Gun-hee Moon

Solar production of H2O2 on reduced graphene oxide–TiO2 hybrid photocatalysts consisting of earth-abundant elements only

A superior cocatalytic behavior of reduced graphene oxide (rGO) was observed for the photocatalytic production of H2O2 in the TiO2-based system. The adsorption of phosphate on TiO2 enhanced the production of H2O2 up to a millimolar level. The in situ formation of cobalt phosphate on rGO/TiO2 enabled the photocatalytic production of H2O2 even in the absence of organic electron donors.

Molecular‐Level Understanding of the Photocatalytic Activity Difference between Anatase and Rutile Nanoparticles

The generation of oxidants on illuminated photocatalysts and their participation in subsequent reactions are the main basis of the widely investigated photocatalytic processes for environmental remediation and selective oxidation. Here, the generation and the subsequent diffusion of (·)OH from the illuminated TiO2 surface to the solution bulk were directly observed using a single-molecule detection method and this molecular phenomenon could explain the different macroscopic behavior of anatase and rutile in photocatalysis. The mobile (·)OH is generated on anatase but not on rutile. Therefore, the photocatalytic oxidation on rutile is limited to adsorbed substrates whereas that on anatase is more facile and versatile owing to the presence of mobile (·)OH. The ability of anatase to generate mobile (·)OH is proposed as a previously unrecognized key factor that explains the common observations that anatase has higher activity than rutile for many photooxidative reactions.

Photocatalytic Synthesis of Pure and Water‐Dispersible Graphene Monosheets

Several research groups are actively investigating graphene, which consists of a one-atom thick planar sheet of sp bonded carbon, in attempts to understand its unusual characteristics, such as outstanding electronic properties, optical properties, thermal conductivity (5000 Wm 1 K ), high mechanical strength (200 times stronger than steel), and large surface area per unit mass (2,630 mg 1 calculated,) Novoselov et al. first obtained high quality monosheet graphene detached from natural graphite by the so-called Scotch tape method in 2004. Chemical vapor deposition (CVD) is frequently used to synthesize graphene in a practical scale, and it is suitable for producing a large area (on the inches scale) of high-quality graphene as good as that obtained from natural graphite. However, the CVD process requires high processing temperatures and etching processes, which remove Ni or Cu catalyst nanoparticle layers to yield mono layers or a few layers of graphene on the substrate. Thus, the process remains expensive. Its applications have focused on replacing indium tin oxide (ITO) or fluorinedoped tin oxide (FTO) in transparent conducting electrodes. If an efficient solution-based chemical reduction method were available, we could produce graphene by reducing graphene oxide (GO) on a large scale at low cost. Unfortunately, these types of graphene intrinsically contain many defects that cannot be removed by reduction or thermal treatment. Still, because reduced graphene oxide (RGO) can be obtained easily as powder or solution forms, its utility in, for example, photocatalysts, fuel cells, batteries, ultracapacitors, and hydrogen storage is highly valuable. And solution forms of RGO can be easily applied for inkjetprinting, spray or spin-coating on various substrates. In the solution phase synthesis of graphene, aggregation is a serious problem. Graphene layers tend to aggregate due to high van der Waals interactions. Li et al. solved this problem by reducing GO by hydrazine (RGOH2N NH2) in a basic solution, where RGOH2N NH2 remained dissolved by electrostatic repulsion between the negatively charged carboxylic groups on the graphene sheets. The advantage of this method is that water-dispersible graphene could be made without the use of surfactants or stabilizers. However, one of the problems is that hydrazine is harmful, explosive, and expensive. Furthermore, nitrogen impurities derived from hydrazine impose limits on the conductivity, and residual hydrazine in graphene solutions make its handling and further processing dangerous. Williams et al. introduced the use of ultraviolet (UV) photocatalysis to produce a composite of TiO2 and graphene. [16]

CHAPTER 5:Photoexcitation in Pure and Modified Semiconductor Photocatalysts

Semiconductor photocatalysis has great promise for applications in solar energy conversion and environmental remediation because it is inexpensive, versatile, and environmentally benign. However, some critical factors such as limited light absorption and fast electron–hole pair recombination restrict the practical applications. Because no pure-phase semiconductors efficiently utilize solar irradiation, semiconductor materials have been modified in various ways to enhance their light-absorption properties. In this chapter, photoexcitation in modified semiconductor systems that include the impurity-doped, coupled, dye-sensitized, ligand-to-metal charge transfer (LMCT)-sensitized, and local surface plasmon resonance (LSPR)-sensitized semiconductors are discussed. In particular, the photoexcitation mechanism, charge transfer pathway, applications, and limitations of each method are described.