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A rapid microwave solid-state synthesis method is systematically investigated to achieve a H 2 producing visible light active spinel photocatalyst. ZnFe 2 O 4 nanocrystallites were obtained by microwave irradiation of precursor compacts under optimized conditions. This investigation led to a uniform sized nanocrystalline photocatalyst that yielded a quantum-yield of H 2 evolution y3.8 times higher than that of conventionally synthesized ZnFe 2 O 4 . The synthesis parameters -microwave power, synthesis temperature, and time, were found to control the physico-chemical properties viz phase formation kinetics, phase purity, crystallinity, specific surface area and photochemical efficiency, of the synthesized photocatalyst. The study reveals that the threshold microwave power of ¢3 kW was necessary to obtain a spinel phase structure, while lower power (,3 kW) could not induce the crystallization even after prolonged low-power irradiation of 180 min. At the threshold power, a minimum of 10 min. synthesis time was enough to obtain uniform sized nanocrystallites, indicating that the synthesis method is y24 times faster than the solid state reaction method, which needs nearly 4 h. The particle morphology evolution with irradiation time from 10-150 min. exhibited de-crystallization phenomena. Longer irradiation displayed a morphological crystallization probably induced due to the simultaneous area and volumetric heating effect. The possible ''formation mechanism'' of these uniform nanocrystallites has been presented here for qualitative understanding. Thus synthesized photocatalysts generated hydrogen from a water-methanol mixture even without the co-catalyst loading. The ferrite photocatalyst was found to decolorize methylene blue dye with a maximum decay constant of 0.232 h 21 , thereby demonstrating its capability in the pollutant decomposition applications, all under visible light photons.

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SummaryThis paper investigates solar radiation‐induced photocatalytic hydrogen generation using spinel ZnFe2O4 (ZFO) photocatalysts fabricated using different routes, viz., solid state reaction (SSR), polymer complex (PC), microwave sintering (μW) and self‐propagating combustion (SPC) method. The physicochemical properties of the photocatalysts like crystallinity, surface area, band gap and band energetics is studied as it influences their photochemical behavior. The study reveals a high crystallinity of the ZFO photocatalysts, those are synthesized using SSR, PC and μW methods, where SSR method yields the larger dimension crystallites of ~53 nm. The nanoparticles obtained from SPC methodology exhibit a relatively large surface area and a smaller crystallite size of around ~18 nm. Monodispersed particles with comparatively large surface area are obtained in the case of PC method. ZFO obtained from μW synthesis exhibits enhanced optical properties, thus favoring high absorption of solar photons. A relatively more negative flat band potential is displayed by the μW samples (−0.543 vs normal hydrogen electrode) as estimated from the electrochemical measurements. Consequently, these samples yield a higher quantum yield (0.19%) for hydrogen evolution even without co‐catalyst loading. On the contrary, the photocatalysts obtained by SSR and PC methods did display an enhancement in the quantum yield as compared to the μW samples but only after Pt co‐catalyst loading. Copyright © 2015 John Wiley & Sons, Ltd.