Tungsten oxide nanocrystals engineered with oxygen vacancies absorb near-infrared light and enhance photocatalytic breakdown of dye pollutants in water.
(Nanowerk Spotlight) Extending the light-harvesting capabilities of photocatalytic materials into the near-infrared (NIR) range has presented a persistent materials science challenge. Conventional semiconductors such as titanium dioxide and stoichiometric tungsten oxide absorb primarily ultraviolet or visible light, excluding a significant portion of the solar spectrum.
Attempts to overcome this limitation using plasmonic metals like gold and silver introduced new problems: these materials are expensive and chemically unstable under operating conditions. This has prompted interest in semiconducting metal oxides that can be chemically tuned to absorb NIR light by altering their electronic structure.
One approach involves introducing oxygen vacancies into the crystal lattice of metal oxides, forming non-stoichiometric compounds such as WO₃₋ₓ (tungsten oxide deficient in oxygen). These vacancies act as intrinsic dopants, increasing the concentration of free electrons—mobile charge carriers not bound to atoms. Higher carrier densities enhance conductivity and enable a material property known as localized surface plasmon resonance (LSPR), where electrons collectively oscillate in response to incoming light. This phenomenon extends optical absorption into longer wavelengths, including the NIR range, and opens new pathways for photocatalytic activation.
Prior studies demonstrated that metal oxides with engineered oxygen deficiencies can exhibit plasmonic absorption, but many relied on simplified theoretical models that ignored the geometry of nanostructures. Moreover, the connection between LSPR behavior and photocatalytic efficiency remained largely speculative. The lack of rigorous modeling made it difficult to predict which structural features most effectively support NIR activity.
A new study published in Scientific Reports (“Enhanced photocatalytic performance of non-stoichiometric WO₃−ₓ nanocrystals via near-infrared localized surface plasmon resonance”) addresses these gaps with a systematic investigation of tungsten oxide nanocrystals. The research combines experimental synthesis, spectroscopic characterization, and LSPR simulations based on Mie-Gans theory—a model that accounts for particle shape, dielectric environment, and orientation to provide more accurate predictions than conventional approaches.
The study prepared several types of WO₃−ₓ nanocrystals by adjusting acid concentration, introducing surfactants, and applying a calcination step at 400 °C. These modifications controlled both the morphology and the degree of oxygen deficiency. Scanning electron microscopy revealed that the resulting particles adopted various shapes, including flower-like nanosheets, nanorods, and microspheres. Crucially, calcination did not drastically alter morphology but led to phase changes and size reductions that indicated increased crystallinity and defect formation.
FESEM images of WO₃−ₓ nanocrystals (a) W3, (b) W3N, (c) W1.5, (d) W1.5N, (e) W3C, (f) W3NC, (g) W1.5C, and (h) W1.5NC. (Image: Reprinted from DOI:10.1038/s41598-025-99138-x, CC BY) (click on image to enlarge)
X-ray diffraction confirmed that the heat treatment introduced non-stoichiometric phases such as W₁₈O₄₉ and WO₂.90, which are known to host oxygen vacancies. The transformation also caused a visible color shift in the powders, an indication of altered electronic states and vacancy-induced mid-gap levels. Infrared spectroscopy supported these findings, showing reduced water content and stronger tungsten–oxygen bonds in the calcined samples.
Optical absorption measurements provided evidence for extended light harvesting into the NIR region. Samples with higher vacancy concentrations displayed broad absorption tails beyond 1000 nm. Band gap energies, estimated using the Tauc method, were consistent with values for indirect semiconductors and varied slightly depending on phase composition and morphology.
The researchers simulated LSPR absorption spectra using Mie-Gans theory, incorporating particle dimensions derived from electron microscopy. These simulations revealed NIR-centered plasmonic peaks for all oxygen-deficient samples, confirming that vacancy engineering induces metallic-like optical behavior. Notably, calcination caused significant redshifts in the LSPR peaks—up to 558 nm for some samples—linked to increased free carrier density. Estimated carrier concentrations reached values above 10²² cm⁻³, approaching those of conventional plasmonic metals.
Photoluminescence spectroscopy identified multiple defect-related emissions in the visible range, consistent with various charged oxygen vacancy states. Lower photoluminescence intensity in some samples, particularly those with high carrier density, suggested reduced electron-hole recombination—a condition favorable for catalytic efficiency.
Photocatalytic performance was assessed by monitoring the degradation of methylene blue dye under light exposure. The most effective sample (W1.5NC) achieved 76% degradation within 125 minutes, approximately four times more efficient than the least active sample. Reaction kinetics followed pseudo-first-order behavior, and the rate constants confirmed the advantage of vacancy-rich, plasmon-active materials.
Further analysis using Mott–Schottky plots revealed that while conduction band levels were not ideally positioned to generate superoxide radicals, the valence band positions supported hydroxyl radical formation. These reactive species, particularly hydroxyl radicals, are instrumental in dye degradation and likely explain the observed photocatalytic trends.
Two mechanisms were proposed. The first is a traditional interband process, where light excites electrons across the band gap, creating holes that oxidize water to form hydroxyl radicals. The second involves NIR-driven LSPR excitation, where hot electrons are generated and transferred into the conduction band, interacting with adsorbed oxygen to form reactive oxygen species. Both pathways contribute to enhanced photocatalytic activity in the presence of sufficient light and engineered vacancy structures.
This study establishes a strong correlation between free carrier density and plasmonic absorption strength. Higher carrier densities promote broader and stronger NIR light absorption, which contributes to more effective photocatalytic degradation of dye pollutants under light exposure. The integrated use of experimental data and Mie-Gans modeling demonstrates how combining defect engineering with theoretical analysis can guide the design of efficient plasmonic photocatalysts.
By clarifying the optical and electronic consequences of oxygen vacancies in tungsten oxide nanostructures, this work provides a framework for developing cost-effective, light-responsive materials for environmental remediation and solar-driven chemical applications.
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