Intrinsic Photodynamic Study on Photocatalytic Materials

To relieve the global energy crisis and environmental pollution caused by the combustion of traditional fossil fuels, developing an environmental-friendly renewable energy to replace fossil fuel is urgent. Among the possible energy sources, solar energy has attracted numerous attentions because of the abundant storage. However, it is challenging to efficiently utilize and store solar energy. One attractive strategy to address this challenge is to convert solar energy to fuel through artificial photosynthesis (e.g. photocatalytic water splitting to generate H2). A technologically significant solar-driven water splitting system requires an efficient photocatalytic system that can not only effectively harvest light but also can perform the subsequent charge separation and catalytic reaction. The objective of my research projects is to develop such photocatalytic materials that can be used as light absorption and charge separation materials for light driven proton reduction to generate hydrogen. The materials that were of interest include semiconductor nanocrystals and porous crystalline materials. To establish their structure and property relationship, a suit of advanced spectroscopic methods including steady state absorption and emission spectroscopy, time resolved optical and X-ray absorption spectroscopy were used to examine their excited state, energy transfer, and charge separation dynamics during photoinduced reaction. One class of semiconductor photocatalytic materials that I have studied were CuInS2 quantum dots. The dependence of carrier dynamics of CuInS2 quantum dots on their sizes are presented in chapter 3. Their photocatalytic activity together with catalytic mechanism for visible light driven hydrogen generation are discussed in chapter 4. Zeolitic imidazolate frameworks (ZIFs), a subclass of metal organic frameworks (MOFs), are the second class of materials that I have investigated. The impact of the chemical compositions on ZIFs on their photophysical and photocatalytic property are discussed in chapter 5 and chapter 6. The energy transfer dynamics from the encapsulated chromophores to ZIFs is discussed in chapter 7 and chapter 8

Composition Effect on the Carrier Dynamics and Catalytic Performance of CuInS\u3csub\u3e2\u3c/sub\u3e/ZnS Quantum Dots for Light Driven Hydrogen Generation

Water soluble CuInS2/ZnS quantum dots (QDs) represent one of the most promising single component photocatalysts for the hydrogen evolution reaction (HER). In this work, we report the effect of cation composition in CuInS2/ZnS QDs on the carrier relaxation and charge separation dynamics as well as their photocatalytic performance for the HER. With decreasing Cu to In ratio (increasing Cu deficiency), we observed slightly faster electron trapping and carrier recombination but significantly improved photocatalytic activity for the HER. This can be attributed to the enhanced electron transfer (ET) from the sacrificial donor to CuInS2/ZnS QDs resulting from the lower valence band (larger driving force for ET) of QDs with higher Cu deficiency. This work not only provides important insight into the mechanistic origins of the HER but also demonstrated that altering the composition in CuInS2/ZnS QDs is a viable approach to further improve their performance for solar to fuel conversion

Unravelling the Correlation of Electronic Structure and Carrier Dynamics in CuInS\u3csub\u3e2\u3c/sub\u3e Nanoparticles

In this work, we report the direct correlation of photoinduced carrier dynamics and electronic structure of CuInS2 (CIS) nanoparticles (NPs) using the combination of multiple spectroscopic techniques including steady-state X-ray absorption spectroscopy (XAS), optical transient absorption (OTA), and X-ray transient (XTA) absorption spectroscopy. XAS results show that CIS NPs contain a large amount of surface Cu atoms with ≪four-coordination, which is more severe in CIS NPs with shorter nucleation times, indicating the presence of more Cu defect states in CIS NPs with smaller size particles. Using the combination of OTA and XTA spectroscopy, we show that electrons are trapped at states with mainly In or S nature while holes are trapped in sites characteristic of Cu. While there is no direct correlation of ultrafast trapping dynamics with NP nucleation time, charge recombination is significantly inhibited in CIS NPs with larger particles. These results suggest the key roles that Cu defect sites play in carrier dynamics and imply the possibility to control the carrier dynamics by controlling the surface structure at the Cu site in CIS NPs

Unravelling the Correlation of Electronic Structure and Carrier Dynamics in CuInS2 Nanoparticles

In this work, we report the direct correlation of photoinduced carrier dynamics and electronic structure of CuInS2 (CIS) nanoparticles (NPs) using the combination of multiple spectroscopic techniques including steady-state X-ray absorption spectroscopy (XAS), optical transient absorption (OTA), and X-ray transient (XTA) absorption spectroscopy. XAS results show that CIS NPs contain a large amount of surface Cu atoms with ≪four-coordination, which is more severe in CIS NPs with shorter nucleation times, indicating the presence of more Cu defect states in CIS NPs with smaller size particles. Using the combination of OTA and XTA spectroscopy, we show that electrons are trapped at states with mainly In or S nature while holes are trapped in sites characteristic of Cu. While there is no direct correlation of ultrafast trapping dynamics with NP nucleation time, charge recombination is significantly inhibited in CIS NPs with larger particles. These results suggest the key roles that Cu defect sites play in carrier dynamics and imply the possibility to control the carrier dynamics by controlling the surface structure at the Cu site in CIS NPs