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• Fenton-like advanced oxidation processes (AOPs) have been receiving lots of attentions in the fields of water treatment due to their low cost and high oxidation potentials [1-4]. Metal oxides exhibited high reactivities towards the catalyzed H₂O₂ propagations, which initiated radical chain reactions for pollutant degradation [5]. Manganese dioxide (MnO₂) was an efficient heterogenous catalyst to activate such processes, activating H₂O₂ to produce large amounts of superoxide anions (O₂^•−) and hydroxyl radicals (^•OH) [6, 7]. Moreover, this catalyzed reaction would be promoted when framing MnO₂ to a nanosized shape with different crystal forms and facet exposures [8]. However, it was not practical to recycle and separate the suspended MnO₂ nanomaterials, thus limiting large-scale applications [9]. Some techniques of solving this issue have been developed, such as magnetic modification [10], support loading [11], and membrane construction [12]. Among these, assembling nanomaterials into macroscopic membrane was considered as the most promising engineered technology. The catalytic membrane not only promoted the potential of nanostructured MnO₂ separation and recovery, but also enhanced the Fenton-like reaction activity by confining such process in a nanodomain membrane pore [13, 14]. The simultaneous filtration and reaction in confined space overcame the mass transfer limitations which often encountered in bulk diffusive mode operations [15, 16]. Therefore, exploring facile approaches to prepare catalytic membrane using MnO₂ nanomaterials was urgent for practical applications.

  Nanomaterials, enabling to be assembled into membrane, should be designed with special architectures to guarantee mechanical strength during the membrane filtration [17]. Assembling one dimensional (1D) MnO₂ nanowire into two-dimensional (2D) membrane by vacuum filtration was extensively used due to its advantages including easy-operation and liable controllability [18]. In that case, it was prerequisite to prepare the well-structured MnO₂ nanowires. Some previous studies employed hydrothermal methods to accelerate the redox reactions of Mn hydrated ions by adding oxidative reagents (e.g., HNO₃ or (NH₄)₂S₂O₈) to obtain MnO₂ nanowires [12, 19, 20]. However, for these current approaches, the reactions always concluded various extra components (mixed crystal) and byproducts (e.g., MnOOH) [21]. Such issues would not only affect the purity of obtained MnO₂ nanowires, but also raised the difficulty of reaction control. In addition, since the vacuum filtration method required very high aspect ratio and good dispersion of MnO₂ nanowires in the solvents, few approaches could be applied in preparing such featured MnO₂ nanowires [12]. Therefore, studies on facile synthetic methods to obtain MnO₂ nanowires for catalytic membrane assembling and practical applications of water treatment were in great demands.

  Herein, we developed a facile method to prepare ultralong α-MnO₂ nanowires with high purity and aspect ratio under 140 ℃ for 12 h through hydrothermal reaction. Specially, the C₂H₅OH and CH₃COOK were used as reductive and control reagents respectively to react with KMnO₄ as manganese source. Compared to previous studies [12, 19, 20], this strategy exhibited three advantages as follows: (1) This synthesis reaction was mild, easily controlled, and insensitive to temperature, reagents ratio, and reaction time; (2) the byproducts in reaction system were less and safer than those in other methods; (3) all reaction reagents were cheap, readily available, and environmentally friendly. We further investigated the formation mechanism of α-MnO₂ nanowires via various characterizations. Moreover, we assembled these α-MnO₂ nanowires into free-standing membrane as a Fenton catalyst for organic degradation in water. By the comparison with α-MnO₂ nanowire powder in aqueous solution, the possible mechanism of enhanced pollution removals and the application potential of α-MnO₂ nanowire membrane in Fenton-like oxidation were comprehensively explored.

  Details of experimental procedures and characterizations were exhibited in Texts S1–S4 (Supporting information). The phase purity and crystal structure of the prepared MnO₂ were examined by XRD in Fig. 1a. All the diffraction peaks could be exclusively indexed as tetragonal α-MnO₂ (JCPDS No. 44-0141, space group I4/m, a = b = 9.784 Å, c = 2.863 Å), which was a type of the well-defined 2 × 2 tunnel structure with a tunnel size of 4.6 Å and was composed of edge shared MnO₆ octahedra with corner sharing double chains. No other characteristic peaks of the impurities were detected in the spectrum, indicating high phase purity of the obtained α-MnO₂ [20]. The elemental composition of α-MnO₂ was detected by EDS mapping (Fig. 1b). The atomic ratio of Mn to O was very close to 2, confirming that the highly purified α-MnO₂ was successfully synthesized. The morphology of α-MnO₂ was characterized by SEM and TEM. As shown in Fig. 1c, the α-MnO₂ were homogenously overlapped in a 1D nanowire shape as expected. The nanowire morphology grew into as long as tens of micrometers with diameter of ~50 nm (Fig. S1 in Supporting information). Such ultralong nanowires were ensured to form a stable membrane with high porosity and flexibility by easily interconnecting with each other [22]. Notably, a little number of bundles made up of ultralong nanowires (inset in Fig. 1c) parallelly coexisted with together, which indicated some details of "nucleation-growth" mechanism [21]. The ultralong nanowires were typical mesoporous structure (type II isotherm, Fig. S2 in Supporting information) and the BET surface area was up to 34.2 m²/g, which would endow the as-prepared α-MnO₂ with excellent catalytic performance [23]. With the help of HR-TEM, the deeper microstructure of α-MnO₂ nanowires was investigated (Fig. 1d). The clear crystal lattices signified that the ultralong nanowire was single-crystalline with crystal orientation along [001] direction (c axis). Along the growth axis, the interplanar spacing was 0.69 nm, corresponding to the (110) facets. Two other interplanar spacings between the adjacent lattice planes were also measured to be 0.31 and 0.23 nm, which could be assigned to the (310) and (211) facets of α-MnO₂. Moreover, the SAED pattern displayed orderly spot arrays which were indexed to the (110), (200), (310) and (211) planes. Comparing the intensities of XRD diffraction peaks, it could be found that the intensities corresponding to exposed facets in α-MnO₂ were relatively stronger. As known, the exposed facets had high percentage of unsaturated atoms and great potential to react with low percentage of unsaturated atoms [24, 25]. In addition, the high-index exposed facets were usually composed of high density of under-coordinated atoms, such as steps, edges, and kinks, serving as active sites. The successful synthesis of α-MnO₂ with high-index exposed facets would contribute to enhancing the catalytic activity [20].

  Figure 1

  

  Figure 1.  (a) XRD pattern, (b) EDS spectrum, (c) SEM images and (d) HR-TEM images and SAED pattern of prepared MnO₂ nanowires.

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  The formation mechanism of α-MnO₂ nanowires was systematically discussed and proposed. C₂H₅OH and CH₃COOK were identified as the key factors in the formation of ultralong nanowire shaped α-MnO₂. C₂H₅OH acted as a mild reducing agent to react with manganese source, MnO₄⁻. Redox potential of MnO₄⁻/MnO₂ (0.588 V in neutral condition) was only slightly higher than that of CH₃COOH/C₂H₅OH (0.31 V in neutral condition) which acted as the reductive electrode couple (Eq. 1) [26]. Such approximately standard electrode potentials made the reaction mild and slow, facilitating the "nucleation-growth" process [12, 27]. However, the reactive solution containing C₂H₅OH and MnO₄⁻ could only form flower shaped α-MnO₂ instead of nanowire shaped (Fig. 2a). The lateral growth of α-MnO₂ needed to be restricted to form nanowires. As seen in Eq. 1, the reaction between C₂H₅OH and MnO₄⁻ did not generate steric reagent, which led to lateral growth into the flower shape of obtained MnO₂.

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  Figure 2

  

  Figure 2.  SEM images of MnO₂ nanowires prepared under different conditions: (a) CH₃CH₂OH as reductive reagent at 140 ℃ for 12 h, (b) CH₃CHO as reductive reagent at 140 ℃ for 12 h, (c) CH₃CH₂OH as reductive reagent and H₂SO₄ as control reagent at 140 ℃ for 12 h, (d) CH₃CH₂OH as reductive reagent and CH₃COOH as control reagent at 140 ℃ for 12 h.

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  Previous study suggested that CH₃COOH could hinder the growth of lateral nanostructure through steric effects due to the formation of hydrogen bonds with -OH groups on MnO₂ [28]. For example, when employing CH₃CHO as a substitute of C₂H₅OH to reduce MnO₄⁻, the formed MnO₂ transferred from flower-shape to nanowire-shape (Fig. 2b) due to the formation of byproduct CH₃COOH (Eq. 2). Moreover, both adding H⁺ (i.e., H₂SO₄) which enhanced the hydrolysis of generated CH₃COO⁻ to be CH₃COOH and directly adding CH₃COOH in C₂H₅OH/MnO₄⁻ system (Eq. 3) could further lead to the nanowire-shaped growth of MnO₂ (Figs. 2c and d). However, in the conditions outlined above, the obtained MnO₂ nanowires were short and seriously bonded with each other (Figs. 2b–d). This was because the gap of redox potential between electrode couple (e.g., CH₃COOH/CH₃CHO (−0.12 V) < C₂H₅OH/CH₃COOH (0.31 V) in the neutral condition, MnO₄⁻/MnO₂ (0.588 V in neutral condition) < MnO₄⁻/MnO₂ (1.510 V in acidic condition)) in the reactive system became larger under these conditions, resulting in rapid formation of MnO₂ nuclei [29]. Too many nuclei of MnO₂ lowered the development of ultralong dendritic structure and further restricted the nanowire-shaped growth of MnO₂. To create a mild redox condition, CH₃COOK, which was also the only byproduct in the reaction system, was introduced into the reactive solution and the ultralong α-MnO₂ nanowires were obtained (Fig. 1c and Fig. S1). On the one hand, the hydrolysis of CH₃COO⁻ could generate CH₃COOH at the beginning of this reaction (Eq. 4). On the other hand, such process could produce extra OH⁻ which further decreased the gap of redox potential between CH₃COOH/CH₃CH₂OH and MnO₄⁻/MnO₂ (0.564 V in base condition). This method created both steric configuration and mild reactive condition for well control of the ultralong growth of α-MnO₂ nanowires, and was insensitive to temperature, reagent ratio, and reaction time (Fig. S3 in Supporting information).

  The formation mechanism of ultralong α-MnO₂ nanowires using our method was depicted in Fig. 3. Firstly, the primary nucleation of MnO₂ occurred slowly in the solution through the reaction between MnO₄⁻ and C₂H₅OH in weak base condition and formed three-dimension MnO₂ cores under hydrothermal condition. Then the hydrolysis of CH₃COOK leaded to the production of CH₃COOH attached to the surface of MnO₂ nuclei. When such reaction proceeded in a mild manner under steric effect of CH₃COOH, the MnO₂ intermediates grew along the c axis and formed ultralong nanowires slowly and homogenously. This approach was not only environmentally friendly and easily implemented, but also forward-looking for developing other ultralong 1D nanostructures of metal oxides.

  Figure 3

  

  Figure 3.  Schematic illustration for the formation mechanism of α-MnO₂ nanowires.

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  MnO₂ can enable Fenton-like chemistry and activate hydrogen peroxide (H₂O₂) to produce hydroxyl radicals (^•OH) for organic pollutant degradation [5]. The catalytic performance of catalysts is intensively related to their application forms [30]. Herein, the catalytic performance of the relevant free-standing α-MnO₂ membrane (Figs. 4a and b) assembled by ultralong α-MnO₂ nanowires towards BPA degradation was deeply investigated. To exhibit the advantage of membrane catalyzation with continuous flow through MnO₂ membrane (MnO₂-M, Fig. S4 in Supporting information) over traditional heterogeneous catalytic process with bulk MnO₂ particle suspension (MnO₂-P, Fig. S4), systemically comparison of the removal process was conducted between MnO₂-M and MnO₂-P towards BPA. As shown in Fig. 4c, the removal of BPA by MnO₂-M, MnO₂-P, or H₂O₂ solution alone within 60 min was negligible (less than 10%) due to respective limited reactivity. When MnO₂ was coupled with H₂O₂, BPA removal was enhanced significantly because Fenton-like reaction was initiated and a large amount of ^•OH were generated to oxidize the aqueous organic contaminants. Particularly, the reactivity of MnO₂-M (k_obs = 0.1112 min⁻¹) confined within nanowire membrane was obviously higher than that of MnO₂-P (k_obs = 0.0441 min⁻¹) in bulk solution (Fig. 4c). Moreover, the catalytic reactivity of MnO₂-M was considerable for most of the model phenolic pollutants (Fig. 4d), in addition of the limited Mn²⁺ release less than 0.05 mg/L in reactive solution (Fig. S6 in Supporting information), indicating a preferable potential in practical application.

  Figure 4

  

  Figure 4.  (a, b) Digital photo and SEM images of obtained MnO₂ membrane, (c) BPA removal efficiency and (d) reactive rate constant for organic pollutants of MnO₂ membrane catalyzed H₂O₂ system. Condition: solution volume, 50 mL; MnO₂-M/MnO₂-P, 1 mg; H₂O₂, 50 mg/L; BPA, 2 mg/L; pH, 6.75; for continuous flow reaction (MnO₂-M), recycle solution speed, 50 mL/min; for batch reaction, stirring speed, 300 r/min.

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  Due to the numerous channels or pores in membrane structure, the nanoconfinement effect is often hypothesized as a crucial factor for tuning membrane performance [30]. Therefore, it is speculated that the nanoconfinement effect could interpret this excellent performance when numerous channels (Fig. 4b) were observed to be developed within the ultralong α-MnO₂ nanowire membrane. Previous studies demonstrated that when chemical reactions occurred within a small or nanoscale confined space, their activity would be improved by altering the dielectric constant of water molecules, enrichment and exitance of reactants, or energy barriers [13, 14, 29, 31]. Based on the second-order rate of BPA with ^•OH (k₂ of 6.9 × 10⁹ L mol⁻¹ s⁻¹) [32], the steady-state ^•OH concentration in MnO₂-M/H₂O₂ system (0.267 pmol/L) was approximately 2.5-fold higher than that in MnO₂-P/H₂O₂ system (0.107 pmol/L) [33]. This result indicated that more ^•OH were exposed within the MnO₂-M to attack organic substrates due to the occurring nanoconfinement effect. When the overlapped α-MnO₂ nanowires created abundant channels or pores, the mass transfer of ^•OH was confined and its exposed concentration became higher as the distance towards pore walls became smaller. The heterogeneous spatial distribution of ^•OH avoided the long-ranged diffusion of short-lived free radicals and promoted the reaction kinetic rate within membrane compared to that on particles in bulk solution [14].

  This proposal could be further identified by electron paramagnetic resonance (EPR) and quenching results. From the EPR results (Figs. 5a and b), it was observed that DMPO-OH intensity (Fig. 5b) changed slightly in MnO₂-M reactor within 60 min, when most of the confined ^•OH near wall within the membrane were captured. However, the DMPO-OH intensity (Fig. 5a) decreased rapidly in MnO₂-P over time due to the fact that most surface generated ^•OH were self-quenched in bulk solution (Eqs. 5 and 6) and were less available for DMPO scavenger.

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  Figure 5

  

  Figure 5.  (a, b) EPR spectrum at different reaction time of MnO₂ particle and MnO₂ membrane catalyzed H₂O₂ system, (c) variances of radical quenching tests under different TBA/H₂O₂ ratio, (d) relationship between adsorption capacity and reactive rate constant for several organic pollutants.

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  TBA was used as a quenching probe to evaluate the reactive efficiency of ^•OH towards BPA in MnO₂-M and MnO₂-P [34]. Results showed that TBA exhibited higher quenching efficiency towards BPA removal in MnO₂-M/H₂O₂ system than in MnO₂-P/H₂O₂ system, regardless of TBA adding concentration (Fig. 5c). This phenomenon indicated that ^•OH was more thermodynamically favorable to react with TBA in a confined interspace than in a bulk solution. The ^•OH confined in the interspace of α-MnO₂ membrane was concentrated so that it possessed more collision probabilities with contaminants, resulting in enhanced efficiency during the reaction. As shown in Fig. 5d, the k_obs of different phenolic pollutants in both MnO₂-M/H₂O₂ (Fig. 4d) and MnO₂-P/H₂O₂ (Fig. S5 in Supporting information) systems were positively correlated with their adsorption efficiencies. Notably, slope of k_obs versus adsorption efficiency in MnO₂-M/H₂O₂ was higher than that in MnO₂-P/H₂O₂ system, indicating that the contaminant removal in MnO₂-M/H₂O₂ was more dependent on the reactions occurring around the surface possessing interface space. In a continuous flow reaction, the mass transportation between ^•OH and phenolic compounds was confined in channels or pores, thus enhancing the oxidized kinetic rate [35].

  In summary, our facile method for the preparation of ultralong α-MnO₂ nanowire membrane exhibited two following advantages. On the one hand, such easy and environmentally friendly method built a mild redox surrounding and steric hindrance to control the growth of α-MnO₂ nanowires. On the other hand, the α-MnO₂ membrane obtained by assembling these α-MnO₂ nanowires could create abundant channels or pores, triggering the nanoconfined effect to enrich the reactive oxygen species (^•OH) available for targeted contaminants in Fenton-like reaction. This study not only established a novel method to prepare ultralong α-MnO₂ nanowires, but also pointed out a potential application form of nanowire-shaped Fenton catalysts in environmental remediation.

  Declaration of competing interest

  The authors declare no conflict of interest.

  Acknowledgments

  The support from National Natural Science Foundation of China (Nos. 52000050, 52100024 and 42007115), Postdoctoral Science Foundation of China (Nos. 2019M663245 and 2020M670913), Heilongjiang Postdoctoral Fund (No. LBH-Z20063) and State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (Nos. 2021TS22 and QAK202111) are greatly appreciated.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.07.007.

  
  
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  Figure 1  (a) XRD pattern, (b) EDS spectrum, (c) SEM images and (d) HR-TEM images and SAED pattern of prepared MnO₂ nanowires.

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  Figure 2  SEM images of MnO₂ nanowires prepared under different conditions: (a) CH₃CH₂OH as reductive reagent at 140 ℃ for 12 h, (b) CH₃CHO as reductive reagent at 140 ℃ for 12 h, (c) CH₃CH₂OH as reductive reagent and H₂SO₄ as control reagent at 140 ℃ for 12 h, (d) CH₃CH₂OH as reductive reagent and CH₃COOH as control reagent at 140 ℃ for 12 h.

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  Figure 3  Schematic illustration for the formation mechanism of α-MnO₂ nanowires.

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  Figure 4  (a, b) Digital photo and SEM images of obtained MnO₂ membrane, (c) BPA removal efficiency and (d) reactive rate constant for organic pollutants of MnO₂ membrane catalyzed H₂O₂ system. Condition: solution volume, 50 mL; MnO₂-M/MnO₂-P, 1 mg; H₂O₂, 50 mg/L; BPA, 2 mg/L; pH, 6.75; for continuous flow reaction (MnO₂-M), recycle solution speed, 50 mL/min; for batch reaction, stirring speed, 300 r/min.

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  Figure 5  (a, b) EPR spectrum at different reaction time of MnO₂ particle and MnO₂ membrane catalyzed H₂O₂ system, (c) variances of radical quenching tests under different TBA/H₂O₂ ratio, (d) relationship between adsorption capacity and reactive rate constant for several organic pollutants.

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