Facile Synthesis of a Nickel Sulﬁde (NiS) Hierarchical Flower for the Electrochemical Oxidation of H 2 O 2 and the Methanol Oxidation Reaction (MOR)

The synthesis of a novel hierarchical ﬂower-like NiS via a solvothermal method for the electrochemcial oxidation of H 2 O 2 on a carbon paste electrode with high catalytic activity for the (MOR) in an alkaline medium has been reported. Novel nickel sulﬁde (NiS) hierarchical ﬂower-like structures were characterized by X-ray diffraction, scanning electron microscope, and transmission electron microscopy. A carbon paste electrode was modiﬁed with the as-prepared hierarchical ﬂower-like NiS, resulting in a high electrocatalytic activity toward the oxidation of H 2 O 2 . The NiS-modiﬁed electrode was used for H 2 O 2 sensing, which was achieved over a wide linear range from 0.5 μ M to 1.37 mM (I/ μ A = − 0.19025 + 0.06094 C/mM) with a low limit of detection (LOD) of 0.3 μ M and a limit of quantitation (LOQ) of 0.8 μ M. The hierarchical ﬂower-like NiS also exhibited a high electrocatalytic activity for the methanol oxidation reaction (MOR) in an alkaline medium with a high tolerance toward the catalyst-poisoning species generated during the MOR. The MOR proceeded via the direct electrooxidation of methanol on the oxidized NiS surface layer because the oxidation peak potential of the MOR was more positive than that of the oxidation of NiS.

Hydrogen peroxide (H 2 O 2 ) has attracted a great attention due to its importance as a reactive oxygen species and its involvement in chemical, biological, food-processing, medical, diagnostic, and environmental related fields. 24,25 Due to its key role, the determination of H 2 O 2 have arisen a great interest, hence many approaches have been developed, such as optical, titration-based, and electrochemical methods. [26][27][28] Among these methods, electrochemical analysis is particularly promising due to its many advantages such as low cost, easy to manipulate and fast analysis. 29 Although many nanostructures-based electrocatalysts such as prussianblue-grafted carbon nanotube/poly (4-vinylpyridine) composites, 30 gold nanostars, 31 35 have been employed for this purpose, the preparation of the hierarchical nanostructure with high surface area is still a challenge for fabricating an efficient sensing platform for H 2 O 2 .
Pt-based materials are currently the most common electrocatalysts used as anodes in direct methanol fuel cells (DMFC) due to their high electrocatalytic activities. 36,37 However, Pt suffers from a high price and tendency to be poisoned by CO that occupies the active sites of the Pt via adsorption and block the transportation of z E-mail: baiqingyuan1981@126.com; c.fernandez@rgu.ac.uk methanol in its oxidation, limiting its utility in commercial applications. Therefore, the development of cost effective catalysts is highly desirable. Ni-based compounds are interesting alternatives to Pt due to their relative low costs and high electrocatalytic activity, which have attracted considerable recent attention 38,39 due to the Ni (II)/Ni (III) redox reactions. 40 To the best of our knowledge, this is the first time that the synthesis of a novel hierarchical flower-like NiS via a solvothermal method for the electrochemical detection of H 2 O 2 on a carbon paste electrode with high catalytic activity for the MOR in an alkaline medium has been reported. The high catalytic activity for the MOR of the NiS nanostructures as an effective electrocatalysts in an alkaline medium opens a new potential application in DMFC.
Apparatus.-The phase of the hierarchical flower-like NiS structure was determined on a Rigaku D/max2550VB X-ray diffractometer (XRD). Scanning electron microscopy (SEM) images were obtained with a Hitachi SU8010 scanning electron microscope. Transmission electron microscopy (TEM) images and high-resolution TEM (HRTEM) images were obtained on an FEI Tecnai G2 F20 transmission electron microscope at an accelerating voltage of 200 kV. The specific surface areas of the samples were measured by nitrogen adsorption on a Gemini VII 2390 Analyzer at 77 K using the volumetric method. A CHI 842C electrochemical workstation (Austin, TX, USA) was used for all electrochemical experiments with a conventional three-electrode system, which included a NiS-modified carbon paste electrode (CPE) as the working electrode, a platinum coil as an auxiliary electrode, and an Ag/AgCl (saturated KCl) as the reference electrode. , and diethylamine (50 μL) was sucessively injected to form a micellar system under vigorous stirring. After stirring for 5 min, excessive sulfur power (4.8 mg) was then added to ensure the loss of sulfur during the reaction, and the mixture was stirred for 20 min. This mixture was transferred to a stainless steel autoclave, which was sealed, heated to 200 • C, and maintained at this temperature for 12 h. The autoclave was cooled to room temperature, the upper layer yellow liquid has obvious stratification with the black solid deposited on the bottom. The liquid was pour away and the products were washed by ethanol and separated by centrifugation at 8000 rpm for 10 min, which ensured the full collection of the product. The black NiS powder can be ensured by being vacuum dried at 60 • C for 1 h.

Fabrication of NiS modified CPE.-A carbon paste containing
of 75:25 graphite powder/liquid paraffin, was packed firmly into one end of a glass tube (1.8 mm inner diameter) to fabiricate the bare CPE. Electronic connection to the CPE was made via a copper wire. To prepare NiS suspension, 2 mg NiS powder was ultrasonically dispersed in water for 10 minutes. For electrode modification, 5.0 μL of a NiS suspension (2 mg/mL) was drop-coated onto the bare CPE, which was then dried in an ambient atmosphere.

Results and Discussion
Characterization of the hierarchical flower-like NiS nanostructures.-Figs. 1a, 1b shows the typical SEM images of a NiS sample at different magnifications. Hierarchical flower-like structures with sizes ranging from 300 to 500 nm are clearly visible in the lowmagnification SEM image. Meanwhile, the high-magnification image (Fig. 1b) reveals that these aggregates are composed of many uniformly distributed nanosheets with the thicknesses of ca. 18 nm. The structure of the flower-like NiS was further investigated by TEM (Fig.  1c) and HRTEM (Fig. 1d). According to these TEM images, the NiS product consisted of multiple nanosheets. The distance of the lattice finger is 0.197 nm, which corresponds to the d spacing of the (102) face of a hexagonal NiS phase (Fig. 1d).   Table I. Overall, the NiS-modified electrode displayed a wider linear range and lower limit of detection for H 2 O 2 than the other electrodes.
Reproducibility and repeatability tests of the NiS-modified electrodes revealed a relative standard deviation of 5.2% for ten successive measurements with the same electrode and 6.8% for measurements  with five different electrodes, illustrating the acceptable reproducibilities and repeatabilities of the electrodes. The long-term stability of the NiS-modified electrode was also investigated under continuous operation. After 800 s of continuous operation, 88% of the initial current response was maintained. After being stored in air for two weeks, the electrode had an 8% decrease in current response, indicating its relatively high long-term stability. The presented method was applied for the detection of H 2 O 2 in disinfectant sample (2.7-3.3%). The disinfectant sample was diluted by water with a ratio of 1:30, after which 6.0 μL of the diluted sample solution was injected into the stirring 0.1 M NaOH solution (8 mL) and detected by amperometric method. The value of H 2 O 2 concentration was found to be 2.9%, which was in accord with the value that obtained by permanganimetric method (3.1%).
Methanol electrooxidation.-The scan-rate dependence of the CVs of the NiS-modified CPE was also investigated, as shown in Fig.  5. Well-defined redox peaks were clearly observed at the different scan rate from 0.01 to 0.5 V/s and the currents increased proportionally with scan rate (I ox (μA) = 35.35 + 355.67 υ; I red (μA) = −1.97 -115.72 υ), indicating that the processes were surface controlled according to the following Eqs. 1.
The NiS-modified CPE was also used for the electrocatalytic oxidation of methanol. Fig. 6 shows CVs of the NiS-modified CPE in   Fig. 6, no enhanced oxidation current appeared over the potential arrange before the oxidation peak potential of Ni (II)/Ni (III) (0.5 V) according to the following Eqs. 1. An obvious oxidation current was observed for MOR at the NiS-modified electrode when the potential was positive than 0.52 V, suggesting that direct electro-oxidation of methanol occurs after the oxidation of Ni (II) in which Ni (III) is used as an active surface for methanol oxidation. Similar oxidation currents were also observed during the reverse scan, these currents were ascribed to the further oxidation of methanol or other MOR intermediates. The forward and reverse oxidation currents nearly overlapped, suggesting that the NiS-modified electrode exhibited a high electrocatalytic activity for the MOR and a high tolerance for poisoning species generated by the MOR.
The CVs shown in Fig. 7, which were recorded at a scan rate of 50 mV/s, reveals the influence of the methanol concentration on the electrocatalytic activity of the NiS-modified. The anodic currents increase with the methanol concentration, suggesting that the oxidation of methanol exhibited a typical electrocatalytic response. Clearly, the anodic peak current in the forward sweep was proportional to the concentration of methanol. Another oxidation current was also observed in the reverse scan. This current also increased with the methanol concentration, suggesting that the methanol and other MOR intermediates that were partially oxidized in the forward scan were further oxidized during the reverse scan. In addition, the oxidation peak potential of the MOR (∼0.52 V) was more positive than that of the oxidation of NiS (∼0.5 V), indicating that the oxidation of methanol occurred through the direct electrooxidation on the Ni (III) surface via the following processes: 50,51 Ni 3+ -methanol = Ni 3+ -intermediate + e − [2] Ni 3+ -intermediate = Ni 3+ -products + e − [3]

Conclusions
Hierarchical flower-like NiS nanostructures were fabricated by a simple solvothermal method. The as-prepared NiS flower-like nanostructures were used for enzyme-free H 2 O 2 sensing and methanol electrooxidation in an alkaline medium. The H 2 O 2 sensor was highly reproducible and had an excellent electrocatalytic activity and a wide linear range from 0.5 μM to 1.37 mM. The hierarchical flower-like NiS also exhibited a high electrocatalytic activity for the methanol oxidation reaction (MOR) in an alkaline medium with a high tolerance toward the catalyst-poisoning species generated during the MOR. The MOR proceeded via the direct electrooxidation of methanol on the oxidized NiS surface layer.