A Novel Method to Fabricate Hierarchical Copper Oxide Photoelectrode and Its Application for Photoelectrochemical Water Splitting
Authors: Hosseini H. Seyed Morteza , Siavash Moakhar Roozbeh , Soleimani Foad , Sadrnezhaad Sayed Khatiboleslam Sadrnezhaad 
Published: May 2020

During the last decades, using fossil fuels has caused energy and environment problems for communities because of uncontrolled CO2 emission. These problems increase environmental concerns over the emissions of the pollutant gases, known as greenhouse gases. Replacing clean fuels instead of fossil fuels and developing technologies to use them is essential to address environmental concerns. Among the various ideas to solve this dilemma, solar energy as a clean, sustainable, renewable, and affordable energy source is an excellent option for energy supply to earth's inhabitants. It can be a suitable solution for resolving global concerns about the energy crisis and environmental pollution.
Hydrogen production from photoelectrochemical (PEC) water splitting is an economical, environmentally friendly and a promising energy source to replace fossil fuels. With the help of this technology, we can convert solar energy directly into H2.
In each PEC cell, a semiconductor material is used as a photocatalyst with specific properties to absorb light and split water on its surface.
The coated photoelectrodes with photoactive materials such as semiconductors have the most critical role in PEC cells. These materials can improve the efficiency of water-splitting reactions under solar light irradiation, thus enhance PEC hydrogen evolution.
Among candidate semiconductors, p-type copper oxide semiconductor is one of the cheapest and most effective semiconductor materials with a narrow band and resulting in higher light absorption, excellent physical and chemical properties, easy fabrication process, and diverse morphologies.
Photocorrosion and the poor charge transfer properties of this inexpensive photocathode are some of the problems that have limited it for applications such as photoelectrochemical water splitting.
Researchers are trying to overcome the limitations of this material for photoelectrocatalytic applications by using techniques such as the creation of heterojunction structure, the addition of suitable elements as dopants to the copper oxide structure, and the formation of a protective layer on it. It should be noted that synthesis processes and the final morphologies are essential factors that influence the final properties of CuO.
Nanostructured copper oxide can be synthesized by various methods such as sonochemical, hydrothermal, chemical bath deposited, sol-gel, laser ablation, and electrochemical method that each of these methods has advantages and disadvantages. These methods may be hard and expensive and not suitable for the production of nanoparticles on a large scale; furthermore, many copper oxide synthesis methods are high-temperature. Some processes have improved the PEC performances of CuO by various nanostructures such as nanoparticles, nanowires, nanorods, urchin-like nanostructure, butterfly-like, honeycomb-like, etc.
Recently, microwave-assisted synthesis has become a popular method due to the very fast synthesis (within several minutes), the simplicity and inexpensive of the process, the high purity obtained nanomaterials and the ability to scale-up production. It can develop the hierarchical CuO nanostructures.
Herein, we introduce a novel and facile in-situ synthesis of three-dimensional hierarchical CuO using by inexpensive precursors via microwave-assisted. Besides, we deposited this nanocomposite on the FTO directly in the microwave and obtained a unique morphology with high stability.
The comparison between our CuO photoelectrode which synthesis via microwave-assisted and other CuO photoelectrode which synthesis via other chemical based method showed that the high crystalline CuO which synthesised via microwave-assisted photoelectrode improves photoelectrochemical properties and increases hydrogen production. The CuO photoelectrode exhibit a photocurrent of ~0.87 mAcm-2 (0V vs. RHE) which is significantly higher than some CuO photoelectrode which synthesis via other chemical based method. Also, the crystalline CuO showed long lifetime stability in the aqueous electrolyte solution and retained its stability about 60 % after 85 min.

Figure 1