Hierarchical heterostructures of Bi2MoO6 microflowers decorated with Ag2CO3 nanoparticles for efficient visible-light-driven photocatalytic removal of toxic pollutants

Abstract

Developing highly active and durable visible-light-driven photocatalysts for the degradation of toxic pollutants is of vital significance. Herein, Ag₂CO₃ nanoparticles were in situ formed on Bi₂MoO₆ microflowers to produce Ag₂CO₃/Bi₂MoO₆ heterostructures via a facile procedure. The morphologies, phases, chemical compositions, and optical properties of Ag₂CO₃/Bi₂MoO₆ were examined by multiple characterization techniques. The Ag₂CO₃/Bi₂MoO₆ heterostructures exhibited substantially improved performance in the removal of industrial dyes (rhodamine B (RhB), methyl orange (MO), and methyl blue (MB)), and the antibiotic tetracycline hydrochloride (TC), compared with bare Bi₂MoO₆ and Ag₂CO₃ under visible-light irradiation. The enhancement of activity was attributed to the high charge-separation capacity, which results from the matched band alignment of the two components. The cycling experiments showed a good durability of Ag₂CO₃/Bi₂MoO₆. Holes were found to be the dominant active species accounting for the pollutant degradation. This compound is a promising candidate for wastewater treatment.

Introduction

Industrial pollutants, such as industrial dyes and antibiotics, in wastewaters pose a huge threat to the environment [1–2]. Thus, many methods for pollutant removal have been established. However, the conventional wastewater treatments are usually accompanied by high cost, low efficiency and other insufficiencies [2]. The decomposition and mineralization of pollutants under sunlight through photocatalysis has been demonstrated to be an effective and green technology for environmental remediation [3–6]. Crucial to photocatalysis is to obtain high-performance photocatalysts [7–8]. Obtaining excellent photocatalysts that can be excited by visible light (43% of the solar energy spectrum) is very important for practical applications [9–14].

Bi₂MoO₆ has been regarded as a promising visible-light-driven (VLD) photocatalyst because of its good activity, chemical stability and nontoxicity [15–17]. However, the low carrier-separation rate and narrow photo-response range of Bi₂MoO₆ substantially lower its photocatalytic performance [18–19]. To overcome this obstacle, various methods have been developed, including doping [20–21] and the construction of heterojunctions [22–33]. Particularly, the combination of Bi₂MoO₆ with other semiconductors to construct heterojunction photocatalysts leads to an enhanced activity of Bi₂MoO₆, which originates from the increased charge separation at the interface [22–30].

Recently, Ag-based compounds (e.g., Ag₃PO₄, Ag₃VO₄, Ag₂CrO₄, and Ag₂CO₃) [34–36] have emerged as good VLD photocatalysts for pollutant removal. Ag₂CO₃ exhibits a high visible-light photocatalytic activity [37]. However, it is unstable under illumination. Previous studies found that the good stability of Ag₂CO₃ could be achieved through the rational construction of heterojunctions, such as Ag₂CO₃/Bi₂WO₆ [37], Ag₂O/Ag₂CO₃ [38], Ag₂CO₃/Bi₂O₂CO₃ [39], Ag₂CO₃/C₃N₄ [40], Ag/Ag₂CO₃/BiVO₄ [41] Ag₂CO₃/AgBr/ZnO [42], and Ag/Ag₂CO₃/Bi2MoO₆ [32]. The band structure of Ag₂CO₃ matches well with that of Bi₂MoO₆ [32]. Moreover, morphology modulation is another significant way to enhance photocatalytic activity. Three-dimensional nanostructures endow materials with unique physicochemical properties, for instance, high specific surface area, good molecular diffusion/transport, and good recyclability and light harvesting ability. To the best of our knowledge, application of Ag₂CO₃ nanoparticles coupled with flower-like Bi₂MoO₆ for photocatalytic degradation of toxic pollutants remains unreported.

Herein, we synthesized flower-like Ag₂CO₃/Bi₂MoO₆ heterostructures, in which Ag₂CO₃ nanoparticles were evenly anchored on Bi₂MoO₆ microflowers to construct novel hierarchical heterojunction photocatalysts by via in situ precipitation. The photocatalytic properties of Ag₂CO₃/Bi₂MoO₆ was measured regarding the photocatalytic degradation of industrial dyes (rhodamine B (RhB), methyl orange (MO), and methyl blue (MB)), and the antibiotic tetracycline hydrochloride (TC) under visible light. The improved performance of the photocatalytic degradation was prominent, and the reasons were rationally analyzed. Also, the photocatalytic mechanism of pollutant degradation over Ag₂CO₃/Bi₂MoO₆ was discussed.

Results and Discussion

Characterization of catalysts

A series of flowerlike Ag₂CO₃/Bi₂MoO₆ heterostructures with different weight ratios (0.1/1, 0.2/1, 0.3/1, and 0.5/1) were constructed and labeled ACO/BMO-10, ACO/BMO-20, ACO/BMO-30, and ACO/BMO-50, respectively. The crystal structure of Bi₂MoO₆, Ag₂CO₃, and Ag₂CO₃/Bi₂MoO₆ heterostructures were determined by XRD technique (Fig. 1). The diffraction peaks of Ag₂CO₃ and Bi₂MoO₆ were indexed as orthorhombic Bi₂MoO₆ (JCPDS 76-2388) and monoclinic Ag₂CO₃ (JCPDS 26-0399), respectively. The XRD pattern of these heterostructures show the characteristic peaks of both Ag₂CO₃ and Bi₂MoO₆, indicating the successful synthesis of Ag₂CO₃/Bi₂MoO₆ heterostructures. In another publication, Ag/Ag₂CO₃/Bi₂MoO₆ nanoplates, composed of three phases, have been described [32].

Figure 1

XRD patterns of Ag₂CO₃/Bi₂MoO₆ heterojunctions, bare Bi₂MoO₆ and Ag₂CO₃.

To visually study the microstructure and morphology of Ag₂CO₃/Bi₂MoO₆, SEM images of the as-prepared catalysts were taken. Bare Bi₂MoO₆ presents a hierarchical microsphere structure (diameter: 1.6–3.5 μm, Fig. 2,b). After Ag₂CO₃ was loaded onto Bi₂MoO₆, the resulting Ag₂CO₃/Bi₂MoO₆ retained the flower-like architecture. The representative ACO/BMO-30 displays the flower-like structure, the surface of which is decorated with Ag₂CO₃ nanoparticles (size: 10–50 nm, Fig. 2,d). In contrast, the previously reported Ag/Ag₂CO₃/Bi₂MoO₆ is composed of Bi₂MoO₆ nanoplates and Ag/Ag₂CO₃ nanorods/nanoparticles [32].

Figure 2

SEM images of (a, b) bare Bi₂MoO₆ and (c, d) ACO/BMO-30.

Further information about the structure of ACO/BMO-30 was collected from TEM images (Fig. 3). The TEM images are in line with the SEM observations, i.e., ACO/BMO-30 exhibits a flower-like architecture loaded with Ag₂CO₃ nanoparticles (Fig. 3,b). The HRTEM displays two different lattice spacings of 0.32 and 0.43 nm, which match well with the (121) planes of orthorhombic Bi₂MoO₆ and the (110) planes of monoclinic Ag₂CO₃ (Fig. 3). Moreover, the energy-dispersive spectroscopy (EDS) pattern confirmed the existence of Ag, C, O, Bi, and Mo in ACO/BMO-30 (Fig. 3).

Figure 3

(a–c) TEM images of ACO/BMO-30; (d) EDS pattern of ACO/BMO-30.

The optical absorption of Bi₂MoO₆, Ag₂CO₃ and Ag₂CO₃/Bi₂MoO₆ heterostructures were measured by using UV–vis diffuse reflection spectra (UV–vis DRS, Fig. 4). The absorption edges of Ag₂CO₃ and Bi₂MoO₆ are around 570 nm and 470 nm, respectively, in line with already reported values [30,33,39]. Compared to pristine Bi₂MoO_6, the absorption of the Ag₂CO₃/Bi₂MoO₆ heterostructures was substantially improved owing through the introduction of Ag₂CO₃ nanoparticles. Ag/Ag₂CO₃/Bi₂MoO₆ [32], Ag₂MoO₄/Bi₂MoO₆ [22], and Ag₂CO₃/Bi₂MoO₆ heterostrutures are VLD photocatalysts.

Figure 4

UV–vis diffuse reflection spectra of bare Bi₂MoO₆, Ag₂CO₃ and Ag₂CO₃/Bi₂MoO₆ heterostructures.

The band gap energy (E _g) can be estimated from the Tauc plot: (αhν) = A(hν − E _g)^{n /2}. Where α, h, ν, and A are absorption coefficient, Planck’s constant, the frequency of light, and a constant, respectively. The value of n depends on the type of electronic transition and n is equal to 1 for Bi₂MoO₆ and Ag₂CO₃. The Tauc plots of Ag₂CO₃ and Bi₂MoO₆ converted from the UV–vis DRS measurements are shown in Figure S1 (Supporting Information File 1). The band gaps are determined to be 2.17 for Ag₂CO₃ and 2.66 eV for Bi₂MoO₆.

The band potentials of Ag₂CO₃ and Bi₂MoO₆ can be estimated by the empirical equations:

[1]

[2]

where the X value for Bi₂MoO₆ is 5.5 eV [43], and that for Ag₂CO₃ is 6.02 eV [44]. The value of E ₀ is ca. 4.5 eV. Hence, the values of E _VB and E _CB of Bi₂MoO₆ were calculated as −0.32 and 2.34 eV, and those of Ag₂CO₃ were calculated as 0.43 and 2.60 eV.

PL spectra were measured to analyze the electron–hole-separation efficiency [45–47]. Fig. 5 shows the contrast between the PL spectra of bare Bi₂MoO₆ and of ACO/BMO-30. The emission peak of Bi₂MoO₆ is centered at ca. 465 nm under an excitation wavelength of 300 nm. Intriguingly, the PL emission intensity of Bi₂MoO₆ was reduced after the introduction of Ag₂CO₃. A similar phenomenon was also found in Ag/Ag₂CO₃/Bi₂MoO₆ [32] and Ag₂MoO₄/Bi₂MoO₆ [22]. This result shows that the charge separation efficiency is enhanced in ACO/BMO-30.

Figure 5

PL spectra of bare Bi₂MoO₆ and ACO/BMO-30.

Photocatalytic performance

The efficiency of Ag₂CO₃/Bi₂MoO₆ heterostructures in the photocatalytic degradation of industrial dyes (RhB, MO and MB), and the antibiotic TC of the under visible light was measured. Fig. 6 displays the degradation of RhB as a function of the time. The RhB concentration remains unchanged in the absence of catalysts. In the presence of bare Bi₂MoO₆ and Ag₂CO₃ only 39.8% and 58.7% of RhB were degraded after 90 min of reaction time. The degradation of RhB was substantially enhanced when a combination of Bi₂MoO₆ and Ag₂CO₃ was used. For instance, the introduction of a low amount of Ag₂CO₃ (10 wt %) resulted in 74.9% degradation of RhB. Obviously, the RhB degradation performance is closely related to the loading amount of Ag₂CO₃. Among the heterostructures, ACO/BMO-30 achieves the best activity in the degradation of RhB, with 100% degradation efficiency after 60 min of reaction time. ACO/BMO-10, ACO/BMO-20 and ACO/BMO-50 showed degradation efficiencies of 60.2%, 69.6%, and 85.1%, respectively. Remarkably, a mixture of 23.1 wt % Ag₂CO₃ and 76.9 wt % Bi₂MoO₆ exhibited a much lower activity than ACO/BMO-30, verifying that the close contact between the components also has a significant influence on the photocatalytic performance of the heterostructures.

Figure 6

(a) Photocatalytic degradation of RhB over different samples under visible light. (b) Rate constants of RhB degradation for different catalysts.

The pseudo-first-order kinetic plots and rate constants of RhB degradation for various catalysts are presented in Fig. 6. The degradation rate constant of ACO/BMO-30 is 0.0562 min⁻¹, which 9.0-, 4.7- and 5.2-fold higher than that of bare Bi₂MoO₆ (0.0056 min⁻¹), Ag₂CO₃ (0.0099 min⁻¹), and the mixture (0.0091 min⁻¹).

Fig. 7 and Figure S2 (Supporting Information File 1) show the degradation of MO and TC as a function of the time. Again, ACO/BMO-30 displayed the highest activity, with degradation efficiencies of 94.9% for MO, 100% for MB, and 78.9% for TC. The photocatalytic activity in the degradation of TC of ACO/BMO-30 was further compared with that of Ag/Ag₂CO₃/Bi₂MoO₆ [32], and of Ag₂MoO₄/Bi₂MoO₆ [22]. As shown in Figure S3 (Supporting Information File 1), ACO/BMO-30 is much more active than Ag₂MoO₄/Bi₂MoO₆, but not as active as Ag/Ag₂CO₃/Bi₂MoO₆ due to the fact that Ag/Ag₂CO₃/Bi₂MoO₆ is a ternary composite.

Figure 7

Photocatalytic degradation of (a) MO and (b) TC over different samples under visible light.

To examine the mineralization ability of ACO/BMO-30, the total organic carbon (TOC) was measured during the RhB degradation, and the result is presented in Figure S4 (Supporting Information File 1). Apparently, the TOC removal efficiency gradually goes up with the increase of reaction time and the final TOC removal efficiency is as high as 80.6% after 5 h of reaction. Hence, ACO/BMO-30 shows a decent mineralization ability.

To test the durability of ACO/BMO-30, repeated runs were carried out under unchanging conditions. As depicted in Fig. 8, after six consecutive runs, the RhB degradation efficiency is still about 92.2%. Additionally, no apparent change of the crystalline structure was found after the photocatalytic reactions, as shown in the XRD pattern (Fig. 8). However, the XPS pattern of the used ACO/BMO-30 suggests that some Ag⁺ is reduced to Ag(0) after the reaction (Figure S5, Supporting Information File 1). It has been recognized that the formation of Ag/Ag₂CO₃ shows a stable structure [32]. These results verify that ACO/BMO-30 is stable. It is rationally speculated that ACO/BMO-30 has great potential for the elimination of toxic pollutants.

Figure 8

(a) The cycling performance of ACO/BMO-30. (b) The XRD patterns of ACO/BMO-30 before and after five reaction cycles.

Photocatalytic mechanism

In general, radicals and other active species (e.g., •O₂ ⁻, •OH and h⁺) produced in the photocatalytic reaction system contribute to the decomposition of pollutant [48–52]. We performed radical-scavenger tests to determine the active species in the degradation of RhB. As displayed in Fig. 9, the addition of isopropanol (IPA) resulted in a slight inhibition of the RhB degradation, signifying that •OH is not the major active species. The addition of p-benzoquinone (BQ) makes the RhB degradation efficiency decline from 100% to 67.3%, suggesting that •O₂ ⁻ plays a minor role. After the addition of ammonium oxalate (AO) the RhB degradation efficiency decreased from 100% to 19.3%, indicating that h⁺ plays a dominant role in the degradation of RhB over Ag₂CO₃/Bi₂MoO₆.

Figure 9

Radical-scavenger tests over ACO/BMO-30.

Based on the above results, a possible schematic mechanism is presented in Fig. 10. In the Ag₂MoO₄/Bi₂MoO₆ system [22], electron–hole pairs can only be generated by visible light in Bi₂MoO₆. In comparison, both Ag₂CO₃ and Bi₂MoO₆ are able to absorb visible light and, and charge carriers are photo-generated on the surface of Ag₂CO₃ and Bi₂MoO₆. The electrons in the CB of Bi₂MoO₆ can partially transfer into that of Ag₂CO₃. The electrons are then captured by the oxygen, producing •O₂ ⁻ radicals, which play a minor role in decomposing pollutants, as confirmed by the radical-scavenger test (Fig. 9). At the same time, the holes in the VB of Ag₂CO₃ can partially flow into that of Bi₂MoO_6. The holes in both components mainly account for the pollutant degradation (Fig. 9). Electron–hole recombination is severely inhibited, as verified by PL results (Fig. 5) [53–54]. Although the VB potential of Ag₂CO₃ (2.6 eV) is more positive than that of H₂O/•OH (2.38 eV/NHE at pH 7), the generated •OH radicals did not act as the major active species for pollutant degradation (Fig. 9). In Ag/Ag₂CO₃/Bi₂MoO₆, Ag(0) can also be excited to generate electrons, reacting with O₂ to form •O₂ ⁻ radicals due to the plasmon resonance (SPR) effect [32]. Based on above analysis, the rational design and construction of Bi₂MoO₆-based heterostructures is favorable for the separation of charges, leading to a superior activity in pollutant degradation.

Figure 10

Proposed photocatalytic degradation mechanism of Ag₂CO₃/Bi₂MoO₆.

Conclusion

We synthesized a novel photocatalyst of Bi₂MoO₆ microflowers covered with Ag₂CO₃ nanoparticles by a facile procedure. The Ag₂CO₃/Bi₂MoO₆ heterostructures, especially ACO/BMO-30, showed a substantially enhanced photocatalytic performance in the removal of pollutants (RhB, MO, MB and TC) under visible light compared to bare Bi₂MoO₆ and Ag₂CO₃. Moreover, ACO/BMO-30 possesses good durability and stability. The enhanced photocatalytic performance is ascribed to the extended optical response and the matched band structure, reducing carrier recombination. This study offers a novel highly efficient VLD photocatalyst with promising applications in environmental remediation.

Experimental

Materials

Bi(NO₃)₃·5H₂O, NaHCO₃, NH₃·H₂O, Na₂MoO₄·2H₂O, rhodamine B (RhB), methyl blue (MB), tetracycline hydrochloride (TC), methyl orange (MO), isopropanol (IPA), AgNO₃, p-benzoquinone (BQ), and ammonium oxalate (AO) were obtained from Chemical Reagent factory (China). All the reagents were used directly without further treatment.

Synthesis

Flower-like Bi₂MoO₆ was synthesized using a solvothermal route. Typically, Na₂MoO₄·2H₂O (1 mmol) was dissolved in a mixture of 40 mL of CH₃CH₂OH and 40 mL of ethylene glycol with the aid of ultrasonic treatment. Then Bi(NO₃)₃·5H₂O (2 mmol) was also dissolved in the above solution in the same way. Subsequently, the solution was magnetically stirred for 1 h, and then loaded into a 100 mL Teflon container and reacted at 160 °C for 25 h. After the reaction system had cooled down to room temperature, the precipitants were washed thoroughly with deionized water and ethanol, dried, and calcined at 350 °C for 1 h.

Ag₂CO₃/Bi₂MoO₆ heterostructures were obtained by wet-chemical deposition. Typically, 0.6 g Bi₂MoO₆ was ultrasonically dispersed in 30 mL of deionized water. Then AgNO₃ solution (0.1 M) was poured into the suspension and kept stirring for 1 h in the dark. Subsequently, NH₃·H₂O (0.05 M) was added into the above mixture. After that, NaHCO₃ solution (0.1 M) was dropped into the above system slowly under magnetic stirring in the dark, followed by stirring for another 2 h. The collected solid was washed, and dried at 70 °C overnight. By varying the added amounts of AgNO₃, NaHCO₃, and NH₃·H₂O, the as-prepared Ag₂CO₃/Bi₂MoO₆ heterojunctions with various weight ratios (0. 1/1, 0.2/1, 0.3/1 and 0.5/1) were obtained and denoted as ACO/BMO-10, ACO/BMO-20, ACO/BMO-30, and ACO/BMO-50, respectively. For comparison, pure Ag₂CO₃ was prepared by the same method without the addition of Bi₂MoO₆. A mixture of Ag₂CO₃ and Bi₂MoO₆, symbolized as mixture, was prepared by simple physical mixing.

Characterization

The microstructure of the samples was observed with a Hitachi S-4800 scanning electron microscope (SEM) and a JEM-2010F transmission electron microscope (TEM). The corresponding chemical compositions were detected by energy-dispersive X-ray (EDX) spectroscopy equipped on the SEM. The crystal structure of catalysts was identified by X-ray diffractometry (XRD, Bruker D8 Advance) with a scanning range of 2θ from 20° to 80°. The UV–vis spectra were recorded using an UV-2600 UV–vis spectrophotometer (Shimadzu). The photoluminescence (PL) emission spectra of the samples were recorded on a Hitachi F-7000 spectrophotometer, employing an excitation wavelength of 300 nm.

Photocatalytic test

The photo-degradation of RhB, MO, TC and MB was carried out in a reactor containing 40 mg of sample, and 100 mL of RhB (10 mg·L⁻¹), MO (10 mg·L⁻¹), TC (20 mg·L⁻¹) or MB (10 mg·L⁻¹) under visible-light irradiation. A 300 W xenon lamp equipped with a UV-cutoff filter (λ > 400 nm) served as the light source. First, the suspension was stirred in the dark for half an hour. During reaction, 2 mL of solution was taken at certain intervals, and centrifuged to obtain the supernatant for UV–vis spectrophotometry measurements. Total organic carbon (TOC) tests were executed by the degradation of RhB (50 mg·L⁻¹, 150 mL) solution with 300 mg of ACO/BMO-30 as the catalyst.