Two-Dimensional Porous SiO2 Nanostructures Derived

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Two–dimensional Porous SiO2 Nanostructures Derived from Renewable Petal Cells with Enhanced Adsorption Efficiency for Removal of Hazardous Dye Ruyu Cui, Yan Lin, Junchao Qian, Yuanyuan Zhu, Nan Xu, Feng Chen, Cheng-Bao Liu, Zhengying Wu, Zhigang Chen, and Xing Zhou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b00170 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on March 3, 2017

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ACS Sustainable Chemistry & Engineering

Two–dimensional Porous SiO2 Nanostructures Derived from Renewable Petal Cells with Enhanced Adsorption Efficiency for Removal of Hazardous Dye Ruyu Cui,a Yan Lin,a Junchao Qian,a Yuanyuan Zhu,b Nan Xu,a Feng Chen,a Chengbao Liu,a Zhengying Wu*,a Zhigang Chen*a and Xing Zhoua a

Jiangsu Key Laboratory for Environment Functional Materials, School of Chemistry, Biology

and Material Engineering, Suzhou University of Science and Technology, 1 Kerui Road, Suzhou 215009, China. b

Faculty of Materials Science and Engineering, Changzhou University, 1 Gehu Road,

Changzhou 213164, China. Corresponding Authors E-mail Address *Zhengying Wu: [email protected]; *Zhigang Chen: [email protected];

ABSTRACT: Diverse microstructures and morphologies from plant cells inspire us with great opportunities for creating the novel nanomaterials. In this work, a biomorphic mesoporous SiO2 with unique two-dimensional (2D) nanostructures was feasibly fabricated by employing

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renewable petal cells as bio-scaffolds. During the structure formation, the hydrolyzed siliceous species initially adhere to, and then penetrated the cell walls, forming the composite of siliceous species/cells. It is followed by the shrinkage and deformation of cell skeleton in subsequently drying process. Then the special 2D SiO2 with abundant mesopores inside was acquired after careful removal of the cells. The concentration of siliceous source (tetraethyl orthosilicate, TEOS) in the impregnation/infiltration steps is a key factor for the replication of the biological morphology for SiO2. The sample prepared with CTEOS of 0.05 mol·L-1 can well duplicate the bio-morphology of petal cells, which has a BET surface area of 177 m2·g-1 and pore size ranging from 4-9 nm. Due to its high accessible pores and large attachable adsorption sites, the biomimetic 2D porous SiO2 displays the significant adsorption capacity for methylene blue (74 mg·g-1), which is higher than those by nonporous SiO2 (14 mg·g-1) and the traditionally hydrothermal synthesized mesoporous SBA-15 (45 mg·g-1).

KEYWORDS: Two-dimensional, Porous silica, Biomimetic technology, Adsorption


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1. Introduction Nanoporous silicas in the mesopore range (2-50nm) are very promising due to their remarkable properties such as high surface area, versatile pore structures, adjustable aperture size and good hydrothermal stability1-3. These basic characteristics, along with the low cost, nontoxicity and easy-preparation make nanoporous SiO2 possess high potential in numerous applications, including adsorption, catalysis, environment protection, drug delivery system4-6. For the traditional synthesis of mesoscale nanoporous silicas, special surfactants are usually employed as soft-templates or structure-directing agents to generate porosity7. Thus, mesoporous silicas with various morphologies like rod8, fiber9, sphere9, polyhedron10, etc. can be fabricated in an easy manner through controlling the reaction conditions or introducing some organic or inorganic additives11. Nevertheless, most of the morphology adjustments and controls are conducted at the three-dimensional (3D) space scale, and there is no report on the fabrication of two-dimensional (2D) nanoporous silicas. Actually, 2D nanomaterials with the thickness on the order of nanometers and lateral sizes of submicro- to micrometers have drawn the considerable attention since the discovery of graphene in recent years12-15. Apparently, 2D nanomaterials have the distinct properties different from their respective 3D structures. So they are expected to have a significant impact on various potential applications such as electronics, separation, catalysis, sensors and others16. For example, 2D graphene-based materials with a few atomic thickness and large planar size, such as graphene oxide (GO), magnetic graphene and its relative composites have already demonstrate their highly adsorptive activities as excellent water treatment agents1719

. However, it is difficult to precisely control the growth of a nanoporous material at two space

dimensions16, 20. It still keeps a great challenge to synthesize a 2D nanoporous material by the traditional bottom-up surfactant-mediated pathways.


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Different from the artificial methodologies, nature has provides the more diverse and sophisticated ways for developing advanced materials21-22. Numerous hierarchical porous structures in all levels of biological organizations are created by nature, which opens up new opportunities for the replication of synthetic materials23. Virtually, biotemplate technology24 has already shown its efficiency and conveniency for the preparation of various structures with special biomimetic morphologies derived from diatoms25, cotton fibers26, luffa sponge27, rose petals28, rice-straw29, green leaves30 and so on. Furthermore, these biological tissues used as templates are renewable, easy to remove and environmentally friendly. Their replicas have the desired structures display excellent performance in the photoelectric conversion, chemical catalysis, and sensor manufacturing25-30. Macroporous SiO2 with ordered superstructure was produced by using bacterial as a biotemplate for the first time in 199731. Subsequently, pollen grain32 and diatoms33 have been used for the biomimetic synthesis of porous SiO2 with special architectures. Chiral self-supporting SiO234 and mesoporous SiO2 nanotube35 can also be formed around cellulose scaffolds at a specific pH or by addition of organic surfactant. Obviously, employing the biological tissues as templates is great promising in generation of structurally well-defined materials, which gives us a clue to construct novel 2D structured nanoporous SiO2 in a nanofabrication route. In this study, a special 2D porous SiO2 combined with regular micro and nano-sized substructures was prepared for the first time through replicating the cell structures of camellia petals. The biotemplate is easy to obtain and the method is simple and accurate. The expected SiO2 shows a distinguishable 2D lamellate morphology with abundant mesopores inside. It is fascinating that the cell-derived 2D porous SiO2 shows an enhanced adsorption efficiency for removing hazardous dye from aqueous solutions when compared to the traditionally synthesized


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mesoporous materials. This is because that the sophisticated 2D porous structure of SiO2 offers more accessible and attachable surface adsorption sites than the conventional mesoporous SiO2 in the adsorption. This work provides a new idea for designing the porous material with the unusual 2D microstructures.

2. Experimental 2.1 Preparation of biomorphic SiO2 nanosheets Hydrochloric acid (HCl, 35-37 wt.%), sodium hydroxide (NaOH), ethanol (EtOH), methylene blue (MB, C16H18ClN3S, MW = 373.9) and tetraethyl orthosilicate (TEOS) were purchased from Sinopharm Chemical Reagent Co. Ltd. of China. All above chemicals were analytical grade and used without further purification. Camellia petals used in this study were collected from nature (our campus) and washed by deionized water before using. Then, the camellia petals were pretreated with an ethanol aqueous solution (EtOH/H2O = 1/1, acidity of the solution was adjusted to pH = 3 by HCl) for two weeks to elute contaminant elements and pigments in the petals. After that, those petals were immersed into the TEOS ethanol solution with different concentration (0.01, 0.05, 0.10 and 015 mol·L-1) for 3 days, and then were washed with anhydrous ethanol and dried under ambient temperature (these dried TEOS-petal composites were denoted as HSxAs). Finally, the petal templates were removed by calcining the composites in air for 5 hours at 550 0C. The obtained biomorphic SiO2 materials were designated as HSx, x varies from 001, 005, 010 to 015, corresponding to the used TEOS concentrations. 2.2 Characterizations Cell morphology of the camellia petal was observed under an Olympus IX71 fluorescence microscope. Scanning electron microscopy (SEM) of the HSx samples was performed on a Hitachi S-4800 electron microscope. Transmission electron microscopy (TEM) of the samples


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was performed using a JEM-2100 electron microscope operating at an accelerating voltage of 200 kV. Atomic force microscopy (AFM) analysis was performed in a Dimension V scanning probe Nanoscope IIIa (Digital Instruments Inc.) operating in the tapping mode. Surface area and pore size distribution of the samples were determined by the Brunauer-Emmett-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) model (according to the adsorption branches) using a Micromeritics Tristar 3020 system. Thermogravimetric analysis (TGA) was performed using a Perkin-Elmer TGA analyzer with a heating rate of 10 0C/min up to 800 0C under an air flow. Fourier Transform Infrared (FT-IR) spectra of powder samples suspended in KBr pallets were recorded on a Nicolet IS 10 spectrometer (Thermo). X-ray photoelectron spectroscopy (XPS) spectra were obtained from a PHI Versaproke 5000 system. Zeta potentials of the HS010 sample were obtained on a Malvern ZEN3690 instrument. The pH of the initial aqueous solution was adjusted by dilute HCl or NaOH solutions. 2.3 Batch adsorption experiments Batch test was typically carried out by adding 20 mg of adsorbent (HSx, SBA-15 or nonporous SiO2) into a set of 50 mL plastic flasks containing 20 mL of MB solutions with different initial concentrations (20 - 120 mg·L-1) to get adsorption isotherms. The mixtures were shaken at 25 0C with the speed of 150 rpm for 24 hours to ensure adsorption equilibrium. For the study of kinetic adsorption behaviour, the mixtures were shaken at appropriate time of 1, 2, 4, 6, 8, 12, 16, 20 and 24 hours, respectively. The pH of the solutions were adjusted by adding HCl or NaOH solutions (0.02 mol·L-1). All adsorption experiments were performed in triplicate and the average value was present in this study. After equilibrium, the suspensions were separated by filtrations. Finally, MB concentration of the filtrate was determined using a UV-2450 spectrophotometer at a maximum wavelength around 665 nm. For the recycling test, five parallel tests were performed


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for the first run. After that, the used adsorbent was collected together by filtration, dried in air, calcined at 550 0C for 2 h in a muffle furnace for the regeneration, and then reused in the next run. The amount of dye adsorption onto HSx sample was calculated from the mass balance equation as: =

− 1

where Qe (mg·g-1) is the amount of MB adsorbed per gram of HSx at equilibrium; C0 (mg·L-1) and Ce (mg·L-1) are the initial and equilibrium liquid-phase concentrations of dye; V (L) is the volume of the solution and M (g) is weight of the dry adsorbent. The adsorption kinetics of MB on HSx samples were fitted by a pseudo-second-order kinetic model36-37, which gives a linear form as follow:

1

= + 2 where k (g·mg−1·h−1) is the rate constant of pseudo-second-order adsorption; Qt (mg·g-1) is the amount of MB adsorbed at time t (h). Rate parameters, k and Qe, can be directly calculated from the intercept and slope of the plot of (t/Qt) against time t. The adsorption isotherms of MB on HSx samples were described by Freundlich-type and Langmuir-type equations38. Both isotherm models are expressed as follows: Langmuir model: =

3 1 +

Freundlich model: ⁄

=

4


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where Qmax (mg·g−1) is the monolayer capacity of adsorbent, KL (L·mg−1) is the Langmuir binding constant. KF [mg·g−1(L·mg−1)1/n]) and 1/nF are Freundlich constants, indicator of adsorption capacity and adsorption intensity39, respectively.

3. Results and discussion 3.1 Formation and structure of the 2D porous HSx samples Just as many flowers, camellia petal cell consists mainly of cellulose, hemicellulose, pectic substances, anthocyanins, lignin and small amount of minerals40-41. Its large number of surface functional groups such as -OH, -COOH, etc. are beneficial for the adsorption of inorganic siliceous species. In addition, camellia petal has its own special morphology and cell structure. The surface of camellia petal looks not flat under the fluorescence microscope. The epidermal cells of fresh


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Scheme 1. Schematic illustration of the fabrication process for HSx by using camellia petal cells as bio-scaffolds; SEM images of HS001 (a), HS005 (b), HS010 (c), and HS015 (d) samples. (Step 1: Pre-treatment of the templates; Step 2: Interactions between the siliceous species and the templates; Step 3: Removal of templates to obtain the final materials.) petals consist of many irregular columnar cells, packing closely to each other, as shown in Step 1 in Scheme 1. Before the replication process, those fresh petals were firstly immersed into ethanol solution for two weeks to decreasing the content of the soluble organic constituents (i.e. pigments, cytoplasm, etc) and the other inorganic oxides41 from the petal cells. As a result, the petals looks transparent and the red color of the epidermal cells becomes lightened. On the microscopic view, those pre-treated petals are not full of cell fluids but the polygon cell shape preserves (Step 1, Scheme 1). Shrinkage and deformation of cell skeletons appear when they


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were impregnated in the TEOS ethanol solution (0.05 mol·L-1) and subsequently dried in air (Step 2, Scheme 1). During the impregnation/infiltration process, TEOS and related hydrolyzed siliceous species can adhere to the cell walls, and penetrate the cells, forming the siliceous species contained petals. With increasing the concentration of TEOS, the siliceous depositions on the cell surface become the thicker. Consequently, the SiO2 with different surface morphologies and porous structures were obtained after calcinations (Step 3, Scheme 1). It should be pointed out that the concentration of the siliceous source in the impregnation/infiltration steps has a significant influence on the replication of the biological morphology for SiO2. As illustrated in SEM images (Step 3, Scheme 1), HS005 (b) and HS010 (c) show the characteristic bio-morphologies with periodic arrays of embossments in their profiles, which is similar to that of the fresh petals (Step 1, Scheme 1). Furthermore, the surface structure of HS005 looks more distinct than HS010 due to the appropriate siliceous concentration (CTEOS = 0.05 mol·L-1) in the synthetic solution. Too low TEOS concentration will cause the unfavourable replication of the cell structures, and the morphology of HS001 prepared with CTEOS = 0.01 mol·L-1 seems very rough and irregular (SEM, a). Too much TEOS (CTEOS = 0.15 mol·L-1) leads to the overlap of SiO2 in the petal cell surface, resulting in a much smoother surface. The hierarchical pores of HS015 generated from the petal cells become obscure (SEM, d). Figure 1 shows the typical TEM images of the porous SiO2 derived from camellia petal cells, illustrating the 2D sheet-like morphologies for the HS005 and the HS010 samples. Lateral size of the nanosheets is about a few micrometres and appears slightly crumpled (Figure 1a, 1c). Atomic force microscopy analysis reveals that the mean thickness for HS005 is 2.7 nm±0.05 nm (Figure 2). Furthermore, it is found that the HS010 lamella looks much thicker than HS005 by carefully


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measuring the edges of those two samples (See Figure 1a, c, red circled areas). This is in agreement with the SEM results that the higher TEOS loading causes the larger SiO2 thickness (Scheme 1, SEM b and c). TEM images also reveal that there are a large amount of inhomogeneous mesopores in the SiO2 nanosheets, and the pore diameters of the HS005 and HS010 samples varies from 4 to 9 nm on high magnifications (Figure 1b, 1d). The special 2D sheet-like morphology and well distributed pores of the samples may be beneficial for the adsorption of organic dye molecules.

Figure 1. TEM images of petal cell templated HS005 and HS010.


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Figure 2. AFM images (a, b) and the corresponding height profiles of the HS005 sample. N2 adsorption-desorption isotherms and pore size distributions of the HSx samples are presented in Figure 3. The isotherm shapes of four HSx samples are similar to each other. They are type IV with H3-type hysteresis loops in the relative pressure range of 0.4~1.0 according to the IUPAC classification. The isotherms can be divided into three different steps: convex at low relative pressure (p/p0 = 0~0.1), nearly linear at the middle section (p/p0 = 0.1~0.4), and continuous adsorption at high relative pressure (p/p0 = 0.4~1.0). There are no obvious adsorption limit at high relative pressure for all samples, indicating the presence of widely distributed mesoporous (2-50 nm) and macrospores42. The pore sizes of all samples are mainly between 2 nm and 10 nm with an average in 4.5~6.8 nm (calculated from the adsorption branches, Figure 3b and Table S1), which is consistent with the TEM results. As listed in Table S1, the HSx samples synthesized with different TEOS concentrations have BET surface area of 78~177 m2·g-1 and pore volume of 0.13~0.21 cm3·g-1. The HS001 with poor bio-morphology and structure shows the lowest surface area while the HS005 sample with the good bio-morphology exhibits the largest surface area and pore volume. BET surface area and pore volume of the HS010 are not as high as those of the HS005. We measured the N2 adsorption


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Figure 3. N2 adsorption–desorption isotherms (a) and pore size distributions (b) of the petal cell templated HSx samples. For clarity, isotherm curves for HS005, HS010 and HS015 are offset 25, 125 and 150 cm-3·g-1, respectively. of HS010 twice and the results are the same. The HS015 synthesized with the highest TEOS concentration has surface area and pore volume of 151 m2·g-1 and 0.17 cm3·g-1, respectively, which are slightly lower than those of HS005.


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3.2 Composition of the 2D porous HSx samples TG and DSC of the as-synthesized (petal impregnated with TEOS) HS001 and HS010 are shown in Figure 4. The total weight losses are around 97% and 80% for HS001As and HS010As, respectively. The weight loss before 200 0C (15% for HS001, 9% for HS010) can be ascribed to the evaporation of physically adsorbed water. The other weight losses are mainly due to the thermal decomposition of the structure directing templates (camellia petals) (Figure 4a). There are two weight loss steps in the range of 200-550 0C for HSxAs samples, corresponding to two exothermic


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Figure 4. TG (a) and DSC (b) curves of the HS001AS and HS010AS samples. peaks in DSC curves (Figure 4b). The first step among 200-380 0C with an exothermic peak at about 365 0C is ascribed to the charring of organic constituents in the petal cells. The other weight loss step with a corresponding exothermic peak near 450 0C can be assigned to the through combustions of the charred matters. It is interesting to find that the second exothermic peak of HS001As (467 0C) moves towards a bit higher temperature compared by HS010As (436 0

C). This is probably due to the more organic constituents exists in HS001As than in HS010As.

In fact, most compositions (97%) in the as-synthesized HS001 are from the camellia petals, so only 3% of solid mass is remained after calcination (Figure 4a). The low SiO2 yield for HS001As also implies its poor replicated structure and imperfect biomorphology (Scheme 1, SEM a). Weight loss ends at 550 0C for HSxAs samples, indicating complete decomposition of the petal templates. So 550 0C was chosen as the calcination temperature to remove the petal templates. FT-IR spectra for the calcined HSx samples synthesized with different TEOS concentration show characteristic adsorption bands at ca. 470, 802 and 1053 cm-1 (Figure S1), which are ascribed to the deformation models of Si-O-Si [δ(Si-O-Si)], symmetric and as symmetric stretching of Si-O-Si [νs(Si-O-Si)], [νas(Si–O–Si)]43-44. The adsorption bands at 1640 and 3445 cm-1 can be assigned to bending and stretching bands of H-O-H [δOH(H-O-H)] and [νOH(H-OH)]. Weak bands at 2912-2855 cm-1 ascribed to the saturated C-H stretching vibrations appear in all HSx samples45-46, indicating a few residual C exists in the samples. Additionally, 1420 cm-1 characteristic band of CO32-

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emerges in the HS001 sample, which is originated from the

organic template. However, this carbonate adsorption band is invisible for other HSx samples (Figure S1). XPS was carried out to further analyse the possible elementary composition and their chemical states of the HS005 sample.


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Whole XPS scan spectrum demonstrates that HS005 consists mainly of O (18.6%) and Si (68.0%). A little other elements, such as Ca (5.1%), C (4.9%), P (1.8%) and Cl (1.7%) coexist in the calcined HS005 (Figure 5a-b). The contaminant C (C 1s peak at 284.8 eV, Figure 5c) was chiefly introduced by the adhesive tape used in the XPS test, and partially came from the residual C after thermal combustion of the template. The Cl 2p3/2 peak at 198.4 eV (Figure 5d) indicated Cl- 48 is probably derived from the HCl ethanol/H2O solution during the pre-treatment process. The mineral elements like Ca and P are originated from the raw petals even they were pre-treated by the dilute HCl ethanol/H2O solution before the replication. The high resolution O 1s peak can be fitted into three smaller peaks located at 531.3 eV, 532.6 eV and 533.2 eV, respectively (Figure 5e). The first binding energy peak at 531.3 eV can be attributed to oxygen of C=O49-50 configuration of the CaCO3, which is coincident with the high-resolution XPS spectra of C 1s (288.3 eV)51 and Ca 2p3/2 (347 eV)52 (Figure 5c). Calcium carbonate is probably originated from the decomposition of calcium oxalate that distributed among all taxonomic levels of photosynthetic organisms53. The second (532.6 eV) and third (533.2 eV) bands of O 1s imply the presence of Si-O-Si (SiO2) and P-O-P ((P2O7)4−) bonds54-55, which are further confirmed by the high-resolution XPS spectra of Si 2p (103.2 eV) and P 2p (133.1 eV) (Figure 5f). Pyrophosphate is unstable in aqueous solution and can be hydrolysed to inorganic phosphate56.


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Figure 5. XPS spectra survey (a), atomic concentration (b) and high-resolution XPS spectra of Ca 2p3/2, C 1s, Cl 2p3/2, O 1s, P 2p, Si 2p (c, d, e, f) for the HS005 sample. 3.3 Adsorption of the 2D porous HSx samples to MB The adsorption experiments of MB on HSx were carried out in order to evaluate the effectiveness of the adsorbents and to gain insight into the underlying mechanisms. Shapes of kinetic curves for four HSx samples are similar to each other (Figure 6a). There is a fast uptake of MB during the initial 2 hours, then the adsorption rate for MB slows down in the subsequent hours. The adsorption equilibrium is established at about 8 hours for HS005 and HS010 samples, while 12 hours for


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Figure 6. Kinetic curves (a) and liner plots for the pseudo-second-order kinetics (b) of MB absorbed on HSx samples; adsorption isotherms of MB on HS005, HS010, SBA-15 and SiO2(c); schematic diagram of MB adsorption on mesoporous SBA-15 and 2D lamellate HS005 nanosheets (d). HS001 and HS015 samples. Moreover, the adsorption capacities of four HSx samples varies due to their different structures and morphologies (SEM images in Scheme 1). The equilibrium adsorption amounts of HS005 and HS010 are near 70 mg·g-1 while that of HS001 and HS015 are 59 and 65 mg·g-1, respectively (Figure 6a). Besides the morphology, textural property also affects the adsorption performance of the sample. HS005 with the largest surface area and pore


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volume has the largest adsorption capability while HS001 with the lowest surface area and pore volume has the smallest adsorption capacity in four HSx samples (Table S1, Figure 6a). HS010 with the good bio-morphology and textural property also has a high adsorption capability comparable to HS005. HS015 with the large surface area but relatively poor bio-morphology shows a MB uptake larger than HS001 but lower than HS010 (Table S1, Scheme 1, and Figure 6a). Liner plots for the pseudo-second-order kinetics of MB adsorption onto HSx samples, the parameters and linear coefficients R2 obtained by fitting are present in Figure 6b and Table S2. The calculated values of equilibrium sorption capacity Qemod are identical to those obtained from experiments (Qe exp) and all the values for R2 are greater than 0.999 (Table S2), suggesting the sorption process of MB onto these lamellate porous silica samples (HSx) can be represented well by the pseudo-second-order model.

Table 1. Adsorption capacities to MB for some mesoporous SiO2, 2D graphene based materials and other related materials reported in literatures.

Mesoporous SiO2 materials

MS6

MSA57

OMS58

HSA57

CM-259

AM-360

S16C-061

Qe (mg·g-1)

25

60

54

38

32

30

345

Graphene based materials

GO series17

MG@ m-SiO218

MrGO/ TiO219

Fe3O4-GS62

mGO/ PVA63

MGC64

CoFe2O4FGS65

Qe (mg·g-1)

49-599

140

425

426

231

66

72

Other related materials

Cellulose -clay hydrogel45

PAni hydrogel66

Keratin nanofibrous membrane67

PAA/ MnFe2O468

g-C3N469

MLS70

HSC71

Qe (mg·g-1)

783

71

170

50

2.5

31

149


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Figure 6c displays the adsorption isotherms of MB on the petal cell derived 2D porous SiO2 (HS005 and HS010), the traditional P123-templated mesoporous SBA-1538 and nonporous SiO2. The adsorbed amounts of MB grow steeply with the initial concentrations increase and then reach maximums. The MB uptake by HS005 is a little higher than that by HS010 at high MB concentrations. Actually, HS005 and HS010 show the best adsorption behaviors in both of isotherm and kinetic studies (Figure 6a and 6c). The adsorption capacities of the lamellate porous SiO2 (HS005 and HS010) are much higher than those of mesoporous SBA-15 and the nonporous SiO2. Moreover, the 2D porous SiO2 also has the higher adsorption capacity than some of other porous SiO2 materials reported in literatures (Table 1). This is due to the special 2D porous structures of HSx samples that were well-replicated from camellia petal cells. The 2D layered structures can provide more open and accessible adsorption sites for the adsorbent to contact. So the graphene-based materials normally have much larger MB uptakes than graphite17. Additionally, the adsorption capacity of the graphene oxide (GO) was found to enhance along with the increase in the oxidation degree and interplanar distance for the GO17. In our work, the 2D porous SiO2 nanosheets with the nanometer-sized thickness (2.7 nm for HS005) also has high amount of accessible pores and attachable adsorption sites which are important to promote adsorption capacity65, as shown in Figure 6d. Consequently, HSx samples have better MB uptakes than SBA-15, even though their BET surface areas and pore volume are lower than those of SBA-1538. On the other hand, some of the residual inorganic species in the calcined HSx samples may also contribute to their good adsorption abilities, which needs to be further studied in the future.


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The isotherm of MB adsorption on SBA-15 can be fitted well by either Langmuir or Freundlich isotherm models38. While sorption isotherms for HS005, HS010 and nonporous SiO2 fit the Langmuir model (RL = 0.96~0.98) well (Figure 6c, Table S3). It implies that the surfaces of HSx materials are homogeneous and the accessible sorption sites are energetically equivalent to the MB molecules during the adsorption. Moreover, the Langmuir monolayer capacity (Qmax) and binding constant (KL) values of HS005 and HS010 are much higher than those of SBA-15 and nonporous SiO2 (Table S3), which confirms that HS005 and HS010 have the larger adsorption capacities and stronger adsorption intensities compared by mesoporous SBA-15 and nonporous SiO2. In practical industrial processes, wastewater may be acidic or alkali. To investigate the effect of initial pH on the adsorption capacity for HSx samples, aqueous solutions were adjust to pH values ranging from 3 to 11. Obviously, the adsorption performance of MB onto HS010 was significant influenced by solution pH (Figure S2). The MB uptake continuously increase with pH varying from 3 to 11. As the initial concentration of MB is 80 mg·L-1, the MB removal efficiency is 35% (Qe = 28.1 mg·g-1) at pH = 3, and it reaches 63, 70, 74 and 88% (Qe =50.2, 56.2, 59.5 and 70.6 mg·g-1) at pH = 5, 7, 9, 11, respectively. Similar results are founded in the MB adsorption process using other mesoporous silicas61, 72. The lower adsorption of MB under acid conditions is probably due to the presence of large number of H+ ions in solution. At pH > 2, the surface of SiO2 in aqueous solution (isoelectric point, pH = 2) is negatively charged73, which is beneficial for the electrostatic interaction between the SiO2 surface and the positively charged dye molecule (MB+). In acidic conditions, large amount of H+ ions in solution will compete with the MB+ cations for the adsorption sites of HS010. However, this competition from H+ will be weakened when solution pH is raised (cH+ is


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decreased). Furthermore, the surface charge negativity of HS010 enhances with the increase in solution pH (see zeta potential results of HS010 in Figure S3), which will bring the stronger interactions between MB+ and the negatively charged surface of HS010. Consequently, the adsorption amount of MB on the HS010 sample tends to increase with the increase in pH. Stability of the 2D porous SiO2 at high pH (pH =11) value was investigated in recycling experiments. After the first 3 runs, the MB removal efficiency is slightly decreased from 88% to 83% (Figure 7). However, the adsorption ability of HS010 was recovered after the 4th and 5th cycles. In general, the difference of adsorption amounts to MB between the five recycles is little, indicating the good stability and recyclability of the petal derived 2D porous SiO2.

Figure 7. Reuse of the HS010 sample for MB removal.

4. Conclusions In summary, a unique porous SiO2 with two-dimensional (2D) microstructure has been fabricated successfully using renewable camellia petal cells as biological templates. Different from the traditional hydrothermal approaches, this biomimetic process conducted in mild


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synthetic condition provides a new scalable and sustainable way for developing 2D porous materials through learning from nature. The concentration of siliceous source is crucial to the effective duplication of the petal cell structures. The special 2D microstructure and high porosity make this SiO2 show the higher adsorption capability than nonporous SiO2 and the commercial surfactant templated mesoporous material (SBA-15). This present study exploit a new and versatile technique for the design and synthesis of promising 2D porous materials using the renewable natural resources.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI. Further characterizations and experimental data (textural properties and FTIR spectra of the HSx samples; kinetic parameters calculated by the pseudo-second-order model for MB adsorption onto HSx samples; Langmuir and Freundlich parameters for adsorption of MB over SBA-15, nonporous SiO2, HS005, and HS010 samples; effect of pH on the adsorption of MB onto HS010 sample); Zeta potentials of HS010 at different solution pH (PDF). AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Phone: +86-512-67374120. Fax: +86-512-67374120; *E-mail: [email protected]. Phone: +86-512-68083175. Fax: +86-512-67374120; Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation (NSF) of China (21407111, 21377090 and 51403148), NSF of Jiangsu Province (BK20151198 and 14KJA430004). Financial support from Jiangsu Collaborative Innovation Center of Technology and Material for Water Treatment, the Open Project of the Jiangsu Key Laboratory for Environment Functional Materials (SJHG1310) and the Suzhou Science and Technology Project (SYG201530) is also gratefully acknowledged. The authors thank Professor Jian Zhu in Shanghai Normal University for the XPS measurements.


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For Table of Contents Use Only Manuscript Title: Two–dimensional Porous SiO2 Nanostructures Derived from Renewable Petal Cells with Enhanced Adsorption Efficiency for Removal of Hazardous Dye Authors: Ruyu Cui, Yan Lin, Junchao Qian, Yuanyuan Zhu, Nan Xu, Feng Chen, Chengbao Liu, Zhengying Wu*, Zhigang Chen* and Xing Zhou TOC

Synopsis A novel two-dimensional porous SiO2 with high adsorption efficiency was feasibly fabricated by a cost-effective sustainable biomimetic method.


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Two-Dimensional Porous SiO2 Nanostructures Derived

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