Cu2O is a narrow band gap material serving as an important candidate for photoelectrochemical hydrogen evolution reaction. However, the main challenge that hinders its practical exploitation is its poor photostability, due to its oxidation into CuO by photoexcited holes. Here, we thoroughly minimize the photo-oxidation of Cu2O nanowires by growing a thin layer of the TiO2 protective layer and an amorphous layer of the VOx cocatalyst using magnetron sputtering and atomic layer deposition, respectively. After optimization of the protective and the cocatalyst layers, the photoelectrode exhibits a current density of −2.46 mA/cm2 under simulated sunlight (100 mW/cm2) at 0.3 V versus reversible hydrogen electrode, and its performance is stable for an extended illumination time. The chemical stability and the good performance of the engineered photoelectrode demonstrate the potential of using earth-abundant materials as a light-harvesting device for solar hydrogen production.

Hierarchical nanostructures have attracted considerable research attention due to their applications in the catalysis field. Herein, we design a versatile hierarchical nanostructure composed of NiMoO4 nanorods surrounded by active MoS₂ nanosheets on an interconnected nickel foam substrate. The as-prepared nanostructure exhibits excellent oxygen evolution reaction performance, producing a current density of 10 mA cm⁻² at an overpotential of 90 mV, in comparison with 220 mV necessary to reach a similar current density for NiMoO₄. This behavior originates from the structural/morphological properties of the MoS₂ nanosheets, which present numerous surface-active sites and allow good contact with the electrolyte. Besides, the structures can effectively store charges, due to their unique branched network providing accessible active surface area, which facilitates intermediates adsorptions. Particularly, NiMoO₄/MoS₂ shows a charge capacity of 358 mAhg⁻¹ at a current of 0.5 A g⁻¹ (230 mAhg⁻¹ for NiMoO₄), thus suggesting promising applications for charge-storing devices.

The inception of high-entropy alloy promises to push the boundaries for new alloy design with unprecedented properties. This work reports entropy stabilisation of an octonary refractory, HfMoNbTaTiVWZr, high-entropy thin film metallic glass, and derived nitride films. The thin film metallic glass exhibited exceptional ductility of ≈60% strain without fracture and compression strength of 3 GPa in micro-compression, due to the presence of high density and strength of bonds. The thin film metallic glass shows thermal stability up to 750 °C and resistance to Ar-ion irradiation. Nitriding during film deposition of HfMoNbTaTiVWZr thin film of strong nitride forming refractory elements results in deposition of nanocrystalline nitride films with compressive strength, hardness, and thermal stability of up to 10 GPa, 18.7 GPa, and 950 °C, respectively. The high amount of lattice distortion in the nitride films leads to its insulating behaviour with electrical conductivity as low as 200 S cm−1 in the as-deposited film. The design and exceptional properties of the thin film metallic glass and derived nitride films may open up new avenues of development of bulk metallic glasses and the application of refractory-based high entropy thin films in structural and functional applications.

A key requirement for semiconductors operating in light-harvesting devices, is to efficiently convert the absorbed photons to electronic excitations while accommodating low loss pathways for the photogenerated carrier’s transport. The quality of this process corresponds to different relaxation phenomena, yet primarily it corresponds to minimized thermalization of photoexcited carriers and maximum transfer of electron-hole pairs in the bulk of semiconductor. However, several semiconductors, while providing a suitable platform for light-harvesting applications, pose intrinsic low carrier diffusion length of photoexcited carriers. Here we report a system based on a vertical network of reduced graphene oxide (rGO) embedded in a thin-film structure of iron oxide semiconductor, intended to exploit fast electron transport in rGO to increase the photoexcited carrier transfer from the bulk of the semiconductor to rGO and then to the external circuit. Using intermodulation conductive force microscopy, we locally monitored the fluctuation of current output, which is the prime indication of successful charge transfer from photoexcited semiconductor to rGO and efficient charge collection from the bulk of the semiconductor. We reveal the fundamental properties of vertical rGO and semiconductor junction in light-harvesting systems that enable the design of new promising materials for broad-band optical applications.

Conversion and storage of the solar radiation into applicable forms of energy, using ubiquitous materials is of central importance that quests several disciplinary fields in both applied technology and fundamental science. Harnessing the solar energy received by the earth has the potential to replace the current sources of energy and it is imperative for sustainable development. 

Since the early development of modern photovoltaics (PVs), based on silicon wafers, a rational step was the substantial development of the new generation PV technologies that can provide lower-cost and higher efficiency than their predecessors. Deliberate solutions involved employing different semiconducting materials that are indispensable, non-toxic and compatible with large-scale fabricating technologies.  

Exploiting metal oxide (MOx) semiconductors, a broad class of non-toxic, cheap and abundant materials, is already promoted as a key component for high-performance optoelectronic devices and can be an ideal solution for inexpensive harnessing of sustainable energy resources like Sun light. The favorable band gap and high absorption cross-section of some MOx semiconductors permit utilizing different spectral region of the solar spectrum. However, at this present, the implication of MOx in high-throughput optoelectronic devices remained on the low side. Some of the main drawbacks that attain to poor performance of the MOx are associated with their poor intrinsic carrier mobility especially in p-type light absorbers and insufficient visible light absorption notably in n-type semiconductors.   

The main aim of this thesis is to further contribute to the development and exploitation of this class of materials with the main focus on their role in optoelectronic devices and energy storage systems. 

The content of this thesis considers two main aspect of the research. 

Substantially, this work analyses the vital role of the interface engineering using nanostructured MOx, where we exploit unique phenomena such as intense electric field confinement in 1dimensional (1D) structures resulting in ample light trapping in the fabricated heterojunctions. Unfortunately, this fact comes at the cost of introducing space charge region (SCR) limits in the fabricated devices attaining for poor derived currents. 

Here I would probably spend couple of words for introducing the Co3O4 NR as the basis for p-n inverted nanorod junction…

Plasmonic metal nanoparticles (NPs) were conventionally used to extend the spectral response of the wide-bandgap semiconductors. Within the scheme of this thesis, we employ the silver plasmonic NPs in a 1D light harvesting structure of zinc oxide (ZnO), where we mediate hot-carrier collection of the charges via controlled illuminations. 

Even further, we provide a comprehensive analysis on the hot-carrier redistribution mechanisms of the plasmonic NPs to semiconductor, providing direct experimental proof using transient pump-probe spectroscopy and time-resolved photoluminescence analysis. Our work resulted in a distinct understanding of the radiative and non-radiative carrier transfer between the active constituents of the system, which have not been corroborated previously.

In a parallel approach, the research activities in this work, take a few steps ahead and investigates the issues related to the disparities in the PV plants. A common prerequisite after conversion of the solar light using PV devices is the electrochemical storage of the energy where it can answer the needs for far-reaching energy requirements. Fostered by the intrinsic capacitance characteristic of the MOx, we interplay the role of the interfacial engineering in Co3O4 porous films and investigate the effect of their lateral architecture on Li+ ion adsorption and desorption properties.

Finally, our findings resulted in the fabrication of a hybrid device with dual functionality as an all-oxide PV system that can directly store the converted Sunlight as in a supercapacitor device. The prospect of this device can provide the over-potential required for direct storage of the converted solar energy into larger high storage systems.

In summary, the results presented in this thesis highlights the potential of the MOx semiconductors for photovoltaic and storage applications. We identify the various step-forward routes, which can provide the possibility of large-scale deployment of this novel class of materials.

Catalysts capable of improving the performance of oxygen evolution reaction (OER) and oxygen reduction reactions (ORR) are essential for the advancement of renewable energy technologies. Herein, Ag-decorated vertically aligned MoS2 nanoflakes are developed via magnetron co-sputtering and investigated as electrocatalyst towards OER and ORR. Due to the presence of silver, the catalyst shows more than 1.5 times an increase in the roughness-normalized rate of OER, featuring a very low Tafel slope (58.6 mv dec⁻¹), thus suggesting that the catalyst surface favors the thermodynamics of hydroxyl radical (OH•) adsorption with the deprotonation steps being the rate-determining steps. The improved performance is attributed to the strong interactions between OOH intermediates and the Ag surface which reduces the activation energy. Rotating ring disk electrode (RRDE) analysis shows that the net disk currents on the Ag-MoS2 sample are two times higher at 0.65 V compared to MoS2, demonstrating the co-catalysis effect of silver doping. Based on the rate constant values, Ag-MoS2 proceeds through a mixed 4 electron and a 2 + 2 serial route reduction mechanism, in which the ionized hydrogen peroxide is formed as a mobile intermediate. The presence of silver decreases the electron transfer number and increases the peroxide yield due to the interplay of a 2 + 2 electron reduction pathway. A 2.5–6 times faster conversion rate of peroxide to OH- observed due to the presence of silver, indicating its effective cocatalyst nature. This strategy can help in designing a highly active bifunctional catalyst that has great potential as a viable alternative to precious-metal-based catalysts.Graphica

Characterizing carrier redistribution due to optical field modulation in a plasmonic hot-electron/semiconductor junction can be used to raise the framework for harnessing the carrier decay of plasmonic metals in more efficient conversion systems. In this work we comprehensively studied the carrier redistribution mechanisms of a 1-dimensional (1D) metal-semiconductor Schottky architecture, holding the dual feature of a hot-electron plasmonic system and a simple metal/semiconductor junction. We obtained a strongly enhanced external quantum efficiency (EQE) of the plasmonic Ag decorated ZnO semiconductor in both the band-edge region of ZnO and the corresponding plasmonic absorption profile of the Ag NPs (visible region). Simultaneously, the insertion of an insulating Al2O3 intermediate layer between Ag NPs and ZnO resulted in a parallel distinction of the two main non-radiative carrier transfer mechanisms of plasmonic NPs, i.e. direct electron transfer (DET) and plasmonic induced resonance energy transfer (PIRET). The multi-wavelength transient pump-probe spectroscopy indicated the very fast plasmonic radiative transfer dynamics of the system in <500 fs below 389 nm. We demonstrate a 13% increase of photogenerated current in ZnO upon visible irradiation as a result of non-radiative plasmonic hot-electron injection from Ag NPs. Overall, our device encompasses several effective solutions for designing a plasmonic system featuring non-radiative electron-electron plasmonic dephasing and high photoconversion efficiencies.

Direct stacking of n‐type and p‐type metal oxide (MOx) semiconductors is one of the appealing directions toward low cost and environmentally friendly photovoltaics (PVs). However, the main shortcoming, hindering the PV performance of MOx heterojunction devices is attributed to the tradeoff between light absorption and maximized carrier extraction in p‐type MOx. In this work, we demonstrate that the nanorod (NR) geometry of Co₃O₄ light absorber with a nearly ideal bandgap of ∼1.48 eV, can remove this hurdle through strong internal light trapping of adjacent one‐dimensional (1D) structure and enhanced carrier mobility. The inverted n‐on‐p configuration of the core‐shell 1D heterojunction, obtained by depositing a thin TiO₂ n‐type layer, resulted in enlarged charge generation compared to the typical p‐on‐n counterpart device. Fine‐tuning of Co₃O₄ NRs length, permits PV investigation of the heterojunctions with respect to absorber layers thickness. The optimized Co₃O₄ NRs/TiO₂ heterojunction (30 nm Co₃O₄ NR length) presented a record high open circuit photovoltage (V_oc) of (0.52 ± 0.03) V under 1 sun irradiation. Impedance analysis of the heterojunctions, indicates formation of the p⁺‐p depletion. The presented work can highlight some vital venues to enhance photoconversion efficiency of the all‐oxide heterojunctions while introducing a pioneer contender as inverted (n‐on‐p) MOx heterojunction.

One of the most intensively investigated directions in the field of photovoltaics is the development of technologies able to provide vacuum-free and low-cost solar cells with decent efficiency, based on earth-abundant and environmentally friendly materials. Solar cells based on oxide materials are a promising candidate for the purpose, being most of the investigated oxides comparatively more stable than most of solar cell technologies alternative to silicon, and composed of harmless materials. While oxides can exhibit high extinction coefficient in the visible and near-infrared spectral region, guaranteeing full absorption of sunlight, the main factor limiting efficiency in such kind of p–n junction devices is the low hole mobility in the p-type oxide, which represents the main challenge to be overcome to make this technology competitive. This chapter illustrates the latest results in the field, including integration of nanowire geometries as viable solution toward fast charge transport and collection.

We investigated the photocatalytic behavior of gold nanoparticles supported on CeO₂–TiO₂ nanostructured matrixes in the CO preferential oxidation in H2-rich stream (photo-CO-PROX), by modifying the electronic band structure of ceria through addition of titania and making it more suitable for interacting with free electrons excited in gold nanoparticles through surface plasmon resonance. CeO₂ samples with different TiO₂ concentrations (0–20 wt %) were prepared through a slow coprecipitation method in alkaline conditions. The synthetic route is surfactant-free and environmentally friendly. Au nanoparticles (<1.0 wt % loading) were deposited on the surface of the CeO₂–TiO₂ oxides by deposition–precipitation. A benchmarking sample was also considered, prepared by standard fast coprecipitation, to assess how a peculiar morphology can affect the photocatalytic behavior. The samples appeared organized in a hierarchical needle-like structure, with different morphologies depending on the Ti content and preparation method, with homogeneously distributed Au nanoparticles decorating the Ce–Ti mixed oxides. The morphology influences the preferential photooxidation of CO to CO₂ in excess of H₂ under simulated solar light irradiation at room temperature and atmospheric pressure. The Au/CeO₂–TiO₂ systems exhibit much higher activity compared to a benchmark sample with a non-organized structure. The most efficient sample exhibited CO conversions of 52.9 and 80.2%, and CO₂ selectivities equal to 95.3 and 59.4%, in the dark and under simulated sunlight, respectively. A clear morphology–functionality correlation was found in our systematic analysis, with CO conversion maximized for a TiO₂ content equal to 15 wt %. The outcomes of this study are significant advancements toward the development of an effective strategy for exploitation of hydrogen as a viable clean fuel in stationary, automotive, and portable power generators.