Chengwu Yang

UV‐Triggered Dopamine Polymerization: Control of Polymerization, Surface Coating, and Photopatterning

UV irradiation is demonstrated to initiate dopamine polymerization and deposition on different surfaces under both acidic and basic pH. The observed acceleration of the dopamine polymerization is explained by the UV-induced formation of reactive oxygen species that trigger dopamine polymerization. The UV-induced dopamine polymerization leads to a better control over polydopamine deposition and formation of functional polydopamine micropatterns.

Surface Faceting and Reconstruction of Ceria Nanoparticles

The surface atomic arrangement of metal oxides determines their physical and chemical properties, and the ability to control and optimize structural parameters is of crucial importance for many applications, in particular in heterogeneous catalysis and photocatalysis. Whereas the structures of macroscopic single crystals can be determined with established methods, for nanoparticles (NPs), this is a challenging task. Herein, we describe the use of CO as a probe molecule to determine the structure of the surfaces exposed by rod-shaped ceria NPs. After calibrating the CO stretching frequencies using results obtained for different ceria single-crystal surfaces, we found that the rod-shaped NPs actually restructure and expose {111} nanofacets. This finding has important consequences for understanding the controversial surface chemistry of these catalytically highly active ceria NPs and paves the way for the predictive, rational design of catalytic materials at the nanoscale.

UV‐Induced Tetrazole‐Thiol Reaction for Polymer Conjugation and Surface Functionalization

A UV-induced 1,3-dipolar nucleophilic addition of tetrazoles to thiols is described. Under UV irradiation the reaction proceeds rapidly at room temperature, with high yields, without a catalyst, and in both polar protic and aprotic solvents, including water. This UV-induced tetrazole-thiol reaction was successfully applied for the synthesis of small molecules, protein modification, and rapid and facile polymer-polymer conjugation. The reaction has also been demonstrated for the formation of micropatterns by site-selective surface functionalization. Superhydrophobic-hydrophilic micropatterns were successfully created by sequential modifications of a tetrazole-modified porous polymer surface with hydrophobic and hydrophilic thiols. A biotin-functionalized surface could be fabricated in aqueous solutions under long-wavelength UV irradiation.

Evidence for photogenerated intermediate hole polarons in ZnO

Despite their pronounced importance for oxide-based photochemistry, optoelectronics and photovoltaics, only fairly little is known about the polaron lifetimes and binding energies. Polarons represent a crucial intermediate step populated immediately after dissociation of the excitons formed in the primary photoabsorption process. Here we present a novel approach to studying photoexcited polarons in an important photoactive oxide, ZnO, using infrared (IR) reflection-absorption spectroscopy (IRRAS) with a time resolution of 100 ms. For well-defined (10-10) oriented ZnO single-crystal substrates, we observe intense IR absorption bands at around 200 meV exhibiting a pronounced temperature dependence. On the basis of first-principles-based electronic structure calculations, we assign these features to hole polarons of intermediate coupling strength.

O2 Activation on Ceria Catalysts - The Importance of Substrate Crystallographic Orientation

An atomic-level understanding of dioxygen activation on metal oxides remains one of the major challenges in heterogeneous catalysis. By performing a thorough surface-science study of all three low-index single-crystal surfaces of ceria, probably the most important redox catalysts, we provide a direct spectroscopic characterization of reactive dioxygen species at defect sites on the reduced ceria (110) and (100) surfaces. Surprisingly, neither of these superoxo and peroxo species was found on ceria (111), the thermodynamically most stable surface of this oxide. Applying density functional theory, we could relate these apparently inconsistent findings to a sub-surface diffusion of O vacancies on (111) substrates, but not on the less-closely packed surfaces. These observations resolve a long standing debate concerning the location of O vacancies on ceria surfaces and the activation of O2 on ceria powders.

Corrigendum: Evidence for photogenerated intermediate hole polarons in ZnO

Nature Communications 6: Article number: 6901 (2015); Published 22 April 2015; Updated 30 June 2015 In the Methods section of this Article, there are three references that do not refer to the correct articles. The sentence ‘The sample surface was cleaned using standard procedures’ incorrectly directs to reference 2, whereas the correct reference is ref.

Carbon Dioxide Adsorption on CeO2(110): An XPS and NEXAFS Study

The adsorption of CO2 on the surface of a CeO2 (110) bulk single crystal was studied by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The high-quality XPS and C K-edge NEXAFS data show that CO2 adsorbs as a carbonate species on both fully oxidized CeO2 (110) and partially reduced CeO2-x (110). No evidence for the formation of a carboxylate (CO2δ- ) intermediate could be found. On the fully oxidized CeO2 (110) substrate, the carbonate decomposes upon heating to above 400 K, leading to the desorption of CO2 . The NEXAFS data reveal the presence of a minor amount of formate (or carboxylate) and bicarbonate species, which are related to reactions of CO2 with surface hydroxyl groups. In the case of reduced CeO2-x (110), the carbonate species completely disappear upon heating to temperatures above 500 K. In contrast to conclusions presented in earlier works, the oxidation state of the surface is unchanged, that is, CO2 does not re-oxidize the reduced CeO2-x (110) surface.

Rendering Photoreactivity to Ceria: The Role of Defects

The photoreactivity of ceria, a photochemically inert oxide with a large band gap, can be increased to competitive values by introducing defects. This previously unexplained phenomenon has been investigated by monitoring the UV-induced decomposition of N2 O on well-defined single crystals of ceria by using infrared reflection-absorption spectroscopy (IRRAS). The IRRAS data, in conjunction with theory, provide direct evidence that reducing the ceria(110) surface yields high photoreactivity. No such effects are seen on the (111) surface. The low-temperature photodecomposition of N2 O occurs at surface O vacancies on the (110) surface, where the electron-rich cerium cations with a significantly lowered coordination number cause a local lowering of the huge band gap (ca. 6 eV). The quantum efficiency of strongly reduced ceria(110) surfaces in the photodecomposition of N2 O amounts to 0.03 %, and is thus comparable to that reported for the photooxidation of CO on rutile TiO2 (110).

IR spectroscopy applied to metal oxide surfaces: adsorbate vibrations and beyond

Abstract Metal oxides are among the technologically most important materials. Almost all metals are covered by one oxide layer under ambient conditions. The characterization of oxide surface properties, therefore, is still attracting an increasing amount of attention in surface science. A challenge is provided by the fact that these materials cannot be studied with standard techniques in a straightforward fashion. In this review, we summarize recent results obtained by IR-spectroscopy applied in a reflection-geometry to macroscopic oxide monocrystals. These results provide new insights in the adsorption and subsequent reactions and photoreactions of molecules on these highly interesting, very complex class of materials. In addition, the IR-spectroscopy can also be used to probe photophysical properties, e.g. the generation and decay of electron or hole polarons.