New insights on photocatalytic hydrogen evolution of ZnFe2-xGaxO4 (0 ≤ x ≤ 2) solid solutions: Role of oxygen vacancy and ZnO segregated phase

The exponential increase in global energy and clean energy demands lead to the necessity for development of efficient photocatalyst for hydrogen evolution. Herein, photocatalytic hydrogen evolution activity with nano-structured ZnFe2-xGaxO4 (0 <= x <= 2) solid solution samples synthesized by citrate-gel method at 550 degrees C is reported. The prepared materials have been characterized for their structure and optical properties. The formation of solid solution with spinel type structure in the complete range of composition and a systematically decreasing trend of unit cell parameters with increasing Ga3+ concentration is observed from powder XRD studies. The band gap of these solid solutions could be tuned from 1.9 to 3.1 eV by increasing the Ga3+ concentration. Without any co-catalyst, ZnFe2O4 shows poor catalytic activity (378 mu mol h(-1) g(-1)) for photocatalytic hydrogen evolution from water. Whereas its activity enhances with loading of co-catalyst and touches a maximum activity of 3089 mu mol h(-1) g(-1) with loading 1 wt% of Pt. The catalytic activity increases systematically with increasing Ga concentration in the solid solutions and reaches to 3989 mu mol h(-1) g(-1) for ZnGa2O4 nanomaterials. Electrochemical impedance spectroscopic and transient photocurrent measurements also support the observed increasing trend. The lower catalytic activity of ZnFe2O4 has been attributed to the faster e-h recombination due to lower band gap and inherent oxygen vacancies. A smaller amount of oxygen vacancies and segregated ZnO phase observed in Ga substituted ZnFe2O4 samples favor the e-h pair separation and that facilitate the hydrogen generation from water. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

This study reported high photocatalytic hydrogen evolution activity of nano-structured ZnFe2-xGaxO4 (0 <= x <= 2) solid solutions synthesized by citrate-gel method, with increasing Ga concentration leading to higher catalytic activity. Co-loading of co-catalyst significantly enhanced the activity of the solid solution samples for photocatalytic hydrogen evolution from water.