Submitted to the Graduate School of the University of Ulsan in partial fulfillment of the requirements for the degree of. The kinetically induced transformation of graphite-plotted layers into graphene nanosheets in one step by the NPDS at room temperature was explained in terms of the mechanical exfoliation of graphite-stacked layers with an impact velocity higher than a critical value. This indicates the need to study the effect of the composition ratio between the 2D NMs and the other functional materials on the electrochemical performance of the fabricated electrodes.
Calibration curves of current-induced response with increasing concentration of H2O2 with pure phase Mn3O4 (b) and NC Mn3O4 with MoS2 (c) and graphene nanosheets (d).
Introduction
Exfoliated multilayer materials in the form of two-dimensional (2D) nanosheets with improved surface-to-volume ratio help improve performance in energy and sensing applications [8, 9]. A crushing process is expected to occur during exfoliation as shown in Figure 1. 1. The force generated by the exfoliation technique can break a large micron powder into smaller ones. The deposition process of nano-sized ceramic and metal films on various types of substrates can be performed by high-speed micron powder impact in one step at room temperature according to the vacuum kinetic sputtering process using different strategies such as cold spray, aerosol deposition technique (AD) and nanoparticle deposition system (NPDS) [64-67]. Dry spray application has general advantages such as a) application at room temperature; b) without thermal damage to the substrate; c) without the use of dangerous chemicals; and d) application in one step in a short time.
However, the derived localized thermodynamic parameters (i.e. the pressure of 2.5 GPa and temperature of 500 K) from the numerical simulation of single particle impact according to Akedo et al., [64] are not enough for interfacial bonding between the fragmented particles and the used substrate.
The detailed plan for this thesis is divided into several chapters that deal with concrete goals.
Chapter 2 demonstrates information about the used materials in the deposition process of pure phases or heterostructure electrodes by the NPDS from the initial micron powders on either metal or
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
Hybrid NCs between MoS2 nanosheets and Co3O4 were deposited in one step at room temperature on NF porous and titanium plate 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. The hybridization between the MoS2 nanoplates and nanostructure Co3O4 in the produced thin films was investigated by various techniques such as XRD, FE-SEM, Raman spectra and XPS. The fabricated Mn3O4 hybrid NCs were used to study the H2O2 reduction in the alkaline medium (0.1 M NaOH).
The mixed micron powder was used in the deposition process on the porous NF substrate by NPDS.
Experimental Conditions
Results and discussion
Furthermore, by comparing peak positions with the corresponding Raman peaks in the spectrum of the Co3O4 powder, Figure 5. Raman spectra of the deposited Co3O4-graphene composites on Ni foam with 25, 50 and 75 wt% Co3O4 content are illustrated in Figure 5. The EDS spectrum and the corresponding microstructure elemental mapping of the deposited graphene thin film in Figure 5.
The surface morphology of the nanostructured Co3O4 thin film deposited on the Ni foam is illustrated in Figure 5. In addition, less agglomeration tendency was observed in the SEM images of the composite thin films compared to pure Co3O4. The observed inhibition of aggregation tendency of Co3O4-graphene/NF thin film composites compared to Co3O4/NF thin film can be understood through the proposed mechanism.
On the other hand, the O 1s XPS spectrum of the pure Co3O4/NF catalyst is in Figure 5. The observed positive shift of the Co-O bond in the hybrid catalyst may be caused by the strong coupling between the nanostructured Co3O4 and graphene nanosheets . In the case of Co3O4-graphene hybrid catalysts, it can be observed that increasing graphene content results in a gradual decrease in onset potential, η and Tafel slope values.
In Co3O4-graphene hybrid catalysts, each of the Co3O4 nanoparticles and graphene nanosheets have a different contribution to the obsd. The enhancement observed in high Co2+ spin states results in an increase in electrocatalytic active states in the reduction of H2O2 at the working electrode interface.
Summary
1(b) shows the XRD patterns of the bare titanium plate and the deposited thin films of pure ZnO and ZnO-graphene NC at different graphite contents (25, 50 and 75 wt%). SEM morphology of deposited ZnO-graphene NCs with 25, 50 and 75 wt. % graphite content is shown in Fig. 6. 9(c, d) reveals the FTIR spectra of deposited ZnO thin films and ZnO-graphene nanosheets with different graphite contents (25, 50 and 75 wt.%) on a titanium plate substrate.
When ID/IG > 1, it represents the transformation of the layered structure of micro-sized graphite to nano-sized graphene nanosheets (i.e., the decrease in domain size in the sp2 plane) [115]. 13(a-d) show the deconvoluted high-resolution O 1s XPS scan of ZnO/Ti-sheet and nano-sized ZnO-graphene NCs/Ti-sheet with 25, 50 and 75 wt% graphite content in the binding energy range of 525 to 545 eV. The deconvolution of C 1s XPS scans of all hybrid ZnO-graphene NCs/Ti sheets in Figure 6.
The modifications of ZnO-graphene NCs/Ti-sheet hybrid photoanodes after the end of the PEC water splitting measurements were investigated using the XPS analysis as shown in Figure 6. 16(c) reveals the valence band (VB) level estimate from the XPS recording spectrum of the titanium-modified photoanodes with nano-sized ZnO and ZnO-graphene NCs in the binding energy range from -3 to 6 eV. The Mott-Schottky plots of the nanosized ZnO and ZnO-graphene NCs hybrid photoanodes with different graphite contents (25, 50 and 75 wt%) are shown in Figure 6.
6, refers to the enhancement of the concentration of minority carriers (P) within the space charge layer as Ef moves towards the Ei level [329]. According to Kelly et al. [330], the spatial separation at the semiconductor/electrolyte interface between the photogenerated electrons and holes is necessary to observe the PEC response current.
Overview
However, the wide band gap of ZnO in the UV region limits the use of the pure ZnO nanostructure in visible light harvesting and conversion applications [354] . The designed rGO/zinc ferrite NCs photocatalyst with a narrow band gap of 1.86 eV showed a high photocatalytic degradation efficiency regarding MB degradation in the presence of H2O2 oxidative agent due to the. The incorporated Cu ions in the prepared NCs led to the enhancement of visible light harvesting and MB photocatalytic degradation efficiency in H2O.
Ahmad et al., [356] synthesized ZnO graphene NCs using a combination of the common Hammer technique for graphene preparation and solvothermal technique for hybridization of graphene and ZnO NPs. The incorporation of graphene nanosheets into ZnO resulted in the improvement of visible light harvesting and in the improvement of MB photocatalytic degradation efficiency. 357] reported the preparation of rGO decorated with ZnO NPs using several sequential ZnO species uptake steps with thermal reflux at 65 °C for 24 h, followed by thermal oxidation in the oven at 100 °C for 5 h.
The synthesized heterostructure ZnO-rGO NCs showed a high photocatalytic degradation efficiency of MB under UV and visible light irradiation. The instantaneous fragmentation of the 2D layered structure from bulk graphite to graphene nanosheets is usually accompanied by a sharp increase in the localized temperature. This provides a suitable environment for strong bonding between the grains and hybridization between the nanostructured transition metal oxide (TMO) and the formed graphene species in the fabricated nanocomposite thin films.
In the present study, we have provided an effective and economically viable technique for the rapid fabrication of nanoscale hybrid photocatalysts using a one-step kinetic spray deposition at room temperature by the NPDS technique. We also evaluated the effect of variation in graphite content on the photocatalytic activity of ZnO-graphene NCs/NF hybrid photocatalysts using the photocatalytic degradation of MB under visible light.
Results and Discussion
2(d) depicts the micro-Raman spectra of the ZnO-graphene NCs/NF hybrid photocatalysts at a graphite content of 25, 50 and 75 wt%. The deposition uniformity of the ZnO/NF thin film is illustrated by the corresponding composition mapping shown in Figure 7. The fragmentation of the micro-sized graphite powder into nano-sized graphene with different morphologies (nanoflakes and nanoflowers) is depicted in SEM images of ZnO- graphene NCs/NF with different graphite contents wt%); see Figure 7.
The spatial uniformity of ZnO-graphene NCs/NF thin film on the lateral area of the NF porous substrate was also demonstrated from the corresponding composition mapping as shown in Figure 7. The carbon signal was also detected in all the ZnO-graphene NCs/NF hybrid photocatalysts. The Zn 2p high-resolution XPS spectra of nanostructured ZnO/NF and ZnO-graphene NCs/NF were further analyzed using the deconvolution process to estimate the exact positions of the peaks; see Figure 7.
The convoluted spectra of the C 1s scan for the ZnO graphene NCs/NF heterostructure with 25, 50 and 75 wt% graphite content are shown in Figure 7. The characteristic C-C bond of graphene nanosheets caused by sp2 hybridization between adjacent carbon atoms was observed in the ZnO-graphene NCs/NF hybrid photocatalysts. This confirmed the strong synergy between the different species presented in the ZnO graphene NCS/NF hybrid photocatalysts [276].
13(a) displays the optical absorption spectra of ZnO nanosheets/NF and ZnO-graphene NCs/NF hybrid photocatalysts at different graphite contents (25, 50 and 75 wt%), in the UV-visible region extending from 200 to 800 nm. In the case of ZnO-graphene NCs/NF hybrid photocatalysts, the main absorption edge was positively shifted to relatively longer wavelengths, and the optical absorption in the visible region was enhanced.
