- Email: [email protected]
β-Ag2MoO4 heterojunctions with enhanced visible-light-driven catalytic activity
ARTICLE IN PRESS
JID: JTICE
[m5G;November 8, 2017;22:6]
Journal of the Taiwan Institute of Chemical Engineers 0 0 0 (2017) 1–7
Contents lists available at ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
AgI/β -Ag2 MoO4 heterojunctions with enhanced visible-light-driven catalytic activity Junlei Zhang, Zhen Ma∗ Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3 ), Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, PR China
a r t i c l e
i n f o
Article history: Received 7 June 2017 Revised 30 September 2017 Accepted 18 October 2017 Available online xxx Keywords: AgI/β -Ag2 MoO4 Heterojunctions Photocatalysis Dyes Tetracycline hydrochloride
a b s t r a c t Visible-light-driven photocatalysis for removing industrial dyes and antibiotics in waste water has attracted much interest. New catalysts with enhanced photocatalytic performance have been actively sought. Herein, via a simple sequential precipitation method, AgI nanoparticles were anchored onto micron-sized β -Ag2 MoO4 particles to form AgI/β -Ag2 MoO4 p–n heterojunctions. These heterojunctions exhibited remarkably upgraded activities in the visible-light-driven photocatalytic degradation of rhodamine B, methyl orange, and tetracycline hydrochloride compared with pristine AgI and β -Ag2 MoO4 . The enhanced photocatalytic activity can be mainly attributed to the tight heterojunction structure and well matched energy band that may facilitate the separation and transfer of photogenerated electron– hole pairs. Photogenerated holes (h+ ) and superoxide radical anions (•O2 − ) were found to be the main active species. A possible photocatalytic mechanism was proposed. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Environmental pollution caused by toxic and harmful organic pollutants is exacerbating the danger to humans and other living species. Photocatalysis, an environment-benign technology, can be used to eliminate organic contaminants [1]. A key research topic of photocatalysis is the development of highly efficient photocatalysts. Semiconductors based on silver halides (AgX, X = Cl, Br, or I) have attracted increasing interest [2–4]. Compared to AgCl and AgBr, AgI exhibits high potential in the development of visible-light-driven photocatalysts because of the relatively high photo-stability and suitable band gap (Eg = ∼2.8 eV) [4,5]. AgX (X = Cl, Br, or I) can be modified by plasmonic silver to obtain stable photocatalysts [6,7]. For instance, Ghosh et al. reported that Ag–AgI composite exhibited higher photocatalytic antibacterial activity than AgI [8]. Wang et al. found that Ag/AgI heterojunction prepared by one-step hydrothermal synthesis exhibited stable and efficient photocatalytic activity [9]. Other AgI-based/containing composites, such as Ag–AgI/Al2 O3 [10], Ag–AgI/Fe3 O4 @SiO2 [11], K4 Nb6 O17 /Ag@AgI [12], Ag/AgI/BiOI [13], Ag/AgI@TNTs [14], g-C3 N4 /Fe3 O4 /AgI [15], AgI/ZnSn(OH)6 [16], AgI/BiOIO3
∗
Corresponding author. E-mail address: [email protected] (Z. Ma).
[17], RGO/BiOI/AgI [18], Ag@AgI/ZnS [19], ZnO/AgI/Fe3 O4 [20], AgI/TiO2 /rGO [21], AgI/BiOI–Bi2 O3 [22], AgI/WO3 [23], AgI/AgVO3 [24], Bi7 O9 I3 /AgI/AgIO3 [25], Ag2 ZnI4 /AgI [26], and AgI/Bi2 O2 CO3 [5], have been developed. It is still interesting to develop AgIbased new photocatalysts for fundamental research and practical applications. Ag2 MoO4 has been used in the preparation of high-temperature lubricants [27], ion-conducting glasses [28], photoluminescent materials [29], and photoswitch devices [30], owing to its good thermal stability, conductivity, and photosensitivity. However, reports on the photocatalytic application of β -Ag2 MoO4 are rare [31], due to the wide band-gap (∼3.3 eV) of β -Ag2 MoO4 [32]. To develop better photocatalysts, β -Ag2 MoO4 has been coupled with other substances to form heterojunctions such as Ag@Ag2 MoO4 – AgBr [33], Ag2 MoO4 /Ag3 PO4 [34], Ag2 MoO4 /Ag/AgBr/GO [35], β -Ag2 MoO4 /Bi2 MoO6 [36], and β -Ag2 MoO4 /g–C3 N4 [37]. However, to the best of our knowledge, AgI/β -Ag2 MoO4 heterojunctions have not been reported. Considering that the band edges between AgI (ECB : −0.40 eV; EVB : 2.40 eV [24]) and β -Ag2 MoO4 (ECB : −0.18 eV; EVB : 3.02 eV [37]) are well matched, in this work we developed AgI/β -Ag2 MoO4 composites by a sequential precipitation method. These catalysts were tested in the visible-light-driven photocatalytic degradation of rhodamine B (RhB), methyl orange (MO), and tetracycline hydrochloride (TC). Relevant samples were characterized, and possible reasons for the enhanced photocatalytic activity were investigated.
https://doi.org/10.1016/j.jtice.2017.10.018 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: J. Zhang, Z. Ma, AgI/β-Ag2 MoO4 heterojunctions with enhanced visible-light-driven catalytic activity, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.018
ARTICLE IN PRESS
JID: JTICE 2
[m5G;November 8, 2017;22:6]
J. Zhang, Z. Ma / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–7
Fig. 1. XRD patterns of AgI/β -Ag2 MoO4 composites and standard XRD patterns of β -Ag2 MoO4 (JCPDS 08-0473) and AgI (JCPDS 09-0374).
4 mmol AgNO3 was dissolved in a 100 mL breaker containing 30 mL deionized water to form solution A, and 2 mmol Na2 MoO4 ·2H2 O was dissolved in another 100 mL breaker containing 30 mL deionized water to form solution B. Solution B was then added dropwise into solution A, under vigorous stirring (1200 r/min), to obtain β Ag2 MoO4 . Then, a certain amount of AgNO3 (0.25, 0.5, 1, 2, 4, 8, or 16 mmol) was added into the above system, and the mixture was magnetically stirred for 30 min to obtain Ag+ -β -Ag2 MoO4 . Subsequently, a solution containing 0.25, 0.5, 1, 2, 4, 8, or 16 mmol KI was added dropwise into the above mixture, and the mixture was continuously stirred for 4 h. The solid collected by filtration was washed with deionized water for three times, and dried at 60 °C for 24 h. These AgI/β -Ag2 MoO4 composites were named as S1, S2, S3, S4, S5, S6, and S7, respectively. The theoretical AgI/β -Ag2 MoO4 molar ratios of these composites (S1–S7 samples) are 1/8, 1/4, 1/2, 1/1, 2/1, 4/1, and 8/1, respectively. For comparison, the same procedure was adopted to prepare pristine AgI (or β -Ag2 MoO4 ) using 2 mmol AgNO3 and 2 mmol KI (or 1 mmol Na2 MoO4 ·2H2 O) as the starting materials.
2. Experimental
2.3. Characterization
2.1. Materials
X-ray diffraction (XRD) experiments were carried out on a MSAL XD2 X-ray diffractometer using CuKα radiation at 40 kV and 30 mA with a scanning speed of 8°/min. X-ray photoelectron spectroscopic (XPS) data were collected via a multifunctional photoelectron spectroscopy instrument (Axis Ultra Dld, Kratos). Scanning electron microscopy (SEM) experiments were conducted on a Shimadzu SUPERSCAN SSX-550 field emission scanning electron microscope. Optical diffuse reflectance spectra were recorded on a UV–Vis–NIR scanning spectrophotometer (Lambda 35, PerkinElmer) with an integrating sphere accessory. Photoluminescence (PL) spectra were collected using a LabRAM HR Evolution in-
AgI JCPDS 09-0374
Intensity (a.u.)
S7 S6 S5 S4 S3 S2 S1 Ag2MoO4 JCPDS 08-0473
20
30
40
50
60
70
2 Theta (degree)
AgNO3 , Na2 MoO4 ·2H2 O, KI, rhodamine B (RhB), and methyl orange (MO) of analytical grade were purchased from Sinopharm Chemical Reagent. Tetracycline hydrochloride (TC) was purchased from Aladdin. All reagents were used as received. 2.2. Synthesis of AgI/β -Ag2 MoO4 AgI/β -Ag2 MoO4 composites were prepared by a sequential precipitation method at room temperature. In a typical synthesis,
Fig. 2. XPS spectra of (A) Ag 3d, (B) I 3d, (C) Mo 3d and (D) O 1s peaks related to the samples.
Please cite this article as: J. Zhang, Z. Ma, AgI/β -Ag2 MoO4 heterojunctions with enhanced visible-light-driven catalytic activity, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.018
ARTICLE IN PRESS
JID: JTICE
[m5G;November 8, 2017;22:6]
J. Zhang, Z. Ma / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–7
3
strument (HORIBA JY company, France) with an excitation wavelength at 355 nm. Electrochemical impedance spectroscopy (EIS) and Mott–Schottky measurements were performed by an electrochemical analyzer (CHI 660B Chenhua Instrument Company). 2.4. Evaluation of photocatalytic activity Photocatalytic degradation of RhB/MO/TC was conducted under visible light at room temperature. 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 source (Fig. S1 in the Supporting Information) [36–40]. Typically, 30 mg photocatalyst was suspended in 50 mL RhB (10 mg/L), 50 mL MO (10 mg/L), or 50 mL TC (20 mg/L) solution in a 100 mL beaker. The mixture (containing a solution and a catalyst) was stirred for 30 min in the dark to establish the adsorptiondesorption equilibrium. The Xe lamp was then turned on, and the solution was still stirred at a speed of 800 r/min. During the reaction, the solution was sampled in certain intervals and centrifuged, and the supernatant was analyzed by a UV-5200 PC spectrometer. 3. Results and discussion 3.1. Characterization of the samples The XRD patterns of the as-prepared AgI and β -Ag2 MoO4 (Fig. S2) show characteristic peaks of hexagonal AgI (JCPDS 09-0374) and cubic Ag2 MoO4 (JCPDS 08-0473), respectively [33,37]. The XRD patterns of AgI/β -Ag2 MoO4 composites (S1–S7) can be indexed to a combination of AgI and Ag2 MoO4 (Fig. 1). The peaks at 2θ = 22.3°, 23.7°, 25.3°, 32.7°, 39.2°, 42.6°, 45.6°, 46.3°, 47.2°, 52.0°, 59.3°, 61.6°, 63.0°, 66.5°, 71.0°, and 73.4° can be assigned to the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (2 0 1), (2 0 2), (2 0 3), (2 1 0), (2 1 1), (1 0 5), (3 0 0), and (2 1 3) planes of hexagonal AgI, while other peaks at 2θ = 16.5°, 27.1°, 31.8°, 33.3°, 38.6°, 42.2°, 47.8°, 50.9°, 55.8°, 63.1°, 65.7°, and 66.6° are assigned to the (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1), (4 2 2), (5 1 1), (4 4 0), (6 2 0), (5 3 3), and (6 2 2) planes of cubic Ag2 MoO4 , respectively [33,37]. The intensities of the AgI peaks enhance gradually as the AgI/β -Ag2 MoO4 molar ratio increases, while the peaks due to Ag2 MoO4 decline. No peaks of any impurity phase can be observed. According to the XPS spectra in Fig. S3, AgI, AgI/β -Ag2 MoO4 (S7), and β -Ag2 MoO4 contain the corresponding elements. For AgI/β -Ag2 MoO4 (S7), two characteristic peaks for Ag 3d are located at 368.68 and 374.69 eV (attributed to Ag 3d5/2 and Ag 3d7/2 of Ag+ , respectively), indicating that no Ag0 exists in S7 (Fig. 2A). The peaks at 619.90 and 631.40 eV (Fig. 2B) are for I 3d5/2 and I 3d3/2 , respectively, confirming the existence of I− in S7. Fig. 2C shows two peaks at 231.90 and 235.00 eV, corresponding to the Mo 3d5/2 and Mo 3d3/2 binding energies of Mo6+ . The O 1s spectrum of S7 (Fig. 2D) shows a peak at 530.30 eV assigned to hydroxyls (−OH) [41] and another peak at 532.10 eV ascribed to oxygen in Ag2 MoO4 [37]. Notably, the binding energies of Ag 3d, I 3d, Mo 3d, and O 1s for AgI/β -Ag2 MoO4 (S7) are slightly higher than those of pristine AgI and/or β -Ag2 MoO4 , probably owing to the interaction between AgI and β -Ag2 MoO4 . Figs. 3 and S4 show the SEM images of β -Ag2 MoO4 , AgI/β Ag2 MoO4 composites, and AgI. β -Ag2 MoO4 prepared by precipitation consists of many irregular particles with sizes ranging from several to ten microns, and all the particles exhibit smooth surfaces (Fig. 3A). With the deposition of AgI in the above system, lots of nanoparticles emerge on the surface of micron-sized β -Ag2 MoO4 particles (Fig. 3B–H). These nanoparticles should be AgI, as seem from the SEM image of pristine AgI (Fig. S4). With the increase of
Fig. 3. SEM images of β -Ag2 MoO4 (A) and AgI/β -Ag2 MoO4 composites (S1–S7 samples for B–H images).
the AgI/β -Ag2 MoO4 molar ratio, more and more AgI nanoparticles are observed in S1–S7 samples (Fig. 3B–H). The TEM image further confirms that AgI/β -Ag2 MoO4 composite (S7) consists of lots of nanoparticles and micrometer-sized particles (Fig. 4A). The corresponding SAED image illustrates the polycrystalline characteristics of both hexagonal AgI and cubic Ag2 MoO4 (Fig. 4B). EDX data were further used to explore the compositions of AgI/β -Ag2 MoO4 composites (S1–S7). The existence of Ag, Mo, I, and O elements in AgI/β -Ag2 MoO4 composites is again confirmed (Fig. S5). The AgI/Ag2 MoO4 molar ratios of AgI/β Ag2 MoO4 composites (S1–S7) are determined as 1/7.95, 1/4.18, 1/1.96, 1/1.03, 2.37/1, 4.48/1, and 8.67/1, respectively (the inset of Fig. S5), in line with the theoretical values. The optimal properties of samples were investigated by UV– Vis–NIR DRS (Fig. 5A). β -Ag2 MoO4 exhibits strong absorption in the UV region with an absorption edge around 389 nm, i.e., it can only be excited by UV light [31,33,36,37,42]. AgI/β -Ag2 MoO4 composites show absorption from visible-light to UV region, and the absorption edges with different AgI/β -Ag2 MoO4 molar ratios somehow change in an orderly pattern, gradually closing to that (463 nm) of AgI. The optical absorption near the band edge of a semiconductor follows the Tauc equation: α hν = A (hν − Eg )n /2 , where A, α , h, ν , and Eg are a constant, absorption coefficient, Planck’s constant, light frequency, and band gap, respectively [5,31]. The power index n equals to 1 for a direct band-gap material and 4 for an indi-
Please cite this article as: J. Zhang, Z. Ma, AgI/β -Ag2 MoO4 heterojunctions with enhanced visible-light-driven catalytic activity, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.018
ARTICLE IN PRESS
JID: JTICE 4
[m5G;November 8, 2017;22:6]
J. Zhang, Z. Ma / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–7
Fig. 4. TEM and SEAD images of AgI/β -Ag2 MoO4 composite (S7).
rect band-gap material. The n value for β -Ag2 MoO4 and AgI is 1. Fig. 5B shows the plots of (α hν )1/2 versus energy (hν ). The Eg values of AgI, AgI/β -Ag2 MoO4 composites (S1–S7), and β -Ag2 MoO4 are calculated to be ∼2.68, ∼2.76, and ∼3.19 eV, respectively.
ied. Before irradiation, the reaction mixture was kept in the dark for 30 min to achieve an adsorption–desorption equilibrium. From Fig. 6A, it seems that AgI/β -Ag2 MoO4 composites can adsorb more pollutant molecules in the dark than pristine AgI and β -Ag2 MoO4 . The specific BET surface area of a typical AgI/β -Ag2 MoO4 composite (S7) is 0.17 m2 /g (Fig. S6), whereas those of AgI and β -Ag2 MoO4 are below the detection limit (i.e., close to 0). The visible-light source (λ >400 nm) was subsequently turned on to start the photocatalytic reaction. As shown in Fig. 6A, RhB dye can hardly be decomposed without a catalyst. RhB can be degraded more quickly using AgI than using β -Ag2 MoO4 , because β -Ag2 MoO4 can only be excited by UV light (Fig. 5A). RhB can be more efficiently removed using AgI/β -Ag2 MoO4 composites. The optimal catalysts are S6 and S7 where the theoretical AgI/β Ag2 MoO4 molar ratios are 4/1 and 8/1, respectively. As shown in Fig. 6B, MO can hardly be removed in the absence of a catalyst. When using pristine β -Ag2 MoO4 or AgI, MO can be degraded very slowly. AgI/β -Ag2 MoO4 composites are much more active than pristine β -Ag2 MoO4 or AgI. The optimal catalysts are still S6 and S7. As shown in Fig. 6C, no degradation of TC happens under visible irradiation in the absence of a catalyst. AgI/β -Ag2 MoO4 composites still exhibit higher photocatalytic activity than pristine AgI and β Ag2 MoO4 . The optimal catalysts are still S6 and S7. The effect of catalyst dosage on TC removal efficiency was studied (Fig. S7A). When the catalyst (S7) dosage is less than 50 mg, the removal efficiency of TC (20 mg/L, 50 mL) increases along with increasing dosage. When the S7 dosage is 70 mg, TC removal efficiency is hardly raised, suggesting that the optimal S7 dosage is around 50 mg under the present condition. When the S7 dosage is 30 mg, TC removal efficiency is gradually improved with the decrease in the initial concentration of TC (Fig. S7B). The stability of catalyst was tested. As shown in Fig. 7A, four cycles of TC degradation experiments verify that AgI/β -Ag2 MoO4 composite (S7) has excellent recyclability. By comparing the XRD patterns of used and fresh S7 samples (Fig. 7B), one can conclude that the crystal phase is relatively stable except for the formation of a few Ag0 species due to photo-reduction [39]. The recyclability of AgI/β -Ag2 MoO4 composite is also confirmed by the degradation of RhB and MO (Fig. S8).
3.2. Photocatalytic activities of samples
3.3. Photocatalytic degradation mechanism
The visible-light-driven photocatalytic degradation of industrial dyes (RhB and MO) and tetracycline hydrochloride (TC) were stud-
Ammonium oxalate (AO), isopropyl alcohol (IPA), and benzoquinone (BQ) were used as scavengers to capture photogenerated
B
Absorption (a.u.)
Ag2MoO4 S1 S2 S3 S4 S5 S6 S7 AgI
200
300
400
500
600
Wavelength (nm)
700
800
Ag2MoO4
(αhv)1/2 (eV)1/2
A
AgI S1 S2 S3 S4 S5 S6 S7
2.76 eV 2.68 eV 3.19 eV
2
3
4
5
6
Energy (eV)
Fig. 5. (A) UV–vis diffuse reflectance spectra of AgI, AgI/β -Ag2 MoO4 composites, and β -Ag2 MoO4 ; (B) the plots of (α hν )1/2 versus the bandgap (eV) of AgI and β -Ag2 MoO4 .
Please cite this article as: J. Zhang, Z. Ma, AgI/β -Ag2 MoO4 heterojunctions with enhanced visible-light-driven catalytic activity, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.018
ARTICLE IN PRESS
JID: JTICE
[m5G;November 8, 2017;22:6]
J. Zhang, Z. Ma / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–7
5
A
A 1.0
1.0
1st run
2nd run
3rd run
4th run
light on blank Ag2MoO4 AgI S1 S2
C/C0
0.6
dark
0.4
0.8
C/C0
0.8
S3 S4 S5 S6 S7
0.2
0.4
0.2
0.0 -30
0
15
30
45
0
60
80
160
240
320
Time (min)
Time (min)
B
B
1.0 blank Ag2MoO4 AgI S1 S2
light on dark
0.6 S3 S4 S5 S6 S7
0.4 0.2
fresh S7
Intensity (a.u.)
0.8
C/C0
0.6
Ag0
used S7
0.0 -30
0
15
30
45
60
Time (min)
C
C/C0
0.8
0.6 S3 S4 S5 S6 S7
0.4
0.2 -30
0
15
30
40
50
60
70
Fig. 7. (A) Cycling runs in photocatalytic degradation of TC (20 mg/L, 50 mL) over S7 (30 mg) under visible light irradiation; (B) XRD patterns of S7 before and after four cycling runs.
blank Ag2MoO4 AgI S1 S2
dark
30
2 Theta (degree)
1.0 light on
20
45
60
Time (min) Fig. 6. The adsorption and degradation of (A) RhB, (B) MO, and (C) TC with or without a catalyst (30 mg) under visible light irradiation.
holes (h+ ), hydroxyl free radicals (•OH), and superoxide radical anions (•O2 − ), respectively [43,44], in order to provide some information for the reaction mechanism. As shown in Fig. S9, the addition of IPA (1 mmol) has virtually no impact on the photodegradation of RhB, confirming that •OH is not the active species. However, when adding AO (1 mmol) or BQ (0.02 mmol), the degradation efficiency of RhB (after 15 min of irradiation) decreases from 92.0% to 45.3% or 73.9%, indicating that h+ and •O2 − are the main active species. This conclusion was again confirmed in the radical-capture experiments involving MO and TC. Photoluminescence (PL) emission was employed to investigate the combination and separation of the photogenerated carriers
[44]. As seen in Fig. S10A, the PL intensity of AgI/β -Ag2 MoO4 composite (S7) is much lower than those of AgI and β -Ag2 MoO4 , indicating that the recombination of photoexcited electron–hole pairs could be efficiently inhibited though the formation of AgI/β Ag2 MoO4 heterojunction. Electrochemical impedance spectroscopy (EIS) can be used to characterize the interfacial charge transfer properties and the separation efficiency of photogenerated carriers. A smaller arc radius indicates higher efficiency in charge transfer [45]. As shown in Fig. S10B, the arc radius of AgI/β -Ag2 MoO4 composite (S7) is smaller than that of AgI, indicating that S7 can facilitate the interfacial charge transfer and separate carriers efficiently. The band edge positions of AgI (Eg = ∼2.68 eV) and β -Ag2 MoO4 (Eg = ∼3.19 eV) were further evaluated using empirical equations [46]: EVB = X – E0 + 0.5Eg ; ECB = EVB – Eg , where EVB is the VB edge potential and X is the electronegativity of the semiconductor (i.e., the geometric mean of the electronegativity of the constituent atoms). The X values for AgI and β -Ag2 MoO4 are calculated to be 5.48 eV [24] and 5.92 eV [33], respectively. E0 is the energy of free electrons on the hydrogen scale (∼4.5 eV) and Eg is the band gap energy of the semiconductor. According to these equations, the EVB and ECB values of AgI are determined to be 2.32 and −0.36 eV, respectively. Those of β -Ag2 MoO4 are 3.02 and −0.17 eV, respectively. A possible photocatalytic reaction mechanism is proposed (Fig. 8), based on the above results and energy band diagram. When p-AgI and n-Ag2 MoO4 semiconductors form p-n heterojunction in the dark, both AgI and β -Ag2 MoO4 semiconductors have
Please cite this article as: J. Zhang, Z. Ma, AgI/β -Ag2 MoO4 heterojunctions with enhanced visible-light-driven catalytic activity, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.018
JID: JTICE 6
ARTICLE IN PRESS
[m5G;November 8, 2017;22:6]
J. Zhang, Z. Ma / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–7
Fig. 8. The possible mechanism of photocatalytic reaction on AgI/β -Ag2 MoO4 heterojunction.
the same Fermi level after reaching the charge equilibrium. The higher Fermi energy of n-Ag2 MoO4 versus p-AgI makes the energy bands of n-Ag2 MoO4 bend upward and those of p-AgI bend downward toward the interface after the two semiconductors are in contact and reach an electrical equilibrium. Thus, the type-II band alignment is formed in AgI/β -Ag2 MoO4 composites, which is very beneficial for the visible absorption and carrier mobility [47]. Under visible light irradiation, the electrons (e− ) in β -Ag2 MoO4 cannot be excited from the VB to CB but the electrons photoexcited in the CB of AgI still drift to the CB of β -Ag2 MoO4 . Since the CB of AgI (higher than −0.36 eV) is more negative than the potential of O2 /•O2 − (−0.33 eV vs. NHE), the photogenerated electrons can reduce O2 to yield •O2 − , further participating in the degradation of organic molecules. The transformation of Ag+ into Ag0 usually takes place under light irradiation. Ag0 can be further excited to produce electrons to be trapped by adsorbed O2 in water to produce •O2 − [48] for the photodegradation. In addition, since the potentials of the •OH/OH− (2.38 eV vs. NHE) and •OH/H2 O (2.72 eV vs. NHE) are more positive than the VB of AgI (lower than 2.32 eV), the enriched holes on the VB of AgI cannot react with OH− and H2 O to form •OH radicals. Such enriched holes can react with the adsorbed molecules on catalyst surfaces. 4. Conclusions Novel AgI/β -Ag2 MoO4 composites with different AgI/β Ag2 MoO4 molar ratios were prepared by sequential precipitation through which AgI nanoparticles were deposited onto the surface of micron-sized β -Ag2 MoO4 particles. Compared to pristine AgI and β -Ag2 MoO4 , these composites exhibited significantly improved photocatalytic performance in the degradation of dyes (RhB and MO) and antibiotic (TC) under visible-light irradiation and good recyclability. Such enhanced and stable photocatalytic activity can be attributed to the strong interfacial interactions between AgI and β -Ag2 MoO4 , which can promote the transfer of the photo-generated electrons from AgI to β -Ag2 MoO4 to generate •O2 − by reducing the surface chemisorbed O2 . The synergistic effect of •O2 − and the photo-generated holes resulted in the enhanced degradation of RhB/MO/TC. Acknowledgment We acknowledge the financial support by National Natural Science Foundation of China (grant no. 21477022).
Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2017.10.018. References [1] Devi LG, Kavitha R. A review on non metal ion doped titania for the photocatalytic degradation of organic pollutants under UV/solar light: role of photogenerated charge carrier dynamics in enhancing the activity. Appl Catal B Environ 2013;140–141:559–87. [2] Fan YY, Ma WG, Han DX, Gan SY, Dong XD, Niu L. Convenient recycling of 3D AgX/graphene aerogels (X = Br, Cl) for efficient photocatalytic degradation of water pollutants. Adv Mater 2015;27:3767–73. [3] Yu HG, Liu L, Wang XF, Wang P, Yu JG, Wang YH. The dependence of photocatalytic activity and photoinduced self-stability of photosensitive AgI nanoparticles. Dalton Trans 2012;41:10405–11. [4] Xiao D, Geng GW, Chen PL, Li TS, Liu MH. Sheet-like and truncated-dodecahedron-like AgI structures via a surfactant-assisted protocol and their morphology-dependent photocatalytic performance. Phys Chem Chem Phys 2017;19:837–45. [5] Zhang LL, Hu C, Ji HH. p-AgI anchored on {001} facets of n-Bi2 O2 CO3 sheets with enhanced photocatalytic activity and stability. Appl Catal B Environ 2017;205:34–41. [6] An CH, Wang ST, Sun YG, Zhang QH, Zhang J, Wang CY, et al. Plasmonic silver incorporated silver halides for efficient photocatalysis. J Mater Chem A 2016;4:4336–52. [7] Ma XC, Dai Y, Guo M, Huang BB. The role of effective mass of carrier in the photocatalytic behavior of silver halide-based Ag@AgX (X = Cl, Br, I): a theoretical study. ChemPhysChem 2012;13:2304–9. [8] Ghosh S, Saraswathi A, Indi SS, Hoti SL, Vasan HN. Ag@AgI, core@shell structure in agarose matrix as hybrid: synthesis, characterization, and antimicrobial activity. Langmuir 2012;28:8550–61. [9] Wang XJ, Wan XL, Li WQ, Chen XN. One-step hydrothermal synthesis of the Ag/AgI heterojunction with highly enhanced visible-light photocatalytic performances. Micro Nano Lett 2014;9:376–81. [10] Hu C, Peng TW, Hu XX, Nie YL, Zhou XF, Qu JH, et al. Plasmon-induced photodegradation of toxic pollutants with Ag-AgI/Al2 O3 under visible-light irradiation. J Am Chem Soc 2010;132:857–62. [11] Guo JF, Ma BW, Yin AY, Fan KN, Dai WL. Photodegradation of rhodamine B and 4-chlorophenol using plasmonic photocatalyst of Ag-AgI/Fe3 O4 @SiO2 magnetic nanoparticle under visible light irradiation. Appl Catal B Environ 2011;101:580–6. [12] Cui WQ, Wang H, Liang YH, Liu L, Han BX. Preparation of Ag@AgI-intercalated K4 Nb6 O17 composite and enhanced photocatalytic degradation of rhodamine B under visible light. Catal Commun 2013;36:71–4. [13] Lin HL, Zhao YJ, Wang YJ, Cao J, Chen SF. Controllable in-situ synthesis of Ag/BiOI and Ag/AgI/BiOI composites with adjustable visible light photocatalytic performances. Mater Lett 2014;132:141–4. [14] Zheng ZF, Chen C, Bo A, Zavahir FS, Waclawik ER, Zhao J, et al. Visible-light-induced selective photocatalytic oxidation of benzylamine into imine over supported Ag/AgI photocatalysts. ChemCatChem 2014;6:1210–14. [15] Akhundi A, Habibi-Yangieh A. Ternary magnetic g-C3 N4 /Fe3 O4 /AgI nanocomposites: novel recyclable photocatalysts with enhanced activity in degradation of different pollutants under visible light. Mater Chem Phys 2016;174:59–69.
Please cite this article as: J. Zhang, Z. Ma, AgI/β -Ag2 MoO4 heterojunctions with enhanced visible-light-driven catalytic activity, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.018
JID: JTICE
ARTICLE IN PRESS
[m5G;November 8, 2017;22:6]
J. Zhang, Z. Ma / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–7 [16] Chen F, Yang Q, Niu CG, Li XM, Zhang C, Zhao JW, et al. Enhanced visible light photocatalytic activity and mechanism of ZnSn(OH)6 nanocubes modified with AgI nanoparticles. Catal Commun 2016;73:1–6. [17] Cui DH, Song XC, Zheng YF. A novel AgI/BiOIO3 nanohybrid with improved visible-light photocatalytic activity. RSC Adv 2016;6:71983–8. [18] Islam MJ, Reddy DA, Ma R, Kim Y, Kim TK. Reduced-graphene-oxide-wrapped BiOI-AgI heterostructured nanocomposite as a high-performance photocatalyst for dye degradation under solar light irradiation. Solid State Sci 2016;61:32–9. [19] Reddy DA, Choi J, Lee S, Kim TK. Controlled synthesis of heterostructured Ag@AgI/ZnS microspheres with enhanced photocatalytic activity and selective separation of methylene blue from mixture dyes. J Taiwan Inst Chem Eng 2016;66:200–9. [20] Shekofteh-Gohari M, Habibi-Yangjeh A. Ultrasonic-assisted preparation of novel ternary ZnO/AgI/Fe3 O4 nanocomposites as magnetically separable visible-light-driven photocatalysts with excellent activity. J Colloid Interface Sci 2016;461:144–53. [21] Vinoth R, Karthik P, Muthamizhchelvan C, Neppolian B, Ashokkumar M. Carrier separation and charge transport characteristics of reduced graphene oxide supported visible-light active photocatalysts. Phys Chem Chem Phys 2016;18:5179–91. [22] Wang Q, Shi XD, Liu E, Crittenden JC, Ma XJ, Zhang Y, et al. Facile synthesis of AgI/BiOI-Bi2 O3 multi-heterojunctions with high visible light activity for Cr(VI) reduction. J Hazard Mater 2016;317:8–16. [23] Wang TY, Quan W, Jiang DL, Chen LL, Li D, Meng SC, et al. Synthesis of redox– mediator-free direct Z-scheme AgI/WO3 nanocomposite photocatalysts for the degradation of tetracycline with enhanced photocatalytic activity. Chem Eng J 2016;300:280–90. [24] Wang XH, Yang J, Ma SQ, Zhao D, Dai J, Zhang DF. In situ fabrication of AgI/AgVO3 nanoribbon composites with enhanced visible photocatalytic activity for redox reactions. Catal Sci Technol 2016;6:243–53. [25] Zeng C, Hu YM, Guo YX, Zhang TR, Dong F, Du X, et al. Achieving tunable photocatalytic activity enhancement by elaborately engineering composition-adjustable polynary heterojunctions photocatalysts. Appl Catal B Environ 2016;194:62–73. [26] Razi F, Zinatloo-Ajabshir S, Salavati-Niasari M. Preparation, characterization and photocatalytic properties of Ag2 ZnI4 /AgI nanocomposites via a new simple hydrothermal approach. J Mol Liq 2017;225:645–51. [27] Liu EY, Gao YM, Jia JH, Bai YP. Friction and wear behaviors of Ni-based composites containing graphite/Ag2 MoO4 lubricants. Tribol Lett 2013;50:313–22. [28] Bhattacharya S, Ghosh A. Silver molybdate nanoparticles, nanowires, and nanorods embedded in glass nanocomposites. Phys Rev B 2007;75:2103. [29] De Santana YVB, Gomes JEC, Matos L, Cruvinel GH, Perrin A, Perrin C, et al. Silver molybdate and silver tungstate nanocomposites with enhanced photoluminescence. Nanomater Nanotechnol 2014;4:1627–35. [30] Cheng L, Shao Q, Shao MW, Wei XW, Wu ZC. Photoswitches of one-dimensional Ag2 MO4 (M = Cr, Mo, and W). J Phys Chem C 2009;113:1764–8. [31] Xu DF, Cheng B, Zhang JF, Wang WK, Yu JG, Ho WK. Photocatalytic activity of Ag2 MO4 (M = Cr, Mo, W) photocatalysts. J Mater Chem A 2015;3:20153–66.
7
[32] Oliveira CA, Volanti DP, Nogueira AE, Zamperini CA, Vergani CE, Longo E. Well-designed β -Ag2 MoO4 crystals with photocatalytic and antibacterial activity. Mater Des 2017;115:73–81. [33] Bai YY, Lu Y, Liu JK. An efficient photocatalyst for degradation of various organic dyes: Ag@Ag2 MoO4 -AgBr composite. J Hazard Mater 2016;307:26–35. [34] Dhanabal R, Velmathi S, Bose AC. High efficiency new visible light driven Ag2 MoO4 -Ag3 PO4 composite photocatalyst towards degradation of industrial dyes. Catal Sci Technol 2016;6:8449–63. [35] Bai YY, Wang FR, Liu JK. A new complementary catalyst and catalytic mechanism: Ag2 MoO4 /Ag/AgBr/GO heterostructure. Ind Eng Chem Res 2016;55:9873–9. [36] Zhang JL, Ma Z. Flower-like Ag2 MoO4 /Bi2 MoO6 heterojunctions with enhanced photocatalytic activity under visible light irradiation. J Taiwan Inst Chem Eng 2017;71:156–64. [37] Zhang JL, Ma Z. Novel β -Ag2 MoO4 /g-C3 N4 heterojunction catalysts with highly enhanced visible-light-driven photocatalytic activity. RSC Adv 2017;7:2163–71. [38] Zhang JL, Liu H, Ma Z. Flower-like Ag2 O/Bi2 MoO6 p-n heterojunction with enhanced photocatalytic activity under visible light irradiation. J Mol Catal A Chem 2016;424:37–44. [39] Zhang JL, Ma Z. Ag6 Mo10 O33 /g-C3 N4 1D-2D hybridized heterojunction as an efficient visible-light-driven photocatalyst. Mol Catal 2017;432:285–91. [40] Zhang JL, Ma Z. Flower-like Ag3 VO4 /BiOBr n-p heterojunction photocatalysts with enhanced visible-light-driven catalytic activity. Mol Catal 2017;436:190–8. [41] Cheng RL, Zhang LX, Fan XQ, Wang M, Li ML, Shi JL. One-step construction of FeOx modified g-C3 N4 for largely enhanced visible-light photocatalytic hydrogen evolution. Carbon 2016;101:62–70. [42] Gouveia AF, Sczancoski JC, Ferrer MM, Lima AS, Santos MRMC, Li MS, et al. Experimental and theoretical investigations of electronic structure and photoluminescence properties of β -Ag2 MoO4 microcrystals. Inorg Chem 2014;53:5589–99. [43] Yue LF, Wang SF, Shan GQ, Wu W, Qiang LW, Zhu LY. Novel MWNTs-Bi2 WO6 composites with enhanced simulated solar photoactivity toward adsorbed and free tetracycline in water. Appl Catal B Environ 2015;176:11–19. [44] Zhang JL, Zhang LS, Shen XF, Xu PF, Liu JS. Synthesis of BiOBr/WO3 p-n heterojunctions with enhanced visible light photocatalytic activity. CrystEngComm 2016;18:3856–65. [45] Zhu TT, Huang LY, Song YH, Chen ZG, Ji HY, Li YP, et al. Modification of Ag3 VO4 with graphene-like MoS2 for enhanced visible-light photocatalytic property and stability. New J Chem 2016;40:2168–77. [46] Zong X, Yan HJ, Wu GP, Ma GJ, Wen FY, Wang L, et al. Enhancement of photocatalytic H2 evolution on CdS by loading MoS2 as cocatalyst under visible light irradiation. J Am Chem Soc 2008;130:7176–7. [47] Pan H. Principles on design and fabrication of nanomaterials as photocatalysts for water-splitting. Renewable Sustainable Energy Rev 2016;57:584–601. [48] Jing LQ, Xu YG, Huang SQ, Xie M, He MQ, Xu H, et al. Novel magnetic CoFe2 O4 /Ag/Ag3 VO4 composites: highly efficient visible light photocatalytic and antibacterial activity. Appl Catal B Environ 2016;199:11–22.
Please cite this article as: J. Zhang, Z. Ma, AgI/β -Ag2 MoO4 heterojunctions with enhanced visible-light-driven catalytic activity, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.018
JID: JTICE
[m5G;November 8, 2017;22:6]
Journal of the Taiwan Institute of Chemical Engineers 0 0 0 (2017) 1–7
Contents lists available at ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
AgI/β -Ag2 MoO4 heterojunctions with enhanced visible-light-driven catalytic activity Junlei Zhang, Zhen Ma∗ Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3 ), Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, PR China
a r t i c l e
i n f o
Article history: Received 7 June 2017 Revised 30 September 2017 Accepted 18 October 2017 Available online xxx Keywords: AgI/β -Ag2 MoO4 Heterojunctions Photocatalysis Dyes Tetracycline hydrochloride
a b s t r a c t Visible-light-driven photocatalysis for removing industrial dyes and antibiotics in waste water has attracted much interest. New catalysts with enhanced photocatalytic performance have been actively sought. Herein, via a simple sequential precipitation method, AgI nanoparticles were anchored onto micron-sized β -Ag2 MoO4 particles to form AgI/β -Ag2 MoO4 p–n heterojunctions. These heterojunctions exhibited remarkably upgraded activities in the visible-light-driven photocatalytic degradation of rhodamine B, methyl orange, and tetracycline hydrochloride compared with pristine AgI and β -Ag2 MoO4 . The enhanced photocatalytic activity can be mainly attributed to the tight heterojunction structure and well matched energy band that may facilitate the separation and transfer of photogenerated electron– hole pairs. Photogenerated holes (h+ ) and superoxide radical anions (•O2 − ) were found to be the main active species. A possible photocatalytic mechanism was proposed. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Environmental pollution caused by toxic and harmful organic pollutants is exacerbating the danger to humans and other living species. Photocatalysis, an environment-benign technology, can be used to eliminate organic contaminants [1]. A key research topic of photocatalysis is the development of highly efficient photocatalysts. Semiconductors based on silver halides (AgX, X = Cl, Br, or I) have attracted increasing interest [2–4]. Compared to AgCl and AgBr, AgI exhibits high potential in the development of visible-light-driven photocatalysts because of the relatively high photo-stability and suitable band gap (Eg = ∼2.8 eV) [4,5]. AgX (X = Cl, Br, or I) can be modified by plasmonic silver to obtain stable photocatalysts [6,7]. For instance, Ghosh et al. reported that Ag–AgI composite exhibited higher photocatalytic antibacterial activity than AgI [8]. Wang et al. found that Ag/AgI heterojunction prepared by one-step hydrothermal synthesis exhibited stable and efficient photocatalytic activity [9]. Other AgI-based/containing composites, such as Ag–AgI/Al2 O3 [10], Ag–AgI/Fe3 O4 @SiO2 [11], K4 Nb6 O17 /Ag@AgI [12], Ag/AgI/BiOI [13], Ag/AgI@TNTs [14], g-C3 N4 /Fe3 O4 /AgI [15], AgI/ZnSn(OH)6 [16], AgI/BiOIO3
∗
Corresponding author. E-mail address: [email protected] (Z. Ma).
[17], RGO/BiOI/AgI [18], Ag@AgI/ZnS [19], ZnO/AgI/Fe3 O4 [20], AgI/TiO2 /rGO [21], AgI/BiOI–Bi2 O3 [22], AgI/WO3 [23], AgI/AgVO3 [24], Bi7 O9 I3 /AgI/AgIO3 [25], Ag2 ZnI4 /AgI [26], and AgI/Bi2 O2 CO3 [5], have been developed. It is still interesting to develop AgIbased new photocatalysts for fundamental research and practical applications. Ag2 MoO4 has been used in the preparation of high-temperature lubricants [27], ion-conducting glasses [28], photoluminescent materials [29], and photoswitch devices [30], owing to its good thermal stability, conductivity, and photosensitivity. However, reports on the photocatalytic application of β -Ag2 MoO4 are rare [31], due to the wide band-gap (∼3.3 eV) of β -Ag2 MoO4 [32]. To develop better photocatalysts, β -Ag2 MoO4 has been coupled with other substances to form heterojunctions such as Ag@Ag2 MoO4 – AgBr [33], Ag2 MoO4 /Ag3 PO4 [34], Ag2 MoO4 /Ag/AgBr/GO [35], β -Ag2 MoO4 /Bi2 MoO6 [36], and β -Ag2 MoO4 /g–C3 N4 [37]. However, to the best of our knowledge, AgI/β -Ag2 MoO4 heterojunctions have not been reported. Considering that the band edges between AgI (ECB : −0.40 eV; EVB : 2.40 eV [24]) and β -Ag2 MoO4 (ECB : −0.18 eV; EVB : 3.02 eV [37]) are well matched, in this work we developed AgI/β -Ag2 MoO4 composites by a sequential precipitation method. These catalysts were tested in the visible-light-driven photocatalytic degradation of rhodamine B (RhB), methyl orange (MO), and tetracycline hydrochloride (TC). Relevant samples were characterized, and possible reasons for the enhanced photocatalytic activity were investigated.
https://doi.org/10.1016/j.jtice.2017.10.018 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: J. Zhang, Z. Ma, AgI/β-Ag2 MoO4 heterojunctions with enhanced visible-light-driven catalytic activity, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.018
ARTICLE IN PRESS
JID: JTICE 2
[m5G;November 8, 2017;22:6]
J. Zhang, Z. Ma / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–7
Fig. 1. XRD patterns of AgI/β -Ag2 MoO4 composites and standard XRD patterns of β -Ag2 MoO4 (JCPDS 08-0473) and AgI (JCPDS 09-0374).
4 mmol AgNO3 was dissolved in a 100 mL breaker containing 30 mL deionized water to form solution A, and 2 mmol Na2 MoO4 ·2H2 O was dissolved in another 100 mL breaker containing 30 mL deionized water to form solution B. Solution B was then added dropwise into solution A, under vigorous stirring (1200 r/min), to obtain β Ag2 MoO4 . Then, a certain amount of AgNO3 (0.25, 0.5, 1, 2, 4, 8, or 16 mmol) was added into the above system, and the mixture was magnetically stirred for 30 min to obtain Ag+ -β -Ag2 MoO4 . Subsequently, a solution containing 0.25, 0.5, 1, 2, 4, 8, or 16 mmol KI was added dropwise into the above mixture, and the mixture was continuously stirred for 4 h. The solid collected by filtration was washed with deionized water for three times, and dried at 60 °C for 24 h. These AgI/β -Ag2 MoO4 composites were named as S1, S2, S3, S4, S5, S6, and S7, respectively. The theoretical AgI/β -Ag2 MoO4 molar ratios of these composites (S1–S7 samples) are 1/8, 1/4, 1/2, 1/1, 2/1, 4/1, and 8/1, respectively. For comparison, the same procedure was adopted to prepare pristine AgI (or β -Ag2 MoO4 ) using 2 mmol AgNO3 and 2 mmol KI (or 1 mmol Na2 MoO4 ·2H2 O) as the starting materials.
2. Experimental
2.3. Characterization
2.1. Materials
X-ray diffraction (XRD) experiments were carried out on a MSAL XD2 X-ray diffractometer using CuKα radiation at 40 kV and 30 mA with a scanning speed of 8°/min. X-ray photoelectron spectroscopic (XPS) data were collected via a multifunctional photoelectron spectroscopy instrument (Axis Ultra Dld, Kratos). Scanning electron microscopy (SEM) experiments were conducted on a Shimadzu SUPERSCAN SSX-550 field emission scanning electron microscope. Optical diffuse reflectance spectra were recorded on a UV–Vis–NIR scanning spectrophotometer (Lambda 35, PerkinElmer) with an integrating sphere accessory. Photoluminescence (PL) spectra were collected using a LabRAM HR Evolution in-
AgI JCPDS 09-0374
Intensity (a.u.)
S7 S6 S5 S4 S3 S2 S1 Ag2MoO4 JCPDS 08-0473
20
30
40
50
60
70
2 Theta (degree)
AgNO3 , Na2 MoO4 ·2H2 O, KI, rhodamine B (RhB), and methyl orange (MO) of analytical grade were purchased from Sinopharm Chemical Reagent. Tetracycline hydrochloride (TC) was purchased from Aladdin. All reagents were used as received. 2.2. Synthesis of AgI/β -Ag2 MoO4 AgI/β -Ag2 MoO4 composites were prepared by a sequential precipitation method at room temperature. In a typical synthesis,
Fig. 2. XPS spectra of (A) Ag 3d, (B) I 3d, (C) Mo 3d and (D) O 1s peaks related to the samples.
Please cite this article as: J. Zhang, Z. Ma, AgI/β -Ag2 MoO4 heterojunctions with enhanced visible-light-driven catalytic activity, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.018
ARTICLE IN PRESS
JID: JTICE
[m5G;November 8, 2017;22:6]
J. Zhang, Z. Ma / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–7
3
strument (HORIBA JY company, France) with an excitation wavelength at 355 nm. Electrochemical impedance spectroscopy (EIS) and Mott–Schottky measurements were performed by an electrochemical analyzer (CHI 660B Chenhua Instrument Company). 2.4. Evaluation of photocatalytic activity Photocatalytic degradation of RhB/MO/TC was conducted under visible light at room temperature. 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 source (Fig. S1 in the Supporting Information) [36–40]. Typically, 30 mg photocatalyst was suspended in 50 mL RhB (10 mg/L), 50 mL MO (10 mg/L), or 50 mL TC (20 mg/L) solution in a 100 mL beaker. The mixture (containing a solution and a catalyst) was stirred for 30 min in the dark to establish the adsorptiondesorption equilibrium. The Xe lamp was then turned on, and the solution was still stirred at a speed of 800 r/min. During the reaction, the solution was sampled in certain intervals and centrifuged, and the supernatant was analyzed by a UV-5200 PC spectrometer. 3. Results and discussion 3.1. Characterization of the samples The XRD patterns of the as-prepared AgI and β -Ag2 MoO4 (Fig. S2) show characteristic peaks of hexagonal AgI (JCPDS 09-0374) and cubic Ag2 MoO4 (JCPDS 08-0473), respectively [33,37]. The XRD patterns of AgI/β -Ag2 MoO4 composites (S1–S7) can be indexed to a combination of AgI and Ag2 MoO4 (Fig. 1). The peaks at 2θ = 22.3°, 23.7°, 25.3°, 32.7°, 39.2°, 42.6°, 45.6°, 46.3°, 47.2°, 52.0°, 59.3°, 61.6°, 63.0°, 66.5°, 71.0°, and 73.4° can be assigned to the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (2 0 1), (2 0 2), (2 0 3), (2 1 0), (2 1 1), (1 0 5), (3 0 0), and (2 1 3) planes of hexagonal AgI, while other peaks at 2θ = 16.5°, 27.1°, 31.8°, 33.3°, 38.6°, 42.2°, 47.8°, 50.9°, 55.8°, 63.1°, 65.7°, and 66.6° are assigned to the (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1), (4 2 2), (5 1 1), (4 4 0), (6 2 0), (5 3 3), and (6 2 2) planes of cubic Ag2 MoO4 , respectively [33,37]. The intensities of the AgI peaks enhance gradually as the AgI/β -Ag2 MoO4 molar ratio increases, while the peaks due to Ag2 MoO4 decline. No peaks of any impurity phase can be observed. According to the XPS spectra in Fig. S3, AgI, AgI/β -Ag2 MoO4 (S7), and β -Ag2 MoO4 contain the corresponding elements. For AgI/β -Ag2 MoO4 (S7), two characteristic peaks for Ag 3d are located at 368.68 and 374.69 eV (attributed to Ag 3d5/2 and Ag 3d7/2 of Ag+ , respectively), indicating that no Ag0 exists in S7 (Fig. 2A). The peaks at 619.90 and 631.40 eV (Fig. 2B) are for I 3d5/2 and I 3d3/2 , respectively, confirming the existence of I− in S7. Fig. 2C shows two peaks at 231.90 and 235.00 eV, corresponding to the Mo 3d5/2 and Mo 3d3/2 binding energies of Mo6+ . The O 1s spectrum of S7 (Fig. 2D) shows a peak at 530.30 eV assigned to hydroxyls (−OH) [41] and another peak at 532.10 eV ascribed to oxygen in Ag2 MoO4 [37]. Notably, the binding energies of Ag 3d, I 3d, Mo 3d, and O 1s for AgI/β -Ag2 MoO4 (S7) are slightly higher than those of pristine AgI and/or β -Ag2 MoO4 , probably owing to the interaction between AgI and β -Ag2 MoO4 . Figs. 3 and S4 show the SEM images of β -Ag2 MoO4 , AgI/β Ag2 MoO4 composites, and AgI. β -Ag2 MoO4 prepared by precipitation consists of many irregular particles with sizes ranging from several to ten microns, and all the particles exhibit smooth surfaces (Fig. 3A). With the deposition of AgI in the above system, lots of nanoparticles emerge on the surface of micron-sized β -Ag2 MoO4 particles (Fig. 3B–H). These nanoparticles should be AgI, as seem from the SEM image of pristine AgI (Fig. S4). With the increase of
Fig. 3. SEM images of β -Ag2 MoO4 (A) and AgI/β -Ag2 MoO4 composites (S1–S7 samples for B–H images).
the AgI/β -Ag2 MoO4 molar ratio, more and more AgI nanoparticles are observed in S1–S7 samples (Fig. 3B–H). The TEM image further confirms that AgI/β -Ag2 MoO4 composite (S7) consists of lots of nanoparticles and micrometer-sized particles (Fig. 4A). The corresponding SAED image illustrates the polycrystalline characteristics of both hexagonal AgI and cubic Ag2 MoO4 (Fig. 4B). EDX data were further used to explore the compositions of AgI/β -Ag2 MoO4 composites (S1–S7). The existence of Ag, Mo, I, and O elements in AgI/β -Ag2 MoO4 composites is again confirmed (Fig. S5). The AgI/Ag2 MoO4 molar ratios of AgI/β Ag2 MoO4 composites (S1–S7) are determined as 1/7.95, 1/4.18, 1/1.96, 1/1.03, 2.37/1, 4.48/1, and 8.67/1, respectively (the inset of Fig. S5), in line with the theoretical values. The optimal properties of samples were investigated by UV– Vis–NIR DRS (Fig. 5A). β -Ag2 MoO4 exhibits strong absorption in the UV region with an absorption edge around 389 nm, i.e., it can only be excited by UV light [31,33,36,37,42]. AgI/β -Ag2 MoO4 composites show absorption from visible-light to UV region, and the absorption edges with different AgI/β -Ag2 MoO4 molar ratios somehow change in an orderly pattern, gradually closing to that (463 nm) of AgI. The optical absorption near the band edge of a semiconductor follows the Tauc equation: α hν = A (hν − Eg )n /2 , where A, α , h, ν , and Eg are a constant, absorption coefficient, Planck’s constant, light frequency, and band gap, respectively [5,31]. The power index n equals to 1 for a direct band-gap material and 4 for an indi-
Please cite this article as: J. Zhang, Z. Ma, AgI/β -Ag2 MoO4 heterojunctions with enhanced visible-light-driven catalytic activity, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.018
ARTICLE IN PRESS
JID: JTICE 4
[m5G;November 8, 2017;22:6]
J. Zhang, Z. Ma / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–7
Fig. 4. TEM and SEAD images of AgI/β -Ag2 MoO4 composite (S7).
rect band-gap material. The n value for β -Ag2 MoO4 and AgI is 1. Fig. 5B shows the plots of (α hν )1/2 versus energy (hν ). The Eg values of AgI, AgI/β -Ag2 MoO4 composites (S1–S7), and β -Ag2 MoO4 are calculated to be ∼2.68, ∼2.76, and ∼3.19 eV, respectively.
ied. Before irradiation, the reaction mixture was kept in the dark for 30 min to achieve an adsorption–desorption equilibrium. From Fig. 6A, it seems that AgI/β -Ag2 MoO4 composites can adsorb more pollutant molecules in the dark than pristine AgI and β -Ag2 MoO4 . The specific BET surface area of a typical AgI/β -Ag2 MoO4 composite (S7) is 0.17 m2 /g (Fig. S6), whereas those of AgI and β -Ag2 MoO4 are below the detection limit (i.e., close to 0). The visible-light source (λ >400 nm) was subsequently turned on to start the photocatalytic reaction. As shown in Fig. 6A, RhB dye can hardly be decomposed without a catalyst. RhB can be degraded more quickly using AgI than using β -Ag2 MoO4 , because β -Ag2 MoO4 can only be excited by UV light (Fig. 5A). RhB can be more efficiently removed using AgI/β -Ag2 MoO4 composites. The optimal catalysts are S6 and S7 where the theoretical AgI/β Ag2 MoO4 molar ratios are 4/1 and 8/1, respectively. As shown in Fig. 6B, MO can hardly be removed in the absence of a catalyst. When using pristine β -Ag2 MoO4 or AgI, MO can be degraded very slowly. AgI/β -Ag2 MoO4 composites are much more active than pristine β -Ag2 MoO4 or AgI. The optimal catalysts are still S6 and S7. As shown in Fig. 6C, no degradation of TC happens under visible irradiation in the absence of a catalyst. AgI/β -Ag2 MoO4 composites still exhibit higher photocatalytic activity than pristine AgI and β Ag2 MoO4 . The optimal catalysts are still S6 and S7. The effect of catalyst dosage on TC removal efficiency was studied (Fig. S7A). When the catalyst (S7) dosage is less than 50 mg, the removal efficiency of TC (20 mg/L, 50 mL) increases along with increasing dosage. When the S7 dosage is 70 mg, TC removal efficiency is hardly raised, suggesting that the optimal S7 dosage is around 50 mg under the present condition. When the S7 dosage is 30 mg, TC removal efficiency is gradually improved with the decrease in the initial concentration of TC (Fig. S7B). The stability of catalyst was tested. As shown in Fig. 7A, four cycles of TC degradation experiments verify that AgI/β -Ag2 MoO4 composite (S7) has excellent recyclability. By comparing the XRD patterns of used and fresh S7 samples (Fig. 7B), one can conclude that the crystal phase is relatively stable except for the formation of a few Ag0 species due to photo-reduction [39]. The recyclability of AgI/β -Ag2 MoO4 composite is also confirmed by the degradation of RhB and MO (Fig. S8).
3.2. Photocatalytic activities of samples
3.3. Photocatalytic degradation mechanism
The visible-light-driven photocatalytic degradation of industrial dyes (RhB and MO) and tetracycline hydrochloride (TC) were stud-
Ammonium oxalate (AO), isopropyl alcohol (IPA), and benzoquinone (BQ) were used as scavengers to capture photogenerated
B
Absorption (a.u.)
Ag2MoO4 S1 S2 S3 S4 S5 S6 S7 AgI
200
300
400
500
600
Wavelength (nm)
700
800
Ag2MoO4
(αhv)1/2 (eV)1/2
A
AgI S1 S2 S3 S4 S5 S6 S7
2.76 eV 2.68 eV 3.19 eV
2
3
4
5
6
Energy (eV)
Fig. 5. (A) UV–vis diffuse reflectance spectra of AgI, AgI/β -Ag2 MoO4 composites, and β -Ag2 MoO4 ; (B) the plots of (α hν )1/2 versus the bandgap (eV) of AgI and β -Ag2 MoO4 .
Please cite this article as: J. Zhang, Z. Ma, AgI/β -Ag2 MoO4 heterojunctions with enhanced visible-light-driven catalytic activity, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.018
ARTICLE IN PRESS
JID: JTICE
[m5G;November 8, 2017;22:6]
J. Zhang, Z. Ma / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–7
5
A
A 1.0
1.0
1st run
2nd run
3rd run
4th run
light on blank Ag2MoO4 AgI S1 S2
C/C0
0.6
dark
0.4
0.8
C/C0
0.8
S3 S4 S5 S6 S7
0.2
0.4
0.2
0.0 -30
0
15
30
45
0
60
80
160
240
320
Time (min)
Time (min)
B
B
1.0 blank Ag2MoO4 AgI S1 S2
light on dark
0.6 S3 S4 S5 S6 S7
0.4 0.2
fresh S7
Intensity (a.u.)
0.8
C/C0
0.6
Ag0
used S7
0.0 -30
0
15
30
45
60
Time (min)
C
C/C0
0.8
0.6 S3 S4 S5 S6 S7
0.4
0.2 -30
0
15
30
40
50
60
70
Fig. 7. (A) Cycling runs in photocatalytic degradation of TC (20 mg/L, 50 mL) over S7 (30 mg) under visible light irradiation; (B) XRD patterns of S7 before and after four cycling runs.
blank Ag2MoO4 AgI S1 S2
dark
30
2 Theta (degree)
1.0 light on
20
45
60
Time (min) Fig. 6. The adsorption and degradation of (A) RhB, (B) MO, and (C) TC with or without a catalyst (30 mg) under visible light irradiation.
holes (h+ ), hydroxyl free radicals (•OH), and superoxide radical anions (•O2 − ), respectively [43,44], in order to provide some information for the reaction mechanism. As shown in Fig. S9, the addition of IPA (1 mmol) has virtually no impact on the photodegradation of RhB, confirming that •OH is not the active species. However, when adding AO (1 mmol) or BQ (0.02 mmol), the degradation efficiency of RhB (after 15 min of irradiation) decreases from 92.0% to 45.3% or 73.9%, indicating that h+ and •O2 − are the main active species. This conclusion was again confirmed in the radical-capture experiments involving MO and TC. Photoluminescence (PL) emission was employed to investigate the combination and separation of the photogenerated carriers
[44]. As seen in Fig. S10A, the PL intensity of AgI/β -Ag2 MoO4 composite (S7) is much lower than those of AgI and β -Ag2 MoO4 , indicating that the recombination of photoexcited electron–hole pairs could be efficiently inhibited though the formation of AgI/β Ag2 MoO4 heterojunction. Electrochemical impedance spectroscopy (EIS) can be used to characterize the interfacial charge transfer properties and the separation efficiency of photogenerated carriers. A smaller arc radius indicates higher efficiency in charge transfer [45]. As shown in Fig. S10B, the arc radius of AgI/β -Ag2 MoO4 composite (S7) is smaller than that of AgI, indicating that S7 can facilitate the interfacial charge transfer and separate carriers efficiently. The band edge positions of AgI (Eg = ∼2.68 eV) and β -Ag2 MoO4 (Eg = ∼3.19 eV) were further evaluated using empirical equations [46]: EVB = X – E0 + 0.5Eg ; ECB = EVB – Eg , where EVB is the VB edge potential and X is the electronegativity of the semiconductor (i.e., the geometric mean of the electronegativity of the constituent atoms). The X values for AgI and β -Ag2 MoO4 are calculated to be 5.48 eV [24] and 5.92 eV [33], respectively. E0 is the energy of free electrons on the hydrogen scale (∼4.5 eV) and Eg is the band gap energy of the semiconductor. According to these equations, the EVB and ECB values of AgI are determined to be 2.32 and −0.36 eV, respectively. Those of β -Ag2 MoO4 are 3.02 and −0.17 eV, respectively. A possible photocatalytic reaction mechanism is proposed (Fig. 8), based on the above results and energy band diagram. When p-AgI and n-Ag2 MoO4 semiconductors form p-n heterojunction in the dark, both AgI and β -Ag2 MoO4 semiconductors have
Please cite this article as: J. Zhang, Z. Ma, AgI/β -Ag2 MoO4 heterojunctions with enhanced visible-light-driven catalytic activity, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.018
JID: JTICE 6
ARTICLE IN PRESS
[m5G;November 8, 2017;22:6]
J. Zhang, Z. Ma / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–7
Fig. 8. The possible mechanism of photocatalytic reaction on AgI/β -Ag2 MoO4 heterojunction.
the same Fermi level after reaching the charge equilibrium. The higher Fermi energy of n-Ag2 MoO4 versus p-AgI makes the energy bands of n-Ag2 MoO4 bend upward and those of p-AgI bend downward toward the interface after the two semiconductors are in contact and reach an electrical equilibrium. Thus, the type-II band alignment is formed in AgI/β -Ag2 MoO4 composites, which is very beneficial for the visible absorption and carrier mobility [47]. Under visible light irradiation, the electrons (e− ) in β -Ag2 MoO4 cannot be excited from the VB to CB but the electrons photoexcited in the CB of AgI still drift to the CB of β -Ag2 MoO4 . Since the CB of AgI (higher than −0.36 eV) is more negative than the potential of O2 /•O2 − (−0.33 eV vs. NHE), the photogenerated electrons can reduce O2 to yield •O2 − , further participating in the degradation of organic molecules. The transformation of Ag+ into Ag0 usually takes place under light irradiation. Ag0 can be further excited to produce electrons to be trapped by adsorbed O2 in water to produce •O2 − [48] for the photodegradation. In addition, since the potentials of the •OH/OH− (2.38 eV vs. NHE) and •OH/H2 O (2.72 eV vs. NHE) are more positive than the VB of AgI (lower than 2.32 eV), the enriched holes on the VB of AgI cannot react with OH− and H2 O to form •OH radicals. Such enriched holes can react with the adsorbed molecules on catalyst surfaces. 4. Conclusions Novel AgI/β -Ag2 MoO4 composites with different AgI/β Ag2 MoO4 molar ratios were prepared by sequential precipitation through which AgI nanoparticles were deposited onto the surface of micron-sized β -Ag2 MoO4 particles. Compared to pristine AgI and β -Ag2 MoO4 , these composites exhibited significantly improved photocatalytic performance in the degradation of dyes (RhB and MO) and antibiotic (TC) under visible-light irradiation and good recyclability. Such enhanced and stable photocatalytic activity can be attributed to the strong interfacial interactions between AgI and β -Ag2 MoO4 , which can promote the transfer of the photo-generated electrons from AgI to β -Ag2 MoO4 to generate •O2 − by reducing the surface chemisorbed O2 . The synergistic effect of •O2 − and the photo-generated holes resulted in the enhanced degradation of RhB/MO/TC. Acknowledgment We acknowledge the financial support by National Natural Science Foundation of China (grant no. 21477022).
Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2017.10.018. References [1] Devi LG, Kavitha R. A review on non metal ion doped titania for the photocatalytic degradation of organic pollutants under UV/solar light: role of photogenerated charge carrier dynamics in enhancing the activity. Appl Catal B Environ 2013;140–141:559–87. [2] Fan YY, Ma WG, Han DX, Gan SY, Dong XD, Niu L. Convenient recycling of 3D AgX/graphene aerogels (X = Br, Cl) for efficient photocatalytic degradation of water pollutants. Adv Mater 2015;27:3767–73. [3] Yu HG, Liu L, Wang XF, Wang P, Yu JG, Wang YH. The dependence of photocatalytic activity and photoinduced self-stability of photosensitive AgI nanoparticles. Dalton Trans 2012;41:10405–11. [4] Xiao D, Geng GW, Chen PL, Li TS, Liu MH. Sheet-like and truncated-dodecahedron-like AgI structures via a surfactant-assisted protocol and their morphology-dependent photocatalytic performance. Phys Chem Chem Phys 2017;19:837–45. [5] Zhang LL, Hu C, Ji HH. p-AgI anchored on {001} facets of n-Bi2 O2 CO3 sheets with enhanced photocatalytic activity and stability. Appl Catal B Environ 2017;205:34–41. [6] An CH, Wang ST, Sun YG, Zhang QH, Zhang J, Wang CY, et al. Plasmonic silver incorporated silver halides for efficient photocatalysis. J Mater Chem A 2016;4:4336–52. [7] Ma XC, Dai Y, Guo M, Huang BB. The role of effective mass of carrier in the photocatalytic behavior of silver halide-based Ag@AgX (X = Cl, Br, I): a theoretical study. ChemPhysChem 2012;13:2304–9. [8] Ghosh S, Saraswathi A, Indi SS, Hoti SL, Vasan HN. Ag@AgI, core@shell structure in agarose matrix as hybrid: synthesis, characterization, and antimicrobial activity. Langmuir 2012;28:8550–61. [9] Wang XJ, Wan XL, Li WQ, Chen XN. One-step hydrothermal synthesis of the Ag/AgI heterojunction with highly enhanced visible-light photocatalytic performances. Micro Nano Lett 2014;9:376–81. [10] Hu C, Peng TW, Hu XX, Nie YL, Zhou XF, Qu JH, et al. Plasmon-induced photodegradation of toxic pollutants with Ag-AgI/Al2 O3 under visible-light irradiation. J Am Chem Soc 2010;132:857–62. [11] Guo JF, Ma BW, Yin AY, Fan KN, Dai WL. Photodegradation of rhodamine B and 4-chlorophenol using plasmonic photocatalyst of Ag-AgI/Fe3 O4 @SiO2 magnetic nanoparticle under visible light irradiation. Appl Catal B Environ 2011;101:580–6. [12] Cui WQ, Wang H, Liang YH, Liu L, Han BX. Preparation of Ag@AgI-intercalated K4 Nb6 O17 composite and enhanced photocatalytic degradation of rhodamine B under visible light. Catal Commun 2013;36:71–4. [13] Lin HL, Zhao YJ, Wang YJ, Cao J, Chen SF. Controllable in-situ synthesis of Ag/BiOI and Ag/AgI/BiOI composites with adjustable visible light photocatalytic performances. Mater Lett 2014;132:141–4. [14] Zheng ZF, Chen C, Bo A, Zavahir FS, Waclawik ER, Zhao J, et al. Visible-light-induced selective photocatalytic oxidation of benzylamine into imine over supported Ag/AgI photocatalysts. ChemCatChem 2014;6:1210–14. [15] Akhundi A, Habibi-Yangieh A. Ternary magnetic g-C3 N4 /Fe3 O4 /AgI nanocomposites: novel recyclable photocatalysts with enhanced activity in degradation of different pollutants under visible light. Mater Chem Phys 2016;174:59–69.
Please cite this article as: J. Zhang, Z. Ma, AgI/β -Ag2 MoO4 heterojunctions with enhanced visible-light-driven catalytic activity, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.018
JID: JTICE
ARTICLE IN PRESS
[m5G;November 8, 2017;22:6]
J. Zhang, Z. Ma / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–7 [16] Chen F, Yang Q, Niu CG, Li XM, Zhang C, Zhao JW, et al. Enhanced visible light photocatalytic activity and mechanism of ZnSn(OH)6 nanocubes modified with AgI nanoparticles. Catal Commun 2016;73:1–6. [17] Cui DH, Song XC, Zheng YF. A novel AgI/BiOIO3 nanohybrid with improved visible-light photocatalytic activity. RSC Adv 2016;6:71983–8. [18] Islam MJ, Reddy DA, Ma R, Kim Y, Kim TK. Reduced-graphene-oxide-wrapped BiOI-AgI heterostructured nanocomposite as a high-performance photocatalyst for dye degradation under solar light irradiation. Solid State Sci 2016;61:32–9. [19] Reddy DA, Choi J, Lee S, Kim TK. Controlled synthesis of heterostructured Ag@AgI/ZnS microspheres with enhanced photocatalytic activity and selective separation of methylene blue from mixture dyes. J Taiwan Inst Chem Eng 2016;66:200–9. [20] Shekofteh-Gohari M, Habibi-Yangjeh A. Ultrasonic-assisted preparation of novel ternary ZnO/AgI/Fe3 O4 nanocomposites as magnetically separable visible-light-driven photocatalysts with excellent activity. J Colloid Interface Sci 2016;461:144–53. [21] Vinoth R, Karthik P, Muthamizhchelvan C, Neppolian B, Ashokkumar M. Carrier separation and charge transport characteristics of reduced graphene oxide supported visible-light active photocatalysts. Phys Chem Chem Phys 2016;18:5179–91. [22] Wang Q, Shi XD, Liu E, Crittenden JC, Ma XJ, Zhang Y, et al. Facile synthesis of AgI/BiOI-Bi2 O3 multi-heterojunctions with high visible light activity for Cr(VI) reduction. J Hazard Mater 2016;317:8–16. [23] Wang TY, Quan W, Jiang DL, Chen LL, Li D, Meng SC, et al. Synthesis of redox– mediator-free direct Z-scheme AgI/WO3 nanocomposite photocatalysts for the degradation of tetracycline with enhanced photocatalytic activity. Chem Eng J 2016;300:280–90. [24] Wang XH, Yang J, Ma SQ, Zhao D, Dai J, Zhang DF. In situ fabrication of AgI/AgVO3 nanoribbon composites with enhanced visible photocatalytic activity for redox reactions. Catal Sci Technol 2016;6:243–53. [25] Zeng C, Hu YM, Guo YX, Zhang TR, Dong F, Du X, et al. Achieving tunable photocatalytic activity enhancement by elaborately engineering composition-adjustable polynary heterojunctions photocatalysts. Appl Catal B Environ 2016;194:62–73. [26] Razi F, Zinatloo-Ajabshir S, Salavati-Niasari M. Preparation, characterization and photocatalytic properties of Ag2 ZnI4 /AgI nanocomposites via a new simple hydrothermal approach. J Mol Liq 2017;225:645–51. [27] Liu EY, Gao YM, Jia JH, Bai YP. Friction and wear behaviors of Ni-based composites containing graphite/Ag2 MoO4 lubricants. Tribol Lett 2013;50:313–22. [28] Bhattacharya S, Ghosh A. Silver molybdate nanoparticles, nanowires, and nanorods embedded in glass nanocomposites. Phys Rev B 2007;75:2103. [29] De Santana YVB, Gomes JEC, Matos L, Cruvinel GH, Perrin A, Perrin C, et al. Silver molybdate and silver tungstate nanocomposites with enhanced photoluminescence. Nanomater Nanotechnol 2014;4:1627–35. [30] Cheng L, Shao Q, Shao MW, Wei XW, Wu ZC. Photoswitches of one-dimensional Ag2 MO4 (M = Cr, Mo, and W). J Phys Chem C 2009;113:1764–8. [31] Xu DF, Cheng B, Zhang JF, Wang WK, Yu JG, Ho WK. Photocatalytic activity of Ag2 MO4 (M = Cr, Mo, W) photocatalysts. J Mater Chem A 2015;3:20153–66.
7
[32] Oliveira CA, Volanti DP, Nogueira AE, Zamperini CA, Vergani CE, Longo E. Well-designed β -Ag2 MoO4 crystals with photocatalytic and antibacterial activity. Mater Des 2017;115:73–81. [33] Bai YY, Lu Y, Liu JK. An efficient photocatalyst for degradation of various organic dyes: Ag@Ag2 MoO4 -AgBr composite. J Hazard Mater 2016;307:26–35. [34] Dhanabal R, Velmathi S, Bose AC. High efficiency new visible light driven Ag2 MoO4 -Ag3 PO4 composite photocatalyst towards degradation of industrial dyes. Catal Sci Technol 2016;6:8449–63. [35] Bai YY, Wang FR, Liu JK. A new complementary catalyst and catalytic mechanism: Ag2 MoO4 /Ag/AgBr/GO heterostructure. Ind Eng Chem Res 2016;55:9873–9. [36] Zhang JL, Ma Z. Flower-like Ag2 MoO4 /Bi2 MoO6 heterojunctions with enhanced photocatalytic activity under visible light irradiation. J Taiwan Inst Chem Eng 2017;71:156–64. [37] Zhang JL, Ma Z. Novel β -Ag2 MoO4 /g-C3 N4 heterojunction catalysts with highly enhanced visible-light-driven photocatalytic activity. RSC Adv 2017;7:2163–71. [38] Zhang JL, Liu H, Ma Z. Flower-like Ag2 O/Bi2 MoO6 p-n heterojunction with enhanced photocatalytic activity under visible light irradiation. J Mol Catal A Chem 2016;424:37–44. [39] Zhang JL, Ma Z. Ag6 Mo10 O33 /g-C3 N4 1D-2D hybridized heterojunction as an efficient visible-light-driven photocatalyst. Mol Catal 2017;432:285–91. [40] Zhang JL, Ma Z. Flower-like Ag3 VO4 /BiOBr n-p heterojunction photocatalysts with enhanced visible-light-driven catalytic activity. Mol Catal 2017;436:190–8. [41] Cheng RL, Zhang LX, Fan XQ, Wang M, Li ML, Shi JL. One-step construction of FeOx modified g-C3 N4 for largely enhanced visible-light photocatalytic hydrogen evolution. Carbon 2016;101:62–70. [42] Gouveia AF, Sczancoski JC, Ferrer MM, Lima AS, Santos MRMC, Li MS, et al. Experimental and theoretical investigations of electronic structure and photoluminescence properties of β -Ag2 MoO4 microcrystals. Inorg Chem 2014;53:5589–99. [43] Yue LF, Wang SF, Shan GQ, Wu W, Qiang LW, Zhu LY. Novel MWNTs-Bi2 WO6 composites with enhanced simulated solar photoactivity toward adsorbed and free tetracycline in water. Appl Catal B Environ 2015;176:11–19. [44] Zhang JL, Zhang LS, Shen XF, Xu PF, Liu JS. Synthesis of BiOBr/WO3 p-n heterojunctions with enhanced visible light photocatalytic activity. CrystEngComm 2016;18:3856–65. [45] Zhu TT, Huang LY, Song YH, Chen ZG, Ji HY, Li YP, et al. Modification of Ag3 VO4 with graphene-like MoS2 for enhanced visible-light photocatalytic property and stability. New J Chem 2016;40:2168–77. [46] Zong X, Yan HJ, Wu GP, Ma GJ, Wen FY, Wang L, et al. Enhancement of photocatalytic H2 evolution on CdS by loading MoS2 as cocatalyst under visible light irradiation. J Am Chem Soc 2008;130:7176–7. [47] Pan H. Principles on design and fabrication of nanomaterials as photocatalysts for water-splitting. Renewable Sustainable Energy Rev 2016;57:584–601. [48] Jing LQ, Xu YG, Huang SQ, Xie M, He MQ, Xu H, et al. Novel magnetic CoFe2 O4 /Ag/Ag3 VO4 composites: highly efficient visible light photocatalytic and antibacterial activity. Appl Catal B Environ 2016;199:11–22.
Please cite this article as: J. Zhang, Z. Ma, AgI/β -Ag2 MoO4 heterojunctions with enhanced visible-light-driven catalytic activity, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.10.018