Calcareous Foraminiferal Shells as a Template for the Formation of

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Calcareous Foraminiferal Shells as a Template for the Formation of Hierarchal Structures of Inorganic Nanomaterials Mahmud Diab,†,‡ Karam Shreteh,† Michael Volokh,† Sigal Abramovich,‡ Uri Abdu,§ and Taleb Mokari*,† †

Department of Chemistry and Ilse Katz Institute for Nanoscale Science and Technology, ‡Department of Geological & Environmental Sciences, and §Department of Life Sciences, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel

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ABSTRACT: A microorganism template approach has been explored for the fabrication of various well-defined threedimensional (3D) structures. However, most of these templates suffer from small size (few μm), difficulty to remove the template, or low surface area, which affect their potential use in different applications or makes industrial scale-up difficult. Conversely, foraminifer’s microorganisms are large (up to 200 mm), consist of CaCO3 (easy to dissolve in mild acid), and have a relatively high surface area (≈5 m2 g−1). Herein, we demonstrate the formation of hierarchical structures of inorganic materials using calcareous foraminiferal shells such as Sorites, Globigerinella siphonifera, Lox-ostomina amygdaleformis, Calcarina baculatus or hispida, and Peneroplis planatus. Several techniques, such as thermal decomposition of single-source precursors of metal oxides or sulfides, reduction of metal salts directly on the surfaces, and redox reactions, were used for coating of different shell materials and several hybrid compositions, which possess nanofeatures. Finally, we examined the role of the prepared 3D structures on the reduction of 4-nitrophenol (4-NP), ethanol electrooxidation, and water purification. A remarkable performance was achieved in each application. The hierarchical structure leads to the reduction of 4NP within several minutes, a 27 mA cm−2 current density peak was obtained for ethanol electrooxidation, and more than 95% of the organic dye contaminants were successfully removed. These results show that using foraminiferal shells offers a new way for designing complex hierarchical structures with unique properties. KEYWORDS: microorganism template, foraminiferal, 3D structures, micro and nanostructures, hierarchical structures

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INTRODUCTION In the last 2 decades, the field of nanotechnology experienced tremendous progress; nowadays, nanomaterials can be readily synthesized in the desired size and shape, and their surface can be engineered and modified to fit the requirements of the desired application. However, assembly and scaling-up routes are still sought after to leverage the potential of nanostructures in various applications (e.g., capacitors, batteries, optic devices, and energy storage). Extensive efforts are invested to overcome this challenge via formation of hierarchal structures, for which different methods were developed such as self-assembly, lithography, and printing.1−4 However, most of these methods suffer from high cost and the necessity of removing the template, which further increases the cost and requires additional effort. Conversely, biotemplates are ready-to-use, and their three-dimensional (3D) structures are highly reproducible. Moreover, most biotemplates can be simply removed via treatment in an acid or a base because of their amphoterism. Nature is rich in microorganisms that possess unique hierarchal structures such as butterfly wings, bacteria, fungi, algae (especially the diatom), viruses, foraminifera, and others. © XXXX American Chemical Society

These templates have attracted attention in science and industry because of their possible utilization for sensors,5 drug delivery,6 (photo)catalysis,7,8 supercapacitors,9 solar cells,10 optical devices,11 and others.12−15 To date, there are various reports that describe a wide range of methods and techniques that use these templates to form 3D structures, such as hydrothermal, sonochemistry, vapor phase, sol−gel, gas/solid displacement, and others.16−21 For example, bacterial pili were coated with metal oxides (Al2O3, TiO2, and ZnO) by atomic layer deposition.22 Au microtubules were formed by exposing the fungal microwires (taken from Aspergillus nidulans) to Au ions followed by reduction and then template removal by extraction in supercritical CO2.23 Nanotubes of the tobacco mosaic virus (TMV) were coated with metal sulfides (CdS and PbS) via incubation of the TMV with cations followed by a reaction with H2S gas.24 Silicon microporous structures based on diatom frustule (Aulacoseira sp.) were formed via gas/solid displacement using Mg gas at 900 °C.5 Crab shells were used Received: December 19, 2018 Accepted: January 16, 2019

A

DOI: 10.1021/acsami.8b22138 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces to form a battery electrode, in which the crab was first coated with carbon followed by sulfur.13 Surprisingly, among the microorganism templates, foraminifera scaffolds have attracted less attention despite their enormous advantages. For example, these templates are abundant, consist of a nontoxic material (CaCO3), thousands of various morphologies exist, and encompass a wide range of sizes (0.1−200 mm). In addition, these templates can be easily grown and can be removed in a mild acidic environment. To date, only few reports have shown the potential of using the foraminifera shells in applications. Chou et al. described the formation of a 3D structure consisting of tricalcium phosphate (β-TCP) and Zn-TCP by using a Calcarina baculatus template (a foraminiferal species). This structure presents an impressive performance in drug delivery, where the drug was released in a controlled form.25−28 Recently, our group reported a general approach for designing 3D structures using a Sorites template (foraminiferal species) where metal, metal oxides, and metal hydroxide were formed [Co, MnO, α-Fe2O3, NiO, and Fe(OH)x]. These structures were used as electrodes for water oxidation and as a filter for water purification. A superior performance was recorded in both fields.7 The design of various hierarchal structures of inorganic materials based on foraminiferal shells as a scaffold is demonstrated in this work. The generality and simplicity of the formation approaches are established by (1) coating different foraminifera species with Co particles, (2) coating Sorites structures with metal sulfides, noble metals, and carbon, and (3) formation of complex multilayered structures by subsequent coating cycles with different materials. Furthermore, we explore the catalytic performance of Sorites@Co and Sorites@Co@Pt for the reduction of 4-nitrophenol (4-NP), Sorites@Co@Au as an electrocatalyst for ethanol oxidation, and the ability of Sorites@C to serve as a filter for water purification. Moreover, we discuss the contribution of the formed 3D structure to the performance in all the experiments.

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The products were washed three times with hexane and dried at 60 °C for 5 min. This procedure was repeated twice. Coating the Sorites with Metal Sulfide Nanostructures. Twenty milligrams of a single-source precursor (SSP) of metal bis(diethyldithiocarbamate) [M(dtcEt2)2], where M = Cd, Cu, or Pb (the SSPs were synthesized based on previously published reports)29,30 was dissolved in 1.0 mL of TOP. Then, a 25 mL beaker which contains the Sorites (10 mg) and 500 μL of the SSP stock solution was heated (on a heating plate inside a glovebox under a N2 atmosphere) to 270 °C for 30 min, except for copper sulfide, when it was heated to 300 °C. The product was cleaned with hexane and dried at 60 °C. This procedure was repeated twice. Coating the Sorites with Metal Nanostructures. (a) A metal salt (5 mg) (AuCl3, H2PtCl6, or AgNO3) was dissolved in deionized water (3.0 mL) and mixed with methanol (1.0 mL). Then, a 20 mL vial, which contains Sorites templates (18 mg) and 500 μL of the metal salt solution was irradiated using a UV light-emitting diode (Thor labs M365L2 driven at 400 mA, λmax = 365 nm) for 1 h. This procedure was repeated again while the Sorites were irradiated on their other side. (b) Sorites@Cu: ∼20 mg of Sorites was placed in 2 mL of CuCl2 solution (40 g L−1) for 1 h (the Sorites’ color turns blue, indicating the formation of copper hydroxide); then, 100 μL of hydrazine solution was added, followed by hand shaking for 1 min (the blue color changes to metallic brownish copper color). Sorites@Co@Metal. A metal salt (3 mg) (AuCl3, H2PtCl6, CuCl2, NiCl2, or AgNO3) was dissolved in ethanol (3 mL). Then, ∼8 mg of Sorites@Co was placed inside the metal salt solution and mixed overnight; the Sorites@Co species were utilized also as a magnetic bar for the mixing process. The product was cleaned with ethanol and dried at 60 °C. Sorites@C. A PVDF solution was prepared by dissolving 0.14 g of PVDF in 3.00 mL of NMP at room temperature (2 h mixing). Sorites (60 mg) was placed inside 1.5 mL of the PVDF solution overnight; then, the Sorites species were transferred to a ceramic boat. Subsequently, the ceramic boat was placed in the middle of a tube furnace and purged with Ar gas for 20 min at room temperature. In the beginning, the furnace was heated to 120 °C to evacuate water molecules, followed by evaporation of NMP at 202 °C for 30 min. The carbonization process was carried out at 700 °C for 30 min, at a heating rate of 8 °C min−1. Dye Degradation. Before all dye degradation experiments, three sets of 30 mg of Sorites (before acid treatment) were collected. Two of these sets were treated with HCl to from the 3D structure. The latter one was marked as a Sorites two-dimensional (2D) structure. Subsequently, Sorites 2D and 3D structures were coated with carbon. Three sets of 7 mL of the dye molecule were mixed with plain 3D Sorites and 2D and 3D structures of Sorites@C. The concentration of the dye was monitored using UV−vis absorbance at λmax (MB 664 nm, RhB 554 nm, and Rh6G 526 nm). 4-NP Reduction. NaBH4 (1.4 mL) (26.0 mM) was mixed with 1.4 mL of 4-NP (0.18 mM) inside a 4 mL cuvette; then, a 3D or a 2D structure of either Sorites@Co or Sorites@Co@Pt was added and manually shaken. The Sorites@Co was cleaned with Ar/H2-based plasma for 2 min (the plasma cleaning was carried out using a GV10x DS Asher source and a 95% Ar/5% H2 gas mixture). Ethanol Electrooxidation. The electrocatalytic activity of the Sorites@Co and Sorites@Co@Au was measured in a 0.5 M NaOH solution containing 1.0 M ethanol, using a PalmSens 3 potentiostat in a three-electrode system. The prepared 3D structures were used as the working electrode, a platinum wire served as the counter electrode, and Ag/AgCl in saturated KCl served as the reference electrode, separated by glass frits. Cyclic voltammetry was performed by applying a potential in the range between −0.4 and 0.45 V versus Ag/ AgCl at a scan rate of 20 mV s−1 Preparation of the Working Electrode. The Sorites@Co and Sorites@Co@Au active materials were pasted on a copper electrode using a CB slurry (100 mg of CB mixed with 100 mg of PVDF in 2 mL of NMP). The prepared electrode was used after drying at 200 °C for 30 min under an inert atmosphere.

EXPERIMENTAL SECTION

Materials. All reagents were purchased from their respective vendors and used without further purification. All solvents and hydrochloric acid (HCl, 32% wt) were purchased from Bio-Lab Chemicals and used as received. Cobalt acetate [Co(ac)2, 99.995%], hydrazine monohydrate (98%), methylene blue (MB), rhodamine 6G (Rh6G, 95%), silver nitrate (AgNO3, 99%), sodium borohydride (NaBH4, 99%), and sodium diethyldithiocarbamate (NaS2CN(C2H5)2, 99%) were purchased from Sigma-Aldrich. Copper(II) chloride (CuCl2, 98%), gold(III) chloride (AuCl 3, 99.99%), chloroplatinic acid hexahydrate (H2PtCl6·6H2O, 99.9%), nickel chloride (NiCl2, 98%), and trioctylphosphine (TOP, 97%) were purchased from Strem Chemicals. Carbon black (CB, >99%), hexadecylamine (HDA, 90%), polyvinylidene fluoride (PVDF), rhodamine B, (RhB), sodium hydroxide (NaOH, 98%), and sodium hypochlorite (11−14% available chlorine) were purchased from Alfa Aesar. 1-Methyl-2-pyrrolidone (NMP) was purchased from J.T. Baker. 4-NP (99%) was purchased from BDH Chemicals. Deionized (DI) water was purified using a Millipore Direct-Q system (18.2 MΩ cm resistivity). Sorites Pretreatment. The top layer of the Sorites was removed by immersion in a 0.05 M HCl solution for 2−3 min at room temperature. Coating the Foraminiferal Species with Co Particles. Five hundred microliters of cobalt solution (40 mg of Co(ac)2 dissolved in 1.0 mL of HDA) was mixed with the foraminiferal species (10 mg) inside a 20 mL vial. Subsequently, the vial was placed on a heating plate under a N2 atmosphere and was heated at 300 °C for 40 min. B

DOI: 10.1021/acsami.8b22138 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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in Figure 1 both in the high-magnification SEM images (a2−f2) and in the EDXS mapping (a3−f3). We have expanded the utilization of the foraminifera template to include metal sulfides, noble metals, polymer, and carbon as shown in Figures 2a−h, S1, and 3a−c. The Sorites shells were chosen as a prototype species to demonstrate the generality and simplicity of our approach. Different methods were utilized to coat the Sorites templates. For instance, thermal decomposition of the SSP in a coordinating solvent (TOP) was used to coat the Sorites with CdS, Cu2−xS, and PbS. A photoreduction process was used to reduce metal ions (Ag+, Au3+, and Pt4+) onto the Sorites surface. A combination of precipitation and reduction was used to form the Cu shell. Moreover, an organic polymer (PVDF) was homogeneously deposited onto the surface of the Sorites, followed by carbonization at 700 °C to form Sorites@C (see more details in the Experimental Section). Figure 2a−h shows the optical images of pristine Sorites and Sorites coated with CdS, Cu2−xS, PbS, Ag, Cu, Pt, and Au, respectively. The images clearly show that the morphology of the structures is preserved after the coating process. The crystalline phases of the Sorites and the coated materials were verified by XRD as shown in Figure 2i−p. The Sorites consist mainly of rhombohedral Mg0.1Ca0.9CO3, in which a new phase (CaCO3) was observed when the Sorites was annealed at high temperature (Figure S2). All the coated materials were crystalline, and their patterns match well with the bulk structures (face-centered cubic for PbS, Ag, Cu, Pt, and Au; hexagonal wurtzite for CdS). In the case of copper sulfide, the obtained pattern cannot be referred to a specific phase because of the overlap in the reflection positions of different copper sulfide phases. On the other hand, previous work showed that thermal decomposition of Cu(dtcEt2)2 in TOP (similar to our procedure) resulted in the formation of Cu2S nanoparticles.29 The homogeneity of the coating process was confirmed by high-magnification SEM images and EDXS mapping, as presented in Figure S3 and in the insets of Figure 2i−p. It is known that the formation of hybrid materials can lead to improved properties and may also result in new features. However, achieving 3D structures that consist of a hybrid material is still challenging. A foraminifera template, amongst the possible scaffolds, allows growing multiple layers of different types of materials while preserving the structure’s original morphology. Herein, we demonstrate a galvanic replacement approach to coat the Sorites@Co with various metals. The difference in the standard redox potential of Co/ Co2+ (0.282 V) and different metals (such as Ni, Pt, Cu, Ag, and Au) is sufficient to drive the redox reaction, in which the Co particle reduces the metal cations on the surface, as presented in Figures 3a,b and S4. Furthermore, a freestanding 3D metal structure can be obtained simply via incubation of the Sorites@Co@metal in 0.1 M HCl for several minutes. Figure S5 shows a hierarchical structure that consists of pure Au particles. The structure’s original morphology was preserved as shown in Figure S5a,b. EDXS analysis of the etched Au structure (after the template was removed) shows and confirms an almost perfect etchingthe elemental composition quantification of the spectrum (Figure S5c) shows >99 atom % Au. Potential Application of the Prepared 3D Structures. In this work, we examined the performance of the obtained 3D structures for three different applications: as a filter for water

Structural Characterization. Scanning electron microscopy (SEM) was performed using a JEOL JSM-7400F ultrahigh-resolution SEM system with a cold field emission gun, which was operated at 3.5 kV. Energy-dispersive X-ray spectroscopy (EDXS) analysis was carried out using a scanning electron microscopy-coupled Thermo Scientific Noran SIX EDX system. For EDXS analyses, an accelerating voltage of 15.0 kV was used. Phase analysis of the samples was carried out using the X-ray diffraction (XRD) method. The data were collected on an Empyrean powder diffractometer (Panalytical) equipped with a position-sensitive X’Celerator detector using Cu Kα radiation (λ = 1.5418 Å), operated at 40 kV and 30 mA. Optical absorbance measurements were made using a Cary 5000 UV−vis− NIR spectrophotometer.

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RESULTS AND DISCUSSION Conformal Coating of Sorties Shells with Various Inorganic Materials. Unique foraminifera species which have precise 3D structures such as Sorites, Globigerinella Siphonifera, Loxostomina amygdaleformis, Calcarina baculatus, Peneroplis planatus, and Calcarina hispida were coated with Co particles according to our previous report.7 Briefly, different morphologies of calcareous shells were immersed in a solution of sodium hypochlorite 12% to clean the traces of sediments from the surface or the pores (the Sorites were pretreated with HCl to remove the top layer). Next, the shells were transferred to a growth solution [40 mg mL−1 of Co(ac)2 in HDA] and heated at 300 °C for 40 min and then washed with hexane. This procedure was repeated twice (for more details, see the Experimental Section). We have achieved a conformal coating of Co nanostructures on the surface of the foraminiferal shells as portrayed in Figure 1. The thickness of the Co layer can be controlled by the concentration of the Co salt or by conducting multiple growth cycles using fresh Co solutions. The quality and homogeneity of the Co layer can be observed

Figure 1. Various foraminifera species coated with Co particles characterized using SEM and EDXS mapping. (a1−3) Sorites@Co, (b1−3) Globigerinella siphonifera@Co, (c1−3) Loxostomina amygdaleformis@Co, (d1−3) Calcarina baculatus@Co, (e1−3) Peneroplis planatus@Co, and (f1−3) Calcarina hispida@Co. C

DOI: 10.1021/acsami.8b22138 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. Structural characterization of the 3D structures, which were coated with metal sulfides and noble metals. (a−h) optical images of pure Sorites and Sorites coated with CdS, Cu2−xS, PbS, Ag, Cu, Pt, and Au, respectively. (i−p) XRD patterns and EDXS mapping (inset) of pure Sorites and Sorites coated with CdS, Cu2−xS, PbS, Ag, Cu, Pt, and Au, respectively. The red, black, and green sticks in the XRD panels correspond to the rhombohedral Mg0.1Ca0.9CO3, rhombohedral CaCO3, and to the relevant phase of the coated materials, respectively.

tion, presented in Figure S7 (black trace). A high rate constant (0.128 ± 0.001 min−1 for Sorites@Co) was derived directly from the slope. This value can be further improved by using a better catalyst than Co, such as Pt. Because the uniformity of the Co shell compares to the shells presented in Figure 2, we decided to use Sorites@Co@Pt. Figure 3d shows that Sorites@ Co@Pt (green trace) in the 4-NP solution results in the reduction of >98% of the 4-NP within 10 min. As expected, a higher rate constant was calculated from the respective slope in Figure S7 (0.371 ± 0.006 min−1). This value is considered high by comparison with other reported works.31−34 The enhanced catalytic activity can be attributed to the high surface area of the hierarchical structures and may also stem from the high metal dosage. Assuming a 100% reaction yield when Sorites@ Co@Pt was formed means that 1.4 mg of Pt was reduced, resulting in a metal dosage of 0.5 g L−1. To evaluate the contribution of the 3D structures in the reduction of 4-NP, we performed the reduction reaction under the same conditions while replacing the 3D structure of Sorites@Co@Pt with a 2D structure of Sorites@Co@Pt. The original structure of the Sorites scaffold is a 2D-like structure. Thus, using the same template to understand the role of the 3D structures should give the most accurate comparison. Two identical sets of Sorites species (8 mg each) were chosen in this experiment (specifically, same amount of Sorites, size, and total weight). One set was treated first with HCl to form the 3D structure, while the other set was used as-is. Then, the two sets were used to form the 2D and 3D structures of Sorites@Co@

purification, as an electrocatalyst for ethanol oxidation, and as a catalyst for the reduction of 4-NP. 4-NP Reduction. The catalytic reactivity of the prepared 3D structure was examined in the reduction of 4-NP to 4aminophenol (4-AP) in the presence of NaBH4 under ambient conditions. Because of the large surface area of the 3D structure, coating it with a catalyst material should result in an excellent performance. Figure 3d (red trace) and Figure S6b show that introducing the Sorites@Co to the 4-NP solution (in the presence of NaBH4) decreases the absorbance intensity of the 4-NP (at 400 nm) by >72% within 10 min and more than 92% within 20 min. The inset in Figure 3d shows clear evidence for the reduction process; the solution color turned unambiguously from yellow (indicating the presence of 4-NP with NaBH4) to colorless. The new peak, which appeared at 300 nm and continuously intensified with the progress of the reaction, indicates the formation of 4-AP. To explore the contribution of the template to the catalytic performance, pure Sorites replaced the Sorites@Co. Figure 3d (black trace) shows a plateau, indicating that the template alone has no apparent effect on the reduction of 4-NP to 4-AP. The 4-NP reduction experiment was carried out with an excess amount of NaBH4 (36 μmol) compared to 4-NP (0.25 μmol) to make the reduction reaction independent of the NaBH4 concentration. It also allows using a pseudo-first-order reaction kinetics approximation to calculate an effective rate constant for the catalytic reaction. A linear ln(C/C0) versus time plot confirms the pseudo-first-order kinetics approximaD

DOI: 10.1021/acsami.8b22138 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 3. (a−c) SEM images of Sorites@Co@Pt, Sorites@Co@Au, and Sorites@C, respectively. Insets in (a,b) show a high-magnification of each structure, while the inset in (c) shows an optical image of carbon-coated Sorites. (d) Reduction of 4-NP on Sorites (black trace), Sorites@Co (red trace), and Sorites@Co@Pt (2D blue trace, 3D green trace); the inset shows an optical image of the 4-NP solution before and after the reaction with the 3D structure. (e) Electrooxidation of ethanol Sorites@Co (black trace) and Sorites@Co@Au (2D green trace and 3D red trace). (f) Three different dye molecules (RhB, Rh6G, and MB bars in pink, orange, and blue, respectively) were adsorbed by plain Sorites and Sorites@C in 2D and 3D configuration, where the solutions’ concentration was measured before and after the filtration using optical absorbance. The bar graph in the inset shows the concentration of the dye molecules before and after filtration using a logarithmic scale.

enhancement. In this case, jf increased by a factor of 3. Furthermore, these values are on the scale of the highest reported values using gold electrodes.35,39,40 Moreover, the ratio jr/jf decreases from 0.35 to 0.27 when a 3D structure was used. The decrease in this ratio represents increased resistance to poisoning of the electrode in ethanol oxidation.35,40 However, we notice that electrooxidation of the ethanol required higher potential with a 3D structure than with a 2D structure (0.28 and 0.22 V, respectively). We believe that the need for high voltage can be associated to structure conductance. The 3D structure consists of Au particles with diameters of few hundred nanometers and with nonuniform spacing between the particles, which affect its performance.35−37 On the other hand, SEM images of the 2D structure show a continuous and close-packed film of Sorites@ Co@Au as presented in Figure S9. The morphology is responsible for the improved conductivity of the structure and the lower required potential. Heating the 3D structure of Sorites@Co@Au at 550 °C in air for 4 h results in reducing the overpotential to 0.17 V (similar to reported values); however, the current density decreased to 5 mA cm−2 (Figure S10a). The heating procedure at high temperature improves not only the conductivity of the gold 3D structure but also results in significant changes to the features of the 3D surface as portrayed in Figure S10b. This result supports the explanation that the decreased conductance of the 3D structure results in the need to apply a higher potential. Water Purification. Herein, we examined the removal of several organic dye molecules from water. The dye molecules were chosen as a case study because of the simplicity of evaluation of the purification process using UV−vis absorption. Previous reports show that carbon can be used to remove dyes (e.g., MB, Congo red, and Rh6G.).41,42 Three sets of 7 mL of dye solutions (Rh6G, RhB, and MB) were mixed with plain

Pt as presented in Figures S8 and 3a, respectively. Subsequently, the two sets were used for the reduction of 4NP (blue and green traces, respectively, in Figure 3d). Using the same active material (Sorites@Co@Pt) ordered as a 2D structure instead of a 3D structure results in the reduction of >82% of the 4-NP within 10 min (compared to >98% in the 3D case) and more than 96% within 20 min. The decreased catalytic activity is evident from the ∼56% lower apparent rate constant, 0.162 ± 0.003 min−1 versus 0.371 ± 0.006 min−1 as shown in Figure S7. The decreased activity can be assigned mainly to the lower specific surface area of the 2D relative to the 3D structures. Nevertheless, the 2D structure of Sorites@ Co@Pt still exhibits better performance (∼27% higher rate constant) than the 3D structure of Sorites@Co because of the importance of the Pt catalyst. Ethanol Electrooxidation. To examine the electrochemical performance of the obtained 3D structures, Sorites@Co@Au was used as an electrocatalyst. Recently, several papers studied the catalytic performance of Au nanoparticles toward alcohol electrooxidation.35−37 They found that Au nanoparticles exhibit a good catalytic performance. Moreover, it was reported that Au is stable for a long time unlike Pt, which exhibits poisoning effects with generated species through the electrooxidation process (e.g., CO).38 Figure 3e shows the cyclic voltammograms of Sorites@Co (black) and Sorites@ Co@Au (3D structurered trace and the corresponding 2D structuregreen trace) in 0.5 M NaOH and 1.0 M ethanol. The Sorites@Co electrode has a negligible performance. Conversely, a 3D hierarchical structure of Sorites@Co@Au (and the corresponding 2D) exhibited good performance, with current density peaks of 27 mA cm−2 (8.5 mA cm−2) and 7.4 mA cm−2 (3 mA cm−2) at forward (jf) and reverse (jr) potential scans, respectively. These results emphasize the impact of the active material arrangement on current E

DOI: 10.1021/acsami.8b22138 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS The authors thank the Interuniversity Institute for Marine Sciences of Eilat, Israel, for the logistic support of the field work and thank Prof. Menny Shalom for support with the electrochemical measurement system. M.D. also appreciates the Israeli Ministry of Science, Technology and Space scholarship.

Sorites and with both 2D and 3D structures of Sorites@C. Figure 3f presents the change in the dyes’ concentration before and after filtration. The initial concentrations of the RhB (pink bars), Rh6G (orange bars), and MB (blue bars) dyes were 13.5, 13.1, and 14.2 μM, respectively. After purification using plain Sorites, the concentrations were reduced to 12.8, 8.5, and 9.6 μM, respectively. Switching to Sorites@C 2D structurebased filtration resulted in concentrations of 1.7, 2.5, and 1.4 μM, respectively. Additional improvement was achieved using 3D structures of Sorites@C, resulting in final dye concentrations of 0.2, 0.7, and 0.1 μM representing degradation of more than 95, 98, and 99% of the initial concentrations of the respective dyes.

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CONCLUSIONS In conclusion, we have shown the potential of using foraminiferal shells as a scaffold to design 3D structures. Coating the template is general, that is, it is not restricted to a specific shape or a particular method because 6 morphologies and 5 different synthetic approaches were used to form more than 14 different compositions. Freestanding structures can be easily achieved by exposure to a mildly acidic environment. We have shown the potential application of the formed 3D structures in three different fields: reduction of 4-NP, electrooxidation of ethanol, and water purification. The role of the 3D structure was discussed and demonstrated to be necessary to achieve the reported high performance in each application. The 3D structure outperformed the 2D counterpart in the 4-NP reduction experiment (apparent pseudo-firstorder kinetic constant more than double), in ethanol electrooxidation (improved current density by more than a factor of 3), and in water filtration from organic dye contaminants (an order-of-magnitude reduction of the final contaminant dye quantity after filtration to nmol levels). We believe that these structures have other potential applications such as supercapacitors, sensors, surface enhanced Raman, antibacterial agents, masks for growth and assembly of nanoparticles, and much more. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b22138.

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Research Article

Optical images, XRD patterns of the Sorites structure, SEM images and EDXS spectra of Sorites coated by inorganic materials, and absorbance spectra of 4-NP and their fitting to pseudo-first-order kinetics (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Taleb Mokari: 0000-0001-7712-1589 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acsami.8b22138 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.8b22138 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX