Oxygen-Vacancy-Mediated Exciton Dissociation in BiOBr for Boosting Charge-Carrier-Involved Molecular Oxygen Activation.

Excitonic effects mediated by Coulomb interactions between photogenerated electrons and holes play crucial roles in photoinduced processes of semiconductors. In terms of photocatalysis, however, efforts have seldom been devoted to the relevant aspects. For the catalysts with giant excitonic effects, the coexisting, competitive exciton generation serves as a key obstacle to the yield of free charge carriers, and hence, transformation of excitons into free carriers would be beneficial for optimizing the charge-carrier-involved photocatalytic processes. Herein, by taking bismuth oxybromide (BiOBr) as a prototypical model system, we demonstrate that excitons can be effectively dissociated into charge carriers with the incorporation of oxygen vacancy, leading to excellent performances in charge-carrier-involved photocatalytic reactions such as superoxide generation and selective organic syntheses under visible-light illumination. This work not only establishes an in-depth understanding of defective structures in photocatalysts but also paves the way for excitonic regulation via defect engineering.

Giant Electron-Hole Interactions in Confined Layered Structures for Molecular Oxygen Activation.

Numerous efforts have been devoted to understanding the excitation processes of photocatalysts, whereas the potential Coulomb interactions between photogenerated electrons and holes have been long ignored. Once these interactions are considered, excitonic effects will arise that undoubtedly influence the sunlight-driven catalytic processes. Herein, by taking bismuth oxyhalide as examples, we proposed that giant electron-hole interactions would be expected in confined layered structures, and excitons would be the dominating photoexcited species. Photocatalytic molecular oxygen activation tests were performed as a proof of concept, where singlet oxygen generation via energy transfer process was brightened. Further experiments verify that structural confinement is curial to the giant excitonic effects, where the involved catalytic process could be readily regulated via facet-engineering, thus enabling diverse reactive oxygen species generation. This study not only provides an excitonic prospective on photocatalytic processes, but also paves a new approach for pursuing systems with giant electron-hole interactions.

Bionanofiber Assisted Decoration of Few-Layered MoSe2 Nanosheets on 3D Conductive Networks for Efficient Hydrogen Evolution.

Molybdenum diselenide (MoSe2 ) has emerged as a promising electrocatalyst for hydrogen evolution reaction (HER). However, its properties are still confined due to the limited active sites and poor conductivity. Thus, it remains a great challenge to synergistically achieve structural and electronic modulations for MoSe2 -based HER catalysts because of the contradictory relationship between these two characteristics. Herein, bacterial cellulose-derived carbon nanofibers are used to assist the uniform growth of few-layered MoSe2 nanosheets, which effectively increase the active sites of MoSe2 for hydrogen atom adsorption. Meanwhile, carbonized bacterial cellulose (CBC) nanofibers provide a 3D network for electrolyte penetration into the inner space and accelerate electron transfer as well, thus leading to the dramatically increased HER activity. In acidic media, the CBC/MoSe2 hybrid catalyst exhibits fast hydrogen evolution kinetics with onset overpotential of 91 mV and Tafel slope of 55 mV dec-1 , which is much more outstanding than both bulk MoSe2 aggregates and CBC nanofibers. Furthermore, the fast HER kinetics are well supported by theoretical calculations of density-functional-theory analysis with a low activation barrier of 0.08 eV for H2 generation. Hence, this work highlights an efficient solution to develop high-performance HER catalysts by incorporating biotemplate materials, to simultaneously achieve increased active sites and conductivity.

Distribution and self-assisted diffusion of Be and Mg impurities in ZnO.

The band gap width of the Be-doped ZnO correlates strongly with the distribution of the dopants. By performing first-principles calculations, it is found that an interstitial Be (Bei) atom preferably migrates in a basal plane. During the migration, such a Bei atom favorably bonds to a substituted Be (BeZn) atom, forming a new defect complex (2Be)Zn, showing a trend of aggregation of Be atoms in ZnO. Furthermore, the stability of the defect complex (2Be)Zn can be weakened by a substituted Mg (MgZn). So, the Mg impurities in Be-doped ZnO might suppress the aggregation of Be, so as to significantly improve the effect of the doped Be on modulating the band gap of ZnO.

Enhanced Exciton Binding Energy of ZnO by Long-Distance Perturbation of Doped Be Atoms.

The excitonic effect in semiconductors is sensitive to dopants. Origins of dopant-induced large variation in the exciton binding energy (E(b)) is not well understood and has never been systematically studied. We choose ZnO as a typical high-E(b) material, which is very promising in low-threshold lasing. To the best of our knowledge, its shortest wavelength electroluminescence lasing was realized by ZnO/BeZnO multiple quantum wells (MQWs). However, this exciting result is shadowed by a controversial E(b) enhancement claimed. In this Letter, we reveal that the claimed E(b) is sensible if we take Be-induced E(b) variation into account. Detailed first-principle investigation of the interaction between dopant atoms and the lattice shows that the enhancement mainly comes from the long-distance perturbation of doped Be atoms rather than the local effect of doping atoms. This is a joint work of experiment and calculation, which from the angle of methology paves the way for understanding and predicting the E(b) variation induced by doping.

Understanding the origin of phase segregation of nano-crystalline in a Be(x)Zn(1-x)O random alloy: a novel phase of Be(1/3)Zn(2/3)O.

The usage of a BexZn1-xO alloy in ultraviolet (UV)-region optoelectronic devices is largely hindered by its intricate phase segregation of crystallites of different sizes. To understand the physical origin of this phase segregation phenomenon on the atomistic scale, we have undertaken an extensive study of the structural evolution of the segregation phases in the BexZn1-xO alloy at finite temperatures by using first-principles calculations combined with the cluster expansion approach. We find that a random alloy of BexZn1-xO tends to segregate into a mix-ordered phase below a critical temperature, by the growth of prototype and nano-sized structures. The segregated phases in BexZn1-xO entail not only ZnO or BeO crystallites, but also two as yet unreported phases with beryllium concentration of 1/3 and 2/3. Both new phases of BexZn1-xO are direct wide-gap semiconductors with band gap values of 4.88 eV and 6.78 eV respectively. We envisioned that the novel Be1/3Zn2/3O crystal is highly promising for solar-blind device applications.

Wide range bandgap modulation based on ZnO-based alloys and fabrication of solar blind UV detectors with high rejection ratio.

Theoretical calculations on formation energies of MgZnO, BeZnO and BeMgZnO alloys are presented. The ternary alloy MgZnO (BeZnO) is found to be unstable with high Mg (Be) contents. However, the quaternary system BeMgZnO is predicted to be stable with small Be/Mg atom ratio. Subsequently, a wurtzite Be0.17Mg0.54Zn0.29O alloy with a bandgap of 5.15 eV has been acquired experimentally. Its bandgap is in the middle of solar blind region and thus it is an ideal material for realizing a high rejection ratio solar blind ultraviolet (UV) detector, which has long been a problem. A metal-semiconductor-metal (MSM) structured solar blind UV detector based on this material is then fabricated, realizing a much higher rejection ratio than reported MgZnO-based detectors. One more interesting thing is, as a complicated quaternary system, BeMgZnO can maintain its crystal quality in a wide compositional range, which is not happening in MgZnO and BeZnO. To get some microscopic insight into the Be-Mg mutual stabilizing mechanism, more calculations on the lattice constants of BeZnO and MgZnO alloys, and the coordination preference of Be ions in alloy were conducted. The a-axis lattice compensation and 4-fold coordination preference of Be atom are confirmed the major origins for Be-Mg mutual stabilizing in ZnO lattice.