PLD growth of strontium titanate thin films on silicon substrate for photoelectrochemical water-splitting

Epitaxial films of metal oxides deposited on silicon substrates represent a new type of material that could be used as protective (or electroactive) layer in the photoelectrochemical water splitting. To understand the influence of crystalline and interfacial properties of oxide layer on the water splitting process a ~10 nm strontium titanate (STO) films have been grown using the PLD method on bare and reduced graphene oxide (rGO) buffered silicon substrate. Our approach relied on the oxide-silicon integration using combination of SrO-assisted deoxidation and controllable coverage of silicon surface with a mono- to threelayer of spin-coated GO. The STO films have been grown at 515 and 700 °C and various experimental techniques were used to examine the surface and crystalline properties of grown films (reflection high energy electron diffraction, atomic force microscopy, scanning electron microscopy, X-ray diffraction, X-ray reflectivity and X-ray photoelectron spectroscopy). The results show that the best the crystallinity of the STO thin films was obtained on rGO/SrO deoxidized silicon surface at 515 °C. Future studies will be devoted to electrochemical characterization of the grown films, that will help to establish clearer link on how the interface and crystalline parameters affect the water splitting process.Workshop “Application-oriented material development”; September 12-14, Bucharest, 2023.Contribution: Poste

PLD growth of strontium titanate thin films on SrO-deoxidized and rGO-buffered Si(001) substrate

Epitaxy represents a process of crystal growth or material deposition in which the new created layers have a high degree of crystallographic alignment with the substrate lattice. In this research 10 nm-thick thin films of strontium titanate (STO) were grown using pulsed laser deposition (PLD) method on Si(001) whose surface was either deoxidized with strontium oxide (SrO) or buffered by reduced graphene oxide (rGO) in combination with SrO deoxidation. In addition to differently prepared Si(001) surface, the effect of deposition temperature on the crystalline structure of the STO thin films was also examined. Reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray reflectivity (XRR) and X-ray photoelectron spectroscopy (XPS) methods were used to examine the properties of the grown films. It was concluded that the STO thin film grown on the rGO-coated Si substrate at 515 °C shows the highest crystallinity with a smooth surface, while the film deposited on the bare silicon has amorphous structure. The STO films grown at 700 °C show textured or polycrystalline structure. Good crystallinity, epitaxial alignment, and clean interface are the major requirements for STO/Si and the STO/rGO/Si heterostructure for making an efficient and stable Si photocathode for the photoelectrochemical (PEC) water splitting. Our future work will be directed toward understanding how the obtained interfaces and crystalline structure of STO films are influencing the PEC process.Twenty-First Young Researchers’ Conference - Materials Science and Engineering: Program and the Book of Abstracts; November 29 – December 1, 2023, Belgrade, Serbi

Robust SrTiO3 Passivation of Silicon Photocathode by Reduced Graphene Oxide for Solar Water Splitting

Development of a robust photocathode using lowcost and high-performing materials, e.g., p-Si, to produce clean fuel hydrogen has remained challenging since the semiconductor substrate is easily susceptible to (photo)corrosion under photoelectrochemical (PEC) operational conditions. A protective layer over the substrate to simultaneously provide corrosion resistance and maintain efficient charge transfer across the device is therefore needed. To this end, in the present work, we utilized pulsed laser deposition (PLD) to prepare a high-quality SrTiO3 (STO) layer to passivate the p-Si substrate using a buffer layer of reduced graphene oxide (rGO). Specifically, a very thin (3.9 nm ∼10 unit cells) STO layer epitaxially overgrown on rGO-buffered Si showed the highest onset potential (0.326 V vs RHE) in comparison to the counterparts with thicker and/or nonepitaxial STO. The photovoltage, flat-band potential, and electrochemical impedance spectroscopy measurements revealed that the epitaxial photocathode was more beneficial for charge separation, charge transfer, and targeted redox reaction than the nonepitaxial one. The STO/rGO/Si with a smooth and highly epitaxial STO layer outperforming the directly contacted STO/Si with a textured and polycrystalline STO layer showed the importance of having a well-defined passivation layer. In addition, the numerous pinholes formed in the directly contacted STO/Si led to the rapid degradation of the photocathode during the PEC measurements. The stability tests demonstrated the soundness of the epitaxial STO layer in passivating Si against corrosion. This study provided a facile approach for preparing a robust protection layer over a photoelectrode substrate in realizing an efficient and, at the same time, durable PEC device

Epitaxial oxides on semiconductors: growth perspectives and device applications

Epitaxial integration of transition metal oxides with semiconductors offers various phenomena for novel device applications, specifically bringing ferroelectric, ferromagnetic, electro-optic, photocatalytic, multiferroic, piezoelectric and other properties to the wellestablished silicon platform. A convenient way of integrating functional oxides with Si(001) substrate is through a SrTiO3 (STO) intermediate layer, which can be fabricated on Si(001) in epitaxial form and with high crystallinity using. The epitaxial growth of functional oxides on silicon substrates requires atomically defined surfaces, which are most effectively prepared using SrO- or Sr-induced deoxidation and passivation. As-prepared surfaces enable overgrowth with various oxides for novel device applications. In our work pulsed laser deposition (PLD) was used to integrate oxides with silicon. We showed the ability to prepare highly-ordered sub-monolayer SrO- and Sr-based surface structures, including two-domain (2×3)+(3×2) pattern at 1/6 ML Sr coverage as determined by the reflection high-energy electron diffraction (RHEED) technique. On the passivated silicon surface epitaxial layers of STO was grown by the method of kinetically controlled sequential deposition. Detailed study of initial deposition parameters proved to be extremely important in achieving epitaxial relation of STO with the underlying substrate. On as-prepared pseudo-substrate different functional films were gown for applications in microelectromechanical systems and electrochemical devices.XI Serbian Ceramic Society Conference - Advanced Ceramics and Application : new frontiers in multifunctional material science and processing : program and the book of abstracts; September 18-20, 2023; Belgrad

The influence of heteroatoms on physicochemical properties of cobalt ferrite nanoparticles

In the last two decades, cobalt ferrite (CoFe2O4, CFO) has attracted extensive attention due to its applicability in data storage, catalysis, energy, environment, and in particular, biomedicine. To further extend applicability and improve understanding of fundamental processes, the present work investigates the influence of heteroatoms on physicochemical properties of CFO. Solvothermal method was used for designing a non-agglomerated particles with uniform morpho-structural properties. The physicochemical properties of Zn2+ and Ga3+ substituted CFO nanoparticles were examined (Co(1-x)ZnxFe2O4 and CoGaxFe(2-x)O4; x=0, 0.1, 0.3 and 0.5). The X-ray diffraction (XRD) measurements confirmed the presence of pure cubic spinel phase in all samples, while the transmission electron microscopy (TEM) showed sphere-like nanoparticles with a mean diameter of 6±1 nm. The amount of adsorbed oleic acid on the surface of the nanoparticles, determined by thermogravimetric (TG) analysis indicates the formation of a complete monolayer of surfactant. The FT-IR analysis substantiated the presence of oleic acid on the surface of the nanoparticles and discovered its covalent bonding to the metal atoms. Substitution of host-atoms was also confirmed by Raman spectroscopy. Magnetic measurements revealed the influence of heteroatoms on saturation magnetization and magnetic anisotropy, showing for all the samples superparamagnetic behaviour at room temperature. The substitution of Co2+ and Fe3+ ions with Zn2+ and Ga3+, respectively, leads to the change in chemical composition and cationic distribution of CFO and consequently to variation of its magnetic properties.Twenty-second Annual Conference YUCOMAT : August 30 - September 3, Herceg Novi, 202

Structural Changes, Dielectric and Ferroelectric Properties of Tribophysically Activated BaTiO3

In order to obtain nanocrystalline material which can be used in MLCC production, the investigations of the influence of BaTiO3 powder tribophysical activation (TPA) on its structural changes, dielectric and ferroelectric properties have been performed. Microstructure development and crystal structure have been studied by mercury porosimetry method, SEM, EDS and X-ray powder diffraction analyses. The modifications of dielectric and ferroelectric properties of sintered samples have been examined and correlated with observed structural changes induced by TPA of starting powders. It has been found that dielectric and ferroelectric properties of tribophysically activated BaTiO3 could be tuned by controlling the grain size and lattice strain of activated nanostructured material

Exploring the impact of calcination parameters on the crystal structure, morphology, and optical properties of electrospun Fe 2 TiO 5 nanofibers

Nanostructured Fe2TiO5 (pseudobrookite), a mixed metal oxide material holds significant promise for utilization in energy and environmental applications. However, its full application is still hindered due to the difficulty to synthesize monophasic Fe2TiO5 with high crystallinity and a large specific surface area. Herein, Fe2TiO5 nanofibers were synthesized via a versatile and low-cost electrospinning method, followed by a calcination process at different temperatures. We found a significant effect of the calcination process and its duration on the crystalline phase in the form of either pseudobrookite or pseudobrookite–hematite–rutile and the morphology of calcined nanofibers. The crystallite size increased whereas the specific surface area decreased with an increase in calcination temperature. At higher temperatures, the growth of Fe2TiO5 nanoparticles and simultaneous coalescence of small particles was noted. The highest specific surface area was obtained for the sample calcined at 500 °C for 6 h (SBET = 64.4 m2 g−1). This work opens new opportunities in the synthesis of Fe2TiO5 nanostructures using the electrospinning method and a subsequent optimized calcination process for energy-related applications

Synthesis, structure and electrochemical performance of NiMn2O4

NiMn2O4, with a cubic spinel structure and numerous and various applications in modern technology, were synthesized with two synthetic routes: sol-gel combustion method with glycine as fuel and electrospinning method with polyvinylpyrrolidone (PVP). Both amorphous powders from sol-gel synthesis and as-spun fibers from electrospinning synthesis were calcined, electrospun fibers at 400 °C and the sol-gel synthesized powders at 800 °C. Electrospun fibers were previously characterized with DTA-TGA to investigate the influence of thermal process on a polymer fiber.The obtained powders were characterized accordingly. Structural analysis was done via X-ray diffraction (XRD) and results show spinel structure with no impurity. The texture and morphology was investigated via N2 physisorption and transmission electron microscopy (TEM), respectively. Chemical states of elements were investigated by X-ray photoelectron spectroscopy (XPS). The electrochemical performance of the synthesized materials as supercapacitors was tested via cyclic voltammetry (CV), electric impedance spectroscopy (EIS), and chronopotentiometry (CP) to aquire galvanostatic charge-discharge (GCD) curves. Experiments were done in 6 M KOH solution with nickel foam as a working electrode. The results show good electrochemical capacity circa 200 F/g, with the potential for further structural improvement of the materials

Synthesis, structure and electrochemical performance of NiMn2O4

NiMn2O4, with a cubic spinel structure and numerous and various applications in modern technology, were synthesized with two synthetic routes: sol-gel combustion method with glycine as fuel and electrospinning method with polyvinylpyrrolidone (PVP). Both amorphous powders from sol-gel synthesis and as-spun fibers from electrospinning synthesis were calcined, electrospun fibers at 400 °C and the sol-gel synthesized powders at 800 °C. Electrospun fibers were previously characterized with DTA-TGA to investigate the influence of thermal process on a polymer fiber. The obtained powders were characterized accordingly. Structural analysis was done via X-ray diffraction (XRD) and results show spinel structure with no impurity. The texture and morphology was investigated via N2 physisorption and transmission electron microscopy (TEM), respectively. Chemical states of elements were investigated by X-ray photoelectron spectroscopy (XPS). The electrochemical performance of the synthesized materials as supercapacitors was tested via cyclic voltammetry (CV), electric impedance spectroscopy (EIS), and chronopotentiometry (CP) to aquire galvanostatic charge-discharge (GCD) curves. Experiments were done in 6 M KOH solution with nickel foam as a working electrode. The results show good electrochemical capacity circa 200 F/g, with the potential for further structural improvement of the materials