Flower-Shaped C-Dots/Co₃O₄{111} Constructed with Dual-Reaction Centers for Enhancement of Fenton-Like Reaction Activity and Peroxymonosulfate Conversion to Sulfate Radical

Abstract

Novel flower-shaped C-dots/Co₃O₄{111} with dual-reaction centers were constructed to improve the Fenton-like reaction activity and peroxymonosulfate (PMS) conversion to sulfate radicals. Due to the exposure of a high surface area and Co₃O₄{111} facets, flower-shaped C-dots/Co₃O₄{111} could provide more Co(II) for PMS activation than traditional spherical Co₃O₄{110}. Meanwhile, PMS was preferred for adsorption on Co₃O₄{111} facets because of a high adsorption energy and thereby facilitated the electron transfer from Co(II) to PMS. More importantly, the Co–O–C linkage between C-dots and Co₃O₄{111} induced the formation of the dual-reaction center, which promoted the production of reactive organic radicals (R•). PMS could be directly reduced to SO₄⁻• by R• over C-dots. On the other hand, electron transferred from R• to Co via Co–O–C linkage could accelerate the redox of Co(II)/(III), avoiding the invalid decomposition of PMS. Thus, C-dots doped on Co₃O₄{111} improved the PMS conversion rate to SO₄⁻• over the single active site, resulting in high turnover numbers (TONs). In addition, TPR analysis indicated that the optimal content of C-dots doped on Co₃O₄{111} is 2.5%. More than 99% of antibiotics and dyes were degraded over C-dots/Co₃O₄{111} within 10 min. Even after six cycles, C-dots/Co₃O₄{111} still remained a high catalytic activity.

1. Introduction

Being attributed to higher oxidative potential (E⁰ = 2.5–3.1 V) and longer half-life (t_1/2 = 30–40 μs), sulfate radical (SO₄⁻•) could degrade organics more efficiently than OH• (E⁰ = 2.80 V, t_1/2 = <1 μs) in neutral solutions. Generally, a Fenton-like reaction was significantly influenced by pH. In neutral/alkaline condition, transition metal ion might form precipitate, resulting in preventing the reaction. On the other hand, SO₄⁻• could react with OH⁻ to produce OH• with lower oxidative ability. To enhance the peroxymonosulfate (PMS) conversion to SO₄⁻•, various transition metals (Fe, Cu, Mn, and Co) were investigated as activator [1,2,3]. Among them, cobalt oxides [CoO, CoO₂, CoO(OH), Co₂O₃, and Co₃O₄] were considered as the most promising catalysts due to high standard reduction potential (E⁰ = +1.92 V vs. NHE) and redox of Co(III)/Co(II) [1]. During the process of a Fenton-like reaction, PMS can not only react with Co(II) as the electron acceptor but also reacts with Co(III) as the electron donor, accompanied by the circulation of Co(III)/Co(II) and the generation of SO₄⁻• or SO₅⁻• [4]. However, similar to classic Fenton reaction, the rate-limiting step [Co(III) + HSO₅⁻ → Co(II) + SO₅⁻• + H⁺] and low conversion PMS to SO₅⁻• seriously restrict the application of cobalt oxides as catalysts [5]. In addition, the active components of catalysts might be converted into low activity substances during the reaction process due to poor stability. For example, cobalt oxides might be corroded and dissolved during the redox process, resulting in deterioration of catalytic performance.

In general, catalysts activity and stability could be influenced by surface morphology and crystal phase. Firstly, the mass transfer between organics and catalyst active sites was affected by the catalysts morphology. Li et al. constructed Fe_xCo_3−xO₄ with porous nanocages structure, which indicated that such morphology significantly improved the catalytic activity and stability [6]. Secondly, atoms arrangement of catalyst, which led to exposure of transition metal with different chemical value, was depended on the crystal structure [7,8]. As for Co₃O₄, both Co(Ⅲ) and Co(Ⅱ) occupied lattice point of octahedron {110} facet, while Co(Ⅱ) entirely occupied the lattice point of tetrahedron {111} facet [8,9]. More importantly, different crystal facets possessed different adsorption energies and electrons transfer properties, which might directly influence the catalytic reaction. Hensen et al synthesized CeO₂ with different crystal planes to investigate its effect on CO oxidation, which found that the free energy barrier of Pd/CeO₂(100) for the CO catalytic cycle was higher than that of Pd/CeO₂(111) [10]. Thus, it is expected that Co₃O₄ constructed by tetrahedron{111} can not only provide more Co(Ⅱ) for PMS activation, but also possess higher adsorption energies and electrons transfer properties for PMS activation.

On the other hand, although increase of Co(Ⅱ) exposed can obviously improve the catalytic activity, Co(Ⅲ) would be still accumulated in the system because of the rate limitation step [Co(III) + HSO₅⁻ → Co(II) + SO₅⁻• + H⁺] with the reaction time. Since the oxidative potential of SO₅⁻• is relatively low, the degradation rate of organics by SO₅⁻• is much lower than by SO₄⁻•. Thus, the selective PMS conversion to SO₄⁻• over cobalt oxides still remained to be improved. In general, cation–π interaction could be formed between the cations and aromatic systems, which could be applied in the classic Fenton system for improvement of transition metal redox [11]. Zubir et al. demonstrated that unpaired π electrons of GO could transfer electrons between GO and iron centers via cation–π interactions, which significantly improved the recyclability of Fe₃O₄ [12]. In addition, the active transition complex [C-H₂O₂], which was formed between covalent carbon networks and H₂O₂, can directly transfer electron from the π-system to H₂O₂ with the generation of OH• [13]. Analogous to g-C₃N₄, carbon quantum dots (C-dots) are considered as a potential carbon-based co-catalyst due to their graphene structure [14,15]. Wang et al. synthesized the SiO₂@C-dots/phosphotungstate catalyst with an inert SiO₂ core and a catalytic active shell made up of the prepared amphiphilic phosphotungstate and C-dots, which proved that C-dots were an efficient co-catalyst in the system [16]. Our previous work also demonstrated that an electron-rich Cu center and electron-deficient π-electron conjugated system could be constructed by doping of g-C₃N₄ in Cu-Al₂O₃, which provided two electron-transfer routes for OH• generation in the presence of H₂O₂ [11]. Meanwhile, addition of C-dots in the hydrothermal system could modulate the formation of crystal structure because of the large number of oxygen functional groups on the surface of C-dots, which act as “surfactants”. Thus, it is expected to simultaneously enhance the catalytic activity and selective PMS conversion to SO₄⁻• by doping of C-dots into Co₃O₄{111}. Notably, C-dots can improve the production of reactive organic radicals (R•) from the reaction of SO₄⁻• and organics. Such reactive organic radicals play an important role in the enhancement of catalytic activity and selective PMS conversion to SO₄⁻•. Some of electrons can be transferred from R• to Co(III) via Co–O–C linkage for Co(III) reduction to Co(Ⅱ) without PMS decomposition, and other electrons can be transferred from R• to PMS over C-dots with the generation of SO₄⁻•.

Based on the above research background and conception, flower-shaped C-dots/Co₃O₄{111} with high active crystal facets are constructed for improvement of catalytic activity and selective PMS conversion to SO₄⁻•. Being attributed to {111} facets, flower-shaped C-dots/Co₃O₄{111} could not only provide large amounts of Co(II) but also possess higher adsorption energies and electrons transfer properties for PMS reduction to SO₄⁻•. More importantly, doping of C-dots on Co₃O₄{111} could facilitate the production of reactive organic radicals (R•) which can act as the important electron donor. On one hand, Co(III) in C-dots/Co₃O₄{111} can be reduced to Co(II) by oxidation of R•. On the other hand, SO₄⁻• can be generated from the reaction of PMS and R• over C-dots. Such synergistic effect significantly improved PMS activation and selective conversion to SO₄⁻•.

2. Results and Discussion

2.1. Characterization of Catalysts

Cetrimonium Bromide (CTMAB) showed significant influence on the surface morphology of Co₃O₄ during the hydrothermal reaction process. Co₃O₄ synthesized by hydrothermal reaction without CTMAB are spherical particles with the diameter of 2 μm (Figure 1a). However, due to a decrease of the surface energy, ethanol and cobalt salts could form nano-sheets in the present of CTMAB. Thus, the product of Co₃O₄ and C-dots/Co₃O₄{111}, which are synthesized in the present of CTMAB, shows a flower-shaped structure (diameter = 5 μm) with a nano-sheets thickness of 67 nm (Figure 1b,c). Notably, the flower-shaped structure increased the surface area and pore volume of catalyst, which could be confirmed by the results of the BET surface area (

Table 1. The BET specific surface area, total pore volume, and average pore diameter of catalysts prepared.

Catalyst	BET Specific Surface Area (m2/g)	Total Pore Volume (cm3/g)	Average Pore Diameter (Å)
Spherical Co3O4{110}	35.45	0.045	66.91
Flower-shaped Co3O4{111}	83.01	0.092	76.25
Flower-shaped C-dots/Co3O4{111}	109.11	0.107	68.6

). The BET surface area of flower-shaped C-dots/Co₃O₄{111} was 109.11 m²/g, which was much higher than that of spherical Co₃O₄{110} (35.45 m²/g). A large specific surface area is beneficial to improving the catalytic activity of the catalyst.

Furthermore, as shown in TEM (Figure 1d), the ‘d’ spacing of lattice fringe corresponding to (111) and (220) planes of spherical Co₃O₄ are respectively 0.464 nm and 0.284 nm, and the inter planar angles between these two planes are 90°, suggesting the {110} crystal structure of the spherical Co₃O₄ [17]. Different from spherical Co₃O₄{110}, the ‘d’ spacing of lattice fringe corresponding to (220) and (442) planes of flower-shaped Co₃O₄{111} are respectively 0.286 nm and 0.167 nm (Figure 1e), and the inter planar angle between these two planes is 30°, suggesting the {111} crystal structure of flower-shaped Co₃O₄{111} [18]. Such difference between spherical Co₃O₄{110} and flower-shaped Co₃O₄{111} was due to that surfactants were preferentially adsorbed on crystal faces, which limited the directional growth of nanocrystals and stabilizing the crystal faces. Various ordered aggregates formed by self-assembly of surfactants in solution can be used as microreactors or templates, which could control the morphology of nano-materials [19]. Thus, addition of CTMAB probably played the dominant role in the formation of {111} crystal structure of Co₃O₄ during the process of hydrothermal reaction. In addition, TEM images (Figure 1f) confirm that the C-dots are successfully doped in the flower-shaped Co₃O₄{111} in which the lattice fringes spacing of 0.322 nm is in accordance with the (002) lattice planes of graphitic carbon [20]. EDS images further demonstrate that the C element is homogenously distributed on the catalyst surfaces (Figure 1g), which show that the C-dots are uniformly doped on Co₃O₄.

XRD spectra clearly describe the crystal structure of catalysts synthesized under different hydrothermal reaction conditions (Figure 2a). The diffraction peaks at 31.4°, 37.1°, 44.9°, 55.9°, 59.7°, and 65.2° were respectively ascribed to the [220], [311], [400], [442], [511], and [440] diffraction planes of Co₃O₄, which could be indexed to a pure phase of spinel Co₃O₄ (JCPDS card NO.43-1003). Since there was no impurity peaks in XRD spectra, which could confirm the excellent crystalline of flower-shaped Co₃O₄{111}, spherical Co₃O₄{110}, and flower-shaped C-dots/Co₃O₄{111} [21]. In general, the Co₃O₄ crystal is a typical spinel structure (Co²⁺(Co³⁺)₂O_2-4), which consists of octahedron (CoO₆) and tetrahedron (CoO₄). Notably, Co(III) species are located in octahedral sites while Co(II) species are in tetrahedral sites (Figure 2b). Since Co(II) is the catalytic site for PMS select conversion to SO₄⁻• [22], tetrahedron structure (CoO₄) with more Co(II) exposed can provide more catalytic sites for PMS activation. Therefore, in perspective of atomic arrangement, accessible Co(II) with PMS is beneficial for improvement of catalytic activity. To a certain degree, it can be considered that the tetrahedron structure (CoO₄) shows higher catalytic activity compared with octahedron structure (CoO₆).

Furthermore, XPS was used to characterize the surface composition and chemical state of flower-shaped Co₃O₄{111}, spherical Co₃O₄{110}, and flower-shaped C-dots/Co₃O₄{111} (Figure 3 and Figure S1). The binding energy peaks near 780 and 795 eV are ascribed to orbits of Co 2p_1/2 and Co2p_3/2, respectively (Figure 3a,c,e). The orbital peak of Co 2p_3/2 can be further separated into two peaks of Co³⁺ (779.8 eV) and Co²⁺ (781.1 eV). Then, the value of Co(II)/Co(III) on the catalysts surface can be calculated according to the peaks area of different valence state of Co (

Table 2. Element molar ratios [Co(II)/Co(III)] on the surface of different catalysts.

Samples	Binding Energy (eV)				Ratios
	Co(II)	Co(III)	OC	OL	Co(II)/Co(III)	OC/OL
Spherical Co3O4{110}	779.8	781.1	532.1	529.1	0.77	0.659
Flower-shaped Co3O4{111}	779.6	781	532	529	1.36	0.635
Flower-shaped C-dots/Co3O4{111}	779.8	781.2	532.1	529.1	1.36	0.844

). The proportion of Co(II) on the surface of flower-shaped Co₃O₄{111} [Co(II)/Co(III) = 1.36] and C-dots/Co₃O₄{111} [Co(II)/Co(III) = 1.36] are much higher than that on spherical Co₃O₄{110} [Co(II)/Co(III) = 0.77]. Thus, compared with spherical Co₃O₄{110}, more active sites of Co(II), which plays the role of activating PMS, were exposed on the surface of flower-shaped Co₃O₄{111} and C-dots/Co₃O₄{111}. In addition, the orbital peak of O 1s can be separated into three peaks. The binding energy of 529 ± 0.2 eV was the characteristic peak of lattice oxygen (O_L) in the Co₃O₄ phase. The peak of O_V (531 ± 0.1 eV) was ascribed to the O²⁻ in oxygen-deficient regions within the matrix of Co₃O₄, and the peak of O_C (532 ± 0.2 eV) was ascribed to OH⁻, chemisorbed or dissociated oxygen species (O²⁻, O₂⁻, or O⁻). Notably, the value of O_C/O_L of flower-shaped C-dots/Co₃O₄{111} was much higher than those of flower-shaped Co₃O₄{111} and spherical Co₃O₄{110}, which indicated that doping C-dots into Co₃O₄ increased the oxygen functional groups on the surface of the catalyst. Since oxygen functional groups on nano-carbons can act as the catalytic sites for PMS activation to generate radicals. Since occupation of the binding sites by surface hydroxyl sites of catalyst could facilitate the oxidation reactions [22], doping of C-dots into Co₃O₄{111} could improve the catalytic performance.

FT-IR spectra can demonstrate the functional groups of spherical Co₃O₄{110}, flower-shaped Co₃O₄{111}, and flower-shaped C-dots/Co₃O₄{111} (Figure 4a). The characteristic peaks at 574.69 cm⁻¹ and 661.48 cm⁻¹ are assigned to the stretching vibrations of Co–O groups of Co₃O₄, and the peak at 3557 cm⁻¹ belongs to the free O–H stretching vibration [23]. In addition, peaks near 1633.4 cm⁻¹ of samples are related to the bending –OH vibrations of physical adsorbed water [24,25]. Furthermore, the bands in the range of 1490.73 cm⁻¹ are ascribed to the C=C aromatic stretching vibrations. The bands at 1307.52 and 1000.89 cm⁻¹ correspond to the C−O stretching absorption and COO– functional groups, respectively. According to the results of XPS and FT-IR, doping of C-dots greatly enriched the oxygen functional groups (–OH, C=O) on the Co₃O₄ surface, which could significantly improve the catalytic performance of the catalyst.

The redox properties of catalysts were characterized by the H₂-TPR (Figure 4b). The β peak represents the reduction of Co(II) to Co⁰ in Co₃O₄ (Equation (2)), while the α peak represents the reduction of Co(III) to Co(II) (Equation (3)). The α peak and β peak of spherical Co₃O₄{110} are 319 °C and 398 °C, respectively. Due to the oxygen vacancy on the surface of flower-shaped Co₃O₄{111} and C-dots/Co₃O₄{111} (Figure S3), the reduction temperature (Co²⁺ → Co⁰, Co³⁺ → Co²⁺) decreased obviously. The surface oxygen vacancy can promote the reduction of the catalyst, which leads to the reduction of the flower-shaped Co₃O₄{111} and C-dots/Co₃O₄{111} at the lower temperature. In addition, the α peak intensity of flower-shaped Co₃O₄{111} and C-dots/Co₃O₄{111} are much lower than that of spherical Co₃O₄{110}. The area ratio of α peak to β peak of spherical Co₃O₄{110} is 0.31 by calculation, while that of flower-shaped Co₃O₄{111} and C-dots/Co₃O₄{111} is only 0.27, which confirmed that there are more CoO contents in flower-shaped Co₃O₄{111} and C-dots/Co₃O₄{111} crystal compared with spherical Co₃O₄{110}. In addition, TPR results of flower-shaped C-dots/Co₃O₄{111} with different C-dots amounts are provided to explore the optimal doping of C-dots for PMS activation (Figure S2). The temperature of Co³⁺ reduction to Co²⁺ decreases gradually as the C-dots content increase from 0.5% to 2.5%, which indicates that doping of C-dots can improve the reducibility of Co₃O₄{111}. However, the temperature of Co³⁺ reduction to Co²⁺ increases as the C-dots content increased from 2.5% to 3.5%, suggesting that excessive doping of C-dots weaken the reduction ability of Co₃O₄. Similar results are also observed in TPR results of Co²⁺ reduction to Co⁰. Such adverse effect can be explained by that excess C-dots could cover the surface of the catalyst, resulting in the inhibition of the Co₃O₄ reduction by H₂. Thus, C-dots (2.5%)/Co₃O₄{111} were selected for further studies.

Cyclic voltammetry (CV) was used to test the charge transfer at the interfacial region of Co₃O₄/PMS in a three-electrode system (Figure 4c). Ag/AgCl was used as reference electrode, platinum was used as pair electrodes, and glassy carbon was used as working electrode. The mixture of 0.1 M Na₂SO₄ and 0.3 mM PMS was used as electrolyte. The CV curves of flower-shaped Co₃O₄{111} and flower-shaped C-dots/Co₃O₄{111} showed distinct reduction peaks beginning at a low potential of −0.25 V (Figure 4c). These reduction peaks indicated that the surface C-dots served as the active sites for the PMS activation. Compared with flower-shaped Co₃O₄{111}, flower-shaped C-dots/Co₃O₄{111} have a higher current density in the cathode, suggesting that C-dots can transfer electrons to PMS. In addition, compared with flower-shaped Co₃O₄{111}, flower-shaped C-dots/Co₃O₄{111} show the smaller semicircle within a higher frequency region in the EIS Nyquist plots (Figure 4d). Due to the similar graphene structure of C-dots, the Dirac Point effect improved the electron transfer of C-dots. Thus, C-dots doped on flower-shaped Co₃O₄{111} could accelerate the redox cycle of Co(II)/Co(III).

3CoO + 3H₂ → 3Co + 3H₂O,

(1)

Co₃O₄ + H₂ → 3CoO + H₂O.

(2)

2.2. Catalytic Activity and Stability

The degradation rate of OTC and ENR by PMS over flower-shaped C-dots/Co₃O₄{111} are much higher than that over other catalysts (Figure 5a,c). More than 99.3% OTC was degraded in 5 min (PMS = 0.075 mM), while more than 99.7% ENR was degraded in 10 min over the flower-shaped C-dots(2.0 wt%)/Co₃O₄{111} (PMS = 0.3 mM). The degradation kinetics by PMS over various catalysts can be well described by pseudo-first-order kinetic model (−lnC/C₀ = kt), where ‘C₀’ is the initial concentration of OTC, ‘C’ is the concentration of reactant at time ‘t’, and ‘k’ is the rate constant. The rate constants followed the order of flower-shaped C-dots/Co₃O₄{111} > flower-shaped Co₃O₄{111} > spherical Co₃O₄{110} (Table S1). Notably, the rate constant of flower-shaped C-dots/Co₃O₄{111} is four times higher than spherical Co₃O₄{110} and flower-shaped Co₃O₄{111} (Figure 5b,d). Accordingly, the addition of C-dots could obviously improve the catalytic activity of Co₃O₄. The optimal content of C-dots doped in flower-shaped Co₃O₄{111} was 2%.

The influences of catalyst dosage and oxidant concentration on the catalytic degradation of ENR were tested. The degradation rate of ENR increases from 56.4% to 99.4% within 10 min while the dosage of flower-shaped C-dots/Co₃O₄{111} increases from 0.01 to 0.05 g·L⁻¹ (Figure S3a). This was because an increase of the catalysts dosage could supply more active sites for catalytic generation of radicals, which promoted the antibiotics degradation. Similar to the catalyst dosage, the increase of PMS concentration can also improve the degradation of ENR (Figure S3b). As the PMS concentration increased from 0.05 to 0.3 mM, the removal rate of ENR increased from 74.2% to 99.8% within 10 min. Since the generation rate of the radicals was related to the catalyst dosage, further increase of PMS concentration from 0.3 to 0.7 mM could not obviously improve the removal rate of ENR anymore. Therefore, the optimal concentration of PMS for ENR degradation was 0.3 mM.

Since high temperature is beneficial to PMS activation and generation of radicals, the removal rate of ENR obviously increases as the temperature increases from 25 °C to 55 °C (Figure S3c). The degradation kinetic constants also followed the order of K_{55 °C} (2.837 min⁻¹) > K_{45 °C} (1.355 min⁻¹) > K_{35 °C} (0.913 min⁻¹) > K_{25 °C} (0.469 min⁻¹). More than 99% of ENR could be degraded at 25 °C within 10 min. In consideration of energy consumption, the room temperature (25 °C) was selected for subsequent catalytic degradation experiments. Furthermore, due to changing the surface charge of catalyst, solution pH also influenced the degradation process obviously. The degradation rate of ENR increases as the initial solution pH increases from 3.0 to 6.0 (Figure S3d). Only 50% of ENR was removed at pH 3.0 within 20 min while ENR was almost completely degraded at pH 6.0. This is because that Co leached from flower-shaped C-dots/Co₃O₄{111} in acid conditions was unfavorable to PMS activation. Notably, the concentrations of Co leached from catalysts in solution could meet the limit of Environmental Quality Standards for Surface Water in China (1.0 mg/L) (Figure S4). However, the degradation rate of ENR decreases as pH increases from 6.0 to 9.0, which indicated that alkaline conditions made adverse effect to catalytic degradation. This was because the large amount of OH⁻ reacted with SO₄⁻• to generate OH•, resulting in lower oxidation ability in neutral/basic conditions.

In order to evaluate the reusability of flower-shaped C-dots/Co₃O₄{111}, OTC (10 mg/L), and ENR(10 mg/L) solutions are used to further test the catalytic degradation stability of C-dots/Co₃O₄{111} in the present work (Figure 6a,b). After six cycling runs, the degradation rate of OTC was still more than 90% within 2 min, and even 100% within 5 min (Figure 6a). Furthermore, the degradation kinetics did not decrease after the sixth cycle runs, showing a perfect catalytic stability of flower-shaped C-dots/Co₃O₄{111}. Similar to catalytic degradation of OTC, flower-shaped C-dots/Co₃O₄{111} also show high catalytic degradation rate of ENR, Rh B, and MB even after six cycles (Figure 6b–d), which also confirm the excellent stability of flower-shaped C-dots/Co₃O₄{111} for PMS activation.

In addition, the surface state of catalyst after six cycling runs is characterized by XPS spectra (Figure 6e). In this work, we propose that Co–O–C bonds conjugated in flower-shaped C-dots/Co₃O₄{111} facilitate electron transfer and correlate well with the transformation from R to R•. Such synergistic interaction between C-dots and Co₃O₄{111} could accelerate the Co(III)→Co(II) redox cycles. The bridge role played by C-dots could transfer electrons to Co(III) for its quick reduction as organic pollutants are oxidized [12]. XPS spectra of catalysts before and after reaction confirmed this inference. The value of Co(II)/Co(III) of flower-shaped C-dots/Co₃O₄{111} did not decreased after catalytic reaction (Figure 6f), which indicated that Co(II) was not oxidized to Co(III). This synergistic effect can not only effectively maintain the number of active sites of catalysts, but also reduce the ineffective decomposition of PMS. As for flower-shaped Co₃O₄{111}, the value of Co(II)/Co(III) decreased from 1.36 to 1.16 obviously after cycled uses (Figure 6f), indicating that some Co(II) of flower-shaped Co₃O₄{111} was oxidized to Co(III). Previous studies have already demonstrated that Co(II) play important role in PMS activation for generation of sulfate radicals. Thus, compared with flower-shaped Co₃O₄{111}, flower-shaped C-dots/Co₃O₄{111} showed higher catalytic activity and selective peroxymonosulfate (PMS) conversion to sulfate radicals.

2.3. Density Functional Theory (DFT) Calculation

In general, adsorption of PMS on catalysts was the premise of activation process. According to the density functional theory (DFT), the adsorption energy of PMS on {110} and {111} facets of Co₃O₄ are calculated by using the Materials Studio 7.0 CASTEP program (Figure S5). The convergence criteria were as follows: the maximal force on the atoms was 0.03 eV Å⁻¹, the stress on the atomic nuclei was less than 0.05 GPa, the maximal atomic displacement was 0.001 Å, and the maximal energy change per atom was 1.0 e⁻⁵ eV. The adsorption energy of PMS (E_abs) on Co₃O₄ can be calculated by the equation of E_ads = E_PMS+Cat − E_abs − E_Cat, where E_PMS+Cat, E_abs, and E_Cat refer to the energy of the optimized adsorption system, PMS and Co₃O₄, respectively. To obtain accurate results, we optimized the atomic coordinates by minimizing the total energy and atomic forces. The optimum adsorption energy of PMS on {110} and {111} facets are −1.12 eV and −5.97 eV, respectively, suggesting that PMS are preferred to be adsorbed on Co₃O₄{111} facet than on Co₃O₄{110} facet. Moreover, the total charge of PMS adsorbed on {111} facet (−0.32 e) was higher than that on {110} facet (−0.16 e), which also indicated that PMS can obtain more electrons from Co₃O₄{111} for activation. Thus, Co₃O₄{111} can exhibit higher catalytic activity and selective peroxymonosulfate (PMS) conversion to sulfate radical.

2.4. The Possible Degradation Pathway of OTC and ENR

Generally, the unsaturated bonds of organic pollutants are easy to be broken in catalytic oxidation process. Based on the Frontier Orbital Theory, the highest occupied molecular orbital (HOMO) acts as the electron donor because of its weak binding force to electrons, while lowest unoccupied molecular orbital (LUMO) acts as electrons acceptor because of its strong binding force to electrons. In the present work, we calculated the bond energy of OTC and ENR by using Gaussian program, which indicates that the overlap of electronic clouds in LUMO and HOMO obviously influence the separation of e⁻ and h⁺ (Figure S6). The large density of the electron cloud in OTC is mainly in the area of 1C, 9C, 14C, 24N. In the case of ENR, the electronic cloud mainly distributed in 3C, 6C, 19C, 20C. The active sites of pollutants were greatly contributed by the frontier orbitals, and unsaturated bonds strongly influenced the activity of antibiotic molecules. Thus, bonds in these positions of ENR and OTC are broken preferentially. In addition, the electron cloud density of OTC is much greater than that of ENR, which caused a higher degradation rate of OTC compared with ENR.

Furthermore, the intermediates of OTC and ENR degradation are identified by LC-MS/TOF, and iron spectra at different retention time (RT) are presented in ESI Scan (Figures S7 and S8). As for OTC degradation intermediates, the original iron at m/z = 461.1([M + H]⁺) could be identified as C₂₂H₂₄N₂O₉. Furthermore, from the mass spectra, some other peaks appeared as the degradation of OTC, which suggested that there were other intermediate products produced. These main products irons at m/z = 460.2 ([M + H]⁺), m/z = 434.2 ([M + H]⁺), m/z = 447.2 ([M + H]⁺), m/z = 432.2 ([M + H]⁺), m/z = 347.2 ([M + H]⁺), and m/z = 218.2 ([M + H]⁺) might be identified as C₂₂H₂₄N₂O₉, C₂₂H₂₈NO₈, C₂₂H₂₇N₂O₈, C₂₂H₂₆NO₈, C₁₉H₂₅NO₅, C₁₂H₂₇NO₂, and C₉H₆O₄, respectively. In addition, m/z = 194.1 ([M + H]⁺) might be identified as C₁₂H₁₈O₂ or C₁₁H₁₄O₃ [20,26,27,28,29]. Finally, all these substances were thoroughly decomposed into CO₂, H₂O, and other gaseous components. Similarly, iron spectra at different retention time (RT) of ENR degradation are presented in ESI Scan (Figure S8). The original iron at m/z = 360.2([M + H]⁺) can be identified as C₁₉H₂₂FN₃O₃. These main product irons at m/z = 344.3([M + H]⁺), m/z = 332.2 ([M + H]⁺), m/z = 361.1 ([M + H]⁺), m/z = 242.3 ([M + H]⁺), m/z = 174.2 ([M + H]⁺), m/z = 202.2 ([M + H]⁺), and m/z = 102.1 ([M + H]⁺) might be identified as C₁₉H₂₃N₃O₃, C₁₇H₁₈FN₃O₃, C₁₇H₁₅FN₃O₅, C₁₃H₁₀N₂O₃, C₇H₁₃N₂O₃, C₁₀H₅N₂O₃, and C₅H₉O₂; m/z = 302.2 ([M + H]⁺) might be identified as C₁₆H₁₉N₃O₃ or C₁₆H₂₂N₃O₂; and m/z = 192.2 ([M + H]⁺) might be identified as C₁₇H₂₄FN₃O₂ or C₁₀H₁₁N₂O₃ [30,31,32,33]. Finally, these substances were decomposed to CO₂, H₂O, and other gaseous components. TOC removal over different catalyst is shown in Figure S9, which indicates that more that 60% organics are mineralized over flower-shaped C-dots/Co₃O₄{111}. Based on the above results, the possible catalytic degradation pathways of OTC and ENR are proposed (Figure 7).

2.5. Proposed Catalytic Mechanism

Generally, sulfate radicals (SO₄⁻•) and hydroxyl radicals (OH•) are considered as the most important radicals for degradation of organic pollutants in the PMS activation system [34,35,36]. To identify which radical played the dominant role for the ENR degradation, classic quenching tests were carried out in which ethanol (EtOH) and tert-Butanol (TBA) were used as radical scavenger. In different quenching conditions, the reaction rate constants of sulfate radical and hydroxyl radical are shown in

Table 3. The reaction rate constants of ethanol (EtOH) and tert-Butanol (TBA) with hydroxyl radical and sulfate radical.

Radical Probo	Reaction Rate Constant (M−1 S−1)
	SO4−•	OH•
Ethanol (EtOH)	(1.6–7.7) × 107	(1.2–2.8) × 109
Tert-Butanol (TBA)	(4–9.1) × 105	(3.8–7.6) × 108

[37,38]. As shown in Figure 8a, about 100% of ENR is degraded in 15 min in the absence of scavenger, while the degradation of ENR obviously decreased as the TBA was added in. Notably, compared with TBA, EtOH showed stronger inhibition to catalytic degradation of ENR. Thus, both hydroxyl radical and sulfate radical played the important role in catalytic degradation process.

To further elucidate the catalytic mechanism, DMPO-trapped EPR signals were detected in different aqueous dispersions of the corresponding samples with the addition of PMS (Figure 8). In the absence of catalysts, signals were not observed in the system of DMPO/PMS/ENR dispersion and the blank experiment (Figure S10). As flower-shaped C-dots/Co₃O₄{111} were added, characteristic signals of DMPO-OH and DMPO-SO₄ were still not observed. However, unexpected characteristic signals which were ascribed to the oxidation products of DMPO (DMPOX) with the intensity ratios of 1:2:1:2:1:2:1 were captured (Figure S10). Since the dosage of oxidant (PMS) was more than pollutants (ENR), the excessive PMS would generate active substance/radicals (OH•, SO₄⁻•, H₂O₂) which could oxidize DMPO to DMPOX [34,39]. As the PMS dosage decreased to 1.67 g/L, the characteristic peaks of DMPO-OH adduct (with hyperfine splitting constants of ɑ_N = ɑ_H = 14.6 G) and DMPO-SO₄ adducts (with hyperfine splitting constants of ɑ_N = 12.7 G, ɑα H = 10.3 G, ɑβ H = 2.1 G, ɑγ H = 1.1 G) are observed, revealing the simultaneous presence of OH• and SO₄⁻•. The characteristic peaks of DMPO–SO₄⁻• and DMPO-OH• were observed in the dispersions of catalysts with their intensities following the order of flower-shape C-dots/Co₃O₄{111} > flower-shaped Co₃O₄{111} > spherical Co₃O₄{110}, which indicated that doping C-dots on Co₃O₄{111} could effectively improve the generation of sulfate radicals.

In addition, in spherical Co₃O₄{110} and flower-shaped Co₃O₄{111} aqueous suspensions, PMS was not only reduced to SO₄⁻• (Equation (7)) but also oxidized to large amounts of SO₅⁻• (Equation (9)) due to the decomposition of PMS following the classic Fenton reaction mechanism. Thus, with the oxidation from Co(II) to Co(III), the value of Co(II)/Co(III) [spherical Co₃O₄{110}, flower-shape Co₃O₄{111}] decreased from 1.36 to 1.16 obviously after cycled uses (Figure 6f). Meanwhile, the accumulated Co(III) was reduced by PMS with the generation of SO₅⁻• in dispersions of flower-shaped Co₃O₄{111} and spherical Co₃O₄{110}, resulting in a low selective PMS conversion to sulfate radicals. Different from spherical Co₃O₄{110} and flower-shaped Co₃O₄{111} aqueous suspensions, PMS was mainly reduced to SO₄⁻• and OH• by the Co(II) and C-dots in the aqueous suspension of flower-shaped C-dots/Co₃O₄{111}. Although some Co(II) was oxidized to Co(III) by PMS with the generation of SO₄⁻• and OH• (Equations (6) and (7)), R• could transfer the electron to Co (III) for its quick reduction via Co–O–C linkage, resulting in a stable value of Co(II)/Co(III) [flower-shaped C-dots/Co₃O₄{111}] before and after catalytic reaction (Figure 6f). Thus, such different mechanism of PMS activation over Co₃O₄ and flower-shaped Co₃O₄{111} led to higher selective PMS conversion to SO₄⁻• over flower-shaped C-dots/Co₃O₄{111}.

In order to further illustrate the co-catalyst effect of C-dots on Co₃O₄{111}, PMS activation rate per second on a single active site was calculated. In classic Fenton reaction system, the concentration of OH• in the aqueous dispersion with H₂O₂ could be directly measured by using the terephthalic acid (TPA) probe method. Accordingly, the turnover frequencies (TOF) of Fenton catalyst could be obtained from the conversion number of H₂O₂ into OH• per second on a single active site [40]. However, TOF cannot be calculated in PMS activation system because it is difficult to accurately quantify the SO₄⁻•. Thus, the turnover numbers (TONs) are selected to evaluate the catalytic efficiency for PMS activation (Equation (3)) [41].

TONs = n (converted reactants)/n (catalyst active sites).

(3)

The number of active sites of catalysts can be calculated by the following method [42]:

$$n=\frac{\mathrm{catalyst}\mathrm{surface}\mathrm{area}}{\mathrm{unit}\mathrm{cell}\mathrm{area}}\times \frac{{{\displaystyle Co}}^{2+}}{{{\displaystyle Co}}^{3+}}.$$

(4)

The specific surface area and Co²⁺/Co³⁺ of the catalyst can be obtained by BET and XPS spectra, respectively. Moreover, the unit cell area can be calculated by the length of Co₃O₄ cell [42]. Then, the PMS concentration was obtained by low-concentration iodide methods (ESI). As expected, C-dots/Co₃O₄{111} show higher TONs than Co₃O₄{111} (Figure S11). Such improvement can be explained as follows: Besides of PMS reduction to SO₄⁻• by Co(II), graphene structure of C-dots could facilitate the production of R• (R•, the product of SO₄⁻•/OH• attacking to organics) (Equation (10)). Such reactive organic radicals promote the PMS conversion to SO₄⁻• (Equation (11)). Electrons were not only transferred from R• to Co(III) via the Co–O–C linkage for Co(III) reduction to Co(II), but also transferred from R• to PMS over C-dots with the generation of SO₄⁻• (Equation (12)). Thus, the PMS conversion rate is obviously increased after C-dots doped on Co₃O₄{111}.

Based on the above results, the possible catalytic mechanism of flower-shape C-dots/Co₃O₄{111} is proposed (Figure 9). Both Co₃O₄ crystal structure and doped C-dots played an important role in PMS activation. On the one hand, flower-shaped C-dots/Co₃O₄{111} exposed more Co(II) for PMS activation with generation of SO₄⁻•. On the other hand, graphene structure of C-dots facilitated the production of reactive organic radicals (R•) (Equation (10)). Some of electrons are transferred from R• to Co(III) via the Co–O–C linkage with the reduction of Co(III) to Co(II) (Equation (11)), which decrease the ‘invalid decomposition’ of PMS (Equation (9)). Other electrons are transferred from R• to PMS over C-dots with the generation of SO₄⁻• (Equation (12)) (R* is the intermediate product from the reaction of R• and Co(III)/HSO₅⁻). Thus, there were three electron transfer routes in the catalytic process: the first route was from Co(II) to PMS with the generation of SO₄⁻•, the second route was from R• to Co(III) via the Co–O–C linkage with the quick reduction of Co(III) to Co(II), and the third route was from R• to PMS via C-dots with the generation of SO₄⁻•. Consequently, high catalytic activity and selective PMS conversion to SO₄⁻• was achieved.

Co(II) + HSO₅⁻ → Co(III) + SO₄²⁻ + OH•,

(5)

Co(II) + HSO₅⁻ → Co(III) + SO₄⁻• + OH⁻,

(6)

OH• + SO₄²⁻/HSO₄⁻ ↔ SO₄⁻•/HSO₄• + OH⁻,

(7)

Co(III) + HSO₅⁻ → Co(II) + SO₅⁻• + H⁺,

(8)

R + SO₄⁻•/OH• → R• + SO₄²⁻/H₂O,

(9)

R• + Co(III)-O-C-dots → R*+ Co(II)-O-C-dots,

(10)

R• + HSO₅⁻ → R* + SO₄⁻•,

(11)

OH•/SO₄⁻• + OTC/ENR →SO₄²⁻ + CO₂ + H₂O.

(12)

3. Materials and Methods

3.1. Chemicals

All chemicals were analytical grade and used without further purification. 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) and Oxone (KHSO₅·0.5KHSO₄·0.5K₂SO₄, ≥95% purity) were purchased from Shanghai Macklin Biochemical Co. Ltd (Shanghai, China). Oxytetracycline (OTC, ≥99%), Enrofloxacin (ENR, ≥99%), Rhodamine B (Rh B, ≥99%), and Methyl orange (MO, ≥99%) were purchased from the Aladdin Industrial Corporation. (Shanghai, China). Acetonitrile (99.9% purity) hypergrade for LC-MS was supplied from the Tedia Company.(Fairfield, OH, USA) Tetrabutylammonium bromide (TBAb, ≥99.0%) was obtained from Aladdin Chemistry Co. Ltd. (Shanghai, China). Other reagents including methanol (99.5%), ethanol (99.7%), acetic acid, tert-Butanol (TBA ≥99.0%), Cobalt acetate Co(CH₃COO)₂·4H₂O, and Cetrimonium Bromide (CTMAB ≥99.0%) were purchased from Nanjing Chemical Reagent Co. LTD (Nanjing, China).

3.2. Synthesis of Catalysts

C-dots: The synthesis method of C-dots is presented in a previous article [43]. Two graphite rods were inserted in a beaker containing 400 mL ultrapure water as two electrodes and the DC voltage of 30 V were attached at both ends of the electrode under continuous stirring. Five days later, the graphite rods were corrode and the water in the beaker turned black. Then, the large particles in the solution were filtered out with a filter paper. The solution was centrifuged with high speed centrifuge at 15,000 rpm for 30 min to remove the small particles. Finally, the solution was evaporated at 60 °C, and C-dots was obtained.

Flower-shaped C-dots/Co₃O₄{111}: Synthesis processes of catalysts are shown as following method in Figure S12. A total of 1.192 g Co(CH₃COO)₂·4H₂O was dissolved in 60 mL ethanol containing a certain amount of C-dots. Then, CTMAB (Co(CH₃COO)₂·4H₂O: CTMAB = 2:1) was added into the above solution, and the mixture was stirred at 120 rpm for 15 min. The mixture was transferred to a Teflon-lined stainless steel autoclave and then heated in a high temperature oven at 170 °C for 48 h. The solid product was dried at 80 °C for 12 h to obtain C-dots/Co₃O₄{111}. The synthesis process of flower-shaped Co₃O₄{111} was similar to C-dots/Co₃O₄{111}, except that C-dots were not added into the hydrothermal reaction system. As for spherical Co₃O₄, C-dots and CTMAB are not added.

3.3. Characterization of Catalysts

The surface morphology of the material is characterized by field emission scanning electron microscopy (FESEM, S-3400NII, Hitachi, Tokyo, Japan). Microstructure and diffraction patterns were characterized by transmission electron microscopy (TEM, JEM-200CX, JEOL, Tokyo, Japan). Furthermore, the specific surface area and pore structure of catalysts were measured by Micromeritics Instrument (Nova 3000, Quantachrome).

The crystal structure of catalysts was characterized by the X-ray diffraction (XRD, Cu-Kα radiation, λ = 1.540562 Å, X’TRA, ARL, Switzerland). The XRD patterns were tested in the range of 30–70° (2θ). The binding energies of Co of the catalyst were measured by X-ray photoelectron spectroscopy (PHI 5000 Versa Probe, UlVAC-PHI, Chigasaki, Japan). In addition, the Fourier Transform Infrared Spectrometer (FT-IR, NEXUS870, NICOLET, Wilmington, NC, USA) was used to analyze the chemical structural functional groups.

To analyze the redox properties of Co in catalyst composites, the H₂-TPR experiments were conducted on the chemisorption analyzer (2920, AutoChem II, Micromeritics). The samples were heated to 400 °C in the Ar atmosphere for 60 min. After cooling down to 50 °C, the samples were heated to 900 °C with temperature ramp of 10 °C min⁻¹ in 10% H₂/Ar. Electrochemical impedance test was carried out by using CHI 760D electrochemical workstation (Austin, TX, USA).

Electrochemical measurements were conducted by an electrochemical workstation (CHI660D Instruments) with a standard three-electrode system. A total of 0.01 g catalyst was ultrasonic dispersed in 1 mL chitosan solution (0.05 M), then the dispersed droplets were placed on the glass electrode and dried in a vacuum oven before use.

DMPO-trapped EPR signals were detected in different aqueous dispersions of the corresponding samples with and without the addition of PMS. A detailed information for the recorded EPR signals is provided in the Figure S10.

3.4. Catalytic Performance

Four aromatic pollutants (OTC, ENR, MB, and Rh B) were selected to evaluate the catalysts activity. Preliminary experiments for ENR degradation indicated that the optimal doses of the catalyst and PMS were 0.05 g/L and 12.5 mM, respectively (Figure 6). Thus, these two doses were used in all catalytic degradation experiments unless otherwise specified. Typically, 200 mL aqueous solutions with certain pollutant and 0.01 g catalyst were placed in 250 mL beaker flasks. To ensure the adsorption–desorption equilibrium, the suspensions were magnetically stirred for 30 min before the catalytic reaction. Then, 12.5 mM PMS was added in the suspensions with magnetic stirring (120 rpm). At given time intervals, 2 mL aliquots were sampled and followed by adding methanol into the sample to quench the radicals for termination of the catalytic reaction, and subsequently filtrated through a Millipore filter (pore size 0.45 μm) prior to the analysis. In addition, to test the catalytic stability, the catalyst was recovered by filtration, washed with deionized water, dried, and reused in the following cycle. The degradation rate of pollutants is calculated. Concentrations of OTC and ENR were measured using Agilent 1200 HPLC (Agilent Technologies, Palo Alto, CA, USA) equipped with a quaternary pump and a UV detector. Chromatographic analysis was performed by a reversed-phase C-18 column (4.6 × 150 mm, 5 μm particle diameter), and the specific liquid conditions are listed in Table S2. The degradation intermediates of OTC and ENR were detected by HPLC coupled with time-of-flight mass spectrometry detection (HPLC–TOFMS, Agilent 1290 Infinity LC/6460 QQQ MS) in positive polarity. Rh B and MB concentrations in solutions were analyzed by the UV−1800 UV–vis spectrometer (Shimadzu, Japan).

4. Conclusions

Novel flower-shaped C-dots/Co₃O₄{111} were constructed for the improvement of the catalytic activity and selective peroxymonosulfate (PMS) conversion to sulfate radicals. Being attributed to the exposure of {111} facets, C-dots/Co₃O₄{111} could provide plenty of Co(II) for PMS activation. In addition, the Co₃O₄{111} facet not only possessed high adsorption energy of PMS on catalyst, but also facilitated the electron transfer from Co(II) to PMS. On the other hand, the graphene structure of C-dots facilitated the production of reactive organic radicals (R•), which provided the electrons for Co(III) and PMS reduction. Thus, C-dots doped on Co₃O₄{111} could obviously enhance the TONs. The catalytic degradation tests indicated that antibiotics and dyes could be efficiently degraded over flower-shaped C-dots/Co₃O₄{111}. Even after six cycling runs, flower-shaped C-dots/Co₃O₄{111} still remained a high catalytic activity. Additionally, according to degradation intermediates identified by LC-MS/TOF and electron cloud density calculated by Gaussian program calculation, the possible degradation pathways of OTC/ENR were proposed. EPR spectra and radical capture experiments further demonstrated that both OH• and SO₄⁻• played a dominant role in catalytic degradation processes. Thus, the present work provided a simple and efficient way for PMS activation, which could be applied in the treatment of wastewater.