Zinc oxide (ZnO) is a typical wide-bandgap semiconductor photocatalyst with a bandgap energy of approximately 3.3 eV. While ZnO demonstrates high chemical stability and strong oxidation capability, its photoresponse is intrinsically limited to the ultraviolet (UV) region, which constitutes only a minor portion of the solar spectrum. This intrinsic limitation severely affects the efficient utilization of solar energy, thereby motivating the development of physical strategies to extend the optical response of ZnO toward longer wavelengths, particularly into the near-infrared (NIR) region.
In this work, a near-infrared-responsive photocatalytic composite system is designed by coupling ZnO nanoparticles with core-shell structured upconversion nanoparticles (UCNPs, NaYF₄:Yb,Tm@NaYF₄). To achieve precise spectral regulation, an organic molecule (C₂₁H₂₁IN₂O₂) is introduced to selectively modulate the upconversion emission profile of the UCNPs. Under 980 nm NIR excitation, this organic molecule effectively suppresses redundant visible emissions while enhancing ultraviolet upconversion emission. Consequently, the dominant emission bands of the modified UCNPs are centered at 345 nm and 361 nm, which exhibit strong spectral overlap with the intrinsic absorption edge of ZnO.
Structural characterization confirms that the UCNPs-C₂₁H₂₁IN₂O₂/ZnO composite is formed without generating new crystalline phases, with both the UCNPs and ZnO retaining their original crystal structures after assembly. Optical analyses reveal that the optimized spectral matching significantly reduces the effective energy transfer distance and enables ZnO to efficiently absorb the upconverted ultraviolet photons generated by the UCNPs under NIR irradiation. In this composite system, the UCNPs function as nanoscale ultraviolet light sources, while ZnO absorbs the converted high-energy photons to generate electron–hole pairs. This indirect excitation pathway effectively extends the photoresponse of ZnO from the UV region into the near-infrared region.
The photocatalytic performance of the composite is assessed using methylene blue as a model organic pollutant. Under 980 nm near-infrared irradiation, the UCNPs-C₂₁H₂₁IN₂O₂/ZnO composite exhibits a degradation efficiency of 93.33% within 70 minutes, significantly higher than that achieved under conventional 365 nm ultraviolet excitation (64.16%). Kinetic analysis shows that the degradation process follows a pseudo-first-order model, with a reaction rate constant of 34.7 × 10^-3 min^-1 under NIR excitation. In addition, cycling tests confirm good structural and photocatalytic stability, with the degradation efficiency remaining above 85.95% after five consecutive cycles.
These results demonstrate that precise modulation of upconversion emission offers an effective physical strategy to overcome the spectral constraints of wide-bandgap semiconductors. The proposed UCNP-based composite system enables the conversion of ZnO into a near-infrared-responsive photocatalyst while preserving its intrinsic crystal structure. This work provides both experimental evidence and physical insight for near-infrared-driven photocatalysis, and proposes a feasible approach to extend the photoresponse of semiconductor toward full-spectrum solar energy utilization.