Free‐standing δ‐MnO2 atomic sheets

δ‐Manganese dioxide (δ‐MnO2) is a 2D material which possesses distinct properties and features due to its unique atomic structure and has already been utilized in numerous disciplines recently, especially in the field of magnetism, energy storage, magnetic resonance imaging, biocatalysts, and fluorescence sensing. Keeping an eye on the huge potential of this 2D material, we report our recent discovery of single‐step synthesis of MnO2 nanosheets via bottom‐up laser crystallization (of aqueous KMnO4 solution) and top‐down sonochemical exfoliation of bulk MnO2 powder. The successful synthesis of δ‐MnO2 nanosheets has been proved through the observation of characteristic Raman peaks at 173 and 634 cm−1 and characteristic X‐ray diffraction peaks. The optical band gap was found to be 1.64 and 1.45 eV for both methods. We also demonstrated that 2D‐MnO2 is a prominent candidate material for ammonia sensing and strain sensing. δ‐MnO2 powder, when employed as cathode material in Li‐ion batteries, results in a stable voltage of ˜0.5 V and in contrast, gives ˜1 V when used in Li‐S batteries and the attained voltage is stable even for >5 h. New methods of synthesis of δ‐MnO2 and its hybrids with graphene will lead to future generation devices, it is expected.


INTRODUCTION
2D materials are among the most actively explored materials systems due to the unique functional properties they offer.Graphene, the first member of 2D materials; exhibits record electronic mobility, quantum Hall effect, record Young's modulus, and excellent thermal conductivity. 1,2Graphene has been deemed as a wonder material of the 21st century due to widespread applications, for example, molecular sensing, 3,4 laser shielding, 5 optoelectronics, [6][7][8] oxidation resistant laminates, 9 in manipulating photonic behavior of nano emitters wrapped by it. 10,11Incidentally, graphene is a semimetal and has compromised carrier concentration and therefore, graphene nanoribbons which are semiconducting have been derived from the unzipping of carbon nanotubes. 12,13The last decade has witnessed the rise of mono-elemental atomic sheets (Xenes).Among Xenes, borophene is metallic but oxidation prone, [14][15][16][17][18] and phosphorene is semiconducting but compromised chemical stability. 19Recently discovered 2D Gold is plasmonic. 20][23] To experimentally realize electronic chips and to explore optoelectronic devices such as LEDs and solar cells, semiconducting 2D materials with suitable band gaps are in demand.MoS 2 has a band gap of 1.4 eV (indirect) in bulk and the band gap keeps on increasing down to monolayer which exists in two phases, one is semiconducting (2H phase) with a band gap of 1.98 eV (direct) and another phase (1T) is metallic. 24Most of the 2D semiconducting metal dichalcogenides, however, face issues with applications where a visible range band gap is crucial.To be precise, they have a visible range band gap only at the monolayer which is difficult to fabricate.It should be noted that MoS 2 , WS 2 , and so forth are not naturally occurring crystals.They are not yet standardized for their synthesis via chemical vapor deposition techniques.Exfoliation techniques face problems as monolayers have multiple phases which are difficult to isolate.Due to these reasons, one needs to explore new 2D materials with a band gap in the visible range and with stable single phase, especially for semiconducting applications in electronics (as active field responsive materials), optoelectronics (solar cells, LEDs), light harvesting, gas/molecular sensing, energy storage, and catalysis.Structurally robust materials with consistency of physical/chemical behavior under extreme thermal processing conditions are always highly desirable.
Oxide 2D materials are often structurally stable against thermal processing and their physical/chemical properties are robust against thermal processing even under elevated temperatures.6][27][28][29] MnO 2 exhibit several polymorphs and incidentally its δ-phase has a band gap of 2.1 eV which is nicely falling in the visible range, very rare though among 2D materials. 30The delta phase is a specific crystallographic phase of MnO 2 that is commonly observed in 2D structures.It is characterized by the arrangement of MnO 6 octahedra forming a layered structure.2D MnO 2 possesses a layered structure in which the MnO 6 octahedra are interconnected, forming two-dimensional sheets.These sheets are stacked on top of each other through weak van der Walls forces.Within each layer, the octahedra share edges, creating a honeycomb-like pattern.The stacking of these layers results in the formation of a three-dimensional structure. 31,32raphene and BN do not exhibit strong spin polarization and magnetism measured in these materials is primarily contributed by edge orbitals or remnant functional groups (if synthesized chemically).Even though defects can give rise to magnetism, it is never reliable up to the extent it is needed for room-temperature magnetic applications.Therefore, one needs to externally inject spin into such systems in order to realize functional spintronic devices.Other 2D materials including graphene, boron nitride, borophene, and so forth, lack ferromagnetic ordering at room temperature.In this regard, MnO 2 having variable valency of Mn along with coupling with oxygen, is expected to be intrinsically ferromagnetic. 33MnO 2 being the most favorite for battery applications, there has been intensive research thrust to experimentally synthesize its 2D sheets in gram scale.5][36] In addition, energy applications have been envisaged for MnO 2 atomic sheets. 37Furthermore, MnO 2 -graphene hybrids are expected to be photosensitive laminates.Such 2D hybrids are expected to have fast responses and are light sensitive.
2D-MnO 2 has been synthesized via various exfoliation strategies including intercalation of potassium ions into MnO 2 voids. 36,38δ-MnO 2 as a 2D material is to date the least experimentally studied among 2D oxides.Moreover, various exfoliation Keeping the spintronic application (as the monolayer itself is supposed to be ferromagnetic at room temperature) and emerging demand of 2D MnO 2 for energy generation and storage applications, gas sensors, photocatalysis and electrocatalysis; it is imperative to explore novel synthesis techniques for it.Controlled synthesis of 2D-MnO 2 is crucial because of their surface-sensitive physical properties and therefore, several approaches to growing 2D-MnO 2 have been developed. 38,39Various synthesis methods already explored include hydro/solvo thermal, sol-gel and template-based techniques.2D-MnO 2 nanosheets were earlier synthesized by multistep processing, involving a high-temperature solid-state reaction for the formation of layered material.Facile and economic synthesis of 2D-MnO 2 sheets in a single-step, scalable, and reproducible manner is the need of the hour to cater to the needs for various targeted applications.
We have employed two strategies of synthesis, one bottom-up laser-induced crystal growth by laser irradiation of KMnO 4 aqueous solution and another one top-down exfoliation of MnO 2 powder to obtain atomically thin MnO 2 nano sheets.Synthesized MnO 2 2D atomic sheets have been characterized in detail, employing microscopy (SEM, AFM, TEM, etc.) and spectroscopies (UV-Vis, PL, Raman etc).Battery applications and gas as well as strain sensing have been explored for 2D MnO 2 .Keeping in mind futuristic applications, graphene-MnO 2 hybrid has been synthesized.

Laser synthesis of 2D-MnO 2
For laser synthesis of 2D-MnO 2 , 6 mg of potassium permanganate (KMnO 4 -Sigma Aldrich [99.5% purity]) was dispersed in 30 mL of isopropyl alcohol (IPA) in a 50 mL quartz vessel.The dispersion was sonicated in a Cole Parmer sonicator for 1 h.The well-dispersed sonicated solution was mounted on a stand for laser exposure.The laser was irradiated on the dispersed solution with a laser fluence of 2 J/cm 2 for a total 5000 and 10,000 shots for each power.Then, the laser-treated dispersion was employed for characterization.

Sonochemical synthesis of 2D-MnO 2
For sonochemical synthesis, 10 mg of manganese dioxide (Sigma Aldrich [99.5% purity]) was dispersed in 25 mL of IPA in a quartz vessel (50 mL).The dispersion was well-shacked and then sonicated in a Cole Parmer sonicator (40 kHz) for a total time of 20 h.The supernatant was separated from the sonicated solution by employing centrifugation.A Remi R-24 centrifuge machine was used at 6 KRPM for 3 min to separate the exfoliated MnO 2 sheets from the remaining part of the sonicated solution.The obtained supernatant was dried at 100 • C for 2 h.The obtained 2D-MnO 2 sheets were used for characterization.

Synthesis of MnO 2 -graphene hybrid
Hybrids of 2D MnO 2 were synthesized by sonochemical technique with graphene (purchased from Ultra Nanotech).
For hybrid synthesis, 10 mg of graphene was dispersed in 20 mL of sonochemically synthesized MnO 2 solution and ultra-sonicated for 8 h.The sonochemically obtained hybrids supernatant was separated and used for further characterization.

Characterization techniques
The supernatant of synthesized MnO 2 sheets was transferred over to a silicon dioxide (SiO 2 ) substrate for the study of microscopy, spectroscopy, and other characterization.For Raman measurements, a Raman spectrometer having an integrated microscope of high resolution (100×) from Olympus (Seki Techno Tron Corporation, Japan) was used for laser beam focusing and to capture the light for micro-optical imaging of MnO 2 sheets.A He-Ne laser (wavelength 633 nm) was used in Raman spectroscopy measurements in backscattering geometry.A scanning electron Microscope (ZEISS GEMINI SEM 500) was used to take high-resolution images by operating it in secondary electron detection mode.A spectrophotometer from SHIMADZU UV-2600 UV-VIS was used in the absorption mode for optical transparency.

AFM and HRTEM microscopy
An atomic force microscope (AFM) (Agilent, Model No. 5500) was operated in a non-contact mode to get high-resolution and large-area topographic images of atomic sheets of MnO 2 using a silicon tip during the measurements.For high-resolution transmission electron microscopy (HRTEM) measurement, MnO 2 sheets from supernatant were spin-coated at 5 KRPM (2 min) onto a carbon-coated copper transmission electron microscope (TEM) grid (Mesh size = 3 mm) following the standard drying protocol.A TEM from JEOL (JEM 2100) was used to study the atomic configuration of 2D-MnO 2 sheets.

Computational details
We have performed all the density functional theory (DFT) based calculations using the Vienna Ab initio Simulation Package (VASP).The calculations were performed under spin-polarized generalized-gradient approximation (SGGA) of the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional with a maximum energy cutoff limit of 150 Ry. 40 The space group R3M with lattice parameter (a = 8.77698 Å, b = 7.60109 Å, and c = 30 Å) and super cell size 3 × 3 × 1 was used for calculations.

DFT band structure calculations
The band gap plot of various 2D oxides is shown in Figure 1A, which demonstrates the visible range band gap of 2D-MnO 2 , desirable for electronic device fabrication.DFT calculations have been performed to illustrate the layer dependence of band gap for MnO 2 sheets (see Figure 1B-D).The band gap obtained for monolayer, bilayers, and trilayer is 1.186, 1.066, and 1.134 eV respectively.Moreover, when the interlayer separation was reduced in trilayer from 3.5 to 3.1 Å, the band gap also decreases to 1.048 eV.The density of states (DOS) for monolayers, bilayers, and trilayers also display a similar behavior as the band structure (see Figure 1E-G).

Bottom-up laser-induced crystallization
The synthesis of MnO 2 employing laser (Nd-YAG laser with wavelength 355 nm) has been investigated by varying laser fluence and the number of laser shots irradiated.Fluence below 2 J/cm 2 , did not yield crystalline sheet formation and therefore, laser irradiation with fluence 2 J/cm 2 was used in detail to synthesize the 2D-MnO 2 sheets and reported in this article.The camera image of the experimental setup of laser synthesis is shown in Figure 2A.For laser synthesis of 2D-MnO 2 sheets, 6 mg of KMnO 4 solution dispersed in 30 mL of IPA, displays pink color (see Figure 2B).The bond dissociation of KMnO 4 is initially slow, however, the laser-localized superheating increases the rate of reaction.Upon laser irradiation with fluence 2 J/cm 2 and 5000 laser shots, the photochemical reaction takes place, and the solution turns out to be red.Usually, high reaction rate results in the production of massive precipitates and non-oriented particles via homogeneous nucleation.Therefore, with an increase in the laser pulse counts to 10,000, the rate of reaction further increased which resulted in a brownish color (see Figure 2B).The growth mechanism of synthesized material by laser with fluence 2 J/cm 2 and 10,000 laser shots was investigated in detail by field emission scanning electron microscope (FESEM) and TEM imaging.The produced MnO 2 sheets are stabilized by CH 3 COO − group. 41nO 4 + (CH 3 ) 2 CHOH → KxMnO 2 + RCOOH Tyndall effect for supernatant containing MnO 2 sheets dispersed in IPA transmits the red and blue laser without any significant scattering, which hints at the better interaction between solvent and material (see Figure 2C.The Flat 2D sheets were obtained in FESEM imaging (see Figure 2D).The sheets with few layers with lateral dimension ∼200 nm were evident in AFM imaging along with its 3D view and line profile (see Figure 2E-G).Lateral dimensions versus thicknesses scatter diagram shows feature of 2D material (Figure S1).
Elemental mapping has been carried out to distinguish the constituent elements of MnO 2 , and we observed a clear signature of both Manganese (Mn) and oxygen (o) (see Figure 2H.Large area HRTEM image displays the atomic configurations of the 2D-MnO 2 sheet (see Figure 2I).Moreover, HRTEM imaging reveals the more periodic arrangements and crystallization of 2D MnO 2 (see Figure 2I, inset SAED pattern hints at better crystallization).The zoomed-in HRTEM image of marked region 1 of Figure 2I reveals the enhanced atomic periodicity and ladder-type atomic configuration (see Figure 2J).The average inter-atomic distance along the array of atoms in Figure 2J was small (see Figure 2L, red plot).While the atomic line profile normal to the linear array of atoms displays two out-of-plane atoms along with one atom lying at the bottom with an average interatomic separation of 0.31 nm (see Figure 2L, green plot)).The zoomed-in region 2 has strained atomic configuration having atoms aligning along a line (see Figure 2K).The average inter-atomic distance in Figure 2K along a linear array of atoms and normal to it was 0.285 (green plot) and 0.366 nm (red plot), respectively (see Figure 2M).The phase determination was carried out by X-ray diffraction (XRD) measurements, which consists of main peaks at 12.34 • , 24.46 • , 37.12 • , and 65.67 • corresponding to (001), (002), (110), and (020) planes, respectively and well coincide with the peaks in reference JCPDS00-042-1317 (see Figure 2N).Hence, XRD analysis confirmed the δ-MnO 2 phase. 34,42The chemical identification was further confirmed by Raman spectroscopy which bestows two major peaks at 173 and 634 cm −1 (see Figure 2O).UV-visible spectroscopy was carried out to measure the optical band gap of synthesized samples.We observed an optical band gap of 1.64 eV (see Figure 2P).In fact, the experimentally obtained morphology of layered MnO 2 is influenced by synthesis parameters which determine defects such as Mn interstitials, oxygen vacancies, remanent functionalities, and residual strain and therefore there is no perfect matching of experimental and theoretical band gaps.
With gradual increase in Nd-YAG laser pulse counts, bond dissociation of potassium permanganate occurs first, and the material is atomized.Formation of Mn 2+ and O 2− is followed by electrostatic clustering of these ions and then they form chemical bonds to form crystals (to stabilize) at the focal hot-spot under laser plasma.Other lasers (e.g., KrF laser) may work too.However, moderate energy window is more favorable.At lower energy of laser, no crystallization, and at very high energy, defect generations and fragmentation of crystals to dots are witnessed.

Top-down sonochemical exfoliation
Sonochemical liquid-phase exfoliation of parent crystal is a facile route to obtain 2D nanosheets.Micro powder of MnO 2 was undergone through powerful ultrasound (40 kHz) treatment.High-intensity ultrasound produces chemical and physical effects that can be used to split/exfoliate the layered materials into individual layers and prolonged treatments yield a significant fraction of monolayers, which can be separated further by centrifugation.Ultrasonic irradiation caused cavitation in a liquid medium, where the formation, growth, and implosive collapse of bubbles occurred.These bubbles with short lifetimes generate high pressure and intense local heating.Those localized hot spots can generate a pressure of above 1800 kPa and a temperature of ∼5000 • C, these spots are reasonable for many chemical reactions. 43MnO 2 powder when sonicated in IPA solvent, makes a good dispersion.Better exfoliation and thinner sheets were achieved with the increase in the timing of sonochemical exfoliation.The dispersion containing MnO 2 sheets obtained after 20 hrs sonification (camera image shown in Figure 3A) was characterized in detail.The layered nature of MnO 2 sheets obtained by sonochemical exfoliation was confirmed by the FESEM image shown in Figure 3B and the AFM images displaying ladder-type pile-up of MnO 2 layer of thickness ∼0.5 nm, witnessed in its line profile (see Figure 3C along with line profile in D).The XRD pattern exhibited peaks at 12.49 • , 24.12 • , 37.09 • , and 65.41 • , which confirms the formation of the δ-MnO 2 phase in obtained sheets (see Figure 3E).The Raman spectrum of the synthesized sample consists of one major peak at 636 cm −1 (see Figure 3F).Tauc plot derived from the UV-visible absorption spectrum yield a band gap of 1.45 eV (see Figure 3G).The properties of 2D MnO 2 can be influenced by the specific layer arrangement and stacking sequence.The reduced dimensionality of 2D MnO 2 introduces the possibility of tailoring its properties by controlling the layer thickness and interlayer interactions.Different stacking patterns can lead to variations in electronic, optical magnetic, and catalytic properties, making layer-dependent properties a topic of interest in the study of 2D MnO 2 .Therefore, we observed the optical band gap difference of ∼0.2 eV for the two synthesis methods.Moreover, theoretical calculations also witnessed the band gap change along the variation in lattice parameters.Elemental mapping was carried out to confirm the presence of both constituent elements manganese and oxygen (see Figure 3H).The large area HRTEM image displays different atomic configurations in different regions (see Figure 3I).The zoomed-in region 1 marked in Figure 3I displays hexagonal atomic configurations (see Figure 3J, inset having fast Fourier transform [FFT] pattern).The average inter-atomic distance for region 1 along the atomic array (cyan line) is 0.27 nm and normal to it is 0.30 nm (blue line) (see Figure 3K).

Gas and strain sensing
Strain among many materials investigated for molecular sensing, gas sensing, and strain sensing, MnO 2 has attracted much attention because it is inexpensive, environmentally friendly, and present in abundance.The schematic of two-probe measurements of synthesized 2D-MnO 2 sheets, employed for gas sensing; is shown in Figure 4A.When ammonia gas was exposed to the two-probe device, we found that MnO 2 is ammonia sensitive and observed that the fall time of resistance was lesser than the rise time (see Figure 4B).Also, as the amount of ammonia gas exposed increased from 26.4 to 105.4 ppm, a sharp fall in resistance was witnessed.Hereby, we can conclude that MnO 2 is a prominent material candidate for ammonia sensing and even sense the very least amount (26.4 ppm) of it.Having out-of-plane atomic symmetry and a high surface area of MnO 2 sheets, nitrogen atoms in NH 3 interact electrostatically with MnO 2 and thus the interactions make it a good surface for ammonia sensing.Apart from possessing atomically transparent sheets, MnO 2 has also a high young modulus of ∼25 GPa. 44As compared to the PVDF, MnO 2 -PVDF nanocomposite display better mechanical strength (see Figure 4C).Enhanced mechanical strength of MnO 2 -PVDF nanocomposite establishes superior interaction between PVDF and MnO 2 sheets.

Li and Li-S batteries
δ-MnO 2 is a highly redox active that could potentially be used as a cathode for Li-ion batteries.To distinguish the effect of MnO 2 on the performance of a battery, we fabricated cells based on electrolyte LiCl and LiCl+MnO 2 .Arduino microcontroller connected to cells to monitor real-time potential gradient (see Figure 4D).The performance of the cell with LiCl + MnO 2 as an electrolyte witnessed better stability in open circuit voltage versus time plot (see Figure 4E).Moreover, along with the voltage stability (∼0.5 V), it also keeps the output voltage very high for a long time (>5 h).To dig further, other cells were also fabricated with the addition of sulfur in both electrolytes.The cell with LiCl + S + MnO 2 as electrolyte performs well and the output voltage reaches ∼1 V and is stable for >5 h.

MnO 2 -G hybrids
The hybridization of various 2D materials has been used to manipulate band gaps and alter materials characteristics properties for a particular application. 45While graphene is semi-metallic and 2D-MnO 2 is semiconducting.Looking at the possibilities in energy storage applications (flexible supercapacitors), we synthesized the 2D-2D hybrid of synthesized 2D-MnO 2 and graphene.To observe the presence of individual elements precisely in the hybrid, elemental mapping was carried out, which confirms the presence of manganese (Mn), oxygen (O) of MnO 2 , and carbon (C) of graphene (see Figure S2a,b).Raman spectrum of the MnO 2 -G hybrid consists of the signatures of both MnO 2 at 436 cm −1 and graphene at 1334 cm −1 (D peak), 1587 cm −1 (G-peak), and 2686 cm −1 (2D peak) (see Figure S3a).Current versus voltage (I-V) and photoconductivity (PC) (employing blue [405 nm] and red [650 nm] laser excitation) measurements of MnO 2 -graphene hybrid reveal the metallic behavior (see Figure S3b).As evident from TEM imaging, the size of graphene is larger, and MnO 2 sheets were hybridized over the sheet of graphene.Moreover, the HRTEM image also displays the overlapping of two different atomic symmetries (see Figure S4a).The zoomed-in region 1 displays the linear atomic configuration which is confirmed by the inset FFT pattern (see Figure S4b).The average inter-atomic distance in Figure S4b marked by the green line was 0.26 nm (see Figure S4d).The zoomed-in region 2 displays the squarish atomic arrangements (see Figure S4c, inset FFT pattern).Moreover, the average inter-atomic distances in two symmetry directions shown by the red and cyan lines in Figure S4c were 0.38 and 0.25 nm, respectively (see Figure S4e,f).Moreover, simple multimeter testing showed 20-100 Ohm/mm for MnO 2 -G nanocomposite, while it was 3000 Ohm/mm for the MnO 2 sample.This simple demonstration hints at the future application of MnO 2 based 2D-2D hybrids.

CONCLUSION
In conclusion, 2D MnO 2 was synthesized by two methods: laser-induced bottom-up synthesis from KMnO 4 and from top-down exfoliation of MnO 2 powder.Various microscopy and spectroscopy carried out on synthesized samples hint at the formation of few-layered sheets of 2D MnO 2 .While characteristic Raman peaks and XRD peaks confirm δ-MnO 2 phase formation, TEM and AFM line profiles prove a few layers formation.UV-Vis absorption spectroscopy for samples synthesized by both methods yields a band gap of 1.45-1.64eV.Defects such as stacking faults, Mn interstitials as well as oxygen vacancies, and strain in the experimental system seem to give higher band gaps as compared to theoretical results.Moreover, DFT underestimates the band gap.Apart from nature (liquid/solid) of precursors, reaction conditions such as temperature/pressure/electric field of laser, surface energy of the solvent, and so forth determine the rate of reaction wherein precursor bonds break and new bonds form and the material crystallize.Therefore, synthesis technique decides the crystalline order and crystal type of the product sheets.Monolayers in general are not stable, evolution of ripples, 3D protrusion, and so forth.are expected due to residual strain in the system.2D MnO 2 has been demonstrated for their ammonia sensing, strain sensing, and Li-ion and Li-S battery applications.These applications have been carried out just as simple demonstrations.The present manuscript describes the exploration of new methods of synthesis of δ-MnO 2 , however can be extended to other 2D oxide materials systems.

F I G U R E 2
(A) Camera photo of laser synthesis. (B) Photographs of 6 mg KMnO 4 solution in 30 mL isopropyl alcohol (from left to right): unirradiated, obtained after laser (laser fluence 2 J/cm 2 ) irradiated 5000 shots and after 10,000 shots.(C) Camera images of the Tindall effect with supernatant imaged with a red and blue laser beam through it.(D) FESEM image of synthesized MnO 2 sheets.(E-G) AFM image and its line profile along with the 3D view of sheet.(H) Elemental mapping of laser synthesized MnO 2 witnessing the elements Mn and O. (I) Large area HRTEM image, inset having SAED pattern of laser synthesized MnO 2 .(J) Zoomed-in HRTEM image of region 1 marked in (I).(G-K) Zoomed-in atomic image from the marked region 2 in (I).(I, M) Atomic line profile of (J) and (K) along two directions marked by corresponding colors.(N) XRD pattern consist of main peaks at 12.34 • , 24.46 • , 37.12 • and 65.67 • .(O) chemical identification was done by Raman spectroscopy which bestows two major peaks at 173 and 634 cm −1 and (P) Tauc plot of laser synthesized 2D-MnO 2 .
Digital image of an aqueous dispersion containing sonochemically exfoliated MnO 2 sheets.(B) FESEM images of layered MnO 2 were obtained by sonochemical exfoliation.(C, D) AFM image along with its line profile.(E) The XRD pattern confirms the δ-MnO 2 phase having peaks at 12.49 • , 24.12 • , 37.09 • , and 65.41 • .(F) The Raman spectrum has one major peak at 636 cm −1 .(G) Tauc plot of 2D-MnO 2 .(H) Elemental mapping of TEM images illustrating the presence of manganese and oxygen.(I) large area HRTEM image of 2D-MnO 2 .(J) Zoomed-in image of region 1 marked in image (I) showing hexagonal arrangements of atoms.(K) Average interatomic distances in the image (J) along two directions as marked by blue and cyan color were 0.2672 and 0.3044 nm, respectively.

F
I G U R E 4 (A) Schematic diagram of ammonia sensing.(B) Ammonia gas sensing of 2D MnO 2 with concentrations 26.4,52.8, 79.4, and 105.4 ppm.(C) Strain sensing of synthesized MnO 2 sheets.(D) Camera image of Arduino microcontroller.(E) The output voltage versus time plot of the cell with LiCl and LiCl + MnO 2 as electrolytes.(F) The output voltage versus time plot of the cell with LiCl + S and LiCl + S + MnO 2 as electrolytes.