Ultrathin Sulfur-Doped Graphitic Carbon Nitride Nanosheets As Metal

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Ultrathin Sulphur-Doped Graphitic Carbon Nitride Nanosheets as Metal-Free Catalyst for Electrochemical Sensing and Catalytic Removal of 4-Nitrophenol Chellakannu Rajkumar, Pitchaimani Veerakumar, Shenming Chen, Balamurugan Thirumalraj, and King-Chuen Lin ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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ACS Sustainable Chemistry & Engineering

Ultrathin Sulphur-Doped Graphitic Carbon Nitride Nanosheets as Metal-Free Catalyst for Electrochemical Sensing and Catalytic Removal of 4-Nitrophenol Chellakannu Rajkumar,† Pitchaimani Veerakumar,*,‡,§ Shen-Ming Chen,*,† Balamurugan Thirumalraj,†,¥, and King-Chuen Lin*,‡,§ †Department

of Chemical Engineering and Biotechnology, National Taipei University of

Technology, Taipei 10608, Taiwan ‡Department §Institute

of Chemistry, National Taiwan University, Taipei 10617, Taiwan

of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan

¥Department

of Chemical Engineering, National Taiwan University of Science and Technology,

Taipei, Taiwan Corresponding Authors *E-mail: [email protected] (S.-M. Chen); Tel.: +886-2-27017147; Fax.: +886-227025238 *E-mail: [email protected] (P. Veerakumar); Tel.: +886-2-23668230; Fax.: +886-223620200 *E-mail: [email protected] (K.-C. Lin); Tel.: +886-2-33661162; Fax.: +886-2-23621483

1 ACS Paragon Plus Environment

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ABSTRACT The graphene-like sulphur containing graphitic carbon nitride (S-GCN) nanosheets were successfully prepared and thoroughly characterized. A simple synthetic method by a thermal condensation approach was reported for preparation of S-GCN with trithiocyanuric acid (TCA) as precursor. The electrochemical performances of the 4-nitrophenol (4-NP) sensor were assessed by cyclic voltammetry (CV), amperometry and differential pulse voltammetry (DPV). Ultrathin SGCN nanosheets have been employed to enhance the electrocatalytic activity showing an excellent remarkable electrochemical behavior towards 4-NP. We thus obtained a wide linear response range from 0.05–90 µM, relatively low detection limit (0.0016 µM) and excellent sensitivity in 0.1 M acetate buffer (ABS, pH 5.5), surpassing the existing modified electrodes in the literature. Moreover, the fabricated S-GCN-electrode is selective over in the presence of many potentially interfering species. As a results, the S-GCN contains (C with N and S) heteroatoms which probably induced the higher electrocatalytic activity and electrical conductivity behavior towards 4-NP. Besides, the structural defect to generate more active sites on the surface of S-GCN could boost the fast electron transfer is provoked during the reduction of 4-NP. As a consequence, it is probably sensitive as well as quantitative detection of 4-NP in real samples. S-GCN was also applied to the hydrogenation of 4-NP by NaBH4 under ambient conditions. Thus, implementation of S-GCN nanosheets offers the advantages of simplicity, reliability, durability, and low-cost.

KEYWORDS: graphitic carbon nitride, 4-nitrophenol, cyclic voltammetry, chronoamperometry, differential pulse voltammetry


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INTRODUCTION Recently, graphitic carbon nitride (GCN) has attracted widespread attention due to its outstanding and unique chemical, electronic and ionic conductivity, photosensitive, electrocatalytic properties as well as increased applications.1-4 Due its low cost, light weight, easy availability and remarkably high stability, it has been applied in water splitting,5 photocatalysis6 sensing7,8 and bio-imaging,9 etc. The polymeric nature of GCN is suitable for electrocatalytic materials, which have been used in a wide range of electrochemical sensing of biomolecules,10-13 phenolic compounds14 and toxic metal ions.15 To enhance its electro and photocatalytic applications, the preparation of these catalytic active materials should be economic and technically feasible.16 Besides, the development of inexpensive, metal-free nanocatalysts is nowadays the prime need in both academic as well as industrial research.17 However, the GCN is one of the most prominent material, because of its suitable band gap  Eg = 2.70–2.90 eV of visible light, the corresponding to wavelength of ( = 460–430 nm), with low electrical conductivity (~0.9 ×10-9 S m-1).18-20 In order to enhance the catalytic performance, the Eg of GCN can be narrowed by sulfur doping. In this manner, Eg of 2.91 eV for GCN was reduced to 2.79 eV for S-GCN, estimated from UV-absorption measurements of ~ 430 nm and ~445 nm for GCN and S-GCN, repectively.21 The sulfur doping effect changes the band gap by stacking its 2p orbitals on the valence band (VB) of bulk GCN. Both GCN and the S-GCN nanosheets belong to metal-free semiconductors, which have served as great optical and electrochemical sensing platforms.22-27 Currently, 4-nitrophenol (4-NP) as a water contaminant and is easily found in the waste water from the industrial effluents, dyes or explosives.28 It is well known that, harmful to human being because of its carcinogenicity, mutagenicity,29,30 and potential damage to the central nervous system, liver, kidney and blood, even in a trace level. Thus, how to remove 4-NP effectively from polluted wastewater is an important and urgent environmental issue. 4-NP was listed as pollutants of the US Environmental Protection Agency (EPA), allowing the upper limit of 4-NP in drinking water only at 60 ppb.31,32 However, the treatment of 4-NP by common methods are mostly ineffective, by using such as enzyme-linked immunosorbent assay,33 high-performance capillary zone electrophoresis,34 flow-injection reflectometry,35 high performance liquid chromatography (HPLC),36 and UV-vis spectrophotometry.37 To date, electrochemical technique have offered great



prospective for 4-NP determinations,38 owing to good sensitivity, environmental friendliness, lowcost, and easy operation etc.39 In the present work, we investigated first time the electrocatalytic behaviour of sulfurdoped graphitic carbon nitride nanosheets which is used to modify screen printed electrode (SGCN/SPCE) towards the 4-NP reduction. The S-GCN nanosheets-modified SPCE thus obtained offer several merits such as a facile and fast electron transfer nature, an extensive π-conjugated electronic structure of S-GCN, and good electrocatalytic activity against the 4-NP. Moreover, such a modified electrode is found to show better detection limit with great sensitivity in monitoring the 4-NP concentrations with respect to various modified electrodes available in the literature. EXPERIMENTAL SECTION Chemicals. Trithiocyanuric acid (C3H3N3S3, 95%, Sigma-Aldrich), melamine (C3H6N6, 98%, Sigma-Aldrich), and 4-nitrophenol (4-NP, 98%, Sigma-Aldrich), were obtained commercially and used without further purification. The supporting electrolyte used for all the experiments was pH 5 acetate buffer solution (ABS, 0.1 M), prepared by using CH3COONa and CH3COOH. All the other pH solutions were adjusted with aqueous solution of 0.5 M H2SO4 and 0.1 M NaOH. All the solutions were freshly prepared using Milli-Q water. Preparation of S-GCN Nanosheets. Sulphur-doped graphitic carbon nitride (S-GCN) nanosheets were prepared by the calcination of trithiocyanuric acid (TCA) in a horizontal furnace at 550 °C for 5 h in static air (temperature ramp rate of 5 °C min-1).40 Typically, 5 g of GCN was prepared under the same experimental procedure using with melamine (MA) as an alternative of TCA. The obtained bulk yellow powder like materials was collected for further usage. Initially, the substance was grounded by mortar pestle, then well dispersed in DI water and was ultra-sonicated for six hours and finally centrifugation (10 000 rpm) for ~10 min. Afterward, light yellow coloured phase in the supernatant was collected and marked as GCN and S-GCN nanosheets (Scheme 1), while the remaining part was discarded.


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Scheme 1. Schematic Diagram of the Preparation Route and Application of the S-GCN

Electrochemical Measurements. In this work, the electrochemical behavior of GCN and S-GCN nanosheets were evaluated by a three-electrode systems. A computerized electrochemical analyzer (CH instruments; CHI 1205b and CHI410a) workstation was used for the electrochemical analysis; a screen printed carbon electrode (SPCE), Pt wire, and a saturated Ag/AgCl electrode were used as a working, counter, and reference electrodes in this experiment. An inert (N2) gas was purged with electrolyte at least 30 min before each measurement. The electrochemical behaviors of the as-prepared composites were examined by electrochemical impedance spectroscopy (EIS, IM6ex ZAHNER, Kroanch, Germany). S-GCN-modified SPCE (hereafter denoted as S-GCN/SPCE) was prepared by the following procedure. Typically, ~2.0 mg of catalyst was dispersed in 1.0 mL water and sonicated for 2 h. Subsequently, 8.0 μL of aliquot was dropped onto SPCE, allowed to dry in air, followed by gentle heating in hot air oven for 20-30 min.



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RESULTS AND DISCUSSION Structural Properties of S-GCN. Powder X-ray diffraction (PXRD) was examined to monitor the crystal phase of MA, TCA, GCN and S-GCN samples (Figure 1a). Both MA and TCA samples shows triclinic layer-like structure, and these layers are interconnected by van der Waals forces.41 Typically, the strongest peak for GCN and S-GCN ca. 27.6° exhibits the characteristic interlayerstacking of aromatic system (002), which provide an interplanar distance (d = 0.325 nm), indicating both samples have better crystallinity. A small diffraction peak at ~13.1◦ was indexed to the (100) plane and assigned to the in-plane aromatic structural packing with an average interplanar distance of d = 0.677 nm. As a result, the two samples (GCN and S-GCN) had similar diffraction peaks in the XRD patterns, indicating that they owned similar crystal structures, despite production by different precursors.21,22 The N2 sorption measurements were conducted to evaluate the BET surface areas and pore size distributions of the as-prepared GCN and S-GCN samples. (Figure 1b, Table 1). The two highly similar N2 adsorption isotherms were of type IV isotherm (cf. Brunauer–Emmett–Teller) with a hysterias loop. Barrett-Joyner-Halenda (BJH) pore-size distribution (Figure 1b, inset) demonstrated that the mesopores (3–140 nm) existed in both samples, confirming the presence of meso/macropores.42 Furthermore, the GCN has a surface area of 48.3 m2 g-1, which is much higher than that of bulk GCN (8 m2 g-1) and the data was not shown. The surface area (SBET), total pore volume (VTot), and pore size (Dp) of the GCN and S-GCN sample was listed in Table 1. Table 1. Textural Parameters, Elemental Compositions for the Samples Studied sample

color

SBETa

VTota

Dpb

content (atom)

BGc

(m2 g1) (cm3 g1)

(nm)

C%

N%

H%

S%

C:N

(eV)

S-GCN

yellow

46.8

0.021

11.8

38.07

60.05

1.02

0.84

0.63

2.66

GCN

yellow

48.3

0.019

11.5

38.61

60.32

1.06



0.64

2.72

aSurface

area from BET method and total pore volume (VTot) calculated at P/P0 = 0.99. bAverage

pore size determined by BJH method. cBand gap calculated from ultraviolet-visible diffuse reflection spectrascopy (UV–DRS).


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Elemental analysis was conducted by CHNOS elemental analyzer (Table 1). The elemental composition was calculated for S-GCN in the atomic ratio of (C, 38.07%), (N, 60.05%), (H, 1.02%) and, (S, 0.84%), respectively; whereas the corresponding result for GCN was 38.61%, 60.32% 1.06%, and 0%, respectively. Between S-GCN and GCN, a decrease in C content and C:N ratio was found from 38.61 to 38.07 % and from 0.64 to 0.63%. The thermal stability of MA, TCA, GCN and S-GCN samples was investigated by thermogravimetric analysis (TGA). All the samples were heated from ambient temperature to 900 °C

at a rate of 10 °C min−1 under air, and TGA curves are shown in Figure 1c. As visualized in

Figure 1c MA and TCA were decomposed quickly within a narrow range of temperatures (from 280 to 350 °C), under air atmosphere.41,43 Typically, ~5–8% weight loss at 100 °C was caused by bound water molecules, whereas weight loss (82–94%) at ~545–640 °C showed an insignificant residual left for the GCN and S-GCN samples; their primary decomposition occurred at ∼510 °C and complete burning at ∼660 °C, which is in good agreement with an earlier report.44 Fourier-transform infrared (FT-IR) analysis is studied to identify the molecular configuration of GCN and S-GCN nanosheets, which are displayed in Figure 1d. The spectra of two sample (Figure 1d) show absorption bands at 1200–1600 cm-1, being typical –C=N stretching vibrations of aromatic heterocyclic rings. The peak at ~800 cm-1 region is due to the breathing mode of the tri-s-triazine units for the two samples.42 However, the FTIR spectra of S-GCN nanosheets had no distinct difference from those of GCN nanosheets, probably because the S content in S-GCN was low and thus its related vibrational spectrum was weak and buried in that of C-N. Indeed, it is evidenced that the unique C-N network remained in GCN nanosheets. In comparison, the FT-IR spectrum of MA shows the absorption at 3330 cm−1 and 3190 cm−1 corresponding to the secondary amine (-NH-) stretching vibrations, (Figure S1, SI). The peaks at 3134–3470 and 1652 cm-1 can be attributed to the primary amine (-NH2) stretching and bending vibrations. Vibrational bands at 1420-1600 cm-1 are assigned to the triazine (C-N) ring in MA molecules, whereas a sharp peak observed for the pristine GCN at 808 cm-1, which is ascribed to out-of-plane skeletal bending modes of the tri-s-triazine units.45 In the FT-IR spectra of TCA (see Figure S1, of the Supporting Information; hereafter denoted as SI), the following major peaks have been identified as N-H stretching vibrations at 2900–3160 cm-1, N-H deformation (1568 cm-1), cyanuric ring vibrations (1537 cm-1), C-N stretching (1363 cm-1) and, C=S stretching (1126 cm-1), which are consistent with those reported in literature.46 7 ACS Paragon Plus Environment


The electronic structures of GCN and S-GCN nanosheets were also characterized by UV– vis diffuse-reflectance spectroscopy (UV-DRS) (Figure 1e) and photoluminescence (PL) spectra, as shown in (Figure 1f). Two absorption peaks are found to center at 340 nm, and one absorption band in the 400-580 nm range indexing to a band gap energy of about 2.72 eV (Figure 1e).47 The absorption peak ~ 300–400 nm is assigned to π–π* transition in the conjugated ring systems, including heterocyclic aromatics. The feature near 500 nm is ascribed to the n–π* transition involving lone-pair electrons on the edge N atoms of the triazine rings, showing a stronger absorption intensity for the S-GCN than the pristine GCN. The Kubelka–Munk plots in Figure 1e (insert) show that the adsorption edge is red shifted with lower bonding energy of 2.66 eV for the S-GCN nanosheets, which is in accordance with the literature.48 PL spectra of GCN and S-GCN nanosheets are given in Figure 1f; the inset shows the photographs taken under max =365 nm UV excitation. Upon irradiation at 365 nm, the two samples yield the PL peaks at around 460 nm, as a result of recombination of electrons and holes, consistent with variation of the band gap energy.49

Figure 1. (a) XRD patterns, (b) N2 adsorption/desorption isotherms, (c) TGA curves, (d) FT-IR spectra, (e) UV-DRS (Inset: plots of αhν2 vs. hν) and (f) PL spectra (insert: photographs taken under max =365 nm UV excitation).


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The PL intensity of S-GCN was apparently lower than GCN, but the result was not easy to explain at the moment. The recombination rate of charge carriers may be influenced by several factors such as concentration, the amount of O2 resolved in the solution, element doping effect, and so on. However, the catalytic activities of GCN was promoted with doping of a little sulfur (S-GCN).50 The morphology and microstructure of GCN and S-GCN samples were examined with field emission-transmission electron microscopy (FE-TEM) as shown in Figure 2a-f. As compared to their parent bulk material which contains solid agglomerates with a size of several micrometers, they are characteristic of many irregular pore sizes with the diameters from 20-50 nm. (Figure S2, SI). The morphology of products obtained after ultra-sonication shows a unique folded graphenelike structure composed of spatially interconnected nanosheets and enormous macrospores with a few hundreds of nanometers (Figure 2d-f).

Figure 2. FE-TEM images of the GCN (a-c) and S-GCN (d-f) with different images. The bars represent (a,d) 0.5 µm, (b,d) 0.2 µm, and (c,f) 100 nm. Elemental mappings of C, N, S and O as well as the overlay images (Figure S3a–e, SI) were further examined to confirm that S atoms are uniformly distributed on the S-GCN nanosheets, in agreement with the X-ray photoelectron spectroscopy (XPS) results. Atomic force microscopy (AFM) was employed to examine the morphology of the S-GCN nanosheets, showing the well9 ACS Paragon Plus Environment


defined sheet-like S-GCN nanosheets as shown in Figure S4a, SI. The topographic height of the S-GCN nanosheets was around 14.7 nm (Figure S4b, SI), indicating the presence of monolayers. XPS analysis was investigated for the chemical composition of GCN and S-GCN nanosheets and elucidation of the corresponding chemical states of the elements (Figure 3). As shown in Figure 3a, the binding energy (B.E) of C 1s and N 1s between these two samples remains approximately the same, indicating that their chemical states of carbon (C 1s) and nitrogen (N 1s) are almost identical. The peak intensity of N element is stronger than those of C and O elements; the C:N ratio for GCN is identical to the theoretical result based on the previous reports.44 The high resolution C 1s XPS spectrum of the S-GCN sample in Figure 3b was deconvoluted into three obvious peaks at 284.5, 286.6, and 288.2 eV. The weak peak at the B.E 284.5 and 286.6 eV was assigned to C-C, and N-C3 bonds, whereas the main peak at 288.2 eV was ascribed to sp2-bonded carbon atom of N=C-N.45–47 As can be seen in Figure 3c, the O 1s peak can be fitted into two peaks at B.E of 531.4 and 532.3 eV for S-GCN sample, which can be attributed to C–O, and C=O coordination, respectively.

Figure 3. (a) XPS spectra of the GCN and S-GCN samples and the corresponding core-level spectrum of (b) C 1s, (c) O 1s, (d) N 1s, (e) S 2p, and (f) solid-state 13C NMR spectra.


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In Figure 3c, the high resolution N 1s XPS spectrum of S-GCN sample was split into three peaks centered at 398.5, 399.6, and 400.6 eV. The former two peaks are stronger and attributed to the tertiary N bonded to carbon atoms in the form of N-C3 and sp2 hybridized aromatic N bonded to carbon atoms of C-N=C, while a board weak peak at 400.6 eV corresponded to the amino functional groups with hydrogen (C-N-H).51 According to Figure 3e, the S 2p spectrum consisted of two major peaks centered at 165.4 and 164.3 eV, belonging to the C–S–C structure. These two peaks could be attributed to the S 2p3/2 and S 2p1/2 spin-orbit states of the C–S covalent bond, confirming existence of S in the S-GCN nanosheets.52 The structure of GCN and S-GCN nanosheets examined by 13C NMR spectra shows two peaks at ~156.5 and 164.4 ppm, which were ascribed to the resonances for the -CN3 and -CN2(NH2) groups of the heptazine units (Figure 3f). These results confirmed the formation of a poly(tri-striazine) structure which is characteristic of graphitic carbon nitride (GCN).53 Electrocatalytic Activity of S-GCN/SPCE for Reduction of 4-NP. Generally, electrochemical impedance spectroscopy (EIS) techniques were used to evaluate their electrochemical behavior. The electrocatalytic performance of the S-GCN/SPCE-modified SPCE was investigated by (EIS) using 5 mM [Fe(CN)6]3-/4- with 0.1 M KCl as the supporting electrolyte vs. Ag/AgCl. The EIS profiles recorded for the bare SPCE, GCN and S-GCN-modified SPCEs are depicted in Figure 4a. A large well defined semicircle at high frequencies is achieved at the GCN-modified SPCE, signifying a less ionic conductivity nature of GCN. In contrast, the lower charge transfer resistance with higher electronic conductivity behavior was observed at S-GCN/SPCE.54 This fact evidenced that the doping S atom can change the electronic properties on the surface of the GCN (S-GCN). Figure 4b presents the cyclic voltammetry (CV) curves recorded in the respective presence of 100 µM of 4-NP analyte in N2 saturated 0.1 M ABS (pH 5.5) with a scanning rate of 100 mV s-1. From the CV investigation, a small reduction peak current intensity of 4-NP can be noticed at bare SPCE due to weak adsorptive properties on the SPCE surface. Essentially, the adsorption process toward the electrode depends on diffusion motion and mobility. However, the bare SPCE cannot attract firmly the 4-NP reactants on the electrode surface. In contrast, for GCN/SPCE and S-GCN/SPCE, the adsorption process is facilitated by electrostatic attraction, apart from diffusion and mobility. Further, the catalyst offers enough site to attract more amount of the reactants with a reduced impedance. Such a reduction peak current increases significantly at -0.8 V for SGCN/SPCE, because some active nitrogen species (pyrolytic and pyridine) stimulate the 11 ACS Paragon Plus Environment


electrocatalytic activities towards the 4-NP. Among them, the S-GCN/SPCE exhibits an excellent reduction and oxidation behavior with higher reduction peak currents at lower reduction potential. The reduction peak current was enhanced at positive shift of reduction peak potential, suggesting that the S-GCN/SPCE exhibits effective electrocatalytic ability towards the reduction of 4-NP, owing to high active surface area, excellent electronic characteristic, good conductivity ability and so on. In addition, the facile electron transfer at the electrode is accelerated, because S atoms create the defects and active sites upon on the GCN surface reducing the space between electrode and electroactive species by the - stacking interaction made between S-GCN and an aromatic structure of 4-NP.55,56 We proposed the electron transfer mechanism which involves the irreversible catalytic reduction of 4-NP tends to lose four electrons and four protons (4H+, 4e-) at first to form 4-hydroxylaminophenol followed by the reversible two electrons and two protons (2H+, 2e-) oxidation-reduction reaction to form 4-nitrosophenol (Figure 4d).32

Figure 4. (a) Electrochemical impedance spectra (Nyquist plots), (b) CV responses of bare SPCE, GCN and S-GCN-modified SPCEs in 100 µM 4-NP recorded at a scan rate of 100 mV s-1 under N2 saturated 0.1 M ABS (pH 5.5) electrolyte, (c) the corresponding plot of electrode material concentration vs current response, and (d) the oxidation-reduction pathway of the 4-NP in the presence of S-GCN nanosheets.


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Effect of Catalyst Dosage, Electrolyte pH and Scan Rate. The S-GCN concentration casted on modified SPCE is crucial towards the reduction of 4-NP. As displayed in Figure 4c the reduction current response gradually rises with the S-GCN concentration from 2 to 5 mg mL-1 owing to increasing accumulation capability of analyte on the surface of the electrode. GCN concentration increases above 5 mg mL-1, the reduction current becomes saturated and then decreases gradually; this fact indicates that the higher concentration of S-GCN made a thick film on the electrode which might resist the diffusion of 4-NP. Hence, the S-GCN concentration is optimized at 5 mg mL-1 for the further electrochemical studies. in the presence of Since the electrocatalytic performance of the S-GCN/SPCE electrode also depends on the pH value of the supporting electrolyte, CV curves recorded in the respective presence of 100 µM of 4-NP analyte in N2 saturated in ABS and varied pH range from 4.5–7.5 of the electrolyte were also studied, as shown in Figure 5a. The reduction peak current greatly enhances with increasing pH and a maximum oxidation peak current was observed with an electrolyte pH of 5.5 over the SGCN/SPCE. While further increasing pH (5.5–7.5) the reduction peaks current contrariwise decreased. As such, a pH value of 5.5 was chosen as an optimal for subsequent electrochemical studies. Figure 5b shows the plot of pH (4.5–7.5) against the irreversible reduction of 4-NP peak irreversible reduction potential (Epc) and peak current (Ipc) at the S-GCN/SPCE; the Ipc apparently shifted negative direction with linear. On the basis of electrochemical studies, a linear regression equation with R2 = 0.9959 was achieved for the 4-NP reduction, which may be expressed as: Epc (V) = −0.0523 pH-0.4121. It is evidenced that an equal number of electrons and protons are transferred the electro-reduction reaction at S-GCN/SPCE.57 CV measurements over the S-GCN/SPCE were recorded in the presence of 4-NP (100 µM) analyte at different scan rates (10–100 mV s-1; Figure 5c) at pH 5.5. The reduction peaks current an enhanced with increasing the scan rate from lower to higher scan rate (10–100 mV s-1), while the irreversible reduction peak potential also shifted to more negative values. This fact evidenced that the electrochemical reduction of 4-NP is an irreversible process over surface of S-GCN/SPCE. Figure 5d shows that irreversible cathodic peak current is proportional to the square root of scan rate with a correlation coefficient (R2) of 0.9987; a linear regression equation may be expressed as: Ipc (µA) = 0.02921/2 (mV s−1) + 0.8205. The above results shown in Figure 5d clearly indicate that the S-GCN-modified SPCE is a desirable nanocomposite material for electrochemical reduction of 4-NP with a typical diffusion-controlled process occurs. Moreover, the relationship 13 ACS Paragon Plus Environment


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between (Epc) and the natural logarithm of scan rate (ln), apparently the Epc changed linearly with ln with a linear regression equation with a correlation coefficient (R2) of 0.9967, which may be expressed as: Epc (mV) = 0.0692 ln v + 0.8205 at varied scan rates (10–100 mV s−1), as shown in Figure S5 (SI).

Figure 5. (a) The CV curves of 100 µM 4-NP was recorded at a scan rate of 150 mV s-1 as a function of pH at S-GCN/SPCE under N2 saturated 0.1 M ABS at varied pH from 4.5–7.5, (b) the plot of pH (4.5 to 7.5) against the irreversible reduction of 4-NP peak potential (Epc) and peak current (Ipc) at the S-GCN-modified SPCE, (c) CV curves of the S-GCN/SPCE in N2 saturated 0.01 M ABS (pH 5.5) containing 100 µM 4-NP at varied scan rates (10–100 mV s-1) and, (d) the corresponding correlations between the peak current and scan rate. Reaction Kinetics and Electrochemical Performances. To investigate the heterogeneous electron rate constant (Ks) of 4-NP was calculated by the following Eq. (1)58-60 𝐾𝑠 = 2.415exp

(

)

―0.02𝐹 1/2 𝐷 (𝐸𝑝 ― 𝐸𝑝/2) ―1/2𝑉1/2 𝑅𝑇


(1)

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where Ep is the peak potential; Ep/2 is the potential at which the current equals a half of the peak current, and the other symbols have the usual meanings. Using above equation, Ks was calculated to be 9.05×10-3cm s-1. Figure 6a displays the amperometric (i-t) response for S-GCN/SPCE in N2 saturated 0.1 M ABS (pH 5.5) in the with and without the addition of 4-NP. Interestingly, the current responses were significantly increased when an increased concentration of 4-NP was added as shown in Figure 6b, indicating the facial electron transfer occurring in an electrocatalytic reduction of 4-NP over S-GCN/SPCE. According to Cottrell equation53 given in eq. 2, a plot of Ip as a function of t-1/2 for each concentration of 4-NP (Figure 6c) yields the diffusion coefficient of 4-NP: 𝐼𝑝 = 𝑛𝐹𝐴𝐶𝑏𝐷1/2𝜋1/2𝑡 ―1/2

(2)

where Ip denotes the current (in A), n is the number of electron, F = 96,485 C mol-1 is the Faraday constant, A is the geometric surface area of the electrode (cm2), Cb represents bulk concentration of the analyte (mol L-1), D is the diffusion coefficient (cm2 s–1), and t is the time (s). Accordingly, by taking the eq 4, the value of D = 2.42 × 10–6 cm2 s–1 for 4-NP from the slop (Figure 6d).



Figure 6. (a) Amperometric responses of the S-GCN/SPCE in N2 saturated 0.1 M (ABS, pH 5.5) in the with and without addition of 4-NP, (b) different concentration of 4-NP: 10, 20, 30 and 40 µM L-1, (c) plots of current vs. time−1/2 and (d) the corresponding calibration plot of slop vs. 4-NP concentration. Figure 7a shows DPV profiles of the S-GCN/SPCE in 0.1 M (ABS, pH 5.5) with different concentrations of 4-NP. As expected, it can be clearly seen that the reduction peak current was gradually increased with the successive addition of 4-NP from the concentrations of 0.05-90 μM, which indicated the rapid electro-reduction of 4-NP at S-GCN/SPCE. In addition, the corresponding linear regression equation can be represented as Ipc (µA)=8.642 µM+ 2.884 with the correlation coefficient of 0.9942, as shown in Figure 7b. The sensitivity and limit of detection (LOD) was estimated to be to be 12.655 µA µM-1 cm-2 and 0.0016 µM, based on the standard formula as (LOD = 3(SD/slope)). The analytical sensing performance of the proposed sensor was compared with previously reported 4-NP sensor, and the results are summarized in Table. S1 (SI). These findings demonstrate that the SGCN/SPCE showed an excellent electrocatalytic activity, good linear range, and lower LOD toward the reduction of 4-NP.


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Figure 7. (a) DPV response of the S-GCN/SPCE under consecutive addition of 4-NP within a total dosage range of 0.05-90 µM, (b) the corresponding calibration plot of response current vs. 4-NP concentration, and (c) Interference study at S-GCN/SPCE with various interference species. All measurements were conducted in N2-saturated ABS (pH = 5.5). Reproducibility, Stability and Interference of the 4-NP Sensor. To estimate an electrode reproducibility, prior to analysis, ten different modified electrodes were prepared and investigated by CV in the presence of 0.1 mM of 4-NP-containing ABS buffer. The relative standard deviation (RSD) was estimated to be 3.05% demonstrates a good reproducibility for the method. The stability was inspected by acquisition of the electrode response with same concentration of 4-NP with 10 days interval; the current response was found to decrease to 92% (after 10 days), 89% (after 15 days) and remain stable at 86% of its original response even after 30 days. The selectivity of the sensor was examined using a S-GCN/SPCE by CV in N2-saturated ABS (0.1 M, pH 5.5) solution with 4-NP together with interferents. A 10-fold excess of some possible interfering substances NB, DNB, DNT, and RS were examined with together with 4-NP at S-GCN/SPCE. Only some electroactive species with the same nitro and phenol groups yield slight interference. In contrast, a 50-fold excess of some inorganic ions Na+, Cu2+, Br−and SO42− have no interference with the detection of 4-NP as shown in Figure 7c. These results further conclude that the proposed sensor exhibits an excellent anti-interference ability toward the detection of 4-NP. The good results of reproducibility, stability and interference tests indicated that S-GCN/SPCE is obviously suitable for analytical application. Hence, the proposed metal-free S-GCN/SPCE shows excellent storage stability, good repeatability, and reproducibility toward the detection of 4-NP Real Sample Analysis. To evaluate the feasibility of an electrode material, the S-GCN/SPCE was examined in real sample analysis. We can use different sewage water samples like industrial, domestic, and underground samples for practical application. The unknown concentration of the 4-NP containing sewage water sample was studied by DPV using the standard addition method under the same experimental condition for Figure. 7a. From this analysis, the succeeded recovery values are summarized in Table 2. In addition, we have also performed the quantitative analysis 4-NP using the high-performance liquid chromatography (HPLC) method. Moreover, the obtained recovery values are compared with traditional HPLC method for concluding the accuracy of the



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proposed sensor. Based on these results, our proposed electrochemical method could be more suitable than the traditional HPLC method towards the sensing of 4-NP in various water samples.

Table 2. Determination of 4-NP in Different Sewage Water Sample at S-GCN/SPCE by DPV sample

added (µM)

found (µM) proposed HPLC methoda

methodb

recovery (%)

recovery (%)

industrial

2.0

2.01

2.06

100.5

103.0

sewage

2.0

1.99

2.04

99.5

102.0

domestic

2.0

1.96

1.99

98.0

99.5

dewage

2.0

1.98

2.01

99.0

100.5

underground

2.0

1.99

2.01

99.5

100.5

water

2.0

2.01

2.03

100.5

101.5

aProposed

electrochemical method; bHPLC method; All measurement is an average of n = 3.

Additionally, the detailed description of the application of metal-free catalytic reduction of 4-NP with help of S-GCN catalyst is referred to Figure S6 and Figure S7 in the SI. In SI, the UV-vis spectra of 4-NP reduction before and after the addition of NaBH4 solution was recalled and the reduction mechanism of 4-NP were also discussed. The catalytic rate constant (k) of 4-NP is plotted as At/A0 vs. reaction time in the presence of GCN and S-GCN as a metal-free catalysts. Compared with our k value, we have compiled the catalytic performance of our catalyst system with other unsupported heteroatom doped metal-free catalysts are compared as listed Table S2 (SI). Notably, our catalysts also show excellent catalytic performance for the reduction of 4-NP in to 4aminophenol (4-AP), showing superior catalytic activity in this work. CONCLUSIONS 18 ACS Paragon Plus Environment

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In summary, a S-GCN based electrochemical reduction of 4-NP was successfully demonstrated for first time. The S-GCN/SPCE has excellent electrocatalytic performance towards 4-NP reduction due to large surface area, excellent electronic characteristic, good conductivity ability, facile electron transfers, and abundant exposed activity sites. Hence, their multiple characteristic of S-GCN/SPCE demonstrated a good linear range in the concentration range (0.05– 90 µM), a LOD of 1.6 nM, with a good current sensitivity of 12.655 µA µM-1 cm-2. In addition, SGCN/SPCE achieved an excellent reproducibility, stability, selectivity for detection of 4-NP, and applicable to 4-NP detection in different sewage water samples and achieved satisfactory recoveries. However, the experimental results have clearly shown that the metal-free S-GCN are better than GCN and commercial activated carbon® in the presence of NaBH4 under same experimental conditions. These results designated that S-GCN nanosheets was reliable, simple, cost-effective, ease of preparation and a good candidate for practical applications. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental results from FT-IR, FE-TEM, CV and UV-vis studies (PDF) AUTHOR INFORMATION *Corresponding Authors *E-mail for S.-M.C.: [email protected] * E-mail for P.V.: [email protected] *Email for K.-C. L.: [email protected] ORCID Shen-Ming Chen: 0000-0002-8605-643X Pitchaimani Veerakumar: 0000-0002-6899-9856 King-Chuen Lin: 0000-0002-4933-7566 Notes The authors declare no conflict of interest. 19 ACS Paragon Plus Environment


ACKNOWLEDGMENTS The authors are grateful for the financial support (MOST 106-2113-M-027-003-MY3 to S.M.C; MOST-102-2113-M-002-009-MY3 to KCL) from the Ministry of Science and Technology (MOST), Taiwan.

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Sulfur-doped graphitic carbon nitride (S-GCN) is proved to perform as an efficient metal-free electrochemical catalyst for the Sensing of toxic 4-nitrophenol.


Ultrathin Sulfur-Doped Graphitic Carbon Nitride Nanosheets As Metal

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