Direct Z-Scheme In₂O₃/In₂S₃ Heterojunction for Oxygen-Mediated Photocatalytic Hydrogen Production

Abstract

We demonstrate the viability of the In2O3/In2S3 heterojunction for efficient photocatalytic H2 evolution from a formaldehyde aqueous solution, which is an alternative renewable hydrogen source, in a direct Z-scheme manner under visible light without any electron mediator and cocatalyst. The optimal In2O3/In2S3 heterojunction shows a fast hydrogen evolution rate of 6.16 mmol·g–1·h–1, which is approximately 166 times and 6 times higher than that of pure In2O3 and pure In2S3, respectively. The superior photocatalytic activity of the In2O3/In2S3 heterojunction compared to single-component In2O3 and In2S3 is attributed to its appropriate band gap, enhanced charge separation, and transfer abilities, as attested by various spectroscopic characterization and photoelectrochemistry experiments. Among the different fabrication approaches, the sequential hydrothermal–calcination technique, which produces a hydrangea-like morphology, affords the most active In2O3/In2S3 heterojunction photocatalyst. Molecular O2 is found to play a key role in instigating the photocatalytic H2 evolution from formaldehyde oxidation as a catalytic electron scavenger, which is not consumed throughout the reaction.

Enhanced adsorption of oxygen species on c/h-In2O3 Z-scheme heterophase junctions for oxygen-mediated photocatalytic hydrogen production

“…It could be seen that p O 2 remained almost constant in all experiments, suggesting the lack of net-consumption or production of O 2 by the reaction, despite its role as a rate promoter, i.e. , a catalyst, similar to the findings in our previous work on the non-photocatalytic 36 and photocatalytic 37 reforming of formaldehyde.…”

Minireview Elaborating S-Scheme Charge Dynamic Photocatalysts: Journey from Z to S, Mechanism of Charge Flow, Characterization Proof, and H2O2 Evolution

“…Henceforth, an alternate heterojunction, or charge dynamic, is very much essential to address the above-discussed challenges for achieving higher redox capabilities and better electron–hole separation. In this regard, the traditional Z-scheme type charge shipment with appropriate redox potentials mimicking natural photosynthesis somehow addresses the drawbacks of type-II route (Figure d) . However, controversy is found to be associated with this Z-scheme charge transfer, as follows.…”

“…First and foremost, the mode of electron transmission is inappropriate as illustrated in Figure a; however, the more authentic electron transmission is depicted in Figure b. From the thermodynamic or electrochemical point of view, the photoproduced electrons in the S2 semiconductor possess greater reduction abilities as compared to the S1 semiconductor . The acceptor (A), Fe 3+ , favors receiving photoelectrons from the S2 rather than S1 semiconductor; likewise, the donor (D), Fe 2+ , is ready to donate the electron to the S1 instead of S2 semiconductor.…”

Review on CeO2-Based Photocatalysts for Photocatalytic Reduction of CO2: Progresses and Perspectives

“…In 1979, Bard first proposed that redox potentials for heterojunction systems could occur at heterojunctions of the Z-scheme . It has also been experimentally demonstrated that two different photocatalysts can form a Z-type heterojunction system. − As shown in Figure e, under light conditions, the electrons in the conduction band of photocatalyst II migrate to the valence band of photocatalyst I, and this process will greatly prolong the time of photogenerated electron–hole pair recombination to obtain enhanced photocatalytic activity for the photocatalytic reduction of CO 2 . , Nanoheterostructures of Cu 2 O/WO 3 /CeO 2 were reported as a ternary oxide Z-scheme by Prajapati et al In the absence of precious metal cocatalysts, its activity for visible-light-driven photocatalytic CO 2 reduction is four times that of the binary oxide component and eight times more active than CeO 2 alone. Furthermore, as shown in Figure a and b, the researchers have also looked at two possible mechanistic scenarios from Type II and Type Z.…”