ceramic substrates. The deposition conditions of 2D NMs pure phases and their nanocomposites (NCs) with transition metal compounds are explained in detail. Also, the experimental measurements for evaluating the electrochemical performance of the fabricated hybrid electrodes toward the water splitting and non-enzymatic H2O2 sensing applications are illustrated in various configurations.
In chapter 3, we investigate the feasibility of the direct mechanical exfoliation and deposition of various layered material types such as graphite, MoS2, and boron nitride (BN) in one step at room temperature under low vacuum conditions using NDPS. The deposited nano-sized thin films are studied by various surface characterization x-ray diffraction (XRD), scanning electron microscope (SEM), and Raman spectroscopy. Based on the obtained results we verified the occurrence of mechanical exfoliation of layered materials to small-size nanosheets due to the micron powder fragmentation and interlayer separation of their stacked structure layers.
Page | 42
After checking the feasibility of mechanical exfoliation of the layered materials pure phases (graphite, MoS2), we will check the ability of NCs formation between either graphene or MoS2 nanosheets with functional transition metal compounds for water splitting based energy conversion applications and non-enzymatic electrocatalytic detection of H2O2. These applications depend strongly on the induced water splitting species (i.e., oxygen or hydrogen) that can be used in different forms such as electrochemical energy storage or directly for the degradation of industrial waste products. For the occurrence of the water- splitting, an external energy source should be applied, which can be either photon, electricity, or hybrid in the form of photo-electrocatalytic water splitting. The formation of functional hybrid NCs with 2D materials nanosheets is widely used for these applications to improve the efficiency of power transferring or accelerating the charge transfer process at the interface between the fabricated electrodes and the used electrolyte. Each graphene or MoS2 can verify this goal but with different synergy mechanisms between the hybrid NCs species. To illustrate this, we investigated the effect of hybridization between one of the 2D materials nanosheets and various types of transition metal compounds. Besides, the formed NCs hybrid electrodes were utilized for improving overall performance by variation of materials composition ratio.
• Graphene nanosheets-based hybrid NCs
Hybrid NCs between graphene nanosheets and either nano-sized Ni(OH)2 or Co3O4 were directly deposited in one step at room temperature on nickel foam (NF) porous substrate to study the electrocatalytic water splitting in the alkaline medium (i.e., 1.0 M KOH) as demonstrated in chapter 4&
5. In both studies, the graphite content in the initial micron powder was changed relative to the used TMs material for thin deposition by the NPDS. To demonstrate the hybridization between the graphene and TMs species in the deposited nano-sized thin films various surface sensitive techniques such as XRD, FE- SEM, Raman spectra, and x-ray photoelectron spectroscopy (XPS) were used. Besides, to study the electrochemical performance of the fabricated heterostructure nano-sized electrodes toward the overall water splitting, the kinetic of each oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) were separately evaluated in the three-electrode configurations. The utilized working electrode (WE) was based on the hybrid NCs at different graphite content. The optimum composition ratio between the graphite species and the TMs materials for each the OER and the HER was recognized, in which the higher graphite content in the fabricated hybrid NCs exhibited better electrochemical performance toward the OER due to the overall improvement in the charge transfer kinetics. Whereas the fabricated electrodes with hybrid NCs that contain a high percentage of TMs showed better electrochemical performance toward the HER. Furthermore, Co3O4-graphene hybrid NCs/NF at different graphite content in chapter 5 was
Page | 43
also, utilized for the investigation of electrochemical reduction of H2O2 in alkaline medium (0.1 M NaOH).
The results revealed the overall improvement of H2O2 reduction with the increase of graphene content in the fabricated hybrid NCs due to the improvement of both the charge transfer kinetics and the density of electrocatalytic active centers. This makes the deposited Co3O4-graphene hybrid NCs/NF with an optimum composition ratio a good candidate as an electrochemical non-enzymatic sensor for H2O2 in an alkaline medium.
Graphene hybrid NCs uses for water splitting applications that depend only on photon excitation or hybrid with electricity in the form of photoelectrochemical was also investigated in chapter 6 & 7, in which ZnO-graphene NCs hybrid photoanodes at different graphite content were deposited in NF porous substrate and titanium sheet substrate in one-step by the NPDS at room temperature. The formation of NCs and the hybridization between the ZnO and graphene nanosheets in the hybrid NCs were characterized by several techniques such as XRD, FE-SEM, as well as various spectroscopic techniques (i.e., Raman, UV- visible absorption, photoluminescence, and x-ray photoelectron spectroscopy. The modified electrodes with ZnO-graphene NCs hybrid photoanodes were utilized for studying the photoelectrochemical water splitting in an alkaline medium (i.e., 0.5 M Na2SO3) as well as photocatalytic degradation of methylene blue under visible-light irradiation. We find that the synergy improvement between the graphene nanosheets and nano-sized ZnO species in the NCs hybrid photoanodes resulted in overall transfer in the photogenerated carrier separation and transfer, which in turn enhanced the solar energy harvesting. This was accompanied by an enhancement in photocatalytic water oxidation as well as the improvement of MB degradation kinetics. Furthermore, the stoichiometric ratio (i.e., 50: 50 wt.%) exhibited the optimum composition with the highest solar energy conversion efficiency.
• MoS2 nanosheets-based hybrid NCs
Hybrid NCs between MoS2 nanosheets and Co3O4 was deposited in one step at room temperature on NF porous and titanium sheet substrate to study the electrocatalytic water splitting in the alkaline medium (i.e., 1.0 M KOH) as well as the electrochemical oxidation of H2O2 in an alkaline medium (0.1 M NaOH) as illustrated in chapter 8 & 9. In both cases, the ratio of Co3O4 to MoS2 content in the micro-sized powder was mixed using the ball milling technique, which is used in the thin film deposition process by the NPDS.
The hybridization between the MoS2 nanosheets and nanostructure Co3O4 in the fabricated thin films was examined by several techniques like XRD, FE-SEM, Raman spectra, and XPS. The fabricated Co3O4- MoS2 NCs/NF was used as an anode for testing the OER in 1.0 M KOH as demonstrated in chapter 8. We
Page | 44
found that the synergy improvement between the Co3O4 and MoS2 species resulted in an improvement of the OER reaction kinetics as well as the decrease of water oxidation overpotential. Furthermore, the hybrid NCs with high MoS2 nanosheets contents exhibited better electrocatalytic activity toward the OER compared with other composites. Whereas Co3O4-MoS2 hybrid NCs with high Co3O4 content exhibited better electrocatalytic activity and selectivity toward H2O2 oxidation compared with other composites, as demonstrated in chapter 9.
In chapter 10, we tried to compare the effect of different synergy behavior between the nano-sized Mn3O4 and various types of 2D materials such as MoS2 and graphene nanosheets, which directly exfoliated and deposited in one step using the NPDS. The fabricated Mn3O4- hybrid NCs were utilized for studying the H2O2 reduction in the alkaline medium (0.1 M NaOH). In both cases, the initial ratio of Mn3O4 to the layered material (i.e., MoS2 or graphite) content was kept 1: 1 in the micro-sized powder, which the mixed process was performed by the ball milling technique. The mixed micron powder was used in the deposition process on the NF porous substrate by the NPDS. The hybridization between the 2D materials nanosheets and the nanosized Mn3O4 in hybrid NCs was examined by several techniques like XRD, FE-SEM, Raman spectra, and XPS. The electrocatalytic reduction of H2O2 was investigated in the 3 electrode cell configuration, in which the synergy improvement between the Mn3O4 and either graphene or MoS2
nanosheets resulted in an improvement of the H2O2 reduction kinetics compared with the pure Mn3O4
phase. The real-time detection of H2O2 was performed using the chronoamperometric (CA) techniques, which revealed a wide linear detection range and high sensitivity toward the H2O2 reduction.