Yanhong Lin

Study on photogenerated charge transfer properties and enhanced visible-light photocatalytic activity of p-type Bi2O3/n-type ZnO heterojunctions

The composite Bi2O3/ZnO photocatalyst, a novel p–n type heterojunction with different molar ratios of Bi to Zn, has been fabricated using a hydrothermal method. The XRD, XPS, UV-vis DRS, SEM, HRTEM studies were used to characterize the as-obtained products. The photocatalytic activities of the p-Bi2O3/n-ZnO heterojunctions were investigated for their efficiency on the degradation of alizarin red (AR) dye under visible light irradiation (λ > 420 nm), and the results showed that the composites possessed remarkable photocatalytic activities, which were conducive to their separation, recycling, and reuse. The photogenerated charge-transfer properties were tested by surface photocurrent (SPC) and surface photovoltage (SPV) experiments and results show that the interfacial electric field located between Bi2O3 and ZnO played an important role in promoting the separation of photogenerated electron and hole pairs. These results are helpful to design and construct high efficiency heterogeneous semiconductor photocatalysts.

Study on formaldehyde gas-sensing of In2O3-sensitized ZnO nanoflowers under visible light irradiation at room temperature

In2O3-sensitized flowerlike ZnO with visible light photoelectric response properties were synthesized by a facile two-step process, and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), EDAX, HRTEM and UV-vis diffuse reflectance spectroscopy. The results revealed that In2O3 nanoparticles have grown and eventually coalesced on the surface of the flowerlike ZnO successfully, and the samples exhibited significant response to visible light. The photoelectric gas-sensing of the In2O3-sensitized ZnO was also studied to formaldehyde (HCHO) under 460 nm light irradiation at room temperature with the help of surface photocurrent technique. It was found that ZnO sensitized with In2O3 could enhance the gas response to HCHO under the visible light illumination. This may be due to the fact that the composite structure of In2O3–ZnO extends the photo absorbing range to visible light area, inhibits the recombination of photo-generated electrons and holes, and thus increases the utilization of photo-generated carriers in photoelectric gas detection, resulting in the higher sensing response in some extent. The gas response to 5 ppm and 100 ppm formaldehyde can reach to 19% and 419% under visible light irradiation at room temperature, respectively. These results should be valuable for designing a new type of visible-light assisted gas sensor at room temperature.

Influence of adsorbed oxygen on the surface photovoltage and photoluminescence of ZnO nanorods

ZnO nanorods have been prepared with sol–gel methods using zinc acetate dihydrate in ethanol in the presence of lithium hydroxide via alkaline hydrolysis. The electron transfer behaviour at the surface and interface in ZnO nanorods was investigated by means of the surface photovoltage technique. The influence of adsorbed oxygen on the surface photovoltage (SPV) response of ZnO nanorods was studied by surface photovoltage spectroscopy (SPS) and field-induced surface photovoltage spectroscopy (FISPS). The results of SPS demonstrate that for ZnO nanorods the built-in electric field should be a main driving force for the separation of the photogenerated electron–hole pairs and its ensuing SPV response. The method of photogenerated charge recombination was also studied with the aid of PL spectroscopy. It is shown that the two methods of energy relaxation in light-excited ZnO nanorods are competitive. When oxygen is adsorbed at the surface and the built-in electric field is formed, the SPV response should be the leading one. Nevertheless, when oxygen is absent, the energy relaxation is mostly carried out by radiative emission.

Construction of Shallow Surface States through Light Ni Doping for High-Efficiency Photocatalytic Hydrogen Production of CdS Nanocrystals

Ni-doped CdS nanowires were synthesized by a simple one-step method. X-ray diffraction, X-ray photoelectron spectroscopy, and photoluminescence spectroscopy confirmed that light Ni doping can form shallow surface states due to the presence of substitutional Ni ions, and heavy Ni doping can form deep surface states due to the presence of interstitial Ni ions. Surface photovoltage spectroscopy and transient photovoltage measurements revealed that the shallow surface states can prolong the lifetime of the photogenerated charge carriers, whereas the deep surface states lead to recombination of the photogenerated charge carriers. The relationship between different surface states and the photocatalytic performance of CdS nanocrystals are discussed. The enhanced density of shallow surface states due to light Ni doping significantly promotes photocatalytic H2 production.

Enhanced photocatalytic degradation of phenol and photogenerated charges transfer property over BiOI-loaded ZnO composites

In this paper, a series of BiOI/ZnO photocatalysts containing various BiOI contents were prepared by a facile two-step synthetic method. The structure and crystal phase, morphology, surface element analysis, optical property of as-prepared samples are measured by X-ray diffraction (XRD), Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and UV-Vis diffuse reflectance spectrometry (DRS). BiOI/ZnO photocatalytic activities of the prepared photocatalysts were evaluated by photocatalytic degradation of phenol under simulated light irradiation. The phenol degradation rate reached 99.9% within 2h under simulated solar light irradiation. The probable photocatalytic mechanism of composites photocatalysts is discussed by active species trapping experiments, the surface photovoltage (SPV), the transient photovoltage (TPV) and photoluminescence (PL) measurements. The results manifest that the superior photocatalytic activity of BiOI/ZnO composites is derived from the strong internal electric field between BiOI and ZnO, which is beneficial for the effective separation and transfer of photogenerated charges in ZnO. Moreover, the loading of BiOI on the surface of ZnO inhibited the recombination of photogenerated charge carriers in ZnO, resulting in excellent photocatalytic activity. On the contrary, the effect of an extension of the light absorption range induced by the introduction of BiOI on the phenol degradation activity is not significant.

Surface charge transfer properties of high-performance Ag-decorated ZnO photocatalysts

Silver-decorated ZnO (Ag/ZnO) chain-like nanoparticle aggregates were prepared by a facile nonaqueous sol–gel method. The influence of Ag decoration on the surface photo-induced charge transfer behaviour of ZnO was investigated by means of surface photovoltage spectroscopy, electric field-induced surface photovoltage spectroscopy and transient photovoltage techniques. The photovoltage spectra revealed that an electronic interaction between Ag and ZnO formed. Ag acted as electron acceptors and effectively inhibited the charge recombination in ZnO with UV light illumination, while under the illumination of visible light a redshift was observed due to the appearance of a plasmon-induced band resulting from the formation of Ag nanoparticle clusters. These were responsible for the significant enhancement in the photocatalytic activity of the Ag/ZnO sample. The results also suggested that the high-performance visible light photocatalytic activity could be manipulated by external biases.

Branched hierarchical photoanode of anatase TiO2 nanotubes on rutile TiO2 nanorod arrays for efficient quantum dot-sensitized solar cells

We report novel hierarchical three-dimensional (3D) TiO2 nanotube-branched rutile TiO2 nanorod arrays (H-TiO2 NRAs) on FTO substrates that serve as model architecture for efficient quantum dot-sensitized solar cells (QDSCs). The newly designed H-TiO2 NRA photoanode offers a large surface area for high QD loading with high light scattering properties. In the meantime, the presence of anatase–rutile heterojunction at the interface helps the rutile nanorods to efficiently collect photo-injected electrons from the anatase nanotubes, reducing electron recombination with the electrolyte and QDs. As a result, the H-TiO2 NRA photoanode with a thickness of only 1 μm, exhibits a solar energy conversion efficiency of 1.04%, which is 2.7 times higher than that found in the pristine nanorod array-based QDSCs, demonstrating the synergistic effect of rutile nanorods and anatase nanotubes for photoelectrochemical solar energy conversion.

Enhanced solar light-driven photocatalytic activity of BiOBr–ZnO heterojunctions with effective separation and transfer properties of photo-generated chargers

With the purpose of high-efficiency separation of photo-generated charges and an improved photocatalytic performance, herein we report novel heterojunctions of p-type BiOBr and n-type ZnO constructed by loading amounts of BiOBr nanoflakes onto the surface of ZnO nanoflowers via a two-step hydrothermal/solvothermal method. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and UV-vis diffuse reflectance spectroscopy (UV-vis DRS) are employed to characterize the as-synthesized nanomaterials. The results indicate that p–n heterojunctions formed between BiOBr and ZnO. When evaluated as a photocatalyst for methyl orange (MO) decolorization, BiOBr–ZnO-40 shows excellent photocatalytic activity, superior to that of pristine BiOBr and ZnO. Notably, the photochemical process for the MO decolorization is elucidated by reactive species trapping and hydroxyl radical quantification experiments, suggesting that ˙OH and O2˙− are the main oxygen active species for BiOBr–ZnO heterojunctions in the MO decolorization. Moreover, the separation and transfer process of photo-generated charges at the surface or interface of the BiOBr–ZnO composites is also investigated by means of surface photovoltage (SPV). On the basis of experimental and theoretical results, the enhanced photocatalytic activity mechanism for the BiOBr–ZnO heterojunctions is proposed. The effective separation and transfer of photo-generated charges are responsible for an improved photocatalytic performance due to the existence of the interfacial electric field located between BiOBr and ZnO.

Direct evidence of the efficient hole collection process of the CoOx cocatalyst for photocatalytic reactions: a surface photovoltage study

The true effect of a cocatalyst on photocatalytic reactions is one of the major issues to resolve in photocatalysis research. Herein, the efficient hole collection role of the CoOx cocatalyst grown on TiO2-nanotubes has been directly proved by surface photovoltage (SPV) spectroscopy. Transient SPV measurements provide evidence that the CoOx cocatalyst could significantly prolong the life-time of photogenerated holes. Furthermore, it is proposed that the hole collection effect can be attributed to the work function difference between the TiO2 and CoOx interface. These findings demonstrate that the development of a cocatalyst with a suitable work function can be a general strategy for photoconversion applications.

Effect of photogenerated charge transfer on the photocatalysis in high-performance hybrid Pt-Co:ZnO nanostructure photocatalyst.

Hybrid Pt-Co:ZnO nanostructure photocatalysts were prepared via a facile two-step synthetic strategy. SPS and TPV investigations demonstrate the existence of the synergetic effect between Pt and Co dopants. Such synergetic effect could make use of visible photons as well as facilitates the separation of photogenerated charges to prevent recombination, effectively prolongating the charges lifetime to participate photocatalytic reaction. The synergetic effect exist in Pt-Co:ZnO inducing as high as 7.7-fold in photovoltaic response and 10-fold in the photo-activity for hybrids compared to Co:ZnO.