Based on the results above presented, the mechanism of the formation of new phases after the AuNPs addition to Zn(OH)2 and ZnGO composite is proposed, as well as the mechanism of H2S adsorption on the new composites.
The addition of AuNPs during the preparation route of Zn(OH)2 led to the formation of some crystalline ZnO. The AuNPs supplied the nucleation sites for the Zn(OH)2, that was formed after the reaction of zinc ions from the ZnCl2 precursor, with NaOH [14]. Zn(OH)2 formed in presence of AuNPs was converted to Zn(OH)42− ions by further reaction with water. Finally, the Zn(OH)42− ions were decomposed to ZnO, and the AuZnO composite nanocrystals were formed. The conversion of Zn(OH)2 to 1D ZnO nanorods via the dissolution–precipitation route by aging was reported [27] and [29]. The Zn(OH)2 releases soluble Zn(II), which is favored for the growth of the ZnO nanorods or cubic crystals as can be seen in Fig. 8.
Apparently, an addition of AuNPs during the composite formation led to different chemical processes than those when GO was not present. The XRD results showed that in the ZnOH and ZnGO samples zinc hydroxide was formed. Knowing that GO does not promote the zinc oxide formation (ZnGO composite), the presence of small crystals of this phase in the AuZnGO composite could be attributed to the effect of AuNPs. It has been previously demonstrated that the presence of metal nanoparticles modifies the ZnO growth [44]. AuNPs when added in a TiO2/ZnO composite also caused a decrease in the intensity of the peaks attributed to ZnO in XRD measurements, due to the decrease of the grain size of the composite product [16]. It seems like the AuNPs increases the number of nucleation centers of ZnO, leading to a drop in the size of the particles as was also found in the case of Al [45].
The above-mentioned mechanism of the composite formation with embedded AuNPs (AuZnOH, AuZnGO) is illustrated in Fig. 10.
Based on the results above presented, the mechanism of the formation of new phases after the AuNPs addition to Zn(OH)2 and ZnGO composite is proposed, as well as the mechanism of H2S adsorption on the new composites.
The addition of AuNPs during the preparation route of Zn(OH)2 led to the formation of some crystalline ZnO. The AuNPs supplied the nucleation sites for the Zn(OH)2, that was formed after the reaction of zinc ions from the ZnCl2 precursor, with NaOH [14]. Zn(OH)2 formed in presence of AuNPs was converted to Zn(OH)42− ions by further reaction with water. Finally, the Zn(OH)42− ions were decomposed to ZnO, and the AuZnO composite nanocrystals were formed. The conversion of Zn(OH)2 to 1D ZnO nanorods via the dissolution–precipitation route by aging was reported [27] and [29]. The Zn(OH)2 releases soluble Zn(II), which is favored for the growth of the ZnO nanorods or cubic crystals as can be seen in Fig. 8.
Apparently, an addition of AuNPs during the composite formation led to different chemical processes than those when GO was not present. The XRD results showed that in the ZnOH and ZnGO samples zinc hydroxide was formed. Knowing that GO does not promote the zinc oxide formation (ZnGO composite), the presence of small crystals of this phase in the AuZnGO composite could be attributed to the effect of AuNPs. It has been previously demonstrated that the presence of metal nanoparticles modifies the ZnO growth [44]. AuNPs when added in a TiO2/ZnO composite also caused a decrease in the intensity of the peaks attributed to ZnO in XRD measurements, due to the decrease of the grain size of the composite product [16]. It seems like the AuNPs increases the number of nucleation centers of ZnO, leading to a drop in the size of the particles as was also found in the case of Al [45].
The above-mentioned mechanism of the composite formation with embedded AuNPs (AuZnOH, AuZnGO) is illustrated in Fig. 10.
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