Water splitting, especially through photoresponsive semiconductors in photoelectrochemical (PEC)
or photocatalytic (PC) cells, produces hydrogen with the utilization of solar energy. Heterojunctionbased metal oxide clusters, including polyoxometalates (POM) and metal-organic frameworks
(MOF), exhibit semiconductor properties with tunable structures and catalytic capabilities,
enhancing solar light absorption and charge transfer efficiency. Integrating these clusters enhances
stability and catalytic efficiency for specific applications.
In this work, various heterojunction structures created with POM and MOF are synthesized
and analyzed with an objective of improving the photoelectrochemical properties. The
heterojunction materials based on Phosphomolybdic acid (PMo12) and Zeolitic imidazolate
framework-9 (ZIF-9) are synthesized by solvothermal method were used in the study. The
instability of PMo12 with the aqueous solution is resolved with the integration of ZIF-9, and the
catalytic properties of ZIF-9 are enhanced with the heterojunction formed between them. The
composite material shows photocatalytic nature with an extended range of solar light absorption,
and the formed p-n heterojunction improves efficient charge transfer, enhancing the PEC properties
[1]. The work is then extended to the formation of a heterojunction between ZIF-9 and a wellknown photocatalyst, titanium dioxide nanorods (TNR). The limitations of TNR, such as reduced
solar light absorption, poor catalytic activity, and charge transfer, are resolved by sensitization of
ZIF-9 upon TNR. The improved PEC properties of the developed photoanode material are
analyzed, and such enhancement is attributed to the efficient p-n heterojunction formed between
ZIF-9 and TNR. The heterojunction formation enhances the extraction of photogenerated holes and
electrons out of TNR, and the sensitization with ZIF-9 improves the efficiency of charge injection
[2]. The third and final part of the work describes the construction of an efficient catalyst through
self-assembly of iron or manganese phosphomolybdic acid (Fe-PMo12 / Mn-PMo12) upon nitrogendoped titanium dioxide (N-TiO2). Nitrogen doping helps in better absorption of solar light and
reduces the work function so that electrons can be easily transferred to the other side containing
doped PMo12. Fe-PMo12 / Mn-PMo12 provide active sites for catalytic activities and rapid electrons
transfer [3]. The work thus paves the way for development of various heterojunction materials for
efficient PEC hydrogen evolution applications.
References:
[1] C.A. Augustine, N. Khatun, R. Bauri, S.C. Roy, Synthesis and photoelectrochemical catalytic
properties of polyoxometalate supported on zeolitic imidazolate Framework, ZIF-9–PMo12,
Mater. Sci. Eng. B Solid-State Mater. Adv. Technol. 291 (2023) 116385.
https://doi.org/10.1016/j.mseb.2023.116385.
[2] S. Yan, S. Ouyang, H. Xu, M. Zhao, X. Zhang, J. Ye, Co-ZIF-9/TiO2 nanostructure for
superior CO2 photoreduction activity, J. Mater. Chem. A. 4 (2016) 15126–15133.
https://doi.org/10.1039/c6ta04620g.
[3] J. Jiang, X. Wang, Y. Liu, Y. Ma, T. Li, Y. Lin, T. Xie, S. Dong, Photo-Fenton degradation
of emerging pollutants over Fe-POM nanoparticle/porous and ultrathin g-C3N4 nanosheet
with rich nitrogen defect: Degradation mechanism, pathways, and products toxicity
assessment, Appl. Catal. B Environ. 278 (2020) 119349.
https://doi.org/10.1016/j.apcatb.2020.119349