Morphology control studies of MnTiO3 nanostructures with exposed

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Functional Nanostructured Materials (including low-D carbon)

Morphology control studies of MnTiO3 nanostructures with exposed {0001} facets as high-performance catalyst for water purification Da Wang, Haodan Xu, Jun Ma, Xiaohui Lu, Jingyao Qi, and Shuang Song ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11132 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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ACS Applied Materials & Interfaces

Morphology control studies of MnTiO3 nanostructures with exposed {0001} facets as high-performance catalyst for water purification

Da WANG†, Haodan XU†, Jun MA*,†, Xiaohui LU†, Jingyao QI*,†, Shuang SONG‡

†

State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin, 150090, China.

‡

College of Environment, Zhejiang University of Technology, Hangzhou, 310032, China.

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Novel single crystal hexagonal MnTiO3 nanosheets with exposed {0001} facets have been synthesized via a simple one-pot hydrothermal method using NaOH as mineralizer and tetraethylammonium hydroxide (TEAH) as morphology controller. The intermediate morphologies of MnTiO3 nanostructures such as nanoparticles, nanowires, nanorods, and nanodiscs are trapped kinetically by adjusting the synthesis conditions. This approach enables us to elucidate the growth mechanisms of MnTiO3 nanosheets based on the tetraethylammonium (TEA+) cations adsorption abilities on different MnTiO3 crystal facets combined with density functional theory calculations. Dissolution and recrystallization processes are involved during the MnTiO3 crystallization. The surface controlled MnTiO3 has been found to be effective as a catalyst for ozonation in the degradation of 4-chlorophenol (4-CP). Within typical experimental conditions (catalyst dosage =0.3 g L−1, [4-CP]0 = 50 mg L−1, [O3] = 20 mg L−1, gas flow = 0.1 L min−1, pH 6.8 and T = 293 K), the TOC removal efficiency of 4-CP in catalytic ozonation with well-structured MnTiO3 (MnTiO3-180-10 sample) was 76.3% after 60 min, compared with only 22.1% and 38.5% TOC removal in the absence of catalyst and with uncontrolled MnTiO3 (MnTiO3-no TEAH sample), respectively. Benefiting from the high exposure percentage of {0001} facet, mixed-valences of manganese, surface hydroxyl groups and the enrichment Lewis acid sites provided by Mn and Ti, the morphology controlled MnTiO3 nanosheets can be applied as heterogeneous catalytic ozonation catalysts which exhibit excellent pollutants degradation. We anticipate that MnTiO3 can be a promising candidate 2


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material for application in remediation of organic pollutants in water.

KEYWORDS: MnTiO3; morphology control; hexagonal nanosheets; catalytic ozonation.

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INTRODUCTION With the rapid development of industrialization and urbanization as well as environmental deterioration, the imperative demand for, but critical lack of clean water sources have drawn immense attention all over the world. A series of industrial products including pesticides, pharmaceuticals and personal care products (PPCPs), and endocrine disrupting compounds (EDCs) are released into aquatic environment, which lead to direct or potential effects on environmental safety and public health.1 Due to the rapid development of engineered material science, various powerful and attractive advanced oxidation processes (AOPs), especially heterogeneous water treatment techniques are applicable to the elimination of a broad range of contaminants.2 Heterogeneous catalytic ozonation, a novel strong oxidation process based on ozonation, has received wide interest in both micro pollutants removal and wastewater treatment field, due to its proven performance in the elimination of refractory organic compounds and low negative effect on water quality.3 So far, various metal oxides such as TiO2, Al2O3 and FeOOH significantly accelerate ozone decomposition and ·OH generation during the catalytic ozonation process, therefore further enhancing the pollutants degradation efficiency.4 Also, seldom researches focus on the crystallization processes during the engineered

materials

synthesis.

Incomplete

synthesis

would

receive

environmental functional materials with low crystallinity, which would cause high ions leaching and catalysts loss issues. Over synthesis not only wastes 4


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energy and increase costs but also reduces the surface area and active sites of the catalyst. Thus, clarify the crystallization processes and synthesis conditions are highly attractive. Considering the wide application, economical efficiency and environmental friendly characteristic of the catalysts used in these common water treatment processes, the engineered materials should be easily prepared using cheap and non-toxic precursors. Manganese (Mn) and Titanium (Ti), two of the earth-abundant elements, are eco-friendly and easily obtained. So far, manganese oxides and titanium dioxides such as MnO2, Mn3O4 and TiO2 are not only widely used as catalysts in AOPs including photocatalysts, electrodes, Fenton-like

catalysts

and

ozonation

catalysts,5,6

but

also

used

as

high-performance asymmetric supercapacitors.7,8 With generating one or several reactive oxygen species (ROS) such as hydroxyl radicals (·OH), superoxide radicals (·O2¯) and singlet oxygen (1O2), AOPs have presented their superior degradation capability and mineralization efficiency for environmental remediation.9 Recently, ilmenite-type (ABO3) catalysts, including FeTiO3, FeSiO3 and NiMnO3,10,11 have attracted attention for photo-degradation or CO oxidation. However, the application of MnTiO3 in AOPs remains a relatively uncharted area in which only a few studies about photocatalysis and Fenton have been published.12,13 So far, most of the studies synthesized MnTiO3 through sintering at high temperatures (700 oC−900 oC),14,15 others acquired MnTiO3 by sol–gel– hydrothermal method or hydrothermal method.13,16 Very recently, Wang et al. 5



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reported the well synthesis of MnTiO3 nanodiscs and investigated the growth mechanism

of

MnTiO3

nucleation−dissolution−recrystallization

nanodiscs

based

processes.13

According

on to

his

characterizations such as XRD, HRTEM and SAED, we deduced that the well-structured intrinsic morphology of MnTiO3 should be hexagonal. However, there have been no previous reports of the synthesis of well-structured hexagonal MnTiO3 nanosheets. It is necessary to find a simple and convenient way to acquire the intrinsic hexagonal MnTiO3 nanosheets. In this paper, well-structured hexagonal MnTiO3 single-crystal nanosheets with exposed {0001} facets were synthesized through one pot hydrothermal method. A strongly hydrated compound tetraethylammonium hydroxide (TEAH) was used to control the morphology formation and crystallization process of MnTiO3 nanostructures under different reaction temperature and time. We put forward a possible synthetic mechanism based on the experimental investigation, characterizations and density functional theory (DFT) calculations. It is found that the addition of TEAH, reaction temperature and reaction time can significantly affect the morphology and crystal form of the prepared MnTiO3 nanosheets. The synthesized MnTiO3 nanosheets behave as superior catalysts in catalytic ozonation processes.

6


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EXPERIMENTAL SECTION Chemicals. All chemicals and solvents were of analytical grade and used as received without further purification. Titanium (IV) oxysulfate (TiOSO4), sodium hydroxide (NaOH), 4-chlorophenol (4-CP), 2,2,6,6-tetramethyl-4-piperidinol (TEMP), NaN3 and potassium indigotrisulfonate were purchased from Sigma-Aldrich, USA. Manganese sulfate monohydrate (MnSO4·H2O), tetraethylammonium hydroxide (TEAH, 25% aqueous solution), RhB, 5,5-dimethyl-1-pyrolin-N-oxide (DMPO), p-benzoquinone (p-BQ), tert-butanol (TBA) and hydrogen peroxide (H2O2) were purchased from Aladdin, China. Deionized (DI) water (18.2 MΩ cm) produced from a Milli-Q water purification system (Millipore, USA) was used in all the experiments. MnO (99.99% metals basis) and Mn3O4 (99.95% metals basis) were purchased from Aladdin, China without further treatment. Preparation of MnTiO3 single-crystal nanosheets. MnTiO3 single-crystal nanosheets were prepared by a one-pot hydrothermal method. Typically, 10 mL TEAH aqueous solution (25%) and 100 mL deionized water were mixed by magnetic stirring. Then certain amount of TiOSO4 and MnSO4·H2O were added to achieve 10 mM Ti4+ and Mn2+ aqueous solution. When the solution became clear by continuous stirring, 40 mL as-prepared NaOH solution were added dropwise to reach the final NaOH concentration as 1M. After stirring for 15 min, the solution became brown and the obtained suspension was transferred into 200 mL Teflon-lined stainless steel autoclaves for hydrothermal treatment at 180 oC for 10 h. After natural cooling, the resultant brown precipitate was washed several times with ethanol and DI water then 7



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dried at 80 oC in air. The prepared sample was named as MnTiO3-180-10. The effect of hydrothermal temperature, hydrothermal reaction time and alkali concentration on the properties of MnTiO3 were further investigated. When one reaction condition was changed, the other reaction conditions remained the same as the sample MnTiO3-180-10. Hydrothermal temperature was changed from 180 oC to 100 oC, 140 o

C and 220 oC. Hydrothermal reaction time was changed from 10 h to 1 h, 5 h and 15

h to investigate the morphological formation process. The above samples were named as MnTiO3-T-t, where T represents hydrothermal temperature and t denotes reaction time. The sample without adding alkali NaOH was called MnTiO3-0M. MnTiO3 synthesized at 180 oC for 10 h in the absence of TEAH were named as MnTiO3-no TEAH. Characterization. The X-ray diffraction (XRD) analysis of the synthesized MnTiO3 single-crystal nanosheets was carried out on diffractometer (max-TTR-III, Rigaku D, Japan) using Cu-Kα radiation source at 45 kV and 40 mA over the 2θ range 20-70o. X-ray photoelectron spectra (XPS) were conducted on photoelectron spectrometer (ESCALAB 250Xi, Thermo Scientific, USA) equipped with a monochromated Al-Kα. Sample sizes, morphology information and crystal structures were observed at field emission scanning electron microscopy (FE-SEM) (Zeiss Sigma 500, Carl Zeiss, Germany)

equipped

with

energy

dispersive

X-ray

spectroscopy

(EDS),

high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) (JEM-2100, JEOL, Japan) with a 200 kV accelerating voltage. The atomic ratio of Mn : Ti was attained by inductively coupled 8


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plasma-optical emission spectrometry (ICP-OES) (7300, Perkin Elmer, USA). Certain amount of MnTiO3 sample was first dissolved by HNO3 and HCl and then diluted to proper concentration before ICP-OES analysis. Brunauer-Emmett-Teller (BET) surface areas were analyzed by nitrogen adsorption and desorption isotherms at 77 K on a surface area and porosity analyzer (ASAP 2020, Micromeritics, USA). Infrared spectra of pyridine (pyridine-IR) were obtained on MnTiO3 after pyridine adsorption using spectrometer (Spectrum 100, PerkinElmer, USA). Performance of advanced water treatment. The water purification abilities of MnTiO3 single-crystal nanosheets in catalytic ozonation processes were evaluated by means of the degradation efficiency of 4-CP. In a typical heterogeneous catalytic ozonation process, semi-batch experiments were carried out with a 1.5 L reactor containing 50 mg L−1 of 4-CP and 0.3 g L−1 of catalyst. Ozone was generated by a laboratory ozone generator (TOGC2B, Degrémont, Germany) from high purity oxygen (99.9%). The inlet flow rate of ozone was 100 mL min−1 and the concentration of ozone was set to be 20 mg L−1. The initial pH of reaction solution was about 6.8. At given time intervals, water samples were collected for further analysis and immediately filtered through a membrane of 0.22 µm pore size. Residual dissolved ozone in the samples was quenched with 10 µL of 1M Na2S2O3 solution. Residual aqueous ozone in the reaction solution was measured by Indigo method. The concentrations of 4-CP was quantified by a high performance liquid chromatography (HPLC, 2695, Waters, USA) with a 2487 UV absorbance detector and symmetry C18 column. Total organic carbon (TOC) was measured on a Multi TOC Analyzer (N/C 9



3100, Analytik Jena AG, Germany). The concentrations of leaching Mn and Ti during the reactions were attained by ICP-OES (7300, Perkin Elmer, USA). The free radicals generated during catalytic ozonation were measured by EPR spectrometer (EMX-8/2.7, Bruker, Germany) using DMPO or TEMP as spin-trapping agents under the following experimental conditions: X-field sweep; center field 3475.00 G; sweep width 200.00 G; frequency 9.76 GHz; power 36.32 mW. The reaction media for the detection of ·OH and 1O2 was DI water while for ·O2− was ethanol because ethanol could prolong the half-life of ·O2−. For chemical scavenger experimentsin the MnTiO3 catalyzed ozonation system, 15 mM TBA, p-BQ and NaN3 were added to clarify the possible existence of ·OH, ·O2− and 1O2 that have been identified in similar systems.17

RESULTS AND DISCUSSION Catalyst characterization Figure 1a shows the XRD patterns of MnTiO3-180-10 and Figure S1 shows the MnTiO3 samples prepared at other different conditions. All the diffraction peaks of MnTiO3-180-10 can be indexed to pyrophanite MnTiO3 (JCPDS NO. 89-3742) with space group R-3 (148). The results indicated the high purity of the MnTiO3 prepared by one-pot hydrothermal method. Figure S2 obtains that the N2 adsorption-desorption isotherms of MnTiO3-180-10 exhibited type II, which corresponded to macroporous materials (pore diameter range from 20 to 100 nm shown in inset of Figure S2). The BET specific surface area (SBET) of prepared samples was listed in Table 1. The SBET of MnTiO3-180-10 was 22.95 m2 g−1, while sample MnTiO3-no TEAH had relatively 10


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lower SBET (9.72 m2 g−1). Figure 1b shows typical top-view FE-SEM images of the as-prepared MnTiO3-180-10 sample and the low magnification FE-SEM image of MnTiO3-180-10 sample was shown in Figure S3a. The well-structured hexagonal MnTiO3 nanosheets were uniform and smooth. The average diameter of the hexagonal MnTiO3 nanosheets was 1 µm while its thickness was about 120 nm (shown in Table 1). The elemental mapping images shown in Figure S4 exhibit uniform distribution of Mn, Ti and O elements on the surface of MnTiO3. The analysis of EDS spectrum (Figure S5) further confirmed the presence of Mn, Ti and O elements. ICP-OES was used to determine the average atomic ratio of Mn : Ti in MnTiO3-180-10 sample and the result was 1 : 0.97, which corresponded well with the elemental ratio of Mn and Ti in MnTiO3.

Table 1. Comparison of physical properties of various MnTiO3 samples. Sample

SBET

average

thickness

(m2 g−1)

diameter (nm)

(nm)

MnTiO3-180-10

22.95

1000

120

MnTiO3-no TEAH

9.72

100

30

MnTiO3-140-10

27.62

800

100

MnTiO3-220-10

16.35

1800

300

MnTiO3-180-1

26.65

400

80

MnTiO3-180-5

23.48

500

100

MnTiO3-180-15

14.52

1300

300

11



Figure 1. (a) XRD pattern, (b) FE-SEM, (c) TEM, (d) HRTEM, (e) SAED and (f) XPS spectra of Mn 3s of MnTiO3-180-10 sample.

The TEM image shown in Figure 1c also gives a feature of a perfect hexagonal MnTiO3 nanosheet. Magnified image (Figure 1d) of the original HRTEM image (Figure 1c) shows the d spacing between two adjacent lattice ത 2) reflection of MnTiO3.18 Figure planes is 0.378 nm, corresponding to the (011 1e gives an SAED pattern of an area selected on a single MnTiO3-180-10 nanosheet along a [0001] zone axis. The single set of periodic array spots clearly indicates a single crystal, which can also be indexed to rhombohedral ത 0} planes while pyrophanite MnTiO3. The inner six spots are assigned to {112 ത 30} and {224 ത 0} the outer six spots can be labelled as the reflections of {03 planes, respectively (full description of planes are shown in Figure S6). Therefore, it can be deduced that the hexagonal MnTiO3 sheet has {0001} 12


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planes as the top and bottom, meanwhile the side planes are constructed by

ത 30}. On the basis of the FE-SEM, TEM and HRTEM results, the percentage {03 of highly exposed {0001} facets for MnTiO3-180-10 nanosheets was estimated to be 88.3% (the percentage of {0001} facets of other MnTiO3 were shown in Table S1). Furthermore, the surface morphology of MnTiO3 was found to significantly vary with the change of reaction temperature and time, which will be discussed in the growth mechanism part. Figure S7a shows the XPS survey spectrum of MnTiO3-180-10 nanosheets. Sharp photoelectron peaks appeared at binding energies of 284.6 eV (C 1s), 458 eV (Ti 2p), 520 eV (O 1s) and 640 eV (Mn 2p). Binding energy scale was calibrated by using the containment carbon (C 1s = 284.6 eV). The Mn 2p region in MnTiO3-180-10 is characterized by a doublet that arose from spin-orbit coupling (2p3/2 and 2p1/2), as shown in the high-resolution XPS spectra (Figure 1f). The peak positions for Mn 2p3/2 and Mn 2p1/2 were observed at binding energies of 640.2 eV and 652.5 eV, respectively, along with two satellite peaks at 646.0 eV and 657.8 eV. The main peak of Mn 2p3/2 is split into two with splitting width about 1.2 eV, due to the existence of the large magnetic moment.19,20 However, it is generally accepted that multiple valence oxidation states (Mn2+, Mn3+ and Mn4+) may exist in manganese oxides so that the Mn 2p usually reveals overlap of energy ranges for the various oxidation states of manganese.21 It is difficult to identify the oxidation states of manganese only by the binding energy shift and peak fitting of Mn 2p.22 13



Galakhov et al.23 proved that the XPS of Mn 3s peaks are more reliable for identifying the valence states of manganese in the manganese contained oxides (Figure S7b and Text S1). By fitting these curves with Gaussian symmetry (data were summarized in Table S2), the molar ratio of Mn2+ to Mn3+ was ca. 1:0.47, no Mn4+ could be identified. The high-resolution spectrum of Ti 2p shown in Figure S7c contains three peaks. The two peaks centered at 457.1 eV and 462.8 eV can be assigned to Ti 2p3/2 and Ti 2p1/2 in line with titanates. These two peaks that appeared asymmetrical in profile fitted well with Ti-O bonds of the TiO6 octahedra, indicating that Ti in MnTiO3 was almost singly presented as +4 valence instead of multi valence states, or may be other Ti states such as Ti3+ had already been oxidized by O2 in air rapidly.20,24 The extra peak at 470.5 eV is the charge transfer satellite peak of Ti 2p.25 Figure S7d shows the high-resolution XPS spectrum of O 1s. By fitting O 1s curve with Gaussian symmetry, the spectrum was decomposed of three peaks. The dominant peak at 529.8 eV agreed with the contribution from the lattice oxygen of MnTiO3. The medium binding energy peak at 531.5 eV was assigned to surface oxygen, such as surface adsorbed oxygen, OH groups or oxygen vacancies. A small peak at 532.8 eV was associated with the H-O-H bonds of chemisorbed H2O. Growth mechanism of hexagonal MnTiO3 nanosheets The morphology formation and crystallization process of MnTiO3 nanosheets by the inorganic manganese and titanium precursors combined with NaOH and TEAH 14


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takes place in various indispensable steps. In order to clarify the formation mechanism of MnTiO3, TEM and SAED patterns of various samples prepared in different temperature and time were shown in Figure 2 (the relevant FE-SEM images of all samples were shown in Figure S3). It should be pointed out that both the inorganic alkali NaOH and organic alkali TEAH play prime role in the MnTiO3 growth process to control the crystal form, shape and size of MnTiO3. In the absence of NaOH, only anatase TiO2 was formed (sample named as MnTiO3-0M and its XRD pattern was shown in Figure S1) while in the absence of TEAH, no hexagonal morphology could be observed (sample named as MnTiO3-no-TEAH and TEM image was shown in Figure S8). So NaOH and TEAH acted as mineralizer and morphology controller during the synthesis of hexagonal MnTiO3 nanosheets, respectively.

Figure 2. TEM and SAED images of (a) MnTiO3-100-10, (b) MnTiO3-140-10, (c) MnTiO3-220-10, (d) MnTiO3-180-1, (e) MnTiO3-180-5 and (f) MnTiO3-180-15 samples.

15



After fixing the concentration of NaOH and TEAH, we interestingly found that the morphologies of MnTiO3 were quite different depending on synthesis temperature and time. Firstly, just considering on the temperature factor, sample MnTiO3-100-10, MnTiO3-140-10, MnTiO3-180-10 and MnTiO3-220-10 were synthesized and put together for comparison. The XRD pattern of MnTiO3-100-10 (Figure S1) revealed no particular peaks at all, indicating that the synthesized powder was amorphous MnTiO3 or still existing as manganese titanium hydroxide. The SAED pattern shown in the inset of Figure 2a manifests characteristic ring pattern, further proving that MnTiO3-100-10 is completely amorphous. This amorphous substance with the size of ca. 20 nm (shown in Figure 2a) was believed to be the very initial state of MnTiO3. Thus, nucleation and growth processes of MnTiO3 could not happen at merely 100 oC. The phase transformation from amorphous material to rhombohedral pyrophanite MnTiO3 (Figure 2b) was achieved at the reaction temperature of 140 oC (sample MnTiO3-140-10). The synthesized MnTiO3-140-10 sample presented single crystal and plate morphology with diameter of 800 nm instead of hexagonal morphology, indicating that the reaction time is efficient for crystallization but insufficient for hexagonal morphology formation. These results accord with the previous work published by Wang et al.,13 indicating that amorphous MnTiO3 nanoparticles and single crystal MnTiO3 nanoplates were two intermediate forms existed during the synthesis of hexagonal MnTiO3 nanosheets. MnTiO3-180-10 sample with the diameter of 1 µm obtained well 16


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hexagonal morphology and high crystallization which was observed in Figure 1c. It is well-known that the hexagonal nanocrystal has both polar and nonpolar facets. The typical crystal habit exhibits top and basal polar {0001} facets and

ത 0} facets. Polar faces with surface dipoles are six lateral nonpolar {01 1 thermodynamically less stable than nonpolar faces. Generally, facet with high surface energy often grew faster than other facets and might disappear quickly due to rearranging itself to minimize surface energy. However, when tetraethylammonium (TEA+) cations and hydroxyl anions formed through hydrolysis, the hydrophobic TEA+ would preferentially be adsorbed on facet which revealed higher surface energy due to the strong electrostatic interactions.26 The surface cleavage of MnTiO3 crystal (Figure 3) also provides further information for explaining the adsorption of TEA+ cations. The {0001} and {011ത0} facets are terminated with Mn, Ti and O atoms, respectively. The zigzag orientations of Mn and Ti octahedral anions on {011ത 0} facets make these facets unfavorable for TEA+ cations adsorption. Besides, the amount of O atoms on the {0001} facets (Figure 3a) is much higher than that on the {011ത0} facets (Figure 3b), suggesting that TEA+ cations would prefer to be adsorbed on the {0001} facets due to the high density of O atoms on them.27,28 By calculating the different surface energies of MnTiO3 using CASTEP combined with DFT calculation (calculation processes were shown in Text S2),29 the surface energy of {0001} facets is 2.73 J m-2 while that of {011ത0} facets is 0.56 J m-2 in the absence of TEAH.30,31 Here, the TEA+ adsorbed on 17



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{0001} facets formed film-like structure which could not only minimize the surface energy of {0001} facets but also reduce the landing speed of crystal units on the {0001} facets. Therefore MnTiO3 would grow faster along [1100] directions than [0001] directions to form intact hexagonal nanosheets. Further increasing reaction temperature to 220 oC (Figure 2c, sample MnTiO3-220-10) made the single crystal MnTiO3 nanosheets grow larger with a diameter of 1.8 µm but the hexagonal morphology became indistinct. However, the red dashed hexagonal area could fit the prepared MnTiO3-220-10

nanosheets to

the

some extent,

indicating

that

MnTiO3-220-10 had the tendency to form hexagonal morphology but failed. The reason might be the fast crystallization rate of MnTiO3 and low adsorption ability of TEA+ on {0001} facets at higher temperature.

Figure 3. The surface structure cleavage of MnTiO3 crystals: (a) {0001} facets and (b) {011ത0} facets (The blue, red and grey balls are denoted as Mn, Ti and O, respectively).

TEM image of sample MnTiO3-180-1 presented in Figure 2d contains both nanowires and nanoparticles, illustrating that the control of reaction time greatly affected the formation of hexagonal nanosheets. A typical polycrystalline SAED 18


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pattern derived from many nanoparticles is presented as an inset in Figure 2d. The nanowires (red arrows in Figure 2d) have a diameter of 15 nm and a length of up to 600 nm while the nanoparticles (red cycles in Figure 2d) with diameters of 15-30 nm agglomerate together and form a ca. 400 nm big plate-like cluster. Kharkwal32 reported the synthesis of pyrophanite MnTiO3 nanowires through method of chemical coprecipitation followed by calcination which proved the reasonable existence of MnTiO3 nanowires. According to the XRD pattern (Figure S1) and HRTEM image (Figure S9), high purity MnTiO3 crystallization was presented and both of the lattice fringes of nanowires as well as nanoparticles belonged to rhombohedral pyrophanite MnTiO3. The appearance of these intermediate morphologies could also be explained by nucleation and crystal growth mechanisms in the presence of TEAH. At the initial reaction stage, the manganese titanium precursor remained the highest concentrations so that nucleation occurs with the formation of many nuclei in a short time. It should be pointed out that at the initial stage, the concentrations of manganese titanium oxide is relatively higher than TEA+, so TEA+ could not be adsorbed on all the formed MnTiO3 nuclei. Those nuclei which were not covered by TEA+ would adsorb more manganese titanium oxide on the {0001} facets, resulting in faster growth along the [0001] direction, and thus, MnTiO3 nanowires were obtained. On the contrary, when the nuclei were covered by TEA+, the growth rate of their {0001} facets was retarded. Then these nuclei would grow along both [0001] and [1100] directions with similar speed and subsequently became nanoparticles. MnTiO3-180-5 synthesized by increasing reaction time to 5h still contains two different morphologies of MnTiO3 as 19



shown in Figure 2e and Figure S3g. Interestingly, sun-like MnTiO3 nanostructures appeared and the former plate-like clusters had already grew into nanosheets with a diameter of ca. 500 nm. It is believed that the disappearance of primary nanoparticles and shortening of the nanowires involved dissolution and recrystallization processes. No nanoparticles could be observed in the image due to the {011ത0} facets of each nanoparticles adhered together to form nanosheets by continuous growth. Furthermore, the previous nanowires also converted to short nanorods (red arrows in Figure 2e) with 10 nm width and 150 nm length. This phenomenon suggests that the unstable MnTiO3 nanowires dissolved along the [1100] direction and recrystallized at the {011ത0} facets of MnTiO3 nanosheets, resulting in the enlargement of nanosheets and shortening of nanowires. Thus, with the optimal reaction time of 10 h as we mentioned in this paper, MnTiO3-180-10 manifested well hexagonal MnTiO3 nanosheets. Further increasing hydrothermal time to 15 h (sample MnTiO3-180-15) led to the destruction of hexagonal morphology of MnTiO3 with a diameter of 1.5 µm shown in the red dashed hexagonal area in Figure 2f and Figure S3h. It could be deduced that when the diameter of hexagonal MnTiO3 reached its upper limit, the insufficient TEA+ on {0001} facets and relatively low concentrations of manganese titanium precursors resulted in unequal growth on the six edges of MnTiO3 nanosheets. Considering all the morphologies of prepared MnTiO3 samples, both reaction temperature and time played the key role on the synthesis of well-structured hexagonal MnTiO3 nanosheets. Low temperature or short reaction time was insufficient for complete single crystal crystallization, while high temperature or long 20


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reaction time would easily cause morphology deterioration and sintering effect. Thus, the tentative synthesis experiments could not only help us find the optimal synthesis conditions, but also clarify the intermediate morphologies of engineered materials. Besides, choosing appropriate mineralizer and morphology controller were really important. In addition, the presence of the geometrical symmetric SAED pattern of MnTiO3-180-10 (Figure 2d) and MnTiO3-180-15 (inset of Figure 2d) further proved the existence of dissolution and recrystallization processes rather than physical cohesion of precursor composites during MnTiO3 nanosheets formation. The various intermediate morphologies such as nanoparticles and nanowires all disappeared in the later reaction stage along with the formation of single crystal MnTiO3 nanosheets. In summary, the schematic view of the growth mechanisms of the single crystal hexagonal MnTiO3 nanosheets can be simply described in Scheme 1. Compared with publication from Wang et al.,13 we discovered three more intermediate morphologies of MnTiO3 and well-structured single crystal hexagonal MnTiO3 nanosheets. A more detailed MnTiO3 growth mechanism was also put forward. In the initial stage of the hydrothermal reaction, precipitates of manganese titanium oxides occurred by dehydration and condensation of manganese titanium hydroxides under basic conditions, leading to the nucleation of MnTiO3. With elongation reaction aging, these nuclei grew into MnTiO3 nanowires and nanoparticles and the latter aggregated as clusters due to electrostatic attraction. At the same time, dissolution and recrystallization processes promoted the formation of MnTiO3 nanosheets by 21



Page 22 of 43

consuming the MnTiO3 nanowires and nanoparticles. The final single crystal hexagonal MnTiO3 nanosheets were thus obtained with the assistance of TEAH as a morphology control agent. When the hexagonal MnTiO3 nanosheets reached the size that TEA+ cannot perfectly control their growth, further consuming the reactants would result in indistinct edges.

Scheme 1. Schematic illustration of the growth process of the MnTiO3 nanocomposites.

Water purification performance The application of MnTiO3 in water treatment fields was evaluated by catalytic ozonation processes. Herein we use MnTiO3-180-10 as the catalyst for all the water purification performance investigations below and the catalytic performance of all other MnTiO3 samples were shown in Figure S10.

22


ozonation

0.6

0.4

0.4

MnTiO3 Adsorption O3 only (TOC) -1 2+ O3 + 1 mg L Mn (TOC) MnTiO3/O3 (TOC) O3 only MnTiO3/O3

0.2 0

10

20

30

40

50

0.2 60

0.0

0.30 0.25 0.20 0.15 0.10 0.05 0.00

♣♣♣♣ ♣♣

Relative Intensity (a. u.)

0.6

0.35

MnTiO3/O3 MnTiO3/O3/TBA MnTiO3/O3/p-BQ MnTiO3/O3/NaN3

0.8

TOC removal (C/C0)

0.8

0.0

b

1.0

-1

a 1.0 4-CP degradation (C/C0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


kapp (min )

Page 23 of 43

-

♣ DMPO-O2

DMPO+MnTiO3+Ethanol+O3 ♦

♦

♦

1

♦ TEMP- O2

TEMP+MnTiO3+O3 ♥

♥

DMPO+MnTiO3+O3

♥

♥

♥ DMPO-OH

Relative Magnetic Value (G)

Time (min)

Figure 4. (a) 4-CP degradation and TOC removal for MnTiO3 catalyzed ozonation processes and (b) degradation rate constants of 4-CP degradation in the presence of different scavengers. The inset of (b) displays the EPR spectra of DMPO and TEMP adducts. Catalyst: MnTiO3-180-10, Catalyst dosage: 0.3 g L−1, [4-CP]0 = 50 mg L−1, [O3] = 20 mg L−1, gas flow = 0.1 L min−1, pH 6.8 and T = 293 K.

Catalytic activities of the MnTiO3 were evaluated in catalytic ozonation of 4-CP with an initial pH of 6.8. Figure 4a includes the decrease efficiency of 4-CP as well as the TOC removal. Control experiment obtained that there are less than 5% of 4-CP adsorbed on the surface of MnTiO3-180-10 after 60 min equilibrium, proving that MnTiO3 was unable to oxidize 4-CP directly neither. For 4-CP degradation using ozonation only, 4-CP was completely degraded in 20 min, revealing the oxidation capability of ozone itself. However, the efficiency of TOC removal within 60 min was less than 20% (Figure 4a), which is to say single ozonation just achieved decomposition of 4-CP molecules, while it could not provide adequate oxidation ability for mineralization of refractory

oxidation

intermediates.

Compared

with

single

ozonation,

MnTiO3-180-10 catalysis displayed its advantages not only in higher 23



Page 24 of 43

degradation capacity of 4-CP, but also in the elevated efficiency of mineralization. At 0.3 g L−1 MnTiO3-180-10 dosage, 100% degradation of 4-CP can be obtained within 10 min and the TOC removal of 4-CP was 76% within 60 min. The catalytic ozonation performance of MnTiO3-180-10 was compared with some previously reported manganese and/or titanium oxides.33-36 As shown in Table 2, MnTiO3 exhibited comparable TOC removal rates with a low catalyst and ozone dosage in a short reaction time. In addition, metal leaching related to the homogeneous catalytic ability was also evaluated. With the introducing of MnTiO3-180-10, 0.69 mg L−1 of Mn was detected in the solution after 60 min of reaction and leaching of Ti was less than 0.01 mg L−1. Therefore, 1 mg L−1 of Mn2+ was added to the reaction solution in order to present the catalytic ozonation ability of leaching Mn. The results in Figure 4a proved that the homogeneous catalytic effect of Mn2+ could be overlooked since its TOC removal rate was almost identical with single ozonation. Therefore the heterogeneous catalysis of MnTiO3 was confirmed since homogeneous Mn2+ scarcely activated ozone.

Table 2. The catalytic ozonation activities of MnTiO3 in this study compared with various manganese and/or titanium oxide catalysts. Material

Target

Reaction conditions

contaminant MnOx/SBA-15

Norfloxacin

C0: 10 mg L−1; [O3]: 1.6 mg min−1; 24


TOC

k

(%)

(min-1)

53.4

0.0036

Lit.

33

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[cat.]: 0.1 g L−1; 60 min; pH: 7. MnOx/γ-Al2O3/

4-CP

C0: 100 mg L−1; [O3]: 2 mg L−1; [cat.]:

94.5

0.0234

34

49.6

0.0188

35

79.2

0.007

36

76.2

0.0238

This

2 g L−1; 120 min; pH: 6.6.

TiO2 TiO2/Zeolite

OH-TiO2

Organic

C0: 7.1 mg L−1; [O3]: 19.2 mg min−1;

compounds

[cat.]: 25 g L−1; 30 min; pH: 7.62.

Oxalic acid

C0: 450 mg L−1; [O3]: 16.5 mg min−1; [cat.]: 1 g L−1; 240 min.

MnTiO3

4-CP

C0: 50 mg L−1; [O3]: 2 mg min−1; [cat.]: 0.3 g L−1; 60 min; pH: 6.8.

The TOC removal abilities of 4-CP for all MnTiO3 samples during catalytic ozonation as well as the metal ions leaching are shown in Figure S10. Compared with ozone alone (22.1% TOC removal after 60 min), all MnTiO3 samples accelerated the TOC removal obviously, proving the catalytic ozonation abilities of MnTiO3. MnTiO3-140-10 sample achieved highest TOC removal rate (92.2%), followed by MnTiO3-180-1 (87.6%) and MnTiO3-180-5 (82.3%). All these three samples obtained higher TOC removal rate than MnTiO3-180-10 sample (76.3%). However, their metal ions leaching potentials were much higher than that of MnTiO3-180-10 sample. The dissolved ions of Mn2+ and Ti4+ for MnTiO3-180-1 sample even achieved 5.78 and 1.24 mg L−1, respectively, which were unacceptable for heterogeneous catalytic water treatment. These results indicated that the inadequate synthesis temperature and 25


study


time for MnTiO3 crystallization not only affected the formation of morphology, but also caused serious metal ions leaching during their application in water treatment. Besides, over synthesis revealed drawbacks on the TOC removal rates for MnTiO3-180-15 (55.2%) and MnTiO3-220-10 (60.1%) samples as shown in Figure S10. Although the metal ions leaching of these two samples were slightly reduced compared with that of MnTiO3-180-10, their TOC removal rates were significantly decreased. Since the homogeneous catalytic ability of Mn2+ ions has been proved to be negligible as shown in Figure 4a, the decreased TOC removal rates of over synthesized MnTiO3 samples were mainly due to the considerable aging and sintering effects.37 Thus, the synthesis conditions as well as morphology control of MnTiO3 samples were closely related to their applications in water purification. In addition, the MnTiO3-no TEAH sample revealed poor TOC removal ability (38.5%) which is lower than all other morphology controlled MnTiO3 samples as presented in Figure S10. In order to further clarify the functions of morphology change and {0001} facets exposure, the effect of SBET should be first normalized. As shown in Table S1, the TOC removal rate was divided by

SBET of each sample to avoid the difference of SBET. It is obviously that the values for all the morphology controlled MnTiO3 samples were between 2.27 to 2.56 while that for MnTiO3-no TEAH sample was only 1.64, indicating that the change of SBET was not the key for water purification performance. The {0001} exposure percentage of each sample was calculated according to the FE-SEM 26


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images and the results were shown in Table S1. The MnTiO3-180-10 presented the highest {0001} exposure percentage of 88.3% and the other morphology controlled MnTiO3 samples achieved at least 79.2% of {0001} exposure percentage. However, the MnTiO3-no TEAH sample only obtained 62.3% of {0001} exposure percentage, proving the morphology control ability of TEAH. As mentioned above, the surface energy of {0001} facet is higher than any other facets existed on MnTiO3 surface according to the DFT calculations. Such a kind of reactive {0001} facet was usually designed to be exposed more on the

ത 0} facets to enhance the surface of engineered materials than inert {01 1 performance of various properties.38 The {0001} facet was predominantly occupied by unsaturated Mn and Ti atoms, which usually exhibited high

ത 0} facet was composed of abundant saturated chemical reactivity, while {011 Mn and Ti atoms, showing low catalytic abilities. During catalytic ozonation, the molecular ozone could be adsorbed and decomposed to reactive oxygen species more rapidly by {0001} facet which could enhanced the 4-CP degradation rate.39,40 On the basis of experimental results, DFT calculations and theoretic mechanisms, the 4-CP degradation trend described above could be interpreted in terms of the influence of the exposed {0001} facet and morphology control combined with crystallization of MnTiO3.41,42 It could be deduced that the higher percentage of {0001} facet existed on the surface of morphology controlled MnTiO3 samples closely correlated to all the AOPs-related water purification processes. 27



TBA was added into the MnTiO3-180-10 catalyzed ozonation system to investigate the active species.43 The degradation rate constants of 4-CP degradation in the presence of different scavengers were shown in Figure 4b and their degradation curves were shown in Figure S11. However, the kapp of MnTiO3/O3/TBA revealed only about 25% (0.1957 min−1) decrease compared with the absence of 15 mM TBA (0.2633 min−1), suggesting that some other active species might also be responsible for 4-CP degradation. Therefore, another two active species scavengers, p-BQ and NaN3 were applied to clarify the possible existence of ·O2− and 1O2 that have been identified in similar systems.17 As shown in Figure 4b, 15 mM of p-BQ significantly inhibited the 4-CP degradation (kapp 0.0646 min−1) and the same amount of NaN3 obtained stronger inhibition ability (kapp 0.0303 min−1). EPR experiments were performed utilizing DMPO and TEMP as the spin-trapping agents to detect the active species directly. The inset of Figure 4b demonstrates the coexistence of ·OH, ·O2− and 1O2 during the MnTiO3-180-10 catalyzed ozonation process, which was consistent with the chemical scavenger probe studies. Based on these results, it is suggested that the ozone decomposed to ·OH, ·O2− and 1O2 with the participation of MnTiO3-180-10 and all of these active species were responsible for the enhanced 4-CP removal.

28


Page 28 of 43

Page 29 of 43

1450

Absorbance (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1490

1620 1545 L

L

B L+B

423 K 623 K

1700

1600 1500 1400 -1 Wavelength (cm )

1300

Figure 5. Pyridine-IR spectra of MnTiO3-180-10 at 423 K and 623 K.

It is generally accepted that several manganese oxides (MnO2, Mn2O3 and Mn3O4) could induce ozone decomposition and further generate various active species such as ·OH, ·O2− and 1O2.44,45 Surface properties of MnTiO3 are important in conferring activity in ozonation reactions in water. The surface hydroxyl groups (-OH), which were confirmed to exist on the surface of MnTiO3 by XPS analysis, were commonly considered as the active sites for ozone decomposition and ·OH generation.36,46 In addition, the multivalence redox couple of Mn3+/4+ as well as the electron transfer between Mn3+/4+ and lattice oxygen of MnTiO3 could also resulted in an enhanced catalytic ozonation performance.47 Furthermore, surface Mn and Ti served as Lewis acid sites and captured O3 to form active species on the surface of MnTiO3.48-50 Pyridine-IR analysis was carried out at 423 K and 623 K to probe the Lewis acid sites on the surface of MnTiO3 as displayed in Figure 5. The characteristic absorption peaks of pyridine can be assigned to Lewis acid sites at the wave 29



number of 1450 and 1620 cm−1, Bronsted acid sites at that of 1545 cm−1, and combined Lewis and Bronsted acid sites at that of 1490 cm−1. After quantification of acid sites from Figure 5 according to the literature,51 the total acid sites and Lewis acid sites were 289.2 and 249.5 µmol g−1. Among all the Lewis acid sites, 10.4% and 47.6% of them were strong Lewis acid sites and medium Lewis acid sites while the rest of them were weak Lewis acid sites. The enrichment of three kinds of surface Lewis acid sites on the surface of MnTiO3 gave it remarkable abilities to adsorb and decompose ozone. 1.0 TOC removal (C/C0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 43

1st cycle 2nd cycle 3rd cycle 4th cycle 5th cycle

0.8 0.6 0.4 0.2 0.0

0

10

20

30

40

50

60

Time (min)

Figure 6. Recycling test of TOC removal of 4-CP for MnTiO3 catalyzed ozonation processes. Catalyst: MnTiO3-180-10, Catalyst dosage: 0.3 g L−1, [4-CP]0 = 50 mg L−1, [O3] = 20 mg L−1, gas flow = 0.1 L min−1, pH 6.8 and T = 293 K.

The stability of MnTiO3-180-10 for catalytic ozonation was also examined. Figure 6 shows that less than 7% decline of 4-CP removal rate was observed after five cycles of catalytic ozonation with MnTiO3-180-10. Besides, the XRD and XPS results of fresh and used catalyst for catalytic ozonation were 30


Page 31 of 43

presented in Figure 7. No remarkable difference of crystal structure and surface element components as well as valence states was found between the fresh and used MnTiO3-180-10 after five successive repeated reactions, indicating the MnTiO3-180-10 performed high stability and remarkable reusability during the

Used in catalyic ozonation

Fresh MnTiO3-180-10

20

30

40

50

60

Mn 2p

Relative Intensity (a. u.)

(1234) (3030)

(0118)

(0224)

(1126)

b (1123)

(0112)

a

(1014) (1120)

catalytic ozonation.

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Used in catalyic ozonation

Fresh MnTiO3-180-10

660

70

655

650

645

640

635

o

2θ ( )

Binding Energy (eV)

Figure 7. XRD patterns (a) and high-resolution XPS spectra of Mn 2p (b) of fresh and used MnTiO3-180-10 sample.

31



CONCLUSION In summary, we have demonstrated a simple one-pot hydrothermal method to synthesize single crystal hexagonal MnTiO3 nanosheets with exposed {0001} facets. The transformation of the MnTiO3 morphologies from primary nanoparticles and nanowires to 2D sheet-like structures has been realized by changing the hydrothermal reaction time and temperature. Dissolution and recrystallization processes occurred during the growth of MnTiO3 nanosheets. Synthesis observations in combination with DFT calculations revealed that the adsorption ability of TEA+ played a key role on the growth of well hexagonal structure of MnTiO3. The superior water purification abilities of MnTiO3 nanosheets were attributed to the high exposure percentage of {0001} facet, surface valence change of manganese accompanying with surface hydroxyl groups and surface Lewis acid sites provided by Mn and Ti. Radical quenching experiments and EPR analysis obtained that ·OH, ·O2¯ and 1O2 were generated during the catalytic ozonation process being responsible for the 4-CP mineralization. Ions leaching analysis and five consecutive cycles of MnTiO3 reuse experiments indicated that the MnTiO3 catalyst also exhibited satisfactory stability and reusability. Such findings elucidated a noble controllable nanostructures growth process and also shed light on the design of available catalysts applied in environmental remediation.

32


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AUTHOR INFORMATION Corresponding authors: Jun MA Tel.: 86-451-86283010 E-mail: [email protected] ; [email protected]

Jingyao QI Tel.: 86-451-86413167 E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript

Notes The authors declare no competing financial interest.

ASSOCIATED CONTENT Supporting Information. Additional information on the XPS analysis of MnTiO3, calculation of surface energy, characterizations of MnTiO3 and structure of MnTiO3.

ACKNOWLEDGEMENTS This work was financially supported by the National Key Research and

33



Development Program (2017YFA0207203, 2016YFC0401107), the National Natural Science Foundation of China (51779065), HIT Environment and Ecology Innovation Special Funds (HSCJ201605) and National Major Science and Technology Program for Water Pollution Control and Treatment (2017ZX07201003-03).

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Graphical Table of Content

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Morphology control studies of MnTiO3 nanostructures with exposed

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