Dual-functional photocatalytic materials represent a significant advancement in sustainable technology, capable of simultaneously producing hydrogen and degrading organic pollutants. These systems address two critical environmental challenges: clean energy generation and wastewater treatment. Among the most studied materials are TiO2/g-C3N4 heterojunctions and doped ZnO, which exhibit unique electronic and catalytic properties enabling coupled redox reactions.

The mechanism of dual-functional photocatalysis relies on the generation of electron-hole pairs under light irradiation. When photons with sufficient energy strike the photocatalyst, electrons are excited from the valence band to the conduction band, leaving holes behind. These charge carriers drive two parallel processes: hydrogen evolution through water reduction and pollutant degradation through oxidation. In TiO2/g-C3N4 heterojunctions, the staggered band alignment enhances charge separation. The conduction band of g-C3N4 lies at a more negative potential than TiO2, facilitating electron transfer to TiO2 for proton reduction. Meanwhile, holes accumulate on g-C3N4, oxidizing organic pollutants. Doped ZnO systems, such as nitrogen or sulfur-doped variants, introduce mid-gap states that narrow the bandgap, improving visible light absorption and charge carrier mobility.

Sacrificial agents play a crucial role in enhancing hydrogen production efficiency. Organic pollutants in wastewater can act as sacrificial electron donors, consuming photogenerated holes and preventing electron-hole recombination. Common sacrificial agents include methanol, ethanol, and glycerol, but real-waste feedstock such as industrial dyes, phenolic compounds, or pharmaceutical residues are increasingly studied. For instance, textile wastewater containing azo dyes has been successfully treated while generating hydrogen using TiO2/g-C3N4 composites. The degradation of methyl orange or rhodamine B not only cleans the water but also supplies electrons for H2 evolution, creating a synergistic effect.

Real-waste feedstock utilization presents both opportunities and challenges. Industrial effluents often contain complex mixtures of organics, heavy metals, and suspended solids that can poison catalysts or block active sites. However, certain components, like chloride ions in dyeing wastewater, can enhance photocatalytic activity by forming reactive chlorine species that aid pollutant degradation. The key is tailoring photocatalyst composition and morphology to withstand harsh conditions. For example, mesoporous TiO2/g-C3N4 structures with high surface area exhibit better resistance to fouling and maintain activity over multiple cycles.

Reaction efficiency remains a primary challenge. Quantum yields for hydrogen production in dual-functional systems are often lower than in dedicated photocatalytic water splitting due to competitive reactions. Pollutant degradation kinetics can also vary widely depending on molecular structure. Persistent organic pollutants like perfluorooctanoic acid require highly oxidative conditions, which may divert holes away from water oxidation, reducing H2 output. Strategies to optimize dual activity include cocatalyst deposition, defect engineering, and heterojunction design. Pt or NiS cocatalysts on TiO2/g-C3N4 improve electron extraction for H2 evolution, while oxygen vacancies in doped ZnO enhance hole mobility for pollutant breakdown.

Scalability is another critical hurdle. Most studies are conducted at lab scale with artificial light sources, but real-world applications demand solar-driven systems and large reactor designs. Photocatalyst immobilization on substrates like glass fibers or alumina foams avoids the need for particle recovery but can limit light penetration and mass transfer. Pilot-scale reactors using compound parabolic collectors or thin-film fixed-bed configurations have shown promise, with reported hydrogen production rates of 0.5–2 mmol/h per gram of catalyst when treating real wastewater streams.

Design principles for optimizing dual-functional materials emphasize balancing redox capabilities. The ideal photocatalyst should have a conduction band position more negative than the H+/H2 redox potential (-0.41 V vs. NHE at pH 7) and a valence band more positive than the pollutant oxidation potential. For instance, g-C3N4’s conduction band at -1.1 V is suitable for H2 generation, while its valence band at +1.6 V can oxidize most organics. Doping with transition metals like Fe or Co introduces additional energy levels that mediate charge transfer, further improving efficiency.

Case studies in industrial wastewater treatment demonstrate the practicality of dual-functional systems. A TiO2/g-C3N4 photocatalyst degraded 90% of phenol in petrochemical wastewater while producing 1.8 mmol H2/g catalyst under visible light over 6 hours. In another example, ZnO doped with 2% nitrogen achieved 75% removal of tetracycline antibiotics from pharmaceutical effluent with concurrent H2 evolution at a rate of 1.2 mmol/h. These results highlight the potential for industrial adoption, particularly in sectors like textiles, pharmaceuticals, and food processing where wastewater is rich in organics.

Despite progress, long-term stability and cost-effectiveness require further research. Photocatalyst deactivation due to surface poisoning or photocorrosion remains an issue, especially in acidic or alkaline waste streams. Advanced materials like carbon-coated heterojunctions or dual-doped semiconductors show improved durability. Economic analyses suggest that coupling hydrogen production with wastewater treatment could offset operational costs, but large-scale deployment depends on developing low-cost, earth-abundant catalysts.

Dual-functional photocatalytic materials offer a compelling pathway toward sustainable energy and environmental remediation. By harnessing sunlight to simultaneously clean water and produce hydrogen, these systems align with circular economy principles. Future advancements in material design, reactor engineering, and process integration will determine their viability for industrial implementation. The intersection of photocatalysis, materials science, and environmental engineering holds the key to unlocking their full potential.