Highly Efficient Hydrogen Production By Photoetched CdS-Pt
Authors: Nagakawa Haruki , Tatsuma Tetsu 
Published: December 2024

Photocatalytic hydrogen production is a promising method for solar energy harvesting. To achieve highly efficient photocatalytic reactions, it is necessary to utilize visible light effectively, which constitutes a significant portion of the sunlight spectrum. However, there are only few reports of photocatalytic materials that exceed an external quantum efficiency (EQE) of 50% in photocatalytic hydrogen evolution under visible light. Therefore, to achieve a higher efficiency, here we combined CdS, one of representative visible-light-responsive photocatalysts, with Pt nanoparticles as co-catalysts suitable for hydrogen production, and controlled the morphology of CdS through photoelectrochemical etching.

We prepared the CdS-Pt photocatalysts through three-steps: (1) synthesis of highly crystalline CdS in a molten salt, (2) loading Pt co-catalysts onto CdS by photoelectrochemical deposition utilizing surface defects, and (3) partial etching of CdS during photocatalytic hydrogen evolution by using excessive holes.

(1) Synthesis of highly crystalline CdS via molten salt treatment: Zincblende CdS particles were prepared from cadmium nitrate and treated further in molten NaCl and CaCl2 to obtain highly crystalline wurtzite CdS (Figure 1a).1

(2) Loading of Pt co-catalysts utilizing surface defects: The prepared wurtzite CdS was irradiated with light (430 nm) in an aqueous solution of lactic acid under a nitrogen atmosphere. In this process, photo-excited electrons are trapped in sulfide ion defects at the CdS surface, while holes are consumed by oxidation of lactic acid. Then, [PtCl6]2- ions were added to the solution in the dark and Pt co-catalysts were deposited onto CdS through reduction reaction of the ions by the trapped electrons (Figure 1b).2

(3) Morphology control of CdS through photoelectrochemical etching: The resulting CdS-Pt was dispersed again in the lactic acid solution and irradiated with light (430 nm) under nitrogen atmosphere. Photo-excited electrons are used for hydrogen evolution through reduction of protons, while holes are consumed by the oxidation of lactic acid and self-oxidation of CdS. In particular, when the light intensity is high, the generation rate of excited carriers as well as the reduction rate of protons exceed the oxidation rate of lactic acid, and partial photoetching of CdS via the self-oxidation occurs significantly. As a result of the photoetching, the surface area of the photocatalyst was increased (Figure 1c).

After the photoetching process, we evaluated the photocatalytic activity of the photoetched CdS-Pt for a hydrogen production reaction in a NaOH solution containing 20 vol% 2-propanol, in which high pH suppresses dissolution of cadmium ions and thereby further photoetching. We found that the hydrogen generation rate was improved by ~8 times. The EQE of the hydrogen production reaction using the photoetched CdS-Pt was calculated to be 91%. The specific surface area of CdS-Pt increases and the distance of photogenerated carriers to reach the surface decreases due to the photoetching. These changes are favorable for the photocatalytic reaction and may contribute to the increased activity. If the rate of carrier excitation is too high in comparison with the rate of carrier consumption by redox reactions, EQE should be low. However, the photoetching process decreases both the particle volume and the generation rate of excited carriers, while the reaction sites are increased. These should be responsible for the improved EQE.

The present technique, which was developed to optimize the morphology of CdS-Pt photocatalyst under given conditions, would lead to highly efficient photocatalytic water splitting and photoreforming reactions, as well as to mechanism analysis of the efficient photocatalytic reactions.

References

H. Nagakawa, T. Tatsuma, ACS Appl. Energy Mater., 2022, 5, 1465–14657.

H. Nagakawa, T. Tatsuma, J. Phys. Chem. C, 2023, 127, 20337–20343.

Figure 1