [18]. Certain perovskite-type ternary compounds (ABO 3 ), such as ZnTiO 3 and ZnSnO 3 , exhibit efficient photocatalytic degradation of organic pollutants at room temperature. In particular, ZnSnO 3 , as a famous functional material, shows promising applications in photoelectrochemical devices [19], photocatalysts [20], and gas sensors [21]. Bi(III)-containing oxides have attracted much attention in the field of photocatalysis due to their better intrinsic properties and special layered structures [22,23]

Experimental part
See the supplementary material for a detailed experimental procedure.

XRD analysis
The crystal structures of the synthesized composites were analyzed using X-ray diffraction In the equation, d is the average grain diameter of the grains, K is constant 0.89, β is the half-height width of the diffraction peak of the sample, λ is X-ray wavelength of 1.54056 nm, θ is the Bragg angle corresponding to the diffraction peak. As shown in Table 1

SEM-EDS
To investigate the morphological features and elemental distribution of the composites, SEM analysis and EDS analysis were performed on Bi 2 MoO 6 /ZnSnO 3 /ZnO composite.

TEM and HR-TEM
To further investigate the morphology of the Bi 2 MoO 6 /ZnSnO 3 /ZnO composite, TEM and HR-TEM analyses were performed. TEM images of the composites are shown in Fig 3 (

XPS analysis
XPS was used to analyze the chemical form of the surface element of the composite

N 2 adsorption-desorption analysis
The surface area can usually impact the adsorption and catalytic performance of photocatalysts.
In order to investigate the surface physicochemical characteristics of the Bi 2 MoO 6 /ZnSnO 3 /ZnO composite, nitrogen adsorption-desorption isotherms were used to characterize ZnO, ZnSnO 3 /ZnO, and Bi 2 MoO 6 /ZnSnO 3 /ZnO composite. Fig S2 and Table S1 show that the Bi 2 MoO 6 /ZnSnO 3 Table S1 shows that the average pore size, specific surface area, and pore volume of Bi 2 MoO 6 /ZnSnO 3 /ZnO composites are larger than those of monomeric ZnO and ZnSnO 3 /ZnO, and the pore size distribution is better than that of monomeric ZnO and ZnSnO 3 /ZnO, indicating that the composite after loading Bi 2 MoO 6 can both effectively improve the specific surface area of the catalysts and optimize the pore size distribution, thereby reducing the substance diffusion resistance. Therefore, it is beneficial to improve the photocatalytic activity of the composite.

Photoelectrochemical performance
The In order to study the separation and recombination efficiency of electron-hole pairs in different materials, electrochemical impedance spectroscopy (EIS) tests are performed, as shown in Fig 7 (a).
The arc radius of Bi 2 MoO 6 /ZnSnO 3 /ZnO composite is smaller than that of ZnSnO 3 /ZnO and ZnO, indicating that the interfacial charge transfer resistance is somewhat reduced and the recombination of electrons and holes in the composites is suppressed . Fig 7 (b) shows the transient photocurrent

Photocatalytic performance
The photocatalytic performance of the composites was analyzed by performing multi-modal  In order to investigate the stability of the composite, Bi 2 MoO 6 /ZnSnO 3 /ZnO is subjected to three cycling degradation tests under UV irradiation, as shown in Fig S5. After three-cycle tests, the Bi 2 MoO 6 /ZnSnO 3 /ZnO composite still has good photocatalytic degradation activity, indicating that the composite shows good stability. In order to further explore the reaction mechanism of CV degradation by composite, 1 mL is sampled every 30 min interval by LC-MS to identify the intermediates of the reaction, and the possible degradation pathways of the corresponding intermediates and CV on Bi 2 MoO 6 /ZnSnO 3 /ZnO are proposed [37,38], as shown in Fig 10. The comparison between the assay results and the standard database shows that the resulting degradation intermediates mainly consist of components (i), (ii), (iii), (iv), (v), (vi), (vii), (viii), etc.

LC-MS analysis
According to the above results and some previous reports, the possible photocatalytic degradation pathways of Bi 2 MoO 6 /ZnSnO 3 /ZnO can be further inferred, as shown in Fig 10. Hydroxyl radicals and metal cations attack the CV in the oxidized lattice formed on the surface of Bi 2 MoO 6 /ZnSnO 3 /ZnO composites, and the chemical names and molecular formulas of the generated components are shown in Table S2. Finally, hydroxyl radicals promote oxidative ring opening reactions to generate some simpler small molecule compounds, which are finally converted to H 2 O, CO 2 , and inorganic ions.

Possible photocatalytic reaction mechanism
To explore the migration path of photogenerated carriers in the composite photocatalysts Bi 2 MoO 6 /ZnSnO 3 /ZnO, the positions of the energy bands are calculated using Eq (4) and (5), as shown in Table 2.
In these formulas, X is the absolute value of the absolute electronegativity of the semiconductor photocatalyst. E c is the standard hydrogen electrode potential (4.5 eV). E VB and E CB are the energy band positions of valence and conduction bands of composites, respectively, and Eg is the band gap value of the semiconductor. The possible photocatalytic mechanism of the Bi 2 MoO 6 /ZnSnO 3 /ZnO composites is speculated from the above calculation results and the trapping experiment results, as shown in Fig 11.

Conclusion
The