Article pubs.acs.org/Langmuir
Cysteine-Assisted Tailoring of Adsorption Properties and Particle Size of Polymer and Carbon Spheres Nilantha P. Wickramaratne,† Vindya S. Perera,† James M. Ralph,‡ Songping D. Huang,† and Mietek Jaroniec*,† †
Department of Chemistry and Biochemistry, Kent State University, Kent, Ohio 44242, United States Saint-Gobain NorPro, 3840 Fishcreek Road, Stow, Ohio 44224, United States
‡
S Supporting Information *
ABSTRACT: A series of cysteine-stabilized phenolic resinbased polymer and carbon spheres were prepared by the modified Stöber method. Cysteine plays a very important role in the proposed one-pot synthesis of the aforementioned spheres; namely, it acts as a particle stabilizer and a source of heteroatoms (nitrogen and sulfur) that can be introduced into these spheres. The diameter of these spheres can be tuned in the range of 70−610 nm by adjusting the cysteine amount and reaction temperature. Since polymer spheres obtained in the presence of cysteine contain sulfur and nitrogen heteroatoms, they were tested for adsorption of copper ions. It is shown that adsorption isotherms recorded for copper ions can be well fitted by Langmuir equation, giving unprecedented adsorption capacities up to ∼65 mg/g.
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INTRODUCTION During the past decade porous and functionalized spheres, including carbon, polymer, silica, and metal oxide, have been actively studied because of their potential applications in drug delivery and cell imaging,1−4 energy storage,5−8 catalysis,9,10 adsorption,11,12 sensors,13,14 and particle templates.15−17 Among these materials, silica spheres gained a lot of attention due to their simple and well-controllable processing based on sol−gel chemistry.18−20 Another advantage of silica spheres is their easy functionalization through postsynthesis grafting based on the reactivity of their silanols with organosilanes.21 However, the postsynthesis grafting adds additional synthesis steps, is time-consuming, and uses expensive chemicals; thus, it is less feasible from an industrial perspective. Until recently, polymer/carbon spheres have been also synthesized using complicated methods like chemical vapor deposition (CVD)22,23 and nanocasting.15,24,25 In general, CVD process takes place in the presence of metal oxide or rare earth metal oxide as catalysts; thus, the purification of carbon spheres is required to remove the metal oxide. Nanocasting method employs silica spheres as a template, which needs to be removed with either NaOH or HF solution. Also, the CVD and nanocasting methods are time-consuming, costly, and thus unfeasible from an industrial viewpoint. It was also shown that the nanocasting strategy is often inappropriate method for the fabrication of polymer/carbon nanospheres due to the aggregation and cross-linking of silica spheres employed as a template and the carbon precursors used.26 Recently, a tremendous effort has been directed toward the synthesis of functionalized polymer spheres. Despite numerous © 2013 American Chemical Society
strategies and products reported, the synthesis of functionalized polymer spheres with controlled particle size has been achieved with a limited success. For instance, polyaniline and polystyrene spheres have been reported but were not examined for adsorption-based applications.27,28 Dong et al.29 reported the synthesis of polymer and carbon spheres (diameter ranging from 30 to 650 nm) using resorcinol/formaldehyde as polymer precursor30 and L-lysine as a basic catalyst. It was shown that the particle size of these spheres can be tuned by changing the molar ratios of catalyst/water and resorcinol/water. However, these polymer and carbon spheres coagulated during the synthesis process, and their dispersion was difficult. Wohlgemuth et al. reported a one-pot hydrothermal synthesis of sulfurand nitrogen-containing carbon microspheres in the presence of waste biomass and amino acid.31 However, these particles were nonuniform and were not examined for adsorption of heavy metal ions. Recently, Liu and co-workers showed that the Stöber synthesis can be extended to the preparation of polymer/ carbon spheres using resorcinol and formaldehyde as precursors.32 It was shown that the diameters of these spheres can be tailored by changing the molar ratio of the catalyst/ solvent/precursors. Later, studies carried out by Choma et al. and Fuertes et al. showed that this method can be further extended to the synthesis of carbon spheres with incorporated metal nanoparticles, silica−carbon core−shell spheres, and Received: January 30, 2013 Revised: March 4, 2013 Published: March 5, 2013 4032
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Figure 1. SEM and TEM images of (A, D) PS*-30, (B, E) PS-0.2-30, and (C, F) PS-0.4-30.
hollow carbon spheres.33,34 So far, there is no report on the Stöber synthesis of heteroatom-containing polymer and carbon spheres. Here we report the synthesis of heteroatom-containing (nitrogen and sulfur) phenolic resin spheres (PS) using Stöber method in the presence of cysteine, which acts as both particle stabilizer and heteroatom precursor. To the best of our knowledge, this work reports the first use of cysteine as a particle stabilizer for the synthesis of phenolic resin-based polymer spheres. We demonstrate that the Stöber synthesis of phenolic resin spheres in the presence of cysteine can be used to incorporate sulfur and nitrogen heteroatoms to these spheres. Further, it is shown that the particle size of these heteroatom-containing polymer spheres can be tailored by adjusting the cysteine amount and reaction temperature. Elemental analysis (EA) data indicate that the amount of sulfur in PS can be enlarged by increasing both the cysteine amount and reaction temperature. Further, the EA data confirm the relatively high loading of sulfur and nitrogen in PS, up to 5.4 and 3.1%, respectively. The size of these spheres can be tailored in the range from about 70 to 600 nm. Importantly, the cysteine-stabilized PS showed very high Cu2+ adsorption capacity, up to ∼65 mg/g.
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PS-x-T and without cysteine as PS*-T, where PS refers to polymer spheres, “x” denotes the cysteine amount used (in grams), and T refers to the reaction temperature. In order to obtain the carbon spheres, PS were subjected to thermal treatment at 600 °C. Thermal treatment was performed in flowing nitrogen in the tube furnace using a heating rate of 1 °C/min up to 350 °C, dwell for 2 h, and resuming heating rate of 1 °C/min up to 600 °C and dwell for 4 h. The resulting carbon spheres were labeled as CS-x-T and CS*-T in the case of synthesis with and without cysteine, respectively; symbols “x” and “T” refer to the amount of cysteine (in grams) and reaction temperature, respectively, whereas “CS” refers to carbon spheres. Adsorption Measurements for Copper Ions. A stock solution of copper (300 ppm) was prepared by dissolving CuSO4 in deionized water. Then dilutions were made with deionized water to obtain working solutions that contain different concentrations of copper (1, 5, 25, 50 100, 150, and 300 ppm). The as-prepared polymer spheres (PS*-30 and PS-0.4-70, 0.025 g) were added to 25 mL of copper solution, and the mixture was stirred for 1 h. Next, an aliquot of the sample was taken out and centrifuged to separate solid and liquid. Finally, the concentration of copper in the supernatant was analyzed by atomic absorption spectrophotometery. The difference in the initial and equilibrium concentration of copper was used to determine the amount of adsorbed copper ions. Equilibrium adsorption isotherms for copper ions on the PS*-30 and PS-0.4-70 samples were measured twice, and the average values of adsorption were used to plot these isotherms. Measurements of Adsorption Kinetics of Copper Ions. The adsorption kinetics of copper ions was examined by using 25 mL of 100 ppm copper solution. 25 mg of polymer particles was added to the above copper solution under stirring. Next, aliquots of copper solution were taken out at different time intervals and centrifuged. The supernatant was diluted with 2% HNO3 acid, and three repeated atomic adsorption spectroscopy (AAS) measurements were made for each aliquot. The average values of the AAS data were used to obtain the concentration of copper ions. Characterization of Polymer and Carbon Spheres. TEM images were obtained using FEI Tecnai F20ST/STEM instrument operated at 200 keV. The preparation of samples for TEM analysis involved their sonication in ethanol for 2 to 5 min and deposition on a 400 mesh lacy carbon coated copper grid. Nitrogen adsorption isotherms were measured at −196 °C on ASAP 2010 volumetric
EXPERIMENTAL SECTION
Synthesis of Polymer/Carbon Spheres. Polymer spheres (PS) were synthesized using a slightly modified recipe reported in our previous study.35,36 Namely, an aqueous−alcoholic solution was prepared by mixing 16 mL of ethanol and 40 mL of distilled water at different temperatures. Subsequently, 0.2 mL of 25 wt % ammonia was added under continuous stirring. Then, 0.4 g of resorcinol was added and stirred until a complete dissolution. Next, various amounts of cysteine were added followed by slow addition of 0.6 mL of 37 wt % formaldehyde and stirring for 24 h at a given temperature. Finally, the reaction mixture was transferred to a 125 mL capacity Teflon container and placed in a sealed metal autoclave vessel, which was placed in an oven at 100 °C for 24 h. The solid product (polymer spheres) was obtained by centrifugation and dried at 100 °C for 12 h. The resulting PS obtained in the presence of cysteine were labeled as 4033
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Table 1. Cu Adsorption Parameters, Elemental Composition and Particle Size of the Samples Studieda sample
DPSc (nm)
Qm (mg/g)
k × 10−2 (min g/mg)
PS*-30 PS-0.2-30 PS-0.4-30 PS-0.4-50 PS-0.4-70 PS-0.4-90 PS-0.2-70 PS-0.3-70 PS-0.3-90
∼610 ∼550+ ∼400# ∼200# ∼120# ∼90+ ∼350#
15.4
4.34
+
64.8 59.4b 29.9b
2.86 3.84 3.65
N% ± std 2.12 2.50 2.32 2.40 3.10 2.50 2.13 3.02 2.50
± ± ± ± ± ± ± ± ±
0.02 0.02 0.03 0.01 0.04 0.12 0.07 0.02 0.08
S% ± std 1.92 4.67 4.81 5.40 5.22 3.10 4.61 4.75
± ± ± ± ± ± ± ±
0.05 0.23 0.02 0.14 0.28 0.03 0.05 0.14
a
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