Dispersibility and Photochemical Stability of Delaminated MXene Flakes in Water.

The environmental stability of 2D MXene flakes must be systematically studied before their further application. Herein, the colloidal dispersibility and photochemical stability of delaminated Ti3 C2 Tx MXene flakes modified with hydrazine (HMH) and KOH and with water as the control (HMH-Ti3 C2 , KOH-Ti3 C2 , and H2 O-Ti3 C2 , respectively) are experimentally and theoretically studied. Modification greatly increases the dispersibility of Ti3 C2 Tx flakes. Their critical coagulation concentrations are 28.7, 106, and 49.1 mm NaCl, and their Hamaker constants are 23.7 × 10-21 , 19.1 × 10-21 , and 37.7 × 10-21 J, respectively; the colloidal interaction follows the classical Derjaguin-Landau-Verwey-Overbeek theory. HMH-Ti3 C2 and KOH-Ti3 C2 exhibit higher photochemical stability, as indicated by their stronger resistance to oxidation under UV and visible light irradiation. Changes in their physicochemical properties and the generation of reactive oxygen species (ROS) are assayed. Spin-polarized density functional theory calculations and molecular dynamics simulations are used to determine the mechanisms underlying the differences in the photochemical stability of Ti3 C2 Tx flakes. K+ ions protect the flakes from oxidation by acting as a middle layer to reduce the coupling between Ti3+ and ROS, while HMH provides stronger protection by absorbing photoelectrons or reacting with ROS. These findings provide new insight into the environmental transformation and design of functional MXenes.

Condition optimization for exfoliation of two dimensional titanium carbide (Ti3C2T x ).

The new class of 2D MXene material exfoliated from transition metal carbides receives increasing research interest due to its extraordinary properties and high potential in energy and environmental applications. However, the exfoliation of Ti3C2T x , a widely studied MXene, from its precursor Ti3AlC2 by chemical etching in HF solution remains to be optimized. This study investigated the optimum exfoliation condition through systematic evaluating potential effects of reaction parameters, including the weight ratio of Ti3AlC2 in HF solution, etching time, reaction temperature, repeating etching, and sonication, on the yield, purity, and structure of produced Ti3C2T x . Results show that a high weight percentage (5 wt%) of Ti3AlC2 etching at 50 °C for 36 h produced highly exfoliated MXene material. Etching at lower weight percentages (0.6-2.5 wt%) of Ti3AlC2 resulted in observable byproduct (AlF3). Degradation of MXene layers with AlF3 enrichment was observed under prolonged etching or higher temperatures. Room temperature etching failed to exfoliate Ti3AlC2 and the repeated etching denatured the MXene material. Introduction of controlled sonication during the etching produced highly exfoliated MXene with minimum etching time, which can be a promising alternative for high quality MXene production.

One-Step Pyrolytic Synthesis of Multiwalled Carbon Nanotubes: The Role of Resupply of Carbon Species on the Quality Control.

An investigation on varying experimental parameters such as solution quantity (2.5, 5 and 7.5 mL) and reaction time (15, 30, 45 and 60 min) was carried out for the production of high-quality multiwalled carbon nanotubes (MWCNTs) in one step pyrolysis. Structural analysis revealed the uniform diameter distribution and the length of nanotubes in the range of 60-80 nm and 0.4-2 µm, respectively. Raman and X-ray diffraction analysis showed a remarkable reduction in defect density with increase in graphitization degree, upon increasing the solution volume and reaction time. MWCNTs prepared at higher solution quantity (7.5 mL) with higher reaction time (60 min) showed higher crystallinity (70% graphitization) and lower defect density (ID/IG: 0.56). The attainment in equilibrium of evaporation cum precipitation in formation of high quality nanotubes structure is evaluated. An effective resupplying of condensed precursors by re-evaporation leads for the achievement of low defect density nanotubes with higher product yield is achieved.

A multi-method analysis of the interaction between humic acids and heavy metal ions.

Understanding of the interaction between humic acids (HAs) and heavy metal ions (HMIs) is essential for the assessment of environmental and health risks of HMIs. Multiple analyses, including fluorescence quenching of HAs; solution pH, zeta potential, and hydrodynamic size changes; and coprecipitation of HAs and HMIs, were carried out to investigate the interaction between two HAs and four HMIs (Ag+, Pb2+, Cd2+, and Cr3+). The HA-HMI interaction mainly included chemical complexation, H+-HMI exchange, electrostatic attraction, and flocculation. The chemical complexation between HAs and HMIs revealed by the Stern-Volmer quenching constant was ordered as Ag < Cd < Pb < Cr. HMIs replaced protons in the acidic functional groups of HAs and thus lowered the pH of the solution. The electrostatic interaction between the negatively charged HAs and HMIs reduced the electronegativity of HAs. Interaction with HMIs, especially the high-valent ions, induced aggregation of HAs, causing precipitation of both HAs and HMIs in the sorptive solution. Cr3+ flocculated and precipitated HAs, but at high concentrations, it reversed the surface charge of HAs and resuspended them. The HA-HMI interaction increased as the HA acidity and solution pH increased.

In vitro bacterial cytotoxicity of CNTs: reactive oxygen species mediate cell damage edges over direct physical puncturing.

Understanding the bacterial cytotoxicity of CNTs is important for a wide variety of applications in the biomedical, environmental, and health sectors. A majority of the earlier reports attributed the bactericidal cytotoxicity of CNTs to bacterial cell membrane damage by direct physical puncturing. Our results reveal that bacterial cell death via bacterial cell membrane damage is induced by reactive oxygen species (ROS) produced from CNTs and is not due to direct physical puncturing by CNTs. To understand the actual mechanism of bacterial killing, we elucidated the bacterial cytotoxicity of SWCNTs and MWCNTs against Gram-negative human pathogenic bacterial species Escherichia coli, Shigella sonnei, Klebsiella pneumoniae, and Pseudomonas aeruginosa and its amelioration upon functionalizing the CNTs with antioxidant tannic acid (TA). Interestingly, the bacterial cells treated with CNTs exhibited severe cell damage under laboratory (ambient) and sunlight irradiation conditions. However, CNTs showed no cytotoxicity to the bacterial cells when incubated in the dark. The quantitative assessments carried out by us made it explicit that CNTs are effective generators of ROS such as (1)O2, O2(•-), and (•)OH in an aqueous medium under both ambient and sunlight-irradiated conditions. Both naked and TA-functionalized CNTs showed negligible ROS production in the dark. Furthermore, strong correlations were obtained between ROS produced by CNTs and the bacterial cell mortality (with the correlation coefficient varying between 0.7618 and 0.9891) for all four tested pathogens. The absence of bactericidal cytotoxicity in both naked and functionalized CNTs in the dark reveals that the presence of ROS is the major factor responsible for the bactericidal action compared to direct physical puncturing. This understanding of the bactericidal activity of the irradiated CNTs, mediated through the generation of ROS, could be interesting for novel applications such as regulated ROS delivery in cancer therapy and the sanitation of potable water supplies.

In situ growth of TiO2 nanoparticles on nitrogen-doped Ti3C2 with isopropyl amine toward enhanced photocatalytic activity.

Construction of heterojunction and nitrogen doping is an effective approach for synthesizing photocatalysts with high quantum yield and efficient electron-hole separation. 2D MXene Ti3C2 has been considered a good carbonaceous nanomaterial for designing heterojunction, while the original surface groups and stacked structure limit the electron-hole separation. Herein, a hybrid of nitrogen-doped Ti3C2 nanosheets and TiO2 nanoparticles (NPs) composed of TiO2 NPs in situ growing on isopropyl amine (iPA) modified Ti3C2 (iN-Ti3C2) was developed for the first time. The novel iN-Ti3C2/TiO2 hybrid exhibited an excellent ultraviolet-light photodegradation of methylene blue (MB), with a degradation rate (0.02642 min-1) significantly higher than that of pure TiO2 NPs, bulk-Ti3C2/TiO2, dimethyl sulfoxide modified Ti3C2/TiO2 hybrid, and hydrazine monohydrate modified Ti3C2/TiO2 hybrid. The formation of heterojunction between iN-Ti3C2 and TiO2 and its role in the photocatalysis were systematically analyzed using various characterization techniques and density functional theory calculation. The iPA modification exfoliated Ti3C2 and doped N on Ti3C2 nanosheets; the in situ grown TiO2 NPs formed efficient heterojunctions with the nanosheets; the N-doping facilitated electron migration in Ti3C2 and inhibited the recombination of photogenerated electron-hole pairs; •OH dominated the photodegradation of MB. This work provides a new approach of constructing efficient photocatalysts for the treatment of organics-polluted water.

Exfoliation and Defect Control of Two-Dimensional Few-Layer MXene Ti3C2Tx for Electromagnetic Interference Shielding Coatings.

Defect-controlled exfoliation of few-layer transition-metal carbide (f-Ti3C2Tx) MXene was demonstrated by optimizing chemical etching conditions, and electromagnetic interference (EMI) shielding coatings were explored. The structural features such as layer morphology, lateral size, layer thickness, defect density, and mechanical stability of the exfoliated f-Ti3C2Tx were strongly dependent on exfoliation conditions. By selecting appropriate exfoliation conditions, moderate etching time leads to the formation of quality f-Ti3C2Tx with lesser defects, whereas longer etching time can break the layer structure and increase defect density, structural misalignment, and oxidative products of f-Ti3C2Tx. The resultant fabricated free-standing flexible f-Ti3C2Tx films exhibited electrical conductivity and electromagnetic interference (EMI) shielding effectiveness (SE) in the X-band of about 3669 ± 33 S/m and 31.97 dB, respectively, at a thickness of 6 µm. The large discrepancy in EMI SE performance between quality (31.97 dB) and defected (3.164 dB) f-Ti3C2Tx sheets is attributed to interconnections between f-Ti3C2Tx nanolaminates interrupted by defects and oxidative products, influencing EMI attenuation ability. Furthermore, the demonstrated solution-processable high-quality f-Ti3C2Tx inks are compatible and, when applied for EM barrier coating on various substrates, including paper, cellulose fabric, and PTFE membranes, exhibited significant EMI shielding performance. Moreover, controlling defects in f-Ti3C2Tx and assembly of heterogeneous disordered carbon-loaded TiO2-Ti3C2Tx ternary hybrid nanostructures from f-Ti3C2Tx by tuning etching conditions could play an enormous role in energy and environmental applications.

Multiwalled Carbon Nanotube Oxygen Sensor: Enhanced Oxygen Sensitivity at Room Temperature and Mechanism of Sensing.

A pyrolysis assisted method was applied for the synthesis of defect controlled carbon nanotubes (CNTs) by varying different growth temperatures. The fabricated resistive devices containing a random network of CNTs were tested for oxygen sensing under standard room-temperature and pressure conditions. Nanotubes grown at moderate growth temperatures (870 °C), when exposed to different concentrations of oxygen, displayed a higher sensitivity (3.6%), with fast response and recovery times of about 60 and 180 s, respectively, compared to nanotubes grown at higher and lower temperatures. A room-temperature oxygen detection concentration as low as 0.3% is achieved. The fast response and recovery of CNTs are explained in terms of physisorption of oxygen molecules at (i) carboxyl functional sites and (ii) graphitic carbon sites (pristine CNT) rather than chemisorption at (iii) defected sites. Interestingly, the density functional theory simulated interaction energies (Eads) of oxygen molecules with defected CNTs (-3.381 eV) and pristine CNTs (-0.753 eV) are higher than that of the carboxyl functional sites (-0.551 eV) and are well correlated with the observed sensing response and recovery times of CNT sensors. Our results show that the carboxyl sites provide lower activation energy or shorter time for desorption of oxygen molecules to yield higher response and fast recovery of the CNT sensors.