Effective nitrogen doping of TiO₂ polymorphs at mild temperatures for visible-light-responsive hydrogen evolution

Area of Science:

• Materials Science
• Photocatalysis
• Surface Chemistry

Background:

• Titanium dioxide (TiO2) is a key photocatalyst, but its efficiency is limited by its wide bandgap, restricting visible-light absorption.
• Developing methods for visible-light-driven photocatalysis is crucial for applications like hydrogen evolution.
• Nitrogen doping is a common strategy to narrow the bandgap of TiO2, enhancing visible-light activity.

Purpose of the Study:

• To develop a mild-temperature nitrogen doping method for TiO2 using urea-derived gaseous species.
• To investigate the role of cyanic acid (HOCN) in nitrogen doping of TiO2.
• To compare the photocatalytic hydrogen evolution performance of mild-temperature doped TiO2 with high-temperature doped samples.

Main Methods:

• Mild-temperature nitrogen doping of anatase and rutile TiO2 using urea.
• Density Functional Theory (DFT) simulations to understand the doping mechanism.
• Photocatalytic hydrogen evolution rate measurements.
• Steady-state and transient surface photovoltage characterizations.

Main Results:

• Mild-temperature nitrogen doping of both anatase and rutile TiO2 significantly enhanced hydrogen evolution rates under visible light.
• DFT simulations identified cyanic acid (HOCN) as the active species for effective nitrogen doping at mild temperatures.
• High-temperature doping, despite increased visible light absorbance, led to defects detrimental to electron transport.
• Mild-temperature doping flattened the band bending without forming Ti3+ defects, improving visible-light-induced electron transport.

Conclusions:

• Mild-temperature nitrogen doping is a superior strategy for enhancing visible-light photocatalytic hydrogen evolution in TiO2 compared to high-temperature methods.
• Controlling defect formation during doping is critical for optimizing photocatalyst performance.
• This approach offers a pathway to bridge the gap between light absorption and photoactivity in wide-bandgap metal oxides.