Yolk-shell ZnO@C-CeO2 ternary heterostructures with conductive N-doped carbon mediated electron transfer for highly efficient water splitting.

Herein, carbon-incorporated yolk-shell ZnO@C-CeO2 ternary heterostructures are employed as visible light responsive photocatalyst for highly efficient photoelectrochemical (PEC) water splitting. Compared to conventional ZnO/CeO2 semiconductors, introduction of a thin PDA shell layer assures the generation of a conductive N-doped graphitic carbon layer after a calcination post-treatment with mesoporous hollow morphologies. The evaluation of PEC water splitting performance of ZnO@C-CeO2 photoanodes reveals the maximum photocurrent density as 7.43 mA/cm2 at 1.18 V RHE under light whereas almost no response is recorded at dark. These superior PEC H2 evolution performance strongly implies efficient charge separation, facilitated charge transfer between photoanode and electrolyte interface as well as within the semiconductor bulk by means of rapid electron transfer ability of N-doped graphitic carbon layer and prolong life time of light inside yolk-shell structure. Furthermore, considerable depression in PL intensity of ZnO@C-CeO2 photoanodes compared to ZnO clearly reveals a higher photon absorption due to the reflection of light in hollow region and increase in electron hole separation efficiency. Moreover, plausible Z-scheme charge transfer mechanism using ZnO@C-CeO2 photoanodes under visible light illumination is verified using radical trapping experiments and X-ray photoelectron spectroscopy (XPS) methods, suggesting new generation of heterostructures for sufficient conversion of sunlight to H2 fuels.

Spherical shape-defined hollow UiO-66 metal-organic frameworks with superior incident photon scattering for enhanced photoelectrochemical H2 evolution.

Herein, spherical hollow N-doped carbon-incorporated UiO-66 metal-organic frameworks (MOF, H-UiO-66) are synthesized using bio-inspired polydopamine (pDA) nanoparticles as multifunctional starting templates. The calculated band properties (ECB = -0.45 eV and EVB = 2.05 eV versus normal hydrogen electrode (NHE)) strongly reveals the visible light absorption of H-UiO-66 nanostructures thanks to the spherical shape-defined morphology as well as cavity of the hollow structure. The evaluation of photoelectrochemical (PEC) water splitting performance of H-UiO-66 photoanodes shows maximum photocurrent density as 10.95 mA/cm2 at 1.53 V versus RHE under LED illumination in which almost no response is recorded at dark. Furthermore, the improved visible-light sensitive PEC water splitting performance of H-UiO-66 photoanodes could be attributed to the main advantages of the one-pot synthesis method of hollow MOFs using multifunctional pDA as follows: i) the hollow morphology provides superior incident photon scattering and multi-reflection of photons inside the MOF cavity; ii) presence of N-doped carbon incorporated morphology facilitates the absorption of water molecules as well as the π-polar interaction between water and carbon; and iii) the reduced bang-gap led to the optical localization of light within H-UiO-66 clusters, suggesting a new generation of heterogeneous well-defined nanostructures for sustainable PEC hydrogen production.