Shuai Shen

Photocatalytic Overall Water Splitting Promoted by an α–β phase Junction on Ga2O3

When Alpha met Beta: a tuneable α-β surface phase junction on Ga(2)O(3) can significantly improve photocatalytic overall water splitting into H(2) and O(2) over individual α-Ga(2)O(3) or β-Ga(2)O(3) surface phases. This enhanced photocatalytic performance is mainly attributed to the efficient charge separation and transfer across the α-β phase junction.

Trap states and carrier dynamics of TiO2 studied by photoluminescence spectroscopy under weak excitation condition

Anatase and rutile TiO(2) were investigated with photoluminescence techniques under the weak excitation condition, where trap states play a vital role in carrier dynamics. The visible emission of anatase and near-infrared (NIR) emission of rutile both exhibit extremely long lifetimes up to milliseconds. The decay processes can be well described by the power-law decay which corresponds to the trapping-detrapping effect. These results indicate that the luminescence processes in both anatase and rutile TiO(2) have a close relationship with trap states. The visible emission band was assigned to the donor-acceptor recombination. Oxygen vacancies and hydroxyl groups mainly serve as the donor and acceptor sites, respectively. The NIR luminescence is originated from the recombination of trapped electrons with free holes, while the trapped electrons were formed through two paths, direct trapping or trap-to-trap hopping. The trap states in anatase and rutile TiO(2) may largely influence the photocatalysis process of TiO(2) and determine the photocatalytic activity under stationary illumination.

Interface Engineering of a CoOx/Ta3N5 Photocatalyst for Unprecedented Water Oxidation Performance under Visible‐Light‐Irradiation

Cocatalysts have been extensively used to promote water oxidation efficiency in solar-to-chemical energy conversion, but the influence of interface compatibility between semiconductor and cocatalyst has been rarely addressed. Here we demonstrate a feasible strategy of interface wettability modification to enhance water oxidation efficiency of the state-of-the-art CoO(x)/Ta3N5 system. When the hydrophobic feature of a Ta3N5 semiconductor was modulated to a hydrophilic one by in situ or ex situ surface coating with a magnesia nanolayer (2-5 nm), the interfacial contact between the hydrophilic CoO(x) cocatalyst and the modified hydrophilic Ta3N5 semiconductor was greatly improved. Consequently, the visible-light-driven photocatalytic oxygen evolution rate of the resulting CoO(x)/MgO(in)-Ta3N5 photocatalyst is ca. 23 times that of the pristine Ta3N5 sample, with a new record (11.3%) of apparent quantum efficiency (AQE) under 500-600 nm illumination.

Effects of Zn2+ and Pb2+ dopants on the activity of Ga2O3-based photocatalysts for water splitting

Zn-doped and Pb-doped β-Ga2O3-based photocatalysts were prepared by an impregnation method. The photocatalyst based on the Zn-doped β-Ga2O3 shows a greatly enhanced activity in water splitting while the Pb-doped β-Ga2O3 one shows a dramatic decrease in activity. The effects of Zn(2+) and Pb(2+) dopants on the activity of Ga2O3-based photocatalysts for water splitting were investigated by HRTEM, XPS and time-resolved IR spectroscopy. A ZnGa2O4-β-Ga2O3 heterojunction is formed in the surface region of the Zn-doped β-Ga2O3 and a slower decay of photogenerated electrons is observed. The ZnGa2O4-β-Ga2O3 heterojunction exhibits type-II band alignment and facilitates charge separation, thus leading to an enhanced photocatalytic activity for water splitting. Unlike Zn(2+) ions, Pb(2+) ions are coordinated by oxygen atoms to form polyhedra as dopants, resulting in distorted surface structure and fast decay of photogenerated electrons of β-Ga2O3. These results suggest that the Pb dopants act as charge recombination centers expediting the recombination of photogenerated electrons and holes, thus decreasing the photocatalytic activity.

Composite Sr2TiO4/SrTiO3(La,Cr) heterojunction based photocatalyst for hydrogen production under visible light irradiation

A composite Sr2TiO4/SrTiO3(La,Cr) heterojunction photocatalyst has been prepared by a simple in situ polymerized complex method. Upon Pt cocatalyst loading, this catalyst shows higher photocatalytic activity towards hydrogen production than individual SrTiO3(La,Cr) and Sr2TiO4(La,Cr) in the presence of methanol sacrificial reagent. Microscopic morphology studies show that well defined heterojunctions are formed by matching the lattice fringes of SrTiO3(La,Cr) and Sr2TiO4(La,Cr), and Pt was preferentially loaded on the surface of the Sr2TiO4(La,Cr) component in the composite Sr2TiO4/SrTiO3(La,Cr) photocatalyst. XPS and EPR analyses show that the composite photocatalyst also has the lowest amount of Cr6+ electron trapping sites. Band structure analysis by combining absorption spectroscopy and Mott–Schottky plots shows that, in the composite photocatalyst, the photogenerated electrons and holes tend to migrate from SrTiO3(La,Cr) to Sr2TiO4(La,Cr) and from Sr2TiO4(La,Cr) to SrTiO3(La,Cr), respectively. This kind of band structure can facilitate charge transfer and separation driven by the minor potential difference between the two components, which is further confirmed by the observation of long lived electrons in the time resolved FT-IR spectroscopic study. It is concluded that the superior photocatalytic activity of the composite heterojunction photocatalyst is due to efficient charge transfer and separation by well defined heterojunctions formed between SrTiO3(La,Cr) and Sr2TiO4(La,Cr), preferential loading of Pt nanoparticles on the Sr2TiO4(La,Cr) component, and the lowest amount of Cr6+ in the composite photocatalyst. The tailored design and synthesis of the composite heterojunction structure is a promising approach for the improvement of the photocatalytic activity of a photocatalyst.

Enzyme Entrapped in Polymer‐Modified Nanopores: The Effects of Macromolecular Crowding and Surface Hydrophobicity

Macromolecular crowding is an ubiquitous phenomenon in living cells that significantly affects the function of enzymes in vivo. However, this effect has not been paid much attention in the research of the immobilization of enzymes onto mesoporous silica. Herein, we report the combined effects of macromolecular crowding and surface hydrophobicity on the performance of an immobilized enzyme by accommodating lipase molecules into a series of mesoporous silicas with different amounts of inert poly(methacrylate) (PMA) covalently anchored inside the nanopores. The incorporation of the PMA polymer into the nanopores of mesoporous silica enables the fabrication of a crowded and hydrophobic microenvironment for the immobilized enzyme and the variation in polymer content facilitates an adjustment of the degree of crowding and surface properties of this environment. Based on this system, the catalytic features of immobilized lipase were investigated as a function of polymer content in nanopores and the results indicated that the catalytic efficiency, thermostability, and reusability of immobilized lipase could all be improved by taking advantage of the macromolecular crowding effect and surface hydrophobicity. These findings provide insight into the possible functions of the macromolecular crowding effect, which should be considered and integrated into the fabrication of suitable mesoporous silicas to improve enzyme immobilization.

Time-resolved photoluminescence of anatase/rutile TiO2 phase junction revealing charge separation dynamics

Junctions are an important structure that allows charge separation in solar cells and photocatalysts. Here, we studied the charge transfer at an anatase/rutile TiO 2 phase junction using time-resolved photoluminescence spectroscopy. Visible (~500 nm) and near-infrared (NIR, ~830 nm) emissions were monitored to give insight into the photoinduced charges of anatase and rutile in the junction, respectively. New fast photoluminescence decay components appeared in the visible emission of rutile-phase dominated TiO 2 and in the NIR emission of many mixed phase TiO 2 samples. The fast decays confirmed that the charge separation occurred at the phase junction. The visible emission intensity from the mixed phase TiO 2 increased, revealing that charge transfer from rutile to anatase was the main pathway. The charge separation slowed the microsecond time scale photoluminescence decay rate for charge carriers in both anatase and rutile. However, the millisecond decay of the charge carriers in anatase TiO 2 was accelerated, while there was almost no change in the charge carrier dynamics of rutile TiO 2 . Thus, charge separation at the anatase/rutile phase junction caused an increase in the charge carrier concentration on a microsecond time scale, because of slower electron-hole recombination. The enhanced photocatalytic activity previously observed at anatase/rutile phase junctions is likely caused by the improved charge carrier dynamics we report here. These findings may contribute to the development of improved photocatalytic materials.

Effect of Pt cocatalyst in Pt/TiO2 studied by in situ FTIR of CO adsorption

In situ transmission infrared spectroscopy was used to study the role of pt cocatalyst in pt/tio2 using co as a probe molecule. an 11 cm(-1) redshift of co adsorbed on pt/tio2 was observed under irradiation in the absence of changes in the co coverage or sample temperature. in contrast, no co shift was detected on pt/al2o3. this indicates that the redshift of the co adsorption peak is due to the photogenerated electron transfer from tio2 to pt, and this accounts for the increased photocatalytic activity by the loaded pt cocatalyst. (c) 2014, dalian institute of chemical physics, chinese academy of sciences. published by elsevier b.v. all rights reserved.

Time-resolved infrared spectroscopic investigation of roles of valence states of Cr in (La,Cr)-doped SrTiO3 photocatalysts

Abstract The kinetics of photogenerated electrons in SrTiO3(La,Cr) pretreated with either H2 or O2 were studied using time-resolved infrared spectroscopy. The X-ray photoelectron and Raman spectra showed that the Cr cations in the sample reduced with H2 were all in Cr3+, whereas those oxidized with O2 were in mixed of Cr3+ and Cr6+. Electrons excited with 355 and 532 nm light pulses showed the absorption of mid-IR light, and this was traced as a function of the time delay in a microsecond domain. The time-resolved results revealed that the decay rate of the photoinduced electrons with Cr3+ was slower than that with Cr6+, implying that trivalent Cr contributed more to retarding recombination of photoinduced electrons and holes, and enhanced photocatalytic H2 production activity.