Tuning the crystallinity of MnO2 oxidant to achieve highly efficient pollutant degradation

Min Zhong,Meng Li,Zixi Fn,Wnsong Hung,Huiru Ho,Zhixun Xi,Qin Zhng,*,Hojin Peng,Yio Zhng

a School of Civil Engineering & Architecture,Wuhan University of Technology,Wuhan 430070,China

b Department of Chemical and Environmental Engineering,University of Cincinnati,Cincinnati,OH 45221,United States

c Hubei Jianke International Construction Co.,Ltd.,Wuhan 430070,China

d School of Urban Construction,Wuchang Shouyi University,Wuhan 430064,China

Keywords:MnO2 polymorphs Oxidation Mn3+Oxygen vacancy Intrinsic relationship K+content

ABSTRACT Manganese dioxide (MnO2),a commonly find oxidant in both natural environment and industrial application,plays a crucial role for various organic compound degradation.Tuning the MnO2 crystal structure is a cost-effective strategy to boost the oxidation reactions,where the challenge remains due to lacking indepth investigation of the crystal properties.Herein,MnO2 with different crystalline structures (x-MnO2)including α-, β- and δ- was prepared through the hydrothermal synthesis for a typical organic pollutant removal.The structural and degradation analysis indicated that the oxidation capacity was originated from Mn3+ and oxygen vacancies (OVs).The intrinsic relationships between oxidation performance and other physiochemical properties such as morphology and electrochemistry were thoroughly discussed,and positive correlations between oxidation capacity and electrochemical properties were found which eventually led to excellent oxidation performance via modulating the above-mentioned properties.Moreover,the K+ content was determined to be the most crucial role in manipulating the structure properties.This work offers a crystal-level insight into the relationship between the crystal structure and oxidative property,promoting rational design of highly efficient oxidant.

Manganese is one of the transition metals and it has attached much attention due to the rich resources,low cost,environmentfriendly and diversified nature.It usually exists in the valence states of +2 to +7,while the manganese oxides mainly appear in Mn state of +2,+3 and +4,such as MnO2,Mn2O3,Mn3O4.Besides,it is common that different Mn states may exist in the oxide phase simultaneously,which provides the oxide with unique characteristics [1].

Manganese dioxide (MnO2),the relatively common oxide among them,displays outstanding performance in adsorption,electrochemistry,oxidation,and so on,while the properties of MnO2behave diversely attributing to the polymorphs in practical terms [2].Attentively,there are numerous crystalline structures of MnO2and the crystallinity such asα-,β-,γ-,δ- andλ- seems to be explored frequently.As for the crystal diversity,it is originated from the stacking or linking variety of [MnO6] units,and the structures could be divided into three types,including one-dimensional(1D) tunnel or chain framework (α-,β-,γ-),two-dimensional (2D)layer or sheet-like (δ-) and three-dimensional (3D) pores structure(λ-),thus the natural fabrication difference determines the various performance in all aspects [3,4].Musilet al.found thatα-,β-,γ-,andδ-MnO2differed in surface appearance,especially the layeredδ-MnO2[5].Shafiet al.employedβ-,γ-,δ- andλ-MnO2for the cyclic voltammetry determination,and the results showed thatδandλ-MnO2displayed the superior electro catalysis behavior [6].Liet al.appliedα-,β- andδ-MnO2for the 1,2,4-trichlorobenzene removal,and the results manifested thatδ-MnO2with the highest Mn3+/Mn4+gained the superior efficiency [7].Hence,there have been numerous researches on MnO2polymorphs,that is,synthesizing MnO2with different crystallinity based on the targeted properties and realizing the desired result.

Fig.1.Physiochemical properties analysis of x-MnO2 and the application for MG degradation: (a) XRD patterns; (b–d) SEM images; (e) LSV and (f) EIS measurement; (g,h)batch experiments for MG degradation comparison.(MnO2 dosage (g): 1–5 mg (pH 4.9±0.1); pH (h): 3–7 (dosage=3 mg)).

To synthesis of MnO2with different crystallography,several methods including thermal,reflux,hydrothermal,and sol-gel methods have been reported before [8].With regard to the hydrothermal synthesis,some previous works pointed out that the K+ions played the role of framework stabilizing and the crystallinity changed with the K+content; meanwhile,K+was closely related to the formation of oxygen vacancies (OVs) that further impacted on the production of reactive oxygen species (ROS) such as O2·-[9–12].Wanget al.claimed that K+amount affected the crystal formation,and layer structure would form under the higher K+content while tunnel skeleton was fabricated with the lower K+[13].Caoet al.revealed thatα- andδ-MnO2exhibited better electrocatalysis activities thanλ-MnO2since higher K+content was presented during the preparation [14].Zhuet al.reported that the higher K+content would slash the formation energy of oxygen vacancies,thus bringing better ozone elimination [10].Therefore,it seems that K+content is the critical factor for the characteristics of MnO2polymorphs in all aspects,such as morphology,electrochemistry,oxidizability.However,the intrinsic relationships among them remain ambiguous and further clarification is demanding to optimize the oxidation efficiency.To shed light on this,MnO2polymorphs includingα-,β-,andδ-MnO2were prepared through the hydrothermal synthesis and investigations including morphology,electrochemistry and oxidizability were conducted.Furthermore,malachite green (MG),a kind of organic compound,was commonly utilized in dyestuff and pigment industries,however,it was proved to be harmful to human beings and other organisms [15,16].Therefore,MG was chosen as the target pollutant,which could not only evaluate the oxidation performance of x-MnO2but also propose efficient methods for the degradation of similar pollutants.Details of experiments were exhibited in Supporting information.

It has been recognized that synthetic MnO2with different crystal structures was actually composed of basic [MnO6] octahedral units,and the bonding motifs diversity resulted in the polymorphs of MnO2,that is,the linking way was varied among the[MnO6] units [17].Obvious difference can be observed from the X-ray diffraction (XRD) peaks of different x-MnO2crystalline in Fig.1a.The characteristic crystal facets ofα-MnO2((110),(200),(310),(211),(301),(411) and (521)),β-MnO2((110),(101),(111),(211),(220),(310) and (112)) andδ-MnO2((001),(002),(200)and (020)) were coincided with XRD patterns of JCPDS No.44–0141 (cryptomelane),JCPDS No.24–0735 (pyrolusite) and JCPDS No.80–1098 (birnessite),respectively [13,18].The crystalline difference was the reflection of K+concentration discrepancy in the preparation,since K+played the important role of cation template and stabilizer,which could not only broaden the crystal cells but also impact on the balance of charge [10,18,19].In short,δ-MnO2with 2D layer structure would be fabricated in the process of chemical reaction and precipitation and acted as the framework precursor,and the sequent hydrothermal was the critical procedure for the crystallinity transformation,since MnO2crystal structure would curl under the elevated temperature and pressure,thus MnO2with 1D structure was formed in this plication[2,13].Consequently,as shown in Fig.S1 (Supporting information),δ-MnO2structure with abundant K+in the interlayer kept the skeleton stable and would be hardly affected by the hydrothermal,thus gaining the most expansive space (0.70 nm) for ions or molecules traversing,such as H2O or OH-species [5,7].As forα-MnO2,crystal structure with limited K+content could not completely resist the force brought by high temperature and pressure.Therefore,it curled to the structure of [2×2] 1D tunnels and occupied the size of 0.46 nm×0.46 nm,which possessed the edgesharing [MnO6] octahedra through the double-chain connection.However,β-MnO2framework without extra K+addition would collapse and transform to the cramped 1D [1×1] tunnels during the hydrothermal process,and the tunnel size was further narrowed to 0.23 nm×0.23 nm as a result of the dense oxygen atom array in the forms of twisted hexagon [20,21].

Based on the discrepancy of crystallinity,characterization instruments such as scanning electron microscope (SEM),transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET)were applied for the observation of morphology difference of x-MnO2.Apart from this,it was accepted that the K+could modulate the ions distribution in crystal cells,which may further have a significant impact on the oxidant’s properties such as electrochemistry and oxidizability.Hence,linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) were employed for physiochemical property analysis,while MG removal efficiency was utilized for the oxidation capacity comparison of x-MnO2.All the experiments above were conducted for expounding the effect of crystallinity discrepancy on the expression of morphology,electrochemistry and oxidizability,thus grasping the mutual influence factors among the above characteristics overall.

As shown in Figs.1b–d,it was obvious that x-MnO2exhibited differently in surface appearance.It could be observed thatβ-MnO2was rod-shaped (Fig.1c) whileα-MnO2was more fluffy and filamentous-like,bringing the larger surface area (Fig.1b).TEM images (Fig.S2 in Supporting information) showed thatβ-MnO2displayed higher aspect ratio at the same multiple thanα-MnO2,which was in line with SEM images.Completely different fromαandβ-MnO2,morphology ofδ-MnO2was similar to the broccoli and it seemed to be composed of numerous micro spheres (Fig.1d),and TEM images showed that it was lamellar microscopically.Correspondingly,it was shown in Table 1 and Fig.S3 (Supporting information) thatδ-MnO2possessed the highest specific surface area (SSA) whileβ-MnO2performed the least,as well as the pore volume (Pv).Although the average pore diameter (Pd) of x-MnO2followed the order ofβ-＞α-＞δ-MnO2,mesopore (2–50 nm) was the dominant structure in all of them (Fig.S3b).It could be acquainted that K2CO3was involved in bothα- andδ-MnO2,which may be the core of discrepancy above,since it had been reported that K2CO3would be transferred to KHCO3through the reaction with H2O and CO2in the autoclave,and then it would decompose quickly between 100°C and 200°C [22].Therefore,CO2gas would be generatedviaKHCO3decomposition in the hydrothermal preparation ofα- andδ-MnO2under the temperature of 180°C and super-atmospheric pressure,which facilitated the formation of abundant pores and made the surface more uneven.Besides,the additional metal cations of K+ofα- andδ-MnO2could incorporate into the structure and improve the surface area remarkably,resulting in the better morphology of them [23].In addition,the extra addition of KOH inδ-MnO2preparation made the solution more alkaline (5 g KOH/60 mL H2O),thus the alkalinous-hydrothermal process could stimulate the structure splitting and enlarge the surface area,explaining whyδ-MnO2was the superlative in surface and pores structure [24].Consequently,δ- andα-MnO2performed much better in SSA and Pv thanβ-MnO2.

Table 1 Morphology characterization and elements analysis of x-MnO2.

Since electrochemistry properties could also reflect the oxidation–reduction reactivity (ORR) to a certain extend,which may provide proper methods for the improvement of organic pollutants degradation,thus LSV and EIS measurements were conducted based on this.Fig.1e exhibited the LSV measurement of x-MnO2,and it could be accepted that onset potential ofα-,βandδ-MnO2was 0.98,0.80 and 0.99 Vvs.SCE while the limiting current density was 2.69,2.08 and 3.11 mA/cm2,respectively,suggesting thatδ-MnO2exhibited the highest ORR kinetics whileβ-MnO2did the worst [25].On the basis of the phenomena above,Tafel slope was drawn into the further analysis,and the results were shown in Fig.S4 (Supporting information).It was apparent that the slopes ofα-,β- andδ-MnO2were 206,224 and 194 mV/dec,respectively,thusδ-MnO2held the optimum catalytic activity due to the lowest slope [26].Integrating all the substances above,it could make the conclusion that the ORR activity obeyed the order ofδ-＞α-＞β-MnO2,which may be due to the special layered structure ofδ-MnO2,thus promoting the capacity of ions traversing and reaction activity [27].In addition,charge transfer numbers (n),which could describe the ORR pathway and reflect the activity,were calculated according to the equation below (Eq.1) [28,29].

whereIdrepresented the current at the disk whileIrwas that at the ring in rotating ring-disk electrode (RRDE) measurement,N was the collection efficiency of ring and disk electrodes (0.37).As exhibited in Fig.1e,charge transfer numbers ofα-,β- andδ-MnO2were 3.93,3.79 and 3.91,respectively,insinuating that ORR pathway of x-MnO2adhered to the 4e transferring,however,the electrocatalytic activity ofβ-MnO2was less prominent than that ofα- andδ-MnO2,which was also verified in LSV and Tafel slop results above.For the fact that K+was conducive for the charge balance with relatively higher electron transfer mobility and favorable for fast ions diffusion,therefore,it was not surprising thatδ-MnO2with the highest K+content displayed the most excellent electrochemical performance exhibited above [30].

In the light of the results above,EIS was adopted for the subsequent inquiry,and the Nyquist spectrums were exhibited in Fig.1f.It could be observed thatδ-MnO2got the lowest semi-circle diameter while that ofβ-MnO2was the highest,suggesting that the charge transfer resistance (RCT) between the electrode and electrolyte interface complied with the order ofδ-＞α-＞β-MnO2,and the results were accordant with that shown in LSV determination.For the more intuitive comparison,ZsimpWin software was applied for the experimental fitting,and the results showed that the electrolyte resistance in solution (RS) ofα-,β- andδ-MnO2were similar,locating in 6.6,7.1 and 6.9Ωaround,respectively.However,RCTvalues were quite different among them according to the fitting results in Fig.S5 (Supporting information).δ-MnO2held the lowestRCTwith 626Ωand that ofα-MnO2increased obviously(5986Ω),yetβ-MnO2possessed the highestRCT,much higher than that of bothδ- andα-MnO2(1.21×104Ω).Therefore,it was much more propitious forδ-MnO2to electron transfer,which was possibly benefited from the superior 2D-sheet structure with K+filling in the interlayer,thus providing the higher surface area as well as allowing the electrolyte species to get through quickly [6].

For the direct comparison of x-MnO2performance diversity,MG removal experiments were proceeded.It could be observed thatδ-MnO2performed the most satisfied MG removal under the different dosage,and the dosing ofδ-MnO2in the experimental range affected the performance slightly because MG removal efficiency was always over 95.00% (from 95.53% to 99.75%) (Fig.1g).As forα-MnO2,MG removal efficacy increased from 74.27% to 96.42% with the dosage increasing.However,it seemed thatβ-MnO2had inferior ability to degraded MG,since MG removal efficiency maintained subtle changes with the dosage increasing (from 38.76% to 46.58%).For the fact that MG was removed through the MnO2solids in the aqueous environment,thus the reaction was actually supposed to be the heterogeneous process.Therefore,pH was an essential factor for the above system since it was not only related to surface charge but also may affect the oxidation of MnO2,since it had been confirmed that MnO2displayed better oxidation capacity in the acid environment [31,32].In order to verify whether MG removal performance of x-MnO2would be restrained by the improper pH,batch experiments were conducted under the pH range of 3～7 for further investigation.It should be noticed that initial pH of the original MG solution was 4.9±0.1,thus pH 5 represented the stock solution.Furthermore,pHpzc of x-MnO2was determined for the verification of the hypothesis that whether the surface repulsion would hold back the heterogeneous reaction,since MG was cationic and existed in the forms of positive charge [33].As shown in Fig.1h and Fig.S6 (Supporting information),it could be concluded that MG removal ofβ-MnO2was always poorer than that ofα- andδ-MnO2although the reaction was conducted under the pH that was favorable forβ-MnO2but detrimental toα- andδ-MnO2,implying that surface contacting interaction was not the resistance for the lower MG removal ofβ-MnO2.Therefore,the conjecture could be proposed that oxidation capacity or some oxygen species related to MG removal ofβ-MnO2was much lower than that ofα- andδ-MnO2,thusβ-MnO2always displayed the inferior MG removal capacity under the favorable conditions even.

Fig.2.Reactive oxygen species detection.(dosage of MnO2=3 mg,pH 4.9±0.1;mole ratio of MnO2 to scavengers=1:20).

Though MnO2was able to remove organic contaminantsviathe common oxidation,it had been also reported that MnO2could degrade contaminants through the radical or non-radical pathway,such as·OH,1O2,and O2·-[34,35].Therefore,quenching experiments were conducted based on the hypothesis that some oxidative radicals and non-radicals were generated for boosting the degradation of MG.Hence,ethanol (EtOH),furfuryl alcohol (FFA)andp-benzoquinone (BQ) were used to capture the·OH,1O2and O2·-,respectively,with the reaction constants ofk(·OH)=1.9×109,k(1O2)=1.2×108andk(O2·-)=9.8×108mol L-1s-1,respectively[36,37].All the molar ratio of x-MnO2to scavengers above was set as 1:20,and the results were shown in Fig.2.It could be noted that MG removal was significantly impeded with the addition of BQ,implying that O2·-was momentous for all of x-MnO2and played a notable role.However,MG removal was hardly affected when the adequate EtOH existed,and the addition of FFA decreased the performance ofα- andδ-MnO2to some extent while it had a negligible impact onβ-MnO2.In order to examine whether the results above were caused by the insufficiency of EtOH and FFA,the ratio of x-MnO2to EtOH or FFA was reset to 1:50,and the results were furnished in Fig.S7 (Supporting information).With the increase of EtOH,MG removal efficacy of all x-MnO2was still in line with the control groups,showing that there may not have any·OH existing in the systems,since·OH was the nonselective radical species and it would react with most organic contaminants.As for FFA,the elevated dosage further slashed MG removal inα- andδ-MnO2while the effect was not obvious inβ-MnO2yet,demonstrating that1O2concentration was so scarce.Hence,it could be comprehended whyβ-MnO2did worst in MG removal,since ROS inβ- was not affluent enough compared withα- andδ-MnO2.Besides,it should be noted that ROS could not obliterate the MG removal performance thoroughly,and it had been also reported that MnO2was capable of removing some organic contaminants through the general redox reaction,thus the sequent experiments were conducted based on the hypothesis above.

It had been mentioned that several Mn states may simultaneously exist in MnO2while Mn3+and Mn4+were usually considered to be the important states species [38,39]).Therefore,Xray photoelectron spectroscopy (XPS) was employed for the subsequent research.Attentively,peaks at 641.3 eV and 643.0 eV represented Mn3+and Mn4+in Mn 2p,respectively; as for O 1s images,peaks of lattice oxygen (Olatt),adsorbed oxygen (Oads) and H2O were determined at 529.8 eV,530.4 eV,and 531.9 eV,respectively [40–44].It could be cognized in Figs.3a–c that the content of Mn3+inα-,β- andδ-MnO2was 53.3%,47.6%,and 54.5%,respectively.It had been reported that Mn3+could be generated at a certain molar ratio of Mn2+/KMnO4in the preparation of MnO2,and the oxygen vacancies formation was companied with the simultaneous process of Mn4+reduction,suggesting that there would be some trivalent Mn existing in MnO2[45,46].Hence,it could be observed that there was a large amount of Mn3+appearing in all of x-MnO2.In addition,it had been mentioned in the previous paragraphs that there were some K+ions filling in the tunnels or interlayer ofα- andδ-MnO2,therefore,more Mn3+would be formed for the charge balance of the foreign ions [2].Therefore,the discrepancy of K/Mn ratio was another reason why Mn3+content adhered the order ofδ-＞α-＞β-MnO2.In addition,it could be noticed that the order of Mn3+content was in line with MG removal efficiency,suggesting that the role of Mn3+was of great concern.It had been reported that O2could be adsorbed on MnO2surface,and then eg1electron of Mn3+could be transferred to the adsorptive O2,and then O2·-was generated [7,17].As a result,the higher Mn3+species content existed,the more prosperous O2·-would be produced,thus accelerating MG degradation.Hence,δ-MnO2with the most affluent Mn3+performed best.Besides,Oadswas another important factor since it could be utilized for O2·-generation.Therefore,it could be discovered in Figs.3d–f that the ratio of Oadsto Olattfollowed the order ofδ-＞α-＞β-MnO2,manifesting thatδ-MnO2possessed the most outstanding capacity for surface oxygen sorption,which was also in accordance with both Mn3+content and MG removing.As for1O2,it was probably formed through the way that O2·-reacted with itself under the existence of H+[47,48].In consequence,α- andδ-MnO2with relatively ample O2·-could produce1O2viathe reaction above while it may be limited inβ-MnO2,since the inadequate O2·-would be consumed by MG and there was too little O2·-left to generate1O2further,thus1O2mass inβ-MnO2was unsatisfied.

In fact,the generation of O2·-was also controlled by the oxygen vacancies (OVs) since OVs could capture the adsorbed O2and motivated it to be converted to O2·-when it got close to the subsurface oxygen defect [49].Therefore,the content of OVs was another essential factor for oxidation capacity of MnO2.With regard to the formation of OVs,it was companied with the loss of Olatt,that is,O atom of Mn(IV)-O-2-Mn(IV) escaped from the framework and then the new structure of Mn(III)-□-Mn(III) formed for the charge balance,consequently,the oxygen vacancy (□) generated while the state of Mn decreased [50–52].Hence,the higher Mn3+content suggested the more presentation of OVs,which further resulted in a higher contaminants removal capacity.Moreover,the ratio of Mn/O for x-MnO2,indicating the potential of O absconding and OVs emergence,was displayed in Table 1.It could be realized that the Mn/O ofα-,β- andδ-MnO2was 0.484,0.434 and 0.492,respectively,which was abided by the OVs production order ofδ-＞α-＞β-MnO2and coincided with the MG removal effi-cacy.Moreover,it should be paid significant attention that K+had a potential for cutting down OVs formation energy (EOV) since K+had the ability to lead to the electrostatic interaction between K+and Olatt,thus boosting the generation of OVs [10,53]).Therefore,OVs were easier to be produced inδ-MnO2with the most abundant K+mass and MG could be degraded more excellently in this paper.Furthermore,it had been reported that OVs on MnO2surface were pivotal in1O2formation,since it favored capturing O2and promoting the mobility,thus facilitating the generation of O2·-and the following conversion of1O2[54].Hence,α- andδ-MnO2with more OVs had abundant sites for1O2fabrication,which was in accordance with the experiments of both MG removal and ROS quenching.

Otherwise,TGA determination,which reflected the surface oxygen species and manganese oxides transformation,was applied for the thermal analysis of x-MnO2,conducting under the determined temperature of 30–1000°C and with the temperature rising rate of 10 K/min.It was revealed in Fig.4 that the decomposition of x-MnO2could be generally divided into four steps as the temperature increased.In step 1 (＜250°C),the mass loss ofα-,β- andδ-MnO2was 4.59%,1.10% and 9.93%,respectively,which was due to the deprivation of physically adsorbed or interlayered water,and the least loss ofβ-MnO2may be attributed to less water entrapped in the parochial tunnel ([1×1]) [55].As the temperature rose (step 2,250–550°C),mass loss ofα- andδ-MnO2were 1.63% and 3.37%,respectively,while that ofβ-MnO2was only 0.53%,signifying that surface active-oxygen disappeared.The total loss ofα-,β- andδ-MnO2below 550°C followed the order ofδ-＞α-＞β-MnO2,implying thatδ-MnO2owned the amplest surface oxygen species,which may be conducive to the generation of O2·-.In step 3 (550–800°C),the conspicuous loss ofβ-MnO2(4.16%) demonstrated that MnO2crystal missed O atom and then transformed into Mn2O3,however,α- (0.35%) andδ-MnO2(0.00% nearly) were relatively stable and maintained the original components.When the temperature was higher than 800°C (step 4),the mass loss was due to the further decomposition from Mn2O3to Mn3O4.It could be discovered that the loss ofδ-MnO2was the lowest,since abundant K+ions in the layers held back the transformation of Mn2O3[56].In general,bothα- andδ-MnO2possessed affluent surface oxygen species while there was little inβ-MnO2,which may be another explanation for MG removal behavior,since more O2·-could be generated onα- andδ-MnO2,thus improving the higher oxidative capability.

Fig.3.XPS analysis of MnO2: Mn 2p (a–c) and O 1s (d–f).

Fig.4.Thermogravimetric analysis of MnO2.

Except for the reactive factors above,Mn3+itself could be also taken into consideration for MG degradation,since Mn3+could receive an electron through the emptyбorbit coordination with MG without changing the Mn spin state [57].As for Mn4+,the process above could be realized through the outer sphere electron transferring and the spin state was required to be transformed,thus it was more difficult compared with Mn3+; meanwhile,Mn3+possessed four 3d orbital electrons and the anti-bonding electron(eg1) would bring about the longer (Jahn-Teller distortion),leading to the weaker Mn-O bond and the higher ORR activity compared with Mn4+[58,59]).Therefore,it could be better understood that superior MG removal was realized under the higher Mn3+rather than Mn4+.Furthermore,it had been mentioned above that Mn3+and OVs were generated simultaneously and the higher K+content was conducive to the richer OVs production,thus generating more Mn3+content.Hence,δ-MnO2with the amplest K+reached the best MG removal owing to the highest Mn3+content whileβ-MnO2did worst.Moreover,MnO2would react with H2O and then turned into the active site (MnOOH) through accepting an electron from H2O,thus MnOOH may be another trigger for MG removal and the ORR kinetics was probably controlled by the MnOOH oxidation [14,60].

According to all the results above,it could be considered that the performance diversity was related to K+content and it seemed that there was some connection among the properties.Therefore,this part was conducted for the thorough expounding of the discrepancy and intrinsic relationship (Fig.5).Firstly,it could be accepted that crystallinity diversity ofα-,β-,andδ-MnO2was attributed to the K+content,since K+ions would act as the template and stabilizer,which was useful for tightening up the 2D layer structure ofδ-MnO2and allowed more ions to pass through the interlayer.However,the limited or the absent K+inα- andβ-MnO2could not prevent the skeleton from curling and collapsing to the 1D tunnel structure with [2×2] or [1×1],respectively,and the capacity of ions containing in the tunnel decreased compared withδ-MnO2.Consequently,electrochemical properties interrelating with ORR kinetics analyzedviaLSV and EIS exhibited thatδ-MnO2gained the most prominent performance,coinciding with the tunnel size or layer distance characteristic above.Secondly,it could be observed that the overall MG removal performance adhered to the order ofδ-＞α-＞β-MnO2,which kept pace with that of ORR activity behavior suggested in LSV and EIS examination.According to the incisive analysis of MG removal in previous paragraphs,it could be acquainted that MG removal was mainly profited from the oxidative attackingviaROS as well as the electrons transferring between Mn3+and MG,thusδ-MnO2with the highest ORR activity favored the interaction with MG and realized the more tremendous removal.Attentively,it had been mentioned that K+was conducive to theEOVdecreasing,thusδ-MnO2with the highest K+content possessed the most fertile Mn3+and OVs and degraded MG in the most conspicuous capacity,since Mn3+and OVs were important sources for the generation of O2·-and1O2,directly oxidizing MG and achieving the rapid degradation.Meanwhile,it had also been mentioned that OVs could facilitate the electrolytic ions diffusion and charges transferring,thusδ-MnO2with the richest OVs displayed the best electrocatalysis and the lowest resistance in LSV and EIS [61].Hence,it could be proposed that the higher K+stimulated the more OVs production,which optimized both electrochemistry and oxidizability of MnO2,thusδ-MnO2gained the best performance.Moreover,it should be admitted that adsorptive O2was another essence for MG removal.Therefore,α- andδ-MnO2with better morphology could provide more active sites for the more excellent O2adsorption capability and MG degradation,which was as a result of the ample K+addition and exhibited in SEM images andSBETdetermination.

Fig.5.Intrinsic relationship among the characteristics of MnO2 polymorphs.

Furthermore,it had been mentioned that crystallinity was connected with ORR activity,and it could be discovered that facets of x-MnO2slackened after reaction (Fig.1a),confirming that they were participated in MG removal process [62].The facets of (001)and (002) inδ-MnO2decreased remarkably since they had a bearing on the formation of OVs,and the more consumption of (001)was a result of the higher activity [27].As forα-MnO2,it could be caught sight of the slight abating of the (110),(200) and (211)facets,signifying that there were utilized in the reaction due to the reactive activity.However,the lessening of facets inβ-MnO2was less visible thanα- andδ- even though the relatively active facet of (110) decreased slightly,which may be due to the finite activity [62].Generally,the crystal facets cutting variation above not only reflected the MG removal performance in essence,but also manifested the crystallinity activity diversification resulted from K+content,which directionally provided the pathway of ORR activity promoting.

In conclusion,MG degradation capacity adhered to the order ofδ-＞α-＞β-MnO2,according with K+content diversity among them,since it could whittle down the formation energy of OVs and enhance the production of both Mn3+and OVs,thus facilitating the sequent ROS generation and MG degradation.In addition,electrochemistry and oxidizability were turned out to be positively correlated due to the reflection of OVs content and ORR activity; meanwhile,the brilliant morphology provided more active sites for O2adsorption,indirectly promoting the oxidation capacity.Therefore,δ-MnO2with the highest K+content was the most ORR active and gained the richest active-sites as well as OVs,thus performing the optimum in electrochemistry and oxidizability.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was financially supported by the Hubei Provincial Key Lab of Water System Science for Sponge City Construction (No.2019–06) and the Fundamental Research Funds for the Central Universities (No.215206002).

Supplementary materials

Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.cclet.2022.01.082.