Probing Perovskite Interfaces and Superlattices with X-ray Photoemission Spectroscopy. In: Woicik J, editor. Hard X-ray Photoelectron Spectroscopy (HAXPES). Cham: Springer International Publishing; 2016. pp. 341-80. [DOI: 10.1007/978-3-319-24043-5_14]

Improved photocatalytic decolorization of reactive black 5 dye through synthesis of graphene quantum dots-nitrogen-doped TiO2

Graphene quantum dots (GQDs), a new solid-state electron transfer material was anchored to nitrogen-doped TiO2 via sol gel method. The introduction of GQDs effectively extended light absorption of TiO2 from UV to visible region. GQD-N-TiO2 demonstrated lower PL intensity at excitation wavelengths of 320 to 450 nm confirming enhanced exciton lifespan. GQD-N-TiO2-300 revealed higher surface area (191.91m2 g-1), pore diameter (1.94 nm), TEM particle size distribution (4.88 ± 1.26 nm) with lattice spacing of 0.45 nm and bandgap (2.91 eV). In addition, GQDs incorporation shifted XPS spectrum of Ti 2p to lower binding energy level (458.36 eV), while substitution of oxygen sites in TiO2 lattice by carbon were confirmed through deconvolution of C 1 s spectrum. Photocatalytic reaction followed the pseudo first order reaction and continuous reductions in apparent rate constant (Kapp) with incremental increase in RB5 concentration. Langmuir-Hinshelwood model showed surface reaction rate constants KC = 1.95 mg L-1 min-1 and KLH = 0.76 L mg-1. The active species trapping, and mechanism studies indicated the photocatalytic decolorization of RB5 through GQD-N-TiO2 was governed by type II heterojunction. Overall, the photodecolorization reactions were triggered by the formation of holes and reactive oxygen species. The presence of •OH, 1O2, and O2• during the photocatalytic process were confirmed through EPR analysis. The excellent photocatalytic decolorization of the synthesized nanocomposite against RB5 can be ascribed to the presence of GQDs in the TiO2 lattice that acted as excellent electron transporter and photosensitizer. This study provides a basis for using nonmetal, abundant, and benign materials like graphene quantum dots to enhance the TiO2 photocatalytic efficiency, opening new possibilities for environmental applications.

Unravelling oxygen-vacancy-induced electron transfer at SrTiO3-based heterointerfaces by transport measurement during growth

Numerous studies have shown that oxygen vacancies play an important role on the formation of two-dimensional electron gas (2DEG) at SrTiO₃-based heterointerfaces. Previously, it is widely believed that the main mechanism is that the oxygen vacancies in SrTiO₃ directly contribute electrons to the 2DEG. Here, we performed transport measurements during the creation of 2DEG for depositing amorphous LaAlO₃ on SrTiO₃ substrates and related heterostructures. Our result suggests that, unlike the previous viewpoint, in this kind of 2DEG the determinant mechanism is the electron transfer from the oxygen vacancies in the film grown on SrTiO₃, rather than the oxygen vacancies in SrTiO₃ themselves. This effect is so striking that an amorphous film of less than 10% monolayer coverage on SrTiO₃, or equivalently 0.04 nm, can already generate a highly conducting 2DEG. The present result may have a general implication and provide a possible way to understand the long-standing debate on the origin of 2DEG at SrTiO₃-based heterointerfaces.

Separating Electrons and Donors in BaSnO3 via Band Engineering

Separating electrons from their source atoms in La-doped BaSnO₃, the first perovskite oxide semiconductor to be discovered with high room-temperature electron mobility, remains a subject of great interest for achieving high-mobility electron gas in two dimensions. So far, the vast majority of work in perovskite oxides has focused on heterostructures involving SrTiO₃ as an active layer. Here we report the demonstration of modulation doping in BaSnO₃ as the high room-temperature mobility host without the use of SrTiO₃. Significantly, we show the use of angle-resolved hard X-ray photoelectron spectroscopy (HAXPES) as a nondestructive approach to not only determine the location of electrons at the buried interface but also to quantify the width of electron distribution in BaSnO₃. The transport results are in good agreement with the results of self-consistent solution to one-dimensional Poisson and Schrödinger equations. Finally, we discuss viable routes to engineer two-dimensional electron gas density through band-offset engineering.

Quantifying small changes in uranium oxidation states using XPS of a shallow core level

Quantification of U(iv), U(v), and U(vi) in UO_2+x using the 5d XPS.