Abstract
Developing highly efficient and stable photoelectrochemical (PEC) water-splitting electrodes via inexpensive, liquid phase processing is one of the key challenges for the conversion of solar energy into hydrogen for sustainable energy production. ZnO represents one the most suitable semiconductor metal oxide alternatives because of its high electron mobility, abundance, and low cost, although its performance is limited by its lack of absorption in the visible spectrum and reduced charge separation and charge transfer efficiency. Here, we present a solution-processed water-splitting photoanode based on Co-doped ZnO nanorods (NRs) coated with a transparent functionalizing metal–organic framework (MOF). The light absorption of the ZnO NRs is engineered toward the visible region by Co-doping, while the MOF significantly improves the stability and charge separation and transfer properties of the NRs. This synergetic combination of doping and nanoscale surface functionalization boosts the current density and functional lifetime of the photoanodes while achieving an unprecedented incident photon to current efficiency (IPCE) of 75% at 350 nm, which is over 2 times that of pristine ZnO. A theoretical model and band structure for the core–shell nanostructure is provided, highlighting how this nanomaterial combination provides an attractive pathway for the design of robust and highly efficient semiconductor-based photoanodes that can be translated to other semiconducting oxide systems.
Highlights
The development of efficient, robust, and cost-effective watersplitting cells[1−7] is key to establishing sustainable hydrogen production as a renewable energy source.[8−11] A viable avenue for this production is photoelectrochemical water splitting, which requires suitable photoelectrodes that can efficiently supply significant current densities over a long lifetime
One example is the growth of the ZIF-8 as a shell around ZnO, which is typically achieved by hydrothermal growth in a methanol or a water/dimethylformamide solution containing a zinc source and the organic linker, which is incorporated with ZnO NRs by the solvothermal conversion of ZnO into ZIF-8.30,31 Even though this integration has been reported in the literature, the interplay between the ZIF-8 shell and ZnO core has not been adequately addressed so far since there remains no conclusive mechanistic model that explains the charge transfer between the core and shell at this time
Cobalt is introduced into the growth solution, where it is incorporated into the ZnO lattice, substituting Zn atoms during growth through competitive reactions, as shown by the X-ray diffraction and X-ray photoelectron spectroscopy (XPS) results in Figures S1 and S2 of the Supporting Information
Summary
The development of efficient, robust, and cost-effective watersplitting cells[1−7] is key to establishing sustainable hydrogen production as a renewable energy source.[8−11] A viable avenue for this production is photoelectrochemical water splitting, which requires suitable photoelectrodes that can efficiently supply significant current densities over a long lifetime. MOFs are a class of organometallic nanomaterials characterized by their organic moieties, large surface areas, and chemical compatibility.[25] Among the many MOF families, the zeolitic imidazolate frameworks are one of the most studied because of their chemical and mechanical properties.[26,27] One of the most studied materials within this family is ZIF-8, which is formed by Zn nuclei tetrahedrally coordinated by 2methylimidazole organic linkers.[28] The integration of a ZIF-8 shell around ZnO cores has shown potential stability, charge generation, and charge transfer improvements.[29,30] One example is the growth of the ZIF-8 as a shell around ZnO, which is typically achieved by hydrothermal growth in a methanol or a water/dimethylformamide solution containing a zinc source and the organic linker, which is incorporated with ZnO NRs by the solvothermal conversion of ZnO into ZIF-8.30,31 Even though this integration has been reported in the literature, the interplay between the ZIF-8 shell and ZnO core has not been adequately addressed so far since there remains no conclusive mechanistic model that explains the charge transfer between the core and shell at this time To date, these two strands have been independently researched as different pathways to enhance the performance of ZnO as a photoanode. A functional model for the synergetic behavior of the combination of these two nanomaterials is given to provide a theoretical framework to consider how their interplay leads to the enhanced functional performance observed
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