Mingi Choi

Hydrogen-doped Brookite TiO2 Nanobullets Array as a Novel Photoanode for Efficient Solar Water Splitting

As a representative photocatalyst for photoelectrochemical solar water splitting, TiO2 has been intensively studied but most researches have focused on the rutile and anatsase phases because brookite, another important crystalline polymorph of TiO2, rarely exists in nature and is difficult to synthesize. In this work, hydrogen doped brookite (H:brookite) nanobullet arrays were synthesized via a well-designed solution reaction for the first time. H:brookite shows highly improved PEC properties with excellent stability, enhanced photocurrent, and significantly high Faradaic efficiency for overall solar water splitting. To support the experimental data, ab initio density functional theory calculations were also conducted. At the interstitial doping site that has minimum formation energy, the hydrogen atoms act as shallow donors and exist as H+. which has the minimum formation energy among three states of hydrogen (H+. H0, and H−). The calculated density of states of H:brookite shows a narrowed bandgap and an increased electron density compared to the pristine brookite. The combined experimental and theoretical results provide frameworks for the exploration of the PEC properties of doped brookite and extend our knowledge regarding the undiscovered properties of brookite of TiO2.

Investigating the Unrevealed Photocatalytic Activity and Stability of Nanostructured Brookite TiO2 Film as an Environmental Photocatalyst

Among three polymorphs of TiO2, the brookite is the least known phase in many aspects of its properties and photoactivities (especially comparable to anatase and rutile) because it is the rarest phase to be synthesized in the standard environment among the TiO2 polymorphs. In this study, we address the unrevealed photocatalytic properties of pure brookite TiO2 film as an environmental photocatalyst. Highly crystalline brookite nanostructures were synthesized on titanium foil using a well-designed hydrothermal reaction, without harmful precursors and selective etching of anatase, to afford pure brookite. The photocatalytic degradation of rhodamine B, tetramethylammonium chloride, and 4-chlorophenol on UV-illuminated pure brookite were investigated and compared with those on anatase and rutile TiO2. The present research explores the generation of OH radicals as main oxidants on brookite. In addition, tetramethylammonium, as a mobile OH radical indicator, was degraded over both pure anatase and brookite phases, but not rutile. The brookite phase showed much higher photoactivity among TiO2 polymorphs, despite its smaller surface area compared with anatase. This result can be ascribed to the following properties of the brookite TiO2 film: (i) the higher driving force with more negative flat-band potential, (ii) the efficient charge transfer kinetics with low resistance, and (iii) the generation of more hydroxyl radicals, including mobile OH radicals. The brookite-nanostructured TiO2 electrode facilitates photocatalyst collection and recycling with excellent stability, and readily controls photocatalytic degradation rates with facile input of additional potential.

Morphological optimization of large-area arrays of TiO2 nanowires & nanotubes for enhanced cold field emission: experiment and theory

Designed by finite elemental modelling, large-area arrays of TiO2 nanowires and nanotubes with differentiated heights mixed together are synthesized on a planar Ti wafer via hydrothermal methods. Experimental measurements reveal that the TiO2 nanowire/tubes arrays with differentiated heights demonstrate a lower shielding effect and their cold field emission (CFE) performances can be further enhanced by increasing their height/diameter ratios for both the nanowires and nanotubes. Theoretically, the TiO2 nanowires and nanotubes are simplified to a “Zero Thickness Charge Disc (ZTCD)” model, based on which their characteristic macroscopic field enhancement factors (γC) are quantified. The theoretically calculated γC values are in good agreement with the experimental ones of the TiO2 nanowires and tubes with a series of geometrical parameters. The TiO2 nanowires and nanotubes have promising potential in CFE. The “ZTCD” model is valuable for future research on quasi-one-dimensional field emitters.

Design and roles of RGO-wrapping in charge transfer and surface passivation in photoelectrochemical enhancement of cascade-band photoanode

Fast charge transfer and anti-photocorrosion are two crucial factors for developing efficient, durable photoanodes for photoelectrochemical (PEC) cells. Reduced graphene oxide (RGO) is a promising photoanode element that can provide both of these. In this study, we elucidated the roles of RGO in the charge transfer and surface passivation of photoanodes by the precise design of a RGO-wrapped photoanode and examination of its PEC properties. Arrays of hetero-nanorods (HNRs) with three different designs were fabricated as photoanodes using RGO, CdSe nanoparticles (NPs), and ZnO nanorods (NRs) as building blocks. CdSe@ZnO HNRs were prepared by decorating ZnO NRs with CdSe NPs. Finite-element analysis and experimental studies demonstrated that in the CdSe@ZnO HNRs, if only the ZnO NRs were wrapped by RGO, the conductivity between CdSe and ZnO was enhanced by RGO to shuttle charges. If RGO only surrounded the outside of the CdSe@ZnO HNRs, the corrosion was slowed owing to the passivation effect of RGO, which increased the electron lifetime of the photoanode. If both CdSe and ZnO were fully wrapped by RGO, the advantages of the two aforementioned cases were both obtained. RGO-wrapped CdSe@ZnO HNRs with position-controlled designs are promising photoanode materials with a high PEC efficiency, and the developed synthesis process can be applied to explore the design and fabrication of next-generation photoanodes using RGO as a building block.