Browsing by Author "Ojha, Abhijeet"

In this study, we report the microstructure, rheology and adsorption characteristics of aqueous suspensions of Graphene Oxide (GO) at a volume fraction (ϕ GO ) = 0.018, which can be transformed into gels by cation induced charge shielding and cross-linking between GO nanosheets. GO nanosheets of average thickness ∼1.5 nm and a lateral dimension of ∼750 nm are synthesized by Hummer's process. At ϕ GO = 0.018, cations of varying size and valence are systematically introduced with electrolytes NH 4 Cl, LiCl, NaCl, KCl, MgCl 2 and FeCl 3 at concentrations ranging from 10 ^-5 –10 ^-1 M to investigate their effect on the rheology of GO suspensions. Our results suggest that depending on the electrolyte concentration, size and the valence of the cation: low viscosity suspensions, fragile gels and solid-like GO-electrolyte gels are formed. The storage modulus (G') of all GO-electrolyte gels increases with the increase in electrolyte concentration and G' follows the order GO-FeCl 3 > GO-MgCl 2 > GO-KCl > GO-NaCl > GO-LiCl > GO-NH 4 Cl. FESEM analysis shows that lyophilized GO-electrolyte gels with 10 ^-1 M electrolytes have a porous morphology resulting from the aggregation of GO nanosheets. The GO-electrolyte gels are shown to adsorb high quantities of oils, with GO-FeCl 3 gels showing a higher adsorption capacity. The GO-NaCl and GO-FeCl 3 lyophilized gels are also shown to adsorb methylene blue dye and follow the pseudo-second-order kinetics of adsorption. Along with higher oil and dye adsorption efficiency, GO-electrolyte gels are easy to recollect after the adsorption, thus avoiding the potential toxicity for bio-organisms in water caused by GO nanosheets.

Superhydrophilic thin films of 21 nm sized non-spherical titania nanoparticles are fabricated from a colloidal suspension by fixed blade flow coating without UV illumination. At a blade angle of a = 36° and a gap of d = 300 µm, hierarchically structured films with increasing surface roughness along with microscopic voids are formed depending on the substrate velocity and the titania volume fraction. Increasing the roughness is shown to be concomitant to an increase in the hydrophilicity, eventually leading to superhydrophilicity or water contact angle less than 5°.

Graphene oxide (GO) is an sp²-bonded single atomic layer of carbon atoms with plenty of oxy-functional groups at its surface. The occurrence of large surface area (~ 2630 m²/g), surface functionalities, electronic, and mechanical properties make graphene-based compounds favorable for remediation applications. We report the synthesis and comparative dye degradation efficiencies of two titania (TiO2)-based composites, GO-TiO2, and reduced GO-TiO2 (rGO-TiO2). Both the composites are characterized using SEM-EDX, TEM, FTIR, and XRD. We study the photocatalysis-mediated degradation of anionic dye Eosin Y and cationic dye Methylene blue (MB) in aqueous dispersions under white light and UV irradiation. Our results demonstrate that rGO(10 wt%)-TiO2 composite shows the maximum degradation (~ 95%) of both 10 ppm Eosin Y and MB dyes in the aqueous dispersion. The enhanced photocatalytic activity of rGO-TiO2 composites as compared to TiO2 and GO-TiO2 can be credited to the presence of efficient electron shuttling from the conduction band of TiO2 into the conductive regions of graphene which eventually restricts further recombination of e⁻/h⁺ pairs. Furthermore, the effects of solution pH and TiO2 loading are studied on the degradation process.

The variation of lateral dimensions of GO nanosheets by subjecting them to different ultrasonication time (t = 30, 60, and 120 min) influences the rheology, microstructure, and adsorption capacity of graphene oxide (GO) suspensions. The average storage modulus (G'p) of GO aqueous dispersions (volume fraction (ϕGO) = 0.018) varies as GO-30 < GO-60 < GO-120, although the C/O of GO nanosheets does not change significantly with ultrasonication. Addition of 10⁻⁵ M – 10^-1 M NaCl and MgCl2 lead to a rise in G'p as compared to electrolyte-free suspensions with the formation of liquid-like, fragile gels and solid gel-like samples. Independent of time of ultrasonication, solid gels are formed at 10^-1 M of electrolytes. The maximum G'p is observed in gels of GO-30 with 10^-1 M MgCl2. We hypothesize that an increase in ultrasonication time causes the formation of new nanosheets with un-functionalized edges. While Na⁺ screens the negative charge of GO causing aggregation, but Mg²⁺ not only screens but also interacts with carboxyl groups of GO nanosheets. Therefore, the degree of Mg²⁺ bridging the GO nanosheets is the lowest in GO-120 and the highest in GO-30 suspensions. The adsorption capacity of methylene blue (MB) on GO-120-electrolyte lyophilized gels is the smallest, in agreement with the weak cross-linking and mechanical strength of these gels.

A series of α-cyanostyrenes bearing anthracene and electron withdrawing trifluoromethyl units were designed and synthesized. The α-cyanostyrene skeleton favors aggregation induced enhanced emission behavior due to the restriction of intramolecular rotations. Remarkably, the anthryl cyanostyrenes bearing simple trifluoromethyl (CF3) substituents form stable organogels with enhanced fluorescence emission compared to their solution state. In water, the CF3 substituted anthrylstyrenes self-assemble into entangled fibrous nano/microstructures through intermolecular H-bonding, π-π stacking and cyano substituent interactions. The morphological features of the aggregates and the gels were substantiated using scanning electron microscopy, TEM, and powder XRD measurements. The stability of the gels was assessed using rheology investigations.

α-Cyanostyrenes bearing a planar pyrene unit and electron withdrawing trifluoromethyl units were designed and synthesized. The conformational restriction due to the presence of the cyano group on the double bond favors aggregation induced emission in aqueous media. The styrylpyrenes aggregate to form microstructures influenced by π-π stacking, cyano and CF3 substituent interactions. Importantly confluence of the pyrene ring and simple trifluoromethyl (CF3) unit allows the formation of a stable organogel with bathochromic shifts in emission. The formation of aggregates and the gel was substantiated using ¹H NMR spectroscopy and scanning electron microscopy. The stability of the gels was assessed using rheology investigations and rationalized by single crystal X-ray data.

Titanium diboride (TiB2), a layered ceramic material, is well-known for its ultrahigh strength, wear resistance, and chemical inertness. In this work, we present a simple one-pot chemical approach that yields sheet-like nanostructures from TiB2. We serendipitously found that TiB2 crystals can undergo complete dissolution in a mild aqueous solution of H2O2 under ambient conditions. This unexpected dissolution of TiB2 is followed by non-classical recrystallization that results in nanostructures with sheet-like morphology exhibiting Ti-O and B-O functional groups. We show that this pathway can be used to obtain an aqueous dispersion of nanosheets with concentrations =3 mg mL-1. Interestingly, these nanosheets tend to transform into a hydrogel without the need of any additives. We found that the degree of gelation depends on the ratio of TiB2 to H2O2, which can be tuned to achieve gels with a shear modulus of 0.35 kPa. We also show this aqueous dispersion of nanosheets is processable and forms hierarchical paper-like macrostructures upon vacuum filtration. Such an ability to assemble into free-standing 3D structures would enable a leap to practical applications. We also show that the high surface area and presence of oxy-functional groups on these nanosheets endow them a superior photocatalytic activity to degrade organic pollutants. This exemplifies the rich potential that TiB2 offers upon nanoscaling. The results presented here not only add a novel material to the 2D flatland but also urge the scientific community to revisit the chemistry of metal borides, that have been traditionally considered as relatively inert ceramics.