Roles of Cocatalysts in Photocatalysis and Photoelectrocatalysis

Abstract

S ince the 1970s, splitting water using solar energy has been a focus of great attention as a possible means for converting solar energy to chemical energy in the form of clean and renewable hydrogen fuel. Approaches to solar water splitting include photocatalytic water splitting with homogeneous or heterogeneous photocatalysts, photoelectrochemical or photoelectrocatalytic (PEC) water splitting with a PEC cell, and electrolysis of water with photovoltaic cells coupled to electrocatalysts. Though many materials are capable of photocatalytically producing hydrogen and/or oxygen, the overall energy conversion efficiency is still low and far from practical application. This is mainly due to the fact that the three crucial steps for the water splitting reaction: solar light harvesting, charge separation and transportation, and the catalytic reduction and oxidation reactions, are not efficient enough or simultaneously. Water splitting is a thermodynamically uphill reaction, requiring transfer of multiple electrons, making it one of the most challenging reactions in chemistry.This Account describes the important roles of cocatalysts in photocatalytic and PEC water splitting reactions. For semiconductor-based photocatalytic and PEC systems, we show that loading proper cocatalysts, especially dual cocatalysts for reduction and oxidation, on semiconductors (as light harvesters) can significantly enhance the activities of photocatalytic and PEC water splitting reactions. Loading oxidation and/or reduction cocatalysts on semiconductors can facilitate oxidation and reduction reactions by providing the active sites/reaction sites while suppressing the charge recombination and reverse reactions. In a PEC water splitting system, the water oxidation and reduction reactions occur at opposite electrodes, so cocatalysts loaded on the electrode materials mainly act as active sites/reaction sites spatially separated as natural photosynthesis does. In both cases, the nature of the loaded cocatalysts and their interaction with the semiconductor through the interface/junction are important. The cocatalyst can provide trapping sites for the photogenerated charges and promote the charge separation, thus enhancing the quantum efficiency; the cocatalysts could improve the photostability of the catalysts by timely consuming of the photogenerated charges, particularly the holes; most importantly, the cocatalysts catalyze the reactions by lowering the activation energy. Our research shows that loading suitable dual cocatalysts on semiconductors can significantly increase the photocatalytic activities of hydrogen and oxygen evolution reactions, and even make the overall water splitting reaction possible. All of these findings suggest that dual cocatalysts are necessary for developing highly efficient photocatalysts for water splitting reactions.

CO2 Reduction: From the Electrochemical to Photochemical Approach

“…A common and often necessary strategy to improve the photocatalytic performance is to introduce auxiliary cocatalysts to the surface of the semiconductor photocatalyst 208. These cocatalysts markedly change the energetics of the charge transfer process at the surface and increase the catalytic turnover rates, making the production rate of solar fuels a dominant process over the charge recombination or reverse reactions 209. In addition, the timely consumption the photogenerated charges on the cocatalyst would also slow down the photocorrosion of semiconductor photocatalysts and improve their stability.…”

“…They are usually deposited onto the photocatalyst surface via either chemical reduction or photochemical reduction of corresponding precursors. These noble metals can often serve as the electron sink to concentrate photogenerated electrons from photocatalysts, and consequently reduce the possibility of electron‐hole recombination 209. Within their presence, the selectivity of CO 2 photoreduction is generally shifted in favor of CH 4 over CO or other hydrocarbon products.…”

Recent Progress in Energy‐Driven Water Splitting

“…In case that the photogenerated electrons and holes possess thermodynamically sufficient potentials for water splitting, the recombination may still occur when there is lack of suitable active sites on the surface for water splitting. Therefore, the loading of cocatalysts on semiconductors is important to introduce the active sites while suppressing the charge recombination and the reverse reaction in water splitting 11. In addition to the cocatalysts, sacrificial agents such as various organic/inorganic electron donors also play a significant role in influencing their photocatalytic activity for water splitting reaction.…”

“…The cocatalysts play a role in extracting photogenerated charge carriers, hosting active sites for photocatalytic water reduction or oxidation reaction, suppressing photocorrosion, and thereby improving the stability of photocatalysts. Noble metals such as Ru, Rh, Pd, Ag, Pt and Au are commonly used as efficient cocatalysts for photocatalytic water reduction reaction 11, 14, 28. As the noble metals have lower Fermi level than that of TiO 2 , the photoexcited conduction band electrons of TiO 2 can transfer to the metals (i.e.…”

Facet‐Engineered Surface and Interface Design of Photocatalytic Materials

“…As such, photocatalytic performance is no longer dependent on the activity of TiO 2 facets 115, 116. Given the role of cocatalysts as reactant adsorption and activation sites, it would be straightforward to tune the photocatalytic performance by tailoring the surface facets of cocatalysts while maintaining the exposed facet of light‐harvesting semiconductor 117.…”