Sima Umrao

A possible mechanism for the emergence of an additional band gap due to a Ti–O–C bond in the TiO2–graphene hybrid system for enhanced photodegradation of methylene blue under visible light

Here we report the experimental and theoretical study of two TiO2–graphene oxide (TG) and TiO2–reduced graphene oxide (TR) composites synthesized by a facile and ecological route, for enhanced visible light (∼470 nm) photocatalytic degradation of Methylene Blue (MB) (99% efficiency), with high rate constant values (1800% over bare TiO2). TG couples TiO2 nanopowder with Graphene Oxide (GO) while TR couples it with reduced graphene oxide (RGO). The present study, unlike previous reports, discusses never-before-reported double absorption edges obtained for both TG (3.51 eV and 2.51 eV) and TR (3.42 eV and 2.39 eV) composites, which represents the reason behind feasible visible light (2.56 eV) induced photocatalysis. TiO2 domains in the composites dominate the higher band edge, while GO/RGO domains explain the lower band edge. Formation of Ti–O–C bonds in both TG and TR drives the shifting upwards of the valence band edge and reduction in band gap. Further, these bonds provide a conductive pathway for charge carriers from TiO2 nanopowder to the degraded species via the GO/RGO matrix, resulting in decreased charge carrier recombination in TiO2 and enhanced efficiency. To attest that the developed theory is correct, density function theory (DFT) calculations were performed. DFT obtained energetics and electronic structures support experimental findings by demonstrating the role of the Ti–O–C bond, which results in double band edge phenomenon in composites. Finally, the mechanism behind MB degradation is discussed comprehensively and the effect of the weight percent of GO/RGO in the composite on the rate constant and photodegradation efficiency has been studied experimentally and explained by developing analytical equations.

Microwave-Assisted Synthesis of Boron and Nitrogen co-doped Reduced Graphene Oxide for the Protection of Electromagnetic Radiation in Ku-Band.

The electromagnetic interference (EMI) shielding of reduced graphene oxide (MRG), B-doped MRG (B-MRG), N-doped MRG (N-MRG), and B-N co-doped MRG (B-N-MRG) have been studied in the Ku-band frequency range (12.8-18 GHz). We have developed a green, fast, and cost-effective microwave assisted route for synthesis of doped MRG. B-N-MRG shows high electrical conductivity in comparison to MRG, B-MRG and N-MRG, which results better electromagnetic interference (EMI) shielding ability. The co-doping of B and N significantly enhances the electrical conductivity of MRG from 21.4 to 124.4 Sm(-1) because N introduces electrons and B provides holes in the system and may form a nanojunction inside the material. Their temperature-dependent electrical conductivity follows 2D-variable range hopping (2D-VRH) and Efros-Shklovskii-VRH (ES-VRH) conduction model in a low temperature range (T<50 K). The spatial configuration of MRG after doping of B and N enhances the space charge polarization, natural resonance, dielectric polarization, and trapping of EM waves by internal reflection leading to a high EMI shielding of -42 dB (∼99.99% attenuation) compared to undoped MRG (-28 dB) at a critical thickness of 1.2 mm. Results suggest that the B-N-MRG has great potential as a candidate for a new type of EMI shielding material useful in aircraft, defense industries, communication systems, and stealth technology.

Synthesis, Characterization, and Tribological Evaluation of TiO2-Reinforced Boron and Nitrogen co-Doped Reduced Graphene Oxide Based Hybrid Nanomaterials as Efficient Antiwear Lubricant Additives

The microwave-synthesized reduced graphene oxide (MRG), boron-doped reduced graphene oxide (B-MRG), nitrogen-doped reduced graphene oxide (N-MRG), boron-nitrogen-co-doped reduced graphene oxide (B-N-MRG), and TiO2-reinforced B-N-MRG (TiO2-B-N-MRG) nanomaterials have been synthesized and characterized by various state-of-the-art techniques, like Raman spectroscopy, powder X-ray diffraction, scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. Furthermore, the tribological properties of prepared nanomaterials as antiwear additives in neutral paraffin oil have been evaluated using a four-ball machine at an optimized additive concentration (0.15% w/v). The tribological parameters, like mean wear scar diameter, coefficient of friction, and wear rates, revealed that these nanomaterials have potential to be developed as environmentally friendly sulfated-ash-, phosphorus-, and sulfur-free antiwear lubricant additives. The friction- and wear-reducing behavior of MRG increased upon successive doping of nitrogen, boron, and both nitrogen and boron. Among these additives, B-N-co-doped MRG shows superior tribological behavior in paraffin base oil. Besides this, the load-carrying properties of B-N-co-doped MRG have significantly improved after its reinforcement with TiO2 nanoparticles. A comparative study of the surface morphology of a lubricated track in the presence of various additives has been assessed by SEM and contact-mode atomic force microscopy. The X-ray photoelectron spectroscopy studies have proved that the excellent lubrication properties of TiO2-B-N-MRG are due to the in situ formation of a tribofilm composed of boron nitride, adsorbed graphene layers, and tribosintered TiO2 nanoparticles during the tribocontact. Being sulfur-, halogen-, and phosphorus-free, these graphene-based nanomaterials act as green antiwear additives, protecting interacting surfaces significantly from wear and tear.

Facile, rapid and upscaled synthesis of green luminescent functional graphene quantum dots for bioimaging

We report here the upscaled synthesis of green luminescent functionalized graphene quantum dots (FGQDs) by using an inexpensive and commonly occurring natural precursor viz. graphite powder. We observed in our sample that photoluminescence increases for excitation wavelengths of 300 nm to 350 nm and then decreases when excited at 375 to 425 nm for FGQDs at neutral pH. We found that the synthesized FGQDs do not show a drastic change in emission properties when kept under different pH conditions, which makes them a potential candidate for in vivo imaging, where the pH of the culture media plays a crucial role in the maintenance of the fluorescence. Water solubility, and excellent photostability along with low cytotoxicity of FGQDs are manifested as a remarkable bioimaging material.

Nanostructured palladium-reduced graphene oxide platform for high sensitive, label free detection of a cancer biomarker

We report the results of studies related to the fabrication of a palladium nanoparticle decorated-reduced graphene oxide (Pd@rGO) based electrochemical immunosensor for the label free ultrasensitive detection of the prostate-specific antigen (PSA), a prostate cancer biomarker. The synergistic electrochemical activities of Pd and rGO result in an enhanced electron transfer used for the development of an ultrasensitive immunosensor. A facile approach was developed for the in situ synthesis of Pd@rGO using ascorbic acid as the reducing agent which enables the simultaneous reduction of both Pd+2 and GO into Pd nanoparticles and rGO, respectively. XRD, FTIR, SEM and TEM investigations were carried out to characterize the Pd@rGO material. A thin film of nanostructured Pd@rGO was electrophoretically deposited on an ITO coated glass electrode that was subsequently functionalized with anti-PSA antibodies. The electrochemical sensing results of the proposed immunosensor showed a high sensitivity {28.96 μA ml ng−1 cm−2}. The immunosensor is able to detect PSA at concentrations as low as 10 pg ml-1. The simple fabrication method, high sensitivity, good reproducibility and long term stability with acceptable accuracy in human serum samples are the main advantages of this immunosensor.

Large-Area Highly Conductive Transparent Two-Dimensional Ti2CTx Film

We report a simple and scalable method to fabricate homogeneous transparent conductive thin films (Ti2CTx, one of the MXene) by dip coating of an Al2O3 substrate in a colloidal solution of large-area Ti2CTx thin flakes. Scanning electron microscopy and atomic force microscopy images exhibit the wafer-scale homogeneous Ti2CTx thin film (∼5 nm) covering the whole substrate. The sheet resistance is as low as 70 Ω/sq at 86% transmittance, which corresponds to the high figure of merit (FOM) of 40.7. Furthermore, the thickness of the film is tuned by a SF6+Ar plasma treatment, which etches Ti2CTx film layer by layer and removes the top oxidized layer without affecting the bottom layer of the Ti2CTx flake. The resistivity of plasma-treated Ti2CTx film is further decreased to 63 Ω/sq with an improved transmittance of 89% and FOM of 51.3, demonstrating the promise of Ti2CTx for future transparent conductive electrode application.

Multi-layered graphene quantum dots derived photodegradation mechanism of methylene blue

The photocatalytic degradation of methylene blue (MB) under visible light in the presence of a multi-layered graphene quantum dots (MLGQDs) photocatalyst has been investigated in aqueous heterogeneous solution. The photodegradation mechanism, following pseudo first order kinetics, was examined for the effects of the existing monomer (MB+)–dimer {(MB+)2} equilibrium in MB solution. The photocatalytic degradation efficiency of MB+ achieved 93.3% with a rate constant (k) of 0.056 min−1 after 60 min irradiation with green light, while less degradation ∼ 89.44% with k ∼ 0.024 min−1, was achieved for (MB+)2. MB+ is perhaps a short-lived species and favors the photodegradation of MB in comparison to the (MB+)2 species. Similar trends have been found under blue light irradiation. MB+ species easily pulls a proton from the functional groups of MLGQDs, resulting in an intermediate product Luco-methylene blue (LMB). Finally, all MB species and intermediate products degrade into an environmental benign product via highly reactive OH radicals. In addition, our ab initio theoretical results reveals that monomers abstract a proton from hydroxyl groups of MLGQDs and formation of LMB takes place, which is weakly bonded with MLGQDs by hydrogen bonds.

Microwave-assisted boron and nitrogen co-doped reduced graphene oxide as a transparent conductive electrode

A crystalline Boron (B)- and Nitrogen (N)-co-doped microwave-assisted reduced graphene oxide (BNMRGO) film was investigated as a potential transparent conducting electrode (TCE) material. X-ray diffraction results revealed the good crystallinity of the BNMRGO film, and the presence of a (0004) reflection plane indicated the formation of a few small domains of hexagonal boron nitride in the microwave assisted reduced graphene oxide (MRGO) sheets under the co-doping process. Raman and X-ray photoelectron spectroscopic results indicated a reduction of sp3 carbon centers upon co-doping. The ID/IG ratio decreased after co-doping from 0.89 to 0.24, indicating a low average defect density of ∼1.01 × 1010 cm−2. Optoelectronic characterization of the BNMRGO film on a glass substrate revealed a high optical transparency of 82% at 550 nm and a low sheet resistance (Rsh) of 355 Ω/sq, which was lower than that observed from the MRGO sheets (Rsh = 719 Ω/sq). BNMRGO provided a ratio between the direct conductivity (σdc)...