Synthesis and Adsorption Properties of Hierarchically Ordered

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Synthesis and Adsorption Properties of Hierarchically Ordered Nanostructures Derived from Porous CaO Network Weijie You, Yali Weng, Xiu Wang, Zanyong Zhuang, and Yan Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11633 • Publication Date (Web): 05 Oct 2016 Downloaded from http://pubs.acs.org on October 12, 2016

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ACS Applied Materials & Interfaces

Synthesis and Adsorption Properties of Hierarchically Ordered Nanostructures Derived from Porous CaO Network

Weijie Youa,b, Yali Wenga,b, Xiu Wanga,b, Zanyong Zhuanga,b*, Yan Yua,b* a

Key Laboratory of Eco-materials Advanced Technology (Fuzhou University), Fujian Province University, China

b

College of Materials Science and Engineering, Fuzhou University, New Campus, Minhou, Fujian Province 350108, China *Corresponding author. Fax: +86 591 22866534; E-mail address: [email protected], [email protected]

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Abstract Using the porous framework of CaO as templates and reagents, we explored a surfactant-free and economical method for preparing the calcium silicate hydrate (CSH) hierarchically ordered nanostructures. Incorporation of SiO2 nanoparticles into the CaO framework, followed by a reaction assisted by hydrothermal treatment, resulted in the formation of CSH with well-defined morphologies. The structural features of CSH were characterized by 3-D hierarchical networks, wherein nanofibers assembled to form nanosheets, and nanosheets to hieratically ordered structures. Investigation of the crystal growth mechanism indicated that the key to forming the CSH ordered assembly structure was by confining the Ca/Si ratio within a small range. Non-classic oriented aggregation mechanism was used to describe the crystal growth of nanosheets, while the porous CaO framework served as template/reagents responsible for the formation of hierarchical structures. The resulting CSH adsorbent exhibited better performance in removing Pb(II) compared with other types of random CSH adsorbents. Additionally, the hierarchical structure of CSH provided more pores and active sites as support for other active functional materials such as zerovalent iron (Fe0). As-produced CSH@Fe nanocomposite with self-supported structures displayed high capacities for removing of Pb(II) after five adsorption-desorption cycles, and high capacities for other heavy metal ions (Cu2+, Cd2+, and Cr2O72-）and organic contaminants. Keywords: Hierarchically ordered structure; Oriented aggregation; Porous CaO 2


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framework; Surfactant-free; Calcium silicate hydrates

Introduction Low-dimensional nanomaterials such as nanowires and nanosheets have a variety

of

applications

in

catalysis,

energy

conversion/storage,

and

environmental remediation.1-3 Nevertheless, tight agglomeration and stacking of these nanounits driven by high surface energies always lead to fast deterioration of their physicochemical properties. To overcome this problem, well-defined 3-D nanomaterials

have

been

fabricated

using

surfactants

such

as

tetrabutylammonium bromide (TBAB), polyvinylpyrrolidone (PVP), and block co-polymers P123 as soft templates for the hierarchical assembly of nanomaterials.4-6 However, it is difficult to choose a suitable template for a specific application and the removal of the template after synthesis can be time-consuming or incomplete, which produces potentially toxic contaminants. As an alternative, hard sacrificial templates have received much attention for the preparation of functional porous materials.7-10 In many studies, SiO2 or carbon microspheres serve as mechanical supports to fabricate hollow or porous nanostructures.9,10 More recently, many researchers shifted their attention to calcium carbonate (CaCO3),11,12 which is an inexpensive, eco-friendly, and naturally abundant material. However, CaCO3 cannot be used as a template due to the tight packing of its surface as well as its size, which is typically over several microns. To overcome these disadvantages, Akiyama et al. reported the co-precipitation and subsequent decomposition of Mn–Ca-carbonates to 3



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synthesize manganese oxide hollow structures.11 Shi et al. also showed that calcination of CaCO3, which releases CO2, could form loose and porous framework of CaO, rendering it a suitable template for creating 3-D graphene foams.13 Despite the improvements of these new strategies, acid etching was usually required to remove the templates, increasing the cost and possibility of environmental pollution. It is still highly desirable to rationally design and synthesize 3-D nano-architectures using a green and facile approach. Over the past decade, hierarchical materials based on calcium silicate hydrate (CSH) have drawn growing attention for their potential applications in bone tissue engineering, drug delivery, and heavy metals extraction.14-18 Nevertheless, there are few reports of the surfactant-free synthesis of CSH hierarchical materials, since this kind of CSH nanomaterials is usually difficult to prepare using most known methods such as coprecipitation, sol-gel, and hydrothermal method. Previously, we reported the transformation of the oyster shells, mainly CaCO3, into CHS by calcination.18-19 The as-synthesized CHS material showed high adsorption capacities toward organic pollutants and heavy metal ions including Cr, Cd, Pb, and Mn.18-19 However, little is known regarding the underlying mechanism of this new approach of fabricating hierarchically ordered CSH. In this work, we report a surfactant-free and economical method for preparing CSH, utilizing the porous framework of CaO resulted from the decomposition of CaCO3 as templates. Incorporation of SiO2 into the CaO framework helped 4


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create the 3D hierarchical CSH networks constructed by nanosheet building blocks. It was found that the key to forming ordered self-assembled structures was a confined Ca/Si ratio within a small range. The composite of CSH@Fe, i.e. nanoscale zerovalent iron (Fe0) supported by the hierarchical CSH structures, displayed outstanding performance for the removal of heavy metal ions including Pb2+, Cu2+, Cd2+, and Cr2O72- as well as organic pollutants. The underlying mechanism of forming hierarchical structure was proposed to be non-classic oriented aggregation pathways of crystal growth of nanosheets, with CaO framework served as template/support. We anticipate the finding of this work to enrich the understanding and pathway of functional materials design.

Experimental Section Materials Fumed silica was purchased from XIBEI Iron Alloy Company in China. Calcium carbonate (CaCO3), sodium borohydride (NaBH4), ferrous sulfate (FeSO4·7H2O), ethanol (>99%), copper sulfate pentahydrate (CuSO4·5H2O), potassium dichromate (K2Cr2O7), lead nitrate (Pb(NO3)2), cadmium nitrate (Cd(NO3)2), methyl orange (MO), and Congo red (CR) were provided by Sinopharm Chemical Reagents Co., Ltd. China. All reagents were of analytical grade, and used as received without further purification. Ultrapure water (18.0 MΩ cm-1) in the experiments was prepared by using an ultra-pure purification system. Synthesis of calcium silicate hydrates (CSHs) 5



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CSHs were prepared via calcination and hydrothermal method. Briefly, different molar ratios of calcium carbonate and fumed silica (Ca/Si=1:1, 1:1.1, 1:1.2 and 1:1.3) were mixed. The mixtures were calcined at 800 °C for 2 h, followed by a hydrothermal treatment at 170 °C for 4 h to obtain a series of products. The as-synthesized CSHs with Ca/Si=1:1, 1:1.1, 1:1.2 and 1:1.3 are designated as CSH1, CSH2, CSH3 and CSH4, respectively. Finally, the CSH1, CSH2, CSH3 and CSH4 were collected by centrifugation, washed with ultrapure water and ethanol by several times, and then dried at 50 oC. Preparation of Fe0 and CSH@Fe composite The composite CSH-supported Fe0 (CSH@Fe) was prepared via a chemical reduction procedure based on a previously reported method.20 Briefly, CSH (0.406 g) was dispersed in 35 mL of ethanol, followed by mixing with 35 mL of 0.1 mol/L Fe(II) solution in a 500 mL three-neck round-bottom flask. Subsequently, NaBH4 solution (70 mL, 0.25 mol/L) was added dropwise to the flask at a rate of 3 mL/min. The solution was stirred for another 30 min under room temperature. Before and throughout the procedure, N2 was introduced to expel the dissolved oxygen and maintain an anaerobic environment. Finally, the black metal particles were collected, washed with ultrapure water and ethanol, and dried under vacuum. The content of doped Fe0 was assayed by dissolving each sample of CSH@Fe in 5% nitric acid, followed ICP analysis of Fe. The unsupported Fe0 was prepared similarly without CSH as the control. Pb(II) removal by CSH and CSH@Fe 6


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An amount of 30 mg of each adsorbent (CSH@Fe, CSH, or Fe0) was added into the Pb(II) solution (50 mL, 1000 mg/L). The reaction was proceeded at room temperature for 6 h. At given time intervals, 1 mL of solution were retrieved for analysis. Additionally, a series of simulated wastewater samples containing only one target contaminant, i.e. Cu2+, Cr2O72-, Cd2+, CR or MO, were prepared to test the removal capacities of CSH@Fe. All tests were conducted in triplicate and the average value was used for calculation. The removal capacity (qe, mg/g) of all samples were calculated according to Eq. (1):3

qe 

(CO  Ce ) V W

(1)

where Co and Ce are the initial and equilibrium concentration (mg/L) of the target contaminant, respectively. V is the volume of the solution (L), and W is the weight of the adsorbent (g). Characterization X-ray diffraction (XRD) analysis of the samples were recorded on a Philips X’pert-MPDX-ray diffractometer. Fourier transform infrared spectra (FTIR) of CSHs were obtained on a TJ270-30A spectrophotometer. Thermogravimetric analysis (TGA) was conducted on a TGA-Q600 (Switzerland) at a heating rate of 5 °C min−1 from 25 to 1000 °C. The morphologies of the CSH and CSH@Fe were collected by a scanning electron microscope (SEM, Philips XL30) and a transmission electron microscopy (TEM, FEI Tecnai G2 F20) operating at an accelerating voltage of 200 kV. BET surface area of samples were measured by a 7



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quantachrome autosorb-1-C-TCD automated gas sorption analyzer. The zeta potentials of samples were measured by a Zetasizer Nano ZS-90. The chemical composition of CSH and CSH@Fe before and after Pb(II) adsorption was determined by an X-ray photoelectron spectrometer (XPS, PHI 5000 Versa Probe). The concentrations of contaminants in solution were determined by inductively coupled plasma atomic emission spectrometry (ICP-MS, Elan-9000, PE) and a Shimadzu UV-1800 spectrophotometer. Reusability study The desorption of metals from CSH3 and CSH3@Fe was performed by putting the spent adsorbent into 0.1 M HCl solution at room temperature. The regenerated CSH3 were washed with ultrapure water, and imbedded with similar amount of Fe0 to form CSH3@Fe as described above. The regenerated CSH3 and CSH3@Fe were used in the next cycle of adsorption experiments.

Results and discussion The composition and structure of CSHs materials The structure and phase of each sample can be determined from the XRD patterns shown in Fig.1a. The starting material CaCO3 can be well indexed to the calcite,21 the most stable phase of naturally occurring CaCO3. After annealing of the mixed CaCO3 and fumed silica (Ca:Si=1:1) at 800 °C for 2 h, the calcite decomposed to form CaO.16 No obvious peaks attributable to CSH can be found, indicating few CSH was formed. Subsequently, hydrothermal treatment of CaO@SiO2 in water at 170 °C for 4 h was performed. As shown in Fig.1a, new 8


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diffractions peaks at 28.8o, 29.3o 29.9o and 31.8o attributable to the CSH15-16 replaced the original diffraction peaks from CaO@SiO2, indicating the complete transformation of CaO@SiO2 into the CSH. No significant change in the diffraction pattern can be observed for CSH1, CSH2, CSH3 and CSH4, which suggests that different Ca:Si ratios did not cause a phase change. The FTIR spectra of CSHs (Fig. S1 in the Supporting Information, SI) confirm the XRD analysis that all the samples have similar structure. The slight difference in relative intensities of the diffraction peaks of among these samples may be associated with the lattice orientation of nanocrystal growth.

Figure 1. a) XRD patterns of CaCO3, CaO@SiO2, CSH1, CSH2, CSH3 and CSH4; b) TGA curves of CSH1, CSH2, CSH3 and CSH4.

As shown in Fig. 1b, the TGA curves of fresh CSH1, CSH2, CSH3 and CSH4 all show a similar two-stage weight loss between 25-1000 oC. The first 7-9 wt% loss took place below 200 oC, assigned to the loss of physically adsorbed water. A gradual loss of 10-12 wt% between 200-700 oC can be attributed to the thermal

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decomposition of crystal water from CSH.22 In general, pure calcite decomposes above 640 oC (with TGA curve shown in Fig. S2). As there is basically no weight loss for all the samples above 700 oC (Fig. 1b), it confirms the XRD pattern that the calcite completely decomposed into CaO, and as-produced samples contained only CSH. It can also be deduced from the weight loss of the structure H2O that the chemical composition of the samples is (CaSiO3)·H2O. The morphologies of CSHs Fig. 2 presents the SEM images of CaCO3, CaO, CaO@SiO2, and CSH1-CSH4. All these samples contain aggregates with diameters of 20-30 µm. The pure calcite particles have smooth surfaces (Fig. 2a-c). Annealing of calcite at 800 oC for 2 h resulted in a 3-D interconnected porous CaO framework (Fig. 2d-f). After further annealing of CaO framework with SiO2 at 800 °C for 2 h, silica particles were attached onto CaO, generating CaO@SiO2 (Fig. 2g-i). After hydrothermal treatment of CaO@SiO2, the CSH1-CSH4 with different morphologies were fabricated. At the Ca:Si molar ratio of 1:1, the individual CSH particles are in the form of nanofibers with an average length of 1-2 µm and an average diameter of dozens of nanometers. These CSH nanofibers are arranged in the form of tightly packed bundles (Fig. 2j-l). As shown in Fig. 2m-r, a slight change of Ca:Si molar ratio to 1:1.1 or 1.1.2 resulted in orderly arrangement of CSH nanofibers to form small pieces of nanosheets. Several pieces of nanosheets assemble to form folded sheets, which further aggregate into hierarchically ordered walnut-shaped 10


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microspheres with an average diameter of 2-3 µm. Nanosheets are ranged in good order to form walnut-shaped microspheres, indicating the formation of ordered nanostructure. At the Ca:Si ratio of 1:1.3, randomly aggregated micron-sized CSH sheets start to form CSH4 as shown in Fig 2s-u. A small fraction of CSH4 containing both the stacked large-sized sheets and nanofibers can be observed as well (Fig. S3).

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Figure 2. SEM images of a-c) CaCO3; d-f) CaO; g-i) CaO@SiO2; j-l) CSH1; m-o) CSH2; p-r) CSH3, and s-u) CSH4.

As shown in Fig. S4, the nitrogen adsorption/desorption isotherms of all samples can all be classified as a type H3 hysteresis loop,23,24 with the adsorption

12


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occurring mainly in the medium and high pressure region (0.4

Synthesis and Adsorption Properties of Hierarchically Ordered

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