As seen water is formed and remains adsorbed on the surface. The analyses of the exhausted samples’ surfaces suggest that the H2S reactive adsorption products on ZnOH, ZnGO, AuZnOH and AuZnGO are zinc sulfide, elemental sulfur, sulfite, and sulfate. While the sulfide can be the product of a direct acid–base reaction with the OH groups of the inorganic phase, the other three species have to be the products of surface oxidation. The oxidation is apparently more efficient on the surface in which more sulfate are present. On the surface of zinc hydroxide light and especially generation of holes might be responsible for the H2S oxidation [18].
In the case of ZnGO, the high dispersion of Zn(OH)2 and more terminal OH groups than on Zn(OH)2 increase the amount of H2S interacting with the surface via acid–base reactions. Its oxidation is more efficient owing to the fact that the graphene-based phase has the ability to activate oxygen and form superoxide ions [49]. Moreover, it increases the efficiency of the electron transfer [50]. Its presence also may delay the recombination of electron–hole pair formed on the zinc hydroxide phase [51]. In the presence of water on the surface, sulfuric acid can react with zinc hydroxide and forms zinc sulfates. The presence of sulfur in the form S2O62− is a positive factor since twice more sulfur is engaged to one zinc ion, increasing in this way the amount adsorbed. These products of surface reactions are more bulky than OH groups and their deposition might close the very small interface pores causing a decrease in surface area and porosity.
Since the incorporation of AuNPs to the composites results in even more terminal OH groups than on the surface of ZnGO the amount of H2S adsorbed can be further enhanced. Moreover, AuNPs increase the efficiency of the electron transfer to O2 adsorbed on the surface [47]. Thus even more superoxide ions can be formed and participate in oxidation reactions. AuNPs can also act as an electron traps and thus enhance the electron–hole separation and the subsequent transfer of trapped electrons to O2, which act as electron acceptor [52]. These cause that on the surface of this composite a variety of oxidation products is detected. Interestingly, an increase in the surface area after adsorption on this material is found. This can be due to the deposition of sulfur species in mesopores resulting in a decrease in their sizes and formation of micropores, In fact this sample has the highest volume of mesopores among all materials studied. The presence of oxidizing agents can also result in oxidation of some graphene-based phase, causing defects and thus forming new small pores.