Combining electrospinning with physical vapor deposition (PVD) offers a powerful approach to fabricate semiconductor nanofiber composites with tailored functionalities. This hybrid technique leverages the advantages of both methods—electrospinning for producing high-surface-area nanofibers and PVD for depositing uniform, conformal coatings of metals or other materials. A notable example is the fabrication of titanium dioxide (TiO2) nanofibers coated with gold (Au), which exhibit enhanced plasmonic and photocatalytic properties due to the synergistic effects between the semiconductor and the metal.
Electrospinning produces polymer or inorganic nanofibers with diameters ranging from tens to hundreds of nanometers. For TiO2 nanofibers, a precursor solution containing a titanium alkoxide and a polymer like polyvinylpyrrolidone (PVP) is electrospun, followed by calcination to convert the fibers into crystalline TiO2. The resulting nanofibers possess a high surface-to-volume ratio and porous structure, ideal for photocatalytic applications. However, pure TiO2 has limitations, such as rapid electron-hole recombination and a wide bandgap that restricts light absorption to the ultraviolet range. Integrating plasmonic metals like Au via PVD addresses these challenges.
PVD techniques, such as sputtering or thermal evaporation, deposit Au onto the TiO2 nanofibers with precise control over thickness and morphology. The Au coating forms nanoparticles or a thin film on the fiber surface, depending on deposition parameters like power, time, and substrate temperature. The interaction between Au and TiO2 creates localized surface plasmon resonance (LSPR) effects, where Au nanoparticles absorb visible light due to collective oscillations of conduction electrons. This phenomenon enhances the composite's light-harvesting capability beyond TiO2's intrinsic bandgap.
The plasmonic effects in Au-TiO2 nanofiber composites significantly improve photocatalytic performance. Under visible light irradiation, LSPR excitation in Au nanoparticles generates hot electrons that inject into the TiO2 conduction band, initiating redox reactions. This process extends the photocatalytic activity of TiO2 into the visible spectrum, enabling applications like water splitting, pollutant degradation, and organic synthesis. Studies have demonstrated that Au-TiO2 composites exhibit higher degradation rates for organic dyes like methylene blue compared to pure TiO2, with enhancements up to 3-5 times under visible light. The exact improvement depends on factors like Au nanoparticle size, loading density, and TiO2 crystallinity.
The hybrid electrospinning-PVD approach also optimizes charge carrier dynamics. The intimate contact between Au and TiO2 reduces electron-hole recombination by providing efficient electron transfer pathways. Additionally, the nanofiber morphology ensures that Au nanoparticles are evenly distributed, maximizing light absorption and active sites for catalysis. For instance, a study reported that Au-coated TiO2 nanofibers achieved a hydrogen evolution rate of 15 µmol/h under visible light, whereas uncoated TiO2 showed negligible activity. The Au loading and nanoparticle size were critical, with optimal performance observed at 2-5 nm diameters and 1-3 wt% Au.
Beyond photocatalysis, these composites find use in plasmon-enhanced sensors and photoelectrochemical devices. The LSPR of Au nanoparticles enhances the local electric field, improving surface-enhanced Raman scattering (SERS) sensitivity for detecting trace molecules. In photoelectrochemical water splitting, Au-TiO2 nanofibers exhibit higher photocurrent densities due to improved charge separation and light absorption. The table below summarizes key performance metrics for Au-TiO2 nanofiber composites in various applications:
Application | Performance Metric | Improvement Over Pure TiO2
Photocatalytic dye degradation | Degradation rate constant (min-1) | 3-5x higher under visible light
Hydrogen evolution | H2 production rate (µmol/h) | 10-15x higher under visible light
SERS detection | Enhancement factor | 10^4-10^6 for target molecules
Photoelectrochemical response | Photocurrent density (mA/cm2) | 2-3x higher under simulated sunlight
The hybrid fabrication method also allows for tuning the composite's properties by adjusting electrospinning and PVD parameters. For example, varying the calcination temperature of TiO2 nanofibers controls their crystallinity (anatase vs. rutile phase), which affects charge carrier mobility and photocatalytic activity. Similarly, PVD conditions influence Au nanoparticle size and distribution, directly impacting plasmonic resonance wavelength and intensity. A balance must be struck between Au loading and light penetration depth—excessive Au coverage can block light absorption by TiO2, while insufficient coating limits plasmonic effects.
Challenges remain in scaling up production and ensuring long-term stability. Agglomeration of Au nanoparticles during PVD or under operational conditions can degrade performance. Strategies like incorporating adhesion layers or using alloy coatings (e.g., Au-Ag) mitigate these issues. Additionally, the cost of Au motivates research into alternative plasmonic materials like silver or copper, though their environmental stability is inferior.
In summary, electrospinning combined with PVD enables the design of advanced semiconductor nanofiber composites with plasmonic enhancements. Au-TiO2 nanofibers exemplify how this hybrid approach can overcome material limitations, unlocking new functionalities in photocatalysis, sensing, and energy conversion. The method's versatility extends to other semiconductor-metal systems, paving the way for next-generation optoelectronic and catalytic materials. Future work may explore multi-component coatings or core-shell architectures to further optimize performance and scalability.