(Invited) Tailoring Zinc Oxide Nanorod-Arrays for Photo-(electro)Chemical Applications
Authors: Kegel Jan , Povey Ian M , Pemble Martyn E 
Published: February 2020

As one of the most studied metal-oxides Zinc Oxide (ZnO) already found its way into many industrial applications. Nevertheless enormous research interest is drawn to this earth-abundant, environmentally-friendly material as it offers interesting material properties such as a high exciton binding energy, a direct bandgap and comparably high charge carrier mobility.1 Furthermore the ability to grow ZnO nanostructures using a wide range of deposition techniques offers new perspectives for the material to be used in opto-electronics.2 Low-temperature solution based methods are of particular interest as possible routes to the low-cost growth of high surface-area nanostructures for the integration of ZnO into novel energy production and storage devices.3

The steadily growing research areas of solar water splitting and photo-catalysis are prominent examples of areas where interest in ZnO-based materials and devices may be found.4-6 For these applications the semiconductor/electrolyte interface plays a crucial role. It is of the highest importance to carefully engineer the materials properties in order ensure effective charge carrier transport across the interface. In turn it must be the goal of materials research to tailor the ZnO towards these target applications, where possible addressing the key issues of short timescale charge carrier recombination over intrinsic defects, low-visible light absorption and photo-corrosion.
This paper describes strategies for the minimisation of the undesirable influences associated with these issues for ZnO nanorod-arrays grown from solution. Firstly it is demonstrated that careful defect-engineering can be used to increase the probability to run photo-(electro)chemical reactions. Here, the specific focus is directed towards orange-luminescent defect-centres. Linear sweep voltammetry in neutral electrolyte solution reveals that ZnO nanorod-arrays exhibiting strong orange emission show a defect-promoted increase in photo-current of ca. 200 % under simulated sunlight as compared to as-grown nanorod-arrays.
As the large bandgap energy of ZnO, some 3.3 eV (< 380 nm), considerably limits the absorption of sunlight a doping strategy is applied in order to shift the materials absorption into the visible region. It is demonstrated that this may be achieved by the incorporation of significant amounts of cobalt into the ZnO host lattice using a newly developed low-temperature solution-based growth method. It is shown that the visible-light absorption of the resulting material, which consists of ZnO:Co nanorod-arrays directly deposited onto seed-layer coated glass substrates, extends up to 700 nm by controlling the cobalt concentration in the growth solution.
Lastly, we address the issue of photocorrosion of ZnO-based photo-electrodes. Previously various attempts have been made to enhance the material`s chemical stability. Over-coating of ZnO with thin layers of e.g. C3N4 or TiO2 using different deposition techniques are among the most applied strategies.7-9 In order to produce effective and stable chemical barriers pin-hole free layers are essential. atomic layer deposition (ALD) offers the advantage of controlled and nominally pin-hole free deposition of ultrathin barrier layers even onto 3D-geometries, thus, making it potentially a powerful technique for the chemical stabilisation of ZnO nanostructures for photo-(electro)chemical applications. In this study the ZnO nanorod-arrays are over-coated with ultrathin ALD TiO2 layers. It is presented that TiO2 shells are able to chemically protect the ZnO core in acidic media. However, it was found that for some samples the TiO2 barriers are not complete – especially at the rod tips - leading to a considerably lower etch rate as compared to uncoated nanorod-arrays. Photo-electrochemically the core-shell structures show a photo-current stabilisation for several hours. Nevertheless the initial photo-current is lower when compared to uncoated nanorod-arrays indicating a higher charge transfer resistance for the ZnO-TiO2/electrolyte interface.
Acknowledgements: This work was funded by Science Foundation Ireland, US: Ireland Grant RENEW- Research into Emerging Nanostructured Electrodes for the Splitting of Water, number 13/US/I2543.
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