Two-dimensional Ultrathin Nanosheets:Electronic Structure Modulation And Their Applications In New Energy Conversion

In the 21st century, the energy crisis caused by the depletion of fossil fuels and the serious environmental problems accompanying their combustion are two major social issues which seriously hinder sustainable development of our society. It is of vital importance for researchers to develop new clean energy and carry out effective controlling on pollution. In recent years, inorganic graphene analogues with high percentage of surface atoms have sparked worldwide interests, which can provided as an ideal model for photoelectrochemical water splitting and CO2 reduction. Pepole can not only obtain clean energy through photoelectrochemical energy conversion, but also control the worsening global climate by the means of CO2 reduction. In this dissertation, two-dimensional ultrathin nanosheets are first put forward as ideal material modes. The modulation of electronic structure based on defect engineering and elemental doping are explored by the first-principles calculations, and the time-resolved spectroscopy was used to understand the effect of surface microstructure on photo-generated electron-hole transport and separation, in order to gain in-depth understanding on the relationship between structure and performance. The main contents of this dissertation include the following aspects:1. Recently, oxygen vacancies in oxide semiconductors have been reported to increase solar light harvesting through narrowing the band gap, thus achieving an improved water splitting efficiency. However, the atomic-level insights into the role of oxygen vacancies during the photocatalytic process is still an open question. This is mainly caused by the lack of an ideal model that matches well with the real catalyst, and hence the knowledge gained from the conventional models cannot be directly applicable to the real catalysts. Finding an ideal model for disclosing the role of oxygen vacancies in photocatalysis remains a huge challenge. Herein, oxygen vacancies confined in atomically-thin sheets is proposed as an excellent platform to study the oxygen vacancy-photocatalysis relationship. As an example, S-atomic-thick In2O3 porous sheets with rich/poor oxygen vacancies are first synthesized via a mesoscopic-assembly-fast-heating strategy, taking advantage of an artificial hexagonal mesostructured In-oleate complex. Theoretical/experimental results reveal that the oxygen vacancies endow 5-atomic-thick In2O3 sheets with a new donor level and increased states density, hence narrowing the band gap from UV to visible regime and improving the carriers separation efficiency. As expected, the oxygen vacancy-rich ultrathin In2O3 porous sheets-based photoelectrode exhibits a visible-light photocurrent of 1.73 mA/cm2, over 2.5 and 15 times larger than that of the oxygen vacancy-poor ultrathin In2O3 porous sheets-and bulk In2O3-based photoelectrodes. This work opens up new opportunities in the field of photocatalysis.2. Researchers have reported that transition-metal dopants can serve as trap sites to capture the photogenerated electrons or holes and hence reduce the electron-hole recombination, thereby achieving improved photocatalytic efficiency. However, the transition-metal dopants can also act as the recombination centers, resulting in limited improvement in photocatalytic activity. Such controversy is probably due to that almost all of the transition-metal dopants are present on the interior of photocatalysts, rather than on the surface, which would adversely affect or cover the effect of elemental doping on photocatalytic activity. Herein, an ideal model of doping confined in atomic layers is first proposed to unravel the atomic-level insights into the effect of doping on photocatalysis. As a prototype, Co doping confined in In2S3 3-atomic-layers is first successfully implemented through a lamellar hybrid intermediate strategy. DFT calculations reveal that the introduction of Co ions brings about several new energy levels as well as increased density of states at conduction band minimum. Furthermore, ultrafast transient absorption spectroscopy results disclosed that the Co-doped InS3 3-atomic-layers achieved 25-fold increase in average recovery lifetime, which is believed to be responsible for the remarkably promoted efficacy of electron-hole separation. As a direct outcome, the synthesized Co-doped In2S33-atomic-layers yield a photocurrent of 1.17 mA/cm2 at 1.5 V vs. RHE, nearly 10 and 17 times higher than that of the perfect In2S3 3-atomic-layers and the bulk counterpart, respectively. This work establishes a clear atomistic correlation between elemental doping and photocatalysis, opening new possibilities for tailoring the photocatalytic properties.3. Ultrathin metal layers show substantially promoted CO2 electroreduction activities. However, for some metals with higher chemical reactivity, the ultrathin structure inevitably results in its oxidization in a non-controlled manner. To address this issue, we construct an ideal model of graphene confined ultrathin layers of highly reactive metals. As an example, highly reactive Sn quantum sheets confined in graphene are first synthesized via a spatially confined reduction strategy. Take the advantage of the protection of graphene, Sn quantum sheets shown non oxidized states which revealed by X-ray absorption fine structure spectroscopy. In addition, the lowered Sn-Sn coordination numbers resulted from XAFS enable Sn quantum sheets confined in graphene to efficiently stabilize the CO2·- intermediate. As a result, the Sn quantum sheets confined in graphene show roughly 2,2.5 and 13 times larger electrocatalytic activity than the 15 nm Sn nanoparticles mixed with graphene,15 nm Sn nanoparticles and bulk Sn, while the former also possesses formate Faradaic efficiency of over 85% during the 50-h period. Briefly, this work provides a promising lead for designing efficient and robust catalysts for electrolytic fuel synthesis.