Titanium dioxide (TiO2) is a widely studied photocatalyst due to its stability, non-toxicity, and strong oxidative capability under ultraviolet (UV) light. However, its large bandgap (approximately 3.2 eV for anatase and 3.0 eV for rutile) limits its absorption to only the UV portion of the solar spectrum, which constitutes about 4-5% of sunlight. To enhance its photocatalytic efficiency under visible light, doping with metals and non-metals has been extensively investigated. These dopants introduce modifications in the electronic structure of TiO2, enabling visible-light absorption while influencing charge carrier dynamics.

Metal doping involves incorporating transition metals or rare-earth elements into the TiO2 lattice. Common metal dopants include iron (Fe), copper (Cu), nickel (Ni), and chromium (Cr). These metals can occupy substitutional or interstitial sites within the TiO2 crystal structure. Substitutional doping occurs when a metal ion replaces a titanium (Ti) atom in the lattice, while interstitial doping involves the metal occupying spaces between the lattice sites.

Iron doping is one of the most studied approaches due to its ability to introduce intermediate energy levels within the TiO2 bandgap. Fe³⁺ ions can substitute Ti⁴⁺ sites, creating defect states just above the valence band. These states act as electron traps, reducing the effective bandgap and allowing visible-light absorption. However, excessive Fe doping can lead to increased charge recombination due to the formation of deep-level defects, which act as recombination centers. Optimal Fe doping concentrations typically range between 0.5% and 2.0% atomic percentage, balancing visible-light absorption and photocatalytic activity.

Copper doping introduces both Cu⁺ and Cu²⁺ species into TiO2. Cu⁺ ions can occupy interstitial positions, while Cu²⁺ tends to substitute Ti⁴⁺ sites. The presence of Cu states creates new energy levels below the conduction band, facilitating electron transitions under visible light. Additionally, Cu doping enhances charge separation by acting as an electron sink, reducing recombination rates. However, high Cu concentrations may lead to phase instability and the formation of secondary phases like CuO, which can diminish photocatalytic performance.

Non-metal doping, particularly with nitrogen (N), carbon (C), and sulfur (S), has also been widely explored. Non-metal dopants modify the valence band structure of TiO2 by mixing their p orbitals with the O 2p orbitals, resulting in bandgap narrowing. Nitrogen is one of the most effective non-metal dopants due to its comparable atomic size to oxygen and its ability to form stable N-Ti-O bonds. Substitutional nitrogen replaces oxygen atoms in the TiO2 lattice, introducing N 2p states above the O 2p valence band. This shifts the valence band edge upward, reducing the bandgap to approximately 2.4-2.8 eV, depending on doping concentration. However, excessive nitrogen doping can introduce localized mid-gap states that promote recombination.

Carbon doping can occur in substitutional or interstitial forms. Substitutional carbon replaces oxygen atoms, while interstitial carbon occupies voids in the lattice. Carbon doping introduces C 2p states near the valence band, extending light absorption into the visible range. Additionally, carbon can enhance charge separation by acting as an electron reservoir. However, high carbon concentrations may lead to structural disorder and reduced crystallinity, negatively impacting photocatalytic efficiency.

Sulfur doping is typically achieved through substitutional replacement of oxygen or titanium atoms. Sulfur incorporation introduces S 3p states above the valence band, narrowing the bandgap. Substitutional sulfur at oxygen sites (S²⁻) is more effective than substitution at titanium sites (S⁴⁺ or S⁶⁺), as the latter can introduce deep-level defects that increase recombination. Optimal sulfur doping levels are critical to avoid excessive defect formation while maintaining visible-light absorption.

The modification of TiO2's band structure through doping directly influences charge carrier dynamics. Dopants can introduce shallow or deep energy levels within the bandgap. Shallow levels near the conduction or valence band edges facilitate visible-light absorption without significantly increasing recombination. Deep-level defects, however, act as trapping centers for electrons and holes, promoting recombination and reducing photocatalytic efficiency. The balance between bandgap narrowing and recombination rates is crucial for optimizing doped TiO2 performance.

Trade-offs exist between visible-light absorption and catalytic efficiency in doped TiO2 systems. While doping extends the absorption range, it may also introduce defects that hinder charge separation. For example, excessive metal doping can lead to the formation of recombination centers, while high non-metal doping concentrations may cause structural disorder. The optimal doping concentration varies depending on the dopant type and synthesis method but generally falls within a narrow range to maximize visible-light activity without compromising charge carrier mobility.

In summary, doping TiO2 with metals and non-metals is a viable strategy to extend its photocatalytic activity into the visible spectrum. Metal dopants like Fe and Cu introduce intermediate energy levels, while non-metal dopants such as N, C, and S modify the valence band structure. The choice of dopant, its concentration, and its incorporation mechanism significantly influence the electronic properties and photocatalytic performance of TiO2. Careful optimization is required to balance visible-light absorption with minimal recombination, ensuring efficient photocatalytic activity under solar irradiation.