3D Flowerlike Î±-Fe2O3@TiO2 CoreâShell Nanostructures: General

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3D Flower-Like #-Fe2O3@TiO2 Core-Shell Nanostructures: General Synthesis and Enhanced Photocatalytic Performances Jun Liu, Shuanglei Yang, Wei Wu, qingyong Tian, Shuyuan Cui, Zhigao Dai, Feng Ren, Xiangheng Xiao, and Changzhong Jiang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b00956 • Publication Date (Web): 27 Sep 2015 Downloaded from http://pubs.acs.org on October 5, 2015

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ACS Sustainable Chemistry & Engineering

3D Flower-Like α-Fe2O3@TiO2 Core-Shell Nanostructures: General Synthesis and Enhanced Photocatalytic Performances Jun Liu1, 2, Shuanglei Yang2, 3, Wei Wu2, 4*, Qingyong Tian1, 2, Shuyuan Cui2, Zhigao Dai1, 2, Feng Ren1, Xiangheng Xiao1*, 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 Printable Functional Nanomaterials and Printed Electronics, School of Printing and Packaging, Wuhan University, Wuhan 430072, P. R. China

3

State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, P. R. China

4

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

Abstract: The three-dimension (3D) flower-like α-Fe2O3@TiO2 core-shell nanocrystals with thorhombic, cubic and discal morphologies are synthesized for photocatalytic application. α-Fe2O3 nanocrystals were prepared via Cu2+, Zn2+ and Al3+ ions-mediated hydrothermal route. The α-Fe2O3@TiO2 core-shell nanocrystals are obtained via hydrothermal and annealing process. The shape-dependent photocatalytic activity of these as-obtained α-Fe2O3@TiO2 core-shell nanocrystals are measured. The results reveal that the discal α-Fe2O3@TiO2 nanocrystals exhibit the best photocatalytic activity than other two core-shell nanocrystals. Because the discal α-Fe2O3 nanocrystals possess more rough surface and surface defects. The fast interfacial charge transfer process and the

*

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

[email protected] (W. Wu), [email protected] (X.H. Xiao), [email protected]. (C. Z. Jiang)

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wide spectral response could be the driving force for the enhanced photocatalytic performance. These core-shell architectures provide a positive example for synthesis of novel composite nanomaterial. Keywords: iron oxide; titanium dioxide; core-shell structure; photocatalyst; heterostructures

Introduction Semiconductor nanomaterials with excellent photoelectric properties have been widely used in photocatalysis and water splitting application.1-5 The semiconductor photocatalyst could absorb photons from both ultraviolet and visible light irradiation due to the different band gaps. A well-defined semiconductor photocatalyst not only improves the transfer efficiency of photoinduced charge carriers, but also widens the spectral response range.6 Recently, more and more catalytic nanomaterials are developed and utilized in photocatalysis.7-8 For example, magnetic materials is an interesting materials which have shown the excellent magnetic properties and semiconductor performances.9-14 The α-Fe2O3 is one of magnetic materials which have been investigated for several decades because of its chemical stability and low cost. As an n-type semiconductor with narrow band gap (2.2 eV), α-Fe2O3 is conventionally used as visible light-driven photocatalyst.15-16 However, in monocomponent α-Fe2O3 materials, the high recombination rate of photogenerated electrons and holes hinder its widespread application in photocatalysis. The main reason is that the lifetime of photogenerated charge carriers are very short (< 10 ps),17 and the hole diffusion length is not enough (2-4 nm) for transferring to the α-Fe2O3 surface.18-19 Therefore, the single-component α-Fe2O3 photocatalyst is not beneficial to separate the photogenerated pairs of electrons and holes. Thus, α-Fe2O3 nanocrystals with shape-dependent catalytic properties have drawn more attentions.20 For example, Hou and co-worker synthesized the oblique and truncated nanocubes for enhanced visible light photocatalytic application, they found that the distinct photocatalytic activity could be attributed to the various morphologies which are caused by different exposed crystal facets.21 2 ACS Paragon Plus Environment

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Obviously, the shape of α-Fe2O3 nanocrystals plays a key role to influence their photocatalytic performance. Additionally, coupling of wide bandgap semiconductor (such as ZnO, TiO2 and SnO222-28) with α-Fe2O3 form the composite photocatalyst to elevate the photocatalytic activities. This kind of composite photocatalyst could accelerate the separation rate of charge carries in photocatalytic application,29 especially for coupling with TiO2 nanocrystals because of its well anti-photocorrosion properties.30 Anatase phase of TiO2 nanocrystals are a familiar semiconductor photocatalyst, the excellent UV light-driven photocatalytic reaction is presented due to its wide bandgap (3.2 eV).31 As an example, Tang and co-worker prepared the Fe2O3-TiO2 nanocomposite for photocatalytic application. 100 mL (50 ppm) of 2, 4-dichlorophenoxyacetic acid could be completely decomposed by 10 mg of the Fe2O3-TiO2 hybrid catalyst under full-arc 300 W solar lamp irradiation for 150 min. The TiO2 in the Fe2O3-TiO2 hybrid structure is composed with both anatase and rutile structure TiO2 inside. This is the main reason for improving the photocatalytic properties.32 In recent year, the anatase TiO2 nanocrystals with exposed (101) facets have been reported for relatively improved photocatalytic activity.33 Therefore, coupling with α-Fe2O3 nanocrystals and TiO2 with more (101) exposed facets to form the core-shell structure is a favorable way to improve the photocatalytic performance under both UV and visible light illumination.34 Herein, three α-Fe2O3 nanocrystals with different shapes and corresponding core-shell α-Fe2O3@TiO2 composite nanocrystals are synthesized and used in photocatalytic application. The α-Fe2O3 nanocrystals are prepared by metal ions-mediated (Cu2+, Zn2+ and Al3+) hydrothermal route. Subsequently, the anatase TiO2 with enriched (101) exposed facets are coated on their surface via hydrothermal route and annealing treatment. The morphologies and composition are determined by various analytic techniques. Moreover, the photocatalytic activity of three naked α-Fe2O3 3 ACS Paragon Plus Environment


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nanocrystals are comparative investigated by degradation of RhB under UV and visible light irradiation, the distinct photocatalytic activities could be attributed to the different morphologies. After coating with anatase TiO2, the shape-dependence and synergetic photocatalysis of 3D flower-like nanocrystals are also carried out. The proposed mechanism and photocurrent test for the charge transfer process of enhanced photocatalytic performances are discussed. Experimental section Materials and chemicals. Iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O, AR), diethylenetriamine (DETA 98%), ammonia (NH3·H2O, 25%) and absolute ethanol (C2H5OH) were purchased from Sinopharm Chemical Reagent Co., Ltd., titanium(IV) isopropoxide and Nafion solution (5 wt%) were purchased from Sigma-Aldrich Co., zinc acetate (Zn(CH3COO)2·2H2O, AR), cupric acetate anhydrous (Cu(CH3COO)2, AR), aluminium acetate (Al(CH3COO)3, AR), Rhodamine B (RhB), Acid Orange 7 (AO7) and Malachite Green (MG) were purchased from Shanghai Aladdin Reagents Co., Ltd. All the reagents were analytically pure (AR) and used as received without further purification. The deionized water (18.2 Ω) was used throughout all the experiments. Synthesis of thorhombic, cubic and discal α-Fe2O3 particles. According to our previous reports,35-36 thorhombic, cubic and disc α-Fe2O3 nanoparticles were prepared by using a metal ions-mediated hydrothermal route. Typically, 0.808 g of Fe(NO3)3·9H2O was dissolved in 10 mL of water under magnetic stirring, 1 mmol of acetate precursor was added to the above solution. Then, 10 mL ammonia solution was added. Finally, the mixture was transferred into a Teflon-lined stainless steel autoclave with a capacity of 50 mL for hydrothermal treatment at temperature of 160 °C for 16 h. After that, the autoclave was allowed to cool down to room temperature naturally. The obtained α-Fe2O3 product were washed several times with ethanol and deionized water before drying under vacuum at 60 °C for 12 h. 4 ACS Paragon Plus Environment

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Synthesis of α-Fe2O3@TiO2 composite nanocrystals. The α-Fe2O3@TiO2 composite nanocrystals were also synthesized via hydrothermal process. 40 mg of as-obtained thorhombic, cubic or disc α-Fe2O3 nanoparticles was dispersed in 20 mL of absolute ethanol under ultrasound. Then, 200 µL of DETA and a certain amount of titanium (IV) isopropoxide (17 µL for synthesis of α-Fe2O3(Cu)@TiO2, (Fe:Ti = 10:1), 11µL for α-Fe2O3(Zn)@TiO2, (Fe:Ti = 15:1) and 38µL for α-Fe2O3(Al)@TiO2, (Fe:Ti = 5:1)) were added in the above-mentioned solution. After vigorous stirring, the mixture was transferred into a Teflon-lined stainless steel autoclave with a capacity of 50 mL for hydrothermal treatment at temperature of 200 °C for 20 h. After the reaction, the autoclave was allowed to cool down to room temperature naturally. Moreover, the obtained solution was centrifuged and washed three times with ethanol and water and then dried under vacuum at 60 °C for 12 h. Finally, the dried products were annealed in annealing furnace with air atmosphere at 450 °C for 2 hours, and the final products were obtained. Characterization. Scanning electron microscopy (SEM) images were obtained by using a cold field emission SEM (Hitachi S-4800). The transmission electron microscopy (TEM) patterns were measured by JEOL JEM-2010 (HT) operated at 200 kV. The high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and Raman measurements were performed by a laser confocal Raman microspectroscopy (LabRAM HR800). The laser with wavelength of 488 nm used as the excitation source. Powder X-ray diffraction (XRD) patterns of the samples were recorded on a D/ruax2550PC (Japan) using Cu Kα radiation (λ = 0.1542 nm) operated at 40 kV and 40 mA and at a scan rate of 0.05° 2θ S-1. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo Scientific ESCALAB 250 Xi system with Al Kα (1486.6 eV) as the radiation source. X-ray spectroscopy (EDX) analysis were performed with a JEOL JEM-2100F. The photocurrent measurement were performed with A potentiostat 273A (Princeton 5 ACS Paragon Plus Environment


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Applied Research Company) and a 500W Xe lamp (100 mW/cm2). The UV-Vis absorption spectra of the samples were carried out on a Shimadzu UV-2550 spectrophotometer. Photocatalytic tests. In the photocatalytic experiments under UV and visible light (mercury lamp light) irradiation (the spectral distribution and relative intensity of the mercury lamp were listed in Table S1 (please refer to Supporting Information)), the control (A 10 mg·L-1 RhB, AO7 and MG solution without added particles) and experimental (A 10 mg·L-1 RhB, AO7 and MG solution with 3 mg obtained samples) groups were carried out (Preparation of the TiO2 shell: 3 mg of α-Fe2O3@TiO2 composite nanocrystals were dispersed in 3 mL distilled water, then 3 mL of hydrochloric acid was added in. The mixture were stirred for 24 hours until the color of samples fade from red to white). Firstly, the absorption of samples was presented in the dark for 30 min with gentle stirring to reach the absorption equilibrium. Then, the solutions were illuminated under a mercury lamp (300 W). The reaction solutions were sampled at 30 min illumination intervals, and the corresponding UV-Visible spectra (measured in the range of 450 ~ 650 nm or 200 ~ 800 nm) were recorded to monitor the progress of the degradation of RhB, AO7 and MG by a Shimadzu 2550 UV-Visible spectrophotometer. Photoelectrochemical tests. The photoelectrochemical measurements was carried out with three α-Fe2O3@TiO2 core-shell nanoparticles as the working electrodes. The working electrodes were prepared as follows: 1 mg as-obtained nanopowders were dispersed in 200 µL of H2O, and 20 µL of Nafion solution (5 wt%) was added into the above solution. Then the mixture was deposited onto the surface of fluorine tin oxide (FTO) glass (size: 1.5 cm×2.5 cm) by spinning at 300 rmp. Finally, these sample were annealed in vacuum at 550 °C for 2 h to remove the Nafion.37 In photoelectrochemical measurements process, the surface area of working electrodes exposed to electrolyte was fixed at 0.785 cm2. A silver chloride electrode was used as a reference electrode and 6 ACS Paragon Plus Environment

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a large area platinum plate was used as a counter electrode. Aqueous solution of Na2SO4 (0.5 M) was prepared as the electrolyte. A potentiostat 273A (Princeton Applied Research Company) and a 500W Xe lamp (100 mW/cm2) were used for photocurrent measurement. Result and discussion The morphology of α-Fe2O3@TiO2 composite nanocrystals The schematic diagram of the entire growth processes of the α-Fe2O3 nanocrystals and their α-Fe2O3@TiO2 nanocomposite are shown in Figure 1. The α-Fe2O3 nanocrystals with thorhombic (Figure 1a), cubic (Figure 1b) and disc (Figure 1c) shapes are synthesized via different metal ion-mediated hydrothermal process, which are reported in our previous works.35-36 The α-Fe2O3 nanocrystals are generated as thorhombic, cubic and disc shapes because of the addition of Cu2+, Zn2+ and Al3+ ions, respectively. The yields the three sample (α-Fe2O3(Cu), α-Fe2O3(Zn) and α-Fe2O3(Al)) are 96.88%, 99.26% and 97.13%, respectively. Moreover, the anatase TiO2 nanosheets are coated on the surface of these nanocrystals via employing titanium (IV) isopropoxide as the titanium source in an alkaline environment by hydrothermal process. The products are calcined at 450 °C in air for 2 h to prepare highly crystalline anatase phase.38 Finally, the 3D flower-like α-Fe2O3@TiO2 architectures with three different shapes are obtained. Figure 2 shows the electron microscope (EM) images and fast Fourier transform (FFT) patterns of α-Fe2O3 nanocrystals. Obviously, three α-Fe2O3 nanocrystals with distinct morphologies are observed. The thorhombic (Figure 2a-c), cubic (Figure 2d-f) and disc (Figure 2g-i) shapes of α-Fe2O3 nanocrystals are tailored by Cu2+, Zn2+and Al3+ ions, respectively. The surfaces of thorhombic and disc α-Fe2O3 nanocrystals are rough, but the surface of cubic α-Fe2O3 nanocrystals is smooth. Moreover, the apparent single-crystal structure are presented in the HRTEM images. In Figure 2c, two lattice spacings of 0.368 nm could be indexed to (012) and ( 102 ) planes of 7 ACS Paragon Plus Environment


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thorhombic α-Fe2O3 nanocrystals, respectively. The FFT patterns is indexed to a rhombohedral α-Fe2O3 crystal along the [ 221 ] zone axis indicating that the exposed facets of the crystal are {012} planes. In Figure 2f, two lattice spacings of 0.270 nm correspond to the (014) and ( 104 ) planes of cubic α-Fe2O3 nanocrystals, respectively. The inset FFT patterns in Figure 2f is indexed to α-Fe2O3 crystal along the [ 441 ] zone axis show that the exposed facets are {104} planes. Two lattice spacings of 0.252 nm in Figure 2i correspond to the (110) and ( 2 10 ) planes of disc α-Fe2O3 nanocrystals, respectively. The FFT patterns along the [001] zone axis show the exposed facets are {110} planes. Obviously, the different exposed crystal facets {012}, {104} and {110} of three shapes of α-Fe2O3 nanocrystals are observed. Indeed, different exposed crystal facets can lead to different electrochemical and catalytic properties.39-40 Various literatures focused on that, especially the single crystal α-Fe2O3 with exposed {104} facets.21, 41 The exposure of high-index crystal planes of α-Fe2O3 nanocrystals can hold more surface defects and high surface energy than that of low-index crystal planes. Therefore, the different exposed crystal planes can impact the photocatalytic activity relevantly.42-43

Figure 3 shows the representative EM images and SAED patterns of TiO2 coated α-Fe2O3 nanocrystals. In SEM images (Figure 3a, 3d, 3g), it is obvious that the TiO2 nanosheets are deposited on surface of α-Fe2O3 nanocrystals. Which suggest that the flower-like α-Fe2O3@TiO2 composite nanocrystals are successful prepared. In Figure 3b, 3e, 3h, the apparent core-shell structures of α-Fe2O3@TiO2 composite nanocrystals are exhibited. The inset SAED patterns reveal that the α-Fe2O3 core is a monocrystal nanomaterials with ordered diffraction spots (red mark), and TiO2 shell is a polycrystalline nanosheets with diffraction rings (yellow mark). The HRTEM images are presented in Figure 3c, 3f, 3i, which are taken from the edge of composite nanocrystals. The lattice spacings of 0.368 nm, 0.169 nm, 0.252 nm and 0.270 nm are corresponded to the (012), (116),


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(110) and (104) planes of α-Fe2O3 nanocrystals (JCPDS No. 33-0664), respectively. But the lattice spacing of 0.352 nm could be attributed to (101) plane of anatase TiO2 (JCPDS No. 21-1272). These results further reveal that the α-Fe2O3@TiO2 composite nanocrystals with defined core-shell structure are successful synthesized.44-45

The structural characterization of α-Fe2O3@TiO2 composite nanocrystals The Raman spectra of as-obtained naked α-Fe2O3 nanocrystals and α-Fe2O3@TiO2 composite 6 nanocrystals are shown in Figure 4a-c. Hematite belongs to the D3d crystal space group and

possesses seven typical Raman active modes, namely two A1g modes (225 and 498 cm−1) and five Eg modes (247, 293, 299, 412 and 613 cm−1).46 The peaks of all these samples are list in Table S2. These peaks could match the A1g and Eg modes, respectively. But the peak positions and intensity of three shapes of α-Fe2O3 nanocrystals are different (Figure 4a-c), which are caused by the different sizes and morphologies of α-Fe2O3 nanocrystals.47 The anatase phase TiO2 is belonged to the D419h crystal space group,48 the positions at 152.3 (a), 150.2 (b) and 146.0 (c) cm-1 of three composite nanocrystals can be assigned as the Eg mode of anatase phase TiO2. The different intensities are presented due to the different amount of TiO2 coating. Furthermore, the inset spectra in Figure 4a-c are the comparison of the magnified spectra from 200 to 240 cm-1 for Fe A1g transitions before and after coating with TiO2. The Gaussian fitting results show a clear shifts because of the phonon confinement effect,49-50 these shifts are 1.2, 0.2 and 1.6 cm-1 in the inset images of Figure 4a-c, respectively. These shifts could be proportional to the intensity of interaction between α-Fe2O3 and TiO2. In addition, the XRD patterns of α-Fe2O3 and α-Fe2O3@TiO2 composite nanocrystals are presented in Figure S1 and Figure 4d. The standard JCPDS card of pure α-Fe2O3 (33-0664, black lines) and anatase TiO2 (21-1272, blue lines) are used for comparison. All positions and intensities of 9 ACS Paragon Plus Environment


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the α-Fe2O3 nanocrystals could be indexed to the standard JCPDS card well. It indicates that as-prepared α-Fe2O3 nanocrystals are rhombohedral structure of hematite with high crystallinity. After coating with anatase TiO2, the apparent diffraction peaks of anatase TiO2 are only presented in α-Fe2O3(Al)@TiO2 composite nanocrystals, and it could be indexed to {101}, {004}, {200} and {105} planes of anatase TiO2(Figure 4d). However, the diffraction peaks of TiO2 in Figure S1a and

S1b are inconspicuous. Because the diffraction peaks of α-Fe2O3(Al) are weaker than α-Fe2O3(Cu) and α-Fe2O3(Zn). As a polycrystalline materials, the TiO2 shell are submerged in strong diffraction peaks of monocrystalline α-Fe2O3(Cu) (Figure S1a) and α-Fe2O3(Zn) (Figure S1b) nanocrystals.51 Therefore, the diffraction peaks of anatase TiO2 could be detected only in Figure 4d. Moreover, the XPS spectra of α-Fe2O3 nanocrystals and α-Fe2O3@TiO2 composite nanocrystals with three different morphologies are shown in Figure S2 and Figure 5, respectively. In Figure S2, the full XPS spectrum (Figure S2a) and the characteristic peaks of Cu2+, Zn2+ and Al3+ (Figure S2b,

S2c, S2d) of three different shapes of α-Fe2O3 nanocrystals are also presented. It suggests that these ions have been introduced into the α-Fe2O3 nanocrystals.34 The Fe/Cu, Fe/Zn and Fe/Al are15.12:1, 23.44:1 and 13.74:1, respectively. Figure 5a shows the full XPS spectrum of α-Fe2O3@TiO2 with three morphologies (thorhombic (a), cubic (b) and disc (c)). The peaks of Fe 2p and Ti 2p are apparent in all curves of the three samples, the α-Fe2O3@TiO2 core-shell structure could be further confirmed. Figure 5b and 5c display the high resolution XPS spectrum of Fe 2p peaks and TiO2 2p peaks. In Figure 5b, the two peaks located at 725.1 and 711.5 eV, which are confirmed to Fe 2p1/2 and Fe 2p3/2 of α-Fe2O3, respectively. In Figure 5c, the observed binding energy for Ti 2p is 464 and 458.4 eV, which are belonged to Ti 2p1/2 and Ti 2p3/2 of Ti (IV), respectively. These results indicate that the shells on the surface of α-Fe2O3 nanocrystals are TiO2. Moreover, the characteristic peaks of Cu2+, Zn2+ and Al3+ of three different shapes of α-Fe2O3 nanocrystals are also distinct in Figure 5d, 10 ACS Paragon Plus Environment

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5e and 5f, respectively. The peaks have further proved that these metallic ions are introduced into α-Fe2O3 nanocrystals during the morphologic shaping processes. Furthermore, the EDX spectra of as-obtained α-Fe2O3 nanocrystals and α-Fe2O3@TiO2 core-shell nanocrystals are measured and shown in Figure S3. The relative amount of Ti element in three samples are distinct, it demonstrates that the amount of TiO2 coating is different among the three samples. The order of atomic ratio is α-Fe2O3(Al)@TiO2 (15.25 %, Figure S3c) > α-Fe2O3(Cu)@TiO2 (5.40 %, Figure S3a) > α-Fe2O3(Zn)@TiO2 (3.48 %, Figure S3b), which reflects that the shape of α-Fe2O3 carriers is an important factor to improve the amount of TiO2 coating.

The catalytic performances of α-Fe2O3@TiO2 composite nanocrystals The UV-Vis spectra of these samples are carried out before the photocatalytic test. In Figure S4, the three α-Fe2O3 (Cu2+, Zn2+ and Al3+) samples and corresponding TiO2 coated samples are shown in Figure S4a, S4b and S4c, respectively. In the three α-Fe2O3 samples, some absorbed intensity could be observed in visible region (λ > 420 nm). After coating with anatase TiO2 nanocrystals, the absorbed intensity in ultraviolet region (λ < 420 nm) have been improved. Therefore, this core-shell α-Fe2O3@TiO2 composite nanocrystals show the photocatalytic activity under both UV and visible light region. The photocatalytic properties of α-Fe2O3 nanocrystals, TiO2 nanocrystals (from the core-shell α-Fe2O3@TiO2 by HCl etching the α-Fe2O3 core) and core-shell α-Fe2O3@TiO2 composite nanocrystals with three different morphologies have been investigated via degrading the RhB under UV and visible light. The typical degradation curves of RhB with α-Fe2O3(Cu)@TiO2 (a), α-Fe2O3(Zn)@TiO2 (b) and α-Fe2O3(Al)@TiO2 (c) are shown in Figure 6a-c. The irradiation time interval is 30 min. The photocatalytic reaction of semiconductor materials can be followed as 11 ACS Paragon Plus Environment


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− ln( C C 0 ) = kt , the k is the apparent rate constant of the degradation. Because of the pseudo first

order reaction happens during the process of degradation. In Figure 6d-f, the comparison of photocatalytic activity, k value and degradation rate of obtained samples are exhibited. The degradation rate is proportional to the slope,52 and the linear fitting of the slope are shown in Figure

6e. After irradiation with UV and visible light for 120 min, the photocatalytic activity of naked α-Fe2O3 nanocrystals is considerable different due to their different morphologies. The main reason is that the morphology would form different exposed crystal planes, and high-index crystal planes are a facilitated factor to improving the photocatalytic activity.53 The planes {012}, {104} and {110} are mainly exposed in α-Fe2O3(Cu), α-Fe2O3(Zn) and α-Fe2O3(Al), respectively. The different exposed crystal facets could induce the different photocatalytic activities. The roughness of α-Fe2O3 surface is also key factor for improving the photocatalytic performance. Because the multiple reflection process of lights could happen in rough surface of α-Fe2O3 nanocrystals. More lights are absorbed for enhanced photocatalytic performance.54-55 The sequence of photodegradation rate of α-Fe2O3 nanocrystals is as follows: α-Fe2O3(Al) (52.7%) > α-Fe2O3(Cu) (49.1%) > α-Fe2O3(Zn) (23.1%). As a consequence, the photocatalytic activity are positively correlated to the different crystal planes exposure and roughness. After coating with TiO2, the order is still the same. The α-Fe2O3(Al)@TiO2 composite nanocrystals show their excellent photodegradation rate (98.1 %) and large k value (3.2×10−2 min-1) than α-Fe2O3(Cu)@TiO2 (87.8 %, 1.69×10−2 min-1) and α-Fe2O3(Zn)@TiO2 (58.0 %, 0.64×10−2 min-1) composite nanocrystals (Figure 6f). But the photocatalytic activity of α-Fe2O3@TiO2 have been improved by compared with the corresponding naked α-Fe2O3 nanocrystals. The increment of degradation rate is 38.7 %, 26.9 % and 45.4 % (red mark in Figure 6f), respectively. The different increment indicate that the different planes coupling with TiO2 nanocrystals could induce the different photocatalytic activities. The photocatalytic 12 ACS Paragon Plus Environment

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activities of TiO2 shells from these three α-Fe2O3@TiO2 samples have been also investigated. The degradation

rates

of

TiO2

(α-Fe2O3(Cu)@TiO2),

TiO2

(α-Fe2O3(Zn)@TiO2)

and

TiO2

(α-Fe2O3(Al)@TiO2) are 55.0%, 45.6% and 70.5%, respectively (Figure 6f). From these results, the composite structures of α-Fe2O3@TiO2 show the better photocatalytic performance than the corresponding α-Fe2O3 and TiO2 monocomponent photocatalysts. The improved photocatalytic activities of α-Fe2O3@TiO2 composite structures demonstrate that the synergistic effect between α-Fe2O3 and TiO2 are contributed to degradation of the RhB under UV and visible light irradiation.56-57 Another two stable chemicals (Acid Orange 7 (AO7) and Malachite Green (MG)) are employed to evaluate the photocatalytic performance. The degradation process of the two chemicals are carried out under the UV and visible light by using 3 mg α-Fe2O3(Al)@TiO2 composite nanocrystals, respectively. Figure 7a shows the UV-vis absorption spectra of 10 mL of AO7 (15 mg/L). About 98.6% AO7 are degraded after 90 min irradiation of UV and visible light. The UV-vis absorption spectra of 10 mL of MG (10 mg/L) are shown in Figure 7b. After irradiation for 150 min, about 90% MG are degraded. Hence, this α-Fe2O3(Al)@TiO2 heterostructures could degrade various organic pollutants. During the photodegradation of RhB, the apparent quantum efficiency of these samples are elaborated. The solution of the sample is 10 mL, the measured average optical power [hν]inc is 11.72 mW·mL-1·per unit time (s), which are measured by irradiatometer (FZ-A, Photoelectric Instrument Factory of Beijing Normal University, wavelength range (400-1000 nm)). The light source is the 300 W high pressure mercury lamp, the intensity and wavelength distribution of this high pressure mercury lamp are shown in Table S3. Only the light with 400, 510, 620 and 720 nm could be measured by irradiatometer (Table S3). According to the relative intensity of the light with different 13 ACS Paragon Plus Environment


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wavelength, the optical power [hν]inc of each Wavelength could be calculated corresponding. Total optical power d[hν]inc/dt of 10 mL solution is 25.86 mW·mL-1×10 mL =258.6 mW per unit time (s) (15.516 W·min-1). The apparent quantum efficiency are calculated by the formula (1) as follow:58-60

ϕx =

m(d [ x] / dt ) d [hν ]inc / dt

(1)

d[x]/dt is the rate of change of the concentration of the reactant and d[hv]inc/dt is the total optical power impinging on the sample. The rate of change of the concentration of the reactant could be the

k value of the − ln(C C0 ) = kt . The k value of all samples are shown in Table S4. The apparent quantum efficiency (ϕx) are calculated correspondingly.

Proposed enhanced catalytic mechanisms The schematic diagram of the photocatalytic mechanism of α-Fe2O3@TiO2 composite nanocrystals is shown in Figure 8. The enhanced photodegradation activities of the as-obtained α-Fe2O3@TiO2 composite nanoparticles should be attributed to the fast interfacial charge transfer of the two semiconductors.34, 61 Under the light irradiation, the photogenerated electrons in α-Fe2O3 are excited from valence band (VB) to conduction band (CB) under irradiation of visible light, and the electrons in TiO2 could be also excited under the UV light. Before contact, as shown in Figure 8a, the CB of TiO2 lie overtop the CB of α-Fe2O3. However, after contact, the two semiconductors could reach to the same Fermi level because of the different work functions of the α-Fe2O3 (5.88 eV) and TiO2 (4.308 eV), the CB of TiO2 would lie below the CB of α-Fe2O3 (Figure 8b).62-63 Therefore, the excited electrons of α-Fe2O3 can overcome the barrier and transferred to the CB of TiO2, these electrons have been captured by dissolved oxygen in aqueous solution to produce the reactive oxygen species (·O2). In contrast, the holes stayed behind in VB will transfer from TiO2 to the VB of 14 ACS Paragon Plus Environment

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α-Fe2O3 expeditiously, the holes are trapped by H2O to form hydroxyl radicals (·OH). These reactive oxygen species (·O2) and hydroxyl radicals (·OH) with high-activities could degrade RhB rapidly. Therefore, in this wide-narrow bandgap composited photocatalysis system, the photogenerated electron-hole pairs are separated at the contact surfaces efficiently and increase the lifetime of the charge carriers.64 The effective separation of electron and hole pairs is the final reason for improving the photocatalytic abilities. The photoelectrochemical measurements was carried out with three α-Fe2O3@TiO2 core-shell nanoparticles as the working electrodes. The Photocurrent-Potential plots of three composite samples are presented in Figure 9. The best photocurrent intensity (35.2 µA·cm-1) are obtained with the presentation of α-Fe2O3(Al)@TiO2 by compared with α-Fe2O3(Cu)@TiO2 (22.3 µA·cm-1) and α-Fe2O3(Zn)@TiO2 (18.3 µA·cm-1), it reveal that this sample possess the largest carrier concentration during the light irradiation, and the more electron-hole pairs are generated for charge transfer process. The order of the photocurrent intensity is α-Fe2O3(Al)@TiO2 (c) > α-Fe2O3(Cu)@TiO2 (a) > α-Fe2O3(Zn)@TiO2 (b), which are in agreement with the result of photodegradation of RhB. The obvious photocurrent demonstrate that the interfacial charge transfer between Fe2O3 and TiO2 is existing in this composite nanoparticles, the electrons transfer could be further testified and quantified by this characterization.

Conclusions In summary, three shapes of α-Fe2O3 nanocrystals and their anatase TiO2 coated composite nanocatalysts are demonstrated. The mental ions-mediated (Cu2+, Zn2+ and Al3+ ions) hydrothermal route is a significant method for controlling the morphologies of α-Fe2O3 nanocrystals. More importantly, hydrothermal method is also used in coating with anatase TiO2 with (101) exposed facets on surface of α-Fe2O3 nanocrystals. The results reveal that the 3D flower-like α-Fe2O3@TiO2 15 ACS Paragon Plus Environment


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core-shell composite nanocrystals are synthesized successfully. Besides, the enhanced photocatalytic performance of 3D flower-like α-Fe2O3@TiO2 are presented by compared with naked α-Fe2O3 nanocrystals. The best photodegradation performance are obtained by using α-Fe2O3(Al)@TiO2 composite nanocrystals as photocatalyst. The different exposed crystal facets is the key factor for photocatalytic improvement. These α-Fe2O3@TiO2 core-shell architectures are expected to provide new perspectives for the fabrication of other core-shell structures for photocatalytic application or other application.65-67

Associated content The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxxxxxxxxxx. XRD patterns of the α-Fe2O3 and their corresponding α-Fe2O3/TiO2 core-shell nanostructures, XPS spectra of different bare α-Fe2O3 products; EDX spectra of the three α-Fe2O3@TiO2 core-shell nanocrystals; UV-Vis spectra of the three α-Fe2O3 and their corresponding α-Fe2O3@TiO2 core-shell nanocrystals; The parameters on spectral distribution and relative intensity of the used mercury lamp in the photocatalytic tests; The peaks of α-Fe2O3 and TiO2 in Raman spectra; The parameters on spectral distribution and relative intensity of the used high pressure mercury lamp and their corresponding optical power (mW·mL-1) in the photocatalytic tests; The apparent quantum efficiency (ϕx) values of samples(PDF).

Author information *Tel: +86-27-68778529. Fax: +86-27-68778433. E-mail: [email protected]. (Wei Wu) [email protected]. (Xiangheng Xiao) [email protected]. (Changzhong Jiang) 16 ACS Paragon Plus Environment

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Acknowledgment This work was supported by the NSFC (51201115, 51171132, 11375134), China Postdoctoral Science Foundation (2014M550406), Hong Kong Scholars Program, Hubei Provincial Natural Science Foundation (2014CFB261), the Fundamental Research Funds for the Central Universities (No. 2042015kf0184) and Wuhan University.

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Nanosci. Nanotechnol. 2014, 14, 4886-4890.



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Figure Captions Figure 1. The schematic illustration of the formation process of α-Fe2O3 and α-Fe2O3@TiO2 with thorhombic (a), cubic (b) and discal (c) shapes. Figure 2. SEM (a), TEM (b) and HRTEM (c) images of as-prepared thorhombic α-Fe2O3(Cu); SEM (d), TEM (e) and HRTEM (f) images of as-prepared thorhombic α-Fe2O3(Zn); SEM (g), TEM (h) and HRTEM (i) images of as-prepared thorhombic α-Fe2O3(Al) (the insert is the corresponding FFT pattern, respectively). Figure 3. SEM (a), TEM (b) and HRTEM (c) images of as-prepared thorhombic α-Fe2O3(Cu)@TiO2; SEM (d), TEM(e) and HRTEM (f) images of as-prepared thorhombic α-Fe2O3(Zn)@TiO2; SEM (g), TEM (h) and HRTEM (i) images of as-prepared thorhombic α-Fe2O3(Al)@TiO2 (the insert is the corresponding SAED pattern, respectively). Figure 4. The Raman spectra of α-Fe2O3 seeds with different ion addition (Cu2+(a), Zn2+(b) and Al3+(c)) and their corresponding α-Fe2O3@TiO2 composite nanocrystals; (d) XRD patterns of as-prepared α-Fe2O3(Al) and α-Fe2O3 (Al)@TiO2, standard PDF cards of α-Fe2O3 (33-0664, black lines), anatase TiO2 (21-1272, red lines). Figure 5. (a) The complete XPS spectra of α-Fe2O3(Cu)@TiO2 (black line, a)), α-Fe2O3(Zn)@TiO2 (red line, b)) and α-Fe2O3(Al)@TiO2 (blue line, c)); (b) Main and satellite peak of Fe 2p3/2 and Fe 2p1/2 for the three samples; (c) Main peak of Ti 2p3/2 and Ti 2p1/2 peak for the three samples; (d) main peak of Cu 2p3/2 and Cu 2p1/2 peak for the α-Fe2O3(Cu)@TiO2, (e) Main peak of Zn 2p3/2 and Zn 2p1/2 peak for the α-Fe2O3(Zn)@TiO2, (f) Main peak of Al 2p peak for the α-Fe2O3(Al)@TiO2. Figure 6. The typical degradation curve of RhB in the presence of α-Fe2O3(Cu)@TiO2 (a), α-Fe2O3(Zn)@TiO2 (b), α-Fe2O3(Al)@TiO2 (c); (d) The normalized concentration changing of as-prepared samples under UV and visible light irradiation; (e) The reaction rate constant versus irradiation time of UV and visible light with different catalysts; (f) The degradability distributions of all the catalysts in (d) under UV and visible light irradiation for 120 min.


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Figure 7. (a) The degradation curve of AO7 in the presence of α-Fe2O3(Al)@TiO2 composite nanocrystals; (b) The degradation curve of MG in the presence of α-Fe2O3(Al)@TiO2 composite nanocrystals. Figure 8. Schematic diagram of the photocatalytic mechanism of α-Fe2O3@TiO2 composite nanocrystals under UV and visible light irradiation. Figure 9. Current-potential plots of the (a) α-Fe2O3(Cu)@TiO2, (b) α-Fe2O3(Zn)@TiO2, (c) α-Fe2O3(Al)@TiO2 under UV and visible light irradiation.



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Figures

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TOC

As-synthesized 3D α-Fe2O3@TiO2 core-shell nanostructures are used to sustainable environmental application and shown the enhanced photocatalytic and photoelectrochemical performances.


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3D Flowerlike Î±-Fe2O3@TiO2 CoreâShell Nanostructures: General

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