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In situ Oxidation and Self-Assembly Synthesis of Dumbbell-Like #-Fe2O3/Ag/ AgX (X = Cl, Br, I) Heterostructures with Enhanced Photocatalytic Properties Lingling Sun, Wei Wu, Qingyong Tian, Mei Lei, Jun Liu, Xiangheng Xiao, Xudong Zheng, Feng Ren, and Changzhong Jiang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01473 • Publication Date (Web): 14 Dec 2015 Downloaded from http://pubs.acs.org on December 21, 2015

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ACS Sustainable Chemistry & Engineering

In situ Oxidation and Self-Assembly Synthesis of Dumbbell-Like α-Fe2O3/Ag/AgX (X = Cl, Br, I) Heterostructures with Enhanced Photocatalytic Properties Lingling Sun1, 2, Wei Wu2, 3*, Qingyong Tian1, 2, Mei Lei1, 2, Jun Liu1, 2, Xiangheng Xiao1, Xudong Zheng1, Feng Ren1, Changzhong Jiang1* 1

Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, School of Physics and Technology, Wuhan University, Wuhan 430072, P. R. China 2

Laboratory of Functional Nanomaterials and Printing Electronics, School of Printing and Packaging, Wuhan University, Wuhan 430072, P. R. China

3

Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, P. R. China

Abstract: In this work, the novel dumbbell-like α-Fe2O3/Ag/AgX (X = Cl, Br, I) heterostructures were successfully synthesized via an in situ oxidation reaction and self-assembly process by using the α-Fe2O3/Ag core-shell nanoparticles (NPs). The as-obtained dumbbell-like α-Fe2O3/Ag/AgX heterostructure contains an individual spindle-like α-Fe2O3 nanoparticle and a single near-spherical Ag/AgX nanoparticle. The morphology, microstructure, component and optical property of as-synthesized α-Fe2O3/Ag/AgX heterostructures were characterized by various analytical techniques. The great changing of morphology and component in such synthesis route make great effect on the final photocatalytic performance. The dumbbell-like α-Fe2O3/Ag/AgX heterostructures exhibit excellent photocatalytic activity for the degradation of RhB dye under simulated sunlight irradiation. In particular, the α-Fe2O3/Ag/AgCl can completely degrade RhB molecules within only 20 min under

*

To whom correspondence should be addressed. Tel: +86-27-68778529. Fax: +86-27-68778433. E-mail:

[email protected] (W. Wu), [email protected]. (C. Z. Jiang) 1

ACS Paragon Plus Environment


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simulated sunlight irradiation, which is superior to the pure α-Fe2O3, α-Fe2O3/Ag NPs and commercial P25. The enhanced activity is attributed to the efficient interfacial charge rectification and faster carrier migration in the α-Fe2O3/Ag/AgX heterostructures. Furthermore, the

degradation rate of as-synthesized

dumbbell-like

α-Fe2O3/Ag/AgX (X = Cl, Br, I) heterostructures follows this order: α-Fe2O3/Ag/AgCl > α-Fe2O3/Ag/AgBr > α-Fe2O3/Ag/AgI. The results can be owing to that the oxidation capability of Cl0 is stronger than Br0 and I0. This unique synthetic work can provide physical insight to prepare novel nanomaterials with special structures and properties, which can apply in photocatalysis, photo-splitting of water and solar cell, etc. Keywords: dumbbell-like structure; noble metal; silver halides; photocatalytic activity.

Introduction The semiconductor-based photocatalysis play a vital role and have a promising development in counteracting the worldwide environmental pollution and energy shortage issues. Various semiconductor materials, such as TiO2, SnO2, Fe2O3, ZnO, WO3, CdS, etc, have attracted extensive attention and intensive research in the field of photodegradation of organic pollution, photo-splitting of water to produce hydrogen and solar cell, because of their advantages of chemical stability, commercial availability and low-toxicity.1-3 However, the monocomponent semiconductor material has a lot of limitation in these application fields such as the low utilization of sunlight, the rapid recombination of photogenerated electrons and holes. Various attempts have been made to overcome the mentioned limitations all the time. The conventional techniques include phase and morphology control, metal or nonmetal ions doping, noble metal deposition, the coupling of two or more semiconductor with different bandgaps, dye-sensitized, etc.4-8 As an example, Xu et al. fabricated Sand C- co-doped anatase TiO2 by a soft synthesis method, which exhibits excellent visible light-driven photocatalytic activity for degradation of methylene blue.9 In addition, the branched SnO2/α-Fe2O3 semiconductor nanoheterostructures were successfully prepared by Niu et al. and 2


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possess excellent visible light or UV photocatalytic performances due to the effective electron-hole separation at the interface of SnO2 and α-Fe2O3.10 Among these strategies to improve photocatalytic activity, noble metal deposition can greatly enhance the visible light photocatalytic activity. The critical reason is that noble metal nanoparticles (i.e. Ag, Au, Pt) can harvest visible light owing to their size- and shape-dependent surface plasmon resonance (SPR) effect.11 Furthermore, noble metal NPs could work as electron traps and active reaction sites, which is beneficial to separate the photogenerated electrons and holes effectively and accelerate the redox reaction.12-13 Therefore, plasmonic photocatalysts consisting of noble metal NPs and semiconductors have been increasingly studied in recent years because of their outstanding performance in photocatalytic field. Especially, the silver/silver halides plasmonic photocatalysts possess excellent photocatalytic performance and high stability under various light irradiations.14-15 For example, An et al. successfully synthesized AgCl:Ag core-shell hybrid nanoparticles by a one-pot approach, the results demonstrated that the as-prepared products exhibit high catalysis efficiency for decomposition of organics under sunlight as well as high stability and recyclability.16 Furthermore, the Ag/AgX-based composite materials were extremely popular with researchers. The introducing of Ag/AgX can greatly enhance the photocatalytic performance for individual semiconductor. For example, plasmonic photocatalyst Ag/AgCl/TiO2 was fabricated by Yu et al. and exhibit enhanced visible-light-driven photocatalytic activity compared to pure TiO2.17 Hu et al. prepared a plasmonic photocatalyst Ag-AgI supported on mesoporous alumina, which exhibits enhanced photocatalytic performance for degradation and mineralization of organic pollutants under visible light irradiation.18 Additionally, magnetic nanomaterials were widely used in the catalysis and biotechnology in recent years because of their advantage of high activities, magnetic recyclability, and reusability.19-24 3



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Especially, α-Fe2O3, as the most common and stable magnetic iron oxide materials, is also an n-type semiconductor with a bandgap of 2.2 eV, which can absorb visible light with the wavelength below 564 nm.25-26 Therefore, Fe2O3 has been used in visible-light driven photocatalysis application, for instance, Fe2O3 hollow spheres with novel cage-like architectures were successfully fabricated by a controlled hydrothermal precipitation reaction for enhanced the photocatalytic ability under visible light.27 In our previous study, α-Fe2O3-based semiconductor composite structure exhibits enhanced photocatalytic activity and well stability.28-30 Herein, a plasmonic photocatalyst was designed and synthesized by a facile wet-chemical route, this heterostructure consists of a single spindle-like α-Fe2O3 particle and an individual Ag and Ag halides (AgCl, AgBr or AgI) composite nanoparticle. In our work, the α-Fe2O3/Ag core-shell NPs were firstly prepared by deposited uniform Ag NPs on the surface of spindle-like α-Fe2O3 NPs. Then the Ag NPs were in situ oxidized into AgX nanocrystals by Fe3+ ions in reaction solution, the α-Fe2O3/Ag/AgX were successfully prepared by self-assembly of Ag and AgX nanocrystals. The as-prepared α-Fe2O3/Ag/AgX possess a dumbbell-like shape, in which a near-spherical Ag/AgX NP were linked to the surface of a spindle-like α-Fe2O3 NP. This process of transformation from the core-shell structure of α-Fe2O3/Ag NPs to the dumbbell-like structure of α-Fe2O3/Ag/AgX NPs is controllable and repeatable. The as-prepared α-Fe2O3/Ag/AgX exhibit enhanced photocatalytic activity for degradation of RhB dye under simulated sunlight irradiation. The sunlight driven photocatalytic activities of dumbbell-like α-Fe2O3/Ag/AgCl and α-Fe2O3/Ag/AgBr were superior to pure α-Fe2O3, α-Fe2O3/Ag and commercial P25. Especially, the α-Fe2O3/Ag/AgCl plasmonic photocatalyst can completely decompose RhB dye in only 20 min. The enhanced mechanisms for photocatalytic activity were discussed in detail. Experimental section 4


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Materials.

Ferric

chloride

hexahydrate

(FeCl3·6H2O),

sodium

dihydrogen

phosphate

(NaH2PO4·2H2O), glutaraldehyde aqueous solution (C5H8O2, 25 %), disodium hydrogen phosphate (Na2HPO4·12H2O), silver nitrate (AgNO3), iron (Ш) nitrate nanohydrate (Fe(NO3)3·9H2O), sodium bromide (NaBr), potassium iodide (KI) and ethanol (C2H5OH) were purchased from Sinopharm Chemical Reagent Co., Ltd. Polyvinyl pyrrolidone (average Mw~10000) was purchased from Sigma-Aldrich. Ammonia (NH3·H2O, 25 %) was purchased from Wuhan Wangsen Chemical Reagent Co., Ltd. 3-Aminopropyltrimethoxysilane (ATPES), Rhodamine B (RhB) were purchased from Shanghai Jingchun Chemical Reagent Co., Ltd. All chemicals were analytical grade without further purification. The deionized water was used throughout the experiments. Synthesis of spindle-like α-Fe2O3 seeds. Spindle-like α-Fe2O3 seeds were fabricated by a forced hydrolysis method in our previous reports.31 The detail synthesis process is as below: Firstly, 7.0 mg of NaH2PO4 was dissolved in 100 mL of deionized water and the solution was heated to 95 °C in a flask. Secondly, 1.8 mL of FeCl3 aqueous solution (1.48 M) was added dropwise into the flask. Finally, the mixed solution was heated to 105 °C and kept for 14 h. The obtained precipitate was collected by centrifugation and cleaned several times with ethanol and deionized water. Synthesis of α-Fe2O3/Ag composite NPs. Ag NPs with uniform size were coated on the surface of α-Fe2O3 NPs by a self-catalytic synthesis method, and the coated process contains three steps.32 The first step is synthesis of amino functionalized α-Fe2O3 NPs. Briefly, 10 mg of as-obtained α-Fe2O3 seeds was dispersed in 100 mL of ethanol in a flask by ultrasonication, and then 0.5 mL of APTES aqueous solution (2 %, v/v) was dropwise added into the flask under rapid stirring. After a few minute, 1 mL of deionized water was also added dropwise into the above solution. The mixed solution was kept at 30 °C for 2 h under rapid stirring. The products were centrifuged and washed twice with ethanol and water, respectively. The second step is the modified of aldehyde group on the 5



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as-obtained amino functionalized α-Fe2O3. The as-obtained amino functionalized α-Fe2O3 NPs were dispersed in glutaraldehyde solution (5 %, diluted with 0.02 M phosphate buffer (PB) solution) under rapid stirring and the mixture was kept at 30 °C for 2 h. The resultants were centrifuged and washed several times with ethanol and water. The third step is depositing Ag NPs on the surface of α-Fe2O3 by a seed growth method. 34 mg of silver nitrate was dissolved into 5 mL of deionized water, and then diluted ammonia (4 %) was gradually added into the AgNO3 solution until the generated precipitates completely vanished. The aldehyde-terminated α-Fe2O3 was dispersed in 3 mL of ethanol and then added the above silver ammonia solution. The mixed solution was heated at 80 °C for 40 min and the product was centrifuged and washed twice with ethanol and water. Synthesis of α-Fe2O3/Ag/AgX composite NPs. In a typical synthesis of α-Fe2O3/Ag/AgCl, the as-obtained α-Fe2O3/Ag was dispersed in an aqueous solution containing 50 mM PVP by ultrasonication at the room temperature. After under stirring for a few minutes, 3 mL of FeCl3 solution (0.74 M) was slowly added to the mixed solution and the reaction was continued for 2 h with rapid stirring at the room temperature.33 For the synthesis of the α-Fe2O3/Ag/AgBr, the experimental process is the same as that of α-Fe2O3/Ag/AgCl except that the FeCl3 solution were replaced by the Fe(NO3)3 solution and NaBr solution in the reaction process. In the synthesis of α-Fe2O3/Ag/AgI, the FeCl3 solution was also replaced by the Fe(NO3)3 solution and KI solution in the reaction process.34 The as-obtained resultants were centrifuged and washed with ethanol and deionized water, then dried in a vacuum for subsequent measure. Characterization. Transmission electron microscopy (TEM) images were measured by JEOL JEM-2010 (HT) operated at 200 kV. High-resolution TEM (HRTEM) images and energy-dispersive X-ray spectroscopy (EDX) were performed on a JEOL JEM-2100F transmission electron microscope. The samples were dispersed in ethanol and dropped on carbon covered copper grids for 6


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characterization. Scanning electron microscopy (SEM) images were obtained using a FEI SIRION-200 field emission scanning electron microscopy (FESEM) operating at working voltage of 12 kV. X-ray photoelectron spectroscopy (XPS) patterns of the samples were recorded on a Thermo Scientific ESCALAB 250Xi system with Al Kα (1486.6 eV) as the radiation source. The absorption spectra of the as-prepared products and the absorption spectra of photodegradation RhB dye in photocatalytic tests were investigated by a Shimadzu UV-2550 spectrophotometer. Photocatalytic

Tests.

Photocatalytic

performances

of

dumbbell-like

α-Fe2O3/Ag/AgCl,

α-Fe2O3/Ag/AgBr and α-Fe2O3/Ag/AgI nanocomposites were evaluated by the degradation of RhB dye under simulated sunlight irradiation. The simulated sunlight sources came from a mercury lamp (300 W) of BL-GHX-V photochemical reaction apparatus, and the parameters on spectral distribution and relative intensity of the mercury lamp were displayed in the Table S1 (see Supporting Information). In a typical procedure, 3 mg of products was dispersed in RhB solution (10 mg/L, 10 mL), and then the mixed solution was stirred for 30 min in the dark to reach adsorption equilibrium between the catalyst and the solution. After tuning on the mercury lamp (300W), the reaction solution was sampled at 5 min illumination intervals, and the corresponding UV−visible spectra (measured in the range of 450 to 650 nm) were recorded to monitor the progress of the degradation of RhB by a Shimadzu 2550 UV−vis spectrophotometer. As a comparison, the according photocatalytic tests of blank sample, pure α-Fe2O3 NPs, α-Fe2O3/Ag core-shell NPs and commercial TiO2 (P25) by photodegradation of RhB dye were also carried out with the identical experiment conditions. Results and discussion The synthesis mechanism of α-Fe2O3/Ag/AgX (X = Cl, Br, I) heterostructures As shown in Figure 1, the overall experimental procedures for the synthesis of novel 7



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dumbbell-like α-Fe2O3/Ag/AgX (X = Cl, Br, I) heterostructures contains three major process. Firstly, the spindle-like α-Fe2O3 seeds were prepared by a forced hydrolysis method. Then, Ag NPs with uniform size were coated on the surface of α-Fe2O3 NPs by a self-catalytic synthesis method. Briefly, the terminal -NH2 functional groups were linked on the surface of α-Fe2O3 and then the terminal -CHO functional groups were modified. By the reaction of α-Fe2O3-CHO beads and Ag[(NH3)2]+ ions, Ag NPs were reduced and deposited on the surface of α-Fe2O3 seeds. Finally, the α-Fe2O3/Ag/AgX heterostructures were synthesized by in situ oxidation and self-assembly basic of Ag NPs on the surface of α-Fe2O3 NPs. According to the redox potentials of E0(AgCl/Ag) = +0.223 V vs. SHE, E0(AgBr/Ag) = +0.007 V vs. SHE and E0(AgI/Ag) = -0.15 V vs. SHE which are all lower than the redox potential of E0(Fe3+/Fe2+) = +0.771 V vs. SHE, Fe3+ ions can directly oxidize the surface Ag NPs into AgX nanocrystals at room temperature.34-35 In this synthesis process, the X- (X = Cl, Br, I) ions not only served as a halide source for the growth of AgX nanocrystals but also served as an activation agent to initiate the replacement reaction. Simultaneously, the PVP molecules in this process were selectively adsorbed onto the AgX and Ag crystals, which can make the crystals self-assembly as an Ag/AgX hybrid nanoparticle deposited on the surface of spindle-like α-Fe2O3 nanoparticle. In this process, PVP plays two crucial roles in the morphologies and structures formation of Ag/AgX hybrid NPs. Firstly, PVP molecules can selectively adsorbed onto the various crystal facets of AgX nanocrystals and Ag nanocrystals. Subsequently, PVP can promote the heterostruture growth of Ag/AgX, and induce the oriented attachment of Ag/AgX hybrid NPs on α-Fe2O3 NPs.36-37 The morphology and composition of α-Fe2O3/Ag/AgX (X = Cl, Br, I) heterostructures The morphologies and microstructure of the as-obtained α-Fe2O3 seeds and α-Fe2O3/Ag core-shell NPs were examined by FESEM, TEM and HRTEM. The SEM and TEM images (Figure 2a and 2b) 8


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reveal that α-Fe2O3 NPs have well-defined spindle-like shape and the average length and diameter are approximately 240 and 50 nm, respectively. And the α-Fe2O3 NPs possess excellent monodispersity. The measured EDX spectrum of α-Fe2O3 NPs was inset in Figure 2b, in which the elements of Fe, O, C and Cu are existed. The appeared of Fe, O elements can demonstrate the successful synthesis of α-Fe2O3 NPs and the elements of C and Cu come from the copper grid used for a substrate in the characterization process. Figure 2c is HRTEM image of the α-Fe2O3 seed. It can be observed that the spacing of 0.2715 nm can be indexed to the (104) plane of α-Fe2O3 (JCPDS No. 33-0664). The inset fast Fourier transformation (FFT) pattern shows that the planes of (012), (104), (110) of α-Fe2O3 are appeared, which is also demonstrate the obtained products are α-Fe2O3 NPs. Subsequently, the Ag NPs are uniformly coated on α-Fe2O3 NPs. The as-obtained α-Fe2O3/Ag core-shell NPs are characterized and show in Figure 2d-e. The corresponding SEM image (Figure 2d) shows that the as-obtained α-Fe2O3/Ag NPs still keep the spindle-like shape and the Ag shell is not dense shell but consist of many small Ag NPs. Compared to the α-Fe2O3 NPs, the surface of α-Fe2O3/Ag became rough because of the existence of small Ag NPs. The TEM image (Figure 2e) also clearly shows that Ag NPs possess a very uniform distribution on the surface of α-Fe2O3. In addition, the average diameter of Ag NPs is ca.10 nm. The insert pattern in Figure 2d is the EDX spectra of α-Fe2O3/Ag NPs, in which the Ag element extra appears in the measured sample compared to α-Fe2O3 NPs. Figure 2f is the HRTEM image of α-Fe2O3/Ag NPs and the inset are different magnification HRTEM images taken from an individual Ag particle on the surface of α-Fe2O3. It turns out that the Ag NPs are formed by means of the facet distances of 0.2047 nm, corresponding to Ag (200) facet (JCPDS No. 04-0783). The dumbbell-like α-Fe2O3/Ag/AgX heterostructures were successfully synthesized by in situ oxidation and self-assembly basic on the as-obtained α-Fe2O3/Ag core-shell NPs. The morphology 9



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and structure were characterized by FESEM, TEM and HRTEM. Figure 3a is the SEM image of α-Fe2O3/Ag/AgCl, which shows that the as-obtained α-Fe2O3/Ag/AgCl NPs possess dumbbell-like heterostructure in which a near-spherical Ag/AgCl NP is linked onto a spindle-like α-Fe2O3 NP. The different magnification TEM images (Figure 3b-c) also clearly shows that the as-obtained α-Fe2O3/Ag/AgCl NPs possess unique dumbbell-like shape. As shown in Figure 3c, the near-spherical Ag/AgCl small particles exhibit a core-shell structure. The inset in Figure 3b is EDX spectra of α-Fe2O3/Ag/AgCl. The analysis results illustrate that the elements of Fe, Ag, O and Cl are from α-Fe2O3/Ag/AgCl. Figure 3d is the HRTEM image of α-Fe2O3/Ag/AgCl, in which the 0.1693 and 0.2732 nm lattices fringes are correspond to the (116) and (104) of Fe2O3. Moreover, the interplanar distances corresponding to (220), (311) and (222) planes of AgCl appear in the small nanoparticle. The corresponding FFT pattern also shows the existence of planes of (220), (311), (222) of AgCl (JCPDS No. 31-1238). The results demonstrate that the α-Fe2O3/Ag/AgCl heterostructure was successfully synthesized. The morphology and structure of α-Fe2O3/Ag/AgBr and α-Fe2O3/Ag/AgI were also characterized and the results display in Figure 4. Form the SEM and TEM images of α-Fe2O3/Ag/AgBr (Figure 4a-b) and α-Fe2O3/Ag/AgI (Figure 4d-e), it can be seen that the morphology of α-Fe2O3/Ag/AgBr and α-Fe2O3/Ag/AgI are all keep dumbbell-like shape, consistent with that of α-Fe2O3/Ag/AgCl. The average diameter of Ag/AgX NPs is ca. 30 nm. The HRTEM image (Figure 4c) of α-Fe2O3/Ag/AgBr shows that the interplanar distances of 0.2700, 0.2042 and 0.2902 nm are corresponding to (104) plane of α-Fe2O3, (311) and (222) planes of AgBr (JCPDS No. 79-0149), respectively. The inset FFT pattern also demonstrates the existence of AgBr. Figure 4f is the HRTEM image of α-Fe2O3/Ag/AgI and the inset is the corresponding FFT pattern. The interplanar distances of 0.2516, 0.2049, 0.2304, and 0.1447 nm could be attributed to the (110) plane of α-Fe2O3, 10


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the (220) and (211) planes of AgI (JCPDS No. 22-1331) and the (220) plane of Ag, respectively. The results illustrate that the α-Fe2O3/Ag/AgI plsmonic photocatalyst was successfully synthesized. The elemental composition and chemical status of as-prepared α-Fe2O3/Ag, α-Fe2O3/Ag/AgX heterostructures were further analyzed by means of XPS. As shown in Figure 5a, the results of the survey

XPS

spectrums

indicate

that

the

samples

of

α-Fe2O3/Ag,

α-Fe2O3/Ag/AgCl,

α-Fe2O3/Ag/AgBr and α-Fe2O3/Ag/AgI all contain the peaks of Ag, Fe and O. Furthermore, the peaks of Cl, Br, I were accordingly appeared in the samples of α-Fe2O3/Ag/AgCl, α-Fe2O3/Ag/AgBr and α-Fe2O3/Ag/AgI, respectively. Figure 5b shows the spectra of Fe 2p of the obtained samples. It can be clearly seen that two well defined peaks at about 710.7 and 724.3 eV in all the samples can be attributed to Fe 2p3/2 and Fe 2p1/2 characteristic of Fe3+ in α-Fe2O3, respectively.38 The results demonstrate that the existence of α-Fe2O3 in all obtained samples (α-Fe2O3/Ag, α-Fe2O3/Ag/AgX). As shown in Figure 5c, the Ag 3d spectrum of α-Fe2O3/Ag consist of two individual peaks at 368.2 and 374.2 eV, which can be attributed to Ag 3d5/2 and Ag 3d3/2 binding energies of Ag0, respectively.39 In the samples of α-Fe2O3/Ag/AgX, two typical peaks of Ag 3d located at ~ 367 and ~ 373 eV can be attributed to the Ag 3d5/2 and Ag 3d3/2 binding energies, respectively. The Ag 3d5/2 peak and 3d3/2 peak can be further divided into two different peaks. The peaks at 367.1 and 373.1 eV are attributed to Ag+ of AgCl, and those at 367.9 and 373.9 eV are attributed to metallic Ag0. The peaks at 366.9 and 372.9 eV are attributed to Ag+ of AgBr, and those at 367.7 and 373.7 eV are attributed to metallic Ag0. The peaks at 367.6 and 373.6 eV are attributed to Ag+ of AgI, and those at 368.4 and 374.4 eV are attributed to metallic Ag0.40 The high-resolution XPS spectra of Cl 2p, Br 3d and I 3d are shown in Figure 5d. In the sample of α-Fe2O3/Ag/AgCl, the peaks at 197.6 and 199.2 eV are attributed to Cl 2p1/2 and Cl 2p3/2 of AgCl, respectively. The spectrum of Br 3d in the α-Fe2O3/Ag/AgBr shows that the binding energies of Br 3d5/2 and Br 3d3/2 are 67.4 and 68.4 eV, in 11



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agreement with literature values. The peaks at 618.8 and 630.3 eV are attributed to I 3d5/2 and I 3d3/2 of AgI, respectively.41 Therefore, the results characterized from XPS confirm the coexistence of Ag0 and AgX on the surface of α-Fe2O3 support. The photocatalytic performances of α-Fe2O3/Ag/AgX (X = Cl, Br, I) heterostructures The corresponding UV-vis adsorption spectra were measured and shown in Figure 6. The bandgap of α-Fe2O3, AgCl, AgBr and AgI is 2.2, 3.25, 2.25 and 2.77 eV, respectively.42-44 Firstly, the α-Fe2O3 NPs have an obvious adsorption at the wavelength shorter than 600 nm. After coated Ag NPs, the strong adsorption peak at about 420 nm is appeared due to the typical SPR of Ag NPs. Simultaneously, the existence of Ag NPs in α-Fe2O3/Ag leads to its stronger absorbance than the pure α-Fe2O3 NPs in UV-vis light region. After the Ag/AgX supported on the α-Fe2O3 by in situ oxide of Ag NPs, the adsorption spectra have an enhancement in UV-vis light region. The adsorption spectra of α-Fe2O3/Ag/AgCl, α-Fe2O3/Ag/AgBr, α-Fe2O3/Ag/AgI all have a slight blue-shift compared to the pure α-Fe2O3 and α-Fe2O3/Ag.45 In addition, the α-Fe2O3/Ag/AgCl possesses stronger adsorption in UV light region than α-Fe2O3/Ag/AgBr and α-Fe2O3/Ag/AgI due to the wide bandgap of AgCl. The photocatalytic activity of the dumbbell-like α-Fe2O3/Ag/AgX heterostructures were evaluated by measuring the decomposition of RhB dye under simulated sunlight irradiation. For comparison, photocatalytic performances of α-Fe2O3, α-Fe2O3/Ag and P25 were also investigated under identical degradation conditions. Figure 7a shows the time-dependent adsorption spectra of RhB in the presence of dumbbell-like α-Fe2O3/Ag/AgCl heterostructure. Before the light irradiation, the RhB solution with photocatalyst was in the dark for 30 min with rapid stirring. The corresponding adsorption spectra of RhB dye were almost not declined. However, after the light turning on, by only 5 min the adsorption spectra of RhB at 554 nm diminished quickly, and the RhB dye is completely 12


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degraded within 15-20 min. The results illustrate that the as-prepared dumbbell-like α-Fe2O3/Ag/AgCl exhibit excellent photocatalytic activity. Figure 7b shows the degradation rate of RhB with time evolution over the as-prepared samples (α-Fe2O3, α-Fe2O3/Ag, α-Fe2O3/Ag/AgX and P25). The self-degradation of RhB dye was measured and the degradation efficiency of bank sample was got to 18.5 % in 60 min. The results illustrate that the as-obtained α-Fe2O3/Ag/AgCl and α-Fe2O3/Ag/AgBr possess superior catalytic activity than pure α-Fe2O3, α-Fe2O3/Ag and P25. Especially the degradation efficiency of α-Fe2O3/Ag/AgCl was almost got to 100 % only in 20 min. Furthermore, the α-Fe2O3/Ag/AgBr also exhibit higher photocatalytic activity, which almost completely degrade of RhB dye after 60 min. In addition, the degradation efficiency of α-Fe2O3/Ag/AgI in 60 min is higher than pure α-Fe2O3 and α-Fe2O3/Ag. Figure 7c displays the photodegradation rate constant (kapp) of RhB over α-Fe2O3/Ag/AgX, which was calculated from a pseudo-first-order reaction kinetic model: ln(C/C0) = k̶ appt, where C0 and C are the concentrations of RhB aqueous solution at the irradiation times of 0 (i.e., after adsorption equilibrium in the dark) and t min, respectively. The k values of α-Fe2O3/Ag/AgCl, α-Fe2O3/Ag/AgBr and α-Fe2O3/Ag/AgI are 14.91×10-2 min-1, 5.28×10-2 min-1 and 1.30×10-2 min-1, respectively. The photodegradation rate constant of RhB over α-Fe2O3, α-Fe2O3/Ag and P25 were also calculated and shown in Figure S2(a-b). The k values of α-Fe2O3, α-Fe2O3/Ag and P25 are 0.44×10-2 min-1, 0.46×10-2 min-1 and 2.88×10-2 min-1, respectively. The results also display that the α-Fe2O3/Ag/AgCl exhibits the highest photocatalytic activity in the α-Fe2O3/Ag/AgX heterostructures and possesses greatly enhancement compared to commercial P25 (~5 times), α-Fe2O3 and α-Fe2O3/Ag. In addition, the corresponding photocurrent of α-Fe2O3/Ag/AgX heterostructures were measured, as shown in Figure S3 (Supporting Information). The photocurrent results demonstrate that the photocurrent intensity of α-Fe2O3/Ag/AgCl is a little smaller than α-Fe2O3/Ag/AgBr but larger than α-Fe2O3/Ag/AgI, which 13



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are partly in agreement with the results of photodegradation of RhB. However, the photocurrent intensity of α-Fe2O3/Ag/AgCl has no distinction compared with α-Fe2O3/Ag/AgBr. Enhancement mechanism The enhanced photocatalytic activity of dumbbell-like α-Fe2O3/Ag/AgX heterostructures are attributed to the efficient interfacial charge rectification and faster carrier migration.46-47 As shown in Figure 8, under simulated sunlight irradiation, electrons (e-) and holes (h+) could be generated from both photoexcited α-Fe2O3, AgX (X = Cl, Br, I) and plasmon-excited Ag NPs. The photoinduced electrons at the Ag NPs are transferred to the conduction band (CB) of α-Fe2O3, while the electrons from the photoexcited AgX could be injected into the Ag and could immediately transfer to the CB of α-Fe2O3. The photogenerated electrons can be trapped by absorbed O2 to form ·O2- and other oxidative species. Simultaneously, the photogenerated electrons in the CB of AgX can be tapped with Ag+ to form the metallic Ag. Furthermore, the photo-produced holes in the valence band (VB) of AgX and the holes from Ag nanoparticles could diffuse to the surface of AgX, which can oxidize X- to X0 (X = Cl, Br, I). Simultaneously, the holes in the VB of α-Fe2O3 can react with OH- and product ·OH. As a result, these active species including ·O2-, ·OH and X0 will decompose the RhB dye molecules. For example, the CB and VB energies of AgCl are -0.09 and 3.16 eV vs NHE (Eg = 3.25 eV), the CB and VB energies of α-Fe2O3 are 0.2 and 2.4 eV vs NHE (Eg = 2.2 eV) and the Fermi level of Ag is 0.3 eV vs NHE in α-Fe2O3/Ag/AgCl heterostructure. Therefore, α-Fe2O3, AgX (X = Cl, Br, I) and Ag NPs can be excited to generate electrons and holesunder simulated sunlight irradiation. The electrons at the CB of AgCl are transferred to Ag NPs, and excited electrons of Ag NPs can be transferred to the CB of α-Fe2O3, while the holes from Ag NPs could be diffuse to the surface of AgCl. The photoelectrons reduce absorbed O2 to form ·O2-, and the holes could react with OH- and Cl- produce ·OH and Cl0 in the photodegradation process, respectively. 14


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In addition, the degradation rate of as-synthesized dumbbell-like α-Fe2O3/Ag/AgX (X = Cl, Br, I) heterostructures follows this order: α-Fe2O3/Ag/AgCl > α-Fe2O3/Ag/AgBr > α-Fe2O3/Ag/AgI. From the above enhancement mechanism, we can propose that the photocatalytic process of α-Fe2O3/Ag/AgX (X = Cl, Br, I) contains three major steps: 1) the light-absorption to generate an electron and a hole, 2) the combination of a photo-generated hole with X- to form X0, 3) the oxidation of pollutant molecules by X0. In the first step, plasmonic photocatalyst α-Fe2O3/Ag/AgX not only can absorb the visible light due to the norrow bandgap of α-Fe2O3 and the SPR of Ag NPs but also can enhance the absorption of UV light due to silver halide. Furthermore, the α-Fe2O3/Ag/AgCl possessed a stronger adsorption in UV+visible light region than both α-Fe2O3/Ag/AgBr and α-Fe2O3/Ag/AgI due to its band gap (~3.25 eV). Subsequently, the Br- and Iwere easily combined a hole to form X0 than Cl-. However, the oxidation ability of Cl0 is stronger than Br0 and I0 in the third step.48-50 Therefore, the photocatalytic activity of α-Fe2O3/Ag/AgCl is better than α-Fe2O3/Ag/AgBr and α-Fe2O3/Ag/AgI because of its stronger absorption and oxidation ability. Conclusion In summary, the dumbbell-like α-Fe2O3/Ag/AgX heterostructures were successfully prepared via an in situ oxidation reaction and self-assembly process by using α-Fe2O3/Ag core-shell NPs. Moreover, their photocatalytic performance studies indicate that the as-obtained α-Fe2O3/Ag/AgX heterostructures possess much higher catalytic activity than α-Fe2O3 and α-Fe2O3/Ag NPs under simulated sunlight irradiation. Especially, the α-Fe2O3/Ag/AgCl can completely degrade RhB dye within only 20min. The enhancement of photocatalytic activity is due to the efficient interfacial charge rectification and faster carrier migration in the heterostructures. Furthermore, the degradation rate of as-synthesized dumbbell-like α-Fe2O3/Ag/AgX (X = Cl, Br, I) heterostructures follows this 15



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order: α-Fe2O3/Ag/AgCl > α-Fe2O3/Ag/AgBr > α-Fe2O3/Ag/AgI, which is due to the oxidation capability of Cl0 is stronger than Br0 and I0. This further work can provide for designing other novel composite materials with special structures and interesting properties, which can apply in multiple fields, including photo-degradation of organic pollution, photo-splitting of water to produce hydrogen and solar cell. Author information *Tel: +86-27-68778529. Fax: +86-27-68778433. E-mail: [email protected] (Wei Wu) [email protected] (Changzhong Jiang) Notes The authors declare no competing ﬁnancial interest. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: The parameters on spectral distribution and relative intensity of the used mercury lamp and the current-potential plots of the dumbbell-like α-Fe2O3/Ag/AgX (X = Cl, Br, I) heterostructures. Acknowledgment This work was partially supported by the NSFC (51471121, 51201115, 51171132, and 11375134), Hong Kong Scholars Program, Young Chenguang Project of Wuhan City (2013070104010011), China Postdoctoral Science Foundation (2014M550406), Independent scientific research fund of Wuhan University (2014202020208), Hubei Provincial Natural Science Foundation (2014CFB261), the Fundamental Research Funds for the Central Universities and Wuhan University. References (1) Liu, G.; Deng, Q.; Wang, H. Q.; Ng, D. H. L.; Kong, M. G.; Cai, W. P.; Wang, G. Z. 16


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Figure Captions Figure 1. Schematic illustration of the formation process of dumbbell-like α-Fe2O3/Ag/AgX (X = Cl, Br, I) heterostructures. Figure 2. (a) SEM image of as-prepared spindle-like α-Fe2O3 NPs, (b) TEM image of α-Fe2O3 NPs, and the insert is EDX spectrum of α-Fe2O3, (c) HRTEM image of α-Fe2O3 NPs, and the insert is the corresponding FFT pattern, (d) SEM image of as-obtained α-Fe2O3/Ag core-shell composite NPs, (e) TEM image of α-Fe2O3/Ag NPs, and the inset is EDX spectrum of α-Fe2O3/Ag, (f) HRTEM image of α-Fe2O3/Ag NPs, and the insets are the high magnification HRTEM images of Ag nanocrystal. Figure 3. (a) and (b) SEM and TEM image of as-prepared dumbbell-like α-Fe2O3/Ag/AgCl hybrid NPs, and the insert in Figure (b) is EDX spectrum of α-Fe2O3/Ag/AgCl, (c) and (d) HRTEM images of α-Fe2O3/Ag/AgCl, and the insert in Figure (d) is the corresponding FFT pattern came from the small oval NP. Figure 4. (a) and (b) SEM image and TEM image of as-prepared dumbbell-like α-Fe2O3/Ag/AgBr hybrid NPs, (c) HRTEM image of α-Fe2O3/Ag/AgBr and the insert is the corresponding FFT pattern, (d) and (e) SEM image and TEM image of as-prepared dumbbell-like α-Fe2O3/Ag/AgI hybrid NPs, (f) HRTEM image of α-Fe2O3/Ag/AgI and the insert is the corresponding FFT pattern came from the black spherical NP. Figure 5. Typical XPS spectra of α-Fe2O3/Ag and α-Fe2O3/Ag/AgX (X = Cl, Br, I) heterostructures, (a) the survey XPS spectra, (b) Fe 2p, (c) Ag 3d, (d) Cl 2p, Br 3d and I 3d. Figure 6. UV-vis absorption spectra of the as-prepared α-Fe2O3, α-Fe2O3/Ag and α-Fe2O3/Ag/AgX (X = Cl, Br, I). Figure 7. (a) UV–vis spectral evolution of RhB as a function of light irradiation time over α-Fe2O3@Ag/AgCl; (b) Photocatalytic degradation of RhB solution over as-prepared samples as a function of irradiation time under simulated sunlight irradiation; (c) kinetic linear simulation curves of the α-Fe2O3/Ag/AgX (X = Cl, Br, I) photocatalysts for the photodegradation of RhB under simulated sunlight irradiation. Figure 8. Schematic illustration of the photocatalytic process for dumbbell-like α-Fe2O3/Ag/AgX (X = Cl, Br, I) 24


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ACS Sustainable Chemistry & Engineering heterostructures under simulated sunlight irradiation.

Figures

Figure 1

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Figure 2

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Figure 3

Figure 4

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Figure 5

Figure 6

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Figure 7

Figure 8

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TOC

Dumbbell-like α-Fe2O3/Ag/AgX (X = Cl, Br, I) heterostructures were synthesized by in situ oxidation and self-assembly basic of α-Fe2O3/Ag core-shell composite NPs and exhibit greatly enhanced photocatalytic properties.

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In situ Oxidation and Self-Assembly Synthesis of ... - ACS Publications

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