UV-Visible Photocatalysis

Updated 13 October 2025

UV-visible photocatalysis is a process that uses photon absorption in semiconductors to generate charge carriers for energy conversion and environmental remediation.
It employs techniques such as doping, defect engineering, and heterojunction formation to extend light absorption from the UV to visible spectrum.
Advanced spectroscopy and nanofluidic methods enable precise characterization of charge dynamics and surface chemistry, boosting photocatalytic efficiency.

UV-visible-light-driven photocatalysis is a process wherein photon absorption from ultraviolet (UV) and visible regions of the electromagnetic spectrum generates charge carriers in a photocatalyst, initiating redox reactions that drive energy conversion and environmental remediation. This field encompasses the design, characterization, and mechanistic understanding of materials—primarily semiconductors—engineered to harvest a broader spectral range of solar radiation, perform photochemical transformations such as water splitting, degradation of hazardous organic compounds, and promote advanced oxidation processes. The development and optimization of UV-visible-light photocatalysts require precise control of electronic structure, bandgap engineering, charge carrier dynamics, surface chemistry, heterojunction formation, and targeted morphological features.

1. Fundamental Mechanisms and Material Engineering

The basic principle of UV-visible-light photocatalysis entails the excitation of electrons from the valence band (VB) to the conduction band (CB) of a semiconductor upon absorption of photons with energy $h\nu \geq E_g$ (where $E_g$ is the bandgap), leaving behind holes in the VB. These photogenerated carriers participate in surface reactions—typically oxidation by holes and reduction by electrons. The efficiency is dictated by effective light harvesting across a wide spectral range, band edge alignment relative to the redox potentials of the targeted reactions, and the suppression of electron–hole recombination.

Bandgap engineering is employed to extend light absorption from the UV ( $\lambda < 400$ nm) into the visible regime ( $400 < \lambda < 700$ nm), utilizing several strategies:

Doping and Defect Engineering: Introduction of aliovalent cations, anions (e.g., N, C), or generation of oxygen vacancies and Ti $^{3+}$ centers generates mid-gap defect states that narrow $E_g$ and allow visible-light activation, as shown in low-temperature NaH-treated TiO $_2$ , yielding a tunable bandgap from 3.04 eV down to 1.82 eV (Wang et al., 2013).
Composite and Heterostructure Formation: Construction of type-II band-aligned or Z-scheme heterojunctions at interfaces between different semiconductor phases (e.g., anatase/rutile TiO $_2$ (Oh et al., 2020), WO $_3$ /Ti-WO $_x$ /TiHyO $_z$ (Kato et al., 2022), In $_2$ S $_3$ /Pt–TiO $_2$ (Wang et al., 2016), TGCN/Janus TMDC (Arra et al., 2019)) drives spatial separation of electrons and holes and exploits interfacial built-in electric fields for enhanced charge migration and suppressed recombination.
Plasmonic and Photonic Enhancement: Integration of plasmonic metal nanoparticles (e.g., Au, Ag) induces localized surface plasmon resonance (LSPR), which enhances visible absorption and generates "hot" electrons that transfer into semiconductors, further boosted by photonic crystal effects that slow photon propagation and maximize absorption at critical energies (Collins et al., 2019, Orlanducci et al., 25 Nov 2024).
Novel Materials Platforms: Development of high-entropy oxides (HEOs) with multiple lattice sites expands configurational entropy and forms a network of heterojunctions, synergistically optimizing visible absorption, charge separation, and redox capability (Edalati et al., 2023).

2. Structural Characterization and Optical Properties

Advanced structural, spectroscopic, and optical techniques are essential for correlating material features with catalytic performance:

X-ray Diffraction (XRD), SEM/EDS: Determine crystallinity, phase composition, grain size, and lattice strain; critical for establishing the presence of mixed phases (e.g., anatase/rutile), defect levels, and composite quality (Wang et al., 2013, Karmakar et al., 2022).
Raman and FTIR Spectroscopy: Provide vibrational signatures specific to structural motifs and confirm the presence of oxygen deficiency, defect phases, intervalent cations (e.g., W $^{5+}$ /W $^{4+}$ ) that initiate new band edge states (Kato et al., 2022).
XPS/UPS and Valence Band Analysis: Quantify doping, defect concentrations, valence/conduction band edge positions, and measure band restructuring (e.g., HOMO–LUMO alignment transition from type-I to type-II heterojunctions) (Oh et al., 2020, Comes et al., 2015).
UV–Vis Absorption and Tauc Analysis: Evaluate optical band gaps, absorption edges, and light-harvesting profiles. Techniques such as the Kubelka–Munk function $F(R)$ , Tauc plots, and diffuse reflectance spectroscopy provide quantitative assessments critical for performance optimization.

These measurements confirm shifts in absorption spectra, reductions in recombination (via photoluminescence quenching), and enhanced visible-light utilization through structural and electronic modification.

3. Surface Chemistry, Charge Dynamics, and Photocatalytic Mechanism

The surface chemistry, active site heterogeneity, and time-resolved dynamics of charge carriers fundamentally control photocatalytic activity:

Surface Termination Effects: Atomic-resolution STM and selective deposition techniques demonstrate that specific terminations (e.g., fivefold coordinated Ti on rutile TiO $_2$ (Tan et al., 2011), SrO/TiO $_2$ on SrTiO $_3$ (Sharma et al., 2022)) provide atomic sites where water adsorption, dissociation, and proton-coupled electron transfer (PCET) steps are energetically and spatially favored, supporting multi-step oxidation or reduction processes.
Charge Carrier Migration and Recombination: Ultrafast pump–probe techniques (transient absorption, reflectance, OPTP spectroscopy) directly measure charge transfer times (e.g., sub-5-ps electron transfer across In $_2$ S $_3$ /TiO $_2$ /Pt interfaces (Wang et al., 2016)) and multi-ns-lived charge populations crucial for catalytic turnover before recombination occurs (Comes et al., 2015).
Radical Formation Pathways: Generation of highly reactive radicals (•OH, O $_2^{\bullet -}$ , H $_2$ O $_2$ ), via VB hole oxidation or CB electron reduction, underpins most degradation and water splitting reactions. The role of adsorbed reactant species, adsorption energies, and molecule–catalyst interactions are quantitatively assessed using DFT-calculated parameters (e.g., $E_\text{ads}$ for dye isomer–TiO $_2$ interactions controlling disinfection efficiency (Elmenaouar et al., 2017)).

A key outcome is the precise mapping of photoinduced elementary steps: $\mathrm{H_2O + h^+ \longrightarrow \cdot OH + H^+}$ for initial water oxidation (Tan et al., 2011), or four-step PCET sequences on SrO surfaces (Sharma et al., 2022).

4. Photocatalyst Designs and Application Domains

UV-visible-light-driven photocatalysts have been adapted for a wide array of applications:

Solar Water Splitting and Hydrogen Evolution: Enhanced visible absorption and efficient charge separation in engineered heterojunctions (e.g., type-II anatase/disordered rutile TiO $_2$ (Oh et al., 2020), La/Cr co-doped titanates (Comes et al., 2015), high-entropy oxides (Edalati et al., 2023)) have resulted in step changes in H $_2$ evolution rates (up to 55-fold over conventional catalysts) and lower noble metal co-catalyst requirements, improving cost effectiveness.
Water and Air Remediation: Supported mixed-phase catalysts (e.g., TiO $_2$ /quartz sand (Hanaor et al., 2014), Sb $_2$ WO $_6$ (Karmakar et al., 2022), WO $_3$ /Ti-WO $_x$ /TiHyO $_z$ (Kato et al., 2022), CoFe $_2$ O $_4$ (Prajapati et al., 29 Jul 2025)) demonstrate high pollutant degradation/genotoxicity abatement under visible or LED illumination due to bandgap narrowing, augmented ROS formation, and improved recyclability provided by magnetic nanoparticle supports.
Indoor Air Quality and VOC Removal: Nitrogen-doped TiO $_2$ photoactive coatings, modeled via comprehensive first-order reaction–diffusion–convection frameworks (Jiang et al., 2018), predict 80% VOC removal in 2–10 days under visible light, with performance strongly dependent on coating geometry, air mixing, and ambient temperature.
Biocompatible and Sustainable Photocatalysis: Protein/silk hybrids (e.g., mKate2-silk (Leem et al., 2018)) and detonation nanodiamond–Au composites (AuNP@DND (Orlanducci et al., 25 Nov 2024)) demonstrate non-toxic, visible-light-activated radical and hydrated electron generation, opening solar-driven routes for green chemistry, nanomedicine, and targeted pollutant reduction.

5. Advanced Spectroscopy, Nanoscale Flow, and Mechanistic Probing

Emerging nanofluidic and spectroscopic methods enable catalytic studies with unprecedented sensitivity and spatiotemporal resolution:

Nanofluidic Scattering Spectroscopy (NSS): This technique achieves label-free, attoliter-scale, real-time monitoring of chemical composition and photoreaction kinetics via visible-light scattering in nanofluidic channels, with online reference correction and reverse Kramers–Kronig analysis for extinction coefficient determination (Altenburger et al., 27 Feb 2025). It is uniquely suited for studying single-particle catalysis and tracking intermediates in continuous flow systems.
Integration with Microfluidics: Continuous flow devices ensure reproducible exposure, rapid solution exchange, and minimize sample consumption—crucial for high-throughput screening or mechanistic elucidation when sample volume is limiting or dynamic environments must be simulated.

6. Challenges, Trade-offs, and Future Prospects

Key challenges in UV-visible-light-driven photocatalysis include:

Balancing bandgap narrowing against unwanted recombination via deep defect states.
Controlling the morphology, crystallinity, and phase purity at scale for optimal performance (e.g., via hydrothermal synthesis (Karmakar et al., 2022), ball-milling (Kato et al., 2022), single-mode microwave oxidation (Kato et al., 2022)).
Ensuring stability and recyclability, especially under fluctuating irradiation and for hybrid or doped systems subject to photo- and chemical degradation.
Achieving comprehensive mechanistic understanding of surface, bulk, and interface processes, facilitating the rational design of novel material systems (e.g., high-entropy oxides (Edalati et al., 2023), van der Waals heterostructures (Arra et al., 2019), plasmon/photonic hybrids (Collins et al., 2019)).
Bridging from laboratory-scale demonstration to practical deployment, including reactor design, light management (spectral overlap, photon flux), and integration with environmental or energy systems.

Future directions emphasize scalable, low-temperature, and green synthesis; computationally guided interface and defect engineering; in situ diagnostic integration; and multiphysics modeling to achieve robust, cost-effective, and high-activity photocatalytic materials for sustainable energy and environmental technologies.