Facile In Situ Self-Sacrifice Approach to Ternary ... - ACS Publications

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Facile In Situ Self-Sacrifice Approach to Ternary Hierarchical Architecture Ag/AgX (X = Cl, Br, I)/AgIO3 Distinctively Promoting Visible-Light Photocatalysis with Composition-Dependent Mechanism Chao Zeng,† Yingmo Hu,*,† Yuxi Guo,† Tierui Zhang,‡ Fan Dong,§ Yihe Zhang,† and Hongwei Huang*,† †

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China ‡ Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China § Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College of Environmental and Biological Engineering, Chongqing Technology and Business University, Chongqing 400067, China S Supporting Information *

ABSTRACT: Three series of ternary hierarchical architecture photocatalysts Ag/AgX (X = Cl, Br, I)/AgIO3 were fabricated for the first time by a facile in situ ion-exchange route. The novel ternary architectures are confirmed by XRD, XPS, SEM, TEM, EDX, and EDX mapping. In contrast to pristine AgIO3, the Ag/AgX (X = Cl, Br, I)/AgIO3 composites show extended absorption edges and highly boosted photoabsorption in the visible region, which are separately ascribed to the intrinsic absorption of AgX and the surface plasmon resonance (SPR) effect of Ag species. The photocatalysis activity of Ag/AgX (X = Cl, Br, I)/AgIO3 composites is studied and compared via photodegradation of methyl orange (MO) under visible-light (λ > 420 nm) irradiation. It is interesting to find that the activity enhancement levels are different for Ag/AgX (X = Cl, Br, I)/AgIO3 with four types of photocatalytic mechanism, which are closely related to the type of AgX or the component content in Ag/AgX (X = Cl, Br, I)/AgIO3. The separation behaviors of charge carrier were also systematically investigated by the PL and EIS. The study may furnish new perspective into controllable fabrication of hierarchical architecture photocatalysts with multiform photocatalytic mechanism. KEYWORDS: Photocatalyst, Ag, AgX (X = Cl, Br, I), AgIO3, Visible-light

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INTRODUCTION

Silver-containing materials show great promise, including Ag3PO4,6 Ag2CO3,7 Ag3VO4,8 AgX (X = Cl, Br, I),9 and so on. They exhibit superior photocatalytic activity for water splitting and pollutant photodegradation. However, the above silvercontaining photocatalysts still suffer from the downsides of high recombination rate of hole−electrons in the phtocatalysis process, which seriously restrict their practical applications. Thus, heterojunction photocatalytsts are fabricated to improve the catalytic performance. For instance, Ag3PO4/TiO2,10 AgPO4/WS2,11 etc. exhibit higher photocatalytic activity than pristine AgPO4 for degrading rhodamine B or methylene blue. Recently, AgIO3 was reported as a new photocatalyst, which can not only efficiently photodegrade dye12 but also convert CO2 to CH4 and CO.13 However, it has very weak absorption in the visible light region. Therefore, efforts should be made to enhance the visible light absorption and inhibit hole−electron recombination of AgIO3. As previously described, developing heterostructure is

Semiconductor photocatalysts have attracted great attention owing to their unique abilities for environmental purification and energy generation.1,2 Among the various semiconductor materials, titanium dioxide (TiO2) has been a widely researched photocatalyst because of its low cost, long-term stability, and nontoxicity. However, pristine TiO2 exhibits low quantum yields and inferior visible-light utilization efficiency due to its wide band gap (3.0−3.2 eV), which extremely restricts its practical application. Therefore, it is urgent to develop novel visiblelight-driven photocatalysts. Currently, there are two main strategies to obtain visible-light-driven photocatalysts. The first one is to broaden the photoresponse of TiO2 into the visible region by doping metal or nonmetal elements, coupling TiO2 with conducting polymer, and building a heterostructure construction.3 Another alternative strategy is to exploit new materials with a small band gap, such as sulfides, oxides, and oxygen acid salts.4,5 However, to date, these photocatalysts are still unappeasable for practical application due to the weak photoreactivity and low stability. © XXXX American Chemical Society

Received: February 18, 2016 Revised: April 5, 2016

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DOI: 10.1021/acssuschemeng.6b00348 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering an effective avenue to realize the above purpose. On the other hand, surface plasmon resonance (SPR) of Ag species can endow the Ag nanoparticles with stronger visible light absorption. For example, Yang et al. prepared a Ag/TiO2 plasmonic photocatalyst with high photocatalytic activity for degradation of gaseous acetone under visible light irradiation.14 Jiang et al. reported a Ag/BiOCl plasmonic photocatalyst that possesses efficient photoactivity under excitation of visible light.15 Besides, Ag/AgCl composite material shows noteworthy photocatalysis activity under visible light irradiation.16 Consequently, it is available to employ the SPR technique to broaden the response region of AgIO3. In a word, combining developing a heterostructure and utilizing an SPR technique is highly desirable and anticipated. In this work, we developed novel Ag/AgX (X = Cl, Br, I)/AgIO3 ternary heterojunction photocatalysts by a facile in situ ionexchange route for the first time, which are heterostructural and contain surface plasmon resonance (SPR). The photocatalysis properties of the Ag/AgX (X = Cl, Br, I)/AgIO3 composite are measured by decomposition of MO removal under visible light illumination (λ > 420 nm). The composite photocatalysts all depict greater photocatalytic activity under irradiation of visible light. The formation of the heterostructure photocatalysts and the diverse photocatalytic mechanism of three series composites are studied in detail.

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EXPERIMENTAL SECTION

Preparation of the Photocatalyst. All starting materials were of analytical grade and used as received. The hydrothermal method was used to prepare AgIO3. Representatively, 2 mmol of AgNO3 and stoichiometric I2O5 were dissolved in 40 mL of deionized water with stirring. Then, the resulting white suspension was transferred into a 50 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 24 h. After natural cooling, the product was collected by filtration, washed repeatedly with ethanol and distilled water, and then maintained at 80 °C for 10 h. The Ag/AgX (X = Cl, Br, I)/AgIO3 composites were synthesized by a facile in situ ion-exchange method between KX (X = Cl, Br, I) and a AgIO3 precursor. A total of 1 mmol of pristine precursor AgIO3 was dissolved in 20 mL of distilled water, and then the aqueous solution KX was added into the AgIO3 solution stepwise under magnetic agitation. After stirring for 5 h at room temperature, the suspension was filtrated, washed, and dried in a desiccator overnight. The as-prepared samples with molar ratios of KX/AgIO3 of 20%, 40%, 60%, 80%, and 100% are marked as 20%, 40%, 60%, 80%, and 100% Ag/AgX (X = Cl, Br, I)/AgIO3, respectively. Ag/AgX (X = Cl, Br, I) was fabricated by a precipitation route with KX (X = Cl, Br, I) and AgNO3. Characterization. X-ray powder diffraction (XRD) was recorded on a Bruker D8 focus with graphite monochromatized Cu Kα radiation (40 kV/40 mA). Scanning electron microscopy (SEM) images were carried out on a Hitachi S-4800 field emission scanning electron microscope operated at 10.0 kV. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were evaluated by JEM-2100 electron microscopy (JEOL, Japan). UV−vis diffuse reflectance spectra (DRS) were performed with a Varian Cary 5000 UV−vis spectrophotometer. The X-ray photoelectron spectroscopy (XPS) was identified on an ESCALAB 250xi (ThermoFsher, England) electron spectrometer. The photoluminescence emission (PL) spectra were conducted using a Hitachi F-4600 fluorescence spectrophotometer PL system with a xenon lamp (400 V, 150 W) as an excitation source. All the above-mentioned measurements were taken at room temperature. Photocatalytic Activity. The photocatalytic activities of Ag/AgX (X = Cl, Br, I)/AgIO3 were tested by photocatalytic decomposition of methyl orange (MO) under visible light with a 500 W Xe lamp (λ > 420 nm). In a typical procedure, 20 mg of photocatalyst was ultraphonically suspended into 40 mL of MO (2× 10−5 mol/L) aqueous solution. Before photoreaction, the suspension was magnetically stirred

Figure 1. XRD patterns of AgIO3, AgX (X = Cl, Br, I), and Ag/AgX (X = Cl, Br, I)/AgIO3 samples. in darkness for 1 h to obtain an adsorption−desorption equilibrium. Later, about 3 mL of the liquid was taken at a certain period time and separated through centrifugation to obtain the supernatant. The concentration of MO liquid was analyzed by measuring the absorbance at the characteristic band of 464 nm on a Cary 5000 UV−vis spectrophotometer. The temporal absorption spectra of MO liquid were obtained by the Cary 5000 UV−vis spectrophotometer. Active Species Trapping Experiment. To detect the active species generated in the photocatalytic process, disodium ethylenediaminetetraacetate (EDTA-2Na), ethylene glycol (IPA), and 1,4-benzoquinone (BQ) were chosen as the hole (h+), hydroxyl radical (•OH), and superoxide radical (•O2−) scavengers, respectively. Typically, 20 mg of photocatalyst with different scavengers (1 mmol) were ultraphonically dispersed in MO aqueous solution (40 mL, 2× 10−5 mol/L), and the following processes were similar to the above MO photodegradation experiment. Photoelectrochemical Measurement. The electrochemical impedance spectra (EIS) are recorded in a standard three-electrode system of the electrochemical station (CHI-660B, China) with Na2SO4 (0.1 M) as the electrolyte solution. The measurement is performed with irradiation of a 300 W xenon lamp equipped with a 420 nm filter at 0.0 V. The saturated calomel electrode (SCE) was chosen to be the reference B


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Figure 2. Typical XPS survey spectra (a) and Ag 3d spectra (b) of Ag/AgX (X = Cl, Br, I)/AgIO3 and a high-resolution XPS spectrum for Ag 3d of 100% Ag/AgCl/AgIO3 (c), 80% Ag/AgBr/AgIO3 (d), 40% Ag/AgI/AgIO3, (e) and 100% Ag/AgI/AgIO3 (f). electrode, and the platinum wires were employed as the counter electrode. The working electrode was the AgIO3, Ag/AgBr, and 80% Ag/AgBr/AgIO3 films coated on ITO.

To investigate the element composition and the chemical states of the as-obtained Ag/AgX (X = Cl, Br, I)/AgIO3 sample, X-ray photoelectron spectroscopy (XPS) analysis was performed and is shown in Figure 2. The main peaks corresponding to Ag 3d, I 3d, O 1s, Cl 2p, Br 3d, and C 1s all can be found in their perspective samples, and the C peak is due to the adventitious hydrocarbon of the XPS instrument (Figure 2a). Due to the distinct binding energy between Ag and X (X = Cl, Br, I), AgCl, AgBr, and AgI exhibit different positions of the Ag peak (Figure 2b). Figure 2c exhibits that the Ag 3d spectra of Ag/100%AgCl/AgIO3 could be divided into two sets of bands. The two peaks at 373.4 and 367.4 eV are attributed to Ag 3d3/2 and Ag 3d5/2 of Ag+; meanwhile the peaks at 374.1 and 368.0 eV can be assigned to Ag 3d3/2 and Ag 3d5/2 of Ag0 species, respectively.17,18 For the 80% Ag/AgBr/AgIO3 sample (Figure 2a), the obvious peaks of Ag, Br, I, O, and C elements can be observed. From Figure 2d, typical peaks of Ag 3d can be deconvoluted into two different peaks. The bands at 373.7 and 367.7 eV are ascribed

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RESULTS AND DISCUSSION The XRD patterns of the pristine AgIO3, Ag/AgX (X = Cl, Br, I), and Ag/AgX (X = Cl, Br, I)/AgIO3 composites are depicted in Figure 1. Take the Ag/AgCl/AgIO3 series as an instance (Figure 1a). It can be found that all the diffraction peaks can be well-indexed to the standard data of AgIO3 (ICSD # 14100) and AgCl (JCPDS # 1−1013), and the characteristic peaks of AgCl gradually strengthen with raising the KCl content. In addition, the existence of metallic Ag has been confirmed through the step scanning XRD (2θ ranging from 37.5° to 38.5°) in samples, which is attributed to the decomposition of AgCl. Similar results are also observed in Ag/AgBr/AgIO3 (Figure 1b) and Ag/AgI/AgIO3 (Figure 1c) series. These results evidenced that the ternary Ag/AgX (X = Cl, Br, I)/AgIO3 composites are obtained. C


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ACS Sustainable Chemistry & Engineering to Ag+, and the other two bands centered at 374.7 and 368.6 eV are ascribed to Ag0 species. With regard to 40% Ag/AgI/AgIO3 and 100% Ag/AgI/AgIO3 (Figure 2a), the XPS signals from Ag, Cl, I, O, and C elements can be seen. The XPS spectra of Ag 3d for 40% Ag/AgI/AgIO3 and 100% Ag/AgI/AgIO3 consist of two peaks at ∼374 and ∼368 eV. In the 40% Ag/AgI/AgIO3 sample (Figure 2e), the peaks at 374.5 and 368.5 eV match Ag0 species, and the other two peaks centered at 373.8 and 367.8 eV are resulted from Ag+. For 100% Ag/AgI/AgIO3 (Figure 2f), the peaks at 374.4 and 368.4 eV are ascribed to Ag0 species, and the other two peaks centered at 373.8 and 367.8 eV are related to Ag+. The XPS analyses confirmed the coexistence of Ag+ and Ag0 species in the Ag/AgX (X = Cl, Br, I)/AgIO3 sample. To study the morphology and structure of the as-prepared ternary composites, scanning electron microscopy (SEM) images of AgIO3, Ag/AgX (X = Cl, Br, I), and Ag/AgX (X = Cl, Br, I)/AgIO3 were obtained (Figure 3). The AgIO3

samples and shown in Figure 4. The Be element is taken as a contrast, and the signal of Al is due to the utilizing of aluminum foil. The result strongly demonstrated the formation of ternary hierarchical architecture of Ag/AgCl/AgIO3. SEM images of Ag/AgX (X= Br, I) and Ag/AgX (X= Br, I)/AgIO3 with different X− (X = Br, I)/IO3− ratios are shown in Figure 5. It also can be seen that the Ag nanoparticles and AgX (X = Br, I) uniformly inlaid on the surface of AgIO3 particles, and their amounts also orderly rise with increasing the molar ratio of X− (X = Br, I)/IO3−. On the basis of the above observation, the formation process of the Ag/AgX (X = Cl, Br, I)/AgIO3 ternary hierarchical composites is illustrated in Scheme 1. When the KX (X = Cl, Br, I) solution was added into a AgIO3 suspension, AgX (X = Cl, Br, I) nanosheets would generate via an ion-exchange reaction. Meanwhile, part of the as-generated AgX sheets would decompose into Ag particles in this process. As the appearance of Ag particles is mainly induced by outer light, they formed on the surface of AgX. Thus, the ternary hierarchical architectures of Ag/AgX (X = Cl, Br, I)/AgIO3 are constructed. To further inspect the elemental composition and distribution of Ag/AgBr/AgIO3, EDX mapping is recorded by taking 80% Ag/AgBr/AgIO3 as an instance. As shown in Figure 6, the O, Br, Ag, and I elements are all homogeneously distributed in the 80% Ag/AgBr/AgIO3 sample. To corroborate the interfacial interaction between the different phases, TEM and HRTEM images of 100% Ag/AgCl/AgIO3 are conducted and displayed in Figure 7. The two sets of adjacent fringes with intervals of 0.30 and 2.37 nm correspond well to the (211) lattice of AgIO3 and (111) lattice of Ag, respectively. However, AgCl cannot be observed because of its instability under electron beam irradiation. This similar phenomenon has been reported in BiVO4/Ag/AgCl.19 The light absorption property is an important factor for photocatalysts, which can be studied by diffuse reflection spectroscopy (DRS). Figure 8 displays the diffuse reflection spectra of AgIO3, Ag/AgX (X = Cl, Br, I), and Ag/AgX (X = Cl, Br, I)/ AgIO3 composites. It can be seen that the absorption edge of AgIO3 is located at about 380 nm, according to the reported value.13 The as-prepared Ag/AgX photocatalysts show strong absorption in the visible range with an obvious absorption peak at ∼550 nm in contrast to the pure AgX (X = Cl, Br, I) photocatalysts as reported,16,20,21 which should be ascribed to the surface plasmon resonance (SPR) of the Ag particle. With increasing the ratio of X− (X = Cl, Br, I) to IO3−, the absorption edges of the Ag/AgX (X = Cl, Br, I)/AgIO3 composites continuously red-shift to the visible light region, which are due to the effect of AgX (X = Cl, Br, I). Meanwhile, the monotonic strengthening of the visible-light response for Ag/AgX (X = Cl, Br, I)/AgIO3 composites is attributed to the SPR effect of incremental Ag particles on the surface of the composite. Light-absorption enhancement of the photocatalyst in the whole visible-light region is believed to be beneficial to photodegradation. In addition, the DRS result also supports the existence of Ag species in the Ag/AgX (X = Cl, Br, I)/AgIO3 photocatalysts. To calculate the band gap of AgIO3, an equation22 on the basis of the classical Tauc method, αhv = (αhv − Eg)n, is employed. AgIO3 is reported as an indirect-transition-allowed semiconductor. Therefore, the value of n for AgIO3 is 2, and band gap Eg of AgIO3 is calculated to be 3.18 eV. Besides, the conduction band (CB) position (ECB) and valence band (VB) position (EVB) of AgIO3 can be calculated through the following empirical equation:23 ECB = X − Ec − 0.5Eg, EVB = ECB + Eg. For AgIO3, electronegativity (X) is estimated to be 6.71 eV.13

Figure 3. SEM images of AgIO3 (a), Ag/AgCl (f), and the full-range Ag/AgCl/AgIO3 composites (b−e). The particles marked by a white arrow are Ag, and sheets marked by a black arrow are AgCl.

single crystal (Figure 3a) possesses a spindly shaped morphology and smooth surfaces with a diameter of ∼5 μm. Figure 3f displays the SEM image of the Ag/AgCl product. It is obvious that the smooth surface of AgCl (∼1um in size) is uniformly covered by Ag nanoparticles with a size of ∼50 nm. SEM images of Ag/AgCl/AgIO3 composites with different Cl−/ IO3− ratios were illustrated in Figure 3b−e. It can be observed that all of the Ag/AgCl/AgIO3 heterostructural composites contain the three phases of Ag, AgCl, and AgIO3. Particularly, the amount of Ag nanoparticles and AgCl assembled on the surface of AgIO3 gradually increases with enhancing the molar ratio of Cl−/IO3−. To provide direct evidence for confirming the coexistence of the three Ag, AgCl, and AgIO3 phases in the Ag/AgCl/AgIO3 composite, EDX was performed on 100% Ag/AgCl/AgIO3 D


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Figure 4. SEM and EDX images of the 100% Ag/AgCl/AgIO3 composite.

Accordingly, the ECB and EVB for AgIO3 are calculated to be 0.68 and 3.86 eV, respectively. Disintegrating MO was selected to evaluate the photocatalytic activities of AgIO3, Ag/AgX (X = Cl, Br, I), and Ag/AgX (X = Cl, Br, I)/AgIO3 composites under visible light irradiation (λ > 420 nm). The absorbance−time curves of those samples are depicted in Figures S1−S3. Without catalyst, MO could only be slightly degraded, which can be neglected. The pseudo-first-order model based on the Langmuir−Hinshelwood (LH) kinetics model was employed to calculate the reaction kinetics of the photodegradation process of MO quantitatively, as shown in the following equation:24

and Figure 9c, the 60%, 80%, and 100% Ag/AgBr/AgIO3 composites show stronger photocatalytic activity than the pristine Ag/AgBr and AgIO3, evidencing that the charge separation and transfer was improved in 60%/80%/100% Ag/AgBr/AgIO3 composites. The 80% Ag/AgBr/AgIO3 composite shows the highest photocatalytic activity, which could degrade 98% of MO molecules after visible light irradiation (λ > 420 nm) for 50 min. The apparent rate constant obtained for the 80% Ag/AgBr/AgIO3 composite is 0.074 min−1, which is 82.2 and 1.62 times higher than that of AgIO3 and Ag/AgBr. For X = I, all the Ag/AgI/AgIO3 composites with different I−/IO3− ratios show higher photocatalytic activity than the AgIO3 and Ag/AgI (Figure 9e and Figure S3). As the ratio of I−/IO3− was increased from 20% to 60%, the rate constant of Ag/AgI/AgIO3 ascends first and reaches the maximum of 0.019 min−1 at 40% Ag/AgI/AgIO3, which is 22.0 times that of AgIO3 and 6.62 times that of Ag/AgI (0.0030 min−1), and then decreases. In particular, the photocatalytic efficiency continuously increases with elevating the I−/IO3− ratio from 60% to 100%. The decomposition rate constant of MO over 100% Ag/AgI/AgIO3 is 0.019 min−1, reaching 21.3 times that of AgIO3 and 6.40 times that of Ag/AgI (0.0030 min−1). All the above experimental results indicate that the photochemical property of AgIO3 can be significantly improved by constructing Ag/AgX (X = Cl, Br, I)/AgIO3 architectures, and they may possess different photocatalytic mechanisms. To test the stability of the as-obtained photocatalysts in the photocatalytic reaction, an 80% Ag/AgBr/AgIO3 sample was chosen for five-cycle recycling experiments for degradation of MO.

ln(C0/C) = kappt

Here, kapp represents the apparent pseudo-first-order rate constant (min−1), C0 is the initial MO concentration (mg/L), and C is the instantaneous concentration (mol/L) of MO solution at time t. As exhibited in Figure 9a and Figure S1, AgIO3 photocatalyst exhibited worse photocatalytic activity than the other samples in the X = Cl series. With increasing the ratio of Cl−/IO3− from 20% to 100%, the photocatalytic activity of Ag/AgCl/AgIO3 gradually goes up, and Ag/AgCl shows the highest decomposition rate (0.074 min−1), which reaches 83.8 times that of AgIO3 (0.00089 min−1) and 1.48 times that of 100% Ag/AgCl/AgIO3 (0.051 min−1). The Ag/AgCl sample can decompose 97% of MO only under 40 min of visible light irradiation (λ > 420 nm). For X = Br, as displayed in Figure S2 E


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Figure 5. SEM images of 20% Ag/AgBr/AgIO3 (a), 40% Ag/AgBr/AgIO3 (b), 60% Ag/AgBr/AgIO3 (c), 80% Ag/AgBr/AgIO3 (d), 100% Ag/AgBr/AgIO3 (e), Ag/AgBr (f), 20% Ag/AgI/AgIO3 (g), 40% Ag/AgI/AgIO3 (h), 60% Ag/AgI/AgIO3 (i), 80% Ag/AgI/AgIO3 (j), 100% Ag/AgI/AgIO3 (k), and Ag/AgI (j).

As shown in Figure S4, there is no large activity loss in the degradetion of MO under the irradiation of visible light (λ > 420 nm). Moreover, compared with the 80% Ag/AgBr/AgIO3 sample before photoreaction, the XRD pattern after the photocatalysis reaction has no changes. These results demonstrate the high stability of 100% Ag/AgBr/AgIO3. Photoluminescence spectroscopy (PL) and electrochemical impedance spectra (EIS) are widely used to monitor the separation

Scheme 1. Schematic Illustration of Fabrication Process for the Ag/AgX (X = Cl, Br, I)/AgIO3 Composites

Figure 6. EDX mapping of 80% Ag/AgBr/AgIO3. F


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Figure 7. TEM and HRTEM images of the as-obtained 100% Ag/AgCl/AgIO3 sample.

and transfer efficiencies of a photogenerated charge carrier. For PL spectra, generally, high emission intensity indicates a high recombination rate of the photogenerated carriers and a low photocatalytic activity.25 The PL spectra of the AgIO3, Ag/AgX (X = Cl, Br, I), and Ag/AgX (X = Cl, Br, I)/AgIO 3 heterojunctional samples at room temperature are exhibited in Figure 10a−c. For the X = Cl series (Figure 12a), they all show similar emission peaks centering around 467 nm. One can see that the Ag/AgCl sample shows the lowest intensity, suggesting the lowest recombination rate of photogenerated charge carrier compared to other samples. The inhibited recombination of the holes and electrons should be assigned to the strong interaction between Ag and AgCl. For X = Br (Figure 10b), with increasing the ratio of Br−/IO3−, the emission intensity of Ag/AgBr/AgIO3 samples decreases monotonically and reaches a minimum when the ratio of Br−/IO3− is 80%. Then, the emission intensity enhances with further increasing the Br−/IO3− ratio. Similarly, 40% and 100% Ag/AgI/AgIO3 show the lowest peak intensities, which also confirm their higher photocatalytic activity (Figure 10c). This is also in good agreement with the MO photodegradation results. Figure 10d displays the EIS Nyquist plots of AgIO3, Ag/AgBr, and 80% Ag/AgBr/AgIO3 composite. It can be found that the arc radius of the Ag/AgBr/AgIO3 composite is apparently smaller than that of AgIO3 and Ag/AgBr samples, revealing a high efficiency of charge transfer on the surface of 80% Ag/AgBr/AgIO3. The results of PL and EIS confirm that the Ag/AgBr/AgIO3 photocatalyst has very efficient charge transfer and a low recombination rate of hole and electron, which may be due to the cooperative action of Ag, AgBr, and AgIO3. As we know, there are various reactive species (•O2−, h+, and •OH) that directly participate in the photocatalytic degradation process. To disclose the effect of reactive species generated over Ag/AgX (X = Cl, Br, I)/AgIO3 composites in the photocatalytic process, active species trapping experiments were carried out, and benzoquinone (BQ), disodium ethylenediaminetetraacetate (EDTA-2Na), and isopropanol (IPA) were utilized to quench •O2−, h+, and •OH in the degradation process, respectively.26 For 100% Ag/AgCl/AgIO3 and 80% Ag/AgBr/AgIO3, it can be found from Figure 11a and b that the addition of 1 mM IPA only slightly affects the photocatalytic degradation of MO, demonstrating that hydroxyl radicals •OH have little influence in the photocatalytic process. However, when benzoquinone (BQ, 1 mM) and disodium ethylenediaminetetraacetate (EDTA-2Na, 1 mM) were added to the reaction system, the degradation of MO was largely suppressed. This result suggests that •O2− and h+ should be the main reactive species for 100% Ag/AgCl/AgIO3

Figure 8. UV−vis diffuse reflectance spectra for the AgIO 3 , Ag/AgX (X = Cl, Br, I), and Ag/AgX (X = Cl, Br, I)/AgIO3 composites. G


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Figure 9. Apparent rate constants for the photodegradation of MO over AgIO3, Ag/AgCl, and Ag/AgCl/AgIO3 composites (a); AgIO3, Ag/AgBr, and Ag/AgBr/AgIO3 composites (c); AgIO3, Ag/AgI, and Ag/AgI/AgIO3 composites (e) under the irradiation of visible light (λ > 420 nm). Temporal absorption spectra of MO under visible light irradiation (λ > 420 nm) for 100% Ag/AgCl/AgIO3 (b), 80% Ag/AgBr/AgIO3 (d), and 40% Ag/AgI/AgIO3 (f).

and 80% Ag/AgBr/AgIO3. And h+ plays a greater role in MO photodegradation than •O2−. With regard to 40% Ag/AgI/AgIO3, the addition of EDTA-2Na and BQ all greatly affected the degradation of MO. However, •O2− plays a more dominant role than h+. In contrast, for 100% Ag/AgI/AgIO3, more h+ participates in the MO photodegradation reaction than •O2−. The active species experiments demonstrated that the Ag/AgX (X = Cl, Br, I)/AgIO3 heterostructural photocatalysts possess a compositiondependent photocatalytic mechanism. Different photocatalytic mechanisms on the basis of band structure over 100% Ag/AgCl/AgIO3, 80% Ag/AgBr/AgIO3, 40% Ag/AgI/AgIO3, and 100% Ag/AgI/AgIO3 are proposed and schematically illustrated in Figure 12. From the previous report,27 one can deduce the band energy level of AgCl (ECB= 0.22 eV, EVB= 2.98 eV), AgBr (ECB= 0.07 eV, EVB= 2.67 eV), and AgI (ECB= −0.15 eV, EVB= 2.65 eV). In the case of 100% Ag/AgCl/AgIO3 heterocatalyst (Figure 12a), AgCl cannot absorb visible light (λ > 420 nm) because of its large band gap. Nevertheless, Ag particles can absorb visible light and thus

induce the appearance of photogenerate electrons and holes owing to the dipolar character and SPR of metallic Ag.28,29 Then, the electrons would transfer to the CB of AgCl and further transfer to the more positive CB position of AgIO3, which would weaken the reducing capacity of electrons. Though the redox potential of CB in AgIO3 is too positive compared to that of O2/•O2− to produce •O2−, the Fermi level of AgIO3 would shift to align the energy level of AgIO3 and its surrounding medium.13 Thus, the reducing reaction of O2 to •O2− can take place. Because of the weakened reducing capacity of electrons, the holes play a more important role than electrons (or •O2−) for photodegradation of MO. As AgCl has a more negative CB level which can endow electrons with stronger reducing capacity, the Ag/AgCl photocatalyst exhibits better photocatalytic activity than the Ag/AgCl/AgIO3 sample. For Ag/AgBr/AgIO3 samples, AgBr can absorb visible light (λ > 420 nm). Thus, both the Ag nanoparticles and AgBr can produce electrons and holes under visible light illumination. The electrons generated from Ag particles and AgBr all transfer to the CB of AgIO3. The relatively H


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Figure 10. PL spectra of pure AgIO3, Ag/AgX (X = Cl, Br, I), and Ag/AgX (X = Cl, Br, I)/AgIO3 composites under an excitation of 350 nm (a−c) and an EIS Nynquist plot (d) of AgIO3, 80% Ag/AgBr/AgIO3, and Ag/AgBr.

Figure 11. Photocatalytic degradation of MO over the 100% Ag/AgCl/AgIO3 (a), 80% Ag/AgBr/AgIO3 (b), 40% Ag/AgI/AgIO3 (c), and 100% Ag/AgI/AgIO3 (d) photocatalysts alone and with the addition of EDTA-2Na, BQ, or IPA.

Different from Ag/AgCl/AgIO3, the electrons originated from AgBr itself in the Ag/AgBr/AgIO3 also transfer to a more positive CB of AgIO3, improving the charge separation. So the existence

positive CB position of AgIO3 decreases the oxidating ability of •O2−. Consequently, the holes take a more important role than electrons in photodegradation of MO, as shown in Figure 12b. I


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composites also originates from the Ag SPR and band charge transfer, two types of photocatalytic mechanisms were observed in Ag/AgI/AgIO3 which are closely associated with their relative content of components. These results are confirmed by the PL, EIS, and active species trapping experiments. Our work may pave a new way to fabrication of hierarchical architecture with tunable photochemical properties.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00348. Figures S1−S5 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 12. Schematic diagrams with different charge-transfer mechanisms of 100% Ag/AgCl/AgIO3 (a), 80% Ag/AgBr/AgIO3 (b), 40% Ag/AgI/AgIO3 (c), and 100% Ag/AgI/AgIO3 (d).

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundations of China (Grant No. 51302251 and 51372233) and the Fundamental Research Funds for the Central Universities (No. 2652013052, 2652015296, 2652015439).

of AgIO3 is crucial to enhancing the photocatalytic performance of the Ag/AgBr/AgIO3 ternary heterostructure, and 80% Ag/AgBr/AgIO3 thus shows better photoactivity than the Ag/AgBr sample. According to the active species trapping experiments, the 40% Ag/AgI/AgIO3 and 100% Ag/AgI/AgIO3 possess different photocatalytic mechanisms. Both Ag and AgI can be excited by visible light (λ > 420 nm) to generate electrons and holes. The as-generated •O2− from electrons would transfer to the CB of AgIO3, and h+ would be accumulated on the VB of AgI. The 40% Ag/AgI/AgIO3 (Figure 12c) has more AgIO3 in components, which permits more •O2− to participate in the photocatalytic process than holes. Consequently, •O2− plays a different role than h+ in photodegradation of MO over 40% Ag/AgI/AgIO3 photocatalyst. As for the 100% Ag/AgI/AgIO3 (Figure 12d), the contents of Ag and AgI covered on AgIO3 increase. This would allow h+ to have more of a platform to directly oxidate the pollutant. Thus, h+ has a more important effect on degradation of MO over 100% Ag/AgI/AgIO3.30 All in all, by coordinating the SPR of Ag and heterostructure as well as components in Ag/AgX (X = Cl, Br, I)/AgIO3, the photocatalytic activity can be optimized, and a different photocatalytic mechanism is understood.

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CONCLUSION In summary, a room temperature in situ ion-exchange synthetic route is developed to fabricate three series of ternary hierarchical architecture Ag/AgX (X = Cl, Br, I)/AgIO3 using AgIO3 as a precursor. Compared to pristine AgIO3, all the Ag/AgX (X = Cl, Br, I)/AgIO3 composites show highly strengthened photoabsorption in the visible region due to the compositing of AgX and Ag. The three series of ternary photocatalysts show different photocatalytic performance for photodegradation of MO under visible-light (λ > 420 nm) with diverse photocatalytic mechanisms. The origin of catalytic activity of Ag/AgCl/AgIO3 series is mainly attributed to the irradiation of Ag nanoparticles. The enhanced activity in the Ag/AgBr/AgIO3 series stems from the SPR of Ag and charge transfer between the bands of AgBr and AgIO3. Though the activity enhancement of the Ag/AgI/AgIO3 J


Research Article


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