Core–Shell TiO2@Au25/TiO2 Nanowire Arrays Photoanode for Efficient Photoelectrochemical Full Water Splitting

Periodic arrays of structurally complex oxide nanoshells and their use as substrate-confined nanoreactors

Sacrificial templates present an effective pathway for gaining high-level control over nanoscale reaction products. Atomic layer deposition (ALD) is ideally suited for such approaches due to its ability to replicate the surface topography of a template material through the deposition of an ultrathin conformal layer. Herein, metal nanostructures are demonstrated as sacrificial templates for the formation of architecturally complex and deterministically positioned oxide nanoshells, open-topped nanobowls, vertically standing half-shells, and nanorings. The three-step process sees metal nanocrystals formed in periodic arrays, coated with an ALD-deposited oxide, and hollowed out with a selective etch through nanopores formed in the oxide shell. The procedure is further augmented through the use of a directional ion beam that is used to sculpt the oxide shells into bowl- and ring-like configurations. The functionality of the so-formed materials is demonstrated through their use as substrate-confined nanoreactors able to promote the growth and confinement of nanomaterials. Taken together, the work expands the design space for substrate-based nanomaterials, creates a platform for advancing functional surfaces and devices and, from a broader perspective, advances the use of ALD in forming complex nanomaterials.

Optimizing the performance of Auy/Nix/TiO2NTs photoanodes for photoelectrochemical water splitting

Water splitting using photoelectrochemical (PEC) techniques is thought to be a potential method for creating green hydrogen as a sustainable energy source. How to create extremely effective electrode materials is a pressing concern in this area. In this work, a series of Nix/TiO2 anodized nanotubes (NTs) and Auy/Nix/TiO2NTs photoanodes were prepared by electrodeposition via cyclic voltammetry and UV-photoreduction, respectively. The photoanodes were characterized by several structural, morphological, and optical techniques and their performance in PEC water-splitting for oxygen evolution reaction (OER) under simulated solar light was investigated. The obtained results revealed the nanotubular structure of TiO2NTs was preserved after deposition of NiO and Au nanoparticles while the band gap energy was reduced allowing for effective utilization of solar light with lower charge recombination rate. The PEC performance was monitored and it was found that the photocurrent densities of Ni20/TiO2NTs and Au30/Ni20/TiO2NTs were 1.75-fold and 3.25-fold that of pristine TiO2NTs, respectively. It was confirmed that the performance of the photoanodes depends on the number of electrodeposition cycles and duration of photoreduction of gold salt solution. The observed enhanced OER activity of Au30/Ni20/TiO2NTs could be attributed to the synergism between the local surface plasmon resonance (LSPR) effect of nanometric gold which increased solar light harvesting and the p-n heterojunction formed at the NiO/TiO2 interface which led to better charge separation and transportation suggesting its potential application as an efficient and stable photoanode in PEC water splitting for H2 production.

Construction of WO3 quantum dots/TiO2 nanowire arrays type II heterojunction via electrostatic self-assembly for efficient solar-driven photoelectrochemical water splitting

Construction of a heterojunction between quantum dots and TiO₂ nanowire arrays via electrostatic self-assembly is rarely reported. In this work, mercury lamp irradiation was used to change the surface potential of WO₃ quantum dots and TiO₂ nanowire arrays, resulting in WO₃ quantum dots tightly attached on the surface of TiO₂ nanowire through electrostatic self-assembly. Photoelectrochemical measurements showed that the WO₃ quantum dots formed a type II heterojunction with the TiO₂ nanowire arrays rather than serving as carrier-trapping sites. In the self-assembly system, the TiO₂ nanowire arrays provide a charge-transfer channel for the WO₃ quantum dots, greatly improving the contribution of the WO₃ quantum dots to the photocurrent. Quantitative calculations showed that the improvement of the bulk carrier-separation efficiency was the reason for the enhanced photoelectrochemical performance of the self-assembled system. The photocurrent density of the optical self-assembled system at 1.23 V (vs. RHE) was ∼5.5 times as high as that of the TiO₂ nanowire arrays. More importantly, the self-assembled system exhibited excellent photoelectrochemical stability.

Energy band alignment at the heterointerface between a nanostructured TiO2 layer and Au22(SG)18 clusters: relevance to metal-cluster-sensitized solar cells

This study is the first to quantify energy band alignments at a nanostructured TiO₂/Au₂₂(SG)₁₈ cluster interface using X-ray photoelectron spectroscopy. The d-band of Au clusters shows band-like character and occupied states at the Fermi level are not detected. The results provide evidence of the existence of a finite optical energy gap in Au₂₂(SG)₁₈ clusters and the molecular-like nature of these clusters. The pinning position of the Fermi energy level at the interface was determined to be 2.8 and 1.3 eV higher than the top of the TiO₂ valence band and the highest occupied molecular orbit level of the Au clusters, respectively. A diffuse reflectance and absorption analysis quantified a 3.2 eV bandgap of the TiO₂ layer and a 2.2 eV energy gap between the highest occupied molecular orbit (HOMO) and the lowest unoccupied molecular orbit (LUMO) levels of the Au clusters. Thus, a cliff-like offset of 0.5 eV between the LUMO level and the TiO₂ conduction band was determined. The cliff-like offset of 0.5 eV provides room for improving the efficiency of metal-cluster-sensitized solar cells (MCSSC) further by lowering the LUMO level through a change in the cluster size. The offset of 0.5 eV between the HOMO level and the 3I^-/I^-3 redox level yields a remarkable loss-in-potential, which implies the possibility of increasing the open-circuit voltage further by properly replacing the redox couple in the MCSSCs.