Photochemistry and the role of light during the submerged photosynthesis of zinc oxide nanorods

Abstract

Recently, metal oxide nanocrystallites have been synthesized through a new pathway, i.e., the submerged photosynthesis of crystallites (SPSC), and flower-like ZnO nanostructures have been successfully fabricated via this method. However, the photochemical reactions involved in the SPSC process and especially the role of light are still unclear. In the present work, we discuss the reaction mechanism for SPSC-fabricated ZnO nanostructures in detail and clarify the role of light in SPSC. The results show that both photoinduced reactions and hydrothermal reactions are involved in the SPSC process. The former produces OH radicals, which is the main source of OH− at the ZnO crystal tips, whereas the latter generates ZnO. Although ZnO nanocrystals can be obtained under both UV irradiation and dark conditions with the addition of thermal energy, light promotes ZnO growth and lowers the water pH to neutral, whereas thermal energy promotes ZnO corrosion and increases the water pH under dark conditions. The study concludes that the role of light in the submerged photosynthesis of crystallites process is to enhance ZnO apical growth at relatively lower temperature by preventing the pH of water from increasing, revealing the environmentally benign characteristics of the present process.

Titania-Based Hybrid Materials with ZnO, ZrO2 and MoS2: A Review

“…Some unquestionable disadvantages of the hydrothermal method include the complexity and cost of the equipment (resulting from the need for the autoclaves to be resistant to high pressure). Moreover, this method does not permit direct observation of the process, and it requires the reagents used to be soluble in water, which serves as the process medium [49,50,51].…”

Fabrication of Iron Oxide Nanoparticles via Submerged Photosynthesis and the Morphologies under Different Light Sources

“…The bandgap energy of Fe(OH) 2 and FeOOH is close to 2 eV; 32) therefore, the electrons can be excited from valence band to conduction band by both visible light and UV under light irradiation conditions in the SPSC process by the photosemiconductive reaction: 33) SC SC o hv e h ( : ) SC semiconductor ... (6) In the former study on SPSC, it is found that the SPSC process is photocatalytic, accompanied by hydroxyl radical generation via water splitting. [24][25][26] The generated electrons build up at the apical portion of the initial rust of iron hydroxides Fe(OH) 2 and FeOOH, whereas the holes left at the bottom of the concave, as shown in Fig. 6.…”

“…At present, flower-like NCs of zinc oxide [24][25][26] and cupric oxide 27) have been successfully synthesized using the SPSC method. The application of SPSC method to the fabrication of other metal oxide nanoparticles has not been reported.…”

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“…The photogenerated charge carriers can readily recombine either in the bulk or at the surface of ZnO through the dissipation of energy in form of light or heat. Photocorrosion arises from the dissolution of ZnO into Zn 2+ ions in aqueous solutions under UV irradiation [84][85][86][87][88][89]. As such, oxygen atoms in the ZnO lattice are oxidized by photogenerated holes to release Zn 2+ .…”

“…As such, oxygen atoms in the ZnO lattice are oxidized by photogenerated holes to release Zn 2+ . In other words, photogenerated holes on the ZnO surface attack the Zn-O bond, leading to the dissociation of Zn 2+ from the surface [85,86]. So the holes would react with ZnO rather than with adsorbed surface water molecules to form hydroxyl radicals.…”