Bhavana Gupta

Energy efficient reduced graphene oxide additives: Mechanism of effective lubrication and antiwear properties

Optimized concentration of reduced graphene oxide (rGO) in the lube is one of the important factors for effective lubrication of solid body contacts. At sufficiently lower concentration, the lubrication is ineffective and friction/wear is dominated by base oil. In contrast, at sufficiently higher concentration, the rGO sheets aggregates in the oil and weak interlayer sliding characteristic of graphene sheets is no more active for providing lubrication. However, at optimized concentration, friction coefficient and wear is remarkably reduced to 70% and 50%, respectively, as compared to neat oil. Traditionally, such lubrication is described by graphene/graphite particle deposited in contact surfaces that provides lower shear strength of boundary tribofilm. In the present investigation, graphene/graphite tribofilm was absent and existing traditional lubrication mechanism for the reduction of friction and wear is ruled out. It is demonstrated that effective lubrication is possible, if rGO is chemically linked with PEG molecules through hydrogen bonding and PEG intercalated graphene sheets provide sufficiently lower shear strength of freely suspended composite tribofilm under the contact pressure. The work revealed that physical deposition and adsorption of the graphene sheets in the metallic contacts is not necessary for the lubrication.

Supercapacitive vertical graphene nanosheets in aqueous electrolytes

Abstract Vertical graphene nanosheets (VGN) is a potential material for high-performance supercapacitors due to its high surface area, open interconnected network and excellent electrical conductivity. Selection of proper aqueous electrolyte is crucial for its utilization as supercapacitor electrode. The present study reports supercapacitive performance of vertical graphene nanosheets in neutral (Na 2 SO 4 ), alkaline (KOH), and acidic (H 2 SO 4 ) aqueous electrolytes. Factors like ionic mobility and electrolyte wettability are found to influence supercapacitor performance. The highest capacitance observed for VGN in KOH (197 μ F/cm 2 ) is attributed to the higher wettability, whereas VGN /H 2 SO 4 system with specific areal capacitance of 188 μ F/cm 2 is found to have highest capacitance retention (96.8%), shortest relaxation time (50 ms), low charge transfer resistance (72 Ω xa0cm 2 ) and minimal change in graphitic structure. The later observation is correlated to cationic mobility. Detailed supercapacitive investigation is presented including equivalent circuit simulation for deduction of electrode/electrolyte interface behavior.

Chemically grafted graphite nanosheets dispersed in poly(ethylene-glycol) by γ-radiolysis for enhanced lubrication

Graphite nanosheets (Gr-NS) dispersed in poly(ethylene-glycol) (PEG200) medium were subjected to various doses of γ-irradiation. Hydroxyl functional groups present in PEG are chemically grafted through hydrogen bonding with hydroxyl, carbonyl and carboxylic groups of Gr-NS. The grafting process is driven by the generation of active radicals from solvent radiolysis. Chemical grafting was investigated using X-ray photoelectron spectroscopy (XPS) and Fourier transform infra-red (FTIR) spectroscopy. The results of spectroscopic studies revealed reduction in oxygen functionality of PEG-Gr-NS at higher doses of γ-irradiation. The γ-irradiation not only bridges the functionalities between PEG and PEG-Gr-NS but edge and basal plane defects in Gr-NS are further reduced as is evident from Raman analysis. The inter-planar sheet distance in Gr-NS is increased due to intercalated chemical grafting with PEG molecules. The chemical grafting between PEG and Gr-NS and reduction in defects enhance the tribological properties with a decrease of 26% and 32% for the friction coefficient and wear, respectively as compared to PEG alone. The lubrication mechanism is mediated through inter-planar weak forces when PEG is chemically grafted with Gr-NS. The electrostatic interaction of PEG with Gr-NS acts as a molecular bridge thus enhancing the sustainability of tribo-stress. Additionally, in the presence of functionalized PEG-Gr-NS tribo-contact conditions, evidence of deposited graphitic tribo-film was observed from micro-Raman spectroscopy inside the steel wear track. This film further enhanced lubrication mediated through low shear strength interlayer graphite sheets and therefore, antiwear properties were synergistically improved.

Anomalous red emission with competition and coexistence of defect and band edge emission in photo-electrochemically active (Zn0.97Ga0.03)(O0.95N0.05) solid solution

Photo-electrochemically active nanostructured (Zn0.97Ga0.03)(O0.95N0.05) solid solution has been synthesized by a solution combustion technique for realizing photocatalytic activity under visible light. A competing free excitonic and defect bound emission is observed from the sample which was investigated by temperature dependent photoluminescence from 4 K to 300 K. The defect bound emission dominates at room temperature providing an anomalous enhanced red emission, not reported before. A broad visible emission confirms the introduction of new defect levels as an impact of solid solution formation as established later by valence band (VB) XPS spectra. VB XPS shows the top of the valence band has shifted without affecting the conduction band, thereby reducing effective band gap of the solid solution from 3.35 eV to 2.8 eV. Tuning of the bandgap is essential to facilitate its activity under visible light irradiation as demonstrated in our present work. The generation of charge carriers, their effective separation and reduced trap states has been demonstrated by photoelectrochemical measurements in order to confirm the potential of the sample for efficient photocatalytic activity. The present sample exhibits a low mean life time with reduced trap states as compared to some previously reported results.

Synthesis of GaN:ZnO solid solution by solution combustion method and characterization for photocatalytic application

GaN-ZnO solid solution has emerged as a successful and reproducible photocatalyst for overall water splitting by one-step photoexcitation, with a bandgap in visible region. When the solid solution is formed, some of the Zn and O ions are replaced by Ga and N ions respectively and there is a narrowing of bandgap which is hypothesized as due to Zn3d-N2p repulsion. The traditional method of synthesis of GaN-ZnO solid solution is by nitridation of the starting oxides under constant ammonia flow. Here we report a solution combustion technique for the synthesis of the solid solution at a temperature about 500 ° C in a muffle furnace with metal nitrates as precursors and urea as the fuel. The as prepared samples showed change in color with the increased concentration of ZnO in the solution. The structural, microstructural, morphological and optical properties of the samples were realized by Powder X ray diffraction, Scanning electron microscopy, Energy dispersive X ray analysis, Transmission electron microscopy ...

Electrochemical performance of brownmillerite calcium ferrite for application as supercapacitor

In the past few years with increase in population and global warming the requirement to store energy from various sources has increased. Recent research have been focused on oxide materials as energy storage device due to their unique structure and interesting properties. Perovskites materials such as La0.5Sr0.5CoO3-δ and SrRuO3 shows capacitive behavior. In previous studies perovskite with different A and B sites have been studied. In this report we have chosen Ca2Fe2O5 perovskite material due to its interesting properties, structure and morphology which changes on introducing cobalt as dopant material. Further we investigated the performance of cobalt doping on the electrochemical behavior of Ca2Fe2-xCoxO5 (x=0, 0.01 and 0.03) using electrochemical characterization such as Cyclic voltammetry (CV), Galvanostatic charge-discharge (GCD) and Electrochemical impedance spectra (EIS).

Two-site H 2 O 2 photo-oxidation on haematite photoanodes

H2O2 is a sacrificial reductant that is often used as a hole scavenger to gain insight into photoanode properties. Here we show a distinct mechanism of H2O2 photo-oxidation on haematite (α-Fe2O3) photoanodes. We found that the photocurrent voltammograms display non-monotonous behaviour upon varying the H2O2 concentration, which is not in accord with a linear surface reaction mechanism that involves a single reaction site as in Eley–Rideal reactions. We postulate a nonlinear kinetic mechanism that involves concerted interaction between adions induced by H2O2 deprotonation in the alkaline solution with adjacent intermediate species of the water photo-oxidation reaction, thereby involving two reaction sites as in Langmuir–Hinshelwood reactions. The devised kinetic model reproduces our main observations and predicts coexistence of two surface reaction paths (bi-stability) in a certain range of potentials and H2O2 concentrations. This prediction is confirmed experimentally by observing a hysteresis loop in the photocurrent voltammogram measured in the predicted coexistence range.Understanding fundamental processes that occur using solar-to-fuel conversion materials is crucial for constructing effective renewable energy collection. Here, authors find the hydrogen peroxide light-driven hole-scavenging mechanism over haematite to proceed with two active sites rather than one

Effect of varying Ga content in ZnO:GaN solid solution synthesized by solution combustion technique for photocatalytic applications

ZnO:GaN (oxy)nitride solid solution has been established as the most efficient non-oxide photocatalyst for water splitting under visible irradiation with one step photoexcitation and also boasts a band gap tunability from 2.8 eV to 2.5 eV[1]. The solid solution of GaN in ZnO is formed by the intersubstitution of few of Zn/O ions by Ga/N ions, and this results in the introduction of new defect levels above the valence band which narrows the effective band gap enabling activity under visible region of spectra. In this work, we report the synthesis of ZnO:GaN solid solution by a solution combustion technique where metal nitrates and urea are used as precursors. The Zn/Ga ratio was varied from 16 to 1 in the precursors. The as synthesized samples were characterized as phase pure by X-ray diffraction, where the wurtzite structure was retained up to Zn/Ga ratio of 5. The Diffuse reflectance spectroscopy studies revealed that as the Ga content in the solid solution increases there is a reduction in band gap, fro...