Bi2MoO6 composites with enhanced

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Journal of the Taiwan Institute of Chemical Engineers 0 0 0 (2018) 1–9

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Ag-Ag2 CO3 /Bi2 MoO6 composites with enhanced visible-light-driven catalytic activity Junlei Zhang a, Zhen Ma a,b,∗ a Department of Environmental Science and Engineering, Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3 ), Fudan University, Shanghai 200433, PR China b Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, PR China

a r t i c l e

i n f o

Article history: Received 11 January 2018 Revised 19 March 2018 Accepted 20 March 2018 Available online xxx Keywords: Heterojunction Ag2 CO3 Bi2 MoO6 Visible-light-driven Photocatalysis

a b s t r a c t Novel Ag-Ag2 CO3 /Bi2 MoO6 composites were obtained through in situ formation of Ag2 CO3 rods/nanoparticles onto Bi2 MoO6 nanosheets, followed by photoreduction. These composites were found to be much more active than Bi2 MoO6 in photocatalytic degradation of rhodamine B (RhB) under visible-light irradiation. In particular, an Ag-Ag2 CO3 /Bi2 MoO6 composite with a theoretical Ag2 CO3 /Bi2 MoO6 mass ratio of ∼27.5% exhibited the highest photocatalytic activity. It was also the most active in photocatalytic degradation of methyl blue (MB) and tetracycline hydrochloride (TC), due to the enhanced visible-light absorption and efficient separation of photogenerated carriers. The good reusability of Ag-Ag2 CO3 /Bi2 MoO6 was verified by cycling degradation experiments. The dominant active species were found to be photogenerated holes (h+ ). A possible photocatalytic mechanism was proposed. © 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Photocatalysis has been studied extensively for hydrogen production [1–4] and treating environmental pollutants [5,6]. However, the most commonly used photocatalyst TiO2 suffers from problems associated with ineffective sunlight capture and conversion [7]. The development of efficient photocatalysts that can make full use of sunlight (especially visible light) has thus become popular in photocatalysis research [8]. Bi2 MoO6 is promising for making photocatalysts because its band-gap is relatively narrow (∼2.8 eV) and it is non-toxic [9–12]. Bi2 MoO6 possesses a unique layered structure built from alternate stacking of corner-shared, distorted MoO6 octahedral slabs and [Bi2 O2 ]2+ layers. This structural feature can facilitate electron conductivity [9–12]. However, Bi2 MoO6 still suffers from unsatisfactory photo-response range and serious recombination of photogenerated carriers. Thus, it needs to be further modified for practical application [13]. Composites with integrated functional components may combine the advantages of different components and overcome the drawbacks mentioned above [13]. Thus, Bi2 MoO6 has been

∗ Corresponding author at: Department of Environmental Science and Engineering, Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3), Fudan University, Shanghai 200433, PR China. E-mail addresses: [email protected], [email protected] (Z. Ma).

combined with other substances such as TiO2 [14], Ag3 PO4 [15], rGO [16], Bi2 S3 [17], g-C3 N4 [18], Ag2 O [19], Fe2 O3 [20], MO (M = Cu, Co3/4 , Ni) [21], BiVO4 [22], Ag3 VO4 [23], AgX (X = Cl, Br, and I) [24–26], and Ag2 MoO4 [27] to make better photocatalysts. The formation of plasmonic composites can not only inhibit the recombination of photo-excited electrons/holes but also enhance visible-light absorption, thus leading to more efficient photocatalysts [28]. For instance, Li et al. found that plasmonic Ag/AgCl-Bi2 MoO6 is much more active than Bi2 MoO6 in the photodegradation of RhB [29]. Meng et al. reported that plasmonic Ag–rGO–Bi2 MoO6 composites showed highly enhanced photocatalytic activity for the removal of phenol [30]. Other plasmonic Bi2 MoO6 -containing composites include Bi-Bi2 MoO6 [31], AuBi2 MoO6 /TiO2 [32], Ag/AgBr-Bi2 MoO6 [33], Ag2 WO4 /Ag/Bi2 MoO6 [34], and Pd-Bi2 MoO6 [35]. Recently, plasmonic Ag-Ag2 CO3 was employed to obtain plasmonic composites Ag-Ag2 CO3 /WO3 [36] and Ag-Ag2 CO3 /BiVO4 [37] with obviously enhanced photocatalytic activities. However, to the best of our knowledge, plasmonic Ag-Ag2 CO3 /Bi2 MoO6 composites were not reported. Herein, novel Ag-Ag2 CO3 /Bi2 MoO6 composites were prepared by the formation of Ag2 CO3 rods/nanoparticles on Bi2 MoO6 nanosheets followed by photoreduction. These composites showed excellent activities in the visible-light-driven degradation of rhodamine B (RhB), methyl blue (MB), and tetracycline hydrochloride (TC). Relevant photocatalysts were characterized, and a possible photocatalytic mechanism was proposed.

https://doi.org/10.1016/j.jtice.2018.03.043 1876-1070/© 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article as: J. Zhang, Z. Ma, Ag-Ag2 CO3 /Bi2 MoO6 composites with enhanced visible-light-driven catalytic activity, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.03.043

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J. Zhang, Z. Ma / Journal of the Taiwan Institute of Chemical Engineers 000 (2018) 1–9

2. Experimental Ag2CO3 JCPDS No. 26-0399

2.1. Materials

Intensity (a.u.)

AgNO3 , NaHCO3 , Na2 MoO4 •2H2 O, Bi(NO3 )3 •5H2 O, rhodamine B (RhB), and methylene blue (MB) of analytical grade were from Sinopharm Chemical Reagent. Tetracycline hydrochloride (TC) was purchased from Aladdin. All reagents were used as received.

S5 S4 S3 S2

2.2. Synthesis of Bi2 MoO6 Bi2 MoO6 were prepared via hydrothermal synthesis under an acidic condition (pH = ∼1). Typically, 4 mmol Bi(NO3 )3 •5H2 O was dissolved in 30 mL HNO3 solution (0.7 mol/L) under vigorously stirring (800 r/min) to form solution A. Meanwhile, 2 mmol Na2 MoO4 2H2 O was dissolved in 20 mL deionized water to form solution B. Solution B and 30 mL deionized water were then dropped into solution A. After being further stirred for 30 min, the mixture was transferred into a 100 mL Teflon lined stainless autoclave. The autoclave was sealed and heated at 160 °C for 20 h. After the autoclave was naturally cooled down to room temperature, the obtained solid product was washed with deionized water for three times, and dried at 60 °C for 24 h. 2.3. Synthesis of Ag-Ag2 CO3 /Bi2 MoO6 Typically, 0.5 g of Bi2 MoO6 powder was well dispersed in 50 mL deionized water with the aid of ultrasonic treatment for 15 min. Subsequently, 10 mL AgNO3 solution (0.1 mol/L) was dropped into the above suspension, and the mixture was stirred for 30 min to form an Ag+ -Bi2 MoO6 system. Then, 5 mL of NaHCO3 solution (0.1 mol/L) was dropped into the above system under vigorously stirring (1200 r/min) to obtain Ag2 CO3 /Bi2 MoO6 . The mixture was further stirred for 30 min. The suspension was irradiated by a 300 W xenon lamp for 2 min to obtain Ag-Ag2 CO3 /Bi2 MoO6 . The solid collected by filtration was washed with deionized water for three times, and dried at 60 °C for 24 h. This Ag-Ag2 CO3 /Bi2 MoO6 composite, with a theoretical Ag2 CO3 /Bi2 MoO6 mass ratio of around 27.5%, was named as S3. Other Ag-Ag2 CO3 /Bi2 MoO6 composites (S1, S2, S4, and S5) with different Ag2 CO3 /Bi2 MoO6 mass ratios (5.5%, 16.5%, 38.5%, and 49.5%, respectively) were prepared by tuning the dosage of AgNO3 (2, 6, 14, and 18 mL) and NaHCO3 (1, 3, 7, and 9 mL) solution. For comparison, pristine Ag2 CO3 was prepared by precipitation using 1 mmol AgNO3 and 0.5 mmol NaHCO3 . Ag-Ag2 CO3 was prepared by photoreduction of Ag2 CO3 suspension.

S1 Bi2MoO6 JCPDS No. 21-0102

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2 Theta (degree) Fig. 1. XRD patterns of Ag-Ag2 CO3 /Bi2 MoO6 composites (S1–S5) and standard XRD patterns of Ag2 CO3 (JCPDS No. 26-0399) and Bi2 MoO6 (JCPDS No. 21-0102).

2.5. Evaluation of photocatalytic activity Rhodamine B (RhB, 10 mg/L), methylene blue (MB, 10 mg/L), and tetracycline hydrochloride (TC, 20 mg/L) were used to compare the photocatalytic activities of catalysts. A Xe lamp (300 W, HSX-F300, Beijing NBeT Technology Co., Ltd.) coupled with a UV-cutoff filter (420 nm) was used as the light resource [19,27,38–41]. The distance between the lamp and the solution was ca. 15 cm. The temperature of system was controlled as around 25 °C. Before illumination, 30 mg catalyst was suspended in 50 mL of RhB, MB, or TC solution in a 100 mL beaker, and the suspension was stirred (800 r/min) for 30 min to establish the adsorption– desorption equilibrium. Afterward, illumination was provided for 60 min for each experiment. 4 mL of slurry (containing the solution and the catalyst) was sampled every 15 min and separated by centrifugation at a speed of 80 0 0 r/min for 15 min. The obtained supernatant was then analyzed by a UV-5200 PC spectrometer. Cycling photocatalytic experiments were carried out to test the recyclability of catalyst. The catalyst collected after each run was washed thoroughly with water and dried. Because the loss of catalyst powders during the sampling and collection/washing processes is unavoidable, several parallel runs were conducted to collect used catalyst powders (in order to replenish the catalyst) and make sure that the amount of catalyst used in each run was the same (30 mg). The mixed catalyst powders used for the nth run were collected from the parallel (n − 1)th runs. 3. Results and discussion

2.4. Characterization 3.1. Characterization X-ray diffraction (XRD) experiments were carried out using a MSAL XD2 X-ray diffractometer with CuKα radiation at 40 kV and 30 mA at a scanning speed of 8°/min. X-ray photoelectron spectroscopic (XPS) data were recorded on a multifunctional photoelectron spectroscopy instrument (Axis Ultra Dld, Kratos). Scanning electron microscopy (SEM) experiments were carried out on a Shimadzu SUPERSCAN SSX-550 field emission scanning electron microscope. Transmission electron microscopy (TEM) experiments were performed using a JEOL JEM-2100F high-resolution transmission electron microscope. Optical diffuse reflectance spectra were obtained on a UV–Vis–NIR scanning spectrophotometer (Lambda 35, Perkin-Elmer) with an integrating sphere accessory. Photoluminescence (PL) spectra were recorded using a LabRAM HR Evolution instrument (HORIBA JY Company, France) with an excitation wavelength of 355 nm. Electrochemical impedance spectroscopy (EIS) experiments were performed by an electrochemical analyzer (CHI 660B Chenhua Instrument Company).

The phase compositions of Bi2 MoO6 , Ag2 CO3 , Ag-Ag2 CO3 , and Ag-Ag2 CO3 /Bi2 MoO6 were analyzed by XRD. As shown in Fig. S1, Bi2 MoO6 and Ag2 CO3 can be indexed to orthorhombic Bi2 MoO6 (JCPDS No. 21-0102) and monoclinic Ag2 CO3 (JCPDS No. 26-0399), respectively. Ag-Ag2 CO3 prepared by photoreduction of a suspension containing Ag2 CO3 for 2 min shows obvious monoclinic Ag2 CO3 peaks whereas Ag0 peaks can be hardly observed, probably due to the low content of Ag0 and/or the small sizes of Ag0 species. As shown in Fig. 1, Ag-Ag2 CO3 /Bi2 MoO6 composites mainly show the obvious peaks of Bi2 MoO6 . This observation is explained by the low contents of Ag-Ag2 CO3 in these samples. In addition, AgAg2 CO3 exists in the form of nanoparticles that can hardly be detected by XRD, whereas Bi2 MoO6 exists in the form of much bigger sheets (to be shown by SEM and TEM data later). With the increase of Ag2 CO3 /Bi2 MoO6 mass ratio (i.e., when going from S1 to S5), the peaks of Bi2 MoO6 become weaker and weaker (as more clearly


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Fig. 2. (A) Survey XPS of Bi2 MoO6 , S3, Ag-Ag2 CO3 , and Ag2 CO3 ; high-resolution XPS data (B-F) from S3.

seen from the change of the most intensive peak at 2θ = 28.3°). The Ag2 CO3 peaks (especially the most intensive one at 2θ = 33.7°) are seen only when the Ag-Ag2 CO3 content is high enough (for the sample S5). XPS was further used to analyze the elemental composition and chemical valence states. Survey XPS data confirm the existence of related elements in samples (Fig. 2(A)). The high-resolution Bi 4f XPS data of S3 show obvious Bi 4f7/2 and Bi 4f5/2 peaks at 158.80 and 164.10 eV, respectively, demonstrating that Bi exists as Bi3+ (Fig. 2(B)). Fig. 2(C) shows Mo 3d5/2 and Mo 3d3/2 peaks at 232.06 and 235.19 eV, respectively, indicating that Mo exists as Mo6+ in S3. The O 1s region can be fitted into two peaks at 529.75 and 531.21 eV (Fig. 2(D)) indexed to the lattice O2− in S3 and surface hydroxyl groups, respectively. In the Ag 3d region (Fig. 2(E)), Ag 3d5/2 and Ag 3d3/2 peaks appear at 367.73 and 373.62 eV,

respectively, verifying the dominance of Ag+ . Another weak peak at 374.20 eV can be indexed to Ag0 . In Fig. 2(F), the major C 1s peak at 288.23 eV is attributed to carbon in Ag2 CO3 , and another peak at 284.6 eV is due to the presence of carbon contaminants [37]. The SEM image shown in Fig. 3(A) confirms that pristine Bi2 MoO6 exists mainly in the form of nanosheets. Pristine Ag2 CO3 is composed of rods and particles (Fig. 3(B)). TEM characterization of S3 shows the coexistence of Bi2 MoO6 nanosheets and Ag-Ag2 CO3 rods/particles (Fig. 3(C)). Now the sizes of AgAg2 CO3 rods/particles are smaller (compared with sizes of pristine Ag2 CO3 in Fig. 3(B)), because the Ag-Ag2 CO3 rods/particles are now supported onto Bi2 MoO6 nanosheets whereas pristine Ag2 CO3 shown in Fig. 3(B) is a bulk material. The corresponding highmagnification TEM image of S3 (Fig. 3(E)) clearly shows three


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Fig. 3. SEM images of (A) Bi2 MoO6 and (B) Ag2 CO3 ; (C) TEM, (D) corresponding SAED, and (E) high magnification TEM images of S3.

kinds of crystal lattice stripes with 0.249, 0.309, and 0.242 nm d-spacing, corresponding to the (1 5 1) plane of orthorhombic Bi2 MoO6 , the (0 1 1) plane of monoclinic Ag2 CO3 , and the (1 0 1) plane of hexagonal Ag0 , respectively. Furthermore, SAED image of another location (Fig. 3(D)) again verifies that Bi2 MoO6 , Ag2 CO3 , and Ag0 coexist in S3, suggesting the successful preparation of AgAg2 CO3 /Bi2 MoO6 .

Fig. 4 shows the elemental distribution in S3. The pictures with distinct color contrast prove that Bi, Mo, O, Ag, and C elements are all dispersed on the sample. The sheet structure can be ascribed to Bi2 MoO6 clearly (as seen from the distribution of Bi, Mo, and O elements), whereas Ag-Ag2 CO3 rods/particles disperse on the Bi2 MoO6 sheet (as seen from the distribution of Ag and C elements, in reference to the STEM image in Fig. 4(A)).


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5

Fig. 4. (A) STEM and (B–F) EDX elemental mapping images of S3.

Bi2MoO6 Ag2CO3 Ag-Ag2CO3 S1 S2 S3 S4 S5

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Fig. 5. (A) UV–Vis diffuse reflectance spectra (DRS) of Bi2 MoO6 , Ag2 CO3 , Ag-Ag2 CO3 , and S1–S5; (B) a plot of (α hν )2 vs. the bandgap (eV) for Bi2 MoO6 and Ag2 CO3 , respectively.

The optical properties of Bi2 MoO6 , Ag2 CO3 , Ag-Ag2 CO3 , and AgAg2 CO3 /Bi2 MoO6 composites (S1–S5) were studied by UV–Vis DRS. All the samples show photo-response from UV to visible-light region (Fig. 5(A)). Ag-Ag2 CO3 /Bi2 MoO6 composites (S1–S5) exhibit stronger absorption towards visible-light than pristine Bi2 MoO6 . Fig. 5(B) shows the plots of (α hν )2 vs. bandgap (eV) of direct transition semiconductors Ag2 CO3 and Bi2 MoO6 . The Eg values of Ag2 CO3 and Bi2 MoO6 are calculated to be 2.58 and 2.83 eV, respectively. The band edge positions of Ag2 CO3 and Bi2 MoO6 were evaluated using empirical equations [42]: EVB = X – E0 + 0.5 Eg ; ECB = EVB – Eg . The X values for Ag2 CO3 and Bi2 MoO6 are 6.02 eV [43] and 5.55 eV [44], respectively. E0 is the energy of free electrons on the hydrogen scale (∼4.5 eV). Thus, the EVB and ECB values of Ag2 CO3 are determined to be 2.81 and 0.23 eV, respectively. Those of Bi2 MoO6 are 2.46 and −0.37 eV, respectively.

3.2. Visible-light photocatalytic activity Rhodamine B (RhB), as one of typical industrial dyes, was first employed to explore the visible-light-driven photocatalytic activities of catalysts. As shown in Fig. 6(A), when no catalyst existed in reaction system (50 mL, 10 mg/L RhB), RhB degradation did not happen either in the dark or under visible-light illumination. Using pristine Bi2 MoO6 as the catalyst, after 30 min in the dark, RhB removal efficiency reached 26.6%, higher than those using other catalysts. This can be ascribed to the relatively high surface area of Bi2 MoO6 (Fig. S2) as well as the interaction between the catalyst and RhB. The RhB removal efficiency was only 51.7% at 60 min of visible-light irradiation. The photodegradation efficiency using pristine Bi2 MoO6 is thus estimated to be 25.1%. When Ag-Ag2 CO3 /Bi2 MoO6 composites (S1–S5) were used, RhB


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Fig. 6. (A) The RhB removal curves and (B) linear transform −Ln (C/C0 ) of the kinetic curves with or without a catalyst; (C, D) the MB and TC removal curves in the absence or presence of a catalyst.

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Fig. 7. (A) Cycling runs in removing RhB over S3; (B) XRD patterns of S3 before and after three cycling runs.

photodegradation efficiencies at 60 min of visible-light irradiation (excluding the removal efficiency values achieved after stirring the catalyst in the dark for 30 min) reached 50.1%, 93.8%, 95.9%, 94.3%, and 87.4%, respectively, obviously much higher than that using pristine Bi2 MoO6 as the catalysts. In particular, S3 exhibited the highest photodegradation efficiency (95.9%), even higher than those (77.8% and 81.4%) obtained by using Ag-Ag2 CO3 or Ag2 CO3 . The photodegradation data were fit using the pseudo-first-order model [17,45]: −Ln(C/C0 ) = Kt, where C is the concentration of RhB at time t, C0 is the original concentration of RhB, and K is the reaction rate constant (Fig. 6(B)). Obviously, S3 shows the highest photodegradation rate (0.110 min−1 ) among Ag-Ag2 CO3 /Bi2 MoO6

composites, even much higher than those (0.0 070 0, 0.0306, and 0.0263 min−1 ) using Bi2 MoO6 , Ag2 CO3 , or Ag-Ag2 CO3 . A similar trend was observed in the degradation of methyl blue (MB, Fig. 6(C)) and tetracycline hydrochloride (TC, Fig. 6(D)). The photocatalytic performance of S3 was further compared with that of commercial TiO2 (P25). As shown in Fig. S3, S3 exhibits much higher removal capability than P25 towards contaminants (RhB, MB, and TC). Additional experiments were carried out. Similar to the experiments in Fig. 6(A), the experiments were conducted using 50 mL RhB solution and 30 mg catalyst. The difference is that now the RhB concentration is 40 mg/L instead of 10 mg/L. As shown in


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Fig. 8. Comparison of the photocatalytic performance of S3 for the degradation of RhB without adding or with isopropyl alcohol (IPA), ammonium oxalate (AO), and benzoquinone (BQ) under visible-light irradiation.

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Fig. 9. (A) Photoluminescence (PL) spectra and (B) electrochemical impedance spectroscopy (EIS) Nyquist plots of Bi2 MoO6 , Ag2 CO3 , Ag-Ag2 CO3 , and S3.

photodegradation and TOC removal curves (Fig. S4), photodegradation using S3 not only leads to decolorization of RhB but also leads to mineralization. S3 is still much more active than Ag-Ag2 CO3 , Ag2 CO3 , and Bi2 MoO6 . Cycling degradation of RhB involving S3 was carried out (Fig. 7(A)). In each run, the catalyst was first stirred in the dark for 30 min, and then the Xe lamp was turned on. The RhB removal efficiency at 75 min of the first run (including 30 min in the dark) was 99% whereas that value was 97% in the third run. The little loss of activity can be attributed to the slight change of a photocatalyst [46]. According to Fig. 7(B), the XRD patterns of the used catalyst (collected after 3 runs) and the fresh catalyst are identical, and the survey XPS data demonstrate the presence of all the related elements. However, high-resolution XPS data show the further reduction of a portion of Ag+ to Ag0 during the cycling experiment (Fig. S5), possibly due to extended photoreduction. The reusability of S3 was further verified by cycling degradation of MB or TC (Fig. S6). 3.3. Possible photocatalytic mechanism Radicals (e.g., •OH, h+ and •O2 − ) generated in the reaction system upon illumination may be responsible for the photodegradation. Scavengers that can capture specific radicals were used to determine which radicals are active species. S3 was used as a catalyst. As shown in Fig. 8, the addition of 1 mmol isopropyl alcohol (IPA), a quencher of •OH, had nearly no effect on the RhB degradation efficiency and photodegradation rate, indicating that

•OH

is not an active species. With adding 1 mmol AO (ammonium oxalate, the quencher of h+ ) into the reaction system, RhB removal efficiency decreased from 99.9% to 5.5%, and the photodegradation rate declined from 0.110 min−1 to 0.0 0 0473 min−1 , indicating that h+ should be responsible for this catalysis. With adding 0.02 mmol BQ (benzoquinone, the quencher •O2 − ), RhB removal efficiency decreased from 99.9% to 92.4%, and the photodegradation rate declined from 0.110 min−1 to 0.0455 min−1 , indicating that •O2 − plays a minor role. These conclusions were confirmed by the radical-trapping experiments involving MB or TC (Fig. S7). Photoluminescence (PL) emission spectroscopy can be used to characterize the recombination of photogenerated carriers. Usually, a lower PL emission intensity corresponds to a higher separation efficiency of electron–hole pairs [36,37]. In Fig. 9(A), S3 shows lower PL emission intensity than Bi2 MoO6 , Ag2 CO3 , and Ag-Ag2 CO3 , indicating that Ag-Ag2 CO3 /Bi2 MoO6 can separate electron–hole pairs more efficiently. Electrochemical impedance spectroscopy (EIS) was further used [47]. As shown in Fig. 9(B), the arc radius of S3 is smaller than that of Bi2 MoO6 , Ag2 CO3 , or Ag-Ag2 CO3 , demonstrating that the construction of AgAg2 CO3 /Bi2 MoO6 can facilitate the interfacial charge transfer and separate electrons/holes more efficiently. These observations can explain the enhanced visible light-driven photocatalytic activity of Ag-Ag2 CO3 /Bi2 MoO6 . On the basis of results obtained and discussed above, a possible photocatalytic mechanism involving Ag-Ag2 CO3 /Bi2 MoO6 was proposed (Fig. 10). Both Bi2 MoO6 and Ag2 CO3 can be excited by visible-light (λ > 420 nm) due to the suitable band gaps (2.83 and


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Fig. 10. The possible mechanism of photocatalytic reaction on Ag-Ag2 CO3 /Bi2 MoO6 .

2.58 eV). Since the CB potential of Bi2 MoO6 (−0.37 eV vs. NHE) is more negative than that of Ag2 CO3 (0.23 eV vs. NHE), the photoexcited electrons (e− ) can flow from the CB of Bi2 MoO6 to that of Ag2 CO3 . Meanwhile, the photo-excited holes (h+ ) can migrate from the VB of Ag2 CO3 to that of Bi2 MoO6 because the VB potential of Ag2 CO3 (2.46 eV vs. NHE) is less positive than that of Bi2 MoO6 (2.81 eV vs. NHE). The separation and migration of photo-excited electrons (e− ) and holes (h+ ) can prolong the lifetime of photogenerated carriers efficiently, resulting in enhanced photocatalytic activity. Since the potential of O2 /•O2 − (−0.33 eV vs. NHE) is less negative than the CB potential of Bi2 MoO6 (−0.37 eV vs. NHE), the photo-excited electrons (e− ) left on the CB of Bi2 MoO6 can react with O2 to produce •O2 − . In addition, Ag0 in Ag-Ag2 CO3 /Bi2 MoO6 can be excited to produce electrons (e− ), which can further react with O2 to produce •O2 − due to the plasmon resonance (SPR) effect [36,37]. These •O2 − radials can remove the pollutants by oxidation. Meanwhile, the enriched holes (h+ ) on the VB of Bi2 MoO6 can react with OH− to form •OH radicals because the potential of the •OH/OH− (2.38 eV vs. NHE) is more negative than the VB of Bi2 MoO6 (2.46 eV vs. NHE). In addition, due to the potential of •OH/H2 O (2.72 eV vs. NHE) is more positive than the VB of Bi2 MoO6 , the enriched holes (h+ ) on the VB of Bi2 MoO6 cannot react with H2 O to form •OH. Accordingly, the holes in the VB of Bi2 MoO6 , •O2 − , and •OH radicals may potentially be able to oxidize the pollutants. However, our radical-trapping experiments indicate that h+ plays a main role, ·O2 − plays a minor role, whereas ·OH is not an active spies in our catalytic system.

4. Conclusions Novel plasmonic Ag-Ag2 CO3 /Bi2 MoO6 composites with different Ag2 CO3 /Bi2 MoO6 mass ratios were successfully prepared through in situ loading Ag2 CO3 rods/particles onto Bi2 MoO6 nanosheets followed by photoreduction. Ag-Ag2 CO3 /Bi2 MoO6 composites were found to be more active than Bi2 MoO6 , Ag2 CO3 , and Ag-Ag2 CO3 under visible-light irradiation. In particular, Ag-Ag2 CO3 /Bi2 MoO6 with a theoretical Ag2 CO3 /Bi2 MoO6 mass ratio of ∼27.5% possessed the highest photocatalytic activity in the degradation of dyes and tetracycline hydrochloride. It also showed good reusability. The synergistic effect of photogenerated holes (h+ ) and superoxide radical anions (•O2 − ) resulted in the quick degradation of pollutant. Such an enhanced photocatalytic activity of Ag-Ag2 CO3 /Bi2 MoO6

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