*3.5. Sacrificial Agent*

The sacrificial agent plays the important role of electron donor, efficiently consuming holes to prevent recombination of charge carriers on the surface of the photocatalyst [100]. Charge recombination is one of the factors that may hinder the performance of photocatalytic reactions. Adding suitable sacrificial agents helps to solve the problem. For different reactions, different sacrificial agents should be used. For photocatalytic hydrogen production reactions, the sacrificial agent acts as the hole scavenger to reduce the charge recombination of the photoexcited electron–hole pairs. Then electrons can react with H<sup>+</sup> and enhance the photocatalytic performance for hydrogen production. Metal sulfide-based photocatalysts exhibit excellent activities in aqueous solution containing sacrificial reagents Na2S and Na2SO3. In addition to Na2S and Na2SO3, some other candidates—such as methanol [31], triethanolamine [20,101], lactic acid [23,44], glycerol [46], and 2-propanol [102]—can act as the sacrificial agents for metal oxysulfide or metal oxide/metal sulfide composite photocatalysts. It will be more constructive from the viewpoints of energy production and environmental protection if the sacrificial agents are sourced from the chemical waste or byproducts of industrial processes. One example is glycerol, which is a byproduct of biodiesel manufacturing. In the chemical industry, sulfur-containing side products and waste are common. In addition to the type of sacrificial agent, its

concentration also affects the photocatalytic activity. As an example, the effect of glycerol concentration on the performance of photocatalysts has been studied [46]. Glycerol can react with photogenerated holes to hinder the recombination of electron–hole pairs. Optimized graphene and glycerol contents can achieve the maximum H2 generation activity (1070 μmol h<sup>−</sup>1g−1). Table 10 lists the photocatalytic activity of photocatalysts using different sacrificial agents.


**Table 10.** Photocatalytic activity of photocatalysts using different sacrificial agents.
