Shengwei Liu

Tunable Photocatalytic Selectivity of Hollow TiO2 Microspheres Composed of Anatase Polyhedra with Exposed {001} Facets

A fluoride mediated self-transformation method is proposed for the synthesis of hollow TiO(2) microspheres (HTS) composed of anatase polyhedra with exposed ca. 20% {001} facets. Importantly, HTS exhibit tunable photocatalytic selectivity in decomposing azo dyes in water. The fluorinated HTS show preferential decomposition of methyl orange (MO) in comparison to methylene blue (MB). In contrast, the surface-modified HTS by either NaOH washing or calcinations at 600 degrees C favor decomposition of MB over MO. The surface chemistry and the surface structure at the atomic level are key factors in tuning the adsorption selectivity and, consequently, photocatalytic selectivity of HTS toward azo dyes.

Improved visible-light photocatalytic activity of porous carbon self-doped ZnO nanosheet-assembled flowers

Hierarchical flower-like C-doped ZnO superstructures (ZnO flowers) assembled from porous nanosheets are obtained by pyrolysis of morphology-analogous Zn5(CO3)2(OH)6 precursors. The prepared ZnO flowers are characterized by X-ray diffraction, thermogravimetic and differential scanning calorimeter analysis, scanning electron microscopy, transmission electron microscopy, N2 sorption measurements, UV-vis diffuse reflectance spectra and X-ray photoelectron spectroscopy. The production of OH radicals on the ZnO surface under visible-light irradiation is detected by a photoluminescence technique using terephthalic acid as a probe molecule. The visible-light photocatalytic activity is evaluated by photocatalytic decomposition of the dye RhB in aqueous solution. The hierarchical organization of nanosheets, the multimode voids between and within porous nanosheets, together with annealing-induced in situ carbon self-doping within the ZnO lattice, account for the enhanced light-absorption capacity, extended light-response range and thus better photocatalytic activity of the ZnO flowers. Furthermore, first-principle density functional theory (DFT) calculation further confirms the C-doping induced red shift in the absorption edges of C-doped ZnO flowers.

Hydrothermal preparation and photocatalytic activity of mesoporous Au―TiO2 nanocomposite microspheres

Au-TiO2 nanocomposite microspheres were prepared by hydrothermal treatment of precipitates of tetrabutyl titanate (Ti(OC4H9)4) in a mixed solution of water, ethanol and Au colloid particles at 180 degrees C for 7 h. The as-prepared products were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, nitrogen sorption, UV-visible diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy and photoluminescence spectroscopy. The photocatalytic activity was evaluated by photocatalytic oxidation decomposition of formaldehyde in air. The results showed that the presence of nanosized Au particles slightly depressed the grain growth of anatase, resulting in smaller crystallite size and greater specific surface areas. Moreover, the absorbance and photoluminescence of anatase TiO2 was modified by those added Au nanoparticles and an appropriate Au amount in Au-TiO2 nanocomposite microspheres led to increase of band gap, decrease of photoluminescence intensity and prolongation of life of photo-generated electrons and holes. The photocatalytic activity of Au-TiO2 nanocomposite microspheres was obviously higher than that of pure TiO2 microspheres and Degussa P25. When the atomic percentage ratio of Au to Ti was below 0.00425, the apparent reaction rate constants increased. When the atomic percentage ratio of Au to Ti reached 0.00425, the sample displayed the highest photocatalytic activity.

Enhanced photovoltaic performance of dye-sensitized solar cells based on TiO2 nanosheets/graphene composite films

Dye-sensitized solar cells (DSSCs) based on TiO2 nanosheets (TiO2-NSs)/graphene nanocomposite films were fabricated, and the effects of graphene on the microstructures and photoelectric conversion performance of the as-fabricated DSSC were investigated. The graphene loading clearly influences the textural properties (including specific surface areas, porosity and pore volume) and the optical absorption properties. Moreover, the charge transfer and transport versus the charge trapping and recombination is also affected by the graphene loading. As a consequence, the photoelectric conversion efficiency of the TiO2-NSs/graphene nanocomposite film electrodes can be improved to a great extent upon graphene loading, dependent on the loading amount of graphene. A moderate amount of graphene ( 0.75 wt%) largely lowered the DSSC performance. Higher graphene loading not only impaired the crystallinity of the TiO2-NSs, but also shielded the light adsorption of the dyes and reduced the number of photogenerated electrons.

A simple cation exchange approach to Bi-doped ZnS hollow spheres with enhanced UV and visible-light photocatalytic H2-production activity

Bi-doped ZnS hollow spheres were successfully synthesized by a facile cation exchange reaction between ZnS hollow spheres and Bi(NO3)3. The samples were characterized by X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, N2 adsorption–desorption isotherms, UV-vis absorption spectroscopy as well as transient photocurrent responses. The photocatalytic H2-production activity was investigated by using Na2S and Na2SO3 as sacrificial reagents in water. Even without a Pt cocatalyst, the as-prepared Bi-doped ZnS hollow spheres exhibited significant visible-light and UV-light photocatalytic activity and good stability for H2-production. The optimal content of Bi dopant was determined to be about 0.3 at% and the corresponding H2-production rate was 1030 and 134 μmol h−1 g−1 under UV and visible-light irradiation, respectively. The apparent quantum efficiency was 0.99% at 420 nm and 4.0% at 365 nm. It is suggested that doping Bi into ZnS generated an isolated state originating from Bi 6s above the top of the valence band of ZnS, and the electron excitation from Bi 6s state to the conduction band occurred upon irradiation with visible light. Furthermore, the UV-light photocatalytic H2 evolution activity over Bi-doped ZnS hollow spheres is even higher than that of Pt/ZnS counterparts. This is ascribed to the fact that the Bi doping facilitates the separation of photogenerated electron–hole pairs and reduces their recombination rate.

Fluorinated semiconductor photocatalysts: tunable synthesis and unique properties.

Semiconductor photocatalysts are of great significance in solar energy conversion and environmental remediation. To overcome serious drawbacks of these materials with respect to narrow light-response range and low quantum efficiency, a variety of strategies have been developed in the past decades to enhance the light harvesting and excitation as well as the charge transfer against recombination. In particular, fluorination of semiconductor photocatalysts can be employed to modify their surface and bulk properties, and consequently, to enhance their photocatalytic performance. This review presents a comprehensive description of the F-mediated synthesis and unique properties of fluorinated semiconductor photocatalysts, in particular titanium dioxide (TiO(2)). The available strategies for the synthesis of fluorinated photocatalysts include post-synthesis fluorination and in-situ fluorination. Depending on the synthesis route and conditions, it is possible to control the chemical nature of incorporated fluorine (such as adsorbed fluoride and lattice-doped fluorine) and the fluoride-mediated crystal modification and organization, which often results in exceptional surface and bulk physicochemical properties, giving rise to unique photocatalytic properties. Significantly, the surface fluorination induces unusual adsorption behavior and interfacial charge transfer dynamics, directly affecting photocatalytic redox properties of the surface-fluorinated photocatalysts. The lattice fluorine-doping, sole or cooperative with other complementary co-dopants, introduces special localized electronic structures and surface defect states, accounting for the exceptional visible-light photoactivity of the fluorine-doped photocatalysts. Finally, recent advances in the synthesis and properties of fluorinated photocatalysts are summarized along with perspectives on further developments in this area of research.

Ionic‐Liquid‐Assisted Synthesis of Uniform Fluorinated B/C‐Codoped TiO2 Nanocrystals and Their Enhanced Visible‐Light Photocatalytic Activity

Exploiting advanced photocatalysts under visible light is of primary significance for the development of environmentally relevant photocatalytic decontamination processes. In this study, the ionic liquid (IL), 1-butyl-3-methylimidazolium tetrafluoroborate, was employed for the first time as both a structure-directing agent and a dopant for the synthesis of novel fluorinated B/C-codoped anatase TiO(2) nanocrystals (T(IL)) through hydrothermal hydrolysis of tetrabutyl titanate. These T(IL) nanocrystals feature uniform crystallite and pore sizes and are stable with respect to phase transitions, crystal ripening, and pore collapse upon calcination treatment. More significantly, these nanocrystals possess abundant localized states and strong visible-light absorption in a wide range of wavelengths. Because of synergic interactions between titania and codopants, the calcined T(IL) samples exhibited high visible-light photocatalytic activity in the presence of oxidizing Rhodamine B (RhB). In particular, 300 °C-calcined T(IL) was most photocatalytically active; its activity was much higher than that of TiO(1.98)N(0.02) and reference samples (T(W)) obtained under identical conditions in the absence of ionic liquid. Furthermore, the possible photocatalytic oxidation mechanism and the active species involved in the RhB degradation photocatalyzed by the T(IL) samples were primarily investigated experimentally by using different scavengers. It was found that both holes and electrons, as well as their derived active species, such as (·)OH, contributed to the RhB degradation occurring on the fluorinated B/C-codoped TiO(2) photocatalyst, in terms of both the photocatalytic reaction dynamics and the reaction pathway. The synthesis of the aforementioned novel photocatalyst and the identification of specific active species involved in the photodegradation of dyes could shed new light on the design and synthesis of semiconductor materials with enhanced photocatalytic activity towards organic pollutants.

Nitrogen-doped TiO2 microsheets with enhanced visible light photocatalytic activity for CO2 reduction

Abstract Nitrogen-doped anatase TiO2 microsheets with 65% (001) and 35% (101) exposed faces were fabricated by the hydrothermal method using TiN as precursor in the presence of HF and HCl. The samples were characterized by scanning electron microscopy, X-ray diffraction, N2 adsorption, X-ray photoelectron spectroscopy, UV-visible spectroscopy, and electrochemical impedance spectroscopy. Their photocatalytic activity was evaluated using the photocatalytic reduction of CO2. The N-doped TiO2 sample exhibited a much higher visible light photocatalytic activity for CO2 reduction than its precursor TiN and commercial TiO2 (P25). This was due to the synergistic effect of the formation of surface heterojunctions on the TiO2 microsheet surface, enhanced visible light absorption by nitrogen-doping, and surface fluorination.

Effect of PSS on morphology and optical properties of ZnO

ZnO micrometer-sized rods with tunable aspect ratios and 3D hollow spherical superstructures are selectively fabricated by a simple poly(sodium 4-styrene-sulfonate) (PSS)-mediated hydrothermal crystallization and assembly strategy. When PSS concentration is relatively low (0-0.5 g L), the aspect ratios of as-obtained microrods steadily decrease with increasing PSS concentration due to the selective adsorption of PSS on the polar ZnO (001) crystal plane. When PSS concentration is relatively high (1 g L), 3D nanosheets-built hollow microspheres form probably due to the organic-inorganic interfacial cooperative assembly. Raman, photoluminescence and UV-vis diffuse reflectance spectra show that the optical properties of as-obtained ZnO microstructures are highly related to their specific morphologies.

Tandem photocatalytic oxidation of Rhodamine B over surface fluorinated bismuth vanadate crystals

BiVO4 crystals with monoclinic-phase and controllable morphologies were synthesized by NaF-mediated hydrothermal processes using Bi(NO3)3 and V2O5 as precursors. The NaF added as a structural controller not only affected the crystal evolution processes of BiVO4 crystals, but also enabled the in situ surface fluorination of the as-synthesized BiVO4 crystals. Interestingly, the photocatalytic oxidation reactions of RhB occurred in a stepwise manner over fluorinated BiVO4 photocatalyst, that is, a faster de-ethylation process (conversion of RhB into rhodamine) followed by a relatively slower mineralization process, involving the destruction of the conjugated structure in rhodamine. Surface fluorination favored the RhB adsorption and hole transfer between RhB molecules and BiVO4 photocatalyst, thus progressively enhancing the initial direct hole transfer mediated de-ethylation process. In contrast, surface fluorination exerts compromised effects on the ·O2− mediated mineralization process, enhancing surface RhB adsorption versus retarding electron transfer from BiVO4 photocatalyst to O2 giving rise to ·O2−, and consequently, moderate surface fluorine coverage is required to balance the aforementioned conflicting effects and achieve the higher mineralization rate. The present study not only demonstrates that the photocatalytic efficiency can be modified by tuning photogenerated active species and photocatalytic reaction processes, but also provides new insights into the fluorination effects on the semiconductor photocatalysis.