Simple and Controllable Synthesis of High-Quality MnTiO3 Nanodiscs

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Simple and Controllable Synthesis of High-Quality MnTiO3 Nanodiscs and Their Application as A Highly Efficient Catalyst for H2O2-Mediated Oxidative Degradation Hao Wang, Qiang Gao, Haitao Li, Min Gao, Bo Han, Kaisheng Xia, and Chenggang Zhou ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00432 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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ACS Applied Nano Materials

Simple and Controllable Synthesis of High-Quality MnTiO3 Nanodiscs and Their Application as A Highly Efficient Catalyst for H2O2-Mediated Oxidative Degradation Hao Wang,† Qiang Gao,*,† Hai–Tao Li,† Min Gao,† Bo Han,‡ Kai–Sheng Xia,‡ Cheng–Gang Zhou*,‡ †

Department of Chemistry, Faculty of Material Science and Chemistry, China

University of Geosciences, Wuhan 430074, PR China ‡

Sustainable Energy Laboratory, Faculty of Material Science and Chemistry, China

University of Geosciences, Wuhan 430074, PR China

*

Corresponding authors:

Tel./Fax: +86 027 6788 3731; E-mail addresses: [email protected] (Q. Gao); [email protected] (C.G. Zhou).

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Abstract: Herein, we reported a simple synthesis of monodisperse, uniform, and highly crystalline MnTiO3 nanodiscs and their application as a new promising catalyst for H2O2-mediated oxidative degradation. Unlike previously reported method that required use of pre-prepared titanate nanowires as precursor, this synthesis of well-defined MnTiO3 nanodiscs was implemented through a one-pot homogeneous reaction process under hydrothermal conditions. The growth of MnTiO3 nanodiscs was revealed to follow the nucleation–dissolution–recrystallization mechanism, and the (001) crystal facet was preferentially exposed since it was the densest and most thermodynamically stable. Two-dimensional flat plane of the MnTiO3 nanodiscs helped minimize the diffusion pathway of guest molecules, thus allowing a fast mass transport. Moreover, the Mn(II)-rich structure and distinct crystallinity endowed MnTiO3 nanodiscs with high activity and stability. In the presence of H2O2, MnTiO3 nanodiscs exhibited a high efficiency in catalytic decomposition of a series of organic pollutants with an excellent recycling durability. About 98.6% of methylene blue was catalytically decomposed within 20 min at 30 oC. When the reaction temperature increased to 40 °C, only 8 min was required. Comparative investigation further confirmed the superior catalytic performance of MnTiO3 nanodiscs, with about 2 times higher removal efficiency than reported under similar conditions. Keywords: MnTiO3 nanodiscs; Growth mechanism; Catalyst; H2O2-mediated oxidative degradation; High activity and stability.


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1. Introduction Dimension- and morphology-tunable nanostructures have attracted rapidly growing interest over the past few decades, mainly because of their unique properties and important technological applications.1,2 The manipulation of nanomaterials with desirable structures possessing excellent physicochemical features, via convenient synthetic routes, is one of the most challenging issues in materials science.3 Recently, considerable attention has been paid to the shape- and size-controlled synthesis of metal titanate nanostructures with versatile performances.4–7 As an important metal titanate, MnTiO3 has recently attracted intense attention due to its promising applications in solar cells,8,9 Li-ion batteries,10 sensors,11,12 and multiferroic materials.13,14 Moreover, it is abundant, non-toxic, and low-cost, which makes it also suitable for dielectric ceramics and photocatalytic applications.15–20 Substantial efforts have therefore been devoted to developing efficient methodologies for controlled synthesis of MnTiO3 nanocrystals. Solid state reaction between the respective oxides or hydroxides of manganese and titanium is usually employed in industrial processes due to its ability of scale production.15,21 However, this method suffers from some disadvantages such as irregular morphologies, non-uniform distribution of grains, and requirement of very high reaction temperatures (≥ 800 oC).21 A range of wet chemical methods are also now publicly available as alternative strategies to synthesize this kind of metal titanate, of which hydrothermal synthesis is more recommendable because of its advantages over other methods such as simple operation procedures, easily adjustable process parameters, and high crystallinity of



the product.22 Moreover, syntheses under hydrothermal conditions usually result in specific morphologies with high particle monodispersity, which are considered fairly beneficial for practical applications. Particularly, it has been recently demonstrated that the novel MnTiO3 nanocrystals with disc-like morphology can be obtained through the hydrothermal treatment of titanate nanowires, MnCl2⋅4H2O, and NaOH solution.20 Such a two-dimensional (2D) nanostructure possesses highly exposed surface and is thus expected to facilitate easy access of guest species.23 However, titanate nanowires need to be pre-prepared before hydrothermal reaction in this method, which will increase the production cost and is also time-consuming. Moreover, reactions occur in heterogeneous solid–liquid systems, which usually have a relatively weak ability to control the uniform growth of product, and it is also somewhat difficult to completely avoid the possible residue of solid precursor. Therefore, it is highly desirable to develop simple methods for controllable synthesis of MnTiO3 with well-defined morphology (e.g. uniform nanodiscs), without the use of any solid precursors. On the other hand, a large number of manganese-based materials (e.g., Mn3O4 and MnSiO3) are proven to be one of the most efficient catalysts for various oxidation reactions due to the remarkably versatile redox chemistry of manganese.24 In the past few years, some of these materials have also been applied to oxidative degradations of organic pollutants in the presence of hydrogen peroxide (H2O2).25–28 Compared to the conventional heterogeneous catalysts (e.g., Fe2O3 and Fe3O4), these manganese-based catalysts usually behave more active in catalyzing decomposition of H2O2 to produce hydroxyl radical (⋅OH), and thus exhibit higher efficiencies for the removal of organic


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pollutants. Nevertheless, their use in H2O2-mediated oxidative degradation is still limited by their relatively low recycling capability. For example, Zhong et al. fabricated a magnetic Mn3O4-based catalyst for sulfamethazine degradation, which exhibited a significant activity loss after five cycles (from 94% (1st) to 60% (5th)).29 This is largely because the active manganese (in reduced form) can be readily oxidized in the presence of H2O2, while the inverse process (i.e., manganese regeneration) is quite difficult owing to the high reduction potential of Mnox/Mnred, which results in the gradual inactivation of manganese and further causes a decreased catalytic activity.30 Very recently, we proposed a strategy for addressing this issue by taking advantages of the unique promoting effect of Ti species on manganese regeneration in the Mn(II)-doped TiO2.31 Through theoretical simulations and experimental investigations, it was revealed that electron cloud in the Ti–O–Mn structure could move to the Mn to compensate for the electron loss of Mn, thereby greatly facilitating the redox cycling of Mn between its oxidized and reduced forms. As a result, the Mn(II)-doped TiO2 catalyst showed excellent reusability for H2O2-mediated oxidative degradation, i.e., retaining 98.40% of its initial activity after five cycles.31 Despite these encouraging results, the Mn content in Mn(II)-doped TiO2 was relatively low (Mn:Ti = 1:10) and hard to achieve a higher level, which somewhat restricted the further enhancement of catalytic performance. Moreover, due to the adverse effect of dopant on the lattice constant, the Mn(II)-doped TiO2 exhibited a weak crystallinity, which might be disadvantages for its long-term stability. Bearing in mind these facts, it was natural to consider the possibility of constructing Mn(II)-rich



and highly crystalline Ti–O–Mn structure for achieving a more excellent performance in H2O2-mediated oxidative degradation. By comparing the difference between Mn(II)-doped TiO2 and MnTiO3, it can easily find that the latter has a significantly higher Mn content (theoretically, Mn:Ti = 1:1). Moreover, as widely reported, highly crystalline MnTiO3 nanocrystals can be easily formed even at relatively low temperatures (≤ 200 oC).10,13,18 Thus, we speculate that the MnTiO3, consisting primarily of Ti–O–Mn structural units with high crystallinity, might hold great potential as a new heterogeneous catalyst for H2O2-mediated oxidative degradation. Herein, we reported a simple synthesis of well-defined nanodisc-like MnTiO3 with pure LiNbO3-type structure through a one-pot homogeneous reaction route under mild hydrothermal conditions. Under optimized conditions, the resulting MnTiO3 nanodiscs exhibited a high dimensional homogeneity with a significantly smaller thickness (ca. 16.2 nm) than that of the reported analogue (~100 nm).20 Furthermore, a possible growth mechanism of MnTiO3 nanodiscs was also proposed based on the experimental results. Interestingly, the prepared MnTiO3 nanodiscs could be used a new promising catalyst for H2O2-mediated oxidative degradation, and showed significantly better catalytic activity than many previously reported heterogeneous catalysts including Mn(II)-doped TiO2. Moreover, the MnTiO3 nanodiscs-type catalyst showed an excellent reusability without significant activity loss after six cycles. 2. Experimental 2.1 Chemicals and Reagents Ethylene glycol (EG), ethylenediamine, manganese chloride tetrahydrate


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(MnCl2⋅4H2O), methylene blue (MB), basic blue 17 (BB), malachite green (MG), alizarin red S (ARS), basic red 5 (BR), sodium hydroxide (NaOH), hydrogen peroxide (H2O2, 30 wt%) were all obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Titanium isopropoxide (TTIP, Ti(OC3H7)4, 98%) was provided by J&K Scientific Ltd. (Beijing, China). All these reagents were used as received without further purification. 2.2 Synthesis of MnTiO3 Nanodiscs The MnTiO3 nanodiscs were synthesized via a simple one-pot hydrothermal route, as illustrated Scheme 1. Typically, 0.394 mL of TTIP (1.25 mmol) was added dropwise into 16 mL of EG under vigorously stirring at ambient temperature to form a homogeneous solution (solution A). Simultaneously, 247.8 mg of MnCl2⋅4H2O (1.25 mmol)

were

dissolved

into

4

mL

of

ethylenediamine

to

get

a

[Mn(ethylenediamine)2]Cl2 solution (solution B) under vigorous stirring. Then, both solution A and solution B were transferred into a Teflon-lined stainless-steel autoclave (50 mL), and subsequently 5 mL of water were added drop by drop under vigorous stirring. After sealed, the autoclave was placed in an oven, heated at 200 oC for 24 h, and then allowed to be cooled to room temperature. The solid product was collected by centrifugation, followed by washing with ethanol and water several times to remove excess solvent and impurities. After drying in a vacuum oven for 6 h at 60 oC, the pale yellow powders (i.e., MnTiO3 nanodiscs) were obtained. 2.3 Characterization of Samples The morphologies of samples were observed by a field-emission scanning



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electron microscope (SEM, SU8010, Hitachi, Japan) and transmission electron microscope

(TEM,

Philips

CM12).

Wide-angle

X-ray

diffraction

(XRD)

measurements were carried out on an X-ray diffractometer (D8-FOCUS, Bruker, Germany). The N2 adsorption/desorption analysis was performed on a Micromeritics ASAP2020 surface area analyzer at 77 K. 2.4 Evaluation of Catalytic Activity Catalytic activity of the typical MnTiO3 nanodiscs was evaluated by oxidative degradation of typical organic pollutants (MB, BB, MG, ARS and BR) in the presence of H2O2. Typically, 10 mL of MB solution (100 mg L−1) containing 4.5 wt% H2O2 was added into a vial that contained 5 mg of catalysts, and the mixture (pH 8.5) was shaken in the dark with a speed of 200 rpm at 30 oC. At predetermined time intervals, the catalyst was separated immediately from the solution with a filter. The concentrations of MB in the course of degradation were measured at the maximum absorption wavelength (664 nm) by using the UV–Vis spectrophotometer. With use of a process similar to that described above, the oxidative degradations of BB, MG, ARS and BR were also investigated. The maximum absorption wavelengths for determining BB, MG, ARS and BR were 629, 617, 518, and 525 nm, respectively. 3. Results and Discussion 3.1 Synthesis and Characterization of MnTiO3 Nanodiscs As depicted in Experimental section, the MnTiO3 nanodiscs were synthesized via a simple one-pot hydrothermal treatment of TTIP, MnCl2·4H2O, EG, ethylenediamine, and H2O. In general, tetra-alkoxyltitanium compounds (Ti(OR)4, e.g., TTIP) are


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highly susceptible to moisture. Once exposed to water, even incompletely dried air, they are likely to be transformed into amorphous titania within a very short period of time.32 Therefore, in order to implement a homogeneous reaction, it is necessary to prevent TTIP from fast hydrolysis. Recently, growing evidence has demonstrated that Ti(OR)4 can readily react with EG to form Ti(OCH2CH2O)2 complex (i.e., Ti(OR)4 + HOCH2CH2OH → Ti(OCH2CH2O)2 + 4HOR), which is stable and highly resistant to hydrolysis.32,33 In our synthesis system, the added amount of EG (16 mL, 286.9 mmol) is significantly higher that of TTIP (1.25 mmol), therefore, it is expected that the transformation of TTIP to Ti(OCH2CH2O)2 should be quite adequate. Indeed, it was confirmed that the mixture of TTIP and EG could be kept in air for several weeks without observing any precipitation from solution. Besides EG, in fact, the ethylenediamine also plays an important role in synthesizing MnTiO3. It will not only regulate the alkalinity of synthesis solution, but also form strong complex with Mn2+ as follows: Mn2+ + 2NH2CH2CH2NH2 → [Mn(NH2CH2CH2NH2)2]2+, which can effectively eliminate the possible formation of manganese oxide or hydroxide precipitation.34 Our recent study has also demonstrated that the ethylenediamine ligand can indeed bind effectively with Mn2+ and the resulting [Mn(NH2CH2CH2NH2)2]2+ complex can maintain stability even at a relatively high temperature (i.e., 200 oC).28 In the present work, in order to assure the adequate coordination between Mn2+ and ethylenediamine, an excess amount (i.e., 4 mL) of ethylenediamine was used. To drive the reaction between Ti(OCH2CH2O)2 and [Mn(NH2CH2CH2NH2)2]2+



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to generate MnTiO3, a certain amount of H2O was added to the synthesis system. Under hydrothermal condition, the presence of H2O might induce the following hydrolysis–condensation reaction: Ti(OCH2CH2O)2 + [Mn(NH2CH2CH2NH2)2]2+ + 3H2O → MnTiO3 + 2OHCH2CH2OH + 2[NH2CH2CH2NH3]+

(1)

Accordingly, the nucleus of MnTiO3 would be generated and further supported formation of the objective product. In order to ensure the morphology and structure of the product to be satisfactorily developed, it is necessary to control the speeds of nucleation and growth by optimizing the reaction conditions. For simplicity, the reaction temperature was fixed at 200 oC, while the dosages of TTIP (1.25 mmol), MnCl2·4H2O (1.25 mmol), EG (16 mL), and ethylenediamine (4 mL) were kept constant, so we needed only to optimize the water content and the reaction time. The water content- and reaction time-dependent nucleation and growth processes will be discussed in detail later. After hydrothermal treatment for 24 h with water content of 5 mL, a typical MnTiO3 sample (designated as MTO-24-5) was successfully synthesized. Its morphology, phase structure, element content, and element distribution were carefully characterized by SEM, EDS, TEM, HRTEM, XRD, and N2 adsorption–desorption measurements (Figure 1). The SEM image (Figure 1a) clearly shows that the as-prepared MTO-24-5 possesses a monodisperse, uniform, and disc-like morphology with an average diameter (D) of ~333 nm and an average thickness (T) of ~16.2 nm. The aspect ratio (D/T) is calculated to be about 20, which implies strong shape


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anisotropy. Recently, Dong et al. reported the synthesis of MnTiO3 through a one-pot heterogeneous reaction using titanate nanowires as titanium precursor.20 The resulting sample showed a hexagonal disc-like morphology, and the thickness of these nanodiscs was ca. 100 nm.20 Evidently, the MTO-24-5 fabricated in this work has significantly smaller thickness than that of the reported sample, which should be somewhat more advantageous for providing sufficiently exposed active sites. Typical EDS elemental mapping analysis (Figure 1b–d) reveals that the MTO-24-5 sample consists entirely of Ti, O, and Mn elements, and these elements are homogeneously distributed throughout the entire parts of the sample. Moreover, the Mn:Ti:O ratio is found to be exactly equal to 1:1:3 (Figure 1e), which agrees well with the theoretical stoichiometric value of MnTiO3. These results provide strong evidence for the high purity of MTO-24-5 sample. In order to gain better understanding of the structural characteristics of MTO-24-5 sample, TEM analysis was also carried out. A typical TEM image of the MTO-24-5 sample exhibits “transparent” nature of the nanodiscs (Figure 1f), implying their ultra-thinness. Furthermore, the exposed facets of the nanodiscs were determined by high-resolution TEM (HRTEM) and the results are shown in Figure 1g. The clear lattice spacing and the selected-area electron diffraction (SAED) pattern indicate that the MnTiO3 nanodisc is a single crystal and owns high crystallinity.35 An larger magnification image of the MnTiO3 nanodisc is shown in Figure 1h. The frontal plane of MnTiO3 nanodisc displays three sets of clear lattice fringes with the same inter-planar distance of about 0.26 nm, and the angle between each pair of



crystal faces is either 120o or 60o. It can be found that both interplanar lattice spacing values and angles labelled in the image are well consistent with the theoretical calculation (Figure 1i). Accordingly, the discrete diffraction spots should be indexed to the (2ത110), (1ത1ത20), and (12ത10) facets, which are all parallel to the [0001] direction. Given that the incident electron beam is perpendicular to the nanodisc plane, it can be further inferred that the top and bottom planes of the MnTiO3 nanodiscs are (001) facets. Phase structure of the MTO-24-5 sample was determined by XRD and the resulting XRD pattern of MnTiO3 is shown in Figure 1j. The diffraction peaks at 2θ at 23.54o, 32.11o, 34.89o, 39.82o, 48.14o, 52.34o, 55.38o, 60.81o, 62.55o, 68.97o can be assigned to the (012), (104), (110), (113), (024), (116), (018), (214), (300), and (1010) crystal facets of LiNbO3-type MnTiO3 structure (JCPDS card No. 29-0902).20 Reflection conditions derived from the indexed reflections of MTO-24-5 are −h + k + l = 3n (n: integer), affording possible space group R3c.36,37 No other diffraction peaks are observed, further confirming high purity of the product. Moreover, all these peaks are very narrow and sharp, which reflects high crystalline nature of the product. In addition, N2 adsorption–desorption analysis demonstrated that the MnTiO3 nanodiscs had a specific surface area of 30 m2 g–1. 3.2 Optimal Synthesis and Possible Formation Mechanism of MnTiO3 Nanodiscs As well documented, the water content and the reaction time are two key factors for the formation of well-defined nanoparticles from hydrolysis and condensation processes.38 Therefore, effect of the water content on the morphology and size of the


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products was first investigated at 200 oC for 24 h. A varied amount of water (2.5, 5, and 7.5 mL) was added to the synthesis system, and the resulting MnTiO3 samples were designated as MTO-24-2.5, MTO-24-5, and MTO-24-7.5, respectively. From the SEM images shown in Figure 2, it can be clearly found that all these samples contain monodisperse and well-defined nanodiscs (Figure 2a, b, d, e, g, and h) and the diameters of nanodiscs have a tendency to become smaller with the increase of water content, i.e., MTO-24-2.5 (ca. 490 nm) > MTO-24-5 (ca. 333 nm) > MTO-24-7.5 (ca. 280 nm) (Figure 2c, f, and i). This phenomenon should be understandable because the hydrolysis rates of metal precursors generally correlate positively with the water content.39 An increase in water content is expected to accelerate precursor hydrolysis and thus produce more nucleus, which results in the decrease in the average particle size.35 In addition, careful observation of high magnification SEM image of the MTO-24-2.5 reveals that this sample, besides possessing nanodiscs, and also contains a significant amount of ultrafine nanoparticles with the diameter below 20 nm around these nanodiscs (Figure 2b). Interestingly, when the reaction time was prolonged for another 48 hours, no ultrafine nanoparticles could be observed and only well-defined nanodiscs with an increasing mean diameter (~ 573 nm) existed alone in this sample (Figure S1). The result implies that the nanodiscs might be formed via a nucleation– dissolution–recrystallization process accompanied by oriented growth, in which small particles dissolve and recrystallize onto a preferred growing direction of the large particles. At low water content, this process proceeds slowly, and the 24 h reaction time is insufficient to allow all the ultrafine particles to evolve into nanodiscs, which



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leads to the coexistence of ultrafine nanoparticles and nanodiscs in the MTO-24-2.5 sample (Figure 2a and b). In contrast, both MTO-24-5 and MTO-24-7.5 samples are found to consist entirely of nanodiscs (Figure 2d, e, g, and h), which might be attributed

to

the

fast

nucleation–dissolution–recrystallization

rate

in

the

high-water-content environments. In order to understand the growth process of MnTiO3 nanodiscs in more detail, time-dependent reactions were carried out. To facilitate discussion, the MnTiO3 samples obtained at different reaction time intervals were designated as MTO-x-5, where “x” and “5” represent the varied reaction time (2, 6, 12, 24, or 48 h) and the fixed water content (mL), respectively. SEM images of the samples are presented in Figure S2 and Figure 3. As shown in Figure S2, only ultrafine nanoparticles smaller than 20 nm appear at 2 h, and they are found to be amorphous by XRD. Moreover, these amorphous primary particles are found to have a relative high molar ratio of Ti/Mn (i.e., ~3:2) as compared to the final product. When the reaction time is up to 6 h, the obtained product (MTO-6-5) is made up of ultrafine nanoparticles (< 20 nm) and nanodiscs with an average diameter of ~288 nm and an average thickness of ~13.3 nm (Figure 3a–c). As the reaction time prolongs to 12 h, nanodiscs become the major product, while the nanoparticles gradually disappear (Figure 3d and e). During this process, it is also observed that the nanodiscs grow in diameter and thickness, i.e., MTO-12-5 (~ 312 nm; ~ 14.5 nm) (Figure 3f). If the reaction time exceeds 24 h, the ultrafine nanoparticles no longer exist and only well-defined nanodiscs with increasing sizes and smooth surfaces can be found (Figure 3g, h, j, and k). The


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obtained samples, i.e., MTO-24-5 (~ 333 nm; ~ 16.2 nm) and MTO-48-5 (~ 335 nm; ~ 16.2 nm) (Figure 3i, and l), show almost identical diameters and thicknesses, suggesting the growth termination of MnTiO3 nanodiscs due to exhaustion of ultrafine nanoparticles. Taken together, these results clearly demonstrate that the formation of MnTiO3 nanodiscs is accompanied by consumption of amorphous nanoparticles, and growth along the disc plane (i.e., (001) facet) far exceeds growth along the direction perpendicular to this plane. Phase structures of MTO-6-5, MTO-12-5, MTO-24-5, and MTO-48-5 were analyzed by XRD and the resulting XRD patterns are shown in Figure S3. All the diffraction lines of these samples are in good agreement with the JCPDS card No. 29-0902 of MnTiO3, indicating that the samples obtained within these investigated time intervals are single phase MnTiO3. Moreover, the degree of crystallinity is gradually enhanced as the reaction time increases from 6 to 24 h, and keep almost unchanged when the reaction time exceeds 24 h, indicating that the 24 h reaction time is sufficient for the formation of highly crystalline MnTiO3 nanodiscs. Interestingly, compared with the standard pattern, the (006) peak is almost nonexistent in all the XRD patterns of these investigated samples, but other diffraction peaks become more and more distinct and intense with the increase of reaction time. This phenomenon strongly implies that the growth of MnTiO3 sample is oriented, where the growth along the [001] direction is severely suppressed.40 As well known, the oriented growth of a nanocrystal is closely related to the anisotropic atomic arrangements in the crystal structure.35 The LiNbO3-type



polymorph of MnTiO3 belongs to the R3c space group with the lattice parameters of a = b = 0.5205 nm, c = 1.3700 nm, α = β = 90o, and γ = 120o.41 Figure 4 shows the atomic arrangements on the commonly exposed facets (i.e., (001), (010), and (100)) of MnTiO3. According to the process described in Supporting Information, the densities of Mn2+, Ti4+, and O2−− on different facets could be figured out, and the results are summarized in Table 1. Obviously, compared to (010) and (100) facets, the (001) facet is found to have the largest atomic packing densities. The packing densities of Mn2+, Ti4+, and O2−− for (001) facet are 4.2621, 4,2621, and 12.7864 nm−1, respectively, which are nearly 1.52-fold higher than those of (010) and (100) facets (Table 1). In general, the most close-packed facets are considered as the most stable facets for various types of crystals, and they tend to be the most exposed facets of the crystals in most cases according to the principle of lowest surface energy.35,42,43 Therefore, it can be inferred that the oriented growth of MnTiO3 nanodiscs is of thermodynamically preferred. On the basis of the above results and explanation, the possible formation mechanism of MnTiO3 nanodiscs can be proposed. As shown in Scheme 2, the process of MnTiO3 nanodisc construction is likely to occur in two main stages. In the early stage, nucleation occurs from the supersaturated solutions of Ti(OCH2CH2O)2 and [Mn(NH2CH2CH2NH2)2]2+ to give ultrafine MnTiO3 nanoparticles (eq. 1). These ultrafine nanoparticles are thermodynamically unstable because of their small size (< 20 nm) and amorphous nature. The prolonged hydrothermal treatment can induce the formation of MnTiO3 nanodiscs at the cost of the ultrafine nanoparticles, due to the


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energy difference in solubility between the large particles and the small particles, according

to

the

well-known

Gibbs−Thomson

law.44

The

thermodynamically-controlled process results in the oriented growth of MnTiO3 nanodiscs, in which the densest and most stable (001) facet of MnTiO3 is preserved under hydrothermal condition. Besides, the large up and down surfaces are terminated by metal atomic layers (Figure 4). During the hydrothermal process, the OH− ions might act as ligands and preferentially bind to the (001) facets through covalent bonds, which will also slow down crystal growth in the direction.45 3.3 Catalytic Performance of MnTiO3 Nanodiscs To illustrate the potential application of the as-synthesize MnTiO3 nanodiscs in environmental remediation, the oxidative degradation of several common organic pollutants (MB, BB, MG, ARS, and BR) was investigated in the dark by employing the typical MnTiO3 sample (i.e., MTO-24-5) as catalyst and H2O2 as oxidant. Figure 5a shows the absorption spectra of MB solution at 30 oC measured at different time intervals after addition of MnTiO3 nanodiscs and H2O2. It is found that the characteristic absorption peak (664 nm) intensity of MB rapidly decreases with increasing reaction time. The removal rate of MB can reach 82.4% within 10 min, and nearly 100% within 20 min. In order to elucidate the contribution of MnTiO3 nanodiscs to the degradation of MB, control experiments were also carried out, and the results are shown in the Figure 5b. Evidently, in the absence of both MnTiO3 nanodiscs and H2O2, the concentration of MB can remain almost unchanged over the whole investigated period, confirming the high stability of MB molecules. Also, it is



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observed that use of H2O2 alone only leads to about 11% removal of MB within 20 min. Moreover, when MnTiO3 is used in the absence of H2O2, less than 5% of MB can be removed after 20 min, indicating a weak adsorption of MB molecules onto the surface of MnTiO3 nanodiscs. These results clearly indicate that the oxidative degradation of MB is mediated by H2O2, which is significantly promoted by MnTiO3 catalyst. In combination with our previous study,31 we can predict that hydroxyl and superoxide radicals can be produced from the following reactions: Ti–O–Mn + H2O2 → Ti–O–Mn–OH + ⋅OH

(2)

Ti–O–Mn–OH + H2O2 → HOO–Ti–O–Mn–OH2

(3)

HOO–Ti–O–Mn–OH2 →Ti–O–Mn + ⋅OOH + H2O

(4)

These produced hydroxyl and superoxide radicals can then oxidize organic compounds by interacting with them. A comparative study of our developed catalyst (MnTiO3 nanodiscs) to other previously reported catalysts for MB degradation was performed, and the results are summarized in Table 2. Considering the catalytic performances are varied by reaction time, catalyst usage, initial concentration of MB, and volume of MB solution, we estimated the removal efficiency toward MB according to the following equation:Error! Reference source not found.

R=

(c0 − ct )V mt

(5)

where R value (mg g–1 min–1) represents the consumption amount (mg) of MB caused by 1 g of catalyst within in 1 min; c0 (mg L–1) and ct (mg L–1) are the concentrations of MB at the beginning and the ending of degradation, respectively; V (L) is the


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volume of MB solution; m and t are the catalyst usage and the reaction time, respectively. As shown in Table 2, the MnTiO3 nanodiscs-based degradation exhibits a high removal efficiency (R = 9.86 mg g–1 min–1), which is about 1.1 ~ 214 times those of processes catalyzed by other different catalysts (R = 0.046 ~ 9.02 mg g–1 min–1). In particular, the R value of MnTiO3 nanodiscs is 2 times higher than that of Mn(II)-doped TiO2 (R = 4.9 mg g–1 min–1), although the latter catalyst had a larger surface area (153.3 m2 g–1) and was used at a relatively higher reaction temperature (35 oC).31 Such a superior performance of MnTiO3 nanodiscs might be partially attributed to the high content of Mn(II) sites. Moreover, the highly open two-dimensional disc-like structure can allow easy access of guest molecules from bulk solution to the surface active sites (e.g., For MB adsorption, the time required to achieve the adsorption equilibrium was only 2.5 min as shown in Figure 5). Temperature has a remarkable influence on the catalytic performance. Figure 6a indicates that the degradation rate of MB increases significantly with the environmental temperature. At 35 oC, the degradation rate of MB reaches 98.2% within 12 min. When the temperature is up to 40 oC, only 8 min are needed to achieve the MB degradation rate of 98.6%. According to the linear relationships between – ln(ct/c0) and t at different temperatures, we calculated the rate constants k values from the slopes of the straight lines, and the results are shown in inset of Figure 6b. Under the similar conditions, the rate constant of MnTiO3 nanodiscs (e.g., 0.16849 min–1 at 30 oC) is also much higher than those of many previously reported catalysts such as MPCMS-500 (0.1058 min–1),Error! Reference source not found. W-Fe/meso-C (0.0159 min–



1 Error! Reference source not found.

),

CuFe2O4/Cu@C (0.10942 min–1),Error! Reference source not found.

and Fe3O4/SiO2/C (0.12 min–1),Error! Reference source not found. which further indicates the high catalytic efficiency of MnTiO3 nanodiscs. Furthermore, the activation energy (Ea) was also calculated from the plots of lnk against 1/T according to the Arrhenius equation.28 The Ea values is estimated to be 62.3 kJ mol–1 (Figure 6c). Generally, the Ea values of ordinary chemical reactions are usually between 60 and 250 kJ mol−1.28 The result presented here implies that the degradation reaction of MB catalyzed by MnTiO3 nanodiscs requires a relatively low activation energy and can be easily achieved. In order to further explore the versatility of the MnTiO3 nanodiscs-type catalyst, its catalytic performance toward oxidative degradation of other typical toxic organic pollutants (i.e., BB, MG, ARS and BR) in the presence of H2O2 was also investigated. All the reactions for these organic pollutants were carried out under the same conditions as that for MB. As shown in Figure 5 and 7, the catalytic degradation rates of the four organic pollutants and MB differed from each other, which may be attributed to the difference in molecular structures and different degradation mechanism. Nevertheless, all the investigated organic pollutants can be degraded completely within relatively short periods of time (i.e., BB (30 min), MG (60 min), ARS (30 min), and BR (80 min)), indicating that the MnTiO3 nanodiscs have a broad potential for catalytic degradation of various organic pollutants. Another advantage of MnTiO3 nanodiscs-type catalyst developed in this work is its high stability. Here, we investigated the recycling stability of MnTiO3 nanodiscs by


Page 20 of 43

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reusing the catalyst under identical reaction conditions. As shown in Figure 8, the catalytic efficiency of MnTiO3 nanodiscs for the degradation of MB only shows a slight decrease after six cycles. Compared to the recently reported catalysts such as titanomagnetite (42.8% activity loss after 5 cycles),Error! Reference source not found. magnetic porous carbon spheres (18% activity loss after 6 cycles),Error! Reference source not found. Fe3O4–MWCNT magnetic nanocomposites (19% activity loss after 5 cycles),Error! Reference source not found.

Ti-HMS (15.4% activity loss after 3 cycles)Error! Reference source not

found.

, Fe/meso-C (51.37% activity loss after 3 cycles)Error! Reference source not found. and

AC/γ-Fe2O3 (15% activity loss after 7 cycles),Error! Reference source not found. our catalyst shows a significantly better durability. Actually, the relatively poor reusability is very common among various transition metals-catalyzed H2O2-mediated oxidation reactions because of the low cycling ability of transition metals between their oxidized and reduced forms in the presence of H2O2.30 Our recent work revealed (both experimentally and theoretically) that Ti in the Ti–O–Mn structure could moderately donate electron to the Mn to compensate for the electron loss of Mn, which could facilitate the redox cycling of Mn(III)/Mn(II) in the H2O2-mediated oxidation process.31 Therefore, it is not surprising that the MnTiO3 nanodiscs, consisting primarily of Ti–O–Mn structural units, exhibits a high level of persistence in catalytic activity. Moreover, the MnTiO3 nanodiscs are highly crystalline, which should be also advantageous for maintaining their structural and chemical stability.

4. Conclusions In summary, we developed a simple one-pot hydrothermal method to synthesize



Page 22 of 43

high-quality MnTiO3 nanodiscs and demonstrated for the first time their potential as a new catalyst for H2O2-mediated oxidative degradation. Under optimized conditions, the resulting MnTiO3 nanodiscs were found to be highly crystalline and exhibited a high dimensional homogeneity with an average diameter of ∼333 nm and a thickness of ∼16.2 nm. Our observations confirmed that the formation of MnTiO3 nanodiscs followed

a

nucleation–dissolution–recrystallization

mechanism

that

was

thermodynamically driven. The oriented morphology was ascribed to the anisotropic atomic arrangement in the crystal structure, in which the (001) was the densest and most stable facet. Benefitting from the two-dimensional open surface, active metal sites-rich structure, and distinct crystallinity, the MnTiO3 nanodiscs exhibited a high efficiency in catalytic decomposition of a series of organic pollutants in the presence of H2O2. About 98.6% of MB was catalytically decomposed within 20 min at 30 oC. When the reaction temperature was up to 40 °C, only 8 min was needed. Moreover, the MnTiO3 nanodiscs could remain high activity after being recycled for several times. Through replacing manganese chloride with other transition metal salts, we believe that the interesting one-pot hydrothermal approach might be extendable to the fabrication of different metal titanate nanomaterials with unique structures as well as advanced physical and/or chemical properties.

Acknowledgements The authors acknowledge the research grant provided by National Natural Science Foundation of China (No. 21303170), Natural Science Foundation of Hubei Province (No. 2015CFB187), Fundamental Research Funds for the Central Universities, China


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University of Geosciences (Wuhan) (No. CUG170101).

Supporting Information Available: Calculation process of the atomic packing densities, SEM images, particle diameter distributions, and XRD patterns.

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Figure Captions Scheme 1. Schematic illustration of synthesis process of MnTiO3 nanodiscs. Figure 1. Characterization on MTO-24-5: SEM images (a); EDS mapping and the corresponding data (b, c, d, and e); TEM image of a typical nanodisc (f); HRTEM image and SAED pattern of the frontal plane of a single nanodisc (g); Larger magnification HRTEM image (h); an indexed pattern corresponding to the zone axis of [0001] (i); and XRD pattern (j).

Figure 2. SEM images and diameter distributions of samples obtained with different water contents: MTO-24-2.5 (a, b, and c); MTO-24-5 (d, e, and f); and MTO-24-7.5 (g, h, and i).

Figure 3. SEM images and diameter distributions of samples obtained at different reaction time: MTO-6-5 (a, b, and c); MTO-12-5 (d, e, and f); MTO-24-5 (g, h, and i); and MTO-48-5 (j, k, and l).

Figure 4. Atomic arrangements on the typical crystal facets of MnTiO3. Scheme 2. Schematic illustration of formation mechanism of MnTiO3 nanodiscs. Figure 5. Normalized UV−Vis spectra of MB vs. reaction time (a); and degradation kinetic curve of MB (100 mg L−1) by different systems (b).

Figure 6. Degradation efficiencies of MB under different reaction temperatures (a); the corresponding fitting curves of kinetic data (b); and the plot of lnk vs. 1/T (c).

Figure 7. H2O2-mediated oxidative degradation of four typical organic pollutants over MnTiO3 nanodiscs: BB (a), MG (b), ARS (c) and BR (d), where the insets show their absorption spectra under different times.



Figure 8. Degradation efficiency of MB during different runs by the “MnTiO3 nanodiscs + H2O2” system.


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Table List Table 1. The atomic packing densities for the (001), (100), and (010) facets.a Ion

(001)/nm−2

(100)/nm−2

(010)/nm−2

Mn2+

4.2626

2.8047

2.8047

Ti4+

4.2626

2.8047

2.8047

O2−

12.7877

8.4141

8.4141

a

The detailed calculation process can be found in the Supporting Information.

Table 2. The catalytic performances of different catalysts in the H2O2-mediated oxidative degradation of MB. Catalyst

mcat. (mg)

VMB

CMB –1

T o

t

R –1

(mL)

(mg L )

( C)

(min)

(mg g min–1)

Ref.

Au−Fe2O3@mesoporous SiO2

10

20

50

25

150

0.667

47

Fc

55.8

100

10

30

60

0.298

48

MPCMSs

20

10

40

30

20

0.98

49

Mesoporous MnO2

25

60

100

30

120

1.476

50

Supported cobalt catalyst

10

25

50

25

80

1.453

51

Fe/M-MnO2

50

100

100

25

20

9.02

52

FeCo2O4−S

100

200

20

30

60

0.6

53

SSWM-supported α-Fe2O3

25

50

50

30

300

0.333

54

W-Fe/meso-C

35

50

30

25

200

0.214

55

Na-OL-1

10

50

30

30

20

7.5

56

100

250

50

25

100

1.25

25

Manganese functionalized silicate nanoparticle CuFe2O4/Cu@C

5

10

20

30

15

2.67

57

γ-Fe2O3/C

10

10

100

30

65

1.461

58

Fe3O4/SiO2/C

20

20

50

35

30

1.583

59

Cu/CuO−Ag

10

10

50

25

36

1.333

60

Titanomagnetite

1200

400

100

35

200

0.167

61

Rattle-type Fe3O4@C composite

10

10

60

30

300

0.2

62

MCSs

20

20

40

30

10

4.0

63

Fe3O4–MWCNT

7.5

25

10

25

720

0.046

64

AC/γ-Fe2O3

20

10

100

25

15

3.333

65

Ti-HMS

100

100

16

25

120

0.128

66

Mn-fTiO2

5

10

100

35

40

4.9

31

MnTiO3 nanodiscs

5

10

100

30

20

9.86


This work


Scheme 1.


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Figure 1.



Figure 2.


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Figure 3.



Figure 4.

Scheme 2.


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Figure 5.

Figure 6.



Figure 7.

Figure 8.


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Table of Contents (TOC)


Simple and Controllable Synthesis of High-Quality MnTiO3 Nanodiscs

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