AgBr nanoparticle surface modified SnO₂ enhanced visible light catalytic performance: characterization, mechanism and kinetics study

Abstract

In this study, a simple hydrothermal procedure and in situ precipitation method were used to prepare SnO₂–AgBr composites, where the molar ratios of SnO₂ and AgBr were 1 : 1, 1 : 2 and 2 : 1. Characterization results showed that the composites had excellent dispersion, crystallinity, and purity. A photocatalytic degradation experiment and first-order kinetic model indicate that SnO₂–AgBr (1 : 1) had the best photocatalytic performance, and the degradation rates of 30 mg L⁻¹ simulated MO and MG wastewater reached 96.71% and 93.36%, respectively, in 150 min, which were 3.5 times those of SnO₂. The degradation rate of MO and MG increases with the dosage. Humic acid inhibited the degradation of MG, while a low concentration of humic acid promoted the degradation of MO, and the composite has good stability with pH. A free radical trapping experiment shows that ·OH and ·O₂⁻ were the main active substances, and h⁺ was the secondary one. According to the results of the characterization and photocatalysis experiments, a Z-scheme mechanism for the SnO₂–AgBr composite was proposed, and the degradation pathway of target pollutants was speculated upon. This study has conceived novel methods for the development of a mature Z-scheme mechanism and in doing so has provided new approaches for the development of photocatalysis for water pollution control.

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1. Introduction

With the rapid development of society and the economy, the world produces about 800 000 tons of synthetic dyes every year, of which about 10–15% are transferred to the natural environment by human activities^1,2 This pollutant will not only cause serious pollution to the water environment, affecting the growth metabolism of aquatic creatures, but is also a potential threat to human health.^3–6 Numerous studies have focused on degradation methods for synthetic dyes, such as adsorption,⁷ microbial degradation,⁸ Fenton reaction,⁹ photocatalysis,¹⁰ and electrochemistry.¹¹ Photocatalytic technology as a green sustainable method that uses solar energy to solve the problem of environmental pollution has received widespread attention due to its low cost and environmentally friendly processing. The strong oxygen reducing active substance produced by the photocatalyst in sunlight can degrade pollutants into intermediate products that are prone to biodegradation in a short period of time, or directly mineralized into H₂O and CO₂.^12–15

Tin dioxide (SnO₂) is a typical semiconductor material, which has the advantages of non-toxicity, excellent photoelectric performance and low cost, but because of its wide band gap (3.6 eV), it has almost no response to visible light, which greatly limits its application as a photocatalytic material.^16,17 Elemental mixing and semiconductor compounding are the two most commonly used modification methods for SnO₂. Elemental mixing is mainly used in the field of battery research, and different types of elements can achieve different modification effects: Sb¹⁸ and Yb¹⁹ can improve the conductivity, Ag²⁰ can improve the arc erosion performance, and Er²¹ and Pb²² can improve the optical emission performance. A semiconductor composite is commonly used in the field of photocatalysis, such as TiO₂,²³ Bi₁₂O₁₅C_l6,²⁴ g-C₃N₄ (ref. 25) and other heterojunctions formed with SnO₂ which reduce the photogenerated hole–electron recombination rate, and broaden the photocatalytic performance in the visible light range. Metal sulfides are important narrow-band-gap semiconductor materials which typically have excellent visible light absorption ability.²⁶ One of them, silver bromide (AgBr), is a good candidate for SnO₂ photosensitization due to its fast charge exchange properties, narrow band gap and high stability as a non-toxic semiconductor.

This study uses a hydrothermal procedure and an in situ precipitation method to prepare an SnO₂–AgBr composite photocatalyst that responds to visible light. It is known that the composite molar ratio between SnO₂ and AgBr is one of the main factors affecting its optical catalytic activity, so using methyl orange (MO) and malachite green (MG) as target pollutants, the effect of a high molar ratio on photocatalytic activity was investigated. Through the influence of composite dosage, pH and humic acid concentration on degradation efficiency, the stability of the photocatalyst to a water environment was demonstrated, and a Z-scheme mechanism of the SnO₂–AgBr composite was proposed, and the degradation pathways of MO and MG were speculated upon.

2. Experiment

2.1 Preparation of SnO₂

All chemicals were of analytical purity and used without further purification. A certain amount of SnCl₄·5H₂O was dissolved in 40 mL of deionized water and magnetically stirred for 60 min. NaOH was added to adjust the pH to 10 and then stirring continued for 30 min. The final product was obtained by hydrothermal reaction at 180 °C for 10 h. The SnO₂ crystals were washed alternately with deionized water and anhydrous ethanol 3 times, and baked at 90 °C until dry.

2.2 Preparation of SnO₂–AgBr

0.46 g of SnO₂ was dissolved in 40 mL of deionized water; then 30 mL of 0.1 mol L⁻¹ AgNO₃ solution was added, and stirred for 30 min. 0.36 g of KBr was dissolved in 40 mL of deionized water, and then the KBr solution was slowly added to the above mixture and stirred away from light for 3 h. After the sediment was filtered and washed with deionized water and heated at 130 °C for 20 h, an SnO₂–AgBr composite with a molar ratio of 1 : 1 could be obtained, which is denoted SnO₂–AgBr (1 : 1). The above steps were repeated to prepare composites with molar ratios of SnO₂ and AgBr of 1 : 2 and 2 : 1, denoted SnO₂–AgBr (1 : 2) and SnO₂–AgBr (2 : 1), respectively.

2.3 Characterization and instrumental parameters

In this experiment, the crystallinity of the sample was characterized with a D8 ADVANCE X-ray diffractometer (XRD), Kα radiation (λ = 1.5406 A) Cu target; the diffraction angle ranged from 10° to 80°. An SU8220 cold field emission scanning electron microscope (SEM) was used to characterize the structure and composition of the samples. The sample crystal was analyzed with a Talos F200S transmission electron microscope (TEM), and the chemical bond of the sample was analyzed by ESCALAB 250 X-ray photoelectron spectroscopy (XPS), where the binding energy in the spectrum was calibrated by C1s (284.62 eV). A 1290II-6460 liquid mass coupling instrument (HPLC-MS) was used to analyze the composition of the samples, in which the current was 10 μA and the voltage was 15 kV. An F-7100 Spectrometer (PL) tested the photoluminescence spectra. The light absorption properties of the materials were analyzed with a UV-3600 type UV-visible spectrophotometer. The electrochemical impedance spectroscopy (EIS) was measured at V_oc one-sun illumination with 0.1 M Na₂SO₄ electrolyte between a Pt-coated film counter electrode and sample film coated on fluorine-doped tin oxide (FTO) as a working electrode, held by two heavy-duty clips.

2.4 Photocatalytic experiments

A 500 W tungsten halogen lamp (λ > 420 nm) was used as the visible light source, and 100 mL (30 mg L⁻¹) of the MO/MG solution were added into the reactor. The temperature of the reaction solutions was kept constant by the actions of a magnetic blender and a cooling system. The SnO₂–AgBr composites were weighed (0.1 g) and evenly dispersed in the solution. The light source was turned off for the first 30 min so that the catalyst and the target pollutants could achieve adsorption equilibrium. A DR6000 ultraviolet spectrophotometer was used to test the absorbance of MO and MG at 464 nm and 618 nm, respectively, and the concentrations at different times were calculated according to the standard curves of MO and MG, to calculate their degradation rates.

2.5 Cycling experiments

The stability of the catalyst was tested by cycling experiments. The same SnO₂–AgBr (1 : 1) compounds were used to degrade MO/MG for four runs. After the degradation reaction the catalyst was collected, washed alternately with anhydrous ethanol and pure water four times, and then fully dried in a constant temperature drying oven at 90 °C before use.

3. Results and discussion

3.1 Material characterization

3.1.1 XRD analysis. The XRD of SnO₂–AgBr composites with different molar ratios are shown in Fig. 1, in which the crystallinity and purity of the composites can be seen. The main diffraction peaks at 26.58°, 33.88°, 51.75° correspond to the tetragonal SnO₂ (110), (101), (211) crystal faces (PDF#41-1445), and the diffraction peaks at 30.96°, 44.35° correspond to the cubic phase AgBr (200), (220) crystal face (PDF#06-0438).^27,28 There are no other peaks, indicating that SnO₂ and AgBr have high purity. The diffraction peaks position of SnO₂ with different molar ratios did not change, indicating that AgBr did not change the crystal shape of SnO₂ through surface loading. The diffraction peak intensity of AgBr increases with its loading capacity, but excessive AgBr led to an increase in the photogenerated hole–electron pair recombination rate and a decrease in photocatalytic degradation activity.²⁹ The XRD results showed that SnO₂ and AgBr were successfully prepared.

Fig. 1 XRD of SnO₂–AgBr composites with different mole ratios.

3.1.2 SEM and TEM analysis. The morphology and structure of the sample can be observed through SEM (Fig. 2a and b), showing that SnO₂ consists mainly of a porous structure composed of irregular ellipsoids with different particle sizes, and the uneven gully structure on the surface provides support for the load of AgBr. As can be seen from Fig. 2c–e, there is local agglomeration of SnO₂–AgBr (1 : 2) due to excessive AgBr, but no significant agglomeration of SnO₂–AgBr (1 : 1) or SnO₂–AgBr (2 : 1) can be observed due to the small size, small load and high dispersion of AgBr.

Fig. 2 SEM and TEM of SnO₂ and SnO₂–AgBr: SEM of (a and b) AgBr, (c) SnO₂–AgBr (1 : 2), (d) SnO₂–AgBr (1 : 1), and (e) SnO₂–AgBr (2 : 1); (f–i) TEM of SnO₂–AgBr (1 : 1).

SnO₂–AgBr (1 : 1) TEM (Fig. 2f–i) proved that SnO₂ and AgBr were successfully combined, but there was a small amount of agglomeration in AgBr (Fig. 3f and g), which was an objective result of the experimental conditions. The lattice spacings of 0.26 nm and 0.33 nm correspond to the (101) and (110) crystal faces of SnO₂, respectively (Fig. 2h); the lattice spacings of 0.26 nm and 0.20 nm correspond to the crystal faces of SnO₂ (101) and AgBr (220), respectively (Fig. 2i).^30–32

Fig. 3 EDS of the SnO₂–AgBr (1 : 1) composite.

3.1.3 EDS analysis. In order to understand the chemical composition and elemental distribution of the composite, EDS of SnO₂–AgBr (1 : 1) was performed (Fig. 3). The results showed that the composite was composed of Sn, O, Ag and Br, and no other elements were detected. AgBr particles are evenly distributed on the surface of SnO₂, and there is no large-scale agglomeration. The theoretical percentage of Ag and Br atoms is relatively low, which may be caused by the existence of a small amount of AgBr agglomeration during the preparation process.

3.1.4 XPS analysis. In order to investigate the elemental composition and valence state of the SnO₂–AgBr composite, as well as the binding state of AgBr and SnO₂ crystals, XPS of SnO₂ and SnO₂–AgBr (1 : 1) was performed; all spectra were based on C1s (284.4 eV). Fig. 4a shows the full spectra of SnO₂ and SnO₂–AgBr (1 : 1), where one can clearly see the binding energies of Sn 3d, O 1s, Ag 3d and Br 3d. The binding energies at 488.1 eV and 496.5 eV correspond to Sn 3d_5/2 and Sn 3d_3/2, respectively, and the pyrolysis energy difference is 8.4 eV, which proves that Sn is Sn⁴⁺ in the composite.^33,34 After loading AgBr, the binding energies of Sn 3d are slightly shifted compared with SnO₂, which may be because the electron cloud density of Sn is changed when AgBr and SnO₂ combine with each other, which affects the electronic structure of SnO₂.³⁵ The binding energy of O1s at 529.6 eV corresponds to the lattice oxygen of Sn–O, and that at 530.9 eV corresponds to water species adsorbed on the surface of the as-prepared sample. These two reactive oxygen species will promote the formation of ·OH and ·O₂⁻ and facilitate the oxidative decomposition of organic matter.^36,37 After loading AgBr, the binding energies of O 1s are shifted in the high-energy direction relative to SnO₂, indicating that the composite has more oxygen vacancies, which can improve the separation of electron–hole pairs and increase the photocatalytic efficiency.³⁸ Binding energies at 367.8 eV and 373.9 eV correspond to Ag3d_5/2 and Ag3d_3/2, respectively.^39,40 In a summary of previous reports,^41,42 binding energies at 367.3 eV and 374.2 eV are attributed to Ag⁰; those at 367.8 eV and 373.8 eV are attributed to Ag⁺. Therefore, the valence state of Ag in the composite is Ag⁺. The binding energies at 67.8 eV and 68.8 eV correspond to Br 3d_5/2 and Br 3d_3/2, respectively, indicating that the valence state of Br in the composite is Br⁻.⁴³ In summary, SnO₂ and AgBr were successfully bonded by chemical bonds.

Fig. 4 XPS of SnO₂ and SnO₂–AgBr (1 : 1): (a) full survey spectra, (b) Sn 3d, (c) O 1s, (d) Ag 3d, and (e) Br 3d.

3.1.5 UV-vis and PL analysis. The degree of response of the composite to light and the recombination efficiency of photogenerated electron–hole pairs are two main factors with which to measure the efficiency of photocatalytic materials. The light absorption range of the composite is reflected by UV-vis, and the photogenerated hole–electron recombination rate by PL emission spectroscopy. The absorption edge of SnO₂ is around 340 nm, and it hardly responds to visible light. The absorption curve of AgBr does not decrease significantly in the range of 200–800 nm, and it still maintains a high absorption intensity (Fig. 5a). Compared with SnO₂–AgBr with different molar ratios, the absorption curve of SnO₂ is obviously redshifted towards the visible region, indicating that the load of AgBr effectively improves the ability of SnO₂ to respond to visible light.⁴⁴ The absorption intensity of the composite for visible light increased first and then decreased with AgBr load, and SnO₂–AgBr (1 : 1) showed the best light absorption intensity and range. The results indicate that visible light can be used more effectively and more photogenerated electron–hole pairs can be generated, which are beneficial to the enhancement of photocatalytic activity.^45,46 The band-gap width is calculated (Fig. 5b according to the Kubelka–Munk function). The band-gap widths of SnO₂, AgBr, SnO₂–AgBr (1 : 2), SnO₂–AgBr (1 : 1) and SnO₂–AgBr (2 : 1) are 3.51 eV, 2.6 eV, 2.75 eV, 2.62 eV and 2.80 eV, respectively.

Fig. 5 (a) UV-vis and (b) band-gap width of SnO₂ and SnO₂–AgBr composite; (c) PL of SnO₂–AgBr composite.

PL results show that SnO₂–AgBr with different molar ratios have diffraction peaks at 460 nm (Fig. 5c), but the SnO₂–AgBr (1 : 1) composite has the lowest diffraction peak intensity, indicating the lowest electron–hole recombination efficiency and the longest photogenerated carrier lifetime.^47,48 In summary, SnO₂–AgBr (1 : 1) has the best photocatalytic performance compared with other molar ratio composites.

3.1.6 Zeta potential test. In order to investigate the charging property of the composite surface and its dispersion in water, a particle size analyzer was used to detect the zeta potential of the SnO₂–AgBr composite, and the results are shown in Fig. 6a. The zero points of SnO₂–AgBr (1 : 2), SnO₂–AgBr (1 : 1) and SnO₂–AgBr (2 : 1) are 4.61, 5.22 and 6.18, respectively. Under neutral conditions, the surface of SnO₂–AgBr is electronegative, which is theoretically more conducive to attracting cationic MB, and has an electrostatic repulsive effect on anionic MO.^49,50

Fig. 6 Zeta potential (a) and electrochemical impedance curve (b) of the SnO₂–AgBr composite.

Electrochemical impedance spectroscopy (EIS) analysis was carried out to determine the electron transport properties of SnO₂–AgBr that eventually influenced the performance of SnO₂–AgBr. It can be seen from Fig. 6b that the impedance radius of SnO₂ is the largest and that of SnO₂–AgBr (1 : 1) is the smallest, indicating that SnO₂–AgBr (1 : 1) has the smallest charge transfer resistance and the highest separation efficiency, which is consistent with the PL result.

3.2 Photocatalytic degradation experiment

3.2.1 Degradation analysis of target pollutants. As shown in Fig. 7a and b, the dark adsorption of the composite with different molar ratios for MO and MG is about 10%, which contradicts the conclusion of the zeta potential analysis. Therefore, it is speculated that electrostatic effect does not play a major role in the adsorption process. In the photoreaction phase, SnO₂ had little degradation effect on MO or MG, which was consistent with the UV-vis analysis. The photocatalytic degradation rate of SnO₂–AgBr was significantly increased because of the formed heterojunction. SnO₂, as a photogenerated electron acceptor, could inhibit the photogenerated electron–hole recombination rate and improve the contact between active substances and pollutants.^51,52 SnO₂–AgBr (1 : 1) had the best degradation effect on MO and MG, and the 150 min treatment rates were 96.74% and 93.36%, respectively. A dynamic analysis of degradation rate (Fig. 7c and d) showed that the k of MO and MG were 2.56 × 10⁻² min⁻¹ and 2.01 × 10⁻² min⁻¹, respectively, which were 1.83 and 1.77 times those of SnO₂–AgBr (2 : 1), 1.53 and 1.22 times those of SnO₂–AgBr (1 : 2). The results indicated that SnO₂–AgBr (1 : 1) had the fastest degradation rate for the two pollutants.⁵³ The photocatalytic degradation rate of SnO₂–AgBr (1 : 2) decreased because the active site on the surface of the composite was shielded due to excessive AgBr attachment to the surface of SnO₂. At the same time, the light penetration was weakened and the absorption of visible light was affected by the composite.^54–56 UV-vis absorption spectra of MO and MG (Fig. 7e and f) reflect the photocatalytic degradation process. The intensity of the absorption peaks at 464 nm and 618 nm gradually decreased and eventually disappeared with reaction time, indicating that the pollutants were degraded.

Fig. 7 Photocatalytic degradation by the composite with different molar ratios for (a) MO and (b) MG; first-order kinetics of (c) MO and (d) MG; UV-vis absorption spectra of (e) MO and (f) MG.

3.2.2 The influence of composite dosage. In the treatment of wastewater, the catalyst is often one of the main costs in the advanced stage of water treatment. Under the condition of pH = 7, 100 mL MO and MG solution with a concentration of 30 mg L⁻¹ were added with 0.05 g, 0.08 g, 0.10 g, 0.12 g and 0.15 g SnO₂–AgBr (1 : 1) composite. The relationship between the dosage of the composite and the treatment efficiency was investigated within 150 min. As shown in Fig. 8, when the dosage increased from 0.5 g L⁻¹ to 1.5 g L⁻¹, the degradation rates of MO and MG increased from 54.42% and 59.85% to 97.76% and 96.32%, and the k increased by 4.58 and 3.65 times, respectively. In this experiment, there was no phenomenon where photocatalytic activity decreased due to excessive composite addition, which affected the transparency of the solution.^57,58

Fig. 8 Effect of composite dosage on degradation rate of (a) MO and (b) MG; first-order kinetics of (c) MO and (d) MG.

3.2.3 Influence of humic acid concentration. Humic acid (HA) is produced by the decomposition of natural organic compounds and exists in all sewage, so it has a great influence on the form of existence and migration performance for some pollutants in water.^59,60 In order to investigate the effect of HA concentration on catalyst degradation efficiency, HA concentrations were set at 0 mg L⁻¹ (blank control), 5 mg L⁻¹, 10 mg L⁻¹ and 20 mg L⁻¹. The experimental results are shown in Fig. 9. When the concentration of HA increased from 0 mg L⁻¹ to 5 mg L⁻¹, it promoted the degradation rate of MO, and inhibited it as the concentration continued to increase. However, it has consistently shown inhibitory effects on MG. The reason is that HA can be used as a photosensitizer to promote the production of active substances, thereby improving the degradation rate of MO and MG. However, it also forms a competitive relationship with MO and MG for active substances, or becomes a quenching agent, reducing the content of active substances.^61–63 In this experiment, the two target pollutants show different experimental results, which is the competitive effect of HA.

Fig. 9 Effect of humic acid on degradation rate for (a) MO and (b) MG; first-order kinetics of (c) MO and (d) MG.

3.2.4 Influence of pH. pH is a very important parameter in water treatment, which can affect the efficiency of water treatment by changing the surface adsorption capacity of a catalyst or the hydrolyzed form of pollutants.^64,65 The relationship between pH and degradation efficiency was investigated when the pH was 3, 5, 7, 9 and 11, and the dosage of the composite was 0.1 g. The results are shown in Fig. 10: with an increase in pH, the photocatalytic degradation rate of MO and MG has little effect, but the k of MO decreases continuously. The reason is that under neutral and alkaline conditions, MO is an azo structure containing N=N double bonds with a bond energy of 456 kJ mol⁻¹, while under acidic conditions, the N=N double bonds are transformed into N–N single bond quinone structures with a bond energy of 159 kJ mol⁻¹. Therefore, the reduction in bond energy under acidic conditions leads more easily to the decomposition of MO.^66,67

Fig. 10 Effect of pH on degradation rate for (a) MO and (b) MG; first-order kinetics of (c) MO and (d) MG.

It was found that under alkaline conditions, MG was converted to colorless tertiary alcohol, resulting in the solution rapidly fading from blue-green to colorless.^68,69 Due to the limitations of the experimental conditions, absorbance cannot be tested, so only acid and neutral conditions were investigated. The degradation effect of MG under acidic conditions is slightly decreased compared with that under neutral conditions, because H⁺ consumes the active substance ·OH or occupies the active site under acidic conditions.⁷⁰ At the same time, with a change in pH, the adsorption properties of the composite for MG and MO showed almost no change, which proved that the adsorption process was not dominated by electrostatic action, but by the hydraulic action of surface complexation or magnetic agitation.

3.3 Repeated experiments

The stability of the SnO₂–AgBr (1 : 1) composite is related to its practicability and is also an important factor to measure its photocatalytic performance.^71,72 After four cycles of experiment, the degradation rates of MO and MG decreased to 66.42% and 70.86%, respectively (Fig. 11a and b). The samples collected after four cycles were tested by XRD, and the characterization results are shown in Fig. 11c and d. The position and intensity of the diffraction peak of SnO₂–AgBr (1 : 1) did not change significantly, but the new diffraction peak at 38.25° corresponds to Ag⁰.⁷³ AgBr is corroded by light and some Ag⁺ is restored to Ag⁰, which covers part of the active site. This is the main reason for the continuous decline in photocatalytic activity with repeated experiments.^74,75

Fig. 11 Composite repeated experiments for (a) MO and (b) MG; (c and d) XRD of composite after repeated experiments.

3.4 Free radical trapping experiment

The main active substances were determined by a free radical trapping experiment, and the degradation mechanism of SnO₂–AgBr (1 : 1) was further studied. Isopropyl alcohol (IPA), p-benzoquinone (BQ) and ammonium oxalate (AO) were used as inhibitors of ·OH, ·O₂⁻ and h⁺, respectively.^76,77 The dosage of BQ was 1 mmol L⁻¹; those of IPA and AO were 10 mmol L⁻¹. The results are shown in Fig. 12. The three inhibitors of active species all reduced the degradation efficiency to varying degrees, indicating that ·O₂⁻, ·OH and h⁺ were involved in photocatalytic degradation. However, after the addition of BQ and IPA, the degradation rate seriously decreased, indicating that ·O₂⁻ and ·OH were the main active substances, and h⁺ played a secondary role. Moreover, these results were further confirmed by electron paramagnetic resonance (EPR) tests with DMPO as the spintrapping reagent. As displayed in Fig. 12c and d, signals of ·OH and ·O₂⁻ were observed in the SnO₂–AgBr (1 : 1) system and the intensities of the signals gradually became intense with prolonged light illumination time.

Fig. 12 SnO₂–AgBr (1 : 1) composite trapping experiments for (a) MO and (b) MG; EPR spectra of the SnO₂–AgBr (1 : 1) composite: (c) DMPO–·OH, (d) DMPO–·O₂⁻.

3.5 Prediction of degradation mechanism

According to the results of the characterization, the SnO₂–AgBr composite reaction mechanism was deduced from the Mulliken empirical formula (eqn (1) and (2)):

E_VB = X − E^e + 0.5E_g (1)

E_CB = E_VB − E_g (2)

where E_VB and E_CB are the potentials at the top of the valence band and the bottom of the conduction band relative to the standard (H₂) electrode potential, respectively; X is the electronegativity of the semiconductor; E^e is the energy of the free electron on the scale of the hydrogen atom (about 4.5 eV); E_g is the band-gap width of the semiconductor.

According to UV-vis, the band gaps of SnO₂ and AgBr are 3.51 eV and 2.6 eV, respectively, and the electronegativities are 6.10 eV and 3.90 eV, respectively. The E_VB and E_CB of SnO₂ are 3.35 eV and −0.16 eV, and those of AgBr are 2.6 eV and 0.0 V.^78,79 SnO₂ has no response to visible light due to its band-gap width of 3.51 eV, and therefore it cannot produce photogenerated electron–hole pairs. Since the E_CB (0.00 eV) of AgBr is higher than that of SnO₂ (−0.16 eV), photogenerated electrons cannot migrate from AgBr to SnO₂ (Fig. 13a). Therefore, if the photoinduced electrons and holes migrate by the traditional type-II charge transfer mechanism, the experimental phenomena cannot be explained.

Fig. 13 (a) SnO₂–AgBr (1 : 1) traditional heterojunctions mechanism; (b) Z-scheme mechanism.

Therefore, a Z-scheme mechanism using Ag as the electronic medium is proposed (Fig. 13b). Under visible light, AgBr is partially reduced to Ag and attached to the surface of the SnO₂–AgBr composite to form Ag@SnO₂–AgBr. Ag has two roles in a Z-scheme mechanism: as an electronic medium and as a photosensitizer.^80,81 From the UV-vis results (Fig. 7a), it can be seen that AgBr responds to visible light, while SnO₂ cannot, so Ag serves as both a photosensitizer and an electronic medium.^82,83 Ag assumed the responsibility for visible light absorption, not SnO₂. The photogenerated electrons (e_CB1⁻) in the AgBr conduction band combine with the photogenerated holes (h_VB2⁺) in the highest occupied orbital of Ag, while the photogenerated electrons (e_CB2⁻) in the lowest unoccupied orbital of Ag migrate to the conduction band of SnO₂. The photogenerated electrons (e_CB3⁻) are enriched in the conduction band of SnO₂ and the holes (h_VB1⁺) are enriched in the valence band of AgBr, and the photogenerated hole–electron separation of AgBr is realized (eqn (3)–(10)).

In addition, since the E_CB of SnO₂ (−0.16 eV vs. NHE) is lower than that of O₂ (−0.046 eV vs. NHE), ·O₂⁻ can be produced. Similarly, the E_VB of AgBr (2.60 eV vs. NHE) is higher than that of OH-/H₂O (2.30 eV vs. NHE), and.

·OH can be formed normally. The Z-scheme mechanism of SnO₂–AgBr composite well reveals its superior photocatalytic performance.

AgBr + hν → e_CB1⁻ + h_VB1⁺ (3)

AgBr + hν → AgNPs (4)

AgNPs + hν → e_CB2⁻ + h_VB2⁺ (5)

e_CB1⁻ + h_VB2⁺ → recombination (6)

e_CB2⁻ + AgNPs → SnO₂ + e_CB3⁻ (7)

h_VB1⁺ + H₂O →·OH + H⁺ (8)

e_CB3⁻ + O₂ →·O₂⁻ (9)

·OH/·O₂⁻/h⁺ + MO/MG → intermediate product + CO₂ + H₂O (10)

3.6 Prediction of photocatalytic degradation pathway

In order to further study the degradation process of MO/MG, samples of 0 min, 15 min, 60 min and 120 min were taken for HPLC-MS analysis, and the results are shown in Fig. 14. At t = 0, m/z 304.1 is the diffraction peak of MO (Fig. 14a), which is an ionic state formed when MO molecules dissolve in water and remove Na⁺, while m/z 329.2 corresponds to the characteristic peak of MG (Fig. 14e).^84,85 With the progress of the reaction, it was found that the diffraction peaks of MO and MG were significantly reduced and gradually transformed into small molecules. The main m/z of MO were 338.0, 175.3, 122.6, 157.0, 108.3 and 94.4, while the m/z of MG were 347.2, 329.2, 315.1, 301.0, 273.0, 226.1 and 150.1.

Fig. 14 Mass spectrum of the intermediates of MO at: (a) 0 min, (b) 15 min, (c) 60 min, (d) 120 min; and of MG at: (e) 0 min, (f) 15 min, (g) 60 min, (h) 120 min.

Based on the m/z generated in different times, the degradation pathways for MO were proposed (Fig. 15 MO). The benzene ring in A (m/z 304.1) is substituted by ·OH to form product B;⁸⁶ B is attacked by active substances; the C–N bond breaks due to low bond energy, forming products C, D and N₂;⁸⁷ C removes ·OH to form product E; and D generates products F and G by demethylation.

Fig. 15 Prediction of degradation pathways of MO and MG.

The degradation pathways of MG (Fig. 15 MG) are: first, A (m/z 329) generates B through demethylation, which is the main pathway of photocatalytic degradation and mineralization.⁸⁸ Subsequently, B is further demethylated to produce C and D; second, A (m/z 329) undergoes decarboxylation to form E; the covalent bond between the benzene ring and the central carbon atom is broken, forming F; G is formed by debenzenization of F. Subsequently, the intermediate of MO/MG will be degraded into CO₂ and H₂O.

4. Conclusions

An SnO₂–AgBr composite showing a Z-scheme mechanism was successfully prepared by a simple hydrothermal procedure and in situ precipitation method. Characterization and photocatalytic experiments showed that the composite performance was best when the molar ratio of SnO₂ to AgBr was 1 : 1. The Z-scheme mechanism reduces the photogenic hole–electron recombination rate of AgBr, and produces a large number of ·OH, ·O₂⁻ and h⁺, and the degradation rates of simulated MO and MG wastewater reach 96.74% and 93.36%, respectively. Different factors have different effects on the degradation efficiency of simulated dye wastewater. The effect of pH on the degradation rate was small, indicating that the composite had good acid–alkali resistance. The degradation rate increased with composite dosage, and the humic acid concentration was negatively correlated with the degradation rate, which played an inhibitory role. According to mass spectrometry analysis, the degradation pathway of MO and MG proceeded through bond-breaking oxidation, ring-opening and deep oxidation decomposition into intermediate products, and finally degradation into CO₂ and H₂O. The repeatability results of SnO₂–AgBr are not very good, because the silver-based photocatalyst easily undergoes photocorrosion. It is hoped that this study can provide a reference scheme for the design and preparation of a new Z-scheme mechanism, and realize the more efficient, economical and green application of photocatalysis in the field of water treatment.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the support of Xiaoliang Yang, Yukai Wang and Fei Tang, and their friends in College of Civil Engineering, Guizhou University, Guiyang, China.

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