Two-Dimensional Porous SiO2 Nanostructures Derived from

Research Article pubs.acs.org/journal/ascecg

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,† Chengbao Liu,† Zhengying Wu,*,† Zhigang Chen,*,† and Xing Zhou†

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†

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 ‡ Faculty of Materials Science and Engineering, Changzhou University, 1 Gehu Road, Changzhou 213164, China S Supporting Information *

ABSTRACT: Diverse microstructures and morphologies from plant cells inspire us with great opportunities for creating novel nanomaterials. In this work, a biomorphic mesoporous SiO2 with unique two-dimensional (2D) nanostructure was feasibly fabricated by employing renewable petal cells as bioscaffolds. During the structure formation, the hydrolyzed siliceous species initially adhere to and then penetrated the cell walls, forming the composite of siliceous species/cells. This is followed by shrinkage and deformation of the cell skeleton in a subsequent drying process. Then, the special 2D SiO2 with abundant internal mesopores 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 duplicate well the biomorphology of petal cells, which has a BET surface area of 177 m2 g−1 and pore size ranging from 4 to 9 nm. Because of its highly accessible pores and large attachable adsorption sites, the biomimetic 2D porous SiO2 displays 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 hydrothermally synthesized mesoporous SBA-15 (45 mg g−1). KEYWORDS: Two-dimensional, Porous silica, Biomimetic technology, Adsorption

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INTRODUCTION Nanoporous silicas in the mesopore range (2−50 nm) are very promising due to their remarkable properties such as high surface area, versatile pore structures, adjustable aperture size, and good hydrothermal stability.1−3 These basic characteristics, along with low cost, nontoxicity, and easy preparation make nanoporous SiO2 very attractive for numerous applications, including adsorption, catalysis, environmental protection, and drug delivery systems.4−6 For the traditional synthesis of mesoscale nanoporous silicas, special surfactants are usually employed as soft-templates or structure-directing agents to generate porosity.7 Thus, mesoporous silicas with various morphologies like rod,8 fiber,9 sphere,9 polyhedron,10 and so forth can be fabricated in an easy manner by controlling the reaction conditions or introducing some organic or inorganic additives.11 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 twodimensional (2D) nanoporous silicas. Actually, 2D nanomaterials with thickness on the order of nanometers and lateral sizes of submicro- to micrometers have drawn considerable attention since the discovery of graphene in recent years.12−15 © 2017 American Chemical Society

Apparently, 2D nanomaterials have distinct properties that differ from their respective 3D structures. Thus, they are expected to have a significant impact on various potential applications such as electronics, separation, catalysis, sensors, and others.16 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 demonstrated their highly adsorptive activities as excellent water treatment agents.17−19 However, it is difficult to precisely control the growth of a nanoporous material in two space dimensions.16,20 It is still a significant challenge to synthesize a 2D nanoporous material by traditional bottom-up surfactant-mediated pathways. Differing from artificial methodologies, nature provides more diverse and sophisticated methods for developing advanced materials.21,22 Numerous hierarchical porous structures at all levels of biological organizations are created by nature, which opens new opportunities for the replication of synthetic Received: January 16, 2017 Revised: February 16, 2017 Published: February 24, 2017 3478

DOI: 10.1021/acssuschemeng.7b00170 ACS Sustainable Chem. Eng. 2017, 5, 3478−3487

Research Article

ACS Sustainable Chemistry & Engineering materials.23 Virtually, biotemplate technology24 has already shown its efficiency and convenience for the preparation of various structures with special biomimetic morphologies derived from diatoms,25 cotton fibers,26 luffa sponges,27 rose petals,28 rice-straw,29 green leaves,30 and so on. Furthermore, these biological tissues used as templates are renewable, easy to remove, and environmentally friendly. Their replicas have the desired structures and display excellent performance in photoelectric conversion, chemical catalysis, and sensor manufacturing.25−30 Macroporous SiO2 with ordered superstructure was produced using bacteria as a biotemplate for the first time in 1997.31 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 nanotubes35 can also be formed around cellulose scaffolds at a specific pH or by the addition of organic surfactant. Obviously, employing the biological tissues as templates is highly promising for the generation of structurally well-defined materials, which gives us a clue for constructing novel 2D structured nanoporous SiO2 in a nanofabrication route. In this study, a special 2D porous SiO2 combined with regular micro- and nanosized 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 that of the traditionally synthesized mesoporous materials. This is because the sophisticated 2D porous structure of SiO2 offers more accessible and attachable surface adsorption sites than those of the conventional mesoporous SiO2 in the adsorption. This work provides a new idea for designing the porous material with unusual 2D microstructures.

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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 PerkinElmer TGA analyzer with a heating rate of 10 °C/min up to 800 °C under air flow. Fourier transform infrared (FT-IR) spectra of powder samples suspended in KBr pellets 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. Batch Adsorption Experiments. Batch tests were typically carried out by adding 20 mg of adsorbent (HSx, SBA-15, or nonporous SiO2) to a set of 50 mL plastic flasks containing 20 mL of MB solutions with different initial concentrations (20−120 mg L−1) to obtain adsorption isotherms. The mixtures were shaken at 25 °C with a speed of 150 rpm for 24 h to ensure adsorption equilibrium. For the study of kinetic adsorption behavior, the mixtures were shaken at appropriate times of 1, 2, 4, 6, 8, 12, 16, 20, and 24 h. The pH of the solutions was adjusted by adding HCl or NaOH solutions (0.02 mol L−1). All adsorption experiments were performed in triplicate, and the average values are presented in this study. After equilibrium, the suspensions were separated by filtration. Finally, the 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 for the first run. After that, the used adsorbent was collected by filtration, dried in air, calcined at 550 °C for 2 h in a muffle furnace for regeneration, and then reused in the next run. The amount of dye adsorption onto HSx sample was calculated from the mass balance equation as Qe =

(C0 − Ce)V M

(1)

−1

where Qe (mg g ) 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, respectively, V (L) is the volume of the solution, and M (g) is the weight of the dry adsorbent. The adsorption kinetics of MB on HSx samples were fitted by a pseudo-second-order kinetic model,36,37 which gives a linear form as

EXPERIMENTAL SECTION

⎛ t ⎞ t 1 = + ⎜⎜ ⎟⎟ 2 Qt kQ e ⎝Qe ⎠

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 of these 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 use. 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 2 weeks to elute contaminant elements and pigments in the petals. After that, the petals were immersed into the TEOS ethanol solution at different concentrations (0.01, 0.05, 0.10, and 0.15 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 h at 550 °C. The obtained biomorphic SiO2 materials were designated as HSx (x = 001, 005, 010, and 015) corresponding to the TEOS concentrations used. 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 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

−1

(2)

−1

where k (g mg h ) is the rate constant of pseudo-second-order adsorption and 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 equations.38 Both isotherm models are expressed as Langmuir model:

Qe =

Q maxKLCe 1 + KLCe

(3)

Freundlich model: Q e = KFCe1/ nF

(4) −1

where Qmax (mg g ) 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 intensity,39 respectively.

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RESULTS AND DISCUSSION Formation and Structure of the 2D Porous HSx Samples. Just like many flowers, a camellia petal cell consists 3479


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ACS Sustainable Chemistry & Engineering

Scheme 1. Schematic Illustration of the Fabrication Process for HSx Using Camellia Petal Cells as Bioscaffoldsa

a

Step 1: pretreatment of the templates; step 2: interactions between the siliceous species and the templates; step 3: removal of templates to obtain the final materials. SEM images of HS001 (a), HS005 (b), HS010 (c), and HS015 (d) samples.

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 unfavorable 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 typical TEM images of the porous SiO2 derived from camellia petal cells, illustrating the 2D sheetlike morphologies for the HS005 and HS010 samples. The lateral size of the nanosheets is a few micrometers and appears slightly crumpled (Figure 1a and c). Atomic force microscopy analysis reveals that the mean thickness for HS005 is 2.7 ± 0.05 nm (Figure 2). Furthermore, it is found that the HS010 lamella looks much thicker than HS005 by carefully measuring the edges of the two samples (see Figure 1a and c, red circled areas). This is in agreement with the SEM results that higher TEOS loading causes larger SiO2 thickness (Scheme 1, SEM b and c). TEM images also reveal that there are a large number of inhomogeneous mesopores in the SiO2 nanosheets, and the pore diameters of the HS005 and HS010 samples vary from 4 to 9 nm in high magnifications (Figure 1b and d). The special 2D sheetlike morphology and well-distributed pores of the samples may be beneficial for the adsorption of organic dye molecules. 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

mainly of cellulose, hemicellulose, pectic substances, anthocyanins, lignin, and a small amount of minerals.40,41 Its large number of surface functional groups such as −OH, −COOH, and so forth are beneficial for the adsorption of inorganic siliceous species. In addition, camellia petals have their own special morphology and cell structure. The surface of the camellia petal looks not flat under a fluorescence microscope. The epidermal cells of fresh petals consist of many irregular columnar cells, packing close to each other, as shown in step 1 in Scheme 1. Before the replication process, these fresh petals were first immersed in ethanol solution for 2 weeks to decrease the content of the soluble organic constituents (i.e., pigments, cytoplasm, etc.) and other inorganic oxides41 from the petal cells. As a result, the petals looks transparent, and the red color of the epidermal cells becomes lighter. In the microscopic view, these pretreated petals are not full of cell fluids, but the polygon cell shape is preserved (step 1, Scheme 1). Shrinkage and deformation of cell skeletons occurs when they 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-containing petals. Upon increasing the concentration of TEOS, the siliceous depositions on the cell surface become thicker. Consequently, the SiO2 with different surface morphologies and porous structures was obtained after calcination (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 biomorphologies with periodic arrays of embossments in their profiles, which are similar to that of the fresh petals (step 1, 3480


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Figure 1. TEM images of petal cell-templated HS005 (a, b) and HS010 (c, d).

steps: convex at low relative pressure (p/p0 = 0−0.1), nearly linear in 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 limits at high relative pressure for all samples, indicating the presence of widely distributed mesopores (2−50 nm) and macropores.42 The pore sizes of all samples are primarily between 2 and 10 nm with an average of 4.5−6.8 nm (calculated from the adsorption branches, Figure 3b and Table S1), which are consistent with the TEM results. As listed in Table S1, the HSx samples synthesized with different TEOS concentrations have BET surface areas of 78− 177 m2 g−1 and pore volumes of 0.13−0.21 cm3 g−1. The HS001 with poor biomorphology and structure shows the

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 by 25, 125, and 150 cm−3 g−1, respectively.

lowest surface area, and the HS005 sample with good biomorphology exhibits the largest surface area and pore volume. BET surface area and pore volume of HS010 are not as high as those of HS005. We measured the N2 adsorption of HS010 twice, and the results are the same. The HS015

Figure 2. AFM images (a, b) and the corresponding height profiles of the HS005 sample. 3481


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biomorphology (Scheme 1, SEM a). Weight loss ends at 550 °C for HSxAs samples, indicating complete decomposition of the petal templates. Thus, 550 °C was chosen as the calcination temperature to remove the petal templates. FT-IR spectra for the calcined HSx samples synthesized with different TEOS concentrations show characteristic adsorption bands at approximately 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 asymmetric stretching of Si− O−Si [νs(Si−O−Si) and νas(Si−O−Si), respectively].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−O−H)]. Weak bands at 2912−2855 cm−1 ascribed to the saturated C−H stretching vibrations appear in all HSx samples,45,46 indicating a few residual C atoms exist in the samples. Additionally, the 1420 cm−1 characteristic band of CO32−47 emerges in the HS001 sample, which originates from the organic template. However, this carbonate adsorption band is invisible for other HSx samples (Figure S1). XPS was carried out to further analyze the possible elementary composition and their chemical states of the HS005 sample. Whole XPS scan spectrum demonstrates that HS005 consists mainly of O (18.6%) and Si (68.0%). A small amount of other elements, such as Ca (5.1%), C (4.9%), P (1.8%), and Cl (1.7%), coexist in the calcined HS005 (Figure 5a and 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 pretreatment process. The mineral elements including Ca and P originate from the raw petals even though they were pretreated with 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, 532.6, and 533.2 eV, respectively (Figure 5e). The first binding energy peak at 531.3 eV can be attributed to oxygen of the CO49,50 configuration of CaCO3, which is coincident with the highresolution XPS spectra of C 1s (288.3 eV)51 and Ca 2p3/2 (347 eV)52 (Figure 5c). Calcium carbonate probably originates from the decomposition of calcium oxalate that is distributed among all taxonomic levels of photosynthetic organisms.53 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−) bonds,54,55 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 hydrolyzed to inorganic phosphate.56 Adsorption of the 2D Porous HSx Samples to MB. The adsorption experiments of MB on HSx were carried out 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 h; then, the adsorption rate for MB slows in the subsequent hours. The adsorption equilibrium is established at approximately 8 h for HS005 and HS010 samples and 12 h for HS001 and HS015 samples. Moreover, the adsorption capacities of four HSx samples vary 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, whereas those of HS001 and HS015 are 59 and 65 mg g−1, respectively (Figure 6a). Besides the morphology, textural properties also

synthesized with the highest TEOS concentration has a 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. 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

Figure 4. TG (a) and DSC (b) curves of the HS001AS and HS010AS samples.

losses are around 97 and 80% for HS001As and HS010As, respectively. The weight loss before 200 °C (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 °C for HSxAs samples corresponding to two exothermic peaks in DSC curves (Figure 4b). The first step among 200−380 °C with an exothermic peak at ∼365 °C is ascribed to the charring of organic constituents in the petal cells. The other weight loss step with a corresponding exothermic peak near 450 °C can be assigned to the combustions of the charred matters. It is interesting to find that the second exothermic peak of HS001As (467 °C) moves toward a slightly higher temperature compared that of HS010As (436 °C). This is likely due to more organic constituents existing in HS001As than in HS010As. In fact, most compositions (97%) in the as-synthesized HS001 are from the camellia petals; thus, only 3% of solid mass remains after calcination (Figure 4a). The low SiO2 yield for HS001As also implies its poor replicated structure and imperfect 3482


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Figure 5. XPS spectra survey (a), atomic concentration (b), and high-resolution XPS spectra of Ca 2p3/2 and C 1s (c), Cl 2p3/2 (d), O 1s (e), and P 2p and Si 2p (f) for the HS005 sample.

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 isotherm and kinetic studies (Figure 6a and c). 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 a higher adsorption capacity than that of some of the other porous SiO2 materials reported in the literature (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. Thus, the graphene-based materials normally have much larger MB uptakes than that of graphite.17 Additionally, the adsorption capacity of the graphene oxide (GO) was found to be enhanced along with the increase in the oxidation degree and interplanar distance for GO.17 In our work, the 2D porous SiO2 nanosheets with nanometer-size thickness (2.7 nm for HS005) also have a high number of accessible pores and attachable adsorption sites, which are important for promoting adsorption capacity,65 as shown in Figure 6d. Consequently, HSx samples have better MB uptakes than that of SBA-15 even

affect the adsorption performance of the sample. HS005 with the largest surface area and pore volume has the largest adsorption capability, and HS001 with the lowest surface area and pore volume has the smallest adsorption capacity in four HSx samples (Figure 6a and Table S1). HS010 with good biomorphology and textural properties also has a high adsorption capability comparable to that of HS005. HS015 with the large surface area but relatively poor biomorphology shows an MB uptake larger than that of HS001 but lower than that of HS010 (Scheme 1, Figure 6a, and Table S1). Linear plots for the pseudo-second-order kinetics of MB adsorption onto HSx samples, parameters, and linear coefficients R2 obtained by fitting are presented in Figure 6b and Table S2. The calculated values of equilibrium sorption capacity Qe mod are identical to those obtained from experiments (Qe exp), and all of the values for R2 are greater than 0.999 (Table S2), suggesting that the sorption process of MB onto these lamellate porous silica samples (HSx) can be represented well by the pseudo-second-order model. 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-15,38 and nonporous SiO2. The adsorbed amounts of MB grow steeply when 3483


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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), and schematic diagram of MB adsorption on mesoporous SBA-15 and 2D lamellate HS005 nanosheets (d).

Table 1. Adsorption Capacities to MB for Some Mesoporous SiO2, 2D Graphene-Based Materials, and Other Related Materials Reported in the Literature mesoporous SiO2 materials Qe (mg g−1) graphene-based materials Qe (mg g−1) other related materials Qe (mg g−1)

MS6

MSA57

OMS58

HSA57

CM-259

AM-360

S16C-061

25 GO series17

60 MG@m-SiO218

54 MrGO/TiO219

38 Fe3O4-GS62

32 mGO/ PVA63 231 g-C3N469

30 MGC64 66 MLS70

345 CoFe2O4− FGS65 72 HSC71

2.5

31

149

49−599 cellulose−clay hydrogel45 783

140 PAni hydrogel66 71

425 keratin nanofibrous membrane67 170

426 PAA/ MnFe2O468 50

compared to those of mesoporous SBA-15 and nonporous SiO2. In practical industrial processes, wastewater may be acidic or alkaline. To investigate the effect of initial pH on the adsorption capacity for HSx samples, aqueous solutions were adjusted to pH values ranging from 3 to 11. Obviously, the adsorption performance of MB onto HS010 was significantly influenced by solution pH (Figure S2). The MB uptake continuously increases 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, and 11, respectively. Similar results are found in the MB adsorption process using other mesoporous silicas.61,72 The lower adsorption of MB under acid conditions is likely due to the presence of a large number of H+ ions in solution. At pH >2, the surface of SiO2 in aqueous solution (isoelectric point, pH 2) is negatively charged,73 which is beneficial for the

though their BET surface areas and pore volumes are lower than those of SBA-15.38 On the other hand, some of the residual inorganic species in the calcined HSx samples may also contribute to their good adsorption abilities, which need to be studied further in the future. The isotherm of MB adsorption on SBA-15 can be fitted well by either Langmuir or Freundlich isotherm models.38 Although 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 that the accessible sorption sites are energetically equivalent to the MB molecules during 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 larger adsorption capacities and stronger adsorption intensities 3484


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ACS Sustainable Chemistry & Engineering electrostatic interaction between the SiO2 surface and the positively charged dye molecule (MB+). In acidic conditions, a large number 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 increased (cH+ is decreased). Furthermore, the surface charge negativity of HS010 is enhanced with the increase in solution pH (see zeta potential results of HS010 in Figure S3), which will provide 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 increasing pH. Stability of the 2D porous SiO2 at a high pH value (pH 11) was investigated in recycling experiments. After the first three runs, the MB removal efficiency is slightly decreased from 88 to 83% (Figure 7). However, the adsorption ability of HS010 was

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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, and zeta potentials of HS010 at different solution pH levels (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: +86512-67374120. ORCID

Ruyu Cui: 0000-0002-1458-9691 Zhengying Wu: 0000-0002-9561-6275 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 at Shanghai Normal University for the XPS measurements.

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

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recovered after the fourth and fifth cycles. In general, the difference of adsorption amounts to MB between the five recycling cycles is small, indicating good stability and recyclability of the petal-derived 2D porous SiO2.

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CONCLUSIONS In summary, a unique porous SiO2 with two-dimensional (2D) microstructure has been fabricated successfully using renewable camellia petal cells as biological templates. Differing from the traditional hydrothermal approaches, this biomimetic process conducted in mild synthetic conditions provides a new scalable and sustainable method for developing 2D porous materials through learning from nature. The concentration of the siliceous source is crucial for the effective duplication of the petal cell structures. The special 2D microstructure and high porosity make this SiO2 have a higher adsorption capability than those of nonporous SiO2 and the commercial surfactanttemplated mesoporous material (SBA-15). This present study exploits a new and versatile technique for the design and synthesis of promising 2D porous materials using renewable natural resources.

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REFERENCES

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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/acssuschemeng.7b00170. Textural properties and FTIR spectra of the HSx samples, kinetic parameters calculated by the pseudosecond-order model for MB adsorption onto HSx 3485


Research Article

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Two-Dimensional Porous SiO2 Nanostructures Derived from

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