CeO2 Hybrid Nanotubes and Their Spontaneous

Urbana−Champaign, Urbana, Illinois 61801, United States. ACS Appl. Mater. Interfaces , 2015, 7 (47), pp 26291–26300. DOI: 10.1021/acsami.5b088...
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Synthesis of Mn3O4/CeO2 Hybrid Nanotubes and Their Spontaneous Formation of a Paper-like, Free-Standing Membrane for the Removal of Arsenite from Water Song Guo,† Wuzhu Sun,† Weiyi Yang,† Zhengchao Xu,‡ Qi Li,*,† and Jian Ku Shang§ †

Environment Functional Materials Division, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P. R. China ‡ Zhangjiagang Green Tech Environmental Protection Equipment Co., LTD., Zhangjiagang 215625, P. R. China § Department of Materials Science and Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States S Supporting Information *

ABSTRACT: One-dimensional nanomaterials may organize into macrostructures to have hierarchically porous structures, which could not only be easily adopted into various water treatment apparatus to solve the separation issue of nanomaterials from water but also take full advantage of their nanosize effect for enhanced water treatment performance. In this work, a novel template-based process was developed to create Mn3O4/CeO2 hybrid nanotubes, in which a redox reaction happened between the OMS-2 nanowire template and Ce(NO3)3 to create hybrid nanotubes without the template removal process. Both the Ce/Mn ratio and the precipitation agent were found to be critical in the formation of Mn3O4/CeO2 hybrid nanotubes. Because of their relatively large specific surface area, porous structure, high pore volume, and proper surface properties, these Mn3O4/CeO2 hybrid nanotubes demonstrated good As(III) removal performances in water. These Mn3O4/CeO2 hybrid nanotubes could form paper-like, free-standing membranes spontaneously by a self-assembly process without high temperature treatment, which kept the preferable properties of Mn3O4/CeO2 hybrid nanotubes while avoiding the potential nanomaterial dispersion problem. Thus, they could be readily utilized in commonly used flow-through reactors for water treatment purposes. This approach could be further applied to other material systems to create various hybrid nanotubes for a broad range of technical applications. KEYWORDS: Mn3O4/CeO2 hybrid nanotubes, redox precipitation reaction, As(III) removal, free-standing membrane, toxicity reduction

1. INTRODUCTION Inadequate supply of clean water is expected to become even worse in the near future throughout the world, which inspires a rapid development of nanotechnology for water treatment and reuse because current technologies and infrastructures are reaching their limit for providing water supply with adequate quality and quantity to meet the needs of both human beings and the environment.1−3 Because of their unique size of smaller than 100 nm in at least one dimension, nanomaterials have novel size-dependent properties different from their bulk counterparts, and could possess excellent performances for water purification. For example, nanomaterials had been explored as highly efficient adsorbents for heavy metals and organics,4−8 disinfectants for biofouling control,9,10 and sensors for water pollutant detection and quality monitoring.11,12 However, it is usually difficult to separate nanomaterials from water bodies, which may cause their dispersion into the aqueous environment. Thus, it may not only increase the water © XXXX American Chemical Society

treatment cost but also cause potential risk to both the public and the environment.13−16 Among various nanomaterials, one-dimensional (1-D) nanostructures such as nanowires, nanorods, nanobelts, and nanotubes have attracted intensive research interests due to their unique properties in mesoscopic physics and nanoscale devices fabrication.17−20 For water treatment purposes, their specific structures with lengths over the nanorange could lead to an easy separation from water by the simple sedimentation or membrane filtration. More preferably, 1-D nanomaterials may organize into macrostructures like thin films or membranes to have hierarchically porous structures, in which interconnected macropores could be beneficial for liquid transport while mesopores and micropores could largely increase their surface Received: September 19, 2015 Accepted: November 10, 2015

A

DOI: 10.1021/acsami.5b08862 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces area for effective contact with contaminants in water.21−24 Thus, they could not only be easily adopted into various water treatment apparatus to solve the separation issue of nanomaterials but also take full advantage of their nanosize effect for enhanced water treatment performance.25,26 Furthermore, these macrostructures composed of 1-D nanomaterials may serve as good templates for creation of hybrid materials that could not only inherit the advantages of their parent materials but also possess the synergistic effect which further enhances their water treatment performance.27−29 Arsenic is one of the most toxic pollutants in water, which could cause various health problems and pose a great threat to human health worldwide.30,31 The World Health Organization had classified arsenic as a carcinogen and recommended that the maximum contaminant level (MCL) for arsenic in drinking water should not be over 10 μg/L.32 In natural water bodies, arsenic is usually found as inorganic As(V) (arsenate) and As(III) (arsenite), between which As(III) is more toxic than As(V) in biological systems33 and it is more mobile and difficult to remove because of its nonionic existence as H3AsO3 in natural water.34 Manganese oxide based adsorbents had been demonstrated to be effective to oxidize and remove various pollutants in water.35,36 Among them, cryptomelane-type manganese oxide (OMS-2) nanowires could self-organize into free-standing membranes,37,38 which was ideal for water treatment purposes. Compared with other highly efficient aresenic nanoadsorbents,39,40 however, the arsenic adsorption capability of OMS-2 nanowires was relatively low. CeO2 nanoparticles had demonstrated a high adsorption capacity for arsenic species.41,42 Thus, if a hybrid material could be created based on CeO2 nanoparticles and OMS-2 nanowires to combine their advantages of high arsenic adsorption of CeO2, good As(III) oxidation capability of manganese oxides, and selforganization to macroscopic membranes of 1-D nanowire structures, it could be readily adopted into various water treatment apparatus for effective arsenic contamination remediation without the nanoadsorbent separation issue. In this work, Mn 3 O 4 /CeO 2 hybrid nanotubes were successfully created by a novel template-based process we developed, in which a redox reaction happened between the OMS-2 nanowire template and Ce(NO3)3 to create hybrid nanotubes without the template removal process. The controlling factors in the formation of Mn3O4/CeO2 hybrid nanotubes were investigated, which demonstrated that both the Ce/Mn ratio and the precipitation agent were critical. Because of their relatively large specific surface area, porous structure, high pore volume, and proper surface properties, these Mn3O4/ CeO2 hybrid nanotubes demonstrated a good As(III) removal performance from water accompanied with the preferable toxicity reduction by oxidizing As(III) to As(V). These Mn3O4/ CeO2 hybrid nanotubes could form paper-like, free-standing membranes spontaneously by a self-assembly process without high temperature treatment, which kept the preferable properties of Mn3O4/CeO2 hybrid nanotubes while avoided the potential nanomaterial dispersion problem. This approach could be further applied to other material systems to create various hybrid nanotubes for a broad range of technical applications.

P. R. China) and used in the synthesis of OMS-2 nanowires. Cerium(III) nitrate hexahydrate (Ce(NO3)3, 98%) and hexamethylenetetramine (C6H12N4, 98.0%) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, P. R. China) and used as the cerium source and the redox-precipitation agent, respectively, in the hydrothermal process to create Mn3O4/CeO2 hybrid nanotubes. Sodium (meta) arsenite (NaAsO2, Shanghai Tian Ji Chemical Institute, Shanghai, P. R. China) was used to prepare the As(III) stock solution. 2.2. Synthesis of OMS-2 Nanowires. OMS-2 nanowires were synthesized with a simple hydrothermal process.35 In a typical synthesis, 10 mmol of K2SO4, 10 mmol of K2S2O8, and 5 mmol of MnSO4·H2O were dissolved in 70 mL of deionized water under magnetic stirring and then transferred to a 125 mL Teflon-lined stainless-steel autoclave. The autoclave was sealed and heated in an oven at 200 °C for 36 h. The resultant brownish black precipitate was repeatedly washed with deionized water until reaching a neutral pH and was resuspended in 500 mL of deionized water under vigorous stirring for 24 h. The precipitates were then filtered and dried at 60 °C for 1 day to obtain OMS-2 nanowires. 2.3. Synthesis of Mn3O4/CeO2 Hybrid Nanotubes. Mn3O4/ CeO2 hybrid nanotubes were obtained with a chemical redoxprecipitation method we developed. First, 0.3 g of OMS-2 nanowires was added into 300 mL of deionized water under ultrasonication for 20 min to obtain a homogeneous suspension. Then, 0.6 g of cerium nitrate was added into the suspension and it was continuously stirred for 10 min. After dissolving 5 g of hexamethylenetetramine into the suspension as the precipitant, the mixture was heated to 90 °C in water bath for 1 h. Finally, the precipitates were filtered and washed thoroughly with deionized water. For comparison purposes, two sets of samples were prepared with similar processes. In one set of experiment, the ratio of cerium nitrate to OMS-2 nanowires was changed to 0:1, 1:2, and 2:1, respectively. In the other experiment, no precipitant was used in the synthesis process. For the As(III) adsorption performance comparison purposes, Mn3O4 and CeO2 nanoparticles were also synthesized in this study. To synthesize Mn3O4 nanoparticles, the proper amount of MnCl2·4H2O was first dissolved in 50 mL of ethanol to form a 50 mM MnCl2 ethanol solution in a 200 mL beaker. Then, 50 mL of 100 mM NaOH ethanol solution was added dropwise into it with magnetic stirring at room temperature. The mixture solution was heated in a Teflon-lined stainless-steel autoclave at 150 °C for 2 h. The precipitates were collected by centrifugation, washed with DI water for several times, and then dried in air at 40 °C for 48 h to obtain Mn3O4 nanoparticles. To synthesize CeO2 nanoparticles, 0.6 g of cerium nitrate was first dissolved into 200 mL of DI water. Then, 5 g of hexamethylenetetramine was added into the solution, and it was heated to 90 °C in water bath for 1 h. The precipitates were collected by centrifugation, washed with DI water for several times, and dried at 40 °C for 48 h to obtain CeO2 nanoparticles. 2.4. Fabrication of the Free-Standing Mn3O4/CeO2 Film. Mn3O4/CeO2 hybrid nanotubes were redispersed in deionized water (0.2 g/500 mL) for 4 h under stirring to obtain a homogeneous suspension. The suspension was placed on a filter paper, and a simple suction filtration was conducted to accelerate the water removal from the suspension. Then, it was dried at 50 °C for 12 h and detached from the filter paper to obtain a paper-like, free-standing membrane composed of Mn3O4/CeO2 hybrid nanotubes. The thickness of the membranes could be modulated by controlling the concentration of Mn3O4/CeO2 hybrid nanotubes in the suspension. 2.5. Material Characterization. X-ray diffraction (XRD) patterns of samples were obtained by a D/MAX-2004 X-ray powder diffractometer (Rigaku Corporation, Tokyo, Japan) with Ni-filtered Cu (λ = 0.15418 nm) radiation at 56 kV and 182 mA. BET surface area was measured by N2 adsorption−desorption isotherms with an Autosorb-1 Series Surface Area and Pore Size Analyzers (Quantachrome Instruments, Boynton Beach, FL, USA), and the pore size distribution was obtained by the Barrett−Joyner−Halenda (BJH) method from the desorption branch data of nitrogen adsorption isotherms. The sample morphology was examined by the transmission

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Potassium persulfate (K2S2O8, 99%), potassium sulfate (K2SO4, 99%), and manganese(II) sulfate (MnSO4, 99%) were obtained from Aladdin Industrial Inc. (Shanghai, B

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electron microscopy (TEM) on a JEOL 2100 transmission electron microscope (JEOL Ltd., Tokyo, Japan) operating at 200 kV. TEM samples were prepared by dispersing them in ethanol, applying a drop of the dispersion on a Cu grid, and drying in air at 60 °C for 3 h. Scanning electron microscopy (SEM) observations were conducted on a LEO SUPRA 55 microscope (ZEISS, Germany). Prior to imaging, the sample was sputtered with gold for 20 s (Emitech K575 Sputter Coater, Emitech Ltd., Ashford Kent, U.K.). X-ray photoelectron spectroscopy (XPS) measurements were conducted with an ESCALAB250 X-ray Photoelectron Spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA) with an Al K anode (1486.6 eV photon energy, 300 W). The XPS spectra were analyzed and fitted with the XPSPEAK software (version 4.1), and the C 1s peak was set at 284.6 eV as a reference. 2.6. As(III) Equilibrium Adsorption Isotherm Experiment. In the As(III) equilibrium adsorption isotherm experiment, 0.05 g of Mn3O4/CeO2 hybrid nanotubes was added into 500 mL As(III) solutions with arsenic concentrations varied from 1 to 80 mg/L (pH ∼ 7). The mixtures were first treated by ultrasonication for 10 min to form homogeneous suspensions and then stirred for 24 h at 500 rpm. A 10 mL aliquot of the suspension was withdrawn and centrifuged at 12 000 rpm for 10 min, and the clear top solution was used for As(III) concentration analysis. One drop of concentrated HCl was added into the clear solution to avoid the potential oxidation of As(III) to As(V). The As(III) concentration was then analyzed by an atomic fluorescence spectrophotometer (AFS-9800, Beijing KeChuangHaiGuang Instrument Inc., Beijing, P.R. China). 2.7. Flow-through As(III) Adsorption Experiment. The freestanding Mn3O4/CeO2 film was fixed in a 25 mm reusable syringe filter to conduct the flow-through As(III) adsorption experiment. The membrane was cut into a round shape with a diameter of ∼25 mm, and its dosage was ∼0.03 g. A 10 mL portion of the As(III) solution was filtered in continuous mode within 2 min through the filter, and the experiment was repeated for 36 times. The initial As(III) concentration was ∼96 μg/L, which is at the high end of arsenic concentration found in natural water bodies. The pH value of the As(III) solution was 7.0 ± 0.2. The As(III) concentration in the filtered solution was analyzed by the atomic fluorescence spectrophotometer.

(CH 2)6 N4 + 10H 2O → 6HCHO + 4NH+4 + 4OH−

(1)

Along with the hydrolysis of hexamethylenetetramine, the pH of the suspension rose from near neutral to 9. Ce3+ ion could remain stable in weak acid solution, but become unstable both in solubility and redox state with the increase of the solution pH. The precipitation of Ce3+ cations (10−3 M) as hydroxides does not occur until the solution pH reaches ∼8.4, while the precipitation of Ce4+ occurs when the solution pH reaches ∼3.1.44 The electrode potential of the Ce4+/Ce3+ couple decreases sharply with the solution pH increase. The standard electrode potential of the Ce(IV)/Ce(III) couple is 1.61 V, and it reduces to −0.225 V when the solution pH reaches the value for the precipitation of Ce4+.45 Thus, a redox precipitation could occur in this approach when the solution pH rose to a proper value, as described by eq 2. 4Ce3 + + 3MnO2 + 12OH− → 4CeO2 + Mn3O4 + 6H 2O (2)

Ce 3+ was oxidized to Ce 4+ and precipitated as CeO 2 nanoparticles on the surface of OMS-2 nanowires, while the OMS-2 template reacted with Ce3+ and was reduced to Mn3O4 simultaneously. As the reaction went on, the template was consumed completely and Mn3O4/CeO2 hybrid nanotubes could be created. Figure 2a shows the X-ray diffraction pattern of the template product obtained by the hydrothermal process, which clearly demonstrated that all diffraction peaks could be indexed to the OMS-2 crystalline phase (JCPDS 42-1348). Figure 2b,c shows the FESEM and TEM images of the OMS-2 template obtained, respectively. The obtained OMS-2 template was composed of nanowires with diameters of ∼20 nm, and their lengths could be over tens of microns, which was critical for their formation of free-standing membranes. The inset image in Figure 2c shows the high-resolution TEM (HRTEM) image on a single OMS-2 nanowire, in which a set of lattice planes could be clearly identified with the interplanar spacing of ∼0.692 nm, corresponding to the (110) plane of OMS-2. After the heat treatment with Ce(NO3)3 under proper reaction conditions, a large crystal structure and morphology changes were observed for the OMS-2 template. Figure 2d shows the X-ray diffraction pattern of the final product, which clearly demonstrated the disappearance of the OMS-2 crystalline phase and the appearance of coexisting Mn3O4 (JCPDS 24-0734) and CeO2 (JCPDS 34-0394) crystalline phases. The large change in crystal structure brought an obvious change in the sample’s morphology after the heat treatment with cerium nitrate under proper reaction conditions. Figure 2e,f shows the FESEM and TEM images of the final product, respectively. The nanowire structure of the OMS-2 template changed to a hollow nanotube structure. As demonstrated in Figure 2f, these nanotubes had an inner diameter of ∼20 nm and their tube wall was composed of nanoparticles with the size of several nanometers. The inset image in the top left of Figure 2f shows the HRTEM image on the wall of the final product, in which a set of lattice planes could be clearly identified with the interplanar spacing of ∼0.493 nm, corresponding to the (101) plane of Mn3O4. The inset image in the bottom right of Figure 2f shows the HRTEM image on the surface of the final product, in which a set of lattice planes could be clearly identified with the interplanar spacing of ∼0.312 nm, corresponding to the (111) plane of CeO2. These observations were in accordance with the XRD analysis results of the final product.

3. RESULTS AND DISCUSSION 3.1. Formation of Mn3O4/CeO2 Hybrid Nanotubes. Figure 1 schematically shows the synthesis of Mn3O4/CeO2

Figure 1. Schematic diagram of Mn3O4/CeO2 preparation.

hybrid nanotubes by the template-based process we developed. The cryptomelane-type manganese oxide (OMS-2) nanowires were first created to serve as the template by a hydrothermal process following a previous report.37 Next, OMS-2 nanowires were dispersed into deionized (DI) water with ultrasonication until a uniform suspension was formed. Then, the proper amount of cerium nitrate and hexamethylenetetramine was added into the above suspension. This suspension was heated to 90 °C and kept at this temperature for 1 h. During the heat treatment, hexamethylenetetramine breaks down into ammonia and formaldehyde following eq 1.43 C

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Figure 2. (a) XRD patterns of the synthesized OMS-2 nanowires. (b, c) FESEM and TEM images of OMS-2 nanowires. (d) XRD patterns of the synthesized Mn3O4/CeO2 nanotubes. (e, f) FESEM and TEM images of Mn3O4/CeO2 nanotubes.

Figure 3. (a) The TEM image of a Mn3O4/CeO2 nanotube. (b−d) The elemental maps of Mn, Ce, and O, respectively.

simultaneously served as the skeleton to provide precipitation sites for reaction products (CeO2 and Mn3O4) to settle on. With the reaction process, OMS-2 nanowires gradually disappeared while Mn3O4/CeO2 hybrid nanotubes were produced, which reserved the 1-D structure as OMS-2 nanowires and developed the hollow nanotube structure based on the nanowire skeleton. Thus, a unique process was developed to synthesize nanotubes by a reactive template, which required no further template removal process and could create hybrid nanotubes for various technical applications. 3.2. Controlling Factors in the Formation of Mn3O4/ CeO2 Hybrid Nanotubes. In this synthesis process, there were two important controlling factors in the formation of Mn3O4/CeO2 hybrid nanotubes. Because of the redox reaction nature, a proper Ce/Mn ratio was critical in the formation of product with nanotube morphology. When no Ce(NO3)3 was present in the reaction mixture, the OMS-2 template could sustain the heat treatment process and no structure and morphology change could be observed in the final product (see Figure S1 in the Supporting Information) because no redox reaction happened without the participation of reductive Ce(NO3)3. When the Ce/Mn ratio was relatively low, no nanotube could be formed after the heat treatment. For example, nanoparticle-covered nanowires were obtained after the heat treatment when the dosage of Ce(NO3)3 was half as that of the OMS-2 in the reaction mixture (see Figure S2 in the

The CeO2/Mn3O4 weight ratio (ω1/ω2) in these Mn3O4/ CeO2 nanotubes could be estimated by the reference intensity method using eq 3 I1 RIR1 ω1 = I2 RIR 2 ω2

(3)

where I1 and I2 are the intensities of the CeO2 (111) and Mn3O4 (211) peaks measured from their XRD pattern, respectively, RIR1 and RIR2 are the reference intensities of CeO2 and Mn3O4 phases compared to the α-Al2O3 standard, respectively, and the sum of ω1 and ω2 is 1. From the XRD analysis data, ω1/ω2 was estimated at ∼0.74/0.26. To examine the chemical composition distribution of our sample, Mn3O4/CeO2 hybrid nanotubes were analyzed with the energy-dispersive X-ray analysis (EDS). Figure 3a shows the STEM image of one single Mn3O4/CeO2 hybrid nanotube, and the distribution maps of Mn, Ce, and O elements in this single nanotube are shown in Figure 3b−d, respectively. It could be observed that Mn, Ce, and O elements distributed relatively uniformly. Thus, Mn3O4 and CeO2 nanoparticles were distributed uniformly in the wall of the nanotube, indicating that they could form a close contact with each other. The uniform distribution of Mn3O4 and CeO2 nanoparticles in the nanotube wall should come from the reaction between Ce(NO3)3 and the OMS-2 template in our synthesis process. The OMS-2 template provided part of reactants and D

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Figure 4. BET measurement curves and pore size distribution of (a) the OMS-2 nanowires and (b) Mn3O4/CeO2 hybrid nanotubes, respectively.

Figure 5. (a, b) XPS survey spectra of OMS-2 nanowires and Mn3O4/CeO2, respectively. (c) The high-resolution XPS spectra of the Ce 3d region. (d) The high-resolution XPS scans over the Mn 3s spectral region of the OMS-2 nanowires and Mn3O4/CeO2 hybrid nanotubes, respectively.

CeO2 nanoparticles. If no precipitation agent was present, the mixture solution pH could not be modulated. It was just ∼pH 3.5, and no morphology and structure change was observed for these OMS-2 nanowires (see Figure S3 in the Supporting Information). Thus, the selection of the proper precipitation agent was also critical in the formation of Mn3O4/CeO2 hybrid nanotubes. 3.3. Surface Properties of Mn3O4/CeO2 Hybrid Nanotubes. The creation of Mn3O4/CeO2 hybrid nanotubes from the OMS-2 nanowires largely changed their composition,

Supporting Information). This phenomenon could be attributed to the excessive OMS-2 presence in the reaction mixture so that the core part of the OMS-2 nanowires could not be reduced to Mn3O4 due to the depletion of Ce(NO3)3 in the outer of nanowires and was preserved as the supporting substrate for the precipitation of both CeO2 and Mn3O4 nanoparticles. Only when the dosage of Ce(NO3)3 was at or over 2 times as that of the OMS-2, Mn3O4/CeO2 hybrid nanotubes could be created from this process. The basic environment was beneficial for the precipitation of Mn3O4/ E

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3.4. As(III) Remediation Performance of Mn3O4/CeO2 Hybrid Nanotubes. These Mn3O4/CeO2 hybrid nanotubes demonstrated a good As(III) adsorption performance from water at near neutral pH environment. Figure 6 compares the

structure, and morphology, which could have remarkable effects on their surface properties. Figure 4a,b shows the BET measurement curves of the OMS-2 nanowires and Mn3O4/ CeO2 hybrid nanotubes, respectively. The BET surface specific area of the OMS-2 nanowires was determined to be ∼30.4 m2/ g, which was less than one-third as that of Mn3O4/CeO2 hybrid nanotubes (∼98.0 m2/g). The pore size distributions of the OMS-2 nanowires and Mn3O4/CeO2 hybrid nanotubes are demonstrated in the inset images in Figure 4a,b, respectively, which were calculated from their desorption data with the Barret−Joyner−Halenda (BJH) model. The total pore volume of the OMS-2 nanowires was ∼0.182 cm3/g, while that of Mn3O4/CeO2 hybrid nanotubes was ∼0.309 cm3/g. Their pore size distributions were quite different. For the OMS-2 nanowires, their pore size distribution had a wide range, and most pores were over 10 nm. This observation suggested that most of their pores should come from pores between individual OMS-2 nanowires, while pores on individual OMS-2 nanowire were limited. For Mn3O4/CeO2 hybrid nanotubes, large parts of their cores ranged from ∼1 nm up to ∼12 nm, which should come from pores on individual Mn3O4/CeO2 hybrid nanotubes, while pores with a larger size should come from pores between individual Mn3O4/CeO2 hybrid nanotubes. Thus, the creation of Mn3O4/CeO2 hybrid nanotubes from the OMS-2 nanowires resulted in much higher specific surface area/pore volume and proper pore structure, which could largely enhance their performance in processes involving surface interactions. XPS study was conducted to compare the surface compositions and elemental states of the OMS-2 nanowires and Mn3O4/CeO2 nanotubes. Figure 5a,b compares the representative XPS survey spectra of the OMS-2 nanowires and Mn3O4/CeO2 hybrid nanotubes. It demonstrated clearly that the OMS-2 nanowires contained K, Mn, and O elements, while the Mn3O4/CeO2 hybrid nanotubes contained Mn, Ce, and O elements. Because of the widespread presence of carbon in the environment, the C 1s peak could also be observed clearly in their XPS survey spectra. This result suggested that the K element in the OMS-2 nanowires was lost during the redox reaction process, which was consistent with the XRD analysis result. Figure 5c shows the high-resolution XPS scan over the Ce 3d spectral region, which demonstrated clearly that it represented a typical Ce4+ 3d spectrum. Peaks centered at 882.9, 888.8, and 899.0 eV could be assigned to Ce4+ 3d5/2, while peaks centered at 901.4, 908.0, and 917.2 eV could be assigned to Ce4+ 3d3/2, respectively.46,47 Thus, the chemical state of Ce changed from Ce3+ in the reactant Ce(NO3)3 to Ce4+ in the final product. Figure 5d compares the highresolution XPS scans over the Mn 3s spectral region of the OMS-2 nanowires and Mn3O4/CeO2 hybrid nanotubes. which could be utilized to determine the chemical state of Mn in these samples.48 For the OMS-2 nanowires, the binding energy width (ΔE) between the separated Mn 3s peaks caused by multiplet splitting was ∼4.7 eV, while that of Mn3O4/CeO2 hybrid nanotubes was ∼5.6 eV. Thus, the chemical status of Mn in the OMS-2 nanowires was mostly +4, while the chemical status of Mn in Mn3O4/CeO2 hybrid nanotubes was the mixture of +2 and +3 as Mn3O4.49,50 XPS analysis results were consistent with the XRD analysis results, which suggested that a redox reaction happened during the heat treatment process to oxidize Ce3+ and reduce Mn4+. Thus, the surface chemical states of samples were largely changed in the synthesis process and could be modulated by incorporating different reaction conditions.

Figure 6. As(III) equilibrium adsorption isotherms of Mn3O4/CeO2 hybrid nanotubes, CeO2 nanoparticles, Mn3O4 nanoparticles, and the OMS-2 nanowires, respectively.

equilibrium adsorption isotherms of Mn3O4/CeO2 hybrid nanotubes, CeO2 nanoparticles, Mn3O4 nanoparticles, and the OMS-2 nanowires. It demonstrated clearly that Mn3O4/CeO2 hybrid nanotubes had a much better As(III) adsorption performance than both Mn3O4 nanoparticles and the OMS-2 nanowires. The As(III) adsorption capacities of Mn3O4 nanoparticles and the OMS-2 nanowires were only ∼80 and ∼28 mg/g, respectively. The As(III) adsorption capacity of Mn3O4/CeO2 hybrid nanotubes was over 160 mg/g, which was higher than that of some recently reported CeO2-based arsenic adsorbents.51−55 For CeO2 nanoparticles, their As(III) adsorption capacity was ∼155 mg/g, much higher than that of Mn3O4 nanoparticles (∼80 mg/g). Thus, the incorporation of CeO2 nanoparticles onto the Mn3O4 nanotube wall could certainly result in a significant increase to the As(III) adsorption capability. In Mn3O4/CeO2 hybrid nanotubes, the CeO2 content was found to be ∼74% and the Mn3O4 content was ∼26%. However, the As(III) adsorption capacity of Mn3O4/CeO2 hybrid nanotubes was over 160 mg/g, which was even higher than that of pure CeO2 nanoparticles. From their As(III) equilibrium adsorption isotherm curves, the As(III) adsorption on CeO2 nanoparticles followed the Langmuir isotherm, while the As(III) adsorption on Mn3O4/CeO2 hybrid nanotubes followed the Freundlich isotherm. These results demonstrated that the As(III) adsorption on Mn3O4/CeO2 hybrid nanotubes was not just a simple addition by the As(III) adsorption from Mn3O4 and CeO2 phases separately, and their enhanced As(III) adsorption was not simply due to the presence of the CeO2 phase. A synergetic effect between CeO2 and Mn3O4 phases of the hybrid material should happen to endow its high As(III) adsorption capacity. Besides the remarkable surface property changes due to the incorporation of CeO2 nanoparticles, the enhanced specific surface area/pore volume and proper pore structure could also facilitate the contact and interaction of As(III) with Mn3O4/CeO2 hybrid nanotubes, and As(III) could be oxidized to As(V) by Mn3O4 and fixed on these Mn3O4/ F

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Figure 7. (a) The high-resolution XPS scans of the As 3d region of Mn3O4/CeO2 hybrid nanotubes after the As(III) adsorption. (b) The highresolution XPS scans over the Mn 3s spectral region of Mn3O4/CeO2 hybrid nanotubes before and after As(III) adsorption, respectively.

CeO2 hybrid nanotubes. All of these specific features of Mn3O4/CeO2 hybrid nanotubes were beneficial to their As(III) adsorption capability. Besides their good As(III) removal performance, these Mn3O4/CeO2 hybrid nanotubes could also reduce the toxicity of arsenic contamination in water by oxidizing As(III) to As(V). X-ray photoelectron spectroscopy (XPS) was used to examine the surface chemical status of Mn3O4/CeO2 hybrid nanotubes after the As(III) adsorption in water. Figure 7a shows the high resolution XPS scan over the As 3d spectral region on Mn3O4/CeO2 hybrid nanotubes after the As(III) adsorption treatment, which could be best fitted by the combination of both the As(III) 3d peak (44.5 eV) and the As(V) 3d peak (45.7 eV). In the initial As(III) water solution, only As(III) existed. After the treatment process, As(III) only accounted for 43.7% of the total As on the surface of Mn3O4/ CeO2 hybrid nanotubes, while As(V) accounted for 56.3%. Figure 7b compares the high-resolution XPS scans over the Mn 3s spectral region of Mn3O4/CeO2 hybrid nanotubes before and after As(III) adsorption. After As(III) adsorption, the binding energy difference (ΔE) between the separated Mn 3s peaks increased from 5.6 to 5.9 eV, which suggested that the oxidation degree of Mn decreased.48,49 Thus, these observations demonstrated clearly that the As(III) removal by the Mn3O4/CeO2 hybrid nanotubes was not only an adsorption process. CeO2 had a strong adsorption of As(III), while Mn3O4 could oxidized it to As(V) after As(III) was adsorbed, which could largely reduce the toxicity of arsenic contamination in water. 3.5. Spontaneous Formation of Paper-like, FreeStanding Membranes from Mn3O4/CeO2 Hybrid Nanotubes and Their As(III) Removal Performance in a Flowthrough Filter. These Mn3O4/CeO2 hybrid nanotubes could form paper-like, free-standing membranes spontaneously by a self-assembly process, as schematically demonstrated in Figure 8a. When the Mn3O4/CeO2 hybrid nanotube suspension was placed on the filter paper, a simple suction filtration was conducted to accelerate the water removal from the suspension. Then, it was dried at 50 °C for 12 h, and a paper-like, freestanding membrane composed of Mn3O4/CeO2 hybrid nanotubes was obtained, which could be easily detached from the filter paper while remaining intact (see Figure 8b). Figure 8c shows the SEM image of the obtained paper-like, free-standing

Figure 8. (a) The schematic illustration of the formation process of the paper-like, free-standing membrane. (b) The optical image of the paper-like, free-standing Mn3O4/CeO2 membrane. (c) The SEM image of the paper-like, free-standing Mn3O4/CeO2 membrane. (d) The simple flow-through filter constructed from a syringe. (e) The As(III) removal percentage of this simple flow-through filter on water samples with an initial As(III) concentration of ∼96 μg/L.

membrane, which demonstrated clearly that it was composed of self-organized nanotubes similar to nonwoven fabrics. The OMS-2 template had a long nanowire length over tens of microns, which was well preserved during the formation of Mn3O4/CeO2 hybrid nanotubes. Thus, these Mn3O4/CeO2 hybrid nanotubes also had a long length, which was critical in the formation of the paper-like, free-standing membrane. This G

DOI: 10.1021/acsami.5b08862 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

ties, which resulted in their good As(III) removal performance and toxicity reduction of arsenic contamination in water. These Mn3O4/CeO2 hybrid nanotubes could form paper-like, freestanding membranes spontaneously by a self-assembly process, in which no high temperature treatment was involved. Thus, the obtained paper-like, free-standing membrane could keep the preferable properties of Mn3O4/CeO2 hybrid nanotubes, while avoiding the potential problem of nanomaterial dispersion into treated water bodies. Thus, it could be readily utilized in commonly used flow-through, fixed-bed reactors for water treatment purposes. With the existence of Mn(II)/ Mn(IV) and Ce(III)/Ce(IV) redox couples, it could also have potential applications in various catalytic reactions. Furthermore, this approach could be applied to other material systems to create various hybrid nanotubes for a broad range of technical applications.

paper-like, free-standing membrane had a dual-pore structure, in which interconnected macropores between self-organized Mn3O4/CeO2 hybrid nanotubes were beneficial for liquid transport and the mesopores on each nanotube could largely increase their surface area for effective contact with contaminants in water. The obtained free-standing membrane had a good mechanical strength to withstand moderate external forces and a good malleability to allow folding. Its shape could be controlled either by changing the substrate shape or by cutting the membrane into the desired ones after its formation. Its thickness could also be tuned by modulating the solid content in the suspension. In our template reaction process, no high temperature treatment was involved, beneficial to keep the structure and surface properties of Mn3O4/CeO2 hybrid nanotubes. Therefore, the large specific surface area, highly porous structure, and functional groups on the surface of Mn3O4/CeO2 hybrid nanotubes could be well preserved during the formation of the paper-like, free-standing membrane. Thus, the obtained freestanding membrane could keep the good removal performance of Mn3O4/CeO2 hybrid nanotubes on As(III) in water due to their nanosize and preferable surface properties, while it could avoid the potential problem of nanomaterial dispersion into treated water bodies and be readily utilized in commonly used flow-through, fixed-bed reactors. Figure 8d shows a simple flowthrough filter constructed from a syringe, and the active component in this filter was the paper-like, free-standing membrane composed of Mn3O4/CeO2 hybrid nanotubes, as shown in Figure 8b, which was used to demonstrate the As(III) removal performance of this free-standing membrane. Figure 8e shows the As(III) removal percentage of this simple flowthrough filter on water samples with an initial As(III) concentration of ∼96 μg/L, which is at the high end of arsenic concentration found in natural water bodies. Every time, a 10 mL water sample with As(III) contamination was injected through the syringe to pass through this free-standing membrane (0.03 g in weight and 40 μm in thickness) with the contact time of just ∼0.07 s. For the first 13 times, the As(III) removal percentage in the treated water samples was mostly stable at ∼99%. With the further increase of the treatment times, the As(III) removal percentage in the treated water samples gradually decreased. When it reached to the 36th time, the As(III) removal percentage in the treated water sample was still ∼85%. This observed As(III) removal percentage change of behavior was typical for the continuous flow-through tests.56,57 For the first 30 treatments, the As(III) concentration in the treated water samples was below 10 μg/L, which met the drinking water standard. After the arsenic concentration in the effluent reached 10 mg/L, the arsenic species on this free-standing membrane could be easily desorbed by injecting NaOH solution through the syringe to wash it, and the simple flow-through filter could then be reused for the removal of As(III).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b08862. TEM images and XRD patterns of samples obtained under various reaction conditions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-24-83978028. Fax: +8624-23971215. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Youth Innovation Promotion Association, Chinese Academy of Sciences (Grant No. Y2N5711171), and the Basic Science Innovation Program of Shenyang National Laboratory for Materials Science (Grant No. Y4N56R1161).

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REFERENCES

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