Wenxin Dai

Photocatalytic Degradation of RhB over TiO2 Bilayer Films: Effect of Defects and Their Location

TiO(2) bilayer films with a normal surface (Ns-TiO(2)), surface defects (Sd-TiO(2)), and interface defects (Id-TiO(2)) were successfully prepared by a combination of cold plasma treatment (CPT) and sol-gel dip-coating technology. The photodegradation of rhodamine B (RhB) over these as-prepared TiO(2) films was investigated via UV-vis irradiation. Results indicate that the three kinds of films exhibit very different photodegradation processes for RhB. A mainly N-deethylation reaction over the Ns-TiO(2) films, whereas an efficient degradation (cycloreversion) of RhB occurs over the Sd-TiO(2) films. In the RhB/Id-TiO(2) system, however, efficient N-deethylation concomitant with the highly efficient cycloreversion of RhB is observed. The efficiency of the complete mineralization of RhB dye follows the order of Id-TiO(2) > Sd-TiO(2) > Ns-TiO(2). It is proposed that the defect sites at the surface or the interface of TiO(2) films promote the separation of photogenerated electron-holes, leading to a higher photoactivity of defective TiO(2) films. Moreover, the higher stability over Id-TiO(2) as compared to Sd-TiO(2) indicates that the interface defect sites in TiO(2) could be applied in environmental photocatalysis.

Organic semiconductor for artificial photosynthesis: water splitting into hydrogen by a bioinspired C3N3S3polymer under visible light irradiation

A novel organic semiconductor photocatalyst mimicking natural light-harvesting antenna complexes in photosynthetic organisms, a disulfide (–S–S–) bridged C3N3S3polymer, was designed and developed to generate hydrogen from water under visible light irradiation. The artificial conjugated polymer shows high H2-producing activity from the half-reaction of water splitting without the aid of a sacrificial electron donor. The H2-producing efficiency and photo-stability of the catalyst could be improved greatly using Ru and single-wall carbon nanotubes as cocatalysts or by adding a sacrificial donor. The results represent a potential and prospective application of the C3N3S3polymer in solar energy conversion and offer significant guidance to develop more stable and efficient photocatalytic systems based on organic semiconductors.

Catalytic Role of Cu Sites of Cu/MCM-41 in Phenol Hydroxylation

Four types of copper-containing MCM-41 mesoporous silicas were synthesized by the surface organometallic chemistry (SOMC) procedure (Cu/MCM-41-S), mechanical mixing (Cu/MCM-41-M), impregnation (Cu/MCM-41-I), and the hydrothermal technique (Cu/MCM-41-H). The resultant samples were characterized in detail by X-ray diffraction (XRD), N(2) physical adsorption, transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (UV-vis DRS), temperature-programmed reduction (TPR), and infrared spectroscopy (IR) of NO adsorption. Catalytic behaviors of these samples for hydroxylation of phenol with H(2)O(2) were evaluated. The results revealed that depending on the preparation methods the samples contain different copper-oxo species and thus show different catalytic behaviors. Among these samples, the one prepared by SOMC contains a predominant amount of isolated Cu(2+) and exhibits the most excellent catalytic activity and selectivity. The amount of isolated copper species decreases in the order of Cu/MCM-41-S > Cu/MCM-41-H > Cu/MCM-41-I > Cu/MCM-41-M, while the amount of copper oxide clusters increases in a reversal order. The difference in the catalytic activity and product selectivity of these four samples could be rationally explained by the distinction of chemical states of copper species. The highly dispersed isolated Cu(2+) species are identified as the active sites in the phenol hydroxylation, while the nonisolated Cu(2+) clusters or oxide are responsible for the deep oxidation of primary product HQ and the decrease of product selectivity. The mechanism of the copper-catalyzed phenol hydroxylation was proposed.

Controlled preparation of In2O3, InOOH and In(OH)3via a one-pot aqueous solvothermal route

Pure cubic In2O3, orthorhombic InOOH and cubic In(OH)3 nanocrystals were separately synthesized via a one-pot aqueous solvothermal route at low temperature by simply regulating the amount of water in the ternary system H2O–DMF–In(NO3)3·4.5H2O.

A heterostructured TiO2–C3N4 support for gold catalysts: a superior preferential oxidation of CO in the presence of H2 under visible light irradiation and without visible light irradiation

In the present study, a simple method, involving precipitation and a subsequent hydrothermal synthesis, was used to prepare a supported gold catalyst on TiO2–C3N4, which was subsequently evaluated for its performance for CO preferential oxidation in the presence of H2. It was found that the supported gold catalyst on TiO2–C3N4 nano-hetero-architecture had a higher catalytic activity than that on the counterpart TiO2 or C3N4 alone under visible light irradiation and without visible light irradiation. A better contact between the gold atom arrangement and heterostructured support was clearly observed by transmission electron microscopy (TEM), which suggested as the physical basis for highly efficient electron transfer. Based on the results of ex situ X-ray photoelectron spectroscopy (XPS), redox couple mode (TCNE/TCNE−) testing, transient photocurrent and Fourier transform-infrared spectra (FT-IR) of CO adsorption, it was proposed that the nano-heterostructure of TiO2–C3N4 and the localized surface plasmon resonance (LSPR) of Au NPs promoted electron transfer among the interfaces of TiO2, C3N4 and Au NPs, resulting in the higher electron density of Au NPs, followed by the activation of CO adsorbed at the Au sites and the formation of O2− radical active species. Moreover, the re-combination of Au–H(ad) with the discharge of molecular hydrogen induced by the higher electron densities of Au NPs and the hydrogen spillover of Au–H(ad) to the support in the formation of surface hydroxyl groups could drive the higher selectivity of oxidizing CO. In view of its electronic effect, the nano-heterostructured TiO2/C3N4 hybrid, which was well attached to Au NPs, could lead to new properties and could consequently promote CO preferential oxidation in the presence of H2.

Controlled synthesis of pure and highly dispersive Cu(II), Cu(I), and Cu(0)/MCM-41 with Cu[OCHMeCH2NMe2]2/MCM-41 as precursor

Cu(II)/MCM-41, Cu(I)/MCM-41 and Cu(0)/MCM-41 materials were prepared with Cu[OCHMeCH2NMe2]2/MCM-41 as precursor under controlled conditions. The states of copper on the resulting samples were characterized by temperature programmed reduction (TPR), UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS), X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy (PL), infrared spectroscopy (FTIR) of NO and CO adsorption, X-ray diffraction (XRD), and N2 adsorption. It was shown that pure Cu(II)/MCM-41 can be acquired by calcining Cu[OCHMeCH2NMe2]2/MCM-41 in pure oxygen at 573 K. Pure Cu(I)/MCM-41 can be obtained by reducing the as-prepared Cu(II)/MCM-41 in a CO/He atmosphere at 473 K, while pure Cu(0)/MCM-41 can be obtained viareduction of the Cu(II)/MCM-41 in a CO/He atmosphere at 673 K. The Cu(II)–O and Cu(I)–O species were showed to be highly dispersed on the surface of MCM-41 as isolated sites. The study provides an alternative method for the preparation of pure copper metal or copper oxide supported materials.

Photocatalytic Oxidation of CO on TiO2: Chemisorption of O2, CO, and H2

On the surface: Adsorption of O(2) at the surface oxygen vacancy (SOV) sites of TiO(2) reconstructs the lattice oxygen (healing SOVs), resulting in a decrease of the photocatalytic activity of oxidizing CO over vacuum-pretreated TiO(2) with increasing temperature (see scheme). Adsorption of H(2) produces new SOVs at the TiO(2) surface and stabilizes the photocatalytic activity. Photocatalytic oxidation of CO over vacuum-pretreated TiO(2) is performed in a series of systems with the introduction of O(2), CO, and H(2) in different orders. The photocatalytic oxidation of CO is dependent on the order of introduction of O(2), CO, or H(2), and introducing O(2) prior to CO promotes the oxidation of CO. Moreover, an increase of reaction temperature suppresses the oxidation of CO, but the preintroduction of H(2) reduces this suppression effect. The results of the chemisorption of O(2), CO, and H(2) at the TiO(2) surface reveal that the adsorbed O(2) heals the surface oxygen vacancy (SOV) sites of TiO(2), while the adsorbed CO and H(2) promote the formation of new SOVs. It is proposed that changes in the amounts of adsorbed O(2) and SOVs are mainly responsible for the differences of CO conversion in different systems.

Probing the Electronic Structure and Photoactivation Process of Nitrogen‐Doped TiO2 Using DRS, PL, and EPR

The electronic structure and photoactivation process in N-doped TiO(2) is investigated. Diffuse reflectance spectroscopy (DRS), photoluminescence (PL), and electron paramagnetic resonance (EPR) are employed to monitor the change of optical absorption ability and the formation of N species and defects in the heat- and photoinduced N-doped TiO(2) catalyst. Under thermal treatment below 573 K in vacuum, no nitrogen dopant is removed from the doped samples but oxygen vacancies and Ti(3+) states are formed to enhance the optical absorption in the visible-light region, especially at wavelengths above 500 nm with increasing temperature. In the photoactivation processes of N-doped TiO(2), the DRS absorption and PL emission in the visible spectral region of 450-700 nm increase with prolonged irradiation time. The EPR results reveal that paramagnetic nitrogen species (N(s)·, oxygen vacancies with one electron (V(o)·), and Ti(3+) ions are produced with light irradiation and the intensity of N(s)· species is dependent on the excitation light wavelength and power. The combined characterization results confirm that the energy level of doped N species is localized above the valence band of TiO(2) corresponding to the main absorption band at 410 nm of N-doped TiO(2), but oxygen vacancies and Ti(3+) states as defects contribute to the visible-light absorption above 500 nm in the overall absorption of the doped samples. Thus, a detailed picture of the electronic structure of N-doped TiO(2) is proposed and discussed. On the other hand, the transfer of charge carriers between nitrogen species and defects is reversible on the catalyst surface. The presence of oxygen-vacancy-related defects leads to quenching of paramagnetic N(s)· species but they stabilize the active nitrogen species N(s)(-).

Comparative Study of Two Different TiO2 Film Sensors on Response to H2 under UV Light and Room Temperature

An anatase TiO2 film sensor was prepared by a facile in-situ method on the interdigitated Au electrode deposited on the alumina substrate. The structure, morphology and the optical properties of the in-situ TiO2 film sensor were characterized by X-ray diffraction, Scanning Electron Microscopy, and UV-vis diffuse reflectance spectra. The photo-assisted gas sensitivities of the prepared film towards H2 gas were evaluated at room temperature in N2 and synthetic air atmospheres. As compared to TiO2 film sensor prepared by drop-coating method, this in-situ TiO2 film sensor exhibited a more compact structure composed of uniform TiO2 microspheres as well as a better gas sensitivity towards H2 under UV irradiation, especially in synthetic air. The photo-electrochemical measurements suggest that these improvements may be associated with the efficient charge transfer in the TiO2 interface induced by the TiO2 microsphere structure. This study might offer a feasible approach to develop photo-assisted gas sensors at ambient temperature.

The correlation between surface defects and the behavior of hydrogen adsorption over ZnO under UV light irradiation

The H2 adsorption behaviors of a series of ZnO samples with different surface defects were characterized using a novel method of testing photo-assisted gas-sensitive response to H2 at room temperature under ultraviolet light irradiation. It was found that two types of H2 adsorption occur according to the electron transfer direction at the interface of the adsorbed H2 and the ZnO sample (type I: electrons transfer from ZnO to H2; type II: electrons transfer from H2 to ZnO). The adsorption type of oxygen vacancies (VOs) and zinc vacancies (VZns) can be changed by adjusting the Fermi level (EF) of ZnO. VOs were beneficial to type I H2 adsorption due to their higher EF, while VZns were beneficial to type II H2 adsorption due to their lower EF. The XRD, XPS and EPR results of the samples showed that VOs and VZns are preferentially formed at the {100} and {002} crystalline planes, respectively. Moreover, the in situ DRIFT results of H2 adsorption and the EPR results of hydroxyl radicals showed that surface hydroxyls and hydroxyl radicals play important roles in H2 adsorption, and two possible mechanisms of H2 adsorption were proposed on VOs and VZns over ZnO, respectively. Furthermore, the adsorbed H2 with different adsorption types induced by VOs and VZns exhibit different oxidation behaviors over ZnO under UV irradiation.