Magnetic Properties and Photocatalytic Applications of 2D Sheets of

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Magnetic Properties and Photocatalytic Applications of 2D Sheets of Non-Layered Manganese Telluride by Liquid Exfoliation Aravind Puthirath Balan, Sruthi Radhakrishnan, Ram Neupane, Sadegh Yazdi, Liangzi Deng, Carlos A. de los Reyes, Amey Apte, Anand B. Puthirath, B Manmadha Rao, Maggie Paulose, Robert Vajtai, Ching Wu Chu, Angel A. Marti, Oomman K Varghese, Chandra Sekhar Tiwary, Maliemadom R. Anantharaman, and Pulickel M. Ajayan ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01642 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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ACS Applied Nano Materials

Magnetic Properties and Photocatalytic Applications of 2D Sheets of Non-Layered Manganese Telluride by Liquid Exfoliation Aravind Puthirath Balan1,2¥, Sruthi Radhakrishnan1¥, Ram Neupane4, Sadegh Yazdi1,7, Liangzi Deng3, Carlos A. de los Reyes5, Amey Apte1, Anand B. Puthirath1, B. Manmadha Rao4, Maggie Paulose4, Robert Vajtai1, Ching-Wu Chu3,6, Angel A. Martí5, Oomman K Varghese4*, Chandra Sekhar Tiwary1,8*, M. R. Anantharaman1,2,9*, and Pulickel M Ajayan1* 1 Department

of Materials Science and NanoEngineering, Rice University, Houston, Texas, USA-77005.

2 Department

of Physics, Cochin University of Science and Technology, Kochi, India-682022.

3Texas

Center for Superconductivity, University of Houston, Houston, Texas, USA-77004.

4

Department of Physics, University of Houston, Houston, Texas, USA-77204.

5

Department of Chemistry, Rice University, Houston, Texas, USA-77005.

6

Lawrence Berkeley National Lab, Berkeley, California, USA, 94720

7Renewable

and Sustainable Energy Institute, University of Colorado-Boulder, Boulder, CO 80309, USA.

8Materials

Science and Engineering, Indian Institute of Technology Gandhinagar, Palaj, Gandhinagar 382355, India 9Inter

University Centre for Nanomaterials and Devices, Cochin University of Science and Technology, Kochi, India-682022

Address of correspondence to: [email protected] (P.M.A.), *[email protected] (MRA), [email protected] (C.S.T), [email protected] (O. K. V.) ¥ These authors contributed equally KEYWORDS: Tellurides, 2D materials, liquid exfoliation, 2D magnetism, photoelectrolysis

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ABSTRACT Metal tellurides are highly sought after for energy and spintronic applications much of which is owed to its catalytic and magnetic properties. Confining it into two-dimensions and improving the surface area enriches its catalytic activity and magnetic properties. Herein, we demonstrate the isolation of ultrathin two-dimensional sheets of Manganese(II)Telluride having an average thickness of ~2 nm and flake size of ~100 nm by means of liquid phase exfoliation presumably for the first time. Vanishing of exchange interactions in two-dimensions results in paramagnetic ordering in Manganese(II)Tellurides sheets while its pristine form prefers antiferromagnetic order. The exfoliated two-dimensional sheets were used to sensitize titania nanotubes to broaden the absorption spectrum and utilize visible light for photoelectrochemical water splitting. INTRODUCTION Titanium Sulfide (TiS), Vanadium Phosphide (VP), Iron Sulfide (FeS), Cobalt Sulfide (CoS), Manganese Arsenide (MnAs), Manganese Selenide (MnSe), Manganese Sulfide (MnS), Manganese monoantimonide (MnSb) and Manganese Telluride (MnTe) belong to the family of transition metal based binary compounds and are well known for their magnetic, optical and transport properties 1-8. Most of these compounds exhibit metallic behavior and are having NiAstype hexagonal crystalline structure.

However, Manganese Telluride (MnTe), unlike other

materials behave as a semiconductor, while a few other binary compounds belonging to the same family (MnSe and MnS) are insulating. The band structure of these materials is in between that of charge transfer and band insulators and hence called “crossroads electronic structure”9. Historically speaking, the thermal and magnetic properties of MnTe have been investigated from the beginning of the 19th century 2, 10. However, it was only in the year 1963 the antiferromagnetic property of MnTe was established beyond doubt by neutron diffraction studies, soon after the 2 ACS Paragon Plus Environment

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Neels two sublattice theory was propounded11-12. The discovery of diluted magnetic semiconductors in the seventies gave a further fillip to the ongoing investigations on MnTe13-14. MnTe is a room-temperature antiferromagnetic material having a Neel temperature (TN) of 307 K15. For many years scientists believed that only ferromagnetic materials have applications in spintronics until recent investigations on materials like MnTe proved that anti-ferromagnetic materials are also promising candidates for spintronic applications16-17. Magnetism in two dimensions (2D) is always fascinating and at the same time, it is not unambiguously explained. 2D magnetism has been an emerging research area ever since the discovery of Graphene18. Earlier investigations on magnetic properties of 2D materials suggested that it is tricky to retain the magnetic order down to monolayers as explained by Mermin-Wagner theorem19. Efforts are on to realize 2D materials having appreciable magnetic properties which involve different strategies namely impurity doping20, introducing defects21, grain boundary engineering22 etc. Despite these efforts, it was only recently that scientists isolated a magnetic 2D material: a ferromagnetic monolayer of CrI323. Recently the authors exfoliated two new non van der Waals materials namely hematene and ilmenene from their ores hematite24 and ilmenite25 respectively. They were found to be exhibiting ferromagnetic ordering quite different from their antiferromagnetic bulk counterparts. MnTe being antiferromagnetic and non-van der Waal in nature, it was thought that its 2D analogue would be interesting from a physics point of view. Semiconducting Tellurides having suitable bandgaps could be ideal candidates for photocatalytic applications. For instance, Sousan Gholamrezaei et. al. and Amrita Ghosh et. al. have shown the photocatalytic performance of hydrothermally synthesized Silver Telluride (Ag2Te) and Cu7Te4 respectively26-27. Designing a stable photocatalytic system with at least 10% efficiency upon solar illumination without an external bias voltage is challenging. Among the 3 ACS Paragon Plus Environment


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existing binary compounds of manganese, MnTe is the only semiconductor having a bandgap energy of 1.27 eV9, 28. Even if its energy bandgap is in the optimum range for obtaining efficient solar energy conversion, it is not sufficient for splitting water. An energy bandgap within the range of 1.8 eV to 2.2 eV would be considered optimum for this purpose29. Furthermore, the conduction band of the semiconductor must be lying at a higher potential than the water reduction potential and valence band at a lower potential than the water oxidation potential. It was anticipated that the dimensional constraints imposed by the 2D structure of MnTe would widen the band gap and position the bands appropriately so that 2D MnTe could be employed for photoelectrochemical water splitting. The layered chalcogenides such as WS2 have been reported to have significant band gap widening upon converting bulk materials to 2D structures (~1.3 eV for bulk and 2.0-2.4 eV for 2D WS2)30-32. Our studies indeed showed that the exfoliated 2D MnTe possessed a much higher bandgap (~2.1 eV from Tauc plots) than bulk MnTe with appropriate band position (determined using Mott-Schottky plots) for water splitting. Nevertheless, due to the poor light absorption in the 2D layer, we used a high surface area wide bandgap support, titania nanotube arrays30. EXPERIMENTAL DETAILS Materials Manganese (II) telluride (MnTe) 99.9% metals basis (Gute Chemie) and N, NDimethylformamide were used without any further modifications. Experimental Work Sample preparation: Commercially available MnTe (99.9% (metals basis): Gute Chemie) was powdered well by employing mortar and pestle using acetone as the wetting medium. 50 mg of 4 ACS Paragon Plus Environment

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the obtained powder was dispersed in 200 mL of N, N-dimethylformamide (DMF) and ultrasonicated for 50 hours to obtain a suspension of MnTe sheets. The ultrathin 2D MnTe sheets were separated by centrifugation at RCF value of 10195 (x g) for 30 minutes. Ultrathin 2D sheets of MnTe were isolated by filtering the supernatant which were used for further characterizations. Formation of MnTe/TNT heterojunction: Highly ordered TiO2 nanotube (TNT) arrays were prepared by anodization of titanium foil (0.3 mm thickness, 99.7% purity) in a two-electrode system, using titanium and platinum foil respectively as anode and cathode31. Titanium foils (2 x 2 cm2) were cleaned in micro-90 solution, washed in de-ionized water, sonicated in acetone and isopropanol and then dried in nitrogen before anodization. The anodization was carried out at 55 V for 6 h. The electrolyte consisted of 0.3 wt.% NH4F and 0.3 volume% water in ethylene glycol. The as-grown amorphous titania nanotube arrays were sonicated in isopropanol for about 5 min. to remove organic compounds and dried in gentle nitrogen flow. The TNTs were annealed at 530 °C for 3 h in oxygen to crystallize them. The annealed nanotubes were immersed in a 2D MnTe dispersed dimethylformamide (DMF) solution in an autoclave that was sealed and heated at 200 °C for 24 hours. The samples were cooled down to room temperature, dried in nitrogen, and annealed at 200 °C for 30 minutes in a tube furnace in nitrogen atmosphere to avoid oxidation. Characterizations The transmission electron micrographs were acquired using an FEI Titan Themis (s) transmission electron microscope. FEI Quanta high-resolution microscope was employed to obtain scanning electron microscopy (SEM) images. A Bruker multimode 8 microscope was employed in scanasyst mode to obtain atomic force microscopy (AFM) images. Raman spectra were measured in a Renshaw inVia Raman microscope using 632.8 nm laser with a spot size of 1μ. X-Ray



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photoelectron spectroscopy (XPS) measurements were carried out in a PHI Quantera model machine. A Shimadzu 2450 UV-Visible spectrophotometer was employed to obtain UV-Visible absorption measurements whereas, magnetic measurements were obtained using a magnetic property measurement system (MPMS 3) by Quantum Design. The current-voltage characteristics were studied by an electrochemical analyzer (CHI_660C) under unit solar intensity (100 mWcm-2) provided by a calibrated AM 1.5 solar simulator in a photoelectrochemical (PEC) cell consisting of MnTe/TNT working electrode, a platinum foil counter electrode, and an Ag/AgCl reference electrode. The active area was 0.34 cm2 and scan rate was 500 mV/s. The electrolyte used was 1 M Na2SO4 (pH~8.6). Mott-Schottky measurements were done using the same setup and electrolyte at a frequency of 1 kHz and in the range -1 to +1 V (vs Ag/AgCl). Incident photon to current conversion efficiency (IPCE) spectra were recorded using a light source-monochromator-photodetector set up (wavelength interval 10 nm). A twoelectrode PEC cell mode with no external bias was used for this study. The nanotube surface images were taken using a LEO 1525 field emission scanning electron microscope (FESEM). RESULTS AND DISCUSSIONS Commercially available manganese telluride is liquid exfoliated in DMF to obtain ultrathin 2D sheets of MnTe (see Figure 1. Schematic).

Pristine MnTe is analyzed using x-ray

diffractometer (XRD). The pristine sample is NiAs type polycrystalline MnTe (Refer Figure S1) corresponding to hexagonal structure having P63/mmc space group and lattice constants of a = 4.1475 Å and c = 6.71 Å. In addition to major MnTe phase, there is a small MnTe2 phase and could be due to a non-stoichiometric excess of tellurium compared to manganese32.


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The surface morphology of the exfoliated samples were analyzed using a scanning electron microscope (SEM). The obtained SEM micrographs (Figure S2a&S2b) shows that the exfoliated sheets resemble a sheet morphology. Atomic force microscope (AFM) measurements (Figure 2a & b) confirm the ultrathin nature of the sheets. A low-resolution AFM image is given in Figure S4 which shows that most of the exfoliated MnTe sheets are in the thickness range 2 nm to 3 nm along with a small number of thick (~10 nm) sheets as well. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images of 2D MnTe show well-defined lattice fringes with its Fourier transform (FFT) in the inset (Figure 2c) confirming the crystallinity of the obtained sheets. (110) planes corresponding to an interplanar distance of 0.207 nm can be clearly noticeable in the HAADF-STEM image33. Low magnification HAADF-STEM (Figure 2d) and TEM (Figure 2e) images indicate the 2D nature of exfoliated MnTe. To verify the compositional details of MnTe sheets, STEM–EDS mapping analysis was carried out (Figure. 2f) and it confirms the purity of the material having the majority of Mn and Te elements. The homogeneous distribution of Mn and Te was also evident from the STEM–EDS mapping analysis (Figure. 2f inset). STEM-EELS mapping of Mn and Te is also given in Figure. S3. A thin organic coating (which has oxygen) produced during exfoliation and the TEM grid are likely to be the source of oxygen peaks in EDS and EELS data as oxygen was also observed in regions of the grid with no sample. Raman spectra of obtained 2D MnTe sheets are shown in Figure 3a. Group theory predicts one E2g Raman Active mode in MnTe around 178 cm-1. In the Raman Spectra of exfoliated MnTe (Figure 3a), we could see two intense modes centred at 120.3 cm-1 and 140.4 cm-1. These peaks correspond A1 and Eu bond stretching34 of crystalline elementary Tellurium precipitates on the surface of MnTe sheets, formed by the decomposition on laser exposure as reported earlier35-36.



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The predicted E2g Raman Active mode corresponding to MnTe centred at around 178 cm-1 is also evident. XPS analysis of the Mn2p and Te3d of exfoliated MnTe sheets are shown in Figure 3b&c respectively. The Mn2p doublets (Mn2p3/2 and Mn2p1/2) at 640.8 and 652.7 eV are from Mn-Te bonding, the doublets at 642.4 and 653.9 are contributed by MnO2 residing on the surface37-38. Te3d spectra, however, has predominant Te-Mn doublets (573 and 583.4 eV) and only traces of Te-O doublets (576 and 586.4 eV). Therefore, Raman spectra together with XPS results confirm the purity of MnTe phase. From the magnetic measurements, it is very clear that the exfoliated samples are paramagnetic whereas pristine MnTe is antiferromagnetic. As the temperature is decreased below 300 K, magnetization and thereby susceptibility exhibits a gradual increase (Refer Figure 4a) which is typical of a paramagnetic material. Room temperature (300K) M-H loop (Figure 4a inset) also confirms the paramagnetic behavior having a linear M-H relationship. However, low temperature (10K) M-H loop with enhanced magnetization (nearly 10 times) and almost zero coercivity (Refer Figure 4a inset) indicate a weak magnetic ordering, a transition which could not be observed in FC-ZFC measurements. The observed change in magnetic behavior could be due to the vanishing of the interplanar exchange interactions among the magnetic Mn ions in exfoliated MnTe sheets. These exchange interactions are responsible for the antiferromagnetic alignment of magnetic cations in pristine MnTe. In the absence of strong exchange interactions, the only interactions that remain in MnTe sheets are dipolar in nature39. Due to the very weak nature of these dipolar interactions, the magnetic moments are oriented randomly even at low temperatures resulting in paramagnetic behavior. A sharp enhancement in magnetization at very low temperatures (< 20 K) further confirms the very weak and dipolar nature of interactions. At very 8 ACS Paragon Plus Environment

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low temperatures, dipolar interactions outweighs thermal energy which facilitates ordering of moments resulting in enhanced magnetization. The pristine MnTe shows antiferromagnetic behavior with an observed anomalous behavior below 87 K (see figure 4b). This sharp increase in magnetization around 87K, both in FC and ZFC curves signifies a ferromagnetic ordering and this abrupt change of slope in FC-ZFC curve is believed to be due to strong inter-planar ferromagnetic interactions as a result of magneto elastic coupling which dominates antiferromagnetic interactions.40 Figure 5a shows the absorbance spectrum of the exfoliated 2D MnTe dispersed in DMF. The Tauc plot (Figure 5a inset) showed a fundamental band gap of ~ 2.1 eV. The band tails, however, extend further so as to have light absorption till about 1.8 eV (~700 nm). The bandgap of bulk MnTe is also determined and is found to be around 1.3 eV (Figure S5). Such an enhancement in visible light absorption property of 2D MnTe stimulated our interest in studying its properties as a photocatalyst. The 2D geometry of the material put constraints on the amount of light absorbed unless multilayers are used. The problem was overcome by forming a heterostructure of 2D MnTe with high surface area titania nanotube arrays (TNTs). Photo-stability, chemical stability, availability, and strong photo-oxidation characteristics made TiO2 the semiconductor of choice for photocatalysis41. It, however, suffers from limitations such as a wide band gap and short diffusion length of minority carriers. The large band gap (3.2 eV for anatase phase) of TiO2 makes it absorb only UV light, which limits the amount of solar photon flux that can be utilized by this material to about 5%. Enhancing the optical absorption and reducing the electron-hole recombination are required for superior photoactivity. When the UV absorbing 1D titania nanotube is combined with a visible light absorbing 2D MnTe to form a



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heterostructure, the 2D and 1D geometries of the materials would take care of the carrier recombination problem and the narrow and wide bandgap combination would facilitate broad spectrum utilization. The broad spectrum light utilization in 2D MnTe/TiO2 structure is evident from incident photon conversion efficiency (IPCE) spectrum given in Figures 5b and 5c. As shown in Figure 5b, pure nanotube array electrodes exhibit high IPCE (~80%) in UV, indicating that 80% of the photons in UV region striking the cell result in the current generation. As expected, IPCE is almost zero at wavelengths greater than ~400 nm, showing poor utilization of visible light by bare nanotubes (Figure 5c). 2D MnTe/1D TiO2 electrodes, on the other hand, showed ~8% IPCE in the 400 nm to 500 nm wavelength region. IPCE enhancement is a result of electron transfer from the conduction band of 2D MnTe to the conduction band of TiO2. The IPCE in the < 400 nm wavelength region decreased after loading MnTe 2D sheets on titania nanotubes. The decreased IPCE of the heterostructure is due to clogging of TNT pores by 2D MnTe (discussed below) decreasing the active surface area of nanotubes, inhibiting the photoactivity. Figure.5d shows the current-voltage characteristics of bare TNT and 2D MnTe/TNT photoanodes, recorded under illumination and dark conditions. Under illumination, the photocurrent density of the bare TNT and 2D MnTe/TNT showed a rise in the current starting at 0.35 and -0.7 V(vs Ag/AgCl), respectively, reaching a maximum value of 0.25 and 0.55 mA cm-2 at about 0.5 and -0.2 V(vs Ag/AgCl) respectively. The solar photocurrent determined by integrating IPCE over standard AM1.5G spectrum is 0.61 mA cm-2, which is close to the measured photocurrent for 2D MnTe/TNT42. The enhanced photocurrent yielded by 2D MnTe/TNT is attributed to visible light absorption by MnTe 2D sheets and transfer of photogenerated electrons to TiO2, reducing the electron-hole recombination. 10 ACS Paragon Plus Environment

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In order to confirm the possibility of electron transfer from 2D MnTe to TiO2 nanotubes, Mott-Schottky measurements were performed. The flat band potentials were found by extrapolating the linear region of the Mott-Schottky curve to intersect the x-axis. As shown in Figure.5e, the flat band potential is shifted toward more negative value (vs NHE), by 0.3 V, for the heterostructure. Therefore, spontaneous water splitting is possible with 2D MnTe/TNT due to its more negative flat band potential (vs NHE) compared to water reduction potential. Figure.5f shows the charge transfer at the interface of 2D MnTe and TNTs. Electrons are excited from the valence to the conduction band of 2D MnTe by photons of energy > 2.1 eV (Figure. 5a). As shown in Figure.5f, the conduction band of 2D MnTe is elevated compared to that of the TNTs and hence, the electrons excited from the valence band to conduction band of 2D MnTe, get transferred to the conduction band of TNTs. At the same time, the holes in MnTe 2D sheets and TNTs (not covered with 2D sheets of MnTe) take part in the water oxidation process. The SEM image (top view) of the bare TiO2 nanotube surface is shown in Figure 5g. The cross-section (Figure 5h) of the MnTe sensitized NT showed any distinct features arising from MnTe due to its 2D nature. Nevertheless, some partially exfoliated sheets blocking the pores of nanotubes could be seen at the top surface (Figure 5i). This geometry could limit the enhancement in the photocurrent because of the large thickness of the sheets and the weak interface they make with TNTs, both causing poor carrier transfer to TNT. The sheets could also obstruct the UV light reaching TNTs. Thus, the material combination, 2D MnTe/1D TiO2, could yield significantly higher photocurrent compared to pure TiO2 nanotube photocatalyst. This higher photocurrent is attributed to the visible light absorption by MnTe 2D sheets and transfer of photogenerated electrons to TNTs, improving the separation of photogenerated electrons and holes. Our experiments show that 2D MnTe is a very useful material for forming heterojunctions with stable wide bandgap 11 ACS Paragon Plus Environment


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semiconductors and enhancing their visible light photoactivity for efficient solar photocatalytic water splitting. CONCLUSIONS In conclusion, 2D MnTe is successfully exfoliated from commercially available bulk precursors. The exfoliated sheets are found to be crystalline and pure with traces of MnTe2. MnTe, being an antiferromagnetic material in its pristine form transforms to paramagnetic on exfoliation and could be due to the vanishing of strong exchange interactions at two dimensions and the magnetic order is characterized by weak dipolar interactions. Modified optical band gap and band positions make 2D MnTe a suitable choice for photocatalytic applications. A heterojunction of 2D MnTe and TiO2 nanotubes is successfully fabricated and the role of 2DMnTe in sensitizing TiO2 nanotubes to utilize visible region of the solar spectrum is demonstrated.


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Figure 1. Schematic: Liquid exfoliation of manganese telluride in dimethylformamide(DMF) solvent. Planar and cross-sectional view of obtained [110] oriented 2D sheets of MnTe is shown. Manganese and Tellurium atoms are coloured violet and orange respectively.



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Figure. 2. Morphology and composition of 2D MnTe: a) AFM image and, (b corresponding height profile of an exfoliated MnTe sheet. c) HAADF-STEM image of 2D MnTe (scale bar, 5nm) with its Fourier transform in the inset. d) HAADF-STEM (scale bar, 100 nm) and e) TEM image of a 2D MnTe sheet (scale bar, 20 nm). f) EDS of the Mn–K, Mn–L, Te-L signals along with EDS-STEM elemental mapping for Mn (K) and Te (L) with scale bar, 10 nm in the inset. Cu signal is from the Cu TEM grid.


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Figure. 3. Characterizations of 2D MnTe: a) Raman spectrum of exfoliated MnTe sheets, b) Te3d XPS spectrum, and c) Mn2p XPS spectrum of exfoliated MnTe sheets.

Figure. 4. Magnetism: a) FC-ZFC M-T diagram (at 10 Oe) of exfoliated MnTe with M-H at 300K and 10K at the inset b) FC-ZFC M-T diagram (at 10 Oe) of pristine MnTe with M-H at 300K and 10K at the inset.



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Figure 5. Optical/Photoelectrochemical: a) Absorbance spectrum of 2D MnTe dispersed in DMF and the Tauc plot (inset) showing the enhanced bandgap of 2.1 eV. Bare and 2D MnTe loaded TiO2 NT characteristics: IPCE spectra in the b) UV-Vis region and c) visible region (zoomed) of the solar spectrum. d) Current-voltage characteristics. e) Mott-Schottky plots (pH = 8.6) f) Schematic diagram showing the charge transfer in the heterojunction photocatalyst. SEM images of g) bare TNT and h) lateral view and i) top view of MnTe loaded TNT respectively


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CORRESPONDING AUTHOR * Address correspondence to: [email protected] (P.M.A.), *[email protected] (MRA), [email protected] (C.S.T), [email protected] (O.K.V.)

PRESENT ADDRESSES B. Manmadha Rao - Deaprtment of Physics, VIT-AP University, Amaravati -522 237, Andhra Pradesh, India. AUTHOR CONTRIBUTIONS A.P.B., S.R., C.S.T., A.A., A.B.P, M.R.A., R.V. and P.M.A envisaged and executed the experiments. S.Y. performed the transmission microscopy experiments. L.D. and C.W.C conducted the magnetic measurements. R.N., B.M.R., M.P. and O.K.V. carried out the photocatalytic experiments and analysis. C.R. and A.A.M. conducted the optical measurements. All the authors contributed to the data analysis and writing. ACKNOWLEDGEMENTS A.P.B. acknowledges University Grants Commission, Govt. of India for BSR Fellowship (Grant No.F.25-1/2013-14 (BSR)/5-22/2007(BSR) dated 30/05/2014). A.P.B., S.R., C.S.T, A.A., R.V. and P.M.A. acknowledge the U. S. Army Research Office MURI grant W911NF-11-1-0362 for the financial assistance. A.B.P. acknowledges the Science & Engineering Research Board (SERB) and Indo-US Science and Technology Forum (IUSSTF) for financial support in the form of postdoctoral fellowship. L.D and C.W.C. thank the US Air Force Office of Scientific Research Grant FA9550-15-1-0236, the T. L. L. Temple Foundation, the John J. and Rebecca Moores 17 ACS Paragon Plus Environment


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Endowment, and the State of Texas through the Texas Center for Superconductivity at the University of Houston for financial support. O.K.V. thanks Shell International Exploration and Production Inc. Game Changer and New Energies Research and Technology group for financial support. M.R.A acknowledges India based neutrino observatory (INO) for the travel grant and University Grants Commission (UGC), India for awarding UGC-BSR Faculty Fellowship (Grant No. F.18-1/2011(BSR)). REFERENCES 1. Franzen, H.; Sterner, C., The X-ray photoelectron spectra of MnS, MnSe, and MnTe. Journal of Solid State Chemistry 1978, 25, 227-230. 2. Squire, C. F., Antiferromagnetism in Some Manganous Compounds. Physical Review 1939, 56, 922-925. 3. Allen, J.; Lucovsky, G.; Mikkelsen, J., Optical properties and electronic structure of crossroads material MnTe. Solid State Communications 1977, 24, 367-370. 4. Decker, D. L.; Wild, R. L., Optical Properties of a-MnSe. Physical Review B 1971, 4, 3425-3437. 5. Oguchi, T.; Terakura, K.; Williams, A. R., Band theory of the magnetic interaction in MnO, MnS, and NiO. Physical Review B 1983, 28, 6443-6452. 6. Neitzel, U.; Bärner, K., Optical and Magnetic Investigations of Some NiAs-Type Transition Metal Compounds. physica status solidi (b) 1985, 129, 707-715. 7. Liu, Y.; Xing, J.; Fu, H.; Li, Y.; Sun, L.; Lv, Z., Structural stability, mechanical properties, electronic structures and thermal properties of XS (X = Ti, V, Cr, Mn, Fe, Co, Ni) binary compounds. Physics Letters A 2017, 381, 2648-2657. 8. Aronsson, B.; Lundström, T.; Rundqvist, S., Borides, silicides, and phosphides: a critical review of their preparation, properties and crystal chemistry. Taylor & Francis: 1965. 9. Youn, S. J.; Min, B. I.; Freeman, A. J., Crossroads electronic structure of MnS, MnSe, and MnTe. physica status solidi (b) 2004, 241, 1411-1414. 10. Kelley, K. K., The Specific Heats at Low Temperatures of Manganese, Manganous Selenide, and Manganous Telluride. Journal of the American Chemical Society 1939, 61, 203207. 11. Komatsubara, T.; Murakami, M.; Hirahara, E., Magnetic Properties of Manganese Telluride Single Crystals. Journal of the Physical Society of Japan 1963, 18, 356-364. 12. Kunitomi, N.; Hamaguchi, Y.; Anzai, S., Neutron diffraction study on manganese telluride. Journal de Physique 1964, 25, 568-574. 13. Furdyna, J., Diluted magnetic semiconductors: an interface of semiconductor physics and magnetism. Journal of Applied Physics 1982, 53, 7637-7643. 14. Dietl, T.; Ohno, H.; Matsukura, F.; Cibert, J.; Ferrand, D., Zener Model Description of Ferromagnetism in Zinc-Blende Magnetic Semiconductors. Science 2000, 287, 1019-1022. 15. Ozawa, K.; Anzai, S.; Hamaguchi, Y., Effect of pressure on the magnetic transition point of manganese telluride. Physics Letters 1966, 20, 132-133. 18 ACS Paragon Plus Environment

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X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) images, Scanning Transmission Electron Microscopy-Electron Energy Loss Spectroscopy (STEM-EELS) images, Atomic Force Microscopy (AFM) images and Absorption Spectrum. ToC GRAPHICS


Magnetic Properties and Photocatalytic Applications of 2D Sheets of

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