Doping engineering of hematite photoanodes by controlling Sn diffusion for efficient photoelectrochemical water splitting

School of Energy and Chemical Engineering (Energy Engineering)Photoelectrochemical (PEC) water splitting is a promising approach for sustainable hydrogen production, utilizing solar energy. This process involves converting water into hydrogen and oxygen gases through the generation of electron-hole pairs. Hematite (??-Fe2O3) has been received significant attention for its low cost, high stability, and unique electrochemical properties, making it a desirable semiconductor for PEC water splitting system. However, due to its low electrical conductivity and poor oxygen evolution reaction (OER) kinetics, the practical solar-to-hydrogen (STH) conversion efficiency is lower than the theoretical maximum value (~15%). Thus, improving the electronic properties of hematite is crucial to achieving higher STH conversion efficiency. This dissertation focuses on the development of hematite through doping engineering, specifically by controlling the diffusion of Sn from the fluorine-doped tin oxide (FTO) substrate. The diffused Sn can act as an n-type dopant, introducing additional charge carriers and enhancing electron and hole transport within the hematite lattice. However, an excessive amount of Sn can cause lattice distortion due to its larger ionic size compared to Fe, and lead to the formation of additional energy levels between the conduction and valence bands, thereby acting as charge recombination centers. In chapter 2, the negative effects of excess Sn on hematite are discussed. Initially, thin film hematite was fabricated for applications such as dual photoanodes and photoanode/solar cells. However, the thin film hematite exhibited lower PEC efficiency compared to the reference thick film, primarily due to the excessive amount of Sn. To maintain the benefits of Sn while mitigating its negative effects, precise control of Sn was achieved through non-metallic Si doping. The controlled Sn and Si co-doped thin film hematite exhibited improved PEC activity due to enhanced carrier concentration and electronic properties. Furthermore, the surface OER kinetics of hematite was accelerated by depositing a co-catalyst of NiFeOx and highly efficient unbiased photoelectrochemical water splitting system was achieved by designing a tandem cell consisted of dual photoanode and perovskite solar cell. In chapter 3, we focus on identifying the optimal dopant between Sn and Si for hematite. Based on the results from chapter 2, it was observed that Sn and Si co-doped hematite demonstrated higher efficiency compared to excess Sn-doped hematite. To determine which dopant, Sn or Si, exhibited superior catalytic activity, additional control of Sn was conducted. Since both Sn and Si diffuse into the hematite lattice through thermal annealing conditions, the Sn diffusion was controlled by adjusting the thermal annealing parameters. The results confirmed that hematite with a higher Si content exhibited better PEC performance, attributed to improved electronic properties at the surface, highlighting the superior catalytic activity of Si compared to Sn. Additionally, the potential use of hematite as an OER electrode in Zn-air batteries was investigated. Hematite exhibited significantly reduced charging potential compared to the novel metal electrode (Ir/C), confirming its potential as an efficient OER electrode in Zn-air batteries. In chapter 4, the research focuses on the design of co-catalysts on the surface of hematite. In addition to the poor electronic properties of hematite, limited hole transfer to the electrolyte is a major challenge for achieving high PEC efficiency. To overcome these challenges, oxygen evolution reaction OER co-catalysts are commonly employed. In this dissertation, we conducted research on 2D MXene sheets as potential co-catalysts for PEC systems, exploiting their high electrical properties and large functional groups. However, the practical utilization of 2D MXene sheets has been impeded by their high reactivity and structural mismatch with hematite, which is typically fabricated as 1D or 3D. To address this structural mismatch, we synthesized 0D nanofragmented MXene (NFMX) using a centrifuge-assisted method. Additionally, we resolved the high reactivity of NFMX by depositing a thin overlayer of NiFe(OH)x. Through this co-catalyst design, we demonstrated the potential of composite co-catalyst design, integrating the exceptional electrical properties of previously challenging-to-use 2D materials, to enhance PEC performance. It provides valuable insights into the promising role of 2D materials with superior electrical characteristics as PEC catalysts. I believe that the results and discussions presented in this dissertation can pave the way for enhancing the PEC efficiency of hematite and exploring its potential for various applications.clos

Boosting hot electron flux and catalytic activity at metal-oxide interfaces of PtCo bimetallic nanoparticles

Despite numerous studies, the origin of the enhanced catalytic performance of bimetallic nanoparticles (NPs) remains elusive because of the ever-changing surface structures, compositions, and oxidation states of NPs under reaction conditions. An effective strategy for obtaining critical clues for the phenomenon is real-time quantitative detection of hot electrons induced by a chemical reaction on the catalysts. Here, we investigate hot electrons excited on PtCo bimetallic NPs during H-2 oxidation by measuring the chemicurrent on a catalytic nanodiode while changing the Pt composition of the NPs. We reveal that the presence of a CoO/Pt interface enables efficient transport of electrons and higher catalytic activity for PtCo NPs. These results are consistent with theoretical calculations suggesting that lower activation energy and higher exothermicity are required for the reaction at the CoO/Pt interfac

NiFeOx decorated Ge-hematite/perovskite for an efficient water splitting system

To boost the photoelectrochemical water oxidation performance of hematite photoanodes, high temperature annealing has been widely applied to enhance crystallinity, to improve the interface between the hematite-substrate interface, and to introduce tin-dopants from the substrate. However, when using additional dopants, the interaction between the unintentional tin and intentional dopant is poorly understood. Here, using germanium, we investigate how tin diffusion affects overall photoelectrochemical performance in germanium:tin co-doped systems. After revealing that germanium is a better dopant than tin, we develop a facile germanium-doping method which suppresses tin diffusion from the fluorine doped tin oxide substrate, significantly improving hematite performance. The NiFeOx@Ge-PH photoanode shows a photocurrent density of 4.6mAcm(-2) at 1.23 V-RHE with a low turn-on voltage. After combining with a perovskite solar cell, our tandem system achieves 4.8% solar-to-hydrogen conversion efficiency (3.9mAcm(-2) in NiFeOx@Ge-PH/perovskite solar water splitting system). Our work provides important insights on a promising diagnostic tool for future co-doping system design. Germanium (Ge) has potential as a dopant suitable for the hematite-based photoelectrochemical water splitting system. Here, the authors report the fabrication of Ge doped porous hematite and demonstrate an efficient tandem system of Ge doped porous hematite and the perovskite solar cell