Mechanistic Insight into Photoelectrochemical Behavior of Nanocluster-Sensitized Photoelectrodes

Abstract

Light harvesting nanomaterials, which possess the visible light-absorption capability, long excited-state lifetimes, and suitable energy levels for charge transfer, hold an important position in the burgeoning research on solar energy conversion and storage applications. These light harvesters primarily extend the spectral response region of wide-band gap semiconductors such as TiO2, make them an effective photoelectrode for the conversion of solar energy and production of solar fuel. In this regard, the efficacy of organic dyes and quantum dots (QDs) have largely been known and worth-appreciated. Despite the enormous advances achieved with these ubiquitous photosensitizers, their stagnant photoconversion efficiency is, however, calling for new and improved light harvesting materials that can provide a breakthrough. Atomically precise metal nanoclusters (NCs) featuring molecular-like discrete electronic structures are receiving a great deal of attention in this respect. Owing to the rich and versatile photodynamics that can be tuned by changing size, composition, and ligand engineering, NCs possess the potential to challenge the dominance of dyes and QDs in energy conversion applications. While the initial proof-of-concept studies have shown the feasibility of metal NCs in photovoltaics and photoelectrochemical (PEC) systems, a comprehensive understanding of various charge-transfer, transport and recombination events in NC-sensitized photoelectrodes is still lacking. This understanding is vital to establish design principles for efficient NC-based light harvesting systems. In the dissertation, we aim to provide the mechanistic understanding of the PEC behavior of NC-sensitized photoelectrodes and its implications in photovoltaics and PEC water splitting applications which will help establish new design principles in NC-based light harvesting assemblies. At first, we carefully designed a comparison study of two representative NCs, Au and Ag, having similar absorption profiles to shed light on the true benefits and limitations of these NC sensitizers in metal-nanocluster-sensitized solar cells (MCSSCs). With the aid of transient absorption spectroscopy and electrochemical impedance spectroscopy, it is revealed that low NC regeneration efficiency is the most critical factor that limits the performance of MCSSCs. The slow regeneration that results from sluggish hole transfer kinetics not only limits photocurrent generation efficiency but also has a profound effect on the stability of MCSSCs. Ag NCs, as compared to Au NCs, possess a greater extinction coefficient and are less vulnerable to charge recombination, however, their superior physical properties cannot be fully realized in photoelectrodes because of the limited hole extracting ability of currently used electrolytes. This finding calls for urgent attention to the development of an efficient redox couple that has a great hole extraction ability and no corrosive nature. Given the pros and cons of Au and Ag NCs as a sensitizer, it is speculated that making efforts to exploit their benefits simultaneously might be able to boost PCE of MCSSCs. In this regard, co-sensitization of Au and Ag NCs or the utilization of AuAg bimetallic NCs would be a remedy that could combine the advantages of Au and Ag and circumvent their shortcomings. In an effort to exploit the benefits of both Au and Ag NC simultaneously in photoelectrodes, we systematically alter the optoelectronic structure of Au18(SG)14 by Ag doping and explain its influence on solar cell performance. Our in-depth spectroscopic and electrochemical characterization combined with computational study reveals that the performance-dictating factors respond in different manners to the Ag doping level, and we determine that the best compromise is the incorporation of a single Ag atom into an Au NC. This new insight highlights the unique aspect of NCs—susceptibility to atomic level doping—and helps establish a new design principle for efficient NC-based solar cells. Besides the intrinsic photophysical behavior of NC sensitizer, realizing the synergy between PEC environment and photoelectrode material for effective utilization of charge carriers is also crucial to understand the true potential of NCs in light harvesting applications. This is particularly important for solar fuel generation in PEC cells whereby the sluggish oxidation kinetics significantly alter the interfacial processes at photoelectrodes. In this respect, we investigate the PEC behavior of Au NC-sensitized TiO2 electrodes in the presence of different hole scavengers in the electrolyte and explain the influence of interfacial trap states during the PEC process. These trap states are not inherent to any particular type of Au NCs but become activated during the PEC process depending upon the electron density in photoelectrode (photoinjected from Au NCs) and accumulation of formed oxidative species at the photoelectrode/electrolyte interface. In addition, we demonstrate a self-consistent explanation of the Mott–Schottky (M–S) plots of Au NC–TiO2 photoelectrodes, demonstrating that M–S measurements can accurately define their photocurrent–voltage behavior and not suitable to determine the flat band potential of sensitized photoelectrodes. Investigating interfaces at photoelectrodes is challenging not only due to complex charge carrier pathways but photo-degradation aggravates this difficulty because interfacial properties are significantly altered by degradation. Unlike dyes and semiconductors that degrade into photo-inactive materials, the photo-degradation of Au nanoclusters (NCs) yields Au nanoparticles (NPs) that are photoactive. Besides, these NPs can form Schottky barriers with TiO2, which can affect interfacial band structures. Hence, the co-presence of this photoactive nano-duo gives rise to unprecedented complexity in understanding the PEC behavior of NC-sensitized photoelectrodes. We unveil that electron injection into TiO2 and subsequent electron trapping at deep surface trap states in TiO2, which are created by sensitization, play a vital role in photo-degradation. We also demonstrate that photocurrent can be enhanced through judicious control over photo-degradation that would otherwise be deleterious. This photocurrent enhancement is attributed to multiple overlooked effects (plasmonic field enhancement and interfacial band bending).