Recent Advances in Metal-Doped Semiconductor Photocatalysts for Water Pollutant Degradation

Introduction

Particularly for the breakdown of organic contaminants from water systems, photocatalysis has become a viable and effective method for environmental restoration. Because of their stability, non-toxicity, and affordability, semiconductors like TiO₂, ZnO, and g-C₃N₄ have been the subject of much research among different photocatalytic materials. However, low visible-light absorption, fast electron-hole recombination, and inadequate quantum efficiency frequently restrict their practical effectiveness.

Metal doping has drawn a lot of interest as a successful modification method to get around these restrictions. By changing band topologies, adding defect states, and enhancing charge separation, transition or rare-earth metals can be incorporated into semiconductor lattices to increase photocatalytic activity in the presence of visible light. According to recent research, metal-doped photocatalysis degrades dyes, medications, and other persistent organic pollutants more quickly than their undoped equivalents.

The design, production, and usage of metal-doped semiconductor photocatalysis for the degradation of water pollutants are the main topics of this paper. It highlights key mechanisms, doping tactics, performance comparisons, and potential challenges associated with their widespread use.

Methodology

A structured literature review was conducted to gather and examine research papers on metal-doped semiconductor photocatalysis for water pollutant degradation published between 2015 and 2025. The actions listed below were taken:

1. Database Search:

Comprehensive searches were conducted in Scopus, Web of Science, Science Direct, and Google Scholar. Keywords included:

Metal-doped photocatalyst

Semiconductor modification

Visible-light photocatalysis

Wastewater Treatment

Organic pollutant degradation

Dye removal

2. Selection Criteria:

When selecting metals for doping semiconductor nanomaterials for photocatalytic applications, researchers consider several technical criteria:

1. Electronic Structure Compatibility

• Instead of increasing electron-hole recombination, the metal dopant should supply energy levels that decrease it.
  Without producing deep trap states that serve as recombination centres, it ought to reduce the band gap (for visible-light activation). 

2. Redox Potential

• For the dopant's redox couples (such as Mⁿ⁺/Mⁿ⁻¹⁺) to efficiently capture electrons or holes and contribute to the production of reactive oxygen species (ROS), they must line up.
  

• The oxidative/reductive processes required for the breakdown of pollutants shouldn't be obstructed.

3. Chemical Stability

• The dopant should remain stable under UV/visible irradiation and avoid leaching or dissolving in aqueous media.

4. Ionic Radius and Lattice Match

• The ionic radius of the dopant should be close to that of the host semiconductor cation to allow substitutional doping without excessive lattice strain or defect-induced instability.

5. Concentration Control

• Too low doping may have a negligible effect; too high doping can form recombination centres or secondary phases (metal clusters) that decrease photocatalytic efficiency.

6. Non-toxicity and Environmental Safety

• Particularly important for water treatment applications, metals like Cd, Pb, or Hg are avoided because of their toxicity.

7. Cost and Availability

• Cheaper and abundant metals (Fe, Cu, Mn) are often preferred over expensive, rare-earth or noble metals unless specific optical properties are required.

3. Data Extraction:

Data on semiconductor type, metal dopant, synthesis process, band gap energy, reaction conditions, and photocatalytic efficiency were documented for each chosen study.

4. Analysis:

• The collected data were compared to identify trends in doping strategies, 
• Improvements in photocatalytic performance
• Mechanisms of enhanced activity and research gaps that need further investigation.
• Photon Absorption and Charge Generation

Mechanism 

M-doped Catalysts + hν = e- {CB} + h+_{VB}

M: doped metal ion.

e⁻: conduction band electron.

h⁺: valence band hole.

Figure 2: Photocatalytic reactions with metal-doped catalysts 

2. Charge Trapping and Reduced Recombination

Mⁿ⁺ + e^-CB = M^(n-1)+

3. Reactive Oxygen Species (ROS) Generation

(a) Oxygen Reduction by Trapped Electrons:

M^(n-1)+ + O₂ = Mⁿ⁺ + O^2•

O^2• + H⁺ = HO^2•

2HO^2• (a) = O₂ + H₂O₂ 

H₂O₂ + e^- = •OH + OH^-

(b) Hole Oxidation:

 h⁺ - VB + H₂O = •OH + H⁺

h⁺ - VB + OH^- = •OH

4. Pollutant Degradation

•OH / O^2•- + Pollutant = Intermediates = CO₂ + H₂O + Minerals