Self-assembly Z-scheme heterostructured photocatalyst of Ag₂O@Ag-modified bismuth vanadate for efficient photocatalytic degradation of single and dual organic pollutants under visible light irradiation

Abstract

Novel Ag₂O@Ag-modified BiVO₄ composites with highly efficient visible light photocatalytic activity were synthesized by a facile pH-mediated chemical precipitation method accompanied by a self-built Z-scheme heterojunction under visible light irradiation in this study. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectrometry (FTIR), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (UV-vis DRS) analysis and electrochemical measurements demonstrated that an acicular sheaf-like heterojunction was formed. Rhodamine B (RhB), methylene blue (MB) and their mixture were selected as the target pollutants to evaluate the enhanced photocatalytic activities of the as-prepared composites. Compared with pure BiVO₄ and Ag₂O, Ag₂O@Ag@BiVO₄ had a high photocatalytic activity because of the efficient separation of electron–hole pairs in the heterojunction construction. The composites with an initial Ag₂O : BiVO₄ mass ratio of 1 : 10 exhibited the highest degradation efficiency for dye. Experimental results indicated that single RhB or MB could be degraded completely within a short irradiation time (15 min for 10 mg L⁻¹ RhB, 48 min for 20 mg L⁻¹ MB), but there was obviously competition with the active species, resulting in a decrease of the degradation rate for both dyes. Regardless of the suppression effect, a relatively higher photocatalytic performance was also achieved for the dye mixture. Several parameters, including initial dye concentration, calcination temperature, initial pH and mineralization ability are discussed in detail. The kinetics study suggested that photocatalytic degradation processes for single or complex dye were well described by first-order kinetics. Due to the generated Ag–Ag₂O self-stability structure, these acicular sheaf-like three-dimensional heterojunction photocatalysts were fairly stable and there was no obvious loss of photocatalytic activity after four consecutive cycles.

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1. Introduction

With the development of modern industry, synthetic dyes are being widely used in various textiles, printing, dyeing and food industries. Colourful dyes have enriched our lives but there also exists a potential hazard to human health and inhabitation for economic development due to the formation of cytotoxic and carcinogenic intermediates.^1,2 Recently, advanced oxidation processes (AOPs) have been widely applied in the degradation of these hard-to-treat pollutants. As a promising AOP technology, photocatalysis has gained increasing attention in organic wastewater treatment owing to its high degradation efficiency, low-cost, and long-term stability compared with other conventional treatment methods.^3–5 Generally, there are two approaches to improve the photoactivity of single semiconductor catalysts: (i) increasing the adsorption capacity of the catalyst for dye molecules and (ii) heterojunction formation, doping and vacancy generation in the photocatalytic system.^6–8 Heterogeneous photocatalytic oxidation (HCO) using semiconductors has attracted considerable interest in air and water purification on account of its strong oxidation power, moderate operation temperature, and green chemistry-related procedures.^9–12 In response to the further exploration of photocatalytic technology, a large number of new-type photocatalysts has been discovered and applied in the degradation of organic pollutants, especially heterostructured visible light-driven (VLD) photocatalysts.¹³

In recent years monoclinic BiVO₄ with a band gap of about 2.4 eV has aroused researchers’ interest, because of its sufficient photocatalytic response and good stability against photocorrosion under visible light irradiation.^14,15 By now, significant efforts have been devoted to achieve the controllable synthesis of BiVO₄ hierarchical nanostructures, improving the photocatalytic performance especially under visible light irradiation.¹⁶ However, the photocatalytic activity of pure BiVO₄ for water splitting and pollutant degradation is still not satisfactory, attributed to its weak surface absorption ability and migration difficulty of photogenerated electron–hole pairs. In order to solve the above existing problems, BiVO₄ was combined with a semiconductor to form an appropriate band position.¹⁷ There are many successful cases such as Cu₂O/BiVO₄,¹⁸ BiVO₄/TiO₂,¹⁹ BiVO₄/BiOCl,²⁰ BiVO₄/Bi₂S₃,²¹ and so on, but it seems to be considerably necessary to find more suitable semiconductors and study the photocatalytic activity and intrinsic mechanism more deeply. Ag₂O is a p-type semiconductor with a direct band gap of about 1.2 eV, which possesses the ability to absorb a large part of visible light.²² According to previous reports, Ag₂O was effectively used as a sensitizer to regulate the light response of relatively wide band gap semiconductors and improve the corresponding visible light photocatalytic activities, such as Ag₂O/TiO₂,²³ Ag₂O/BaTiO₃,²⁴ Ag₂O/Bi₂WO₆,²⁵ and so forth. The well matched overlapping band structures were successfully built due to the introduction of Ag₂O, resulting in effective interface transfer of photogenerated electrons and holes. Taking the applicable energy band gap of Ag₂O and BiVO₄ into consideration, it is reasonable to construct a p–n heterojunction with the appropriate valence band and conduction band. In addition, it has been documented that the loaded Ag₂O could be effectively transformed into a stable Ag₂O/Ag structure in the photodecomposition of organic pollutants under illumination.²⁶ Both Ag₂O and the formed noble metal Ag could act as traps to capture photogenerated electrons, which were beneficial to the efficient separation of electron–hole pairs in the photocatalytic system.²⁷

Herein, a highly efficient VLD heterogeneous photocatalyst, Ag₂O/BiVO₄ was firstly synthesized by a simple chemical precipitation method, followed by a self-built Z-scheme photocatalyst Ag₂O@Ag@BiVO₄ under visible light irradiation. RhB, MB and RhB + MB were used as the target pollutants to evaluate the photocatalytic activities of the Ag₂O@Ag@BiVO₄ heterojunctions. The influence of initial dye concentration, calcination temperature, and initial pH, and a kinetics study for both single and mixed dye degradation were investigated in detail. A possible reaction mechanism for the improved photocatalytic activities was discussed thoroughly and distinctly presented as graphs coupled with equations.

2. Experimental

2.1. Chemical materials

Bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O), ammonium metavanadate (NH₄VO₃), silver nitrate (AgNO₃), sodium hydroxide (NaOH), glacial acetic acid (CH₃COOH), aqueous ammonia (NH₃·H₂O) and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. Rhodamine B (RhB, cationic dye) and methylene blue (MB, cationic dye) and their mixture were chosen as the target substances to evaluate the photocatalytic activity of the as-prepared samples. The structural formulae of RhB and MB are illustrated in Scheme 1. All chemicals were analytical reagent grade, commercially available and used without additional purification or treatment. Deionized water was used as the solvent throughout the experiment.

Scheme 1 Structural formula of model pollutants: rhodamine B (RhB) and methylene blue (MB).

2.2. Preparation of 3D sheaf-shaped BiVO₄

The typical preparation procedure of 3D sheaf-shaped BiVO₄ was as follows: 2 mmol Bi(NO₃)₃·5H₂O was firstly dissolved in a 50 mL mixed solution (v(C₂H₅OH)/v(CH₃COOH)/v(H₂O) = 1 : 1 : 3), and the formed solution was named as solution A. Simultaneously, 2 mmol NH₄VO₃ was dispersed in a 20 mL ammonia solution (v(NH₃·H₂O)/v(H₂O) = 3 : 1), and the solution was marked as solution B. After constant stirring for 20 min, solution B was added into solution A drop by drop and the pH was adjusted to 7.0 by NH₃·H₂O. Afterwards the obtained mixture was aged at 80 °C for 3 h under water bath conditions. After naturally cooling down, the yellow precipitate was collected by filtering and washing with deionized water and ethanol several times to remove the residual ions and surfactants in the products. At last, the products were dried at 80 °C in air for 8 h.

2.3. Preparation of Ag₂O/BiVO₄ heterojunctions

Ag₂O/BiVO₄ heterojunctions were fabricated by a pH-mediated chemical precipitation method. Typically, 0.2 g of BiVO₄ was added into 50 mL deionized water and ultrasonicated for 30 min to achieve a homogeneous dispersion. The desired amount of AgNO₃ was added into the above solution and stirred at room temperature for 30 min in a dark place. The weight ratios of Ag₂O/BiVO₄ were controlled to be 1 : 1, 1 : 5, 1 : 10 and 1 : 20. Then NaOH (1 M) aqueous solution was adopted to adjust the pH of the suspensions to be about 12.5, and the dispersion was continuously stirred for 2 h at room temperature. Finally, the obtained dark brown Ag₂O/BiVO₄ was thoroughly washed with deionized water three times and dried at 60 °C for 8 h. The series of samples was labelled as Ag₂O/BiVO₄ (1 : 1), Ag₂O/BiVO₄ (1 : 5), Ag₂O/BiVO₄ (1 : 10), and Ag₂O/BiVO₄ (1 : 20), respectively. As a reference, pure Ag₂O nanoparticles were synthesized from AgNO₃ and NaOH aqueous solution by the above-mentioned precipitation method without adding BiVO₄. Furthermore, the prepared samples were calcined at 60 °C, 100 °C, 200 °C and 300 °C for 1 h with a heating rate of 2 °C min⁻¹, and named as 60 °C/1 h, 100 °C/1 h, 200 °C/1 h and 300 °C/1 h. A brief description of the preparation process is illustrated in Scheme 2.

Scheme 2 The schematic diagram of the formation process of Ag₂O/BiVO₄ fabricated by a facile pH-mediated chemical precipitation method.

2.4. Characterizations

The phase compositions of the as-prepared samples were measured on a Rigaku D/max 2500v/pc X-ray diffractometer with Cu Kα radiation at a scan rate of 0.1° 2θ s⁻¹. The working voltage and the applied current of the diffraction were 40 kV and 40 mA, respectively. FTIR spectra were collected on an IR Prestige-21 spectrometer (Shimadzu, Japan) at room temperature by the standard KBr disk method. Morphological analysis was performed using field emission scanning electron microscopy (FESEM, Hitachi S-4800) with 5.0 kV scanning voltages. The TEM analyses were conducted using transmission electron microscopy (TEM, FEI Tecnai G20) at an accelerating voltage of 200 kV. The UV-vis absorption spectra were obtained on a UV-vis spectrometer (Shimadza UV-4100) in the range of 300–700 nm, using BaSO₄ as the reference. The XPS measurements were carried out on a Thermo ESCALAB 250XI spectrometer with an Al Kα source. The total organic carbon (TOC) assays were tested using a Shimadzu TOC-VCPH analyzer. Photoluminescence (PL) measurements were monitored using a transient fluorescence spectrometer (Edinburgh FLsp920 full functional state). Photocurrent–time (PT) measurement was acquired using a conventional three-electrode electrochemical cell with a working electrode, a platinum wire counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The working electrode was immersed in a sodium sulfate electrolyte solution (0.5 M) and illuminated under visible light. Electrochemical impedance spectroscopy (EIS) was carried out on a potential Autolab PGSTAT-30 equipped with a frequency analyzer module using a three-electrode system in 0.5 M Na₂SO₄ solution. The electrochemical experimental results were all recorded with a CHI 660B electrochemical system.

2.5. Evaluation of visible light photocatalytic activity

The photocatalytic degradation of RhB, MB as well as their mixtures under visible light irradiation was adopted to evaluate the photocatalytic activities of the as-prepared Ag₂O/BiVO₄ samples. A 300 W Xe lamp with a cut-off filter (λ > 420 nm) was used as the light source and the average light intensity striking the surface of the reaction solution was about 100 mW cm². Briefly, 0.03 g of photocatalyst was added to a 50 mL aqueous solution of RhB (10 mg L⁻¹), MB (20 mg L⁻¹), RhB + MB (5 mg L⁻¹ + 10 mg L⁻¹) or RhB + MB (10 mg L⁻¹ + 20 mg L⁻¹). Before illumination, the suspension was constantly stirred for 30 min in the dark to achieve the adsorption–desorption equilibrium. Afterwards, the mixture was exposed to visible light irradiation with magnetic stirring. At given time intervals, 2.5 mL of the suspension was periodically collected and analyzed after centrifugation. The concentrations of RhB and MB were determined by monitoring the absorbance at ca. 554 nm and 664 nm, respectively.

3. Results and discussion

3.1. Characterization of the as-prepared photocatalysts

3.1.1 XRD analysis. The crystallographic structure and phase purity of the as-obtained samples were examined by XRD analysis. Fig. 1A shows the XRD patterns of Ag₂O/BiVO₄ heterostructures with different weight ratios of Ag₂O. All of the diffraction peaks for BiVO₄ could be indexed as the body-centered monoclinic phase of BiVO₄ (JCPDS File No. 14-0688).^28,29 The major characteristic XRD peaks of pure BiVO₄ and Ag₂O/BiVO₄ heterostructures detected at 2θ values of 18.67°, 28.82°, 30.55°, 34.49°, 35.22°, 39.78°, 42.46°, 45.43°, 46.71°, 49.96°, 53.32°, 55.27° and 59.26° could be indexed as (110), (011), (121), (040), (200), (002), (211), (051), (231), (240), (202), (161), (321) and (123) respectively. The diffraction peaks at 2θ values of 26.77°, 32.79°, 38.11°, 55.02° and 65.56° belong to the (110), (111), (200), (220) and (311) planes of cubic Ag₂O with JCPDS File No. 41-1104.²⁵ The XRD patterns of the Ag₂O/BiVO₄ composites possessed the characteristic peaks of both Ag₂O and BiVO₄ crystalline phases. From Fig. 1A, the intensity of the diffraction peaks of Ag₂O in the composites becomes weaker and the level of weakening was in accordance with the reduction of Ag₂O dosage. A high degree of crystallinity was achieved and there was no detectable peaks ascribable to the impurities, implying that a high purity of Ag₂O and BiVO₄ was obtained. Furthermore, the XRD pattern comparison of the fresh and used Ag₂O/BiVO₄ (1 : 10) composite is given in Fig. 1B; the crystal structure of the catalyst remained unchanged, and only a small peak of metallic Ag was discovered in the used samples. The XRD pattern of the first cycle was similar to that of the fourth cycle, confirming the stable construction of a Ag–Ag₂O bridge in the composite and leading to a stable Z-scheme photocatalyst Ag₂O@Ag@BiVO₄.

Fig. 1 XRD patterns of pure BiVO₄, pure Ag₂O and Ag₂O/BiVO₄ composite photocatalysts with different weight ratios (A), XRD patterns of the fresh and used Ag₂O/BiVO₄ (1 : 10) (B), and XRD patterns at different calcination temperatures (C).

3.1.2 SEM and TEM analysis. The representative SEM images of the as-prepared BiVO₄, Ag₂O and Ag₂O/BiVO₄ composites are presented in Fig. 2. Fig. 2A and B show the morphology of Ag₂O obtained by the precipitation method and the corresponding sizes were about 100–500 nm. Both BiVO₄ and Ag₂O/BiVO₄ are presented as 3D acicular sheaf-shaped structures (Fig. 2C–F). Pure BiVO₄ had uniform size and shape distributions. For the Ag₂O/BiVO₄ composite, the general sheaf-shaped morphology could be also observed, while the surfaces of the catalysts were rough providing numerous nucleation sites for the growth of Ag₂O. Fig. 2F shows clearly that Ag₂O nanoparticles dispersed homogeneously on the surface of BiVO₄. The co-existence of Ag₂O and BiVO₄ in the forming heterojunction structure is proven by the TEM images for pure BiVO₄ (Fig. 3A and C) and Ag₂O/BiVO₄ (1 : 10) (Fig. 3B, D and E). As depicted in Fig. 3A–D, BiVO₄ should be responsible for the emergence of a sheaf-like structure, and a large amount of small nanoparticles on the surface of the Ag₂O/BiVO₄ composite should be Ag₂O. Fig. 3F is a representative HRTEM image of Ag₂O/BiVO₄ (1 : 10); the clear lattice fringes present are indicative of high crystallinity. The interplanar spacings of 0.475 nm and 0.309 nm are clearly observed, which should correspond to the (110) and (121) crystallographic planes of BiVO₄ (JCPDS File No. 14-0688). The d-spacing measured to be 0.272 nm was in good consistency with the lattice spacing (111) of cubic Ag₂O (JCPDS File No. 41-1104).

Fig. 2 SEM images of the samples: pure BiVO₄ (A and B), pure Ag₂O (C and D), and Ag₂O/BiVO₄ (1 : 10) (E and F).

Fig. 3 TEM images of the samples: BiVO₄ (A and C) and Ag₂O/BiVO₄ (1 : 10) (B, D and E), and HRTEM image of Ag₂O/BiVO₄ (1 : 10) (F).

3.1.3 FTIR and XPS analysis. The components of the as-prepared composite catalysts were confirmed by FTIR analysis. Fig. 4A shows the FTIR spectra of the Ag₂O/BiVO₄ heterostructures with different weight ratios of Ag₂O. For pure Ag₂O, two bands at 3410 and 1671 cm⁻¹ could be ascribed to O–H stretching and deformation vibration of physisorbed and/or chemisorbed water molecules,²⁷ while the peak at 1380 cm⁻¹ [F–Ag] belongs to the H–O–H bending vibration of the absorbed water molecules on the surface.³⁰ The band at about 600 cm⁻¹ could be assigned to Ag–O, which was achieved by analysis with Ag₂O and the Ag₂O/BiVO₄ composites.²⁷ The intensity of Ag–O in the Ag₂O/BiVO₄ composites became weaker accompanied with the reduction of the Ag₂O content. Moreover, the broad band at 700–850 cm⁻¹ for both the pure BiVO₄ and Ag₂O/BiVO₄ heterojunctions [F–Bi] is considered as the bending vibration of VO₄³⁻.³¹

Fig. 4 FTIR spectra of the samples: pure BiVO₄, pure Ag₂O and Ag₂O/BiVO₄ composites (A); XPS spectra of Ag₂O/BiVO₄ (1 : 10) (B–F). ​(Additional information: in Fig. 4A, a–f represented for Ag₂O/BiVO₄ (1 : 20), Ag₂O/BiVO₄ (1 : 10), Ag₂O/BiVO₄ (1 : 5), Ag₂O/BiVO₄ (1 : 1), pure BiVO₄ and Ag₂O, respectively.)

XPS measurements were carried out to analyze the surface chemical states and the bonding modality among the atoms of the Ag₂O/BiVO₄ composites. Fig. 4B shows the XPS survey spectra of pure BiVO₄ and Ag₂O/BiVO₄ (1 : 10). The co-existence of C, Bi, V and O elements for pure BiVO₄ could be observed, while another Ag element was discovered in Ag₂O/BiVO₄ (1 : 10) without any other impurities. The emergence of C (the peak at 284.57 eV) could be assigned to the adventitious hydrocarbon from the XPS instrument itself.²⁰ The peaks located at 159.44 eV and 164.68 eV belong to Bi 4f_7/2 and Bi 4f_5/2, respectively (Fig. 4C).³² Two main peaks appearing at the binding energies of 517.02 eV and 524.18 eV correspond to V 2p_1/2 and V 2p_3/2, respectively (Fig. 4D).³³ Typical O 1s spectra of pure BiVO₄ and Ag₂O/BiVO₄ (1 : 10) exhibit the predominant characteristic peaks at 530.2 eV (Fig. 4E). The peaks at 368.1 eV and 374.1 eV in Ag₂O (1 : 10) revealed the presence of Ag⁺, which are homologous to the Ag 3d_5/2 and Ag 3d_3/2 binding energies (Fig. 4F). This suggests the existence of a Ag₂O phase in the composite, which is in good agreement with the XRD and FTIR analysis.

3.1.4 UV-vis analysis. The UV-vis diffuse absorption spectra of the as-prepared samples were measured using a spectrophotometer, as shown in Fig. 5. As for the pure BiVO₄ composite, only the band gap absorption with an absorption edge at ca. 517 nm could be observed. Accompanied by the introduction of Ag₂O nanoparticles, the obtained Ag₂O/BiVO₄ heterojunction exhibited a broader visible light absorption in the range of 500–700 nm and the intensity strengthened with the increase of the Ag₂O content. The optical band gap for semiconductor photocatalysts could be estimated using the following equation:^13,19

(Ahv) = α(hv − E_g)^n/2 (1)

where α, v, E_g and A are the absorption coefficient, light frequency, band gap energy, and a constant, respectively. n is determined by the type of optical transition of a semiconductor (n = 1 for direct transition and n = 4 for indirect transition). The n value for pure BiVO₄ is 4, and the corresponding energy band gap was measured and estimated to be about 2.4 eV.

Fig. 5 UV spectra of pure BiVO₄, pure Ag₂O and Ag₂O/BiVO₄ composites (A), and plot of (Ahv)² versus hv for pure BiVO₄ (B).

3.2. Photocatalytic activity study

3.2.1 Effect of Ag₂O content. The photocatalytic activities of the as-synthesized samples were evaluated through the decomposition of RhB, MB and RhB + MB under visible light irradiation. Fig. 6A and B show the photodegradation of RhB (10 mg L⁻¹) and MB (20 mg L⁻¹), respectively with different weight ratios of Ag₂O to BiVO₄. It could be perceived that there was a slight loss in the dye concentration after 30 min treatment in the dark, verifying the weak absorption ability of the catalysts for the dye. The photocatalytic performance of pure BiVO₄ was poor while it was greatly enhanced with the introduction of Ag₂O. RhB and MB were completely decolourized in 15 and 48 min by Ag₂O : BiVO₄ (1 : 10), while for pure BiVO₄ only 12.9% of RhB and 7.3% of MB could be degraded within the same irradiation time. However, different weight ratios of Ag₂O to BiVO₄ showed different degradation rates for RhB and MB, which were in the order of Ag₂O : BiVO₄ (1 : 10) > Ag₂O : BiVO₄ (1 : 5) > Ag₂O > Ag₂O : BiVO₄ (1 : 20) > Ag₂O : BiVO₄ (1 : 1) > BiVO₄. So the optimized weight ratio of Ag₂O in the Ag₂O/BiVO₄ heterostructures should be 1 : 10. These results suggested that a feasible mass ratio of Ag₂O in the constructed heterojunction was good for the enhancement of photoactivity. Excess Ag₂O deposited on the BiVO₄ surface would increase the recombination rate of photogenerated electron–hole pairs, leading to a poorer degradation efficiency. Wan and Zhang also found that excess AgI deposited on the surface of Bi₂SiO₅ would decrease the photocatalytic activities of Bi₂SiO₅/AgI nanoplates.¹²

Fig. 6 Photodegradation of RhB and MB over Ag₂O/BiVO₄ (1 : 10) composite with different weight ratios (A and B) (experimental conditions: initial RhB concentration: 10 mg L⁻¹, initial MB concentration: 20 mg L⁻¹, pH = 6.3, m/V = 0.600 g L⁻¹, visible light: λ > 420 nm) and initial dye concentrations (C and D) (experimental conditions: pH = 6.3, m/V = 0.600 g L⁻¹, visible light: λ > 420 nm).

3.2.2 Effect of initial dye concentration. Fig. 6C and D display the RhB and MB degradation with different initial dye concentrations using Ag₂O : BiVO₄ (1 : 10). As shown in Fig. 6C, the RhB degradation efficiency decreased with the increase of initial dye concentration in the investigated range (10 to 40 mg L⁻¹). When the initial RhB concentration was below 30 mg L⁻¹, the degradation efficiency maintained a high level (over 90%), but observably declined to 42.1% at 40 mg L⁻¹. A similar variation also appeared in the MB photocatalytic degradation process (Fig. 6D). Higher initial pollutant concentrations could affect the light penetration through the solution in the photocatalytic process.³⁴ The reduced light penetration at a high initial concentration solution resulted in less arrival of photons to the catalyst surface. Meanwhile dye molecules and the produced intermediates tended to compete with each other for initial absorption and catalytic sites on the Ag₂O/BiVO₄ surface. Many studies validated that the dye degradation process was restricted for the formed intermediates during the photocatalytic oxidation process.^35,36 So more production of intermediates in the photocatalytic process would in turn aggravate the competitive adsorption on the catalyst surface between the dye compound and intermediates under high dye concentration conditions.

3.2.3 Effect of different calcination temperatures. The effect of calcination temperature on the photocatalytic activity of the as-prepared Ag₂O : BiVO₄ (1 : 10) was investigated and the results are exhibited in Fig. 7A and B. With the calcination temperature increasing from 60 to 200 °C, the photocatalytic activity of the as-prepared Ag₂O : BiVO₄ (1 : 10) for the decomposition of dye deteriorated. The degradation efficiency of RhB and MB was only 25.8% and 15.2% when the calcination temperature went up to 300 °C. The reason should be attributed to the destruction of the Ag₂O/BiVO₄ heterojunction structure. Yu et al. found that Ag₂O in Ag₂O/Bi₂WO₆ decomposed to form Ag nanoparticles at a calcination temperature of 450 °C.²⁵ The XRD patterns of Ag₂O : BiVO₄ (1 : 10) calcined at 300 °C also demonstrate that almost all of the Ag₂O was decomposed and formed metallic Ag (Fig. 1C). Ag-modified BiVO₄ exhibited a slightly better photocatalytic performance than that of pure BiVO₄, because the electron–hole separation rate was also improved by introducing metallic Ag.²⁵ The excessive Ag⁰ would lower the photocatalytic activity of Ag₂O/BiVO₄. In order to maintain the 3D sheaf-like heterojunctions of Ag₂O/BiVO₄, the calcination temperature should be controlled at a relatively lower level.

Fig. 7 Photodegradation of RhB and MB over Ag₂O/BiVO₄ (1 : 10) composite with different calcination temperatures (A and B) (experimental conditions: initial RhB concentration: 10 mg L⁻¹, initial MB concentration: 20 mg L⁻¹, pH = 6.3, m/V = 0.600 g L⁻¹, visible light: λ > 420 nm), and with different initial pH (experimental conditions: initial RhB concentration: 10 mg L⁻¹, initial MB concentration: 20 mg L⁻¹, m/V = 0.600 g L⁻¹, visible light: λ > 420 nm).

3.2.4 Effect of initial pH. The initial pH is an important parameter for controlling the photocatalytic process. As displayed in Fig. 7C and D, a series of pH was controlled by NaOH (0.1 M) or HNO₃ (0.1 M) to investigate the effect of initial pH in the photocatalytic process. The highest photodegradation efficiencies of RhB and MB could be achieved at an initial pH of 6.3. The pH of an aqueous medium can influence the photocatalytic process through absorption between dyes and the catalyst surface and redox processes of photocatalysts. The optimum photodegradation efficiencies of RhB and MB appeared at nearly neutral, and the photocatalytic activity was depressed under strong acid and alkali conditions, especially in an acidic environment. When the initial pH was below 6, the surface charge of the as-prepared catalysts became increasingly positive as the pH decreased resulting in increasingly stronger electrostatic repulsion with dye molecules. However with an initial pH of 8–10, the negatively charged photocatalyst surface prevented the sorption of hydroxide ions, thus reducing the formation of hydroxyl radicals and the photo-oxidation efficiency.^18,29 Herein, the optimum pH value for both RhB and MB degradation was 6.3.

3.2.5 Degradation and mineralization of mixed dyes. It is well known that real-world wastewater from dye industries usually is various organic dye mixtures.^37,38 Thus it is meaningful to investigate the superior photocatalytic performance of the Ag₂O/BiVO₄ heterojunction for mixed dye wastewater. The experiments were carried out in RhB (5 mg L⁻¹) + MB (10 mg L⁻¹) and RhB (10 mg L⁻¹) + MB (20 mg L⁻¹) aqueous solution in the presence of Ag₂O/BiVO₄ (1 : 10). Compared with single RhB or MB degradation, the degradation efficiencies of a mixture of dyes descended to some extent. As exhibited in Fig. 8A and B, RhB and MB were completely degraded in 20 and 48 min, respectively, while in Fig. 8C and D, we could find that MB achieved a high decomposition rate in both mixed dye solutions and the RhB degradation was significantly hindered, especially at high initial concentrations. As displayed in Fig. 8E, the degradation rate of MB was much faster than that of RhB, which could be explained by the fact that MB possessed better affinity to the catalyst than RhB did, and good absorption resulted in the promotion of dye photodegradation. There is usually a competition mode between two cationic dyes in the photodegradation process of a mixture of dyes, which also was proven in the visible light-driven degradation of RhB and MB catalyzed by InVO₄–BiVO₄ and its supported noble metal nanoparticles.³⁹ Total organic carbon (TOC) analysis is an efficient way to evaluate the degradation degree of organic dyes, which requires a long irradiation time.⁴⁰ Fig. 8F presents the TOC removal in single and mixed dye degradation. The TOC removal for single RhB and MB degradation reached 90.2% and 93.2%. However, the corresponding values for mixed dyes obviously decreased and the TOC removal for the RhB (10 mg L⁻¹) + MB (20 mg L⁻¹) solution was only 32.5%, which should be attributed to the high initial dye concentration. The above information suggested that the TOC removal in single dye degradation was more thorough than that in the mixture of dyes. Meanwhile, it could be noticed that although the whole degradation time in RhB (5 mg L⁻¹) + MB (10 mg L⁻¹) solution was longer than that of any single dye, the final TOC removal could reach 88.2% and was desirable. The relatively high TOC removal was acquired in both single and mixed dye degradation, confirming that the Ag₂O/BiVO₄ heterojunction possessed a preferable mineralization ability.

Fig. 8 The temporal absorption spectrum changes of RhB (A), MB (B), and RhB + MB (C and D) aqueous solutions, and the photodegradation of RhB + MB mixture (E) and TOC removal (F) in the presence of the Ag₂O/BiVO₄ (1 : 10) composite under visible light irradiation.

3.3. Photocatalytic reaction kinetics

Moreover, the photocatalytic decomposition of RhB, MB and RhB + MB in this study followed the pseudo-first-order kinetics model. The rate constant (k_app) could be calculated by the following equation:⁴¹

ln(C_t/C₀) = −k_appt (2)

where C₀ and C_t are the concentration of the dye at time 0 and t, respectively (mg L⁻¹), and k_app represents the rate constant (min⁻¹). The kinetic plots of the different samples are displayed in Fig. 9A–C. The corresponding plots of −ln(C_t/C₀) ∼ t exhibited a good linearity. Table 1 summarizes the kinetic parameters of the samples occurring in the photocatalytic process. High correlation coefficients (R² > 0.98) indicated that the dye photodegradation catalyzed by Ag₂O/BiVO₄ followed a pseudo-first-order reaction. Based on the rate constant (k_app), the photocatalytic performance of BiVO₄ was greatly enhanced with the introduction of Ag₂O, in which Ag₂O/BiVO₄ (1 : 10) presented the highest photocatalytic activity. The corresponding k_app was 0.5208 and 0.1008 min⁻¹ for RhB and MB degradation, which was 49/2.5 and 72/2.2 times higher than that of pure BiVO₄ and Ag₂O, respectively. This result demonstrated the advantage of building 3D Ag₂O/BiVO₄ heterojunctions. Furthermore, compared with the single dye degradation, the value of k_app for RhB (or MB) deceased to 0.0222 (0.0684) and 0.0038 (0.0432) min⁻¹ in RhB (5 mg L⁻¹) + MB (10 mg L⁻¹) and RhB (10 mg L⁻¹) + MB (20 mg L⁻¹) solutions. Obviously, RhB degradation was more affected than MB in the mixed dye solution. This phenomenon further revealed that a competitive mode existed in the multi-component dye photodegradation process. The discrepancy in the absorption ability between RhB and MB and the advantages of good absorption served as a promoter for dye degradation.³⁹

Fig. 9 Kinetic plots and the recycling efficiencies for dye degradation: RhB (A and D), MB (B and E), and RhB + MB (C and F).

Table 1 Correlation coefficients and rate constants for the degradation of dyes in the different photocatalytic systems

Catalyst	RhB		MB
	kapp (min^−1)	R^2	kapp (min^−1)	R^2
a For the degradation of RhB + MB (5 mg L^−1 + 5 mg L^−1).b For the degradation of RhB + MB (10 mg L^−1 + 10 mg L^−1).
BiVO4	0.0106	0.9982	0.0014	0.9958
Ag2O	0.2101	0.9986	0.0465	0.9969
1 : 1	0.0344	0.9879	0.0039	0.9905
1 : 5	0.2931	0.9901	0.0579	0.9904
1 : 10	0.5208	0.9806	0.1008	0.9899
1 : 10^a	0.0222	0.9801	0.0684	0.9821
1 : 10^b	0.0038	0.9821	0.0432	0.9919
1 : 20	0.2448	0.9901	0.0433	0.9958

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3.4. Recyclability and reusability of the photocatalysts

In addition, the stability and recyclability of the photocatalysts are also crucial issues in their practical applications. The photodegradation of RhB, MB and RhB + MB using Ag₂O/BiVO₄ (1 : 10) was investigated for four consecutive runs under identical experimental conditions. As shown in Fig. 9D–F, after four successive cycles, the photocatalytic activity for both single and mixed organic dye degradation had no obvious loss. The XPS spectra of the fresh and used Ag₂O/BiVO₄ (1 : 10) (Fig. 10A and B) reveal that there were two peaks at 374.1 eV and 368.1 eV, which belonged to Ag⁺ in the composite. Another two peaks appeared at 373.2 eV and 367.3 eV which could be attributed to metallic Ag (Ag⁰ (ref. 10 and 33)), which is in accordance with the description in the XRD analysis. The used samples for the first and fourth cycle presented a similar structure and intensity in their XRD patterns, confirming the desirable stability of the as-prepared Z-scheme photocatalyst. The above-mentioned information indicates the excellent reusability performance longevity of Ag₂O@Ag@BiVO₄ under visible light irradiation, which makes it a promising candidate in practical composite wastewater treatment.

Fig. 10 The XPS spectra of the fresh and used Ag₂O/BiVO₄ (1 : 10) (A and B) and the effects of different scavengers on the degradation of RhB (C) and MB (D) over Ag₂O/BiVO₄ (1 : 10) under visible light irradiation.

3.5. Photocatalytic mechanism

It is generally recognized that photo-induced reactive species including trapped holes (h⁺), hydroxyl radicals (˙OH) and superoxide radicals (˙O²⁻) are expected to be involved in the photocatalytic process. In order to reveal the photocatalytic mechanism more clearly, the main oxidative species produced in the photocatalytic process were detected through trapping experiments using benzoquinone (BQ), triethanolamine (TEOA) and isopropanol (IPA) as the scavengers of ˙O₂⁻,⁴² h⁺,⁴³ and ˙OH,⁴⁴ respectively. The results are illustrated in Fig. 10C and D and Table 2, and the degradation efficiency of RhB or MB was hardly inhibited with the addition of IPA, implying that ˙OH was not mainly produced in the photocatalytic system of Ag₂O@Ag@BiVO₄. On the contrary, the degradation efficiencies of RhB changed from 99.80% to 3.12% and 26.55% in the presence of TEOA and BQ, respectively. The same variation happened in MB degradation, and the whole degradation process was almost inhibited and the decomposition rate remarkably decreased with the addition of TEOA and BQ. It could be elucidated that ˙O₂⁻ and h⁺ were the dominant active species generated in the photocatalytic process, which should be responsible for the highly enhanced photocatalytic performance for dye degradation.

Table 2 Degradation efficiencies of RhB or MB in the presence of different scavenger

Scavenger	RhB		MB
	Initial	Final	Initial	Final
No scavenger	99.80%	99.80%	99.52%	99.51%
IPA	99.80%	99.51%	99.51%	96.32%
BQ	99.80%	26.55%	99.51%	11.68%
TEOA	99.80%	3.12%	99.51%	1.59%

---

The possible transfer routes of photo-excited electrons and holes were employed to explain the enhanced photocatalytic activity based on the conduction band (CB) and valence band (VB) potentials of Ag₂O and BiVO₄. The band energy (CB and VB) positions of each semiconductor could be calculated by Mulliken electronegativity theory as exemplified below:^19,33

E_CB = X − E_c − 0.5E_g (3)

E_VB = E_CB + E_g (4)

where X is the electronegativity of the semiconductor, E_c is the energy of free electrons on the hydrogen scale (∼4.5 eV), and the CB and VB potentials of Ag₂O are approximately +0.2 eV and +1.4 eV, respectively.²⁵ The CB and VB potentials of BiVO₄ are estimated to be +0.46 eV and +2.86 eV, respectively.

It is generally acknowledged that the photocatalytic reaction is completed by the collection of efficient charge carrier trapping, migration and transfer. A low PL intensity is indicative of a high separation efficiency of electron–hole pairs, in other words, a good photocatalyst should have a minimum electron–hole recombination. The corresponding PL spectra of BiVO₄, Ag₂O/BiVO₄ (1 : 10) and Ag₂O/BiVO₄ (1 : 1) are clearly revealed in Fig. 11A. From Fig. 11A, the PL intensity of the Ag₂O/BiVO₄ composites is significantly reduced as compared to that of pure BiVO₄, implying a lower recombination feasibility of free charges in the Ag₂O/BiVO₄ heterostructures. Moreover, the lower PL intensity achieved by the fresh Ag₂O/BiVO₄ (1 : 10) composite suggested that photo-induced electron–hole recombination processes were truly hindered. Notably, the used Ag₂O/BiVO₄ (1 : 10) (self-built Z-scheme photocatalyst Ag₂O@Ag@BiVO₄) exhibited the lowest recombination probability of electron–hole pairs, which could be assigned to the formed metallic Ag, further indicating that a Z-scheme system was constructed under visible light.

Fig. 11 The PL spectra (A), PT spectra (B) and EIS analysis (C and D) of the samples: BiVO₄, Ag₂O/BiVO₄ (1 : 10) and Ag₂O/BiVO₄ (1 : 1).

The effect of the Ag₂O/BiVO₄ (1 : 10) composite on the separation efficiency of electron–hole pairs was investigated by photocurrent–time (PT) experiments, as demonstrated in Fig. 11B. The transient photocurrent responses of pure BiVO₄, Ag₂O/BiVO₄ (1 : 10) and Ag₂O/BiVO₄ (1 : 1) were recorded for several on–off cycles under visible light irradiation. For pure BiVO₄, a small amount of photocurrent was generated while for Ag₂O/BiVO₄ composites, a significantly enhanced photocurrent intensity was detected. Especially, Ag₂O/BiVO₄ (1 : 10) possessed the highest current intensity, suggesting more efficient separation of the photogenerated electron–hole pairs over Ag₂O/BiVO₄ (1 : 10). Meanwhile, the above photocurrent order was in good agreement with the result of the photocatalytic activity. The results indicated that there existed good energy band matching in the formation of Ag₂O/BiVO₄ heterojunctions, due to highly efficient electron–hole separation at the interface.

Electrochemical impedance spectroscopy (EIS) was employed to signify the transfer and migration processes of the photo-excited electron–hole pairs in the photodegradation process. Based on previous studies,^45,46 the corresponding radius of the arc reflects the interfacial layer resistance at the electrode surface, and a smaller arc radius indicates higher efficiency in charge transfer. Under the same light irradiation conditions (Fig. 11C), the diameter of the arc radius of the Ag₂O/BiVO₄ composites was much smaller than that of pure BiVO₄. In particular, for Ag₂O/BiVO₄ (1 : 10), the smallest arc radius was acquired, indicating the highest separation of photogenerated carriers over Ag₂O/BiVO₄ (1 : 10). Moreover, the EIS measurements of Ag₂O/BiVO₄ (1 : 10) which were either under light irradiation or dark conditions were also investigated for the produced photogenerated carriers. As displayed in Fig. 11D, a smaller arc radius was achieved for Ag₂O/BiVO₄ (1 : 10) under light irradiation, which further verifies the more efficient generation of photogenerated carriers over Ag₂O/BiVO₄ (1 : 10). In all, the formed p–n heterojunction facilitated migration of charge carriers among the constructed Ag₂O/BiVO₄ systems.

Based on the above results and analysis, the possible mechanism, energy bands and charge separation of the Ag₂O/BiVO₄ heterostructures are proposed as illustrated in Scheme 3. Both BiVO₄ and Ag₂O could be excited under visible light illumination and produced photogenerated electrons and holes. Since the conduction band (CB) position of Ag₂O was lower than that of BiVO₄, the photogenerated electrons on the CB of Ag₂O could directly transfer to the CB of BiVO₄. Consequently, the accumulated electrons in the CB of BiVO₄ could capture the dissolved O₂ and induce it to form ˙O₂⁻. Subsequently, dye molecules can be degraded by an oxidation reaction with the formed ˙O₂⁻ on the catalyst surface. Meanwhile, the corresponding valence band (VB) position of BiVO₄ is more negative than that of Ag₂O, and the photogenerated holes on the VB of BiVO₄ could spontaneously migrate to the VB of Ag₂O.^14,25 The holes on the VB of Ag₂O could directly degrade the organic compounds under visible light irradiation. The photo-excited electrons were mainly collected by BiVO₄ and holes were accumulated by Ag₂O, which were finally consumed by the photo-oxidation process. Unlike the conventional mechanism for a two-phase heterojunction under visible light irradiation, the generated metallic Ag immediately constructed a cross-linking bridge for two semiconductors,²⁷ thus a novel Z-scheme heterostructured photocatalyst Ag₂O@Ag@BiVO₄ was constructed. Similar results have also been confirmed by previous reports, where a Z-scheme system could be generated in Ag compound-based photocatalysts.⁴⁷ For instance, He et al. discovered that a small amount of metallic Ag could be formed under visible light irradiation and a Z-scheme mechanism would work in the Ag₃PO₄/g-C₃N₄ hybrid due to their matching band gaps and positions of Ag₃PO₄ and g-C₃N₄.⁴⁸ The recombination of electrons from BiVO₄ and holes from the VB of Ag₂O on the Ag surface may strongly increase the electron–hole lifetime in each material. This could bring about the activation of two substrates towards electron-rich and electron-deficient intermediates, respectively, therefore the recombination process of electron–hole pairs could be hampered leading to effective charge separation. The high reducing electrons located on the CB bottom of Ag₂O will react with molecular oxygen to form ˙O₂⁻ that could further oxidize dyes. Simultaneously, the holes situated in the VB of BiVO₄ will photocatalytically oxidize dyes directly due to a high positive potential of the E_VB of BiVO₄. The holes could also oxidize water molecules to form ˙OH active species (E⁰ for ˙OH/OH⁻ = 2.38 eV vs. NHE). But the slight amount of the formation of ˙OH active species only participated in the photocatalytic degradation of dyes. The well-matched overlapping band structures and closely contacting interfaces between BiVO₄ and Ag₂O as well as the high-speed separation of the photogenerated electron–hole pairs across the interconnection of the constructed Z-scheme heterojunction were of utmost importance for the enhanced photocatalytic activity under visible light irradiation.

Scheme 3 A schematic illustration of the photocatalytic mechanism of dye degradation over Ag₂O@Ag@BiVO₄ composite photocatalysts under visible light irradiation.

The primary reactive species of Ag₂O/BiVO₄ could be summarized as follows:

Ag₂O + hv → Ag₂O (h⁺) + Ag₂O (e⁻) (5)

BiVO₄ + hv → BiVO₄ (h⁺) + BiVO₄ (e⁻) (6)

Ag₂O (e⁻) + BiVO₄ → Ag₂O + BiVO₄ (e⁻) (7)

BiVO₄ (h⁺) + Ag₂O → BiVO₄ + Ag₂O (h⁺) (8)

BiVO₄ (e⁻) + O₂ → BiVO₄ + ˙O₂⁻ (9)

˙O₂⁻ + organic pollutant → CO₂ + H₂O (10)

h⁺ + organic pollutant → CO₂ + H₂O (11)

The major reactions for when metallic Ag is produced in the photocatalytic process are listed as below:

Ag₂O + hv → Ag₂O (h⁺) + Ag₂O (e⁻) (12)

BiVO₄ + hv → BiVO₄ (h⁺) + BiVO₄ (e⁻) (13)

BiVO₄ (e⁻) + Ag → Ag (e⁻) + BiVO₄ (14)

Ag₂O (h⁺) + Ag → Ag (h⁺) + Ag₂O (15)

e⁻ + h⁺ → heat (16)

Ag₂O (e⁻) + O₂ → ˙O₂⁻ + Ag₂O (17)

˙O₂⁻ + h⁺ + organic pollutant → CO₂ + H₂O (18)

4. Conclusions

In summary, Ag₂O@Ag@BiVO₄ heterostructures were prepared using a facile pH-mediated chemical precipitation method along with a self-built Z-scheme under visible light. The as-synthesized samples exhibited much superior photocatalytic activities compared to pure Ag₂O and BiVO₄ in the degradation of single organic dye (RhB or MB) and multi-component organic dyes (RhB + MB). The optimum weight ratio of Ag₂O in the Ag₂O/BiVO₄ composite was found to be 1 : 10 under identical experimental conditions. The effect of initial dye concentration, calcination temperature, and initial pH on the photodegradation of dye TOC removal was investigated in detail. Both the single and mixed dye degradation matched well with first-order kinetics reactions. The p–n 3D acicular sheaf-shaped heterojunction combined with the Ag–Ag₂O self-stability structure ensured the excellent photocatalytic activity, mineralization ability and high stability. The superior photoelectric properties of the Ag₂O@Ag@BiVO₄ heterostructures ensured a promising application in energy conservation. A Z-scheme charge transfer mechanism in the constructed heterojunction was presumed and interpreted in detail. This work may provide a research strategy to design more heterojunction photocatalysts in the visible light degradation of dye wastewater, with further extension to multiple contaminant treatment.

Acknowledgements

This research was financially supported by the project of National Natural Science Foundation of China (NSFC) (No. 51278175, 51378188, 51478170) and Doctoral Fund of Ministry of Education of China (20130161120021).

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