Nanostructured Sulfur-Doped Porous Reduced Graphene Oxide for

Mar 29, 2018 - sulfur is exploited for the electrochemical detection and removal of Hg(II). ... adsorption process follows the Langmuir isotherm, and ...
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Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Nanostructured Sulfur-Doped Porous Reduced Graphene Oxide for the Ultrasensitive Electrochemical Detection and Efficient Removal of Hg(II) Bhaskar Manna and C. Retna Raj* Functional Materials and Electrochemistry Laboratory, Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, West Bengal, India S Supporting Information *

ABSTRACT: The selective detection and efficient removal of toxic Hg(II) are very challenging for environmental remediation and industrial processing. Herein, we demonstrate the selective electrochemical detection of Hg(II) and its removal using porous three-dimensional sulfur-doped reduced graphene oxide (pS-rGO). The thermal annealing of graphene oxide in the presence of dibenzyl disulfide and silica template at a controlled temperature of 900 °C yields pS-rGO, and it has a large surface area of 449.43 m2 g−1 and 9.96% sulfur in the carbon network. The favorable interaction of Hg(II) with pS-rGO owing to the porous structure and presence of a large amount of thiophenic sulfur is exploited for the electrochemical detection and removal of Hg(II). The selective electrochemical detection of Hg(II) is demonstrated at the potential of 0.2 V without any interference from coexisting other metal ions. The sensing platform could detect as low as 0.1 ppb (0.5 nM; S/N = 14) with a sensitivity of 11.98 ± 0.26 μA ppb−1 cm−2. The platform is highly stable, and only a 9% decrease in the initial current was noticed after 7 days of use. Removal of Hg(II) from water is successfully demonstrated with pS-rGO. It has a high Hg(II) uptake capacity of 829.27 ± 7.19 mg g−1, which is higher than the uptake capacity of undoped porous rGO and nonporous S-rGO. It can be repeatedly used at least four times without any compromise in the Hg(II) uptake capacity. The adsorption process follows the Langmuir isotherm, and the thermodynamic parameters (ΔG°, ΔH°, and ΔS°) obtained evidence that the adsorption is spontaneous and endothermic in nature. Practical utility is demonstrated by developing a prototype Hg(II) decontaminant column packed with pS-rGO, and the column could successfully remove 99.99% of Hg(II). The results obtained with a pS-rGO-based electrode and prototype decontaminant column are authenticated with atomic absorption spectroscopic measurements. The performance of undoped nonporous rGO toward detection and removal is inferior to that of pS-rGO. The remarkable performance of pS-rGO highlights the role of both porosity and S doping. KEYWORDS: Sulfur-doped rGO, Porous, Electrochemical Hg(II) sensing, Adsorbent, Hg(II) removal

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INTRODUCTION Mercury(II) is a highly toxic heavy metal ion, and it is an extremely harmful pollutant in aquatic ecosystems.1−4 Even trace exposure to mercury can cause kidney and respiratory failure, neurological damage, etc.5−7 Mercury can enter into the food chain through industrial human activities and the microbial biomethylation process. The industrial mercury poisoning in Minamata Bay8 is a horrific environmental disaster that brought neurological disorders, loss of motor function, etc. (Minamata disease) in humans who ingested mercury contaminated fish and shellfish. Human activities around the globe emit ∼10 000 tons/year of mercury,9 and it is a serious environmental concern, severely contaminating the atmosphere and water bodies.10 The U.S. Environmental Protection Agency (EPA) has set the maximum contaminant level of Hg(II) in drinking water to be 2 parts per billion (ppb), though the WHO limit is 6 ppb. Sensitive and selective detection and efficient removal of Hg(II) are very important in Hg(II) decontamination of drinking water and water bodies. The © XXXX American Chemical Society

detection of Hg(II) at the ppb level requires highly sensitive and selective sensors as the coexisting other metal ions and analytes should not interfere. On the other hand, the removal of Hg(II) requires economically viable materials of high uptake capacity. Development of inexpensive and simple analytical methods for the detection and removal of Hg(II) is of significant importance in the protection of public health and ecosystems. Various analytical methods such as atomic absorption, atomic emission, atomic fluorescence, ion coupled plasma mass spectrophotometry, microwave induced plasma atomic emission, X-ray fluorescence, neutron activation analysis, surface plasmon resonance,11−16 colorimetry,17−19 electrochemical,20 etc. have been developed in the past for Hg(II) detection. Time consuming tedious procedures and the requirement of Received: December 26, 2017 Revised: March 13, 2018

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DOI: 10.1021/acssuschemeng.7b04884 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Scheme Illustrating the Synthesis of S-rGO and pS-rGO

expensive instruments are some of the serious concerns for practical applications. Electrochemical methods draw significant interest due to their inherent high sensitivity and fast and easy on-site operation, and they do not involve tedious procedures and use of expensive instruments. Au-based electrochemical transducers have been extensively used for the sensing and quantification of Hg(II).20 The Au nanoparticle and Au nanoparticle modified with oligonucleotides are shown to have high sensitivity toward Hg(II).20−23 Recently, the graphene-Au nanoparticle-based transducer with amplification strategy has been used to achieve the electrochemical detection of Hg(II) at the attomolar level.24 Field-effect transistor-based sensors have also been developed for the sensing of Hg(II) at the ultratrace level using graphene and DNA/aptamer.25,26 Although these methods are highly sensitive, and the limit of detection has been achieved at the attomolar level, it requires careful handling of the delicate expensive biomaterials and involves multiple steps. A small compromise in the integrity of these biomaterials would not yield the expected results. It is highly desirable to develop simple analytical methods without using such biomaterials. The carbon-based materials having suitable binding sites are ideal for the electrochemical detection of Hg(II). For the removal of Hg(II), various methodologies such as ion exchange, chemical precipitation, coagulation, amalgamation, adsorption, etc. have been developed.27−31 The analytical methods based on adsorption are very promising as they involve simple procedures and are inexpensive and highly efficient. Traditionally, activated carbon, clays, zeolites, thiolfunctionalized materials of a large surface area, and metal organic frameworks have been used as adsorbents.31−33 Recently, graphene oxide (GO) and graphene-based materials and their composites are emerging as effective adsorbents for toxic metal ions.34 The composite materials based on GO and magnetic metal oxides have been widely used for the removal of Hg(II) and other metal ions. For instance, attempts have been made to remove Hg(II) using GO/Fe−Mn, GO/Fe3O4, etc. composites and thiol functionalized graphene oxide.32,35,36 However, their poor uptake capacity and lack of reusability limit their practical applications. The thiol functionalized materials are susceptible to oxidative dimerization to yield the corresponding disulfide, which has very weak binding capacity toward Hg(II). The ideal candidate should be inexpensive, have a large surface area, have a high affinity toward Hg(II), and have a high stability for repeated use. Porous sulfur doped reduced graphene oxide (pS-rGO) would be a promising candidate for the detection and removal of Hg(II). Sulfur has a high affinity toward Hg(II) due to the favorable soft−soft interaction. Unlike the thiol functionalized materials, the sulfur-doped reduced graphene oxide cannot undergo oxidative dimerization. Herein, for the first time, we demonstrate the selective and sensitive detection and efficient removal of Hg(II) using highly

porous sulfur-doped reduced graphene oxide as an electrochemical interface and an adsorbent. The results obtained with our materials are authenticated with traditional atomic absorption spectroscopic (AAS) measurement.

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EXPERIMENTAL SECTION

Reagents and Materials. Graphite, H2O2 (30%), dibenzyl disulfide (BDS), cetyltetramethylammonium bromide (CTAB), triblock copolymer F127 (Pluoronic 127, EO106PO70EO106), HgCl2, and tetraethyl orthosilicate (TEOS) were obtained from Sigma-Aldrich. All other chemicals used in this investigation were of analytical grade (99.9%) and used without further purification. All the solutions used in this investigation were prepared with Millipore water (Milli-Q system). Instrumentation. Fourier transform infrared spectroscopic (FTIR) measurements were performed with an RX1 PerkinElmer FTIR spectrophotometer. Raman measurements were performed with a HORIBA JOBIN YVON (France, model no. T64000) equipped with a thermoelectric cooled CCD detector. The samples were excited with an air cooled argon−krypton mixed ion gas laser of wavelength 514.5 nm with a power of 120 mW. X-ray diffraction (XRD) analysis was carried out with Panalytical high resolution X’pert PRO X-ray diffraction unit using Ni-filtered Cu Kα (λ = 1.54 Å) radiation. Xray photoelectron spectroscopic (XPS) measurements were performed with a PHI 5000 VersaProbe II scanning XPS microprobe (ULVACPHI, inc.) using an energy source of Al (Kα, hν = 1486.6 eV). Transmission electron microscopic (TEM) images were recorded with Analytical TEM (FEI-TECNAIG2 20S-TWIN) instruments operating at 200 kV. Energy dispersive X-ray spectroscopic (EDX) measurements were carried out with FEI-TECNAIG2 20S-TWIN instruments with EDX attachments (model no. 942409761751, standard 2). A carbon-coated copper grid (Pelco International, USA) was used for the sample preparation. Scanning electron microscopic measurements were performed using an FEI Nova NanoSEM 450 field-emission scanning electron microscope (FESEM) at 5 kV. All the voltammetric and amperometric measurements were carried out with a CHI643B electrochemical analyzer attached to a current booster (CH Instruments, Austin, TX). A two-compartment, three-electrode electrochemical cell with glassy carbon working electrode (0.07 cm2), platinum wire auxiliary, and Ag/AgCl (3 M KCl) reference electrode was used. Adsorption isotherms were measured for nitrogen at liquid nitrogen temperature (77 K) using a Micromeritics 3FLeX adsorption analyzer. The surface area from the adsorption data was obtained using the Brunauer−Emmett−Teller (BET) equation, and pore-size distribution was obtained from Barrett−Joyner−Hanlenda (BJH) analysis. An AAnalyst 700 atomic absorption spectrometer equipped with MHS-15 (PerkinElmer, USA) was used to measure the concentration of Hg2+ and to authenticate the validity of our results. Synthesis of GO and 3D pS-rGO. GO was synthesized from pristine graphite according to modified Hummers method37 (Supporting Information). The mesoporous silica template, MCM48, was synthesized using the known procedure38 (Supporting Information). The synthesis of 3D pS-rGO involves the thermal annealing of GO with a sulfur doping agent BDS in the presence of a silica template. In a typical procedure, GO (50 mg) and the structural template mesoporous silica (250 mg) were dispersed in water and subjected to sonication for 1 h. Water was removed from the mixture by evaporation, and the resulting brown crispy GO/SiO2 composite B

DOI: 10.1021/acssuschemeng.7b04884 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. FESEM (a) and TEM (b) images of pS-rGO. was mixed with BDS using a mortar and pestle. The weight ratio of BDS/GO/SiO2 was kept at 5:1:5, and the mixture was annealed at 900 °C in an Ar atmosphere for 5 h (Scheme 1). SiO2 nanoparticles in the annealed material were removed by hydrofluoric acid treatment, and black colored pS-rGO was obtained. The porous undoped rGO (prGO) was synthesized under identical conditions in the absence of doping agent BDS. S-rGO and rGO were also synthesized (Scheme 1) in the absence of a silica template in order to compare the results obtained with the pS-rGO. Sensing of Hg(II). Glassy carbon (GC) electrodes of 3 mm diameter were polished well with fine emery paper and alumina (0.05 μm) slurry and sonicated in Millipore water for 10 min to remove the physically adsorbed impurities. It was repeatedly washed with copious amounts of Millipore water and dried under ambient conditions. pSrGO or S-rGO (1 mg) was dispersed in 1 mL of ethanol by ultrasonication for 1 h. An aliquot of 10 μL of the dispersion was drop casted uniformly on the GC electrode and dried at room temperature. A square wave anodic stripping voltammetric (SWASV) technique has been used to detect/quantify Hg(II) under optimized conditions. The deposition of Hg(II) was achieved at the potential of −0.35 V (100 s) in 0.1 M KCl, and the stripping current was measured with the optimized parameters: frequency, 40 Hz; amplitude, 20 mV; potential increment, 4 mV. Removal of Hg(II). The adsorption of Hg(II) onto the adsorbent (pS-rGO) was investigated by adding the aqueous Hg(II) solution into the dispersion of the adsorbent in water and stirring for 5 h. The mixture was centrifuged, and the supernatant solution was collected for Hg(II) analysis. The adsorption experiments were carried out with different metal ion concentrations and times. The amount of Hg(II) removed was obtained by measuring the Hg(II) concentration before and after adsorption. The adsorption capacity, qe, of the materials was calculated using the equation39 qe =

(co − ce)V m

nm (Figure S1). At an elevated temperature, the oxygen functionalities of GO are removed and S is doped into carbon network.40 The dissolution of SiO2 in the composite produces a porous structure. The Raman spectral profile shows characteristic D and G bands (Figure S2). The D band is associated with the structural defects, and the G band corresponds to the E2g mode of vibration in the sp2 carbon domain. The intensity ratio of D and G bands (ID/IG) for pS-rGO is significantly high with respect to other synthesized materials (Figure S2). Such a high increase in this ratio reflects the increase in the defects due to the high degree of doping and 3D porous structure. More importantly, a significant down-shift (7−9 cm−1) in the G band was observed for S-rGO and pS-rGO. The G band of rGO appears at 1587 cm−1, while it appears at 1580 and 1578 cm−1 for S-rGO and pS-rGO, respectively. Such a down-shift in G band is characteristic of the n-type substitutional doping of carbon-based materials.41 The doping of S onto the graphene framework was further ascertained with XPS measurement (Figure S3). The surface survey XPS profile of GO (Figure S3A) shows signatures for C and O, whereas pS-rGO shows the presence of S, O, and C (Figure S3I). The deconvoluted C 1s spectrum of pS-rGO (Figure S3J) evidences the reduction of oxygen containing functionalities of GO, and doping of S onto the carbon network. The decrease in the intensity of the peaks corresponding to oxygen functionalities confirms the successful reduction of oxygen functionalities. The peak centered at 285.18 eV corresponds to sp3C, whereas the new peak observed at 285.63 eV is ascribed to the C−S moiety in the carbon network.42 The deconvoluted S 2p spectrum (Figure S3K) shows the S 2p3/2 (163.75 eV) and S 2p1/2 (164.96 eV) peaks corresponding to the −C−S−C− type of sulfur43,44 (thiophenic sulfur) in pS-rGO. The less intense peaks between 167.2 and 169.7 eV are attributed to the sulphoxo group (−SOx−) generated during the doping of S onto the rGO sheet.43,44 The weight percentage of sulfur in pS-rGO was quantified from the XPS profile and was 9.96%. The electron microscopic measurements show that pS-rGO has 3D porous features (Figure 1) ideal for the adsorption of Hg(II) ions. Such porous features were not observed for S-rGO and rGO (Figure S4). The surface area of pS-rGO was obtained from the adsorption isotherm, and the adsorption−desorption profile shows type IV features. The surface area of pS-rGO was found to be 449.43 m2 g−1 (Figure S5), which is significantly (>2 times) larger than SrGO (219.11 m2 g−1) and rGO (218.17 m2 g−1) and is close to that of p-rGO (Table S1). The use of a silica template favors such a large surface area for pS-rGO and prGO. Such a large surface area actually originates from the porous morphology of pS-rGO. The removal of SiO2 by HF leaching creates a porous

(1)

where co and ce are the initial and equilibrium concentrations of Hg(II) (mg L−1), V is the volume of the solution (L), and m is the mass of the adsorbent (pS-rGO; mg). For the practical demonstration of the removal of Hg(II), a small column made of glass (internal diameter: 6 mm) was filled with the pS-rGO up to a height of 4 cm. Then, an aqueous solution of Hg(II) (10 ppm) was poured into the column. The eluent was collected at a flow rate of 0.25 mL min−1, and the concentration of Hg(II) in the eluent was determined electrochemically using a pS-rGO modified electrode and by the AAS technique.

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RESULTS AND DISCUSSION Synthesis and Characterization of pS-rGO. The synthesis of pS-rGO involves the thermal annealing of a 5:1:5 weight ratio mixture of BDS, GO, and SiO2 in an inert atmosphere at 900 °C for 5 h. SiO2 functions as a template to obtain porous structure, and BDS is a S doping agent. The assynthesized SiO2 particles have a size distribution of 40 to 80 C

DOI: 10.1021/acssuschemeng.7b04884 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering morphology. As expected, BJH analysis shows that prGO and pS-rGO have a pore-size distribution from 30 to 800 Å (Figure S6), whereas GO, rGO, and S-rGO have a pore-size distribution from 30 to 40 Å (Figure S6). The capacitive property of GO and rGO-based materials highly depends on the electronic environment and the surface area of the materials. The capacitance of prGO and pS-rGO is ∼2 times higher than rGO and S-rGO (Figure S7). The higher capacitance of prGO and pS-rGO than those of rGO and SrGO is ascribed to the large surface area, and the porous morphology favors the facile diffusion of electrolytes. SWASV Sensing of Hg(II). The main focus of this work is to demonstrate the use of S-doped porous reduced graphene oxide in the voltammetric sensing and removal of Hg(II) and to highlight the role of (i) the sulfur heteroatom in the carbon network and (ii) the porosity and surface area of the material. It is well-known that the electrochemical sensing of Hg(II) largely depends on the surface morphology and chemical nature of the electrode materials. The porous materials capable of complexing/adsorbing Hg(II) are ideal for the development of the Hg(II) sensing interface. Figure 2A illustrates the analytical

network as well as the porous nature of the material. Though the surface area of prGO is close to that of pS-rGO, the magnitude of the peak current is significantly lower than that of pS-rGO, further highlighting the role of sulfur doped onto the carbon network. The stripping current on pS-rGO linearly increases with an increase in the concentration of Hg(II) (Figure 2B) with a sensitivity of 11.98 ± 0.26 μA ppb−1 cm−2. A linear response up to 600 nM has been achieved. The electrode could detect as low as 0.1 ppb (0.5 nM; S/N = 14). This is well below the maximum permissible level set by the U.S. EPA and WHO. The high sensitivity and low limit of detection indicate that pS-rGO is capable of detecting Hg(II) at well below the U.S. EPA and WHO level in drinking water. The long-term storage stability of the pS-rGO-based electrode was tested by subjecting the same electrode to Hg(II) detection with an electrolyte containing 100 ppb Hg(II) for 7 days. Only a 9% loss in the SWASV response was noticed after 7 days (Figure S8), highlighting the storage stability, and the electrode can be repeatedly used for the detection of Hg(II). The response of pS-rGO is highly reproducible, and a relative standard deviation of