Enhancing Photocatalytic Activity via a Z-scheme Heterojunction at the Interface of Hierarchical TiO2 Framework and g-C3N4 Nanoparticles

Significance 

Recent advancement in photocatalytic technology has increased its application in various fields including removal of toxic gases and organic pollutants degradation using natural sunlight. This has attracted significant attention of researchers and especially in development of semiconductor photocatalysts for effective chemical reactions.

Among the available photocatalysts, graphitic carbon nitride (g-C3N4) which comprise mostly of carbon and nitrogen is widely used as a metal-free conjugated photocatalyst in the production of hydrogen owing to its excellent thermal, chemical and optical properties. For instance, it has a proper band gap that enables it to perceive visible sunlight of larger photon flux. Unfortunately, various limitations like low electrical conductivity and high recombination rate of the charge carriers affect its photocatalytic activities. To this note, researchers have been looking for alternatives to increase the photocatalytic properties and have identified combining g-C3N4 with other semiconductor materials at the heterojunction interface as promising solutions.

Presently, titanium oxide (TiO2) photocatalysts have been combined with other semiconductors such as g-C3N4 to form heterojunction photocatalysts with high photon absorption capability and low recombination rate of the charge carriers. There are numerous methods of preparing TiO2/g-C3N4 composites like hydrothermal and calcination techniques. However, these conventional methods result in Type-II heterojunctions with low oxidation capability of holes and low reduction capability of electrons. The challenges can be addressed by forming hierarchical TiO2/g-C3N4 heterojunction photocatalysts by calcinating tetrabutyl titanate and melamine precursors. It provides efficient diffusion transport channels due to the high internal surface area. However, little has been reported about the effect of calcinating temperature on the formation of effective TiO2/g-C3N4 heterojunction.

Recently, Professor Lianying Lu, Professor Guohong Wang, Dr. Juan Wang and a graduate student Min Zou in Hubei Normal University, in collaboration with Professor Jun Li at Kansas State University, investigated the effects of calcinating temperature on hierarchical TiO2/g-C3N4 heterojunction photocatalysts. Their main objective was to illustrate the critical conditions for the formation of effective TiO2/g-C3N4 heterojunction. Their work is published in the journal, Applied Surface Science.

The research team commenced their experimental work by calcinating tetrabutyl titanate and melamine precursors to synthesize hierarchical TiO2/g-C3N4 photocatalyst. They employed various techniques including scanning electron microscopy (SEM) and photoelectron spectroscopy (XPS) to characterize the samples. Eventually, photodegradation of Rhodamine B (RhB) was used to determine the photocatalytic activities of the samples.

The authors observed that calcinating temperature exhibited significant effects on the surface area, catalytic properties, interface structure and microstructure of the samples. Consequently, calcinating at an optimum temperature of 550℃ resulted in TiO2/g-C3N4 with the highest photocatalytic activity in the presence of visible light irradiation for decomposing Rhodamine B. The apparent reaction rate constant of the heterjunction catalyst TiO2/g-C3N4 was found to be 55.0 x 10-3 min-1, which is 16.2 fold of pure TiO2 and 3.4 fold of pure g-C3N4 catalyst, respectively. The enhanced photocatalytic activity was attributed to the formation of Z-scheme TiO2/g-C3N4 heterojunction at the internal surface of the porous TiO2 framework. This was due to the nitrogen or carbon doping at the junction which enhanced the separation of electrons and holes. Furthermore, from the radical trapping experiments, the authors concluded that photo-generated holes and superoxide ions played a significant role in the photo-degeneration of Rhodamine B as compared to hydroxyl radicals. The study by Lianying Lu and colleagues will, therefore, advance the development of efficient photocatalysts.

“It was a surprise when we observed the formation of Z-scheme heterojunction between TiO2 framework and g-C3N4 nanoparticles. They typically form type-II heterojunction, which works well in improving charge separation, but the oxidation and reduction capabilities are not as high as our reported Z-scheme heterojunction. The trace amount of N or C doping at the interface unexpectedly changed the interface to form a better catalyst.” Said Professor Jun Li to Advances in Engineering in a statement. He then added: Science is all about explore unknowns and scientist are rewarded with good surprises.

Enhancing Photocatalytic Activity via a Z-scheme Heterojunction at the Interface of Hierarchical TiO2 and g-C3N4 - Advances in Engineering

About the author

Guohong Wang received his B.S. and M.S. in Chemical Engineering from East China University of Science and Technology and Wuhan University of Science and Technology, respectively, and his Ph.D. in Materials Science in 2008 from Wuhan University of Technology. In 2013, he became a professor at Hubei Normal University. In 2010, he was a visiting scholar at Murdoch University at Australia.

His current research interests include semiconductor photocatalysis, photocatalytic hydrogen production, CO2 reduction to hydrocarbon fuels, and so on.

About the author

Lianying Lu received her B.S. in Chemical Engineering from Wuhan Institute of Technology. In 2014, she became a professor at Hubei Normal University. Her current research interests include materials (e.g., catalysts and adsorbents) for air and water pollution control.

About the author

Jun Li received B.S. degree in 1987 from Wuhan University in China and Ph.D. degree from Princeton University in 1995. He became a professor at Kansas State University in 2012.

His current research interests include various nanomaterials and nanodevices for electrical energy storage, solar energy conversion, electrocatalysis, biosensors and pathogen detection.

Reference

Lu, L., Wang, G., Zou, M., Wang, J., & Li, J. (2018). Effects of calcining temperature on formation of hierarchical TiO2/g-C3N4 hybrids as an effective Z-scheme heterojunction photocatalyst. Applied Surface Science, 441, 1012-1023.

Go To Applied Surface Science

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