Understanding the photoelectrochemical properties of a reduced graphene oxide-WO3 heterojunction photoanode for efficient solar-light-driven overall water splitting

WO3-reduced graphene oxide (WO3-RGO) heterojunction electrodes were prepared for photoelectrochemical (PEC) overall water splitting. The WO3 photoanode incorporated with RGO showed significantly enhanced PEC properties and, hence, photocatalytic water splitting, compared to the bare WO3 at a bias larger than 0.7 V vs. Ag/AgCl, while a decrease in the PEC properties of WO3-RGO compared to the WO3 electrode was observed at a bias smaller than 0.7 V vs. Ag/AgCl. RGO could play a favorable role in enhancing the electron-hole separation due to the presence of interface states according to the Bardeen model, but it could also provide active sites for the electron-hole recombination. A more positive applied bias is in favor of effective electron-hole separation, by means of quick collection and transport of electrons by RGO. As a result, a higher PEC performance of WO3-RGO can only be realised at a relatively more positive bias. This study gives insights into the complex nature of a RGO-semiconductor heterojunction, and its implications on the overall photoconversion efficiency

Rational design of highly efficient visible light driven photocatalyst for enhanced H2 evolution

187 p.This thesis focuses on the development of highly active nanomaterials for H2 evolution based on the rationale design of semiconducting materials. Core shell Au@TiO2 nanocomposite was designed by incorporating the Au (nanoparticles) NPs into the TiO2 hollow cavity, and their H2 activity was further compared with conventional Au-P25. Higher H2 rates generated by the Au@TiO2 can be attributed to its larger surface area and the encapsulation of Au NPs inside the hollow nanocomposites. The encapsulated Au NPs were developed to increase the degree of light absorption through multiple light scattering and induced a stronger localized SPR effect for higher H2 generation. Based on the excellent H2 photocatalytic activity exhibited by the Au@TiO2 hollow spheres, these photocatalysts were fabricated into a photoelectrode, and further employed in a solar assisted microbial hybrid device (Au-TiO2-hybrid device) for H2 generation. This hybrid device, which was driven by solar energy (from PECs) and the metabolism of microorganisms (in MFCs), exhibited higher photocurrent output than conventional standalone PEC devices. The hybrid system employing Au-TiO2 hollow spheres and DSSN+ generated a photocurrent of ~0.35 mA/cm2 at zero bias (0 V vs. Pt), while the conventional stand-alone PEC cell employing only Au-TiO2 hollow spheres was ~0.04 mA/cm2. The enhancement was attributed to the additional electrons originated from the MFC in the hybrid system. Results also show that when these hybrid devices were chemically modified with conjugated oligoelectrolytes (COEs), their photocurrent output and H2 evolution were further enhanced due to better charge collection of the MFCs in the modified hybrid system. The H2 performance of a photocatalyst can also be enhanced by morphological control because the catalytic properties of nanocrystals depend strongly on the enclosed facets derived from a particular morphology. NaInS2 nanostructures with different morphologies, including octahedral nanocrystals (NCs), nanosheets and microspheres were prepared, and the effect of morphologies on the performance for H2 evolution was investigated. The octahedral NaInS2 NCs exhibited the highest H2 evolution and the superior performance can be attributed to the active facets enclosed on these NCs, which provided more active sites to accelerate the water splitting process for H2 generation. To further improve the performance of the octahedral NaInS2 NCs, a quaternary sulphide (Ca-ZnIn2S4) NCs was synthesized via a cation-exchange template strategy. As the morphology and the crystallinity of the NCs were well preserved during the cation-exchange reaction, these obtained quaternary Ca-ZnIn2S4 NCs were able to produce new enhanced properties, while inheriting properties from its parent ternary NaInS2 structure. Higher H2 rates were observed for the Ca-ZnIn2S4 NCs and the enhancement can be attributed to the incorporation of the Ca2+ and Zn2+ cations in the NaInS2 structure, which results in a significant improvement to its light absorption, charge separation and stability of the material. In summary, high performance novel photocatalyst were developed by encapsulating Au NPs in hollow nanocomposites, morphological control and through the formation of quaternary-based nanocomposites.Doctor of Philosophy (IGS

Synthesis, characterization of energetic polymers

Branched polymers are known to provide advantages in terms of viscosity and glass transition temperature over their linear analogues. This study investigates the potential of branched energetic polyurethanes as energetic binders for composite propellants. Such systems have not been explored as energetic binders yet. The highly branched polyurethanes were synthesized by the A2 + B3 approach, where A2 is the diisocyanate end modified energetic glycidyl azide polymer (GAP) diol and B3 is the trimethylol propane (TMP). Linear polyurethanes were synthesized by reacting A2 with 1, 4 butane diol. Both reaction systems were catalyzed by dibutyltin dilaurate (DBTDL). The molecular weights, intrinsic viscosities, glass transition temperatures, decomposition energies and thermal stabilities of the polymers were measured for both the branched and linear polymers and the properties compared. It was found that, the branched systems have lower glass temperatures compared to the linear analogues. This property will provide definite advantages on the low temperature mechanical properties of the branched energetic polyurethanes. The molecular weights of the branched polyurethanes were not high enough to produce observable changes in the intrinsic viscosities. The decomposition energies and thermal stabilities of the polyurethanes were found to be independent of the macromolecular architecture.Bachelor of Engineering (Chemical and Biomolecular Engineering

Bandgap engineering of ternary sulfide nanocrystals by solution proton alloying for efficient photocatalytic H-2 evolution

Bandgap engineering is an important strategy for tailoring the optical and electronic properties of semiconductor nanocrystals. This work describes the first solution proton alloying process for tuning the bandgap energy of ternary sulfide nanocrystals at room temperature. The proposed strategy circumvents the use of toxic heavy metal ions, while maintaining the size and morphology of the nanocrystals, through a seamless tuning of the bandgap over a wide range. It was shown that proton alloying exhibited different effects on the bandgap energies of ternary sulfide nanocrystals and this could be explained by Density-Of-States (DOS) calculations. Using this approach, enhanced optoelectronic properties of ternary sulfide semiconductor nanocrystals were achieved and proton alloyed ZnIn2S4 showed eight times higher photocatalytic H-2 evolution rate than that of the untreated ones due to increased carrier density and decreased charge transfer resistance. (C) 2016 Elsevier Ltd. All rights reserved.</p

A "uniform'' heterogeneous photocatalyst: integrated p-n type CuInS2/NaInS2 nanosheets by partial ion exchange reaction for efficient H-2 evolution

Single-crystalline-like P-N type CuInS2/NaInS2 heterogeneous nanosheets were synthesized by partial cation exchange reaction and show highly improved photocatalytic H-2 evolution activity attributed to the increased efficiency of interfacial charge transfer

Ion-induced synthesis of uniform single-crystalline sulphide-based quaternary-alloy hexagonal nanorings for highly efficient photocatalytic hydrogen evolution

Uniform single-crystalline quaternary sulphide nanoring photocatalysts are synthesized via the copper-ion-induced Kirkendall effect and is followed by a cation exchange reaction. The obtained Cu2+-doped ZnIn2S4 nanorings show highly preserved morphology, and demonstrate high visible-light-driven photocatalytic activity for H2 evolution in water splitting

Operando Investigation of Mn₃O_4+δ Co-catalyst on Fe₂O₃ Photoanode: Manganese-Valency-Determined Enhancement at Varied Potentials

The development of efficient catalysts containing earth-abundant elements for the oxygen evolution reaction (OER) in photoelectrochemical (PEC) systems is highly desired for low-cost energy storage and conversion. In this work, mesoporous α-Fe₂O₃ thin film photoanodes coated with manganese oxide (Mn₃O_4+δ) co-catalysts are prepared by a dip-coating method. The co-catalyst coating significantly enhances PEC water oxidation performance as compared with the uncoated α-Fe₂O₃. To understand the origin of this enhancement, in situ X-ray absorption spectroscopy is employed to monitor the valence state of Mn in the Mn₃O_4+δ co-catalyst as a function of applied potential. It is found that the enhancement of the photocurrent is governed by the Mn valency, and the most prominent enhancement takes place at the valency of ∼3.4+, which is due to the optimal e_g electron filling in Mn cations as the electrocatalyst for OER. Our investigation indicates that the contribution of Mn₃O_4+δ co-catalyst to OER kinetics is variable at different applied potentials