Facile synthesis of porous NiCo₂O₄ nanoflakes as magnetic recoverable catalysts towards the efficient degradation of RhB

Abstract

In this work, a class of flake-shaped magnetic NiCo₂O₄ material is fabricated by a facile hydrothermal reaction followed by a calcination treatment process. The prepared flake-like NiCo₂O₄ displays porous features that endow it with a large specific surface area (142.48 m² g⁻¹) and a narrow pore size distribution (3.70 nm). Furthermore, the NiCo₂O₄ samples were used as high-performance heterogeneous catalysts in the activation of peroxymonosulphate (PMS) to produce active radicals SO₄⁻˙ and HO˙. Then, the produced SO₄⁻˙ and HO˙ can further attack and degrade organic dyes. The catalytic results show that the magnetic flake-like NiCo₂O₄ catalyst can completely degrade rhodamine B (RhB) dye within 30 min with the assistance of PMS. In addition, the catalysts could be magnetically recovered, displayed high catalytic activity and excellent cycling stability. It is believed that the specific porous features, including high specific surface area and tailored pore size distributions, and surface defects can ensure the high activation of PMS for the catalytic oxidation of RhB. More importantly, the present synthetic method is facile, controllable and scalable, which highlights its potential in energy-storage, environmental treatment, and biology-related fields.

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Introduction

Wastewater, containing tens of thousands of types of dyes and other pigments, is produced in large quantities in the world, of which 20% is randomly discharged without undergoing decontamination treatment.^1,2 The treatment of dye wastewater is a serious environmental issue to industries. Currently, physical adsorption and catalytic oxidation are two effective technologies for the removal of organic dyes. The former is easy to operate, but the dyes cannot be radically removed and additional steps are needed for the regeneration of adsorbents (e.g., the use of chemical agents to extract organic dyes from the adsorbent). In contrast, the latter can oxidize the dyes radically into H₂O and CO₂ that are harmless to the environment and the catalyst can be reused without extra treatment. To address this problem, several advanced oxidation processes (AOPs) have been developed for the removal of the dye pollutants. Hydroxyl radicals (HO˙) are generated during the oxidation processes of AOPs, leading to an effective degradation of organic pollutants due to their excellent oxidizing capability.^3,4 The Fenton process is an AOP, and some studies have focused on the treatment of dye wastewater by the Fenton method.^5–7 However, the required acidic pH, slow reaction rate and poor mineralization are defects of the Fenton process.⁸ Previous results demonstrated that the SO₄⁻˙ with higher oxidation potential compared with hydroxyl radicals has degradation ability over a wide pH range.^9,10 Meanwhile, PMS (2KHSO₅·KHSO₄·K₂SO₄) can generate reactive sulphate radicals (SO₄⁻˙) to degrade or mineralize refractory organic pollutants, which has attracted attention due to its water-soluble, environmentally friendly, and high stability.^10–12

The activation of PMS to produce SO₄⁻˙ and OH˙ can be initiated under photochemical, thermal (also with metal catalysts) or chemical conditions, then the produced SO₄⁻˙ and OH˙ can rapidly attack organic pollutants with high rate constants in the range of 10⁵ to 10⁹ M⁻¹ s^−1.9. Noted that, homogeneous catalytic oxidation by PMS coupled with transition metals shows efficient decontamination of organic pollutants because each catalytic entity can act as a single active site.¹³ However, the dissolved metal ions have been recognized as possible potential health hazards, which limit their scalable application.

Compared to homogeneous catalytic systems, heterogeneous Fenton-like systems for PMS activation have proven to be promising alternatives. Recently, bimetallic oxides (composite oxides of cobalt and another metal element) as excellent heterogeneous Fenton-like catalysts in degradation of organic pollutants have received attention due to their specific nanometer size, large surface area to volume ratio, superparamagnetic behavior, and high catalytic efficiency.^14–17 For example, the recovery of the magnetic MFe₂O₄ materials can be achieved through using an external magnet for the final solution, providing an attractive and cost-effective method for practical operation.¹⁸ Recently, spinel-type CuFe₂O₄ (ref. 19) and CoFe₂O₄ (ref. 20) were found to be active in PMS activation for the oxidation of organic pollutants. Su et al.²¹ successfully prepared heterogeneous Co_xFe_3−xO₄ catalysts and found the intimate Fe–Co interactions are critical for efficient heterogeneous activation of oxone. Zhou et al. reported that LuFeO₃ particles formed under ultrasonic irradiation has the ability to depredate RhB within 90 min.²² Yang et al. found that the iron-cobalt mixed oxide nanocatalysts can catalytically activate PMS for removing 2,4-DCP.²³ Although spinel-type MFe₂O₄ nanoparticles (NPs) exhibited high catalytic performance, the MFe₂O₄ NPs with high surface area and the unique magnetic properties always lead to their aggregation, resulting in lower catalytic efficiency.²⁴ To solve these issues, some carbonaceous materials with high electrical conductivity have been widely employed as matrices to introduce into the MFe₂O₄-based materials that can provide a larger surface area for the reactants' diffusion onto the active sites and further enhance their catalytic performances.^25,26

NiCo₂O₄, as one of typical spinel-type materials, is considered a promising candidate for catalysis because of its multiple convertible valence states, low toxicity, high stability, and easily controllable morphology.^27–30 More importantly, spinel-phase NiCo₂O₄ can be built around a closely packed array of O²⁻ ions, with Ni²⁺ and Co³⁺ cations occupying part or all of the tetrahedral and octahedral sites, respectively. More importantly, the solid-state redox couples of Ni³⁺/Ni²⁺ and Co³⁺/Co²⁺ are always present and could provide some notable catalytic activity sites; thus, they would possess potential applications in such as environment catalytic and electrocatalytic aspects.^31–33 However, the study on the catalytic degradation of organic pollutants using NiCo₂O₄ as activated catalysts has been seldom reported, which maybe restrict from its uncontrolled morphology and dispersity. Therefore, the rational design and synthesis of high efficiency and reusable NiCo₂O₄ based heterogeneous catalysts for the effective removal of organic pollutants is very urgent.

In this work, we report that porous flake-like NiCo₂O₄ was prepared via a simple hydrothermal reaction followed by a calcination process. The detailed preparation process is shown in Fig. 1. The NiCo₂O₄/PMS act as heterogeneous catalysts can completely degrade RhB within 30 min, which is superior to most previously reported catalytic results. These findings provide a feasible synthesis route for the production of highly effective heterogeneous bimetallic catalysts with specific surface architectures.

Fig. 1 Schematic illustration of the formation of flake-like NiCo₂O₄.

Experimental section

Synthesis of flake-like NiCo₂O₄

In a typical synthesis, 3 mmol of Co(NO₃)₂·6H₂O, 1.5 mmol of Ni(NO₃)₂·6H₂O, 40 mg of ethylenediamine tetraacetic acid (EDTA) were dissolved in 40 mL of deionized water (DIW) and DMF (1 : 1 v/v) mixed solution with constant magnetic stirring for 30 min at room temperature. Then, the resulting homogeneous solution was transferred into a 60 mL of Teflon-lined stainless steel autoclave and maintained at 180 °C oven for 18 h. After cooling to room temperature naturally, the sample was collected via centrifugation and then washing with water and absolute ethanol for 4 times, next, the sample was dried at 60 °C oven for 8 h, then sintered at air atmosphere at 500 °C furnace for 5 h with a heating rate of 2 °C min⁻¹. Finally, the black product can be obtained. Similarly, when altering mixed solvents types, for example, DIW/methanol or DIW/ethanol, then the NiCo₂O₄ samples with different morphologies can be obtained, wherein; other reaction conditions are kept constant.

Characterization

Samples structure are characterized via powder X-ray diffraction (XRD, Bruker AXS-D8), Fourier transform infrared spectroscopy (FTIR) was recorded with Bruker EQUINOX-55 infrared spectrophotometer on KBr pellet. Samples morphology was observed by field emission scan electron microscope (FE-SEM; Hitachi, S-4800) with accelerating voltage of 15 kV. The structural and composition analysis were taken via using high-resolution transmission electron microscopy (HRTEM) (Philips Tecnai G2 F20) at 200 kV that equipped with energy-dispersive spectroscopy (EDS). The specific porous structures of the prepared cobalt based samples were determined by Brunaur–Emmett–Teller (BET) surface analyser (Tri Star-3020, Micromeritics, USA). X-ray photoelectron spectroscopy (XPS) was conducted using Model VG ESCALAB apparatus with an Al Kα X-ray source to analyse the surface composition of the catalysts before and after catalytic reaction.

Catalytic degradation of organic dye RhB

In a typical degradation reaction, a certain amounts of catalyst and PMS are added into 60 mL of RhB (0.05 mM) solution. The mixture was stirred for 0.5 h at room temperature. After that, at a given time interval, 5 mL of sample was taken for analysis. The catalyst was separated and reused by magnet. The concentration of RhB was determined by a UV-vis spectrophotometer (Hitachi U-3010, Japan) to evaluate the catalytic degradation efficiency of RhB, in which, the maximum absorbance wavelength of RhB is about 554 nm. At the end of experiment, the mixed solution was separated via magnet to collect the residual catalyst for reusing in repeated tests. To determine the active species during batch experiments, tert-butyl alcohol (TBA) and MeOH were employed to distinguish HO˙ and SO₄⁻˙ radicals. Quenching experiments were performed by adding the quencher into the reaction solution before the addition of PMS. In this study, the NiCo₂O₄ catalyst was gathered after completion of the reaction, washed, dried, and evaluated for its reusability.

Results and discussion

Herein, the structure of the obtained black powder in the case of DMF/DIW mixed-solvent system was characterized using XRD analysis, as shown in Fig. 2. All diffraction peaks in Fig. 2a were assigned to pure spinel-phase NiCo₂O₄ (JCPDS card no.73-1704). No impurity peaks were detected in the XRD pattern. A hierarchical, porous, flake-like morphology was observed in the DMF/DIW system, as shown in Fig. 2b.

Fig. 2 (a) XRD pattern; (b, c) SEM images with different magnifications; (d) TEM image; (e) HRTEM image; (f) ED pattern of the prepared flake-like NiCo₂O₄ sample.

The mean thickness of the flake-like structure was approximately 20 nm. Typical TEM images revealed well-defined, porous network-like features for individual nanoflake units (Fig. 2d). The specific flake-like structure was composed of numerous small-sized nanoparticles. Further TEM observations showed that the average pore size was approximately several nanometers, as shown in Fig. 2e. In addition, the nanoflake displayed a clear crystal lattice stripe, with a spacing of 2.15 Å, corresponding to (311) plane. In addition, the electron diffraction (ED) analysis showed a visible multiple diffraction circle pattern (Fig. 2f), indicating good polycrystalline nature. Moreover, the energy-dispersive spectroscopy (EDS) pattern can further identify the presence of the Ni, O, and Co elements for the prepared sample, which indicates pure NiCo₂O₄ species (ESI Fig. S1†). For comparison, while altering the mixed solvents systems, other reaction conditions are kept constant; then, NiCo₂O₄ samples with different morphologies can be produced in the case of the methanol/DIW and/or ethanol/DIW systems. Their morphology, structure and porous feature information are shown in Fig. S2–S4.†

According to the present experimental observations, we propose a possible formation process for the flake-like structure. Co²⁺ and Ni²⁺ first react with EDTA via specific coordination interactions to form coordinated complex intermediates, including Co²⁺ and Ni²⁺ ions. As the reaction proceeds, the preformed coordinated complex intermediates can form initial crystal nuclei at the early reaction stage. Because the formed nanonuclei are thermodynamically unstable due to their high surface energy, they tend to gather to form stable crystal nuclei. Moreover, the nuclei can undergo orientation growth in the presence of EDTA to form a kind of flake-like structure. Consequently, the porous nanoflake structure can be shaped via calcination removal of EDTA species. The Ostwald ripening process can contribute to the formation of the flake-like morphology; details of the formation process can be seen in Fig. 1.

An isothermal N₂ adsorption–desorption analysis was performed to evaluate the porous features of the samples. The N₂ adsorption–desorption isotherm at 77 K and the Barrett–Joyner–Halenda (BJH) adsorption pore size distribution plots of the prepared NiCo₂O₄ sample in the case of the DMF/DIW system are plotted in Fig. 3. The data show that the nitrogen adsorption isotherm is a typical type-IV curve. Additionally, the hysteresis in the nitrogen adsorption isotherm suggests uniform mesoporous features. The specific BET surface area and pore volume of the obtained NiCo₂O₄ sample were 142.48 m² g⁻¹ and 0.2522 cm³ g⁻¹, respectively. In addition, according to the corresponding BJH pore size distribution curve, the pore size was approximately 3.7 nm (Fig. 3, inset).

Fig. 3 Typical isothermal nitrogen adsorption/desorption curve of NiCo₂O₄ in the case of DMF/DIW system at 77 K and pore-size-distribution curve (inset).

To further understand the surface elements state of the as-prepared NiCo₂O₄, XPS testing was implemented, and the corresponding results were presented in Fig. 4. The survey spectrum (Fig. 4a) indicates the presence of Ni, Co, and O species in the sample. By using a Gaussian fitting method, the Ni 2p emission spectrum (Fig. 4c) is split into two spin–orbit doublets characteristic of Ni²⁺ and Ni³⁺ and two shakeup satellites. The fitting peaks at 855.7 and 873.1 eV are indexed to Ni²⁺ species, whereas the other fitting peaks at 854.2 and 871.7 eV are ascribed to Ni³⁺ species.³⁴ In addition, two kinds of Co species can be observed and assigned to the species containing Co(II) and Co(III) ions (Fig. 4b). Specifically, the two fitting peaks at 780.7 and 795.8 eV are attributed to Co²⁺, whereas the other two fitting peaks at 779.4 and 794.3 eV belong to Co³⁺, as shown in Fig. 4b. The high-resolution spectrum for O 1s (Fig. 4d) suggests three oxygen species marked as O1, O2, and O3. The fitting peak at 529 eV is a typical metal–oxygen bond, corresponding to the O1 component. With respect to the O2 component, the fitting peak at 531 eV is commonly associated with defects, contaminants, and a number of surface species, including hydroxyls, chemisorbed oxygen, and under-coordinated lattice oxygen, of the sample. The O3 species, corresponding to peaks at 532.8 eV, could be attributed to multiplicity of physical and chemo-absorbed water at the sample suface.^35,36

Fig. 4 XPS patterns of the prepared NiCo₂O₄ sample before and after catalytic degradation reaction, (a) summary of the XPS; (b) Co 2p; (c) Ni 2p; (d) O 1s.

Generally, several types of reactive radicals can be generated from PMS activated by transition metal oxides, such as sulphate, peroxy-sulphate and hydroxyl radicals. Then, SO₄⁻˙ and HO˙ could attack the organic dye molecules.^23,37 RhB has a major absorption band at approximately 554 nm.¹⁹ The colour of the RhB solution completely disappears after 30 min in the presence of NiCo₂O₄ and PMS, as shown in Fig. 5a, implying that the chromophoric structure of RhB is completely decomposed. A comparison between the spectrum after 20 min and the initial spectrum showed that approximately 95% of the RhB was degraded. Furthermore, the degradation efficiency of RhB as a function of the catalyst time under various catalysts was determined (Fig. 5b). The degradation efficiency of RhB under NiCo₂O₄ formed in the DMF/DIW system is improved compared with the other two catalysts and literature results,^19,25 for example, Yu et al. using Biomass of baker's yeast as biosorbent for the removal of methylene blue and rhodamine B, and the dyes-loaded biomass was regenerated by acid TiO₂ hydrosol. The removal rate was gradually decreased with time and reach equilibrium at approximately 4 h.³⁸ Han et al. synthesised of ZnFe₂O₄ nanoplates by succinic acid-assisted hydrothermal route and further photocatalytic degradation of rhodamine B under visible light. The degradation of RhB under Xe lamp irradiation can be completed within 120 min.³⁹ Wang et al. used MoS₂/Bi₂O₂CO₃ composites as catalyst to photodegradation of RhB under UV light radiation, the RhB can be completely degraded within 75 min.⁴⁰ these data clearly demonstrated that catalysts with specific morphologies and surface microstructure and porous features play a key role in the degradation of RhB.

Fig. 5 UV-vis spectral changes of RhB in the presence of NiCo₂O₄/PMS (a); comparison of the catalytic performances of samples under different conditions (b), reaction conditions are the following: [RhB] = 0.05 mmol L⁻¹, [catalyst] = 0.2 g L⁻¹, [PMS] = 0.5 mmol L⁻¹, initial pH is 3.28, room temperature.

Meanwhile, we have investigated the degradation efficiency of the prepared NiCo₂O₄ catalyst for the different organic dyes under similar conditions, as shown in Fig. 6a, results indicated that most organic dyes can be completely degraded under 40 min, which is superior to most previously reported results.^20–23 In addition, different concentrations of PMS were chosen as the catalyst, and the degradation efficiency of RhB was optimal when the concentration of PMS ranged from 0.5 to 1 mmol L⁻¹, as shown in Fig. 6b. The effects of different concentrations of NiCo₂O₄ catalyst formed in the DMF/DIW system on the catalytic reaction rate were investigated, and the results are shown in Fig. 6c. The catalytic reaction rate exhibited obvious enhancement with the increase of the concentrations of NiCo₂O₄. Furthermore, the plots of the reaction time (t) versus Co/C at different catalytic systems displayed a nearly linear relation. The determined reaction rate constants (K) were measured to be 0.10939, 0.11738, 0.1193, and 0.00222 min⁻¹, respectively (Fig. 7).

Fig. 6 The catalytic degradation of different organic pollutants by the NiCo₂O₄ formed in DMF/DIW system (a); the effect of catalyst concentrations (b); the effect of PMS concentrations (c). Catalytic conditions: [organic dyes] = 0.05 mmol L⁻¹, [catalyst] = 0.2 g L⁻¹, [PMS] = 0.5 mmol L⁻¹, initial pH is 3.28.

Fig. 7 Kinetic studies of RhB dye degradation in the presence of different mixture solvent systems or the absence of catalysts, (a) methanol/DIW + PMS; (b) ethanol/DIW + PMS; (c) DMF/DIW + PMS; (d) only PMS.

The homogeneous NiCo₂O₄ sample prepared in the DMF/DIW system can be easily recycled by a simple magnet separation. After five cycles of RhB degradation, the catalyst does not exhibit any catalytic activity loss, as shown in Fig. 8. After ten cycles, the NiCo₂O₄ suffers from a little catalytic activity loss, suggesting high catalytic stability. The robust structural stability of NiCo₂O₄ was further investigated by XRD, SEM, and FTIR analysis. No considerable change in XRD profiles was observed after for 10 catalytic cycles, in which the porous flake-like morphology was preserved after 10 cycles, as shown in Fig. 9. The FTIR spectra of the NiCo₂O₄ catalyst before and after catalytic reaction are similar (see ESI Fig. S5†). These data fully confirmed that the prepared NiCo₂O₄ catalyst possesses robust structure integrity.

Fig. 8 Catalytic cycling curves of the prepared NiCo₂O₄ catalyst towards degradation of RhB. The NiCo₂O₄ catalyst was prepared in the case of DMF/DIW system.

Fig. 9 Structure and morphology analysis of the NiCo₂O₄ catalyst after 10 cycle's reaction, (a) XRD pattern; (b) SEM image.

Moreover, the Co 2p3/2 peak after the catalytic oxidation process is composed of two peaks at 779.90 eV for Co^II and 781.15 eV for Co^III, and their atom ratio was changed to 53.82 : 46.18 (Fig. 4b). After the reaction, three oxidation states of Ni species were still present in the catalyst, but their atom ratio was changed to 66.25 : 33.75 (Fig. 4c), indicating that the valence of Ni on the surface of the used catalyst had a noticeable change. The electron couples of Co^III/Co^II coexist in the spinel NiCo₂O₄ structures, which can provide chemical activity. The high-resolution O 1s spectra of NiCo₂O₄ before and after catalytic oxidation are resolved into two individual peaks (Fig. 4d). O1 and O2 are active oxygen species for the radical's generation and oxidation decomposition.⁴¹ after the catalytic reaction, the relative intensity of O1 is reduced from 40.58 to 40.15%, and the relative intensity of O2 is increased from 37.29 to 45.33%, indicating that both O1 and O2 are involved during the degradation process. The increase of the O2 concentration is due to the formation of M–OH groups (M: Co or Ni) or O²⁻ adsorbed on the NiCo₂O₄ surface, which may contribute to the enhancement of the Fenton-like process.⁴² The decrease of lattice oxygen in NiCo₂O₄ may be oxidized by M³⁺ with its reduction to M²⁺. As a consequence, we conclude that the NiCo₂O₄ indeed possesses high catalytic activity and durability in the degradation of RhB. The cycled NiCo₂O₄ can be reused for the next catalytic reaction through direct magnet separation and sequent washing, which suggests that the prepared NiCo₂O₄ exhibits certain paramagnetic features, wherein the measured coercivity and saturation magnetization of the NiCo₂O₄ are 15.2 Oe and 3.2 emu g⁻¹, respectively, as shown in Fig. 10.

Fig. 10 Magnetic performances analysis of the prepared NiCo₂O₄ sample in the case of DMF/DIW system.

In our work, radical quenching studies are conducted to identify the dominant reactive oxygen species in the heterogeneous NiCo₂O₄/PMS system. According to literature reports, SO₄⁻˙, HO˙, and SO₅⁻˙ radicals can be generated for the degradation of organic pollutants by the catalyst-mediated activation of PMS.^43,44 MeOH is utilized to scavenge both HO˙ and SO₄⁻˙, due to their high reactivity for oxidation species (KOH˙: 1.2–2.8 × 10⁹ M⁻¹ s⁻¹; KSO₄⁻˙: 1.6–7.8 × 10⁶ M⁻¹ s⁻¹). In addition, TBA is used as a quenching agent for HO˙ because of its high reactivity (KHO˙: 3.8–7.6 × 10 ⁸ M⁻¹ s⁻¹) but not for SO₄⁻˙ radicals (KSO₄⁻˙: 4–9.1 × 10⁵ M⁻¹ s⁻¹).^45–47 Meanwhile, a peroxomonosulphate radical (SO₅⁻˙) might also be formed, but would not contribute to RhB degradation due to its lower redox potential (1.1 eV).^22,46 Therefore, quenching tests were performed using TBA as a scavenger for HO˙ and MeOH as a scavenger for HO˙ and SO₄⁻˙. As shown in Fig. 11, when no quenching agent was added, approximately 100% RhB was completely degraded within 30 min. However, with addition of 1 M TBA or MeOH to the reaction system, the degradation efficiency of RhB (in 30 min) was approximately 75 and 25%, respectively, indicating an excellent inhibition effect. The inhibition effect was enhanced with the increase of the concentration, indicating HO˙ and SO₄⁻˙ radicals were involved in the oxidation degradation of RhB. MeOH inhibited RhB degradation more significantly compared with TBA at the same concentration, indicating that the main radical species generated during the activation of PMS by NiCo₂O₄ were SO₄⁻˙ radicals. A small amount of HO˙ was also generated in the catalytic oxidation, which comes from the reaction of sulphate radical with OH⁻ in water. These results confirmed that the HO˙ and SO₄⁻˙ radicals are involved in the NiCo₂O₄/PMS oxidation process, which agrees with recent findings in the literatures.^23,44,46,48

≡Ni^II + HSO₅⁻ → ≡Ni^III + SO₄²⁻ + HO˙ (1)

≡Co^II + HSO₅⁻ → ≡Co^III + SO₄²⁻ + HO˙ (2)

≡Ni^II–⁻OH + HSO₅⁻ → ≡Ni^III–⁻OH + SO₄⁻˙ + OH⁻ (3)

≡Ni^III–⁻OH + HSO₅⁻ → ≡Ni^II–⁻OH + SO₅⁻˙ + H⁺ (4)

≡Co^II–⁻OH + HSO₅⁻ → ≡Co^III–⁻OH + SO₄⁻˙ (5)

SO₄⁻˙ + H₂O → HO˙ + HSO₄⁻ (6)

SO₄⁻˙ + OH⁻ → HO˙ + SO₄²⁻ (7)

SO₄⁻˙ (or HO˙) + RhB → […many steps…] → CO₂ + H₂O (8)

Fig. 11 Inhibition analysis of TBA and MeOH on RhB degradation by NiCo₂O₄/PMS system at room temperature, the reaction conditions are the following: [RhB] = 0.05 mmol L⁻¹, [catalyst] = 0.2 g L⁻¹, [PMS] = 0.5 mmol L⁻¹, initial pH is 3.28.

Based on the above analysis results, the main processes during the catalytic activation of PMS by NiCo₂O₄ are proposed as follows. First, ≡Ni and ≡Co ions act as Lewis sites and combine with the dissociative adsorption of water molecules to generate ≡Ni–⁻OH and ≡Co–⁻OH.⁴⁹ After the addition of PMS, ≡Ni and ≡Co on the catalysts first react with PMS to generate HO˙ (eqn (1) and (2)). Moreover, ≡Ni^II–⁻OH species on the NiCo₂O₄ surface activate PMS to generate surface-bound SO₄⁻˙ (eqn (3)) and some more ≡Ni^II–⁻OH species can be produced from the formed ≡Ni^III–⁻OH species with the reaction of PMS (eqn (4)). Similarly, ≡Co^II–⁻OH and ≡Co^III–⁻OH species on the catalyst surface react with PMS to produce surface-bound SO₄⁻˙ (eqn (5)). The standard reduction potential of Ni^III/Ni^II is less than the standard reduction potential of Co^III/Co^II.^11,49 Therefore, the reduction of Ni(III) by Co(II) and Co(III) is thermodynamically favourable, which means the generated Ni(II) can be regenerated on the surface of NiCo₂O₄. Herein, ≡Ni^III/≡Ni^II, ≡Co^III/≡Co^II are redox couples, behaving similarly to the Fenton reaction according to the Harber–Weiss cycle,^50–52 which was evidenced by the XPS results. These couples may result in more active sites on the catalyst surface and contribute to the catalytic activity of NiCo₂O₄. In addition, SO₄⁻˙ can react with water or OH⁻ to produce HO˙ (eqn (6) and (7)). The regeneration of the catalyst can continuously proceed until PMS is consumed completely.^23,53,54 Furthermore, RhB in aqueous solution was enriched continuously on the surface of NiCo₂O₄ and was broken down by SO₄˙⁻ and HO˙ (eqn (8)). In addition, NiCo₂O₄ possesses a high specific surface area of 142.48 m² g⁻¹ and a suitable pore size of 3.54 nm, which could decrease the mass transport resistance and allow easier access of the reactants to the active surface sites and further generate more active radicals, such as SO₄˙⁻ and HO˙. As a consequence, the prepared NiCo₂O₄ has excellent activity ability to activate PMS and effectively degrade RhB.

Conclusions

In summary, porous flake-like NiCo₂O₄ nanostructures have been fabricated via a facile hydrothermal reaction followed by a subsequent high-temperature annealing treatment. The prepared NiCo₂O₄ acts as catalyst towards the activation of PMS for the degradation of organic dye RhB. The results demonstrated that the NiCo₂O₄/PMS heterogeneous catalysts display high catalytic activity and excellent cycling stability. This novel catalyst design strategy can be easily extended to other binary or ternary metal oxides with unique hierarchical nanostructures.

Acknowledgements

We thank the funding support from Program for New Century Excellent Talents in University (NCET-13-0953), the Science and Technology Committee of Shaanxi Province (Grant No. 2014KW09-03, 2011KGXX47).

Notes and references

1. Q. Q. Chen, P. X. Wu, Y. Y. Li, N. W. Zhu and Z. Dang, J. Hazard. Mater., 2009, 168, 901.
2. E. Forgacs, T. Cserhati and G. Oros, Environ. Int., 2004, 30, 953.
3. M. H. Hamdi and A. A. Ashraf, J. Hazard. Mater., 2009, 168, 1070.
4. J. Madhavan, P. Maruthamuthu, S. Murugesan and S. Anandan, Appl. Catal., B, 2008, 83, 8.
5. Y. S. Ma, C. F. Sung and J. G. Li, J. Hazard. Mater., 2010, 178, 320.
6. J. H. Sun, S. H. Shi, Y. F. Lee and S. P. Sun, Chem. Eng. J., 2009, 155, 680.
7. M. Tekbas, H. C. Yatmaz and N. Bektas, Microporous Mesoporous Mater., 2008, 115, 594.
8. G. P. Anipsitakis and D. D. Dionysiou, Environ. Sci. Technol., 2003, 37, 4790.
9. P. Avetta, A. Pensato, M. Minella, M. Malandrino, V. Maurino, C. Minero, K. Hanna and D. Vione, Environ. Sci. Technol., 2015, 49, 1043.
10. P. R. Shukla, S. B. Wang, H. Q. Sun, H. M. Ang and M. Tadé, Appl. Catal., B, 2010, 100, 529.
11. G. Lente, J. Kalmaír, Z. Baranyai, A. Z. Kun, I. Keík, D. V. Bajusz, M. Takaícs, L. Veres and I. N. Faíbiaín, Inorg. Chem., 2009, 48, 1763.
12. T. Zhang, Y. Chen, Y. R. Wang, J. Le Roux, Y. Yang and J. P. Croué, Environ. Sci. Technol., 2014, 48, 5868.
13. F. Ji, C. L. Li, X. Y. Wang and J. Yu, Chem. Eng. J., 2013, 231, 434.
14. Y. H. Guan, J. Ma, Y. M. Ren, Y. L. Liu, J. Y. Xiao, L. Q. Lin and C. Zhang, Water Res., 2013, 47, 5431.
15. C. Cai, H. Zhang, X. Zhong and L. W. Hou, J. Hazard. Mater., 2015, 283, 70.
16. M. Stoyanova, I. Slavova, S. Christoskova and V. Ivanova, Appl. Catal., A, 2014, 476, 121.
17. Z. Y. Wang, G. W. Hu, J. Liu, W. S. Liu, H. L. Zhang and B. D. Wang, Chem. Commun., 2015, 51, 5069.
18. S. J. Yang, H. P. He, D. Q. Wu, D. Chen, X. L. Liang, Z. H. Qin, M. D. Fan, J. X. Zhu and P. Yuan, Appl. Catal., B, 2009, 89, 527.
19. L. Y. Wang, G. W. Hu, Z. Y. Wang, B. D. Wang, Y. M. Song and H. Tang, RSC Adv., 2015, 5, 73327.
20. W. B. Shi, X. D. Zhang, S. H. He and Y. M. Huang, Chem. Commun., 2011, 47, 10785.
21. S. N. Su, W. L. Guo, Y. Q. Leng, C. L. Yi and Z. M. Ma, J. Hazard. Mater., 2013, 244, 736.
22. M. Zhou, H. Yang, T. Xian, R. S. Li, H. M. Zhang and X. X. Wang, J. Hazard. Mater., 2015, 289, 149.
23. Q. J. Yang, H. Choi, R. Al-Abed Souhail and D. D. Dionysiou, Appl. Catal., B, 2009, 88, 462.
24. L. J. Xu and J. L. Wang, Environ. Sci. Technol., 2012, 46, 10145.
25. X. Wang, X. D. Han, M. F. Lim, N. Singh, C. L. Gan, M. Jan and P. S. Lee, J. Phys. Chem. C, 2012, 116, 12448.
26. F. Z. Deng, L. Yu, G. Cheng, T. Lin, M. Sun, F. Ye and Y. F. Li, J. Power Sources, 2014, 251, 202.
27. Y. Q. Wu, X. Y. Chen, P. T. Ji and Q. Zhou, Electrochim. Acta, 2011, 56, 7517.
28. Q. Lu, Y. P. Chen, W. F. Li, G. G. Chen, J. Q. Xiao and F. Jiao, J. Mater. Chem. A, 2013, 1, 2331.
29. X. C. Dong, H. Xu, X. W. Wang, Y. X. Huang, M. B. ChanPark, H. Zhang, L. H. Wang, W. Huang and P. Chen, ACS Nano, 2012, 6, 3206.
30. Y. Lei, Y. Y. Wang, W. Yang, H. Y. Yuan and D. Xiao, RSC Adv., 2015, 5, 7575.
31. D. Lee, B. Kim and Z. W. Chen, J. Mater. Chem. A, 2013, 1, 4754.
32. H. Y. Zhu, S. Zheng, Y. X. Huang, L. H. Wu and S. H. Sun, Nano Lett., 2013, 13, 2947.
33. Y. Xiao, C. G. Hu, L. T. Qu, C. W. Hu and M. H. Cao, Chem.–Eur. J., 2013, 19, 14271.
34. J. Bao, X. D. Zhang, B. Fan, M. Zhou, W. Yang, X. Hu, H. Wang, B. C. Pan and Y. Xie, Angew. Chem., Int. Ed., 2015, 54, 7399.
35. X. H. Lu, X. Huang, S. L. Xie, T. Zhai, C. S. Wang, P. Zhang, M. H. Yu, W. Li, C. L. Liang and Y. X. Tong, J. Mater. Chem., 2012, 22, 13357.
36. X. W. Xie, Y. Li, Z. Q. Liu, M. Haruta and W. J. Shen, Nature, 2009, 458, 746.
37. G. K. Zhang, Y. Y. Gao, Y. L. Zhang and Y. D. Guo, Environ. Sci. Technol., 2010, 44, 6384.
38. J. X. Yu, B. B. Li, X. M. Sun, Y. Jun and R. A. Chi, Biochem. Eng. J., 2009, 45, 145.
39. L. J. Han, X. Zhou, L. N. Wan, Y. F. Deng and S. Z. Zhan, J. Environ. Chem. Eng., 2014, 2, 123.
40. Q. Z. Wang, G. X. Yun, Y. Bai, N. An, J. H. Lian and H. H. Huang, Appl. Surf. Sci., 2014, 313, 537.
41. Z. F. Huang, H. W. Bao, Y. Y. Yao, W. Y. Lu and W. X. Chen, Appl. Catal., B, 2014, 154, 36.
42. Y. Ren, L. Lin, J. Ma, J. Yang, J. Feng and Z. Fan, Appl. Catal., B, 2015, 165, 572.
43. F. Ji, C. Li, X. Wei and J. Yu, Chem. Eng. J., 2013, 231, 434.
44. Y. B. Ding, L. H. Zhu, N. Wang and H. Q. Tang, Appl. Catal., B, 2013, 129, 153.
45. J. Zou, J. Ma, L. W. Chen, X. C. Li, Y. H. Guan, P. C. Xie and C. Pan, Environ. Sci. Technol., 2013, 47, 11685.
46. J. Yang, C. Misuk and Y. Lee, Water Res., 2013, 47, 5431.
47. T. Zhang, H. B. Zhu and J. P. Croué, Environ. Sci. Technol., 2013, 47, 2784.
48. J. A. Khan, X. He, H. M. Khan, N. S. Shah and D. D. Dionysiou, Chem. Eng. J., 2013, 218, 376.
49. Y. M. Ren, L. Q. Lin, J. Ma, J. Yang, J. Feng and Z. J. Fan, Appl. Catal., B, 2015, 165, 572.
50. S. Fronaeus, J. Berglund and L. I. Elding, Inorg. Chem., 1998, 37, 4939.
51. T. Lan, L. C. Lei, B. Yang, X. W. Zhang and Z. J. Li, Ind. Eng. Chem. Res., 2013, 52, 4740.
52. G. Y. Hao, W. Wang, G. F. Gao, Q. Zhao and J. P. Li, Environ. Sci. Technol., 2012, 46, 10145.
53. R. Yuan, S. N. Ramjaun, Z. Wang and J. Liu, J. Hazard. Mater., 2011, 196, 173.
54. J. Deng, Y. S. Shao, N. Y. Gao, C. Q. Tan, S. Q. Zhou and X. H. Hu, J. Hazard. Mater., 2013, 262, 836.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra12706a

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