3.1 Properties of HOCO
First, morphologies of the as-prepared materials were visualized. Figure 1(b) and (e) present electronic microscopic figures of the original Co-MOF, indicating successful formation of 2-dimensional oval-like morphology. Its crystalline structure was also determined using XRD analysis as displayed in Fig. 2(a), which is consistent with reported literature [21]. After this pristine Co-MOF was modified with tannium via a chemical-engraving process, the resultant product still retained the oval-like appearance of the original Co-MOF (Fig. 1(c)). However, it can be seen that the interior part of this product was removed, creating a huge hollow-structure inside whereas the thin outer layer (size ca. of 20–25 nm) was remained as shown in Fig. 1(f), verifying the successful construction of hollow-structured oval-like Co-MOF. The XRD pattern of this product was also characterized as depicted in Fig. S1.
When this hollow-structured Co-MOF was thermally-oxidized in air, the resulting substance still showed the oval-like morphology and hollow configuration (Fig. 1(c) and (g)). Moreover, densely-packed and ultrafine spherical particles could be observed, which were distributed evenly throughout this calcined product (Fig. 1(g)); a closer observation in Fig. 1(h) then shows that the particles would be nano-sized, ranging from 7 nm to 13 nm (in diameter) with a principal size of 10 nm (Fig. 1(h)). Besides, its HRTEM (Fig. 1(i)) unveils two particular d-spacings of 0.285 nm and 0.241 nm, which corresponded to the plane (2 2 0) as well as (3 1 1) in Co3O4. This result validated that those nano-sized particles or nanoparticles (NPs) should be Co3O4, demonstrating the successful formation of hollow-structured oval-like cobalt oxide (HOCO).
The crystalline structure of HOCO was also measured (Fig. 2(b)); and its crystalline patterns exhibited several notable diffraction peaks located at 19o, 31.4o, 36.8o, 38.6o, 44.8o, 55.5o, 59.3o, 65.1o, 74.1o and 77.3o, which could be well indexed to those signature planes of Co3O4 (JCPDS #42-1467), asserting the full conversion of the oval-like cobaltic MOF to Co3O4. The electron diffraction pattern of HOCO in Fig. 1(j) reveals the polycrystalline structure of HOCO, which is correlated with its XRD profile. In addition, the elemental composition in HOCO was also identified using EDX analysis in Fig. S2(a), where only Co and O could be noted. Besides, the elemental mapping of HOCO was also determined (Fig. 2(b-d)), showing that oxygen and cobalt were certainly present and uniformly distributed throughout the oval-like outer layer.
Moreover, the functional groups of the pristine oval-like Co-MOF and HOCO were determined through IR profiles (Fig. 2(c)). For the original Co-MOF, various absorption bands located at 500 ~ 1500 cm− 1 shall be attributed to the ligand (i.e., an imidazole ring) [22]. Besides, an absorption band positioned at ~ 1584 cm− 1 could be the C = N bond in 2-MIM, whereas two noticeable peaks located at 2929 and 3138 cm− 1 could be corresponded to the C-H sp3 in the aromatic ring and the C-H sp2 in 2-MIM, respectively, further indicating the formation of Co-MOF [22]. The IR analysis of HOCO showed distinct absorption bands. Particularly, several notable bands detected in 565 and 660 cm− 1 were attributed to Co2+ and Co3+ in cobalt oxide, respectively [23]. Besides, the bands located at 1630/3410 cm− 1 were ascribed to the O-H bond, which was probably due to adsorbed H2O on HOCO [24].
In addition, Raman spectroscopies of Co3O4 NP and HOCO were further characterized as shown in Fig. 2(d). It can be noticed that both materials exhibited five noticeable peaks, which could be assigned to the Raman active vibration modes (i.e., A1g + Eg + 3F2g) of Co3O4 [25]. This further validates the formation of Co3O4 in HOCO. However, the peaks at F2g(1) and A1g regions of HOCO (Fig. 2(e-f)) were shifted to the lower positions than that of commercial Co3O4 NP. Specifically, the band at F2g(1) region of HOCO was switched from 191 to 189 cm− 1, whereas the band at A1g region was moved from 678 to 669 cm− 1. As these two specific regions were assigned to the oxygen mobility of Co3O4 [26], these shifted peaks in HOCO suggested that HOCO might possess more structural defects compared with Co3O4 NP, enabling HOCO to exhibit a more reactive surface [27], thereby enhancing activities of HOCO for MPS activation.
Furthermore, the chemical states of all elements in HOCO were identified by XPS. Figure 3(a) presents a whole-range scan, showing that only notable Co and O signals were detected, which was in line with EDX/mapping results. Besides, the Co2p spectra of HOCO were then deconvoluted into a number of peaks. The peaks detected in 779.4 and 794.3 eV were assigned to Co3+ species, whereas the peaks located at 781.4 and 796.5 eV were assigned to Co2+ species; and co-existence of these Co species strongly confirm the presence of Co3O4 in HOCO [28]. Moreover, the O1s spectra of HOCO was also examined, and a number of notable peaks were detected at 529.4 and 530.9 eV, derived from O species in metal-O bond (Olatt) and R-O bond (Oads), respectively [29].
Additionally, since both commercial Co3O4 NP and HOCO were comprised Co3O4 and HOCO possessed more reactive surface, it was crucial to probe into the distinct reducibility between these two materials using H2-TPR analysis. As displayed in Fig. 3(d), H2-TPR profiles of HOCO and Co3O4 NP are quite different, in which HOCO showed three reduction peaks and Co3O4 NP displayed two reduction bands. Typically, Co3O4 exhibits two reducting steps: the first step is the reducing step from Co3+ to Co2+ and the second step is the reducing step from Co2+ to Co0 [30]. In the case of the commercial Co3O4 NP, these reduction bands were overlapped, in which the band at 465 oC was due to the first step, and the band at 501 oC was due to the second step. On the other hand, TPR analysis of HOCO showed three reductive bands at 296 oC, 334 oC, and 365 oC. Specifically, the peak at 296 oC could be due to the reducing step from Co3+ to Co2+ while these bands at 344 oC and 365 oC could correspond to the reducing step from Co2+ to Co0 in the cluster form and bulk form, respectively [31]. This result showed that the reduction temperatures of Co species in HOCO were much lower than that of Co3O4 NP, further illustrating the higher degree of active surface in HOCO compared with Co3O4 NP, which subsequently contribute to different activities during oxidative reactions [32].
Next, the physical properties of both HOCO and Co3O4 NP were then characterized in Fig. 3(e). HOCO exhibited an isotherm with hysteresis loop, suggesting mesopores contained in HOCO. The pore size analysis can be seen in the inset of Fig. 3(e) further validated the presence of mesopores and macropores in HOCO. The surface area of HOCO was then calculated as 43.57 m2/g with a total porous volume = 0.26 cm3 per gram. On the other hand, the commercial Co3O4 NP showed a relatively low N2 sorption capability; thus, its surface area would be merely 2.5 m2/g with a certain small total porous volume of 0.001 cm3 per gram, probably due to the critical aggregation of these Co3O4 NPs as depicted in Fig. S3. This result further unveils the distinct textural properties between HOCO and Co3O4 NP, which would then influence to their catalytic activities. These textural properties of HOCO and Co3O4 NP were also summarized in Table S1.
Moreover, since the degradation of ARS would be executed via batch-type degradation in water, zeta potentials of HOCO under numerous pH conditions were necessarily measured. Figure 3(f) shows the zeta potential profile of HOCO, illustrating that the zeta potentials of HOCO would be positive under acid and neutral conditions; and switched to negative charges at basic conditions. The zeta potential of HOCO would be + 23.7 mV at pH = 3, and it would then decrease to + 18.3 and + 7.4 mV when pH increased to 5 and 7, respectively. When pH = 9, the zeta potential of HOCO would be -15 mV, and it was further dropped to -31.6 mV at pH = 11 with its pHzpc = 7.6.
3.2 ARS degradation using HOCO-activated MPS
ARS degradation was investigated by different catalytic systems as shown in Fig. 4(a). Initially, it was essential to test if ARS might be eliminated via the adsorption pathway. It can be noticed from Fig. 4(a) that ARS remained steady within 30 min with HOCO only, demonstrating the negligible adsorptive capability of ARS on HOCO surface. On the other hand, ARS was also hardly degraded in the presence of MPS individually since Ct/C0 just approached 0.9, suggesting that MPS self-activation without catalysts would be ineffective, leading to low ARS degradation efficiency. Nevertheless, when HOCO was employed for MPS activation, ARS would be drastically reduced and wholly removed in 30 min. This result reveals that MPS could be activated by HOCO to result in reactive oxygen species (ROS) for degrading ARS. Moreover, as HOCO would be constituted of Co3O4, the commercial Co3O4 was also utilized for comparison with HOCO; and ca. 40% of ARS was eliminated after 30 min, proving that the commercial Co3O4 NP was also able to activate MPS for eliminating ARS. However, the ARS degradation efficiency by the commercial Co3O4 NP + MPS system was relatively low in comparison with that of HOCO + MPS system, indicating the benefit of HOCO for activating MPS to eliminate ARS.
On the other hand, for further comparing degradation efficiencies of ARS by the above-mentioned catalytic systems, two particular indices (i.e., degradation kinetics (k) (using the pseudo 1st order equation) [33] and reaction stoichiometric efficiency (RSE)) [34] were further calculated as the following equations (Eq. (1–2)):
C t = C0 exp(-kt) (1)
RSE = \(\frac{ARS degraded \left(mol\right)}{Amount of MPS used \left(mol\right)}\) (2)
where k denotes the pseudo 1st -order reaction rate. In Fig. 4(b), the corresponding k of HOCO + MPS was calculated as 0.215 min− 1, which would be substantially higher than that of Co3O4 NP + MPS (0.015 min− 1), MPS individually (0.0036 min− 1) and adsorption (0.0017 min− 1). Besides, HOCO + MPS exhibited the highest RSE value of 0.306, followed by Co3O4 NP + MPS (0.122), MPS individually (0.03) and adsorption (0.001). This further validated the excellent activity of HOCO to activate MPS, enabling it to be an effective heterogeneous activator for degrading ARS.
3.3 Effects of HOCO and MPS dosages, and different ARS concentrations
Since ARS was effectively degraded using HOCO-activated MPS, the effects of HOCO dosages and MPS dosages should play critical roles in ARS degradation. Thus, different amounts of HOCO and MPS were then investigated to examine their influence for obtaining the optimal condition for further investigations. As shown in Fig. 5(a), ARS degradation efficiency would be improved by the increase of HOCO concentrations. Particularly, at 50 mg/L of HOCO, ARS gradually degraded as Ct/C0 approached 0.05 in 30 min, and its corresponding k would be 0.095 min− 1 (the inset of Fig. 5(a)). As the dosage of HOCO was raised to 100 mg/L, ARS would be completely eliminated in 30 min, wand it would be reached in an even less reaction time (i.e., 20 min) at 150 mg/L of HOCO. The calculated k would be also measured as 0.215 min− 1 as well as 0.343 min− 1 at the HOCO dosage of 100 and 150 mg/L, respectively. This verifies the advantage of higher HOCO concentrations on ARS degradation, owing to the contribution of abundant active sites for accelerating ROS generated from MPS activation.
In addition, the influence of MPS concentrations on ARS degradation was also studied as illustrated in Fig. 5(b). As a small dosage of MPS (i.e., 50 mg/L) was added, only 60% of ARS was eliminated in 30 min, probably because of insufficient ROS generated. Once MPS concentration was augmented to 100 mg/L, complete ARS degradation would be achieved in 30 min. A higher MPS dosage at 150 mg/L slightly enhanced the ARS degradation efficiency (Fig. 5(b)). This result revealed that the lower dosage of MPS with deficient generated ROS could not fully eliminate ARS while higher MPS dosages could effectively and completely remove ARS owing to sufficient ROS produced. Besides, the corresponding k with various MPS dosage would be also measured inside Fig. 5(b). The corresponding k value at 50 mg/L of MPS was computed to be 0.095 min− 1, and it was then substantially raised to 0.215 as well as 0.27 min− 1 at MPS dosage of 100 and 150 mg/L, which further validated the enhancing effect of relatively high MPS concentrations on ARS degradation.
Furthermore, it was practical for evaluating the activity of HOCO for catalyzing activation of MPS to eliminate different ARS concentrations. In Fig. 5(c), the full degradation of ARS was still afforded in 30 min, even in a high-concentrated ARS solution (200 mg/L), confirming a superior performance of HOCO for activating MPS. In addition, the k values (Fig. 5(c)) were measured to be 0.215, 0.198, and 0.186 min− 1 at the ARS concentration of 50, 100, and 200 mg/L, respectively. This further indicates that HOCO could retain its exceptional performance for MPS activation to eliminate ARS at various concentrations.
3.4 Influences of other factors
As ARS was efficiently degraded using HOCO-catalyzed MPS, the effects of other parameters on ARS elimination should be further investigated. First, the effect of temperatures was examined because temperatures are essential in the reaction kinetic (Fig. 5(d)). Generally, the degradation efficiency of ARS would be significantly boosted when temperature increases. Particularly, at 30 oC (Fig. S4(a)), the complete degradation of ARS was afforded within 30 min with its corresponding k of 0.215 min− 1. When the temperature raised to 40 and 50 oC, ARS degradation efficiencies were significantly enhanced as Ct/C0 reached zero within shorter reaction times. And the corresponding k values were also drastically increased to 0.307 as well as 0.46 min− 1 at 40 and 50 oC. This result assuredly proves the promoting effect of elevated temperatures on ARS elimination using HOCO-activated MPS.
Moreover, as the corresponding k raised with increasing temperatures, the relationship between k and T would be further correlated via the following equation (Eq. (3)) [35]:
Lnk – lnA = –Ea/RT (3)
where Ea represents the calculated activation energy (kJ/mol). The inset of Fig. 5(d) further shows the relationship between k and temperatures was fitted linearly with R2 = 0.996. Thus, the activation energy (Ea) was then computed as 30.9 kJ/mol. This calculated Ea value from HOCO-activated MPS was comparatively lower than most reported Ea values in literature (Table S2), further validating the exceptional catalytic activity of HOCO, enabling it a highly-competitive catalyst for MPS activation.
The influence of different initial pH values on ARS degradation was then investigated. Figure 5(e) shows that ARS was completely degraded in 30 min at a neutral condition of pH = 7 with k of 0.215 min− 1 (Fig. S4(b)). When the pH in ARS solution would be tuned to pH = 5, a weakly acidic environment, ARS degradation was slightly interfered as Ct/C0 approached 0.1 after 30 min. The interference became more pronounced once pH was further tuned to a strongly acidic environment (i.e., pH = 3) since merely 50% of ARS was eliminated, and the corresponding degradation kinetic was significantly reduced to 0.024 min− 1. ARS degradation using HOCO-activated MPS was thus unfavorable in the acidic environment, which possibly due to the fact that MPS was indolent under acidic conditions [36], thereby suppressing MPS activation. Besides, the generated ROS (e.g., SO4•‒, •OH) might be quickly consumed by free protons as follows (Eq. (4–5)) [37], lowering the degradation of ARS.
H+ + •OH + e‒ → H2O (4)
H+ + SO4•‒ + e‒ → HSO4‒ (5)
Additionally, the degradation efficiency of ARS would be also affected at higher pH. Particularly, under a weak basic condition of pH = 9, ARS degradation would be minimally influenced as Ct/C0 still reached 0.15 in 30 min (Fig. 5(e)), and its corresponding k was 0.094 min− 1. Nevertheless, as pH would be changed to 11, a strongly basic environment, only 30% of ARS was removed whereas the degradation kinetic at pH = 11 was noticeably decreased to 0.011 min− 1. This confirms the adverse influence of basic environment on ARS degradation, which was probably attributed to the much stronger revulsion occurred from the negatively charged of HOCO surface and SO5•‒, subsequently restraining the generation of ROS [38]. Moreover, in the basic conditions, SO4•‒ radicals would readily react with excessive OH‒ ions (Eq. (6)) to become •OH radicals which possess a shorter half-life and lower standard redox potential [39], leading to fewer SO4•‒ radicals involved in ARS degradation.
SO4•‒ + OH‒ → SO42‒ + •OH (6)
On the other hand, in actual water systems, various compounds (including natural and artificial substances) are frequently detected, which potentially interfere ARS elimination using HOCO-activated MPS. Therefore, it was important to scrutinize the effect of these compounds on ARS elimination (Fig. 5(f)). The effect of inorganic salt was first examined. In this study, sodium chloride (NaCl) at different concentrations was particularly selected as an inorganic salt model. With 500 mg/L of NaCl, ARS was still effectively degraded since almost 100% ARS was achieved in 30 min whereas ARS elimination was negligibly influenced at a higher concentration of NaCl (i.e., 1000 mg/L), suggesting the minimal interference of NaCl on ARS degradation using HOCO + MPS. This could be explained that the resultant SO4•‒ radicals would interact with free Cl‒ ions to produce chloride radicals as following equations (Eq. (7–8)) [40] that possess a relatively low redox power than that of SO4•‒ radicals, thereby diminishing ARS degradation efficiencies. Nonetheless, HOCO-activated MPS system still efficiently eliminated ARS, even at a higher NaCl concentration, validating that HOCO could retain its preeminent catalytic activity for MPS activation. The corresponding k with NaCl was also calculated as shown in Fig. S4(c).
SO4•‒ + Cl− → SO42‒ + Cl• (7)
Cl• + Cl− → Cl2•− (8)
Moreover, it was also reported that the existence of surfactants may also interfere the activation of MPS, decreasing the degradation effectiveness of the organic pollutants. Thus, it is useful to determine the influence of surfactants in water on ARS elimination using HOCO + MPS. Specific surfactants were chosen, including CTAB and SDS as shown in Fig. 5(f). In the case of CTAB, it is interesting that ARS elimination was improved since Ct/C0 approached zero in a shorter reaction time with k of 0.226 min− 1 (Fig. S4(c)). This reveals positive influences of CTAB to ARS degradation by HOCO-catalyzed MPS, which was probably because CTAB, a typical cationic surfactant, may occupy HOCO surface, resulting in attraction between positively-charged HOCO surface and ARS molecules where ROS were produced [41], thus noticeably enhancing the degradation of ARS. With SDS (500 mg/L), a typical anionic surfactant, ARS degradation efficiency was certainly influenced since solely ca. 50% of ARS was eliminated in 30 min with a much lower k of 0.0234 min− 1. The degradation of ARS using HOCO + MPS was much less effective in the co-existence of SDS, which was possibly because the surface of HOCO became negatively after SDS was introduced, creating a stronger electrostatic repulsion between negatively-charged HOCO surface and ARS molecules, then restricting the degradation of ARS [41].
3.5 Recyclability of HOCO
Since HOCO could effectively activate MPS to degrade ARS in water, it would be more intriguing to examine the reusability of HOCO for multiple ARS degradation cycles. It can be noticed from Fig. 6(a) that almost 100% of ARS was still afforded over 5 degradation cycles, indicating that HOCO could retain its exceptional performance for continuous activation of MPS. Besides, the crystalline pattern of the spent HOCO would be further determined (Fig. 6(b)), which was consistent with the pristine HOCO, showing that the highly-crystalline structure of HOCO was still retained. This indicated the outstanding durability of HOCO, enabling it to be an useful catalyst for MPS activation to eliminate ARS in water.
3.6 Identification of ROS involved in ARS degradation
In order to reveal the generation of potential ROS (e.g., SO4•‒, •OH) from HOCO-activated MPS as the following equations (Eq. (9–11)) [42, 43], and their contribution to the degradation of ARS, the scavenging test by two specific radical scavengers, such as methyl alcohol (MeOH) and tert-butyl alcohol (TBA), was performed.
Co2+@HOCO \(+\) HSO5− \(\to\) Co3+@HOCO \(+\) SO4•‒ \(+\) OH− (9)
Co3+@HOCO \(+\) HSO5− \(\to\) Co2+@HOCO \(+\) SO5•‒ \(+\) OH− (10)
SO4•− + H2O → SO42− + •OH + H+ (11)
Figure 6(c) shows that ARS elimination was inhibited by either MeOH or TBA. It can be noted that TBA comprises no α-hydrogen in its structure, TBA would promptly react with •OH; thus, it was selected to be an •OH quencher. Figure 6(c) reveals that only 65% of ARS was degraded in 30 min by TBA with k decreased to 0.036 min− 1 (Fig. S4(d)), demonstrating the existence of •OH produced during the activation of MPS.
With α-hydrogen in the structure, MeOH can react with both •OH and SO4•‒. The inhibitory effect became even more remarkable when MeOH was then added as Ct/C0 merely reached 0.98, and k was significantly dropped to 0.0005 min− 1, implying that SO4•‒ was also generated from HOCO + MPS system. To confirm these ROS, EPR was further adopted using DMPO as a radical trapping probe. From Fig. 6(d), it can be observed that there was no obvious pattern when MPS was individually introduced in DMPO solution. Nevertheless, once HOCO and MPS were simultaneously added, notable patterns were particularly detected, which was attributed to DMPO-OH and DMPO-SO4 adducts [44, 45]. This further proves the generation of SO4•‒ and •OH species involved in ARS degradation using HOCO + MPS system (as shown Fig. 6(e)).
3.7 A potential degradation process for ARS
For elucidating ARS elimination process, computational calculations of ARS molecule according to the density functional theory (DFT) were executed. Detailed computational settings would be described in SI, and the geometrically-optimized ARS would be then revealed in Fig. 7(a) by atom labels. Moreover, the highest occupied and lowest unoccupied molecular orbitals (MOs) of the geometrically-optimized ARS are also rendered in Fig. S5. Specifically, red and green-colored isosurfaces of these MOs represent electron-abundant and electron-deficient zones, respectively. Moreover, the highest occupied MO located on the naphthalene group bound to the sulfonate group tended to denote electrons. Therefore, ARS would possibly attract electrophilic attacks by the electrophilic ROS.
For further examining the degradation pathway by identifying the most-likely reaction spots of ARS, the Fukui function would be then adopted to provide insights [46]. To this end, the dual descriptor of the Fukui function was calculated, and rendered by Multiwfn to forecast potential reaction spots of ARS [47]. For offereing intuitive visualization of reactive spots of ARS, its corresponding isosurface of electron density derived from the Fukui function is then depicted, and Fig. 7(b), (c), and (d) displays the f −, f 0 and f + isosurfaces of ARS, respectively.
Theoretically, a spot with a huger green-colored isosurface would imply a higher possibility for reactions. Therefore, the electrophilic reaction spots of ARS (e.g., C27, C32, C33, N14, N15, etc.) would withdraw attacks by electrophilic radicals. Besides, Fig. 7(c) also displays possible spots for non-radical reactions, including C27, N14, and N15. These aforementioned spots might withdraw attacks to initiate the degradation process of ARS. On the other hand, the electrostatic potential (ESP) of ARS (Fig. 7(e-f)) also indicate that C19, C24, and C27 might exhibit affinities towards the anionic sulfate radical.
To clarifying the degradation process of ARS, degradation derivations from ARS would be then determined in Fig. S6 and probable intermediates were then outlined (Table S3). According to these intermediates, a probable ARS elimination process using HOCO was then illustrated in Fig. 8 [48, 49]. In the beginning, ARS would be transformed to M1 upon dissolving in water, which was then attacked by the electrophilic ROS on those sites adjacent to the sulfonate group to afford an intermediate, M2. Through the cleavage of N = N bond, M2 would be then decomposed to produce M3, and M4, respectively. M3 was then further oxidized, and decomposed to generate M5, whereas M4 would be oxidized to result in M6 as well as M7 afterwards. On the other hand, M5 would be continuously oxidized to M6, and then M7, which was eventually decomposed to CO2 and H2O.