The insights from a paper published by Mechanical Engineering PhD student Xuan Li could guide the material design and performance improvement of direct Z-scheme systems and lead to increased interest in the field
A paper by Khalifa University Engineering PhD student Xuan Li has been published in Materials Today, providing a comprehensive and timely review on the mechanisms, material systems, and optimizing strategies of a type of photocatalyst, which is a catalyst that generates a chemical reaction using light.
“The rapid economic development and massive use of fossil fuels has caused the environmental problems we’re seeing today,” explained Li. “Air pollution has already become the fourth leading risk factor for human mortality and contributes to around five million deaths per year around the world. Even at low concentrations, the pollutants released from solvents, paints, building materials and furnishings can cause illness, underscoring how the need to produce clean fuels and protect the environment is also crucial for human health.”
Photocatalysis, Li says, is one way these pollutants could be removed from the atmosphere sustainably. With Dr. Lianxi Zheng, Professor, Dr. Corrado Garlisi, Postdoctoral Fellow, Qiangshun Guan, Graduate Student, Dr. Shoaib Anwer, Postdoctoral Fellow, Dr. Khalid Al-Ali, Assistant Professor, and Dr. Giovanni Palmisano, Associate Professor, Li provides an extensive review and discussion of the design process of direct Z-schemes, a type of photocatalyst inspired by natural processes, to combat atmospheric pollution.
What are Photocatalysts?
Photocatalysts are often made with semiconductors and use solar energy to generate electron-hole pairs on the surface of the catalyst. When exposed to sunlight, the ground-state electrons in the semiconductors become excited and “jump” to a higher energy level, leaving behind positively-charged holes. These electrons and holes then interact with the organic molecules in the atmosphere around them.
Two simultaneous reactions occur during photocatalysis: oxidation (when a molecule loses an electron) from the photogenerated holes and reduction (when a molecule gains an electron) from the photogenerated electrons. As the photocatalyst reduces and oxidizes the water and oxygen molecules in the atmosphere around it, several reactive species are created that can break organic pollutants down into clean end-products.
However, there are some fundamental shortcomings in conventional photocatalysts that are preventing them from being used at an industrial scale, which Li believes Direct Z-scheme photocatalysts can overcome. These shortcomings stem from the semiconductors needed, with only a few materials such as titanium oxide and zinc oxide meeting the requirements for producing the necessary reactive radicals with energy efficiency.
What are Direct Z-Scheme Photocatalysts?
Direct Z-scheme photocatalysts are inspired by natural plant photosynthesis. In photosynthesis, plants use two photosystems to separate electrons and holes for efficient reactions in converting carbon dioxide and water into sugar and oxygen. The Z-scheme is a photosystem coupling layout for electron transfer in the light reactions of photosynthesis, where plants transform light energy into chemical energy.
Direct Z-scheme catalysts attempt to mimic the same charge-transfer pathways that plants follow during photosynthesis by using two-semiconductor structures. They are designed in a way that the pathway travelled by electrons and holes follows a ‘Z-scheme’ pathway. This special pathway enhances the photocatalyst’s redox potential (its ability to carry out the oxidation and reduction reactions) and extends the lifetime of electron-hole pairs, making it more efficient.
While there are three types of Z-scheme photocatalysts, direct Z-schemes can maximize the solar energy harvested and can also be used in both the liquid and gas environments.
However, constructing a direct Z-scheme catalyst remains challenging. Various approaches and numerous materials have been proposed in the hope of achieving a highly efficient photocatalyst with broad practical applications. Li’s work focuses on the driving forces of charge transfer to guide the material design.
Reviewing Direct Z-Scheme Photocatalyst Designs
“The core idea of direct Z-schemes is to leverage the synergistic effects that occur between the two semiconductors of a photocatalyst by regulating the charge transfer direction,” explained Li.
“The formation mechanisms, suitable applications, and their performance are strongly dependent on the properties of the two semiconductors and their interactions. For example, the surface nature of the photocatalysts will affect the adsorption and selectivity of the reactant molecules.”
Because there are so many potential materials for developing these photocatalysts, and because their properties influence their performances so greatly, there is no systematic comparison of all developed materials.
In her paper, Li offers universal guidelines on material design of direct Z-schemes, by identifying the main formation mechanisms, considering emerging materials, and noting modification strategies for performance improvement.
“Considering the fact that new materials and new material properties always play fundamental and promoting roles in technology development, it is vital to provide a materials-focused review for direct Z-scheme photocatalysts,” said Li.
Among all available material systems, wide band-gap semiconductors remain the most popular in building reliable direct Z-schemes. Their high potentials on both reductive and oxidative reactions allow use in a wide range of applications from carbon dioxide reduction to organic pollutant degradation, but they generally show a low efficiency in solar light utilization.
In contrast, some visible-light semiconductors show a much higher energy efficiency but suffer from issues with photocorrosion. The researchers indicate that coupling a wide band-gap semiconductor and a narrow band-gap semiconductor could possibly lead to maximizing the light spectrum and notable photocatalytic efficiency.
Organic materials offer great flexibility in building Z-schemes due to being able to tune their morphology and physiochemical properties. Metal-organic frameworks could boost the effectiveness of the material system by modifying the metal and organic crosslinkers, boosting their surface area and extended light absorbance.
“Direct Z-scheme systems are extremely promising and present unique advantages, but the field is still in its early stages with the main emphasis on concept demonstration and efficient light utilization,” said Li. “Considering the enormous variety of known and unknown pollutants in the environment, future studies should focus more on developing direct Z-scheme catalyst systems with high redox capabilities that can degrade a broad range of organic pollutants.”
The insights discussed in this work could help guide engineers to design better photocatalysts with optimized materials and improved performance. Improved photocatalysts can in turn contribute significantly to global efforts to produce clean energy and clean the air.
Another advantage of Li’s review is that it could attract more material scientists to work in and contribute to this area, so that the maximum potential of direct Z-schemes can be achieved in multiple applications.
Jade Sterling
Science Writer
8 June 2021
