研究目的
To grow and characterize defect-free, mixed-phase-free metastable molybdenum trioxide (β-MoO3) and MoO2, and study the phase transformation from β-MoO3 to MoO2 to Mo by thermal reduction in ultra-high vacuum (UHV) using molecular beam epitaxy (MBE) technique, including morphological evolution, elemental quantification, work function, and band gap studies.
研究成果
The thermal reduction in UHV successfully transforms β-MoO3 nanoribbons to MoO2 nanoparticles and then to Mo nanoparticles, with phase transitions occurring at 400°C and 750°C. In-situ XPS confirms the reduction in oxidation states, while ex-situ measurements show decreases in work function and band gap. This provides insights into nanostructural phase transformations and their electronic properties, with implications for applications in optoelectronics and catalysis.
研究不足
The study is conducted under UHV conditions, which may not represent ambient environments. Temperature constraints in the MBE system limit direct comparison between XPS and microscopy measurements. The reduction process may involve residual phases, and the bimodal size distribution of Mo nanoparticles requires further mechanistic understanding.
1：Experimental Design and Method Selection:
The study uses molecular beam epitaxy (MBE) for growth under ultra-high vacuum (UHV) conditions to synthesize β-MoO3 nanostructures and induce phase transitions via thermal annealing. Techniques include in-situ XPS, ex-situ microscopy (FESEM, TEM, GIXRD), KPFM, and UV-Vis-NIR spectroscopy for comprehensive characterization.
2：Sample Selection and Data Sources:
Atomically cleaned Si(100) substrates and commercially available α-MoO3 powder (99.995% purity) are used. Samples are prepared at various substrate temperatures (350°C to 600°C) and annealed up to 750°C.
3：995% purity) are used. Samples are prepared at various substrate temperatures (350°C to 600°C) and annealed up to 750°C. List of Experimental Equipment and Materials:
3. List of Experimental Equipment and Materials: MBE system (Omicron GmbH), HTEZ effusion cell, pyrolytic graphite crucible, FESEM (Carl Zeiss Neon 40), TEM (JEOL JEM-2010), GIXRD with Cu Kα source, UV-Vis-NIR spectrometer (Shimadzu UV-3101PC), in-situ XPS (Omicron XPS instrument with Al Kα source), KPFM (Asylum Research MFP-3D AFM), and various tips and heaters.
4：Experimental Procedures and Operational Workflow:
Substrates are cleaned and degassed. MoO3 is deposited at controlled rates and temperatures. Annealing is performed in UHV. Morphology and structure are analyzed using SEM, TEM, XRD. Chemical composition is monitored with in-situ XPS. Work function is measured with KPFM, and band gap with UV-Vis spectroscopy.
5：Data Analysis Methods:
XPS data are fitted with Gaussian components. SEM and TEM images are analyzed for size distribution using log-normal fitting. Work function is calculated from KPFM measurements. Band gap is determined from Tauc plots of UV-Vis data using Kubelka-Munk equation.
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FESEM
Neon 40
Carl Zeiss
Used for ex-situ surface morphology characterization with high-resolution imaging.
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TEM
JEM-2010
JEOL
Used for high-resolution transmission electron microscopy to analyze nanostructure morphology and crystallography.
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UV-Vis-NIR spectrometer
UV-3101PC
Shimadzu Corporation
Used to study optical properties and measure band gaps of the samples.
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MBE system
Custom-built
Omicron GmbH
Used for growing β-MoO3 nanostructures under ultra-high vacuum conditions.
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XPS instrument
Omicron XPS with SPHERA energy analyzer
Omicron
Used for in-situ X-ray photoelectron spectroscopy to analyze chemical composition and oxidation states.
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AFM
MFP-3D
Asylum Research
Used for Kelvin probe force microscopy (KPFM) to measure local work function.
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Pyrolytic graphite crucible
Used for evaporation of molybdenum oxide in the MBE system.
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Quartz crystal microbalance
Used to measure the thickness of deposited material, pre-calibrated by Rutherford back scattering spectrometry (RBS).
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Type K thermocouple
Used to measure substrate temperature during deposition.
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Optical pyrometer
Used to check and monitor temperatures during experiments.
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Dual tungsten filament e-beam heater
Used to anneal specimens during in-situ XPS measurements.
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Infrared pyrometer
Used to monitor temperature during in-situ heating experiments.
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