MXenes: A New Family of Two-Dimensional Materials and its Application as Electrodes for Li-ion Batteries

Two-dimensional, 2D, materials, such as graphene, possess a unique morphology compared to their 3D counterparts, from which interesting and novel properties arise. Currently, the number of non-oxide materials that have been exfoliated is limited to two fairly small groups, viz. hexagonal, van der Waals bonded structures (e.g. graphene and BN) and layered transition metal chalcogenides. The MAX phases are a well established family of layered ternary transition metal carbides and/or nitrides, with a composition of Mn+1AXn, where M is an early transition metal, A is one of A group elements, X is C and/or N; with n = 1, 2, or 3. The aim of this work is to exfoliate the MAX phases and produce 2D layers of transition metals carbides and/or nitrides by the selective etching of the A layers from the MAX phases. We labeled the resulting 2D Mn+1Xn layers "MXenes" to emphasize the loss of the A group element from the MAX phases and the suffix "ene" to emphasize their 2D nature and their similarity to graphene. The etching process was carried out using aqueous hydrofluoric acid at room temperature. Thirteen different MXenes were produced as a result of this work, viz., Ti2C, Nb2C, V2C, Mo2C, (Ti0.5,Nb0.5)2C, (Ti0.5,V0.5)2C, Ti3C2, (Ti0.5,V0.5)3C2, (V0.5,Cr0.5)3C2, Ti3CN, Ta4C3, Nb4C3 and (Nb0.5,V0.5)4C3. The as-synthesized MXenes were terminated with a mixture of OH, O, and/or F groups. Sonicating MXenes resulted in separating the stacked layers to a small extent. When Ti3C2 was intercalated with dimethylsulfoxide, however, followed by sonication in water, large-scale delamination occurred, which resulted in aqueous colloidal solutions that could in turn be fabricated into MXene "paper". MXenes were found to be electrically conductive, hydrophilic and stable in aqueous environments, a rare combination indeed, with huge potential in many applications, from energy storage, to sensors to catalysts. This work focused on the use of MXenes as electrode materials in Li-ion batteries. They exhibited excellent capability to handle high cycling rates with good gravimetric capacities. The lithiation and delithiation were found to be due to redox intercalation/deintercalation reactions.Ph.D., Materials Science and Engineering -- Drexel University, 201

HIV/AIDS behavioural challenges in Lagos state schools: An empirical study

This paper evaluates the extent of the behavioural challenges concerning HIV/AIDS among senior secondary school (SSS) and tertiary institution students in Lagos, Nigeria. The study, part of a large research project evaluating knowledge-based HIV/AIDS education initiatives in Lagos, has been carried out in collaboration with the Lagos State Ministry of Education. Questionnaires (n=958 copies) were administered to selected senior secondary schools in the Lagos State administrative divisions (urban and rural) and the Lagos State University (LASU). The study aimed to record information about students’ behaviour related to HIV/AIDS as this would provide information relating to the factors responsible for its transmission. This included (a) behavioural change information – BCI, (b) sexually transmitted infections (STIs) and clinical understanding (c) sexual activities (d) condom use and (e) fear of HIV/AIDS-related stigma and discrimination (HASD). The results show that the behaviour of the respondents with regard to HIV/AIDS puts them at risk of contracting the virus. We conclude by suggesting the use of Knowledge Management (KM) techniques as an appropriate modern approach to tackling the problem and that the availability of behavioural change communication (BCC) materials in all Lagos State schools should be increased

Evaluation of HIV health services in Lagos State: an empirical study

This paper investigates the efficiency of a particular health service (specifically HIV/AIDS counselling and testing centres (HCT) in Lagos State, Nigeria). A survey of 958 students, aged between 15-25, was carried out in Lagos State secondary schools and Lagos State University, in conjunction with the Lagos State Ministry of Education, Guidance and Counselling Unit. It was found that there was a low awareness about HIV testing and counselling activities in all the Lagos State divisions. This result suggests that Lagos State students remain vulnerable and that HCT centres are less organized than they could be and it proposes that knowledge management (KM) initiatives be adopted to increase both efficiency and effectiveness of efforts for this essential health service

Dielectrophoretic Assembly of Carbon Nanofiber Nanoelectromechanical Devices

We report a technique for the assembly of bottom-up nanomechanical devices. This technique employs the dielectrophoretic manipulation of nanostructures within a multiple layer lithography process. Mechanical resonators were specifically produced by assembling and clamping tubular carbon fibers onto prefabricated pads. Our preliminary results showed that an assembled cantilevered fiber with length L = 5 µm and width of W = 180 nm possessed a resonant frequency of f = 1.17 MHz. A shorter L = 3-µm-long singly clamped resonator of similar width showed a resonance of f = 3.12 MHz. This frequency range is in agreement with the low gigapascal bending moduli previously reported for carbon structures showing extensive volume defects. This technology would allow the integration of bottom-up nanostructures with other more established fabrication processes, thus allowing the deployment of engineered nanodevices in integrated systems

Intercalation and Delamination of Layered Carbides and Carbonitrides

Intercalation and delamination of two-dimensional solids in many cases is a requisite step for exploiting their unique properties. Herein we report on the intercalation of two-dimensional Ti3C2, Ti3 CN and TiNbC - so called MXenes. Intercalation of hydrazine, and its co-intercalation with N,N-dimethylformamide, resulted in increases of the c-lattice parameters of surface functionalized f-Ti3C2, from 19.5 to 25.48 and 26.8 Å, respectively. Urea is also intercalated into f-Ti3C2. Molecular dynamics simulations suggest that a hydrazine monolayer intercalates between f-Ti3C2 layers. Hydrazine is also intercalated into f-Ti3 CN and f-TiNbC. When dimethyl sulphoxide is intercalated into f-Ti3C2, followed by sonication in water, the f-Ti3C2 is delaminated forming a stable colloidal solution that is in turn filtered to produce MXene \u27paper\u27. The latter shows excellent Li-ion capacity at extremely high charging rates

MXene: a promising transition metal carbide anode for lithium-ion batteries

Herein we report on Li insertion into a new two-dimensional (2-D) layered Ti₂C-based material (MXene) with an oxidized surface, formed by etching Al from Ti₂AlC in HF at room temperature. Nitrogen sorption of treated powders showed desorption hysteresis consistent with the presence of slit-like pores. At 23 m² g-¹, the specific surface area was an order of magnitude higher than untreated Ti₂AlC. Cyclic voltammetry exhibited lithiation and delithiation peaks at 1.6 V and 2 V vs. Li+/Li, respectively. At C/25, the steady state capacity was 225 mAh g-¹; at 1C, it was 110 mAh g-¹ after 80 cycles; at 3C, it was 80 mAh g-¹ after 120 cycles; at 10C, it was 70 mAh g-¹ after 200 cycles. Since Ti₂C is a member of the MXene family - where M is an early transition metal and X is C and/or N - that to date includes Ti₃C₂, Ta₄C₃, TiNbC, and (V₀.₅,Cr₀.₅)₃C₂, our results suggest that MXenes are promising as anode materials for Li-ion batteries

Impaired receptivity of thin endometrium: therapeutic potential of mesenchymal stem cells

The endometrium is a resilient and highly dynamic tissue, undergoing cyclic renewal in preparation for embryo implantation. Cyclic endometrial regeneration depends on the intact function of several cell types, including parenchymal, endothelial, and immune cells, as well as adult stem cells that can arise from endometrial or extrauterine sources. The ability of the endometrium to undergo rapid, repeated regeneration without scarring is unique to this tissue. However, if this tissue renewal process is disrupted or dysfunctional, women may present clinically with infertility due to endometrial scarring or persistent atrophic/thin endometrium. Such disorders are rate-limiting in the treatment of female infertility and in the success of in vitro fertilization because of a dearth of treatment options specifically targeting the endometrium. A growing number of studies have explored the potential of adult stem cells, including mesenchymal stem cells (MSCs), to treat women with disorders of endometrial regeneration. MSCs are multipotent adult stem cells with capacity to differentiate into cells such as adipocytes, chondrocytes, and osteoblasts. In addition to their differentiation capacity, MSCs migrate toward injured sites where they secrete bioactive factors (e.g. cytokines, chemokines, growth factors, proteins and extracellular vesicles) to aid in tissue repair. These factors modulate biological processes critical for tissue regeneration, such as angiogenesis, cell migration and immunomodulation. The MSC secretome has therefore attracted significant attention for its therapeutic potential. In the uterus, studies utilizing rodent models and limited human trials have shown a potential benefit of MSCs and the MSC secretome in treatment of endometrial infertility. This review will explore the potential of MSCs to treat women with impaired endometrial receptivity due to a thin endometrium or endometrial scarring. We will provide context supporting leveraging MSCs for this purpose by including a review of mechanisms by which the MSC secretome promotes regeneration and repair of nonreproductive tissues