Structure Tuning Of Layered Bismuth Oxychlorides And Their Photocatalytic Activity Enhancement

Recent years have witnessed worldwide research interests in layered materials such as atomically-thick graphene, hexagonal boron nitride, transition metal dichalcogenides, perovskites, layered double hydroxides, graphitic carbon nitrides, because they are fundamentally and technologically intriguing for a variety of potential applications in electronics, catalysis, and energy storage. Of these layered materials, bismuth oxychlorides are nontoxic, noble-metal-free, and chemically and optically stable, and thus attract tremendous research interests for photocatalytic energy conversion and environment remediation. Bismuth oxychloride is a layered material that consists of [Cl-Bi-O2-Bi-Cl] monolayers stacked together by van der Waals interaction between neighboring halogen atoms along the c-axis. The intriguing layered characteristics, featuring the strong intralayer covalent bonding within the [Bi2O2] layers and the weak interlayer van der Waals interaction between halogen double slabs, endow bismuth oxychlorides with outstanding electrical, magnetic, optical, chemical, mechanical and catalytic properties and thus as a promising candidate for photocatalytic wastewater and indoor-gas purification, water splitting, organic synthesis, and selective oxidation of alcohol. The most intriguing characteristic of bismuth oxychloride lies in the strong capability of the layered structure to induce internal electric fields, which can boost the charge separation and thus enhance the photoactivity. Because of their unique layered-structure mediated fascinating physicochemical properties, bismuth oxychlorides showcase layered structure-dependent photocatalytic properties, such as layered structure-dependent photocatalytic activities and molecular oxygen activation. Despite these great progresses, the further developments of bismuth oxychloride photocatalysis are still precluded by some key fundamental issues that are unsolved. First, although the photocatalytic activities of bismuth oxychlorides highly depend on their layered structures, there are limited research attentions on developing the methods to boost their photoactivities by effectively tune the numbers and chemical compositions of the layered structures. Second, it remains unknown how the layered structures affect the photocatalytic activities of bismuth oxychlorides. Third, although the internal electric field has been proven to be capable of effectively promoting the separation and transfer of charge carriers, the details of how the internal electric field favors the charge separation remain unclear and the strategies to increase the magnitude of the internal electric field are still lacking. Fourth, the photocatalytic efficiencies of bismuth oxychlorides are thwarted to a large extent by their poor controls over the separation, transportation and consumption of charge, which favors the frequent emergence of the undesirable electron-hole recombination arisen from the random charge flow after the electron-hole separation.To address these issues, in this dissertation, we employ liquid-phase exfoliation, epitaxial growth, carbon doping, and heterojunction construction to tune the number and composition of the layered structures of bismuth oxychlorides to boost their photocatalytic activities. Using bismuth oxychloride nanosheets of the layered structures with tunable layer numbers and chemical compositions as the experimental probes, we clarify the growth rationales of the layered structures, the correlations between the layered structures and the internal electric field and between the layered structures and the photocatalytic activities, and the details of how the internal electric field favors the charge separation. Benefiting from the above nanoarchitecture modulations and mechanistic investigations, we attain the favorable internal electric field magnitude, ultrahigh bulk charge separation efficiency, atomic level charge flow control, and outstanding photocatalytic hydrogen-and oxygen-evolving activities in bismuth oxychlorides, which may render them a class of flagship photocatalysts for both fundamental research and technical application. The main contents of this dissertation are as follows:1. We prepared layered Bi3O4Cl single-crystalline nanosheets with high{001} facet exposure percentages via a facile hydrothermal route. Then we tuned their layer numbers by liquid-phase exfoliation and epitaxial growth, thus modulating the exposure percentages of the{001} facets. We found that their photoreactivities strongly depended on the LEF magnitude, which was correlated to the{001} facet exposure percentage. Bi3O4Cl nanosheets with less layer numbers enabled their more {001} facet exposure to induce the generation of stronger IEF, which favored the photogenerated charge separation and transfer and thus enhanced the photoreactivity.2. We developed a general homogeneous carbon doping strategy comprised of pre-hydrothermal carbonization processing and subsequent thermal treatment, which could effectively tailored the chemical compositions of the layered structures of BiOCl. With using the well-defined BiOCl nanosheets of high{001} or{010} facet exposure, we clarified the homogeneous carbon doping mechanism at a crystal facet level, which shed mechanistic insights into the growth rationales of the layered structures of BiOCl. The first-step hydrothermal carbonization processing enabled the implantation of dopant precursor of carbonaceous nanoclusters into the shallow lattice of BiOCl, which was regulated by the facet-related surface atomic structure and vital for the effective homogeneous doping. The subsequent thermal treatment provided sufficient energy to trigger the decomposition of carbonaceous species into carbon dopant and simultaneously overcome the energy and space barrier for the subsequent substitution of lattice chlorine atoms with carbon atoms; the facet-related arrangement of bulk atoms was identified as the key factor to determining the diffusion and substitution of carbon dopant, which governed the ultimate concentration of carbon dopant and thereby the efficiency of solar-to-hydrogen conversion in BiOCl nanosheets.3. On the basis of our understanding on the correlation between the layered structures and the internal electric field of Bi3O4Cl nanosheets, we utilized our developed carbon doping strategy to tailor the layered configuration of Bi3O4Cl to enhance its internal electric field magnitude. We experimentally showed that carbon doping could enhance the internal electric field magnitude by 126 times. The charge density contour plots and electrostatic potential calculation theoretically demonstrated that Cl substitution by C could intensify electrostatic potential difference between [Bi3O4] and [CxCl1-x] layers, which was responsible for the giant internal electric field increase. Through incorporating carbon into the Bi3O4Cl lattice to increase internal electric field, we tuned its bulk charge separation efficiency to a new record up to 80%. Femtosecond-resolved TAS demonstrated mat strong IEF could separate electrons and holes effectively, and also respectively confine them within [Bi3O4] and [Cl] slices to restrict their recombination during their migration from the bulk to the surface. Because plenty of holes could reach the surface, C-doped Bi3O4Cl enabled photocatalytic water oxidation under visible light in the absence of any noble-metal and electron-scavenger.4. On the basis of our experience in tuning the layer numbers of Bi3O4Cl nanosheets, we successfully exfoliated the layered Bi12O17CI2 nanosheets into their single-layered counterparts by organolithium chemistry. By means of oxygen vacancy chemistry, we further assembled MoS2 monolayers selectively and chemically on [B112O17] end-faces of B112O17Cl2 monolayers to craft Janus [Cl2]-[Bii2Oi7]-[MoS2] bilayer junctions with the Bi-S bonds existing at the interfaces. This atomic level structural and interfacial design allowed us to steer all the charge separation, transportation, and consumption at the atomic level. Electrons originating from visible-light irradiated Bi12O17Cl2 were first driven by the internal electric field between [CI2] and [Bi12O17] end-faces to [Bi12O17] end-faces, and further transferred via the Bi-S bonds formed between Bi12O17Cl2 and MoS2 monolayers to MoS2 monolayers to finally catalyze the hydrogen evolution. Meanwhile, the internal electric field drove holes to [CI2] end-faces where the organic scavenger was oxidized. Such an atomic level steering of charge flow offered a visible-light PHE rate of 33 mmol h-1 g-1 with a quantum efficiency of 36% at 420 nm, superior to any reported MoS2, or monolayer, or bismuth oxyhalide based PHE systems.