Hydrothermally Synthesized h-MoO3 and Î±-MoO3 Nanocrystals: New

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Hydrothermally Synthesized h-MoO and #-MoO Nanocrystals: New Findings on Crystal Structure Dependent Charge Transport Angamuthuraj Chithambararaj, N. Rajeswari Yogamalar, and Arumugam Chandra Bose Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01571 • Publication Date (Web): 03 Feb 2016 Downloaded from http://pubs.acs.org on February 4, 2016

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Crystal Growth & Design

HYDROTHERMALLY SYNTHESIZED h-MoO3 AND

α-MoO3

FINDINGS

ON

NANOCRYSTALS: CRYSTAL

NEW

STRUCTURE

DEPENDENT CHARGE TRANSPORT A. CHITHAMBARARAJa, N. RAJESWARI YOGAMALARb AND A. CHANDRA BOSEa, † a

NANOMATERIALS LABORATORY, DEPARTMENT OF PHYSICS, NATIONAL

INSTITUTE OF TECHNOLOGY, TIRUCHIRAPPALLI – 620 015, INDIA. b

DEPARTMENT OF PHYSICS, VELAMMAL ENGINEERING COLLEGE, SURAPET,

CHENNAI 600 066, INDIA KEYWORDS.Hexagonal-MoO3, Orthorhombic, molybdenum oxide, Hydrothermal method, Structural transformation, Electrical transport, Impedance spectroscopy ABSTRACT The charge transfer characteristics of meta-stable phase hexagonal molybdenum oxide (h-MoO3) and stable phase orthorhombic MoO3 (α-MoO3) nanocrystals has been investigated for the first time using impedance spectroscopy. The results imply that the meta-stable phase h-MoO3 displays a 550 fold increase (at 150 °C) in the electrical conductivity than the stable phase α-MoO3. The

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conductivity also increases as the temperature increases from 130 °C to 170 °C whereby analysis shows a thermal activation energy (Ea) of ~ 0.42 eV. The investigation clearly identifies that the presence of intercalated ammonium ions (NH4+) and crystal water molecules (H2O) in the internal structure of h-MoO3 plays a vital role in enhancing the charge transfer characteristics and showing an ionic conductive nature. Before the impedance investigations, the h-MoO3 and α-MoO3 nanocrystals were successfully synthesized through a wet chemical process. Here, a controlled one step hydrothermal route adopted to synthesize stable phase α-MoO3 nanocrystals sequentially from meta-stable phase h-MoO3 nanocrystals. The hydrothermal reaction conditions such as choice of precipitant, amount of precipitant, reactant solvent medium, reaction time and reaction temperature take part in a significant role in defining the crystal structure, crystallite size, and particle morphology. Based on the crystal structure, size and morphology evolution with respect to the hydrothermal reaction conditions, the possible formation mechanism of MoO3 nanocrystals is proposed. INTRODUCTION Molybdenum trioxide (MoO3) has attracted considerable interest as a promising candidate for broad technological applications such as batteries, super capacitors, chromic devices, gas sensors and catalysis due to its structure, size and shape dependent material properties.[1-14] The crystalline MoO3 has three polymorphous: thermodynamically stable orthorhombic (α-MoO3) phase and two meta-stable phases such as monoclinic (β-MoO3) and hexagonal (h-MoO3). Figure S1 shows the polymorphous of MoO3 and their crystal alignment. The phase stability and the corresponding crystal structure of MoO3 has been determined by the position of MoO6 octahedra known as the basic building unit of MoO3, where molybdenum atom coordinates with six oxygen atoms to form MoO6 octahedra. As shown in the Fig. S1 (a), α-MoO3 has an orthorhombic crystal structure


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consisting of unique double layers of distorted MoO6 octahedral units. The distorted MoO6 octahedra are held together by covalent forces along the a- [100] and c- [001] directions (joined by sharing edges to form zigzag rows) and by weak van der Waals forces along the b- direction [010] (connected by corners to form layers).[15-17] The monoclinic structure of β-MoO3 is markedly different from the crystal structure of α-MoO3 and is related to the cubic Rhenium trioxide (ReO3) structure. Here, the MoO6 octahedral units share corner oxygen atoms in the direction of the c-axis and edge sharing occurs in the direction of the a-axis (Fig. S1 (b)).[18-20] In addition to the above two phases, another form of MoO3 is the hexagonal phase (h-MoO3). The phase is built up by the zigzag chains of MoO6 octahedra linked to each other by corner sharing along the c-axis. The phase is generally formulated as (A2O)x·MoO3·(H2O)y, where A = alkali metal ions or ammonium ions. The exact values of x and y depend on the details of the preparation and subsequent treatment. Here, a different connectivity between chains (connecting through adjacent rather than through opposing oxygens) gives rise to the hexagonal symmetry. This results in the formation of hexagonal tunnels (radius of sphere ~ 1.5 Å to 1.6 Å) comprising the alkali metal ions and large 1D channels as schematically represented in the Fig. S1 (c).[21-23] Thus, the thrust towards the selective phase synthesis of nano-sized MoO3 particles with well-defined morphologies and attractive material properties have been growing over a decade. Despite Mo-O of different crystal structures explored in literature; the understanding on the formation chemistry of MoO3 and structural information have been scarce and somewhat inadequate. Recently, we have developed a simple method to synthesize MoO3 by conventional precipitation and solid state methods under varying reaction parameters.[24-25] Interestingly, the synthesis reaction conditions favors the formation and growth of meta-stable phase h-MoO3 or stable phase α-MoO3 alone. Also, an additional process of calcinations is required to structurally transform the ordered hexagonal phase


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to highly stable orthorhombic phase. The heat treatment process inevitably have resulted in an uncontrolled particle growth and particle morphology.[26] Thus, the synthesis of such distinct crystallographic features becomes quite important. It is a complicated process as it requires fundamental understanding of the interfacial interactions among the solid-state and the solution chemistry during the synthesis processes.This motivated our interest to develop a facile and single step approach towards the synthesis of meta-stable and stable phase MoO3 material. The hydrothermal method is one of the most extensively employed, promising a solution based chemical method and is used for the synthesis of all kinds of nanostructured materials. The method has outstanding advantages like easy control over the crystal structure (meta-stable and stable crystal structures), morphology and size by tuning the different reaction parameters, such as reactant source, reaction temperature, reaction time, reactant solvent medium, and additives. Moreover, the synthesis is environmental friendly and inexpensive; giving high degree of chemical homogeneity on the molecular scale.[27-31] Thus, an autoclave mediated hydrothermal method has been adopted to synthesize meta-stable h-MoO3 and stable α-MoO3 nanocrystals sequently and selectively. Further, the paper systematically investigates the role of experimental parameters such as choice of precipitant, amount of precipitant, reactant solvent medium, reaction time and reaction temperature on the formation of different crystal structures of MoO3 nanocrystals. The physical and thermal characterizations are carried out using X-Ray Diffraction (XRD), Fourier Transform Infrared (FTIR) Spectroscopy, Field Emission Scanning Electron Microscopy (FE-SEM), Transmission Electron Microscopy (TEM), Selected Area Electron Diffraction (SAED), Energy Dispersive X-ray Spectrscopy (EDS), elemental mapping and Thermo Gravimetric Analysis (TGA). More importantly, the charge transfer behaviour of different crystal phases of MoO3 are not yet explored and thus greater attention has to be paid to gain better understanding on the


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electrical conduction properties in MoO3 material. For the first time, a correlation between the structural properties of MoO3 and the charge transfer behaviour are established. Moreover, the frequency and temperature dependence of the electrical response are evaluated using impedance spectroscopy (IS) analysis. EXPERIMENTAL PROCEDURE CHEMICALS Ammonium hepta-molybdate (AHM), (NH4)6Mo7O24.4H2O, concentric nitric acid (HNO3), hydrochloric acid (HCl), deionized water (H2O), ethanol (C2H5OH) and heptane (C7H16) were purchased from Merck and the reagents were used without further purification. For the synthesis of MoO3 nanocrystals, 2.43 g of AHM was dissolved in 10 mL of deionized water. After stirring for 15 min, 3 mL of concentric HNO3 was added slowly in the aqueous AHM solution. The reactant mixture was then transferred to Teflon-lined stainless steel autoclave (100 mL) and heated at 90 °C for 3 h. The system was then allowed to cool naturally to room temperature. The obtained precipitate was collected by centrifugation and washed several times with deionized water and ethanol. Finally, the powder was dried in vacuum oven at 70 °C for 12 h. To optimize the synthesis conditions and establish a control on the crystal structure, crystallite size and morphology, the synthesis conditions were tuned and summarized in the Table 1. INSTRUMENTATION The crystallographic structures of the as-synthesized samples were determined with Ultima III Rigaku X-ray diffractometer at a scanning rate of 0.2°/min in the range between 5° and 60° with Cu Kα1 radiation (1.5406 Å) operated at 40 kV and 35 mA. The crystallite sizes (Dhkl) of the


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synthesized samples were estimated using Scherrer’s method[14] from the width of the most intense reflections: (021) for hexagonal and (040) for orthorhombic phase. The morphological studies and elemental analysis were carried out using FE-SEM (Hitachi S-4800 and S-3000) operated at an accelerating voltage of 5 kV - 10 kV and TEM (JEOL JEM FXII 2000) operated at an accelerating voltage of 100 kV. FTIR spectra were recorded by Perkin Elmer, Spectrum RX1 spectrometer in the range of 4000 cm-1 - 400 cm-1 using KBr as the standard reference sample. The differential thermal analysis (DTA)/differential thermal gravimetric (DTG) and thermo gravimetric analysis (TGA) were studied using EXSTAR6200 thermal analyzer and the measurements were performed under nitrogen atmosphere at a heating rate of 10 °C/min from room temperature to 550 °C. The AC impedance measurements were obtained by Solartron-1260, impedance/gain-phase analyzer with AC amplitude of 1 V at the frequency range between 1 MHz and 1 Hz and the measurement temperature was varied from 130 °C to 170 °C with 10 °C interval. Pellets were prepared by weighting 0.3 g of MoO3 powders and by pressing uniaxially at 5 ton/cm2 for 1 min. Silver paste was applied on both sides of the pellet and the measurements were performed in air atmosphere using the platinum electrodes. RESULTS AND DISCUSSION EFFECT OF PRECIPITANT AND AMOUNT OF PRECIPITANT Choice of precipitant (HNO3 and HCl) and the amount of precipitant (3 mL, 5 mL, 10 mL and 15 mL of HNO3) on the synthesis of MoO3 nanocrystals by hydrothermal method were examined initially (ref. Table 1). Fig. S2 (a-d) shows the XRD patterns of powder samples synthesized for various amount of HNO3. The samples synthesized with 3 mL, 5 mL and 10 mL HNO3 (Fig. S2 (a-c)) show identical diffraction patterns. The strong diffraction peaks at 2θ = 9.7°, 19.6°, 25.99°


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and 29.5° are indexed as (100), (200), (210) and (300) crystal planes of h-MoO3 phase, respectively with reference to the JCPDS data card no. 21-0569. The obtained samples are single crystalline hexagonal phase without any secondary phases. On the contrary, in the XRD pattern of sample synthesized with 15 mL HNO3 (Fig. S2 (d)), additional peaks are observed at 12.85°, 25.9° and 39.1°, corresponding to the (020), (040) and (060) crystal planes of α-MoO3 phase, respectively with reference to the JCPDS data card no. 35-0609. The results clearly demonstrate that the reactions carried out with 3 mL, 5 mL and 10 mL HNO3 tend to develop primary h-MoO3 nuclei, while the reaction using 15 mL HNO3 forms the secondary crystal phase α-MoO3 nuclei. The relative diffraction peak intensity and estimated average crystallite size as a function of amount of HNO3 are depicted in Fig. S3. In order to calculate the relative diffraction peak intensity, two diffraction peaks are considered. For h-MoO3, diffraction peaks corresponding to (210) and (100) planes are considered while for α-MoO3, (040) and (020) planes are used. Increasing the amount of HNO3 from 3 mL to 10 mL, the relative intensity of the diffraction peaks corresponding to hMoO3 and the estimated average crystallite size values (59 nm to 39 nm) are gradually decreased which implies the hindrance of the crystal formation and growth of h-MoO3. Moreover, for 15 mL HNO3, the synthesis condition promotes the formation of α-MoO3 with large crystallite size (42 nm) which can be inferred from the disappearance of h-MoO3 diffraction peaks and the emergence of α-MoO3 diffraction peaks with high intensity. Thus, the observed enhancement in crystallinity and average crystallite size for high amount of HNO3 can be assigned to the resultant of phase transformation in MoO3 material. The low and high magnified SEM images of the samples synthesized under varying amount of HNO3 are displayed in Fig. S4. When the amount of HNO3 is kept as 3 mL, the sample comprises of irregular hexagonal rods with wide particle size distribution ranging between 4 μm


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and 5 μm (Fig. S4 (a-c)). The corresponding TEM image (inset of Fig. S4 (b)) shows that the particles are one dimensional (1D) rods and are distributed randomly. Increasing the HNO3 amount to 5 mL and 10 mL, monodispersed hexagonal particles are obtained as shown in Fig. S4 (d-f) and Fig. S4 (g-i), respectively. In addition, the average length and the breadth of the particles obviously decrease with the increase in amount of HNO3. The length of the rods is found to be in the range of 1 μm to 2 μm and the breadth of the rods is in the range of 600 nm to 700 nm. Further, an increasing the amount of HNO3 to 15 mL, the extremely small size nanoparticles are observed in addition to large sized rectangular particles as depicted in Fig. S4 (j-l). The particle size and shape evolution are supported by TEM image as shown in the inset of Fig. S4 (k). To identify the crystal phases of these two morphologies, SAED was carried out and the resultant dot patterns are shown in Fig. S5. The hexagonal dot pattern obtained from the small sized particles corresponds to hMoO3 phase and the cubic dot pattern obtained from the large sized particles (refer S2(b)) can be indexed as α-MoO3 phase. To affirm the significance of the precipitant, 5 mL HCl was utilized and the same experimental studies were performed and compared. Fig. S6 and Fig. S7 show the XRD pattern and SEM images of the sample synthesized with HCl. The XRD result shows the existence of single phase h-MoO3 without any secondary phases. The particle morphology analysis shows the appearance of irregular and broken hexagonal rods which are distributed randomly. Thus, the hydrothermal synthesis reaction performed with HCl, promotes inhomogeneous growth and random dispersion of hexagonal rods. From the above results, we clearly conclude that the crystal structure, crystallite size, morphology and distribution of the MoO3 particles are highly dependent on HNO3 and the amount. HNO3 with an appropriate amount (in our case, 5 mL or 10 mL HNO3)


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offers a suitable reaction environment for the synthesis of h-MoO3 crystallites with 1D hexagonal shape. EFFECT OF REACTANT SOLVENT MEDIUM Besides the choice of precipitant and amount of precipitant, the role of reactant solvent medium on the synthesis of MoO3 crystallites was further investigated using water, ethanol and heptane. The Fig. S8 (a-c) shows the XRD patterns of the samples synthesized under water, ethanol and heptanes, respectively. Using water and ethanol as hydrothermal reactant solvent medium, the formation of mata-stable phase h-MoO3 occurs. Whereas, in the case of heptane, the synthesis condition leads to the formation of stable α-MoO3 crystal phase. Also, the crystallinity (relative peak intensity variation) and the average crystallite size (peak broadening analysis) varies with respect to reactant solvent medium (Fig. S9). The reaction carried out using water shows better crystallinity and the crystallite size is estimated to be 53 nm. Whereas, for ethanol and heptane, the relative intensity of diffraction peaks tends to decrease, implying the reduced crystallinity of the samples. Further, the crystallite size is found to be 48 nm and 43 nm, respectively. The SEM images of the samples (Fig. S10) clearly depicts that in the case of water (Fig. S10 (a-c)) and ethanol (Fig. S10 (d-f)), monodispersed and well-defined hexagonal rods are obtained. The particles are highly dense and show particle size distribution ranging between 600 nm and 700 nm. On the other hand, with heptane as reactant solvent medium, highly agglomerated and relatively very thin rod-like particles having large aspect ratio are formed (refer Fig. 5(g-i)). Thus, the characterization results reveal the significance of the reactant solvent medium in tuning the formation of MoO3 nanocrystals. EFFECT OF REACTION TIME AND REACTION TEMPERATURE


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To study the impact of reaction time and temperature on crystal phase, crystallite size and morphology of MoO3, a series of time and temperature dependent experiments were carried out while other hydrothermal reaction parameters were kept constant. Fig. 1 represents the XRD patterns of powder samples synthesized under various reaction times and temperatures. The XRD patterns of samples synthesized at 90 °C and 120 °C for different reaction times (Fig. 1 (a-h)) exhibit pure and single phase h-MoO3. However, on increasing the reaction temperature to 150 °C, two different crystal phases are observed. The samples synthesized at 150 °C for short reaction time such as 3 h and 6 h (Fig. 1 (f and g)), the patterns show the formation of h-MoO3, while increasing the reaction time to 9 h and 12 h (Fig. 1 (h and i)), the conditions lead to partial phase transformation from h-MoO3 to α-MoO3 which are confirmed from the new peaks appearing at 12.85°, 25.9° and 39.1°. Further, increasing the hydrothermal reaction temperature to 180 °C, the diffraction peaks intensity of h-MoO3 decline while the intensity of diffraction peaks corresponding to α-MoO3 raise. Such mixed phases were completely convert to pure and single phase α-MoO3 as the reaction temperature was increased to 210 °C, confirming that the higher hydrothermal reaction temperatures favor the formation of α-MoO3. Additionally, it is worth to note that for the samples synthesized at 210 °C for 3 h, 6 h, 9 h, and 12 h, the XRD patterns show narrowing of (0b0) plane with increasing reaction time, suggesting that a large proportion of the crystals preferentially grow along b-direction. Fig. S11 (a-c) shows the relative diffraction peak intensity and average crystallite size with respect to the reaction time and temperature. From the results, we can observe that the reactions carried out at low reaction temperature with short reaction time yields small crystallites of MoO3 products with less crystallinity. With long hydrothermal reaction temperature and time durations, an intense diffraction peaks with narrow widths are


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obtained, suggesting the enhancement of crystallinity and increased average crystallite size of MoO3 as inferred from Table 1. Further, the phase evolution of MoO3 is well supported by the FTIR analysis. The FTIR spectra of the samples synthesized at different reaction times and temperatures are shown in Fig. 2. The samples synthesized at 90 °C for 3 h and 12 h, 120 °C for 12 h and 150 °C for 3 h, 6 h and 9 h show the characteristic vibrational peaks of h-MoO3 (Fig. 2(a-f)). Here, the Mo-O peaks are observed between 1000 cm-1 and 400 cm-1. The peaks observed at 974 cm-1 and 919 cm-1 are attributed to the stretching vibration of molybdenum atom double bonded to oxygen atom υ(Mo=O) and the peak at 603 cm-1 corresponds to Mo-O stretching bond.[32, 33] In addition to the Mo-O peaks, the peaks observed at 3445 cm-1 and 1600 cm-1 are attributed to the stretching and bending vibrations of water molecules. The peaks located at 3255 cm-1 and 1400 cm-1 are assigned to the stretching and bending vibrations of N-H related groups.[34,

35]

It is apparent that by

increasing the hydrothermal reaction time and reaction temperature such as 150 °C for 12 h, 180 °C for 12 h and 210 °C for 3 h and 12 h, the vibrational peaks corresponding to -OH and N-H gradually decline and disappear as seen in the Fig. 2 (g-j). Also, the characteristic vibrational peaks of h-MoO3 (976 cm-1, 917 cm-1 and 601 cm-1) disappeared and new peaks emerged at 993 cm-1, 875 cm-1 and 557 cm-1 are corresponding to the characteristic vibrational peaks of α-MoO3. The peak observed at 993 cm-1 is attributed to the stretching vibration of molybenyl bond υs(M=O). Two additional broad peaks at 875 cm-1 and 557 cm-1 are due to the mutual interaction of oxygen atom with two υ(O-2Mo) and three Mo metal atoms υ(O-3Mo), respectively.[36, 37] Thus, the FTIR results are well consistent with the XRD results for the phase transformation of meta-stable hMoO3 to the stable α-MoO3 phase.


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The reaction time and temperature dependent crystal morphology and microstructure details are further examined by SEM (Fig. 3, Fig. 4 and Fig. 5) and TEM (Fig. 6) analysis. From the results, it is identified that the aggregation of small sized particles and large sized incomplete micro-rods are formed at an early stage of the hydrothermal reaction (refer Fig. 3 (a-b)), at 90 °C for 3 h. With an increase in hydrothermal reaction time to 6 h, 9 h and 12 h, the irregular particles continue to grow and forms monodispersed hexagonal rods as observed from the SEM images displayed in Fig. 3 (c-h). TEM images of the sample synthesized at 90 °C for 12 h as shown in Fig. 6 (a-c) which depict that the length of the rods are in the range of micrometers and the diameter lie between 600 nm and 700 nm. Whereas, the samples synthesized at 150 °C show the presence of hexagonal rods and rectangular shaped MoO3 particles. Here, well-developed hexagonal microrods are formed at 150 °C for reaction time 3 h and the so-formed rod structure beging to collapse as the reaction time is extended to 6 h (Fig. 4 (a-d)). On further increasing the reaction time to 9 h and 12 h, incomplete and rugged hexagonal particles are developed with a small fraction of rectangular shaped particles (SEM images in Fig. 4 (e-h) and TEM images in Fig. 6 (d-f)). A complete change in morphology is observed for the samples synthesized at 210 °C for reaction time 3 h and 6 h, where all the hexagonal particles are completely transferred to rectangular particles as shown in Fig. 5 (a-d). The as-formed rectangular particles grow longer and show welldeveloped belt-like structures as the reaction time is further extend to 9 h and 12 h (Fig. 5 (e-h)). Thus, for the samples synthesized at 210 °C, a large quantity of belt-like structure with typical length of about 40 μm and a few particles in the millimeter scale are observed. Each belts exhibit almost uniform cross section along its entire length. The width of the belts is in the range of 300 nm to 400 nm as inferred from the TEM observation (Fig. 6 (g-i). In addition, the belts exhibit certain flexibility and a ripple-like contrast due to the strain resulting from the bending of the belts.


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EDS spectra and elemental mapping of MoO3 samples synthesized at 90 °C and 210 °C for 9 h are shown in Fig. S12 and Fig. S13, respectively. EDS spectrum of h-MoO3 (Fig. S12 (a)) clearly confirms the presence of oxygen (O) and molybdenum (Mo) atoms along with small fraction of nitrogen (N) atom. The content of nitrogen in the h-MoO3 is estimated to be 4.5 wt. %. Fig. S13 (a) demonstrates the elemental mapping of h-MoO3 and the bright spots corresponding to the presence of each element. The result reveals that the composition of h-MoO3 is oxygen, molybdenum and nitrogen. It further support the homogeneous distribution of Mo and O throughtout the measurement area, whereas the presence of N detected only at the limited sites which clearly evidence that a low content of nitrogen related species are successfully incorporated at its favorite sites within the MoO3 framework. On the other hand, Fig. S12 (b) and S13 (b), the EDS spectrum and elemental mapping of α-MoO3 show only elemental oxygen and molybdenum without any other signals corresponding to the existence of nitrogen in the matrix. The above results are well consistent with the XRD, FTIR and EDS data. Thus, by increasing the hydrothermal reaction time and reaction temperature, the formation of stable crystal structure of α-MoO3 from h-MoO3 with increased crystallite size (45 nm to 89 nm) and morphology evolving from1D hexagonal rods to 1D micro-belts are highly favorable. THERMAL ANALYSIS Fig. 7 (a and b) represents the TG/DTG/DTA curves for h-MoO3 and α-MoO3 samples synthesized at 90 °C and 210 °C for reaction time 12 h, respectively. From the TGA curves, the total mass loss is measured and found to be 7.4 % and 0.6 % for h-MoO3 and α-MoO3 samples, respectively. In order to find out the removal compounds, DTG is further used. The DTG curve of h-MoO3 indicates four major mass loss peaks. As expected, no mass loss peak is observed for α-MoO3.


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Based on the experimental results, the origin of mass losses in h-MoO3 can be assigned as follows: the first mass loss in the temperature between 100 °C and 150 °C is due to desorption of water molecules that are physically adsorbed on the surface of the h-MoO3 particles. The second mass loss between 200 °C and 250 °C may be related to the elimination of surface adsorbed nitrates and ammonium ions. The third and fourth mass losses recorded at 390 °C and 410 °C account for the loss of structurally intercalated ammonium ions (NH4+) and crystalline water molecules (H2O) from the internal structure of h-MoO3.[38-40] Thus, the results confirm that the presence of ammonium ions (NH4+) and water molecules (H2O) in h-MoO3 facilitate to stabilize the hexagonal phase upto 400 °C meanwhile, above this temperature the removal of the above constituents from h-MoO3 results an unstable propagation of phase transformation from h-MoO3 to α-MoO3 as identified from the presence of sharp exothermic peak at 413 °C in DTA curve. Whereas, TG and DTG curves of α-MoO3 show no such mass loss peaks confirming the absence of both ammonium ions and crystal water. Thus, from the above results, it is well identified that the α-MoO3 is the thermodynamically most stable structure of MoO3 and has been build up by MoO6 octahedral units without the presence of ammonium ions (NH4+) and crystalline water molecules (H2O). GROWTH MECHANISM OF h-MoO3 MICRO-RODS AND α-MoO3 MICRO-BELTS From the above experimental results, one of the main observations inferred is the structural phase transformation from hexagonal to orthorhombic MoO3 by varying the hydrothermal reaction conditions. Under condition I as represented in Fig. S14 (low HNO3 amount, for water and ethanol as reactant solvent medium, low reaction time and temperature), the crystallization of hexagonal phase takes place through the self-assembly of MoO6 with the assistance of NH4+ and H2O. From the structural point of view, the NH4+ and H2O are incorporated in the active growth sites within the MoO3 framework that act as structure directing agents contributing higher free energy content


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along c-axis as depicted in Fig. 8 under the condition I. Changing the hydrothermal reaction conditions from condition I to condition II (large HNO3 amount, for heptanes as reactant solvent medium, high reaction time and temperature), the density, acidity, dielectric nature and ionic product of supercritical fluid are altered, creating an extreme pressure in the autoclave thus, allowing one to vary the reaction equilibrium as well as surface free energy. Under these conditions, the supercritical fluid provides substantial amount of driving force for the removal of structure directing agents such as NH4+ and H2O from the internal structure of h-MoO3. This involves a rapid release of the stored chemical potential, resulting in lattice relocation through dissolution and recrystallization in-situ. The process leads to a change of crystal symmetry from meta-stable h-MoO3 to stable α-MoO3. Finally, the continuous growth yields long 1D micro-belt like structures of α-MoO3 as schematically represented under the condition II (Fig. 8). Based on the above discussion, the complete growth mechanism of α-MoO3 includes (i) the formation of hMoO3, (ii) removal of H2O / NH4+ ions, (iii) dissolution of h-MoO3 and recrystallization of αMoO3 and (iv) Ostwald ripening process for the final growth of long micro-belts.[41] On the other hand, it reveals that there are several other factors (choice of precitant, amount of precipitant, reactant solvent medium, reaction time and reaction temperature) responsible for the final crystal structure (meta-stable hexagonal to stable orthorhombic), crystallite size (39 nm to 86 nm) and morphology (hexagonal rod to micro-belts) of MoO3 crystals. In general, tuning the hydrothermal reaction conditions such as ionic strength, coordination environment and the interactions of anions (NO3− and Cl−) with the precursors (choice of precipitant); number and surface coverage of fraction of reactants (amount of precipitant); polarity (ethanol – polar solvent, water – neutral solvent and heptane – non-polar solvent), dielectric constant, boiling point, viscosity and density (reactant solvent medium); kinetic energy (reaction time) and thermodynamic energy (reaction temperature)


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affect the various solution state reaction process including the reaction affinity, reactant collision, diffusion, surface tension and migration. The above factors determine the rate of solubility, degree of supersaturaton, surface and interfacial energy, dissociation and association process that strongly affect the reaction, formation, mass transfer and growth rate of crystals which has the tendency to define the crystal structure, morphology and final growth of MoO3 particles.[25, 42, 43] IMPEDANCE PROPERTIES To understand the charge transfer characteristics of h-MoO3 and α-MoO3, frequency and temperature dependent electrical measurements were carried out using impedance spectroscopy (IS) analysis. The measurements were performed at various temperatures from 130 °C to 170 °C in a wide frequency range of 5 MHz to 100 Hz. Fig. 9 and Fig. 10 show the typical Bode plots (real (𝑍𝑍 ′ ) and imaginary (𝑍𝑍 ′′ ) Vs log frequency (log(𝑓𝑓))) and Nyquist plots (𝑍𝑍 ′ Vs 𝑍𝑍 ′′ ) of h-MoO3

and α-MoO3 samples under various operating temperatures, respectively. The Bode plots of hMoO3 (Fig. 9 (a)) show two non-plateau regions (regions 1 and 3, where there was a linear increase of 𝑍𝑍 ′ ) and one plateau region (regions 2, where the 𝑍𝑍 ′ remains relatively constant) with respect to

decreasing frequency. The imaginary part of impedance data demonstrates two maxima (𝑍𝑍 ′′ 𝑚𝑚𝑚𝑚𝑚𝑚 ) where one observe at higher frequencies and another at lower frequency regions reveal that the

impedance behaviour of h-MoO3 is highly dependent on frequency and exhibiting two successive polarization relaxation processes. Fig. 9 (b) demonstrates the corresponding Nyquist plots of hMoO3 which consist of two successive semicircle arcs describing the successful relaxation of a particular class of charge entity. The semicircle arc observed at higher frequency region can be attributed to the successive charge transfer and relaxation due to the effective contributions of crystal interior (CI) and the another semicircle arc observe at the low frequencies can be due to


16

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effect of crystallite boundaries (CB). On the other hand, the Bode and Nyquist plots corresponding to α-MoO3 (Fig. 10 (a) and (b)) show only one non-plateau region and single semicircle arc in the entire frequency regions which confirms the existence of single polarization relaxation process occurred only by the effective contribution of crystal interior.[44, 45] The impedance spectra measured were further fitted by means of equivalent circuit model using Zview software. The equivalent circuit for h-MoO3 consists of two R and CPE circuits (inset in Fig. 9 (b)) in series with a resistor (RS) which representing two semicircle arcs. Here, R represents a resistance and CPE, a constant phase element. The first circuit in the series (RCI/CPECI) represents the effect of crystal interior and the second one (RCB/CPECB) represents the effect of crystallite boundaries. In other case, the equivalent circuit for α-MoO3 consists of single RCI/CPECI circuit (inset in Fig. 10 (b)) representes the crystal interior contribution to the overall conduction phenomenon. In the equivalent circuit model, CPE is placed instead of pure capacitance component (C) and this can be due to the depression in semicircle arcs, where the centre of the semicircle arcs shifted below the real axis. The observed depression behavior can be assigned to the non-Debye type relaxation process in the MoO3 samples. Thus, the impedance of CPE can be written as (ref. Eqn. 1), 1

𝐶𝐶𝐶𝐶𝐶𝐶 = 𝑍𝑍(𝑗𝑗ω)𝑛𝑛

(1)

where, 𝑍𝑍 is the impedance, ω is the angular frequency (2π𝑓𝑓) and 𝑛𝑛 is the coefficient used to describe the CPE behaviour. The corresponding capacitance C (CCI and CCB) can be derived from the 𝑛𝑛 and CPE values by using the Eqn. 2, 𝐶𝐶 = 𝐶𝐶𝐶𝐶𝐶𝐶(𝜔𝜔𝑚𝑚𝑚𝑚𝑚𝑚 )𝑛𝑛−1

(2)

The fitted and estimated parameter values are summarized in Table 2 and Table 3. The results demonstrate that the value of RCI at 150 °C obtained for α-MoO3 is nearly 550 times higher


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than that of RCI obtained for h-MoO3. The 𝐶𝐶 values obtained from the high and low frequency

semicircles found in the range of pF and nF, respectively, prove that the observed semicircles

represent the crystal interior and crystallite boundaries response of the system. The value of n corresponding to crystal interior for h-MoO3 varies between 0.60 and 0.68 and for α-MoO3, n changes between 0.93 and 0.98. From the above results, it is clear that the value of n closer to 1 for α-MoO3 showing an ideal capacitive behaviour whereas the low value of n in the case of hMoO3 showing non-ideal capacitive behavior. The non-ideal capacitive behavior in h-MoO3 can be assigned to the resultant of ion migration within the crystal interior which causes loss of capacitance value.[46-52] a.c. CONDUCTIVITY The angular frequency (ω) dependent conductivity (σ) at various temperatures for h-MoO3 and αMoO3 samples are shown in Fig. 11 (a) and (b), respectively. The conductivity (σ) is calculated using the imaginary part of dielectric data (𝜀𝜀′′) and the equation states that (ref. Eqn. 3), 𝜎𝜎 = 𝜀𝜀о 𝜀𝜀′′ 𝜔𝜔 𝑡𝑡𝑡𝑡𝑡𝑡 𝛿𝛿

(3)

Where, 𝜀𝜀о is the permittivity in free space and 𝜔𝜔 is the angular frequency. Two distinctly different

regions are observed in variation of 𝜎𝜎 with 𝜔𝜔 such as (i) frequency dependent linear increase in

conductivity (dispersive regions at higher frequencies) i.e. a.c. conductivity and (ii) frequency independent conductivity (plateau regions at low frequencies) i.e. d.c conductivity. In the conductivity graph of h-MoO3 (Fig. 11 (a)), the observed two plateau regions (Regions 1 and 3) and two dispersion regions (Regions 2 and 4) confirm the presence of crystal interior and crystallite boundary activated conductivities. The corresponding conductivity values show ranges in the order of 10-1 S/cm within the temperature range investigated. However, the conductivity graph in the case α-MoO3 (Fig. 11 (b)), the conductivity curves exhibit a single plateau and single dispersion


18

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region that corresponds to the contribution of crystal interior effect. Also, the conductivity values are found in the range between 10-5 S/cm and 10-3 S/cm. Thus, the freuquency dependent conductivity dispersion and frequency independent plateau region of MoO3 in an a.c. fields can be written as (ref. Eqn. 4), 𝜎𝜎(𝜔𝜔) = 𝜎𝜎𝑑𝑑𝑑𝑑 + 𝜎𝜎𝑎𝑎𝑎𝑎

(4)

where, 𝜎𝜎(𝜔𝜔) is the total conductivity of the sample, 𝜎𝜎𝑑𝑑𝑑𝑑 is the dc component, which is temperature

dependent and 𝜎𝜎𝑎𝑎𝑎𝑎 is the component of conductivity depending upon the frequency of the applied

field. This type of conductivity can be analyzed using Joncher’s power law which states that, (ref. Eqn. 5), 𝜎𝜎(𝜔𝜔) = 𝜎𝜎𝑑𝑑𝑑𝑑 + 𝐴𝐴𝜔𝜔𝑛𝑛

(5)

where, 𝐴𝐴 is a temperature dependent constant value, 𝑛𝑛 is frequency dependent power exponent roughly treated as constant less than 1 and represents the degree of interaction between mobile

ions and the environment surrounding them. The 𝑛𝑛 values for h-MoO3 lies in the ranges of 0.72 to 0.79 (Fig. S15) and increases with temperature suggests that h-MoO3 is a type of ion conducting solid and the conduction takes place through small polaron hopping (SPH) process. The high value of 𝑛𝑛 (0.99 to 0.79) and decreasing nature with temperature in α-MoO3 correspond to correlated

barrier hopping process (CBH).[53-57] d.c. CONDUCTIVY

The temperature dependent study reveals the nature of the relaxation process (shift in the value of Z"max or ωmax or σac) and the mobility of charge carriers (intercept values of Z′ with the real axis) in the MoO3 material (Fig. 9, 10 and 11). As the temperature increases, the value of Z"max or ωmax or σac shifts towards higher frequency indicating thermally activated loss in relaxation process and decrese in hopping length. On the other hand, the values of Z′ decreases with temperature can be


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assigned to the increased number of thermal activated mobile charge carriers and suggesting the existence of negative temperature coefficient of resistance (NTCR) behavior. The temperature dependent relaxation time and d.c conductivity were determined using the relation ((ref. Eqn. 6 and Eqn. 7), 𝜏𝜏 = 𝜔𝜔

1

(6)

1 𝑙𝑙

(7)

𝑚𝑚𝑚𝑚𝑚𝑚

𝜎𝜎𝑑𝑑𝑑𝑑 = 𝑅𝑅 𝐴𝐴

Where, 𝑙𝑙 is the thickness and 𝐴𝐴 is the area of the sample pellet. The corresponding activation

energies for relaxation time (Eaτ) and for d.c. conduction (Eaσ) were calculated using the Arrhenius law ((ref. Eqn. 8 and Eqn. 9), 𝜎𝜎𝑑𝑑𝑑𝑑 𝑇𝑇 = 𝜎𝜎0 exp( 𝜏𝜏𝜏𝜏 = 𝜏𝜏0 exp(

−𝐸𝐸𝑎𝑎𝑎𝑎 𝐾𝐾𝐾𝐾

−𝐸𝐸𝑎𝑎𝑎𝑎 𝐾𝐾𝐾𝐾

)

)

(8) (9)

where, σ0 and τ0 are the pre-exponential factor, K is the Boltzmann constant, and T is absolute temperature. By the linear fit of the temperature dependent relaxation data (log(τT) versus 1000/T) and conductivity data ((log(σT) versus 1000/T)) (Fig. 12 (a) and (b)), the activation energies for relaxation time (Eaτ) and for d.c. conduction (Eaσ) for crystal interior conduction in h-MoO3 and αMoO3 are estimated. It is noted that the Eaσ matches well with the Eaτ of h-MoO3 and α-MoO3. A close resemblance of the activation energies indicate that the same type of charged carriers may be involved in both conduction and relaxation processes.[58-62] CHARGE TRANSFER MECHANISM From the above results, it is clealy identified that the conductivity behaviour of meta-stable phase h-MoO3 is nearly 550 times higher than that of stable phase α-MoO3. Based on the conductivity behaviour in MoO3 of two different crystal structures, the mechanism can be quantitatively


20

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explained by ion diffusion model for h-MoO3 and polarization model for α-MoO3. The observed superior change transport characteristics in meta-stable phase h-MoO3 than stable phase α-MoO3 is predominantly by the mobile species i.e. the presence of intercalated ammonium ions (NH4+) and crystal water molecules (H2O) in h-MoO3. Upon applying a.c. frequencies, the easy migration and diffusion of NH4+ ions through the channels, cavities and crystallite boundaries of the h-MoO3 framework which result in super ionic conductive nature of h-MoO3. Here, the interactions of NH4+ ions and H2O molecules with the MoO6 octahedra form non-periodic free energy barriers and shallow potential barrier of low free energy thus facilitate a by-pass channel for charge transfer kinetics and allow the facile diffusion of ions through the matrix as schematically represented in Fig. 13. Based on the above results, the following scheme has been proposed for the conduction mechanism occurring in h-MoO3 (i) via low frequency dependence of ionic transport due to the motion of NH4+ ions and crystal water molecules which are located between MoO6 octahedral units (ii) via high frequency dependence of dipole conduction corresponds to coordination between Mo-O and (iii) via temperature dependence of electronic conduction. Whereas in the case of stable phase α-MoO3, the absence of NH4+ and crystal water units in the crystal structure of α-MoO3 promotes only polarization related AC conductivities and thus result high resistive behavior than that of meta-stable phase h-MoO3. Thus, the scheme for the conduction mechanism occurring in α-MoO3 becomes (i) via frequency dependence of dipole conduction corresponds to coordination between Mo-O and (ii) via temperature dependence of electronic conduction where, the equilibrium sites are of equal and high energy barrier and are spaced regularly in the crystal lattice (ref. Fig. 13). Thus, the above results strongly confirm the intercalation of ammonium ions (NH4+) and crystal water molecules (H2O) in MoO3 sample directly reflects the changes in the crystal phase, phase stability and electrical properties of the sample. The presence of NH4+ ions and crystal


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water molecules in the meta-stable phase h-MoO3 exhibit super ionic conductive behaviour with substantially higher charger transfer characteristics than that of stable phase α-MoO3. To the best of our knowledge, this is the first attempt to perform impedance spectroscopy measurements to correlate structural properties of MoO3 and their charge transfer kinetic behaviour. CONCLUSIONS We have succeeded in the selective synthesis of meta-stable h-MoO3 and stable α-MoO3 nanocrystallites through a facile single step autoclave mediated hydrothermal method under well defined conditions. Their characterizations and the study of their charge transfer kinetic characteristics are presented and interpreted in the light of the structural information and phase transition. The overall results reveal that the distribution of monovalance cation such as NH4+ ions and crystal water (H2O) molecules in MoO3 architectures play a vital role in directing and stabilizing the meta-stable hexagonal structure. We established that the h-MoO3  α-MoO3 transition is driven by the removal of H2O / NH4+ ions from the internal structure of h-MoO3. The electrical conductivity results indicate that the enhanced conductivity of meta-stable phase h-MoO3 than that of stable phase α-MoO3 is most probably associated with the presence of NH4+ ions in the matrix which promote faster ionic conduction behaviour. The overall research strongly suggest that the meta-stable phase h-MoO3 can act as a new and promising ionic conductor for future electrical devices such as batteries, supercapacitors, sensors and chromic devices. SUPPORTING INFORMATION. Supporting Information is available. This material is available free of charge via the Internet at http://pubs.acs.org


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AUTHOR INFORMATION Corresponding Author †

E. Mail.: [email protected]

ACKNOWLEDGMENT One of the author N. Rajeswari Yogamalar acknowledges UGC, New Delhi, India for the financial support via the Dr. D. S. Kothari Postdoctoral fellowship 2012 (PH/11-12/0091). REFERENCES (1)

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Brahma, S.; Choudhary, R. N. P.; Thakur, A. K. Physica B 2005, 355, 188-201.

(59)

Chandra Bose, A.; Thangadurai, P.; Ramasamy, S. Mater. Chem. Phys. 2006, 95, 72-78.

(60)

Badapanda, T.; Senthil, V.; Rout, S. K.; Cavalcante, L. S.; Simoes, A. Z.; Sinha, T. P.; Panigrahi, S.; De Jesus, M. M.; Longo, E.; Varela, J. A. Current Appl. Phys. 2011, 11, 1282-1293.

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Chithambararaj, A.; Rameshbabu, N.; Bose, A. C. Sci. Adv. Mater. 2014, 6, 1302-1312.

(62)

Das, P. R.; Pati, B.; Sutar, B. C.; Choudhury, R. N. P. J. Mod. Phys. 2012, 3, 870-880.

FIGURE CAPTIONS: Figure 1. XRD patterns of the as-synthesized samples under varying reaction time and temperature: (a-d) 90 °C for 3 h, 6 h, 9 h and 12 h; (e) 120 °C for 12 h; (f-i) 150 °C for 3 h, 6 h, 9 h and 12 h; (j) 180 °C for 12 h and (k-n) 210 °C for 3 h, 6 h, 9 h and 12 h, respectively Figure 2. FTIR spectra of as-synthesized samples synthesized under hydrothermal synthesis at (ab) 90 °C for 3 h and 12 h; (c) 120 °C for 12 h; (d-g) 150 °C for 3 h, 6 h, 9 h and 12 h; (h) 180 °C for 12 h and (i-j) 210 °C for 3 hand 12 h, respectively Figure 3. SEM images of the samples synthesized at 90 °C for (a-b) 3 h, (c-d) 6 h, (e-f) 9 h and (g-h) 12 h Figure 4. SEM images of the samples synthesized at 150 °C (a-b) 3 h, (c-d) 6 h, (e-f) 9 h and (gh) 12 h Figure 5. SEM images of the samples synthesized at 210°C (a-b) 3 h, (c-d) 6 h, (e-f) 9 h and (gh) 12 h


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Figure 6. Low and high magnified TEM images of the samples synthesized at 12 h under varying reaction temperatures: (a-c) 90 °C; (d-f) 150 °C; and (g-i) 210 °C Figure 7. TGA/DTA/DTG graphs of (a) h-MoO3 and (b) α-MoO3 samples synthesized at 90 °C and 210 °C for 12 h, respectivley Figure 8. Schematic illustration of h-MoO3 and α-MoO3 nanocrystals growth Figure 9. Frequency dependent (a) Bode and (b) Nyquist plots of h-MoO3; Inset Fig. (b) shows the equivalent circuit model Figure 10. Frequency dependent (a) Bode and (b) Nyquist plots of α-MoO3; Inset Fig. (b) shows the equivalent circuit model Figure 11. Frequency dependence AC conductivity of (a) h-MoO3 and (b) α-MoO3 as a function of temperature Figure 12. Activation energy for (a) relaxation time and (b) d.c conductivity Figure 13. Schematic illustration of charge transfer kinetics in h-MoO3 and α-MoO3


29

(060)

(021)

(040)

(110)

Page 30 of 47

α -MoO3

(n)

210 °C - 12 h

α-MoO3- JCPDS (35-0609)

(m)

210 °C - 9 h

(l)

210 °C - 6 h

(k)

210 °C - 3 h

(j)

180 °C - 12 h

h/α -MoO3

10

20

(h)

150 °C - 9 h

(g)

150 °C - 6 h

(f)

150 °C - 3 h

(e)

120 °C - 12 h

(d)

90 °C - 12 h

(c)

90 °C - 9 h

(b)

90 °C - 6 h

(a)

90 °C - 3 h

(310)

(300)

(210)

(200)

(110)

(100)

h-MoO3

150 °C - 12 h

30

40

50

(218)

h-MoO3- JCPDS (21-0569)

(i)

(008)

h/α -MoO3

(320) (410)

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

(020)


60

2θ (°)

Figure 1. XRD patterns of the as-synthesized samples under varying reaction time and temperature: (a-d) 90 °C for 3 h, 6 h, 9 h and 12 h; (e) 120 °C for 12 h; (f-i) 150 °C for 3 h, 6 h, 9 h and 12 h; (j) 180 °C for 12 h and (k-n) 210 °C for 3 h, 6 h, 9 h and 12 h, respectively


30

Page 31 of 47

O-3Mo (j)

210 °C−12 h 210 °C−3 h

O-2Mo

Mo=O (α-MoO3)

(i) (h)

180 °C−12 h

-O Hs N-H tre str tch etc ing hin g -O NHB HB en en din din g g

Transmittance (%)

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


4000 3000

1600

150 °C−12 h

(g)

150 °C−9 h

(f)

150 °C−6 h

Mo- O

Mo=O (h-MoO3)

150 °C−3 h

(e) (d) (c)

120 °C-12 h

(b) 90 °C-12 h

(a)

90 °C-3 h

1200

800

400

Wavenumber (cm-1)

Figure 2. FTIR spectra of as-synthesized samples synthesized under hydrothermal synthesis at (ab) 90 °C for 3 h and 12 h; (c) 120 °C for 12 h; (d-g) 150 °C for 3 h, 6 h, 9 h and 12 h; (h) 180 °C for 12 h and (i-j) 210 °C for 3 hand 12 h, respectively


31


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Page 32 of 47

Figure 3. SEM images of the samples synthesized at 90 °C for (a-b) 3 h, (c-d) 6 h, (e-f) 9 h and (g-h) 12 h


32

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Figure 4. SEM images of the samples synthesized at 150 °C (a-b) 3 h, (c-d) 6 h, (e-f) 9 h and (gh) 12 h


33


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Page 34 of 47

Figure 5. SEM images of the samples synthesized at 210°C (a-b) 3 h, (c-d) 6 h, (e-f) 9 h and (gh) 12 h


34

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Figure 6. Low and high magnified TEM images of the samples synthesized at 12 h under varying reaction temperatures: (a-c) 90 °C; (d-f) 150 °C; and (g-i) 210 °C


35


Wt. loss = 0.6 % 100

100 DTA

α-MoO3

crystal water

80

60

40

40

20

20

98 97 96 95

TG (% )

(b) α-MoO3

TGA

60

100 99

DTG (µg/min)

DTA (normalized) (µV)

80

94 93

DTG 0

0 92 50

100

150

200

250

300

350

400

450

500

550

Temperature (°C)

DTA

oO h-M

3

to

oO 3 α-M

H2O 123 °C

40

60

40

229 °C

370-440 °C

N-O

20

NH3 H2O

20

98 97 96 95 94 93

DTG

0

99 80

Wt. loss = 7.4 %

60

100

TG (% )

TGA 80

100

413 °C

DTG (µg/min)

(a) h-MoO3

100

DTA (normalized) (µV)

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 36 of 47

0 92

50

100

150

200

250

300

350

400

450

500

550

Temperature (°C) Figure 7. TGA/DTA/DTG graphs of (a) h-MoO3 and (b) α-MoO3 samples synthesized at 90 °C and 210 °C for 12 h, respectivley


36

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Figure 8. Schematic illustration of h-MoO3 and α-MoO3 nanocrystals growth


37


5

maximum 2

3

3

-Z"x10 (Ω)

130 °C 140 °C 150 °C 160 °C 170 °C

(a) h-MoO3

4

2

maximum 1

1 0 102

103

104

105

106

Rs+RCI+RCB

130 °C 140 °C 150 °C 160 °C 170 °C

Z'x103(Ω)

10.0 3

7.5

Rs+RCI

5.0

2

1

2.5

Rs

0.0 102

103

104

105

106

log (f) (Hz) 7.5

-Z"x103(Ω)

130 °C 140 °C 150 °C 160 °C 170 °C Fitting curve for 130 °C Fitting curve for 140 °C Fitting curve for 150 °C Fitting curve for 160 °C Fitting curve for 170 °C

5.0

(b) h-MoO3

Low Fre que ncy

2.5 Hi g Fr h eq ue nc y

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 38 of 47

Rs

0.0 0.0

Rs+RCI+RCB

Rs+RCI 2.5

5.0

7.5

10.0

12.5

15.0

Z'x103 (Ω)

Figure 9. Frequency and temperature dependent (a) Bode and (b) Nyquist plots of h-MoO3; Inset Fig. (b) shows the equivalent circuit model


38

Page 39 of 47

3

-Z"x10 (Ω)

2500

130 °C 140 °C 150 °C 160 °C 170 °C

(a) α-MoO3

2000 1500 1000 500 0 102

103

104

105

106

Z'x103(Ω)

1000

130 °C 140 °C 150 °C 160 °C 170 °C

800 Rs+RCI 2 600 1

400 200 0 102

Rs 3

10

4

5

10

10

6

10

log (f) (Hz) 500

(b) α-MoO3

400

300

200

Hi g Fr h eq ue nc y

-Z"x103(Ω)

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


100

130 °C Low 140 °C Fre 150 °C que ncy 160 °C 170 °C Fitting curve for 130 °C Fitting curve for 140 °C Fitting curve for 150 °C Fitting curve for 160 °C Fitting curve for 170 °C

0 0

200

400

600

800

RCI 1000

Z'x103 (Ω)

Figure 10. Frequency and temperature dependent (a) Bode and (b) Nyquist plots of α-MoO3; Inset Fig. (b) shows the equivalent circuit model


39


(a) h-MoO3

4 3

10-1 Crystal interior

σac (S/cm)

2

1

130 °C 140 °C 150 °C 160 °C 170 °C

10-2 101

102

103

104

ω (Hz)

105

106

107

(b) α-MoO3 10-3 2

σac (S/cm)

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 40 of 47

Crysatal interior -4

10

1

130 °C 140 °C 150 °C 160 °C 170 °C

10-5

102

103

104

105

106

107

ω (Hz) Figure 11. Frequency dependence AC conductivity of (a) h-MoO3 and (b) α-MoO3 as a function of temperature


40

Page 41 of 47

ln(τT) (sec K)

-2

(a)

-4

V 8e 1.8 = oO 3 Ea α-M

-6

) ior ter n i al yst (Cr

-8

al interior) h-MoO3 (Cryst

-10 2.20

2.25

2.30

Ea=0.425 eV

2.35

2.40

2.45

2.50

-1

1000/T (K ) -2

(b) h-MoO (Cryst al interior) 3

-4

ln(σT) (Ω-1cm-1K)

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


Ea=0.48 eV

-6

-8 α-M oO 3 (Cr Ea=1.68 ysta eV l int erio r)

-10

-12 2.25

2.30

2.35

2.40

2.45

2.50

-1

1000/T (K )

Figure 12. Activation energy for (a) relaxation time and (b) d.c conductivity


41


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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 ACS Paragon Plus Environment

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Figure 13. Schematic illustration of charge transfer kinetics in h-MoO3 and α-MoO3 Table 1. Synthesis conditions and the resultant crystal phase, morphology, and crystallite size Precipitant / HNO3 reactant solvent (mL) medium HCl/water

HNO3/water

Temp. (°C)

Time (h)

Crystal phase

Morphology

Crystallite Size, Dhkl (nm)

90

9

Hexagonal

Distorted hexagonal rods

60

3

Hexagonal

59

5

Hexagonal

Distorted and Nonuniform hexagonal rods Uniform hexagonal rods Non-uniform hexagonal rods and layered rods

39

Uniform hexagonal rods Widely distributed hexagonal rods Agglomeration and Non-hexagonal rods

53

Distorted and Nonuniform hexagonal rods Uniform hexagonal rods Distorted and Nonuniform hexagonal rods Disordered hexagonal structure Non-uniform hexagonal rods and rectangular particles Non-uniform hexagonal rods and rectangular particles Non-uniform hexagonal rods and layered rods

48 48

5

10

90

9

15

Hexagonal Hexagonal /Orthorhombic

HNO3/Water

Hexagonal

HNO3/ethanol

Hexagonal

HNO3/heptane

5

90

9

Hexagonal /Orthorhombic

90

3 6

Hexagonal Hexagonal

9 12 12 3

Hexagonal Hexagonal Hexagonal Hexagonal

6

Hexagonal

9

Hexagonal

12

Hexagonal /orthorhombic

12

Hexagonal /Orthorhombic

3 6 9

Orthorhombic Orthorhombic Orthorhombic

120

150 HNO3/water

5

180

210

Layered micro-belts

52

42

48 43

53 56 53 43 45 46

64

60

52 56 65


43


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12

Orthorhombic

Page 44 of 47

86


44

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Table 2. The estimated resistance, constant phase element and n values corresponding to crystal interior and crystallite boundaries of h-MoO3 synthesized at 90 °C for 9 h.

Temp. (°C)

Crystal interior 𝑹𝑹𝑪𝑪𝑪𝑪

x103 (Ω)

𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪 x10-9

𝒏𝒏

Crystallite boundary 𝑪𝑪𝑪𝑪𝑪𝑪

(pF)

(F)

𝑹𝑹𝑪𝑪𝑪𝑪

𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪

x103

𝒏𝒏

x10-7

(Ω)

𝑪𝑪𝑪𝑪𝑪𝑪

(nF)

𝑬𝑬𝒂𝒂𝒂𝒂

(eV)


(eV)

(F)

130

3.14 9.49

0.6833

77.21 12.33 1.76

0.86405 68.86

140

2.30 15.1

0.6594

79.00 9.09

2.01

0.85372 68.45

150

1.68 23.3

0.63648 72.41 6.89

2.38

0.83816 67.09 0.48 0.425

160

1.27 30.3

0.62189 68.46 5.51

2.69

0.82629 69.10

170

0.99 40.5

0.60451 63.13 4.58

3.15

0.80979 68.07

Table 3. The estimated resistance, constant phase element and n values corresponding to crystal interior of α-MoO3 synthesized at 210 °C for 9 h. Temp. (°C)

Crystal interior 𝑹𝑹𝑪𝑪𝑪𝑪

x103 (Ω)

𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪 x10-11

𝒏𝒏

𝑪𝑪𝑪𝑪𝑪𝑪



(pF) (eV) (eV)

(F)

130

19000.20 3.23

0.97743 27.24

140

5000.03

3.69

0.96944 28.14

150

930.00

4.16

0.96365 28.36 1.68 1.88

160

260.00

4.85

0.95608 28.89

170

170.00

6.90

0.93537 31.42


45


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Page 46 of 47

“For Table of Contents Use Only”

Hydrothermally Synthesized h-MoO3 and α-MoO3 Nanocrystals: New Findings on Crystal Structure Dependent Charge Transport A. Chithambararaja, N. Rajeswari Yogamalarb and A. Chandra Bosea, † a

Nanomaterials Laboratory, Department of Physics, National Institute of Technology, Tiruchirappalli – 620 015, iIdia.

B

Department of Physics, Velammal Engineering College, Surapet, Chennai 600 066, India

The crystallization of hexagonal phase takes place through the self-assembly of MoO6 with the assistance of NH4+ and H2O. The NH4+ and H2O are incorporated in the active growth sites within the MoO3 framework that act as structure directing agents. However, the removal of structure directing agents such as NH4+ and H2O from the internal structure of h-MoO3 leads to a change of crystal symmetry from meta-stable h-MoO3 to stable α-MoO3.


46

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Impedance of h-MoO3 and α-MoO3 295x261mm (150 x 150 DPI)


Hydrothermally Synthesized h-MoO3 and Î±-MoO3 Nanocrystals: New

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