Comparison of two different nickel oxide films for electrochemical reduction of imidacloprid

A nickel oxide (NiO) thin film was successfully prepared on Ni foil via a sol–gel method and a reduced state nickel oxide (r-NiO) thin film was obtained by etching NiO with hydrazine hydrate solution. Structure characterization through X-ray diffraction and scanning electron microscopy revealed the growth of nanostructure films on the surface of nickel foil. Cyclic voltammetry, linear sweep voltammetry and electrochemical impedance spectroscopy were used to assess the performance of the two films. Electroreduction of alkaline imidacloprid solution under potentiostatic conditions was carried out in a three-electrode system. The removal efficiencies of 80.2% (r-NiO) and 66.3% (NiO), and the current efficiencies of 67.3% (r-NiO) and 58.9% (NiO) were much higher than 41.7% (removal efficiency) and 0.003% (current efficiency) on the bare Ni electrode. This study prepared two novel thin films with composites of NiO or r-NiO, and thus provided feasible and efficient electrochemical degradation of imidacloprid. The degradation products were characterized and the possible degradation pathways were proposed.


Introduction
Imidacloprid (IMD), a systemic chloronicotinyl insecticide with remarkable water solubility, high toxicity, and good stability in water, is mainly applied to agriculture for controlling sucking insects in crops such as aphids, white ies and termites, and can cause IMD pollution in water sources and soil. 1,2 To solve the pollution problem, various treatment processes have been studied. While diverse microorganisms may potentially degrade insecticides, IMD is none biodegradable due to its high toxicity. 3,4 A lot of work has focussed on exploring effective methods for IMD treatment. Ultraviolet (UV) photolysis with photocatalysis and photo-Fenton's reagent are the most intensively applied technologies. However, photolysis is usually not a competitive option due to the low quantum efficiency as most pesticide compounds are only partially degraded. 5,6 Alternative to advanced oxidation processes with production of hydroxyl radicals ($OH) through hydroxyl radical attack, 7-9 direct or indirect electrochemical generation of $OH in electrochemical advanced oxidation processes (EAOPs) was proposed as a potential option to remove pesticides from contaminated waste water. [10][11][12] This electrochemical process based on redox reactions overcomes the drawbacks of consuming extra chemical reagents or producing secondary pollution in conventional chemical or physical treatments. 13 However, the need for expensive electrodes and thus the relatively large capital investment limits their practical application. 14 Much effort is still needed to employ highly efficient and cost effective electrodes in order to broaden the applicable eld of electrochemical degradation for IMD and other pesticide contaminated wastewater.
In these reports, electrochemical oxidation was carried out using expensive anode to resist the strong oxidative effect of hydroxyl radical. In parallel to the electrochemical oxidation, electrochemical reduction on the low-cost cathode has been scarcely reported for IMD degradation. The reasons of choosing electrochemical reduction instead of oxidation for IMD removal lies in: (i) inexpensive materials can be used as cathode electrodes due to their much less destruction by cathodic processes; (ii) cathodic reduction reactions feasibly occur with lower overvoltage, compared to anodic oxidation with higher overvoltages for $OH generation; and (iii) species of IMD, difficultly degraded by electrochemical advanced oxidation might be feasibly removed through electrochemical reduction.
In this study, two novel materials of NiO based on Ni foils via a sol-gel method and r-NiO by etching NiO with hydrazine hydrate solution were prepared as cathodes and assessed for efficient IMD degradation in alkaline aqueous solution. NiO has been regarded as a promising electrode candidate due to its cost effectiveness, superior chemical and electrical stability, and transparently p-type semi-conductivity based high efficiency. 20,21 The characterization of the oxides is investigated by X-ray diffraction (XRD), scanning electron microcopy equipped with energy dispersive spectrometer (SEM-EDS) and electrochemical tests. IMD degradation products were characterized by high performance liquid chromatograph-mass spectrometer (HPLC-MS) and the possible degradation pathways were proposed.

Preparation of NiO and r-NiO lms
Specimens with dimensions of 70 Â 10 Â 0.1 mm cut from Ni plates were utilized as substrates. The Ni plates were sequentially and ultrasonically pretreated with acetone, hydrochloric acid (1.0 mol L À1 HCl), and deionized water.
The Ni plates were put at the bottom of a beaker. Then, 3.5 g of NiCl 2 $6H 2 O was dissolved in an n-butyl alcohol-ethanol (100 mL : 50 mL) solvent and 18.9 g of citric acid solution was added with magnetic stirring for 30 min. Aerwards, deionized water was added drop by drop. Light green sol-gel was then formed and coated on Ni plate under 90 C heating reux. Aer 60 min, the Ni plate coated sol-gel was taken out from the beaker and calcined at 300 C for 20 min for preparation of NiO. Repeat the coating steps several times until NiO lm on the surface of Ni plates were uniform. Finally, the NiO lm coated on Ni plate was sequentially and ultrasonically washed with acetone and deionized water.
The as-prepared NiO lm coated Ni plate was put in 10% NaBH 4 solution for 240 min at room temperature. The r-NiO lm was then obtained, and sequentially and ultrasonically washed with acetone and deionized water.

Electroreduction of imidacloprid
Stock solution of IMD (40 mg L À1 ) in alkaline (0.1 mol L À1 NaOH) was prepared with deionized water.
Electroreduction of IMD under potentiostatic condition was carried out at a series of potential (À1.0 V, À1.1 V, 1.2 V, À1.3 V and À1.4 V) in 50 mL basic solution containing 100 mg L À1 IMD and 0.1 mol L À1 NaOH for 1000 s at room temperature. The removal efficiency of imidacloprid was calculated based on eqn (1): where C 0 is the initial concentration of imidacloprid, C t is the concentration aer a certain time of electroreduction using NiO, r-NiO or Ni electrodes. Current efficiency was calculated as eqn (2) h where, 96 500 is Faraday constant (C mol À1 ); h is the current efficiency (%); R is removal efficiency calculated according to eqn (1) (%); Q t is the theoretical charge needed to reduce IMD (C); Q p is the practically consumed charge for IMD reduction (C); c is the molar bulk concentration of imidacloprid, here is 0.000156 mol L À1 (100 mg L À1 ); V is the electrolyte volume, here is 0.05 L (50 mL); n is the number of electrons, here is 1.

Liquid chromatography analysis
The concentration of imidacloprid and its transformation products were monitored by liquid chromatography (Agilent HP 1200, Agilent Technologies, Inc., USA), equipped with an Agilent model pump, autosampler, column compartment, and an ultraviolet detector with a C-18 column with the dimensions 4.6 Â 150 nm. The mobile phase used a mixture of acetonitrile and water (40 : 60) at a ow rate of 0.5 mL L À1 . The injection volume of the sample was 5 mL, and the ultraviolet detection was l ¼ 270 nm. The samples were ltered through 0.45 mm lter paper before injection. An HPLC-MS (Agilent 1100-6224) system was used to analyze the transformation products obtained in imidacloprid degradation, with an LC column Luna 5C18100A. The samples were ltered through 0.45 mm lter paper before injection. In this case, the isocratic eluent was 98% (1 nmol L À1 aqueous sodium formate and 0.1% formic acid) and 2% acetonitrile (0.1% of formic acid), which was pumped at a rate of 0.4 mL min À1 for 80 min. Detection was carried out with the diode array detector set at 270 nm and the column temperature maintained at 35 C.

Structure characterization and electrochemical tests
The as-prepared NiO and r-NiO were characterized through XRD performed on a Panalytical Empyrean diffractometer with Cu Ka radiation. The morphology and the elements of the surface of NiO and r-NiO were analyzed by SEM-EDS (JSM-6360LV).
For electrochemical tests, a three-electrode system was employed. NiO, r-NiO lm coated Ni plates or the bare Ni plate were used as working electrodes, whereas a platinum plate and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively in electrolyte of NaOH aqueous solution (0.1 mol L À1 ). Cyclic voltammetry (CV) was performed at a scan rate of 50 mV s À1 between À0.4 V and 0.3 V. Linear sweep voltammetry (LSV) was performed at À1.4 V to 0 V at a scan rate of 5 mV s À1 . The electrochemical impedance spectroscopy (EIS) test was carried out in a frequency range of 0.01 Hz to 100 kHz. All the electrochemical measurements were performed on an electrochemical work station (PARSTAT2273, Princeton Applied Research, USA) at 20 C.

Structural characterization of NiO and r-NiO lms
The structure characterizations of the NiO and r-NiO lms were analyzed by XRD (Fig. 1). The peaks at 2q angle values of 37.2 , 43.2 , 62.8 , 75.4 and 79.3 corresponded to characteristics of nickel oxide, which could be assigned to (101) The grain sizes of NiO and r-NiO were calculated using Scherrer equation: where D is crystallite size, l is the wavelength of X-ray radiation, b is the full width at half height of symmetrical shape of the diffraction peak and q is the Bragg angle. The average grain size of NiO lm was about 50 nm, while r-NiO lm was about 30 nm, suggesting r-NiO ner nanocrystalline lm than NiO by reduction of NaBH 4 solution. Fig. 2 showed the SEM images of the surface views and the corresponding tilted views of Ni substrate, and NiO and r-NiO lms. Both NiO and r-NiO lms were composed of granular particles with the size of about 0.2-2 mm. However, r-NiO lm exhibited thinner thickness and smaller particles than NiO lm, meaning better conductivity for the former. 22 The morphology of even r-NiO thin lms demonstrated that the etching by NaBH 4 played an important role in the formation of r-NiO nanocrystals.
EDS analysis showed that the NiO thin lms consisted of 80.5% (w) of Ni element and 19.5% (w) of O element. However, r-NiO lm consisted of higher Ni elements of 87.5% (w) and lower O element of 12.5% (w) due to the reduction effect of NaBH 4 . This result was well consistent with the result in the XRD patterns (Fig. 1), indicating that r-NiO can effectively improve the structure characteristics and thus the enhanced the electrochemical performance demonstrated hereinaer.

Electrochemical performance of NiO and r-NiO electrodes
The electrochemical tests of CV, LSV and EIS were carried out to investigate the characteristics of NiO and r-NiO lms.   larger and more symmetric current response than the Ni electrode, suggesting the faster electron-transfer kinetics and the better electrochemical catalytic activity than the bare electrode by the formation of the crystallization lms. In addition, the r-NiO lm displayed much larger current values than the NiO lm, mainly ascribed to the former thinner lm and favorable morphology ( Fig. 1 and 2).
LSV curves demonstrated that Ni electrode showed a larger current sharp than NiO or r-NiO at potentials more negative than À1.15 V (Fig. 4), mainly attributed to strong hydrogen evolution in this region. These results precisely demonstrated that hydrogen evolution was greatly reduced by the NiO or r-NiO lms, compared to the bare Ni. This merit of the NiO or r-NiO lms favored for the electroreduction of imidacloprid and improved the current efficiency because of the alleviation of the side reaction of hydrogen evolution at the negative potentials. 23 Moreover, the crystallization of the NiO and r-NiO lms exhibited much better electrochemical catalytic activity for imidacloprid reduction than the bare Ni demonstrated herein-aer. The r-NiO lm exhibited a larger current response than NiO lm, presumably due to its thinner thickness, higher conductivity and better electrochemical activity than the former.
The Nyquist plots of EIS spectra for NiO and r-NiO lms (Fig. 5) were characteristic of semicircle, which was signicantly different from the bare Ni, suggesting the growth of nanostructure lms on the surface of nickel foil. An intercept at high frequency region with real axis of Z 0 is inner resistance (R s ) and distorted semicircle is charge-transfer resistance (R ct ) across the electrode-electrolyte interface. 24,25 From the tting equivalent circuit (inset of Fig. 5), it can be concluded that the R s of NiO thin lm electrode is about 0.89 U, while the R s of r-NiO is about 0.85 U. The span of the semicircle along the x-axis from high to low frequency represented the R ct . The smaller the diameter of the semicircle, the better the electrochemical activity. The r-NiO shows much smaller R ct of 220 U cm 2 than 465 U cm 2 for NiO. The better conductivity and less R ct of the r-NiO lm can be ascribed to its thinner thickness and smaller particles. As a consequence, the r-NiO thin lm exhibited better electrochemical activity and higher electrochemical reduction ability than the NiO lm, as subsequently demonstrated.

Electroreduction of imidacloprid
For all electrodes of Ni, NiO and r-NiO, degradation efficiency of IMD increased with the more negative potential (Fig. 6d), due to the abundant electrons supply at more negative potentials. At the highly negative potentials, hydrogen was observed to evolve drastically on the bare Ni electrode (Fig. 6a), consistent with the much larger current response obtained on Ni electrode (Fig. 6b  and c), while hydrogen evolution were slightly generated on NiO and r-NiO electrodes. This result implied the undesired side reaction of hydrogen evolution has been greatly reduced on the NiO or r-NiO lms. 26 In addition, better electrochemical catalytic activity could be achieved by the formation of crystallization lms of NiO or r-NiO. As a result, the removal efficiencies of 80.2% (r-NiO) and 66.3% (NiO), and the current efficiencies of 67.2% (r-NiO) and 58.9% (NiO) were much higher than 41.7% and 0.003% on the bare Ni electrode at À1.3 V (Fig. 6d and Table  1). The values obtained on r-NiO were higher than on NiO, explaining the better electrochemical catalytic activity and the lower charge transfer resistance in the former.   This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 3040-3047 | 3043 Comparing these results with literature, the removal rate of IMD (equivalent to 4.81 mg L À1 min À1 ) observed in this study at an initial IMD of 100 mg L À1 and much lower current density of 0.10 mA cm À2 , was much higher than the rate achieved on a boron-doped diamond anode at a much higher current density by EAOPs. [27][28][29] In addition, $OH generation on anodic oxidation during EAOPs processes at high-voltage anodes means high electric price or low current efficiency compared to the results of this study with fairly high current efficiency.
The main possible reactions involved in the electrochemical reduction processes were as follows: 30 Imidacloprid + e / reduction products (5) In alkaline solution, the IMD was reduced by direct reduction with accepting electrons (eqn (5)) or by indirect reduction by reaction with the freshly generated atom hydrogen ([H]) (eqn (6)) generated on the surface of the electrode, and then turned into transitional products and nally terminal products.   In alkaline solution, the IMD was reduced by direct reduction with accepting electrons (eqn (5)) or by indirect reduction by reaction with the freshly generated atom hydrogen ([H]) (eqn (6)) generated on the surface of the electrode, and then turned into transitional products and nally terminal products. The occurrence of the side reaction of more hydrogen evolution through water hydrolysis (eqn (4)) on the surface of the Ni electrode than that on the NiO or r-NiO lms, explained the higher current efficiencies in the latter (Table 1). Fig. 7 showed the intermediates identied by HPLC-MS aer electroreduction of IMD with r-NiO electrode. Apparent decrease in peak height of IMD along with the appearance of some new peaks was observed, demonstrating the decrease of IMD and the appearance of new intermediates by electrochemical degradation.
Based on the detected intermediates, a pathway for electrochemical degradation of IMD with r-NiO electrode was proposed (Fig. 9). The charge transfer led to the formation of alkene IMD upon elimination of H atoms. 31 The alkenes were further degraded to yield the 6-chloronicotinic acid, which exhibited a higher insecticidal activity than IMD. However, this 6-chloronicotinic acid displayed a large degree of mineralization and lost its acute toxicity with extended time. 32 IMD urea was the principal degradation product by hydrolysis and in weak alkaline media, which has been proven to have lower toxicity than the parent compound and can be easily mineralized. 33,34 IMD guanidine was formed by the loss of the -NO 2 group aer H + attacking the moiety of N-NO 2 of IMD, 35 which showed higher mammalian toxicity than the parent compound. However, this intermediate could eventually be converted into nontoxic molecular fragments in the natural environment. The 6-chloronicotinic acid was the major degradation product of IMD with simple chemical structure, and had less ecotoxicity than the IMD. 36 More importantly, this compound can be easily mineralized into CO 2 and H 2 O under acid conditions. Fig. 9 The proposed pathway for electroreduction of imidacloprid by r-NiO electrode. This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 3040-3047 | 3045

Conclusions
The novel lms of NiO prepared by sol-gel method and the r-NiO by reduction of NiO lm were successfully explored for electrochemical reduction of IMD. While NiO and r-NiO crystallization lms were composed of granular particles, the r-NiO lm exhibited thinner thickness, smaller particles and better conductivity than the NiO lm. The NiO and r-NiO lms also displayed the merit of the alleviation of the side reaction of hydrogen evolution at the negative potentials. Electroreduction of alkaline IMD solution under potentiostatic conditions was successfully carried out in a three-electrode system, achieving removal efficiencies of 80.2% (r-NiO) and 66.3% (NiO), current efficiencies of 67.3% (r-NiO) and 58.9% (NiO), much higher than 41.7% (removal efficiency) and 0.003% (current efficiency) on the bare Ni electrode. The higher removal efficiencies were mainly ascribed to the better electrochemical catalytic activity and the lower charge transfer resistance of the r-NiO and the NiO lms, whereas the higher current efficiencies were due to the alleviation of the side reactions of hydrogen evolution during the IMD removal process. Four degradation intermediates were detected and the possible pathway for electrochemical degradation of IMD was proposed. The novel nickel oxide prepared on Ni foil was proved to be feasible and cost effective for efficient electrochemical degradation of IMD.

Conflicts of interest
There are no conicts to declare.