Synchronously Achieving Plasmonic Bi Metal Deposition and I

Nov 25, 2015 - The optical absorption of the pure BiOIO3 and BIO@X (X = 0.1, 0.2, 0.4, 0.8, and 1.6) was investigated by DRS (Figure 6a). The absorpti...
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Synchronously Achieving Plasmonic Bi Metal Deposition and I− Doping by Utilizing BiOIO3 as the Self-Sacrificing Template for HighPerformance Multifunctional Applications Shixin Yu,† Hongwei Huang,*,† Fan Dong,‡ Min Li,† Na Tian,† Tierui Zhang,§ and Yihe Zhang*,† †

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 ‡ Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College of Environmental and Bio-logical Engineering, Chongqing Technology and Business University, Chongqing 400067, China § Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ABSTRACT: Herein, we uncover simultaneously achieving plasmonic Bi metal deposition and I− doping by employing wide-band-gap BiOIO3 as the selfsacrificing template. It was synthesized via a facile NaBH4-assisted in situ reduction route under ambient conditions. The reducing extent as well as photocatalytic levels can be easily modulated by controlling the concentration of NaBH4 solution. It is interesting that the band gap of BiOIO3 can be continuously narrowed by the modification, and the photoresponse range is drastically extended to cover the whole visible region. Bi/I− codecorated BiOIO3 not only exhibits profoundly upgraded photoreactivity in comparison with pristine BiOIO3 but also shows universally strong photooxidation properties toward decomposition of multiple industrial contaminants and pharmaceutical, including phenol, 2,4-Dichlorophenol (2,4-DCP), bisphenol A (BPA), dye model Rhodamine (RhB), tetracycline hydrochloride, and gaseous NO under visible light (λ ≥ 420 nm) or simulated solar light irradiation. It also outperforms the well-known and important photocatalysts C3N4, BiOBr, and Bi2WO6 for NO removal. The cooperative effects from Bi SPR and I− doping endow BiOIO3 with a narrowed band gap and highly boosted separation of charge carriers, thus responsible for the outstanding catalytic activity. The present study provides an absorbing candidate for practical environmental applications and also furthers our understanding of developing high-performance photocatalysts by manipulating manifold strategies in a facile way. KEYWORDS: Bi deposition, I doping, photocatalysis, in situ reduction, photoabsorption

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photocatalytic activity.16 Nonetheless, doping always involves incorporation of foreign elements, which are subjected to a complex synthesis process or thermal instability. Noble metal (Au, Ag, and Pt) deposition has recently attracted tremendous interests for the surface plasmon resonance (SPR) effect, which is generated by the resonant photon-generated free electrons collective vibration in conducting materials. SPR can offer multiple advantages, such as reinforced visible-light absorption, charge separation promotion, resonant energy transfer, and electron injection. These virtues allow noble metals to act as excellent cocatalysts and greatly contribute to photocatalysis properties.17−20 Recently, it was demonstrated that the semimetallic bismuth (Bi) also shows the plasmonic properties similar to these noble metals.19 The collective excitation induced by free electrons of Bi semimetal causes strong SPRmediated effects, such as intensive resonant visible-light

INTRODUCTION Semiconductor photocatalysis exhibits huge potentials in pollutants disposal, energy conversion, selective oxidation and organic synthesis.1−4 Though TiO2 was once the most attractive photocatalyst, it suffers from many serious problems, such as low utilization of solar spectrum and high recombination of charge carriers.5−8 Because visible light accounts for 43% of the solar spectrum, exploration of highperformance visible-light-active photocatalytic materials is of significance.9,10 To overcome the aforementioned drawbacks existing in photocatalysts, particularly wide-band-gap (WBG) semiconductors, various strategies have been developed, including fabrication of heterojunctions, element doping, and noble metal deposition, among others.11−14 Among these, element doping is an efficient and frequently used approach by introducing metal or nonmetal elements into the lattice or surface of substrates.15 In addition to being able to broaden the scope of light absorption, this route could facilitate high charge separation and transfer, thus rendering photocatalysts enhanced © XXXX American Chemical Society

Received: October 20, 2015 Accepted: November 25, 2015

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DOI: 10.1021/acsami.5b09994 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces absorption and scattering or near-field enhancements, which endow Bi with absorbing potentials applied in many fields, including photocatalysis, fluorescence, surface-enhanced spectroscopy, and sensors.21 Specifically, it furnishes a promising way to improve the low efficiency of WBG semiconductors. Most recently, bismuth iodate (BiOIO3) has been studied as a new Bi-based photocatalyst that exhibits a layered structural topology composed of (Bi2O2)2+ layers and interbedded (IO3)− anions.20 The manifold advantages of this material, including the noncentrosymmetrical crystal structure, layered configuration-induced internal static electric fields, and polar (IO3)− groups resulting in polarized electric fields, are all beneficial to the separation of photogenerated charge carriers. Thus, BiOIO3 shows highly efficient photocatalytic degradation activity of organic liquid and gaseous pollutants (e.g., NO).8,22−25 Nevertheless, the relatively large band gap of ∼3 eV means it almost only responds to UV light, which badly restricts its applications.25 Considering the structural benefits of BiOIO3, manufacturing visible-light-active BiOIO3 materials is meaningful and highly indispensable. Herein, we demonstrate the simultaneous realization of plasmonic Bi metal deposition and I− doping in BiOIO3 via an in situ reduction method at room temperature using NaBH4 as reducing agent. The reduction extent can be easily controlled by the concentration of the NaBH4 solution. It is found not only that the band gap of BiOIO3 can be narrowed but also that the photoresponse range is drastically extended which covers the whole visible region. The photocatalysis properties are systematically evaluated by degradation of diverse industrial pollutants and pharmaceuticals, such as phenol, Rhodamine (RhB), 2,4-dichlorophenol (2,4-DCP), bisphenol A (BPA), tetracycline hydrochloride, and NO in gaseous phase under visible light (λ ≥ 420 nm) or simulated visible light irradiation. In addition, the charge separation as well as the photocatalytic mechanism of Bi/I− comodified BiOIO3 is also investigated. The present work established that the cooperative effect of plasmonic Bi metal deposition and I− doping can act as an efficacious strategy for improving the photocatalytic activity.

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Figure 1. Flow and schematic diagrams of preparation process. electron microscopy (SEM, S-4800 Hitachi, Japan). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were carried out on a JEM-2010 electron microscopy (JEOL, Japan). The composition and surface properties of the as-synthesized samples were investigated by X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Xi ThermoFisher, UK). The optical properties were measured by UV−vis diffuse reflectance spectra (DRS) using a UV− vis spectrophotometer (Varian Cary 5000, USA). Photoluminescence excitation (PLE) and emission (PL) were investigated on a fluorescence spectrophotometer (F-4600 Hitachi, Japan) with a photomultiplier tube operating at 400 V and a 150 W Xe lamp used as the excitation source. Photoelectrochemical Measurements. The photoelectrochemical properties, including photocurrent response and electrochemical impedance spectrum, were measured in a standard three-electrode system with using an electrochemical workstation (CHI660E, Chenhua Instruments Co., Shanghai, China). In this three-electrode system, a saturated calomel electrode (SCE) was utilized as reference electrode, platinum wires were used as counter electrode, and the electrolyte solution was a 0.1 M Na2SO4 solution. The working electrode was the samples film-coated on indium−tin oxide (ITO) sheet glass. A 300 W xenon lamp with a 420 nm cutoff filter was employed as the visible light source. There was no applied voltage between the electrodes, and the measurements were recorded at room temperature. Photocatalytic Evaluation. The photocatalytic activity of the BIO@X (X = 0, 0.1, 0.2, 0.4, 0.8, and 1.6) series photocatalysts was systematically assessed by photocatalytic degradation of diverse industrial pollutants and pharmaceutical, such as typical dye RhB, 2,4-DCP, BPA, phenol, and tetracycline hydrochloride, and gaseous NO under visible light (λ ≥ 420 nm) or simulated visible light irradiation. To study systematically the catalytic activity of samples further, two types of light sources are used. Photocatalytic degradation of RhB and BPA is carried out under visible light (500 W Xe lamp, λ ≥ 420 nm) and decomposition of phenol, 2,4-DCP, and tetracycline hydrochloride is carried out under the irradiation of simulated solar light (500 W Xe lamp). Specifically, the photocatalyst powders (50 mg) were ultrasonically dispersed in 50 mL of aqueous solution containing the above-mentioned pollutants in quartz tubes. Before starting the photocatalytic degradations, the mixtures in quartz tubes were stirred for 0.5 h in darkness in order to achieve an absorption−desorption equilibrium between the powders and aqueous solution. After illumination, 2.5 mL of the suspensions were extracted each hour or half an hour and separated by centrifugation to obtain the supernatant solutions. The concentrations of as-obtained solutions were measured by testing the absorbance at the characteristic bands of the contaminants. The UV−vis spectra of the supernatant solutions were recorded on a U-3010 spectrophotometer.

EXPERIMENTAL SECTION

Synthesis. The chemicals used in this work are of analytical grade and were used without further treatment. All Bi/I− comodified BiOIO3 samples are prepared by a simple in situ NaBH4 reduction method with bare BiOIO3 as precursors. First, the pure BiOIO3 were obtained in a typical hydrothermal process. Bi(NO3)3·5H2O (30 mmol) and I2O5 (15 mmol) were dissolved in a mixture of 25 mL of deionized water and 4 mL of concentrated HNO3 (∼65%) in a 50 mL Teflonlined autoclave and stirred for 1 h at room temperature. The mixture was heated at 180 °C for 24 h in an oven. Then, the products were collected and washed thoroughly with ethanol and deionized water 4 times and dried at 60 °C. Afterward, 0.5 g of BiOIO3 is added into 50 mL of deionized water to form a homogeneous suspension. Then, 25 mL of NaBH4 solution with NaBH4/BiOIO3 molar ratios of 0.1, 0.2, 0.4, 0.8, and 1.6 are separately dropwise added into the above BiOIO3 suspensions under strong stirring and nitrogen atmosphere. After that, the products are washed with deionized water 3 times and dried at 50 °C for 12 h in a vacuum drying oven. According to this method, the samples with differently modified levels were obtained and were named as [email protected], [email protected], [email protected], [email protected], and [email protected], respectively. The flow diagram of the preparation process is shown as Figure 1. Characterization. The phase structure of the BiOIO3 series samples was recorded by X-ray diffraction (XRD) in a Bruker D8 Advance diffractometer with Cu Kα radiation. The microstructure and morphology of the photocatalysts were researched by scanning B

DOI: 10.1021/acsami.5b09994 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 2. XRD patterns of the pure BiOIO3 and BIO@X (X = 0.1, 0.4, 0.8, and 1.6) photocatalysts. Photocatalytic removal of parts per billion (ppb) levels of NO gas is conducted in a 4.5 L (30 cm × 15 cm × 10 cm) organic glass reactor with continuous NO flow. The light source is a 150 W commercial tungsten halogen lamp that is vertically placed toward the reactor and coupled with a 420 nm cutoff filter to exclude the UV light in the light beam. Photocatalyst (0.2 g) was dispersed in 50 mL of distilled water by ultrasonic for 10 min and coated on two glass dishes with the diameter of 12 cm. Dishes were dried at 70 °C and then put in the reactor. The 600 ppb NO gas in the measurement was acquired by diluting 100 ppm compressed gas cylinder source via flowing air. Before the photocatalytic reaction, an adsorption−desorption equilibrium was achieved in darkness. After equilibrium, the tungsten halogen lamp was turned on. After every minute, the concentrations of NO2 and NOx (NOx includes NO and NO2) were measured by a NOx analyzer (Thermo Scientific, 42i-TL), and the concentration of NO was be determined.

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analyzed by XPS. Figure 3a shows the survey spectra of the [email protected]. The peaks of three constituent elements Bi 4f, O 1s,

RESULTS AND DISCUSSION

Crystal Structure and Chemical States of Surface Composition. Figure 2 shows the XRD patterns of the pure BiOIO3 and Bi/I− comodified BiOIO3 (BIO@X, X = 0.1, 0.4, 0.8, and 1.6) photocatalysts. All the diffraction peaks of pure BiOIO3 could be indexed into that of orthorhombic BiOIO3 (Inorganic Crystal Structure Database (ICSD) # 212019), and the strongest peak at about 27.4° was attributed to the (121) crystal plane of BiOIO3. By introducing NaBH4 into the BiOIO3 suspensions, an obvious change in XRD pattern was observed. With increasing NaBH4 concentration, the intensity of (121) peak of BiOIO3 gradually decreased, which implies that the crystallinity of crystals was damaged. Notably, the characteristic peaks of Bi metal, such as (012), (104), (110), and (202) peaks, appeared when the concentration of NaBH4 solution was high, as observed for [email protected] and [email protected]. This observation strongly evidenced that elementary Bi metal was reduced from BiOIO3. Another fascinating phenomenon observed from the XRD patterns is that the (121) peak of BiOIO3 undertakes a slight left-shift with increasing NaBH4 concentration (right panel of Figure 2). In accordance with Bragg law (nλ = 2d sin 2θ), the decreased 2θ means an enlarged lattice spacing of a crystal. Thus, a certain impurity was believed to be doped into the lattice of BiOIO3 matrix. The surface composition and chemical states of related elements of pristine BiOIO3 and [email protected] samples are

Figure 3. XPS spectra: (a) overall XPS spectra of [email protected], (b) I 3d of [email protected], (c) Bi 4f of BiOIO3 and [email protected], and (d) O 1s of BiOIO3 and [email protected].

and I 3d all can be detected in [email protected]. The C 1s peak is caused by the adventitious hydrocarbon of the instrument. The high-resolution XPS spectra of two constituent elements I 3d and Bi 4f26 are shown in Figure 3b,c, respectively. Different from the normal XPS peaks of I5+ in BiOIO3 or other iodates, four strong peaks with binding energies at 635.13, 630.63, 623.83, and 619.03 eV are observed in the I 3d high-resolution XPS spectrum of [email protected] (Figure 3b), which are separately attributed to the I5+ 3d3/2, I− 3d3/2, I5+ 3d5/2, and I− 3d5/2. The lines in blue and red represent the XPS spectra of I5+ and I−, respectively. Thus, the I− ions are reduced from I5+ of BiOIO3 because of the strong reducibility of NaBH4. Combined with the XRD result, it can be concluded that the as-generated I− species may be incorporated into the crystal lattice of BiOIO3. The Bi 4f XPS spectra of BiOIO3 and [email protected] were shown in Figure 3c, revealing Bi 4f7/2 and Bi 4f5/2 characteristic peaks at 164.5 and 159.0 eV, respectively. In comparison with BiOIO3, a C

DOI: 10.1021/acsami.5b09994 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 4. (a−e) SEM images of BiOIO3 and [email protected], 0.4, 0.8 composite photocatalysts; (f−h) EDX mapping of [email protected].

slight binding energy shift of Bi 4f7/2 peaks occurred over [email protected], suggesting that the coordination environment of Bi3+ ions may be changed. In addition to the two above characteristic peaks, two tiny peaks located at 162.8 and 167.2 eV were found, which can be assigned to the metallic Bi on the surface of the [email protected]. In Figure 3d, the O 1s spectra of the BiOIO3 and [email protected] can be deconvoluted into two peaks at 530.0 and 530.9 eV, which are ascribed to the Bi−O bonds and the hydroxyl groups, respectively.27 The XPS result is consistent with the conclusion from XRD patterns and reveals the coexistence of metallic Bi deposition and I− doping in the modified BiOIO3. Microstructure and Morphology of Bi/I− Comodified BiOIO3. The microstructure and morphology of as-obtained photocatalysts have been investigated by SEM, TEM, and HRTEM. Figure 4a−d shows the SEM images of pure BiOIO3 and [email protected], [email protected], and [email protected] composite photocatalysts, respectively. As shown in Figure 4a, bare BiOIO3 presents regular cuboid−flake structure with smooth surface. With raising the concentration of NaBH4, the regular morphology of BiOIO3 flakes was seriously destroyed. As seen from Figure 4b−d, the surface of BiOIO3 flakes became more and more rough and is peeled to expose many small crystals because of the strong reducing action of NaBH4. Figure 4e depicts the schematic illustration on morphology and surface change of BiOIO3 with increasing the NaBH4 concentration. EDX mapping of [email protected] was conducted to inspect the elemental composition and distribution (Figure 4f−h). It is obvious that there is a homogeneous distribution of I, Bi, and O elements in [email protected]. Further exploration of microstructure was carried out by TEM and HRTEM. Figure 5a−d shows the TEM and HRTEM images of BiOIO3 and [email protected] samples. Figure 5a confirms the regular nanoflake structure and smooth surface of unmodified BiOIO3. After being reduced by NaBH4 solution, the BiOIO3 nanoflake became rugged, and numerous irregular particles emerged across the bulk crystal (Figure 5b). The HRTEM images shown in Figure 5c,d further verify the difference between pure BiOIO3 and BIO@0. The lattice spacing in pure BiOIO3 is 0.289 nm, corresponding well to the

Figure 5. TEM and HRTEM images of BiOIO3 and [email protected] samples.

(002) plane of the orthorhombic BiOIO3. Comparatively, this lattice spacing in [email protected] is measured at 0.292 nm, which is supposed to result from I− doping. In addition, another distinct lattice fringe with a spacing of 0.367 nm was found on the edge of the [email protected] flake as shown in Figure 5d. This lattice fringe matches well with the (012) plane of metallic Bi. Combining the XRD and XPS results further corroborates that the metallic Bi and I− doping comodified BiOIO3 materials are successfully obtained. Enhanced Photoabsorption and Tunable Band Gap. The optical absorption of the pure BiOIO3 and BIO@X (X = 0.1, 0.2, 0.4, 0.8, and 1.6) was investigated by DRS (Figure 6a). The absorption edge of pure BiOIO3 was about 400 nm, indicating that BiOIO3 is almost unresponsive to visible light, which agrees with the previous report.28 After being reduced, D

DOI: 10.1021/acsami.5b09994 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

gap, respectively. The parameter n is a variable number depending on the types of electronic transitions: n = 1 or 4 for direct-allowed or indirect-allowed transitions, respectively. BiOIO3 is an indirect-allowed semiconductor so that the value of n is 4.8,24,28 As displayed in Figure 6c, the band gap values of the pure BiOIO3, [email protected], and [email protected] were, respectively, estimated to be 2.99, 2.18, and 1.58 eV. Photocatalytic Performance for Multiple Applications. The photocatalytic properties of pure BiOIO3 and BIO@X (X = 0.1, 0.2, 0.4, 0.8, and 1.6) were systematically characterized by photocatalytic degradation of diverse industrial pollutants or pharmaceutical, including RhB, 2,4-DCP, BPA, phenol, tetracycline hydrochloride, and gaseous NO under the illumination of visible light (λ ≥ 420 nm) or simulated solar light. The degradation curves of RhB are shown in Figure 7a, and the pure BiOIO3 exhibits a very low photodegradation efficiency under visible light (λ ≥ 420 nm). Comparatively, the modified samples showed highly improved photocatalytic performance during the experiment. Specifically, [email protected] exhibits the most markedly enhanced photocatalytic activity among the photocatalysts, in which the RhB degradation efficiency reaches 76.8% with irradiation of visible light for 4 h. To compare the photodegradation rate of the photocatalysts quantitatively, the pseudo-first-order equation was applied.30 ln(C0/C) = kappt

where t, C0, and C are the reaction time, initial RhB concentration (mg/L), RhB concentration at time t (mg/L); kapp represents the apparent pseudo-first-order rate constant (h−1). According to the equation, the apparent rate constants kapp were calculated as 0.074, 0.080, 0.140, 0.344, 0.092, and 0.075 h−1, respectively, for the pure BiOIO3 and BIO@X (X = 0.1, 0.2, 0.4, 0.8, and 1.6). The highest photocatalytic efficiency obtained in [email protected] is approximately 4.7 times that of the pure BiOIO3. Figure 7c shows the UV−vis absorption spectra of RhB solution catalyzed by [email protected], and it is clear that the concentration of RhB solution decreased gradually with irradiation time. The band shift of the maximum absorbance of RhB solution from 554 to 500 nm within 4 h irradiation is ascribed to the N-demethylation and de-ethylation processes of RhB.9 To confirm the enhanced photocatalytic activity and meanwhile exclude the dyesensitization effect, phenol, a colorless and intractable contaminant, is used here as the target with illumination of simulated solar light. As shown in Figure 7d, the degradation on phenol by BiOIO3 and BIO@X series shows the very similar trend with RhB, and the optimum photocatalysis capability was also observed for [email protected]. Besides, in contrast to pristine BiOIO3, the [email protected] also showed much promoted catalysis activity toward other stubborn pollutants, such as 2,4-DCP, BPA, and tetracycline hydrochloride (Figure 7e). These evidence apparently demonstrated that the photocatalysis properties of BiOIO3 are highly improved by the comodification of metallic Bi and I doping. NO is a gaseous pollutant that is stable and can be removed with the presence of photocatalytic materials.31 To confirm further the universally strong photooxidation ability of modified BiOIO3, NO removal is monitored over BiOIO3 and [email protected] under visible light (λ ≥ 420 nm), and the well-known and important photocatalysts BiOBr, Bi2WO6, and C3N4 are employed as references. The variations of NO concentrations with irradiation time for these samples are shown in Figure 7f.

Figure 6. (a) DRS spectra of BIO@X (X = 0.1, 0.2, 0.4, 0.8, and 1.6). (b) Color evolution of the series of samples. (c) Band gap values of the pure BiOIO3, [email protected], and [email protected].

the photoabsorption of BiOIO3 undergoes significant enhancement in the whole visible light region. It should be ascribed to the two following reasons, corresponding to the two parts in the optical absorption spectra (Figure 6a): Parts I (400−500 nm) and II (>500 nm). The absorbance improvement in part I originates from the extension of the absorption edge of BiOIO3 to longer wavelength, which should be caused by impurity level formed by the I− dopant. In contrast, the drastically and gradually boosted photoabsorption in part II is due to the contribution of metallic Bi deposited on the surface of BiOIO3. The remarkable enhancement on the optical absorption of BiOIO3 is also verified by the color change of the BIO@X series samples. The color change from white to gray to black, as displayed in Figure 6b, also demonstrated the orderly strengthened response to visible light with increasing the concentration of NaBH4 solution. On the basis of the analyses from DRS, the cooperative contributions from metallic Bi and I− dopant are responsible for the significant improvement on visible light absorption of BiOIO3. Optical absorption and band gap of semiconductors adhere to the following Kubelka−Munk (KM) equation,29 where the band gap can be determined

αℏv = A(ℏv − Eg )n /2

(2)

(1)

herein ℏ, v, A, α, and Eg are the Planck’s constant, photon frequency, a constant, optical absorption coefficient, and band E

DOI: 10.1021/acsami.5b09994 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 7. (a) Photocatalytic degradation curves of RhB over pure BiOIO3 and BIO@X (X = 0.1, 0.2, 0.4, 0.8, and 1.6) under the irradiation of visible light (λ ≥ 420 nm). (b) Apparent rate constants of all the samples for RhB photodegradation under the irradiation of visible light. (c) Absorption spectra of RhB at different irradiation times (visible irradiation) in the presence of [email protected]. (d) Photocatalytic degradation curves of phenol over pure BiOIO3 and BIO@X (X = 0.1, 0.2, 0.4, 0.8, and 1.6) under simulated solar light. (e) Photocatalytic degradation curves of 2,4-DCP, tetracycline hydrochloride, and BPA over pure BiOIO3 and [email protected]. (f) Variations of NO concentrations with irradiation time for BiOIO3, [email protected], BiOBr, Bi2WO6, and C3N4.

OH), respectively.32−35 As seen from Figure 8a, the photocatalytic degradation of RhB was remarkably inhibited with addition of EDTA-Na2 and modestly hindered by adding BQ. It demonstrated that h+ plays the most critical role and ·O2− serves as a secondary active species in the photocatalytic degradation process Photocurrent generation, which has a close correlation with photocatalytic activity, is employed to elucidate the charge separation and transfer dynamics of photoelectrons. The high photocurrent response usually signified the high separation efficiency of electrons and holes.36 Figure 8c shows the photocurrent response of pure BiOIO3 and [email protected] with illumination of visible light (λ ≥ 420 nm). When they are exposed to light, rapid photocurrent response was observed for both BiOIO3 and [email protected] photoelectrodes. Obviously, the photocurrent density generated by [email protected] is much higher than that by BiOIO3. This result means that more photoexcitons are produced and more efficient charge separation occurred over [email protected].

In comparison with pristine BiOIO3 with a NO removal rate less about 8%, [email protected] shows an unprecedented enhancement of photoreactivity with a removal rate as high as 55%. It also far excels BiOBr (85%), Bi2WO6 (77%), and C3N4 (91%). The systematical photocatalytic assessment evidently disclosed that the Bi/I− codecoration on BiOIO3 induced by the one-step reduction reaction enables powerful visible-light-active photooxidation ability. Investigation on Charge Separation and Photocatalytic Mechanism. Photoelectrochemistry, photoluminescence, and active species trapping measurements were carried out to survey the charge movement behavior as well as the photocatalytic mechanism. The trapping experiment was implemented to detect the main active species generated in the photodegradation process. Three different scavengers, benzoquinone (BQ), ethylene diaminetetraacetic acid disodium salt (EDTA-Na2), and isopropanol (IPA) were added in the photocatalytic system of RhB degradation as quenchers of three active species, superoxide radicals (·O2−), holes (h+), and hydroxyl radicals (· F

DOI: 10.1021/acsami.5b09994 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 8. (a) Apparent rate constants of RhB over [email protected] in the trapping experiment under the irradiation of visible light (λ ≥ 420 nm). (b) Photoluminescence spectra of BiOIO3 and BIO@X (X = 0.1, 0.2, 0.4, 0.8, and 1.6). (c) Photocurrent response of pure BiOIO3 and [email protected] under visible light (λ ≥ 420 nm) illumination. (d) Electrochemical impedance spectroscopy (EIS) of pure BiOIO3 and [email protected].

Figure 8d displayed another important photoelectrochemical property, electrochemical impedance spectroscopy (EIS). EIS can detect the charge transfer efficiency at the interface of electrodes. A higher charge transfer efficiency reflects a smaller arc radius. In contrast to BiOIO3, [email protected] shows an evidently smaller arc radius in the EIS plot. It demonstrated that the emigration and transfer of charge carriers are promoted in [email protected] in contrast to pristine BiOIO3. Photoluminescence (PL) is another valid way to survey the separation efficiency of electrons and holes. The recombination of electron−hole pairs usually causes the generation of fluorescence. Generally, high recombination of electron−hole pairs leads to the high fluorescence intensity and low photocatalytic activity.37 Figure 8b shows the emission spectra and excitation spectra of pure BiOIO3 and BIO@X (X = 0.1, 0.2, 0.4, 0.8, and 1.6). The series shows emission peaks at 400 nm with excitation of 252 nm ultraviolet light. All the BIO@X samples display emission intensity lower than that of pure BiOIO3, and the intensity gradually decreases with the increase of X (Figure 8b). This variation trend indicates that the recombined rate can be decreased by the current modification. On the basis of the PL result, the in situ deposition of elemental Bi and I− doping can effectively suppress the recombination of electron hole pairs. The reduction degree controlled by NaBH4 concentration can optimize the photocatalytic performance of BiOIO3. According to the above results, the schematic diagram of photogenerated charge carriers separation and the photocatalytic mechanism of BIO@X photocatalysts under visible light irradiation were portrayed in Figure 9. BiOIO3 almost cannot respond to visible light, whereas I− doping could result in a redshift of the optical absorption edge of BiOIO3 on account of the DRS spectra, which means some impure energy levels may be formed beyond the valence band (VB) or under the conduction band (CB) of BiOIO3. Thus, electron−hole

Figure 9. Schematic diagram of charge separation and the possible reaction mechanism of BIO@X under visible light irradiation.

pairs can be activated and separated with illumination of visible light because less photo energy can be required to complete the transition. The photoinduced holes remain in the VB, and the electrons further reacted with O2 absorbed on the surface of photocatalyst to produce robust ·O2− in the CB. Both holes and ·O2− are active species and exert strong oxidation ability in the photochemical reactions. In contrast, because of the SPR effect, the metallic Bi assists the photocatalyst system to absorb more light, inducing generation of more active photoexcitons. Importantly, the SPR effect could facilitate the separation and transport of charge carriers, thus greatly promoting the photocatalytic activity. Moreover, as the Fermi level of metallic Bi (−0.17 eV) is much more negative than the CB of BiOIO3, the electrons can migrate from Bi to the CB of BiOIO3 owing G

DOI: 10.1021/acsami.5b09994 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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to the potential difference, which also favors the accumulation of abundant ·O2−. Benefiting from the above-mentioned advantages, Bi/I− modified BiOIO3 furnishes powerful photocatalysis capability in decomposing diverse contaminants.

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CONCLUSIONS Metallic Bi and I− self-doping comodified BiOIO3 was successfully synthesized by a facile in situ reduction method with NaBH4 solution used as the reductant at room temperature. The presence of Bi metal and I− dopant is verified by multiple techniques. The I− self-doping could result in a redshift of the optical absorption edge of BiOIO3 and consequently a narrowed band gap. The presence of Bi metal not only profoundly enhances the photoresponse in visible region but also facilitates the separation and transport of charge carriers, greatly promoting the photocatalytic activity. On the basis of these benefits, the Bi/I−-modified BiOIO3 exhibits superior photocatalysis properties pertaining to degrading diverse pollutants, including phenol, 2,4-DCP, BPA, tetracycline hydrochloride, RhB, and NO in gaseous phase under visible light (λ ≥ 420 nm) or simulated visible light irradiation. It also excels the well-known photocatalysts BiOBr, Bi2WO6, and C3N4 in a NO removal test. These findings open up a new avenue for manufacturing high-activity visible-light semiconductor photocatalysts via multiple factor coordination.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-10-82322247. E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundations of China (grant no. 51302251), the Fundamental Research Funds for the Central Universities (nos. 2652013052 and 2652015296).

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DOI: 10.1021/acsami.5b09994 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.5b09994 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX