Effect of Zinc Acetate Concentration on Optimization of Photocatalytic Activity of p-Co3O4/n-ZnO Heterostructures

Abstract In this work, p-Co3O4/n-ZnO heterostructures were fabricated on Ni substrate by hydrothermal-decomposition method using cobaltous nitrate hexahydrate (Co(NO3)2·6H2O) and zinc acetate dihydrate (Zn(CH3COO)2·2H2O) as precursors with zinc acetate concentration varying from 5.0 to 55.0 mM. Structure and morphology of the developed samples were characterized by X-ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM). Effect of zinc acetate concentration on the photocatalytic activity of p-Co3O4/n-ZnO heterostructures was investigated by degradation of methyl orange (MO) under the UV light irradiation. The fabricated p-Co3O4/n-ZnO heterostructures exhibited higher photocatalytic activity than pure Co3O4 particles. In order to obtain the maximum photocatalytic activity, zinc acetate concentration was optimized. Specifically, at 35 mM of zinc acetate, the p-Co3O4/n-ZnO showed the highest photocatalytic activity with the degradation efficiency of MO reaching 89.38% after 72 h irradiation. The improvement of photocatalytic performance of p-Co3O4/n-ZnO heterostructures is due to the increased concentration of photo-generated holes on Co3O4 surface and the higher surface-to-volume ratio in the hierarchical structure formed by nano-lamellas. Graphical abstract Electronic supplementary material The online version of this article (10.1186/s11671-018-2604-4) contains supplementary material, which is available to authorized users.


Background
The rapid development of various industries at the beginning of the twenty-first century has been leading to the fast growing of wastewater at the speed which never been observed in the past. The consequent deterioration of the water quality has been greatly affecting the health of aquatic ecosystems and vast majority of people living in such ecosystems. Hence, the effective water treatment has become one of the major global concerns for the time being [1]. Several modern technologies including physical, chemical, and bio-chemical methods have been developed for the efficient water treatment [2,3]. Among them, the photocatalysis process has recently gained great attention due to the superior properties of the developed semiconductor catalysts, which have been utilized for the efficient decomposition of various organic pollutants into the smaller and less harmful substances such as CO 2 , H 2 O, and organic short-chain acids [3][4][5][6][7][8]. Specifically, various micro-and nano-structured semiconductors, such as TiO 2 , MnO 2 , SnO 2 , WO 3 , Fe 2 O 3 , Co 3 O 4 , and ZnO, and different range of their heterojunctions are utilized as functional photocatalysts for the water treatment [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]. It is obvious that different photocatalysts have their own benefits and drawbacks. For example, TiO 2 is so far the most widely employed photocatalyst effective against a wide range of microorganisms co-existing in water. However, it can only absorb UV light for its wide bandgap [12,13]. On the contrary, ZnO is low cost and nontoxic, but possesses rapid recombination of photo-induced electron-hole pairs [29]. Fe 2 O 3 has short hole diffusion length (2-4 nm), poor conductivity, and charge recombination [30].
Generally speaking, photocatalysis is based on the reaction between adsorbed molecules (oxygen, surface hydroxyls groups) or water and photo-generated electron/hole pairs excited by the photon with equal or higher energy than the bandgap of semiconductor. However, the electron/hole recombination is blamed for the low quantum yields, which is still a big obstacle for the photocatalytic activity improvement. In order to overcome this obstacle, the development of efficient p-n heterojunctions has been proposed and attempted with the different levels of success during last few years. For instance, it was found that the fabricated p-n heterojunctions could effectively reduce the recombination rate of the photo-generated electron/hole pairs, which subsequently enhanced the overall photocatalytic activity [31,32]. Thus, the combination of pand n-type semiconductor oxides has paved the way for further development of the p-n heterojunctions and optimization of their photocatalytic activity [33].
As an intrinsic p-type semiconductor, cobalt oxide (Co 3 O 4 ) has been used in the different photocatalytic applications owing to its chemical stability, nontoxicity, low cost, environmental friendliness, etc. [34]. It was reported that Co 3 O 4 with specific band structure can adsorb oxygen much more efficiently compared to the other p-type oxide semiconductors [35]. Other strategies, i.e., doping and heterojunctions, have been developing recently in order to improve the photocatalytic efficiency and properties of the doped Co 3 O 4 -based catalysts such as Co 3 O 4 /Bi 2 WO 6 [36], Co 3 O 4 /TiO 2 [37], and Bi 2 O 3 /Co 3 O 4 [38] have been reported. On the other hand, ZnO (wide bandgap n-type semiconductor) has also been intensively studied as one of the best photocatalytic materials due to its high photochemical activity, nontoxic nature, and relatively low cost. Moreover, it was reported that its photocatalytic activity could also be enhanced significantly by modifying its textural characteristics [39]. Therefore, the combination of p-type Co 3 O 4 and n-type ZnO represents the right approach for successful development of p-n heterojunctions as these heterojunctions can provide built-up inner electric field at the p-n interface that can subsequently enhance the overall photocatalytic activity of fabricated composite material. In fact, several different approaches and synthesizing methods for fabrication of these p-Co 3 O 4 / n-ZnO heterojunctions have recently been reported with reasonable performances [39][40][41]. However, the optimization of photocatalytic activity of fabricated p-n heterostructure, which could be linked to the specific micro-or nano-structural variations, to the best of our knowledge, has rarely been addressed.
Their photocatalytic performance was investigated by taking methyl orange (MO) as an example under the UV light irradiation. The developed p-Co 3 O 4 /n-ZnO heterostructures showed enhancement of the photocatalytic activity in degradation of MO compared to the single Co 3 O 4 component, as they facilitated more photocatalytic sites and accelerated the surface electron transfer rate due to their much higher surface-to-volume ratio. In addition, the effect of zinc acetate concentration on the photocatalytic activity of p-Co 3 O 4 /n-ZnO heterostructures was comprehensively investigated and their photocatalytic activity was optimized.

Results and Discussion
Characterization of Heterostructures   [42]. It was found that all identical peaks of the precursor (Fig. 2a) [44]. These measurements indicated that pure Co 3 O 4 is derived from the Co(OH) 2 after 2 h heating at 250°C.
XRD pattern of the developed p-Co 3 O 4 /n-ZnO heterostructure shows that the intensities of diffraction peaks for both Ni substrate and Co 3 O 4 decreased, which could be caused by the new substance loaded on the surface. In addition, a double-peak can be observed at 2θ = 36.5°. Figure 2b displays the partially enlarged pattern of Co 3 O 4 /ZnO, in which (101) peak of ZnO and (311) peak of Co 3 O 4 are clearly separated. This fact unambiguously confirmed the successful synthesis of the p-Co 3 O 4 / n-ZnO heterostructures. Moreover, no diffraction impurity peak was detected, which also indicated that the synthesized heterostructures are only made of Co 3 O 4 and ZnO. Figure 3 shows typical Raman spectra of the pure Co 3 O 4 and the fabricated p-Co 3 O 4 /n-ZnO heterostructure taken at the room temperature. In these Raman spectra, five different Raman active modes A 1g + 3F 2g + E g of the Co 3 O 4 could be observed. It is well known that Co 3 O 4 has a spinel structure Co 2+ (Co 3+ ) 2 O 2− 4 with Co 2 + and Co 3+ positioned at tetrahedral and octahedral sites, respectively [45]. A 1g mode is a characteristic of the octahedral sites, and the E g and F 2g modes are related to the combined vibrations of tetrahedral site and octahedral oxygen motions [46] Fig. 4. The peaks centered at about 3452 and 1634 cm − 1 are attributed to the O-H stretching and bending modes of the hydrated oxide surface and the adsorbed water [48,49]. The IR absorption peaks at about 660 and 568 cm − 1 confirm the formation of the phase of spinel Co 3 O 4 [50]. Compared with the FTIR spectrum of Co 3 O 4 nanoparticle, new peak at 432 cm − 1 appears in all Co 3 O 4 /ZnO FTIR spectra, which is attributed to the existence of ZnO [51]. In addition, the characteristic peak of ZnO at 432 cm − 1 becomes sharper with the increasing concentration of zinc source, which confirms the coexistence of ZnO and Co 3 O 4 and verifies the successful synthesis of ZnO on the Co 3 O 4 nanoparticles.
The surface morphology of the precursor and as-prepared pure Co 3 O 4 are presented in Fig. 5. From the low-magnification SEM image of the Co(OH) 2 precursor, it is clearly visible that the flower-like layers of precursor, consisting of many sunflower-seed-like petals, have grown uniformly on the surface of the Ni substrate (Fig. 5a). The "petal" size was approximately 10 μm in length, and the whole surface of the porous Ni substrate was covered by Co(OH) 2 precursor. Furthermore, low-magnification SEM image ( Fig. 5b) depicts that the synthetized Co 3 O 4 crystals are also uniformly and densely covered the porous Ni substrate. High-magnification SEM image (Fig. 5c) shows highly dense structure with lots of "sunflower-seed like" Co 3 O 4 crystal stacked together to form Co 3 O 4 spheres. A single sphere size was approximately~20 μm in length. The "sunflower-seed-like" crystals indicated the morphological hereditability of the Co 3 O 4 from its precursor.
The morphologies of Co 3 O 4 /ZnO heterostructures, which were fabricated with the different zinc acetate concentrations, were also investigated by SEM, and the main results are summarized in Fig. 6. It is clear evidenced from this figure that the changes in zinc acetate concentration during preparation of Co 3 O 4 /ZnO heterostructures play the crucial role in the development of the morphology variations. For example, the morphology of Co 3 O 4 /ZnO-5 ( Fig. 6a) is very similar to the morphology of pure Co 3 O 4 (Fig. 5c), as the concentration of zinc acetate is low. However, as the concentration of zinc acetate increased from 5.0 to 25.0 mM, the crystallization of sunflower-seed-like small crystals intensified as presented in Fig. 6b, c. What is also interesting is that the size of the sunflower-seed-like crystals  (Fig. 6d). It was discovered that the inner part of Co 3 O 4 /ZnO sphere is assembled by the numerous nano-lamellas with thickness of 100-200 nm, as clearly indicated by the higher resolution SEM image in Fig. 7. The nano-lamellas are stacked together along the radial direction in interpenetrating network to form Co 3 O 4 /ZnO heterostructural spheres, which ultimately provided higher surface-to-volume ratio in this particular morphology. It is worthwhile to note that as zinc acetate concentration increased further to 45.0 mM, the sunflower-seed-like Co 3 O 4 /ZnO crystals reappeared again in smaller sizes and the new morphology of Co 3 O 4 /ZnO is established (Fig. 6e). In this morphology, Co 3 O 4 /ZnO nanorods have a diameter of approximately 700 nm. Thus, two kinds of crystal morphologies, sunflower-seed-like and nanorod crystals coexisted in Co 3 O 4 /ZnO-45 heterostructure. Finally, when the zinc acetate concentration reached 55.0 mM, the proportion and the size of Co 3 O 4 /ZnO rods increased significantly accompanied by their heavy agglomeration (Fig. 6f).  Consequently, all the above material characterization techniques signify the successful and uniform development of the Co 3 O 4 /ZnO heterostructures. Thus, these p-Co 3 O 4 /n-ZnO heterostructures were formed without any impurity by the decomposition of Co(OH) 2 and zinc acetate Zn(CH 3 COO) 2 precursors calcined and annealed at 250 and 400°C, respectively, by the following reactions [52,53]: Zn The BET surface areas of pure Co 3 O 4 nanoparticles and Co 3 O 4 /ZnO heterostructures are presented in Table 2, and the corresponding nitrogen adsorption-desorption isotherms are depicted in Figure S1    Larger surface area with more adsorption centers is more beneficial for the degradation of organic dyes [54].
To get further confirmation of the development p-Co 3 O 4 /n-ZnO heterostructures, part of the Co 3 O 4 / ZnO-35 structure was peeled off from the Ni substrate to perform XPS analysis. XPS measurements were performed to investigate the chemical binding states of the developed p-Co 3 O 4 /n-ZnO heterostructure. Figure 9 shows the results of XPS measurements, which were carried out to investigate the chemical binding states of the developed p-Co 3 O 4 /n-ZnO heterostructure. Figure 9a depicts the oxidation states of Co 2p in the XPS spectrum. Two main peaks Co 2p 3/2 and Co 2p 1/2 were clearly determined at 780.28 and 795.76 eV, respectively. Noteworthy, owing to complete coating of the porous Ni substrate, some noise level has been recorded at Co 2p 1/ 2 . In addition, the Zn 2p spectrum was also recorded during XPS measurements for p-Co 3 O 4 /n-ZnO heterostructure and this spectrum is presented in Fig. 9b. Two peaks for Zn 2p were also identified as Zn 2p 3/2 and Zn 2p 1/2 at binding energies of 1021.8 and 1044.9 eV, respectively. These results were in the line with other survey [39]. Figure 9c illustrates the O 1s regions for the p-Co 3 O 4 /n-ZnO heterostructure. Employing the Shirley background two deconvoluted Lorentzian-Gaussian peaks were obtained in O 1s spectrum. These peaks for p-Co 3 O 4 /n-ZnO heterostructure were clearly pronounced at 530.2 and 531.4 eV, respectively. The recorded peaks are comparable to the other peaks reported for lattice oxygen and chemisorbed oxygen of the surface hydroxyls [41,55].

Photocatalytic Activity
The photocatalytic degradation of MO under the UV light irradiation (λ = 254 nm) was carried out at room temperature to evaluate the photocatalytic activity of the developed Co 3 O 4 and p-Co 3 O 4 /n-ZnO heterostructures and specify the effect of zinc acetate concentration on the performance of p-Co 3 O 4 /n-ZnO heterostructures. The temporal spectral changes of MO aqueous solutions are displayed in Fig. 10. The corresponding relative concentration of MO with irradiation time and the performance of various p-Co 3 O 4 /n-ZnO heterostructures towards the MO degradation are presented in Fig. 11. As clearly visible from Fig. 10a, MO shown only negligible degradation with increasing irradiation time without catalysts and the degradation efficiency after 72 h of UV irradiation was only 11.66% (Fig. 11a). ZnO also shown poor photocatalytic activity ( Figure S2 of Additional file 1). Pure Co 3 O 4 demonstrated slightly better photocatalytic activity and the degradation efficiency was~17.64% after 72 h irradiation (Fig. 11a). On the contrary, for the developed p-Co 3 O 4 /n-ZnO heterostructures utilized as catalysts, the main characteristic absorption peak (λ = 465 nm) of MO decreased with the increase of the irradiation time (Fig. 10c-h), which caused significant MO degradation. The first-order plot was fitted with this experiment, and the rate constant of MO degradation was obtained by the following equation where t is the irradiation time, C 0 is the initial concentration at time t = 0, C t is the concentration at time t, and k is the first-order rate constant. As can be observed  Table 3.
The photocatalytic degradation of dyes mainly involves several active radical species such as hydroxyl radicals (·OH), holes (h + ), and electrons (e − ) [29]. In order to investigate the active species in the photocatalytic process to better understand the mechanism of photocatalysis, a series of scavengers were employed during the photo-degradation processes. Isopropanol (IPA), triethanolamine (TEOA), and silver nitrate (AgNO 3 ) were used as scavengers for hydroxyl radicals (·OH), photo-generated holes, and electrons in degradation of MO, respectively [55][56][57]. The concentration of the three kinds of scavengers was 10 mM. Figure 12 shows the photocatalytic degradation of MO over Co 3 O 4 / ZnO-35 heterostructure catalyst was 74.30, 30.55, and 90.25% with 10 mM IPA, TEOA, and AgNO 3 , respectively. This result means the photo-generated holes play much more important roles in MO degradation process, compared to ·OH and photo-generated electrons. Photoluminescence (PL) technique is widely used to investigate the recombination rate of the photo-induced electron-hole pairs in photocatalyst. Figure 13 shows the room temperature PL spectra of the synthesized Co 3 O 4 , ZnO, and Co 3 O 4 /ZnO-35 heterostructure (PL spectra of all samples are presented as Figure S3 in Additional file 1). There are two peaks in the PL spectra of Co 3 O 4 , ZnO, and Co 3 O 4 /ZnO heterostructures: one is called near band edge emission (NBE), which is in UV region and due to the recombination of free excitons through an exciton-exciton collision process; and the other one is called deep level emission (DPE, in visible region), which is caused by the impurities and structural defects in the crystal [58,59] [29].

µm
According to the results above, the improvement of photocatalytic activity of Co 3 O 4 by additional incorporation of ZnO is mainly caused by two ways. The first  one is based on the fact that the increasing concentration of photo-generated holes in Co 3 O 4 accelerates the photocatalytic rate. As illustrated in Fig. 15 Fig. 12, photo-generated holes play the most important role in photo-degradation process of MO on p-Co 3 O 4 /n-ZnO heterostructures. Thus, the increasing concentration of photo-generated holes in the Co 3 O 4 VB could lead to its highest photocatalytic activity. The second way of improvement of photo-catalytic activity is caused by the high-specific surface area of the p-Co 3 O 4 /n-ZnO heterostructures. The molecules' absorption-desorption on the surface of catalyst is the first step in degradation process [54,62]. Consequently, higher surface-to-volume ratio in the developed morphology of p-Co 3 O 4 /n-ZnO heterostructures provides more unsaturated surface coordination sites, as shown in Table 2. The p-Co 3 O 4 /n-ZnO heterostructures possess higher specific surface area caused by numerous ultrathin nano-lamellas, as confirmed by SEM characterizations. Therefore, high surface-to-volume ratio and suitable interfaces obtained for the Co 3 O 4 /ZnO-35 heterostructure resulted in its outstanding photocatalytic activity towards the efficient MO degradation.
It needs to note that with the zinc acetate concentration increasing higher than 35.0 mM, the photocatalytic activities of Co 3 O 4 /ZnO heterostucture decreases. This could be caused by the decrease of their specific surface area (as presented in Table. 2). The similar trend was  also observed for the tetracycline (TC) degradation by Mn-doped SrTiO 3 nanotubes with the increase of Mn dopant concentration [63]. Thus, with certain increase of the zinc acetate concentration, the quantity of ZnO increases and the mass of electron-hole pairs within the space charge region is efficiently separated by the   Figure S4 of Additional file 1). No MO adsorption peak appeared in the FTIR spectrum of Co 3 O 4 /ZnO-35 heterostructure immersed 72 h in MO solution, indicating that MO molecules are degraded to the smaller molecules [64,65].
All the above experiments relevant to investigation of the photocatalytic activity of fabricated p-Co 3 O 4 /n-ZnO heterostructures undoubtedly confirmed that the MO degradation under the UV light illumination is relatively slow without the presence of catalyst. The presence of p-Co 3 O 4 /n-ZnO heterostructures as catalysts significantly increased the rate of MO degradation under the same UV light irradiation conditions. The 35 mM of zinc acetate concentration used in preparation of p-Co 3 O 4 /n-ZnO heterostructures has provided the essential prerequisite for development of unique and well-structured morphology with high-surface-to-volume ratio, which subsequently resulted in the maximum photocatalytic activity of the p-Co 3 O 4 /n-ZnO heterostructure for the MO degradation. These experimental results indirectly confirmed the fact that the catalytic process was mainly related to the adsorption and desorption of molecules on the large surface area of catalysts. High-surface-to-volume ratio provided more unsaturated surface coordination sites, which in turn endowed p-n heterojunction with enhanced photocatalytic activity [66].

Conclusions
Different p-Co 3 O 4 /n-ZnO heterostructures were successfully fabricated by the hydrothermal decomposition method on the porous Ni substrate with the different zinc acetate concentration varying from 5.0 to 55.0 mM as a ZnO source. The resulted p-Co 3 O 4 /n-ZnO heterostructures illustrated various structural morphologies. The synthesized p-Co 3 O 4 /n-ZnO heterostructures were subjected to the water treatment as photocatalysts under the UV light irradiation. The reaction rate of MO degradation at the room temperature and at the presence of these photocatalysts was substantially promoted. In fact, p-Co 3 O 4 / n-ZnO heterostructures exhibited much higher photocatalytic activity than that of pure Co 3 O 4 for MO degradation. It was discovered that the photocatalytic activity of p-Co 3 O 4 /n-ZnO heterostructures is greatly affected by the zinc acetate concentration. The optimum zinc acetate concentration was found to be at 35%. At this concentration, the synthesized Co 3 O 4 /ZnO displayed unique

Materials Synthesis
All solvents and reagents were purchased from the commercial sources and represented analytical grade. They were used and received without further purification. p-Co 3 O 4 /n-ZnO heterostructures were prepared by two-step fabrication method on the porous Ni substrate (25 mm × 25 mm × 1 mm). Initially, the Ni substrates were thoroughly cleaned by acetone and deionized water at the room temperature. Then, they were immersed into 6 M hydrochloric acid and 0.1 M nickel chloride solution for 10 min. After that treatment, the cleaned Ni substrates were dried for further use. For fabrication of p-Co 3 O 4 /n-ZnO heterostructures on the Ni substrates, 1.7463 g Co(NO 3 ) 2 ·6H 2 O was firstly dissolved in 18 mL of deionized water and stirred for approximately 5 min until the solution turned pink and gradually turned into black by the addition of 12 mL 28 wt.% ammonia solution. pH of solution was 12. Then, both the solution and cleaned Ni substrate were transferred into 50 mL Teflon-lined stainless steel autoclave with subsequent heat-treatment at 120°C for 10 h. Upon completion of the reaction, the autoclave was cooled to the room temperature and the pH of solution becomes 10

Characterization
The crystal structure of precursor, Co 3 O 4 particles, and p-Co 3 O 4 /n-ZnO heterostructures fabricated on the Ni substrates were characterized by D/Max-rB X-ray diffractometer (XRD) with a Cu-K α1 radiation (λ = 0.1542 nm) operating at 100 mA and 40 kV and a scan rate of 5°/min. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy were carried out by a SU-5000 microscope equipped with EDX attachment. The Raman spectra were recorded on a Renishaw in Via Raman microscope, and a 514.5-nm Ar + laser line with a power output of 20 mW was used for excitation with a spectral resolution of 2 cm − 1 . Fourier Transform Infrared (FTIR) spectra were taken using a NEXUS Thermo Nicolet IR-spectrometer in the range 4000-500 cm − 1 with a spectral resolution of 2 cm − 1 by KBr disk method. X-ray photoelectron spectroscopy (XPS) was employed in order to investigate the surface chemistries of the developed samples in ESCALAB system with AlK X-ray radiation at 15 kV. All XPS spectra were accurately calibrated by the C 1s peak at 284.6 eV to compensation of the charge effect. Brunauer-Emmett-Teller (BET, JW-BK122F, China) was applied to analyze the specific surface area. Room temperature photoluminescence (PL) spectra of the synthesized Co 3 O 4 and p-Co 3 O 4 /n-ZnO heterostructures were performed on an F-4600 fluorescent spectrophotometer (Hitachi Corp., Tokyo, Japan), the maximal excitation wavelength was 200 nm, and the filter was 300 nm.

Photocatalytic Activity Evaluation
The photocatalytic activity of both as-fabricated Co 3 O 4 and p-Co 3 O 4 /n-ZnO heterostructures developed on the Ni substrates for the MO (C 14 H 14 N 3 NaO 3 S) degradation in aqueous solution under the UV light was evaluated by measuring absorbance of the irradiated solution. For this study, Ni substrates attached with the different p-Co 3 O 4 /n-ZnO heterostructures were placed into 100 mL of MO solutions with a concentration of 6 mg/L and pH of 6.5. The solutions were continuously stirred in dark for 2 h before illumination in order to reach the absorption-desorption equilibrium between MO and the p-Co 3 O 4 /n-ZnO heterostructures. Then, the solutions were irradiated by 30 W low-pressure UV lamp (λ = 254 nm), which was located at the distance of 50 cm above the top of the dye solution. During the process, 5 mL solutions were pipetted every 12 h for the absorbance determination by a UNIC UV-2800A spectrophotometer using the maximum absorbance at 465 nm. All experiments were performed under the ambient condition and room temperature. The degradation efficiency of MO was defined as: where D is degradation efficiency, A 0 is the initial absorbance of MO solution, and A t is the absorbance of MO solution after UV irradiation within the elapsed time t.

Photo-Electrochemical Characterization
The photocurrent measurements were carried out at an open circuit potential using an electrochemical workstation (CHI-660e, Chenhua Instrument Corp., China). A three-electrode system was used with the prepared Co 3 O 4 or Co 3 O 4 /ZnO samples, Pt plate, and saturated calomel electrode (SCE) acted as working, counter, and reference electrodes, respectively. A 300 W Xe lamp with an optical filter (AM 1.5 G) was employed as the excitation light source and NaOH solution (1 M) was used as the electrolyte.