Heterointerface Engineering of ZnO/CdS Heterostructures through ZnS Layers for Photocatalytic Water Splitting

Solar-driven water splitting to produce clean and renewable hydrogen offers a green strategy to address the energy crisis and environmental pollution. Heterostructure catalysts are receiving increasing attention for photocatalytic hydrogen generation. ZnO/ZnS/CdS and ZnO/CdS heterostructures have been successfully designed and prepared according to two different strategies. By introducing a heterointerface layer of ZnS between ZnO and CdS, a Z scheme charge-transfer channel was promoted and achieved superior photocatalytic performance. A highest hydrogen generation rate of 156.7 μmol g^-1  h^-1 was achieved by precise control of the thickness of the heterointerface layer and of the CdS shell. These findings demonstrated that heterostructures are promising catalysts for solar-driven water splitting, and that heterointerface engineering is an effective way to improve the photocatalytic properties of heterostructures.

Bridge engineering in photocatalysis and photoelectrocatalysis

Solar driven photocatalysis and photoelectrocatalysis have emerged as promising strategies for clean, low-cost, and environmental-friendly production of renewable energy and removal of pollutants. There are three crucial steps for the photocatalytic and photoelectrochemical (PEC) processes: light absorption, charge separation and transportation, and surface catalytic reactions. While significant achievement has been made in developing multiple-component photocatalysts to optimize the three steps for improved solar-to-chemical energy conversion efficiency, it remains challenging when weak interfacial contact between components/particles hinders charge transfer, restricts electron-hole separation and lowers the structural stability of catalysts. Moreover, owing to the mismatch of energy bands, an undesirable charge transfer direction leads to an adverse consequence. To tackle these challenges, bridges are implemented to smoothen the interfacial charge transfer, improve the stability of catalysts, mediate the charge transfer directions and improve the photocatalytic/PEC performance. In this review, we present the advances in bridge engineering in photocatalytic/PEC systems. Starting with the definition and classifications of bridges, we summarize the architectures of the reported bridged photocatalysts. Then we systematically discuss the insight into the roles and fundamental mechanisms of bridges in various photocatalytic/PEC systems and their contributions to activity enhancement in various reactions. Finally, the challenges and perspectives of bridged photocatalysts are featured.

Electrochemical synthesis of n-type ZnS layers on p-Cu2O/n-ZnO heterojunctions with different deposition temperatures

Metal oxide p-n heterojunctions consisting of p-Cu2O/n-ZnO/n-ZnS nanostructures were deposited on an ITO substrate by three-step electrodeposition. The effect of ZnS layer deposition temperature on the properties of the heterojunction was investigated by different techniques. The Mott-Schottky analysis confirmed the n-type conductivity for ZnO and ZnS and p-type conductivity for the Cu2O layer, respectively. Also, it showed a decrease of ZnS donor concentration with increasing deposition temperature. The X-ray diffraction (XRD) analysis confirms a pure phase of hexagonal ZnO, cubic ZnS and cubic Cu2O structures, respectively. The heterojunction with ZnS deposited at 60 °C shows high crystallinity. The morphological measurements by scanning electron microscopy (SEM) indicate that the deposition temperature has a significant influence on the morphology of ZnO and the atomic force microscopy (AFM) images revealed the improvement of Cu2O morphology by increasing the ZnS deposition temperature. The UV-Vis response shows strong absorption in the visible region and the profile of optical absorption spectra changes with the ZnS deposition temperature. The current-voltage (I-V) characteristics of the Au/p-Cu2O/n-ZnO/n-ZnS/ITO heterojunction display well-defined rectifying behavior for the heterojunction with ZnS deposited at 60 °C.

Photosensitive Schottky barrier diode behavior of a semiconducting Co(iii)–Na complex with a compartmental Schiff base ligand

The opto-electronic properties of an X-ray characterized, end-to-end azide bridged cobalt(iii)–sodium complex, [(N₃)CoLNa(N₃)]_n, have been investigated in detail. The complex is found to be a direct semiconductor material as confirmed by determining the band gap of this complex by experimental as well as theoretical studies. The complex has also been used to construct a photosensitive Schottky device. Optical conductivity, calculated from the DFT study, has been used to analyze how the conductivity of the material changes upon illumination. The electrical conductivity and concomitantly, the photoconductivity of the material increase as a consequence of photon absorption.

The opto-electronic properties of a cobalt(iii)–sodium complex have been studied with the help of both experimental and theoretical investigation. The complex has also been used to fabricate an effective photosensitive Schottky device.

A tetranuclear nickel/lead complex with a salen type Schiff base: synthesis, structure and exploration of photosensitive Schottky barrier diode behaviour

A tetranuclear nickel(ii)/lead(ii) complex has been synthesized and characterized. The complex based device behaves as a Schottky diode. The charge transfer kinetics of the complex is enhanced after light soaking.

Enhanced Photosensitive Schottky Diode Behavior of Pyrazine over 2-Aminopyrimidine Ligand in Copper(II)-Phthalate MOFs: Experimental and Theoretical Rationalization

Two novel Cu(II)-based metal-organic frameworks [C₄₀H₃₄Cu₂N₆O₁₈ (1) and C₂₀H₁₈CuN₂O₁₀ (2)] have been synthesized using 2-aminopyrimidine or pyrazine ligands and phthalate ion and characterized spectroscopically and by X-ray single-crystal diffraction. Both 1 and 2 show electrical conductivity and photosensitivity, evidencing their potentiality in optoelectronic device applications. Experimental and theoretical investigations revealed that the electrical conductivity under irradiation of visible light increases compared to that under dark condition (photosensitive Schottky barrier diode behavior), especially in complex 2. Both 1 and 2 have been successfully applied in technologically challenging thin-film active devices.

Efficient Electron Transfer across a ZnO-MoS2 -Reduced Graphene Oxide Heterojunction for Enhanced Sunlight-Driven Photocatalytic Hydrogen Evolution

The development of noble metal-free catalysts for hydrogen evolution is required for energy applications. In this regard, ternary heterojunction nanocomposites consisting of ZnO nanoparticles anchored on MoS₂ -RGO (RGO=reduced graphene oxide) nanosheets as heterogeneous catalysts show highly efficient photocatalytic H₂ evolution. In the photocatalytic process, the catalyst dispersed in an electrolytic solution (S^2- and SO₃^2- ions) exhibits an enhanced rate of H₂ evolution, and optimization experiments reveal that ZnO with 4.0 wt % of MoS₂ -RGO nanosheets gives the highest photocatalytic H₂ production of 28.616 mmol h^-1  g_cat^-1 under sunlight irradiation; approximately 56 times higher than that on bare ZnO and several times higher than those of other ternary photocatalysts. The superior catalytic activity can be attributed to the in situ generation of ZnS, which leads to improved interfacial charge transfer to the MoS₂ cocatalyst and RGO, which has plenty of active sites available for photocatalytic reactions. Recycling experiments also proved the stability of the optimized photocatalyst. In addition, the ternary nanocomposite displayed multifunctional properties for hydrogen evolution activity under electrocatalytic and photoelectrocatalytic conditions owing to the high electrode-electrolyte contact area. Thus, the present work provides very useful insights for the development of inexpensive, multifunctional catalysts without noble metal loading to achieve a high rate of H₂ generation.

CdS–MoS2 heterostructures on Mo substrates via in situ sulfurization for efficient photoelectrochemical hydrogen generation

CdS–MoS₂ photoelectrode with a double layers core shell structure was prepared on Mo substrate through using a simple electrodeposition and– in situ sulfurization method on Mo substrate.

Synthesis of rGO–Zn0.8Cd0.2S via in situ reduction of GO for the realization of a Schottky diode with low barrier height and highly enhanced photoresponsivity

Application of rGO–Zn_0.8Cd_0.2S in Schottky barrier diode with low barrier height and highly enhanced photoresponse.

ZnO Hierarchical Nanostructure Photoanode in a CdS Quantum Dot-Sensitized Solar Cell

A hierarchical array of ZnO nanocones covered with ZnO nanospikes was hydrothermally fabricated and employed as the photoanode in a CdS quantum dot-sensitized solar cell (QDSSC). This QDSSC outperformed the QDSSC based on a simple ZnO nanocone photoanode in all the four principal photovoltaic parameters. Using the hierarchical photoanode dramatically increased the short circuit current density and also slightly raised the open circuit voltage and the fill factor. As a result, the conversion efficiency of the QDSSC based on the hierarchical photoanode was more than twice that of the QDSSC based on the simple ZnO nanocone photoanode. This improvement is attributable to both the enlarged specific area of the photoanode and the reduction in the recombination of the photoexcited electrons.

Facile synthesis of CeO2 nanoparticle sensitized CdS nanorod photocatalyst with improved visible-light photocatalytic degradation of rhodamine B

A heterostructure photocatalyst consisting of one-dimensional (1D) CdS nanorods (NRs) and cerium dioxide (CeO₂) nanoparticles (NPs) was successfully synthesized via a solvothermal method.

Efficient improvement of photoelectrochemical activity for multiple semiconductor (CdS/PbS/ZnS) co-sensitized TiO2 photoelectrodes by hydrogen treatment

In the present work we report a simple and viable approach to improve the photoelectrochemical activity of TiO₂ photoelectrodes.