Rational Design of Nanostructured Surfaces for Energy Conversion and Wettability

Abstract

Nanoscale semiconductor materials offer many potential benefits for energy conversion. 3D nanostructures can provide light trapping and antireflective properties, while increased surface area reduces local flux and provides increased capacity for catalyst loading. Core-shell structures can improve transport phenomena and enable the use of a more diverse range of materials. However, these geometries present new challenges. Increased surface area can lead to higher recombination rates or surface degradation and many nanoscale fabrication methods can be difficult to scale-up. Engineering heterogeneous nanostructures with precise control over physical and material properties is an important step in understanding the impact of these parameters on the performance of energy conversion devices.

In this thesis, ZnO nanowires are used as a model system for engineering heterogeneous nanostructures. Atomic layer deposition (ALD) surface modification was used to direct the hydrothermal growth of ZnO, enabling arrays of nanowires and branched nanowires with precise control of density, orientation, and size. ALD is well suited to thin film deposition and surface modification for nanostructured surfaces. ALD consists of self-limiting gas-phase surface reactions that are not line-of-sight limited resulting in conformal coverage of even high aspect ratio 3D nanostructures with angstrom scale thickness control. The utility of this approach was demonstrated through the rational design of three-level hierarchical structures for structural omniphobicity, producing surfaces that repel both high and low surface tension liquids.

While ZnO is a semiconductor, its wide band gap limits applications for solar energy conversion. To address this limitation, ALD of bismuth vanadate (BVO), a mid-band gap semiconductor, was developed to enable core-shell nanostructures. BVO was deposited using ALD of alternating films of bismuth and vanadium oxides. A novel Bi-alkoxide precursor was used to enable precise control of stoichiometry along the spectrum of Bi-rich to V-rich compositions, and monoclinic BVO films were obtained after post-annealing. Photoanodes for photoelectrochemical water oxidation were created from the ALD BVO in both planar and nanowire configurations. A planar photoanode of 42 nm thick BVO produced a photocurrent density of 2.24 mA/cm2 at 1.23 V vs. RHE.

Core-shell-shell nanowire arrays consisting of ZnO nanowires, ALD SnO2, and ALD BVO were used to create nanostructured BVO photoanodes. The geometry of these photoanodes was rationally designed to investigate the influence of different geometric parameters and to maximize their photocurrent. The thickness of the BVO was optimized to improve charge separation and transport while the length and density of the nanowire arrays were controlled to optimize light absorption. A photocurrent of 3.76 mA/cm2 at 1.23 V vs. RHE was achieved on high density 1.4 µm ZnO nanowires with a 20 nm ALD BVO film as the primary absorber. These values are the highest reported to date for any ALD photoanode.

In summary, a nanoscale “toolbox” consisting of ALD thin films, ZnO nanowires, and complex branched nanostructures has been demonstrated as a method to study the influence of different materials and geometric properties on performance of nanostructured surfaces and devices. Understanding these properties enables the rational design of nanostructures tailored to individual applications.