The tandem of rapidly growing energy demand, increasingly serious environmental pollution, and climate change problems is putting forward great challenges to the modern world. Solar energy, which is abundant, clean and renewable, is a hopeful candidate for tackling our energy and environmental issues. Nanomaterial-based solar energy conversion and utilization systems have drawn intensive research interest for the past decades. For the core functional nanomaterials, their photoelectrochemical (PEC) properties reflect their abilities of making use of the solar energy. In this thesis, noble metal based nanosystems [nanoparticles (NPs) and nanoclusters (NCs)] anchored on TiO2 substrates are developed and engineered, to achieve new kinds of hybrid nanomaterials with good PEC properties, for solar energy utilization.

This dissertation begins with a brief introduction to provide the readers with necessary background information, including the mechanism of PEC, and criteria of functional properties of PEC materials. Following the introduction, related literature in the research field is reviewed: Firstly, studies on one-dimensional TiO2 nanostructures, i.e. nanotube arrays and nanorod arrays, are discussed, in terms of growth mechanism, morphology engineering, performance evaluation, etc.; Secondly, the recent advancement in material modifications by means of the promising surface plasmonic resonance (SPR) mechanisms is reviewed; The principles of SPR and its effects on the photonic efficiency of photovoltaics and photocatalysis are discussed; Thirdly, noble metal-thiolate nanoclusters, a newly emerged type of hybrid nanomaterial, and their applications in PEC field, are reviewed.

TiO2 nano-substrates with one-dimensional aligned structure are developed for anchoring functional sensitizers. TiO2 nanotube arrays are fabricated using electrochemical etching anodization method, and are characterized using FE-SEM and XRD. The effects of the synthetic parameters, including the duration of anodization and the applied anodic voltage, on the nanotube arrays’ morphology and the PEC performance are addressed. Post-treatments and annealing result in neat nanotubes in anatase phase with traces of rutile phase. The sample with best performance, which is obtained with 12 h of anodization at 40 V, and annealed at 773 K for 6 h in air, is chosen as the standard substrate for the following research. Besides, TiO2 nanorod arrays grown on transparent FTO glass are synthesized via a hydrothermal method as an alternative substrate candidate. It is found that a pretreatment of the FTO glass using a TiCl4 solution can effectively improve the PEC performance.

A one-step in situ hydrothermal method is designed for depositing Ag@Ag2S hybrid nanoparticles onto the TiO2 nanotube substrates. The Ag@Ag2S nanoparticles are evolved from an Ag(I)-thiolate complex. Characterizations are carried out using FE-SEM, HRTEM, XRD, XPS and UV-vis DRS. The Ag@Ag2S nanoparticles are found evenly distributed on the surface of the nanotube substrates. The resultant nano-composites exhibit high visible light-sensitivity and photocurrent output as photoanodes in PEC applications. The mechanism is discussed and the outstanding performance is attributed to the SPR effect of Ag, which is further enhanced by an Ag2S outer-layer.

Based on the fabrication strategy of the Ag@Ag2S NPs, a modified synthesis method is designed to fabricate MPA protected Ag-, Au-, and Ag Au bi-metallic nanoclusters, as an improved new class of sensitizer for PEC. The metal NCs exhibit unique optical properties, with vanished SPR absorption peaks, and presence of photoluminescence (PL) emission in visible to near IR range. These unique optical properties are brought about by the discrete energy levels resulted from quantum confinements for the NCs’ ultra-small size. The Ag- and bi-metallic- NCs show good performance as sensitizers for PEC. Especially, the bi-metallic NCs demonstrate superior photostability under constant illumination. Long-lived excited states are identified through transient PL decay measurements and open-circuit voltage decay analysis. These ligand−metal charge-transfer states allow photoinduced charge transfer upon illumination, and are proposed as the foundation for the NC sensitization.