Structural characterizations of photo-catalytic titanium oxide nanoparticles made from amorphous building blocks

TiO2 [titanium dioxide] nanoparticles (NPs) exhibit a variety of properties substantially departed from the optical and catalytic properties of the known bulk phases. A special interest in the engineering of functionality is how structures and bandgaps interact to enhance the photo-activity of TiO2. However, fundamental understanding of bandgap-related changes is difficult at these length scales because of scarcity of methods capable of probing the bandgap and the structure of isolated small particles. In this research, we adopt a series of highly spatially resolved transmission electron microscopy (TEM) techniques to characterize the structures of amorphous TiO2 nanoparticles (NPs) used as precursors and the TiO2 NPs with narrowed band gap derived from these amorphous precursors. The results show: (1) The amorphous TiO2 precursors consist of TiO6 [titanium atom surrounded by six oxygen atoms] octahedra randomly connected with each other; (2) An unconventional phase transformation occur wherein anatase and TiO2(B) coexist when annealing the amorphous TiO2 NPs in a temperature range from 400°C to 900°C in oxygengas; (3) Chromium and nitrogen co-doped TiO2 NPs have significantly narrowed band gaps originating from extra states on the top of valence band; (4) Black rutile NPs are produced by annealing amorphous TiO2 NPs at 700 °C in argon gas, which have a (crystalline TiO2) core-(amorphous Ti2O3 [titanium sesquioxide]) shell structure. Formation of Ti2O3 originates from diffusion of oxygen vacancies towards vacuum in the amorphous precursors; (5) The black anatase NPs are produced by annealing amorphous TiO2 NPs at 400 °C in argon gas, whose structures are (i) core-shell, or (ii) randomly distributed phases. These two structures originate from different degrees of crystallization and diffusion rate of oxygen vacancies; (6) Cubic-TiO2 is induced by the electron beam, which originates from the migration of interstitial Ti atoms grown on a rutile template; (7) We also discuss the origins of deviations from known phases’ EELS spectra which is the most important tool of electronic structure characterizations in this research; (8) Finally, we found a group of intense peaks in the interface between silicon and TiO2. These peaks may originate from interface plasmon excitations

Hybrid Nanocomposites of Nanostructured Co3O4 Interfaced with Reduced/Nitrogen-Doped Graphene Oxides for Selective Improvements in Electrocatalytic and/or Supercapacitive Properties

Performance enhancements in next-generation electrochemical energy storage/conversion devices require the design of new classes of nanomaterials that exhibit unique electrocatalytic and supercapacitive properties. To this end, we report the use of laser ablation synthesis in solution (LASiS) operated with cobalt as the target in graphene oxide (GO) solution in tandem with two different post treatments to manufacture three kinds of hybrid nanocomposites (HNCs) namely, (1) Co3O4 nanoparticle (NP)/reduced graphene oxide (rGO), (2) Co3O4 nanorod (NR)/rGO, and (3) Co3O4 NP/nitrogen-doped graphene oxide (NGO). FTIR and Raman spectroscopic studies indicate that both chemical and charge driven interactions are partially responsible for embedding the Co3O4 NPs/NRs into the various GO films. We tune the selective functionalities of the as-synthesized HNCs as oxygen reduction reaction (ORR) catalysts and/or supercapacitors by tailoring their structure–property relationships. Specifically, the nitrogen doping in the NP/NGO HNC samples promotes higher electron conductivity while hindering aggregation between 0D CoO NPs that are partially reshaped into Co3O4 nanocubes due to induced surface strain energies. Our results indicate that such interfacial energetics and arrangements lead to superior ORR electrocatalytic activities. On the other hand, the interconnecting 1D nanostructures in theNR/rGO HNCs benefit charge transport and electrolyte diffusion at the electrode–electrolyte interfaces, thereby promoting their supercapacitive properties. The NP/rGO HNCs exhibit intermediate functionalities towards both ORR catalysis and supercapacitance

Antiferroelectric negative capacitance from a structural phase transition in zirconia

Crystalline materials with broken inversion symmetry can exhibit a spontaneous electric polarization, which originates from a microscopic electric dipole moment. Long-range polar or anti-polar order of such permanent dipoles gives rise to ferroelectricity or antiferroelectricity, respectively. However, the recently discovered antiferroelectrics of fluorite structure (HfO$_2$ and ZrO$_2$) are different: A non-polar phase transforms into a polar phase by spontaneous inversion symmetry breaking upon the application of an electric field. Here, we show that this structural transition in antiferroelectric ZrO$_2$ gives rise to a negative capacitance, which is promising for overcoming the fundamental limits of energy efficiency in electronics. Our findings provide insight into the thermodynamically 'forbidden' region of the antiferroelectric transition in ZrO$_2$ and extend the concept of negative capacitance beyond ferroelectricity. This shows that negative capacitance is a more general phenomenon than previously thought and can be expected in a much broader range of materials exhibiting structural phase transitions

In-situ heating experiments in TEM/STEM

Presented on June 25, 2020 from 11:00 a.m.-12:00 p.m. SENIC Technical Webinar Series: Session 10.The Southeastern Nanotechnology Infrastructure Corridor (SENIC) housed at the Institute for Electronics and Nanotechnology at Georgia Tech hosted a series of online technical seminars, open to the academic and industrial community with an interest in cleanroom fabrication and processing for materials, biological, and electronics research.Dr. Mengkun Tian received his Ph.D. degree in the department of Materials Science and Engineering at University of Tennessee, Knoxville (UTK) in 2015, supervised by Prof. Duscher (UTK) and Dr.Geohegan (Oak Ridge National Laboratory). His dissertation is related to structural evolution of photocatalytic TiO2 made by ultra-small amorphous building blocks. During 2015 and 2018, he worked as a post-doc research associate in Dr.Zawodzinski’s group at UTK to fabricate and investigate the corrosion resistant cathode materials for solid acid fuel cell. He was a visiting scientist at Oak Ridge National Laboratory from 2011 to 2018. He joined Georgia Tech as a post-doc in May, 2018 and was promoted to research scientist II last October. His current research interests include in-situ corrosion testing on metals, electron beam induced phase transformation, and phase transformation of high entropy alloys.Runtime: 52:50 minutesIn-situ heating experiment performed in the scanning/ transmission electron microscopes (STEM/TEM) allows us to directly observe the dynamic behaviors of the materials with sizes ranging from micron- to atomic level in real time. The Materials Characterization Facilities (MCF) at IEN currently has two microscopes (FEI Tecnai F30 and Hitachi HD2700) with in-situ heating capabilities. The TEM techniques including (large-scale or atomic) imaging, phase/elemental analysis and diffraction that we could perform in those facilities for in-situ heating will be introduced briefly. A few examples made by the users and manufactures will be given to show how useful the in-situ heating experiments can help us to understand the structural evolution of materials fundamentally. Finally, it will be discussed a strategy to deal with preparation of TEM samples for high temperature heating

Gel hybrid copolymer of organic palygorskite and methyl methacrylate electrolyte coated onto Celgard 2325 applied in lithium ion batteries

Separator is a critical component in the rechargeable lithium-ion battery. The continuous improvement in the performance of the separator is usually focused on reducing its thermal shrinkage and improving ionic conductivity. A new composite separator poly(organic Pal-co-methyl methacrylate) [p(OPal-MMA)@CPM] was developed by coating the hybrid polymer [p(OPal-MMA)] of organic palygorskite and MMA onto both sides of a Celgard 2325 [polypropylene (PP)/polyethylene /PP, CPM] in this research. The ionic conductivity and liquid electrolyte uptake of the p(OPal-MMA)@CPM were dramatically improved compared to CPM. The thermal shrinkage percentage of the composite separators was significant decreased compared to the pure CPM. The Li+ transference number of Li/p(OPal-MMA)@CPM/Li cell (0.893) was higher than that of Li/CPM/Li cell (0.483). Lithium iron phosphate/p(OPal-MMA)@CPM/Li cell showed high capacity on charge-discharge cycles compared with poly(MMA) and CPM. (c) 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47970

The Effect of Different Mixed Organic Solvents on the Properties of p(OPal-MMA) Gel Electrolyte Membrane for Lithium Ion Batteries

A solvent is a key factor during polymer membrane preparation, and it is directly related to application performance as a separator for lithium ion battery (LIB). In this study, different mixed solvents were employed to prepare polymer (p(OPal-MMA)) membranes by the phase inversion technique. The polymer membrane then absorbed liquid electrolytes to obtain gel electrolytes (GPEs). The surface morphologies and porosities of these membranes were investigated, and lithium ion transferences and electrochemical performances of these GPEs were also measured. The membrane displayed an interconnected three-dimensional framework structure with uniformly distributed pores when using DMF as a porogen. When combined with acetone as the component solvent, the prepared GPE displayed the largest lithium ion transference number (0.706), the highest porosity (42.6%) and ion conductivity (3.99 × 10−3 S/cm). Even when assembled as Li/GPE/LiFePO4 cell, it exhibited the highest initial specific capacity of 167 mAh/g and retained most capacity (162 mAh/g) after 50 cycles. The results presented here probably provide reference for choosing an appropriate mixed solvent in fabricating polymer membranes

Preparation and performance of p(OPal-MMA)/PVDF blend polymer membrane via phase-inversion process for lithium-ion batteries

Poly (methyl methacrylate) (PMMA) based gel electrolyte has excellent chemical stability and low interfacial resistance but its mechanical strength is poor. In this study, organic palygorskite modified PMMA (p(OPal-MMA)/poly(vinylidene fluoride)) (PVDF) blend based electrolyte membrane was prepared via phase inversion casting technique. The physical and electrochemical properties of the blend membrane were investigated by XRD, SEM, tensile test and electrochemistry workstation. The results showed that PVDF can significantly improve the mechanical strength. The crystal phase of PVDF disappeared after blending. The porosity and ionic conductivity of the p(OPal-MMA)/PVDF membrane was 57.73% and 4.93 x 10(-3) S/cm respectively. The electrochemical stability window of Li/gel B-PVDF electrolyte/SS cell was stable up to 4.7 V (vs. Li+ /Li), and the cell of Li/gel B-PVDF 43% electrolyte/LiFePO4 exhibited good cycling performance and the Coulombic efficiency was maintained above 98.5% during all the cycling process