Huayao Li

Selectively enhanced UV and NIR photoluminescence from a degenerate ZnO nanorod array film

Energy band engineering is a promising method to tune the photoelectric properties of semiconductors. In this paper, we report an un-element-doped ZnO nanorod array film with a degenerate energy band via annealing in a hydrogen atmosphere. Due to the energy band modification, the photogenerated carrier transitions in the degenerate energy band, involving the valance band, the defect bands and the degenerate conduction band, cause unique photoelectric properties in the degenerate ZnO film. The degenerate ZnO film performs with outstanding conductivity and exhibits an obvious optical absorption in the visible region. Its photoluminescence and photoresponse properties are investigated to understand the fundamentals of photogenerated carrier excitation and recombination in the degenerate ZnO film. Interestingly, the degenerate ZnO film performs with poor photoresponse and has poor photocurrent efficiencies for ultraviolet, blue, and green illuminations, but the photoresponse for near-infrared illumination is attractive. Our results also demonstrate the selectively enhanced down-conversion photoluminescence both at the UV band and at the near-infrared band from the degenerate ZnO film after being excited by an ultraviolet laser source.

Improvement of gaseous pollutant photocatalysis with WO3/TiO2 heterojunctional-electrical layered system.

Since the photogenerated holes play a much more important role than electrons in gas-phase photocatalysis, it is better to enrich the holes in the surface of a material system. Here, a novel [interdigital electrode/WO(3)/TiO(2)] heterojunctional-electrical layered (HEL) system is proposed to realize this attempt. The HEL system consists of interdigital electrode, WO(3) layer and TiO(2) layer, and they are orderly printed onto the alumina substrate from bottom to top using the technology of screen printing. It is surprise that the synergistic effect of layered heterojunction and external low bias can strengthen the separation of electron-hole pairs in both TiO(2) and WO(3), and enrich the TiO(2) surface layer with photogenerated holes to degrade the gaseous pollutants. In comparison with the pure TiO(2) film, a 6-fold enhancement in photocatalytic activity was observed using the HEL system by applying a very low bias of 0.2V. Furthermore, the results also showed that the remarkable improvement could not be obtained when either the WO(3) layer or the low external bias was absent.

Synthesis of TiO2/WO3/MnO2 Composites and High-Throughput Screening for Their Photoelectrical Properties

On the basis of the idea of equilateral ingredient triangle, a material library of the TiO(2)/WO(3)/MnO(2) composite material system was designed, which consisted of 66 ingredient points. Each point in the library corresponded with a device. To fabricate the device, the technology of screen printing was used. The pastes which were suitable for this technology were prepared by ball milling. After we printed the pastes onto the alumina substrate which had been preprinted with Au interdigital electrodes, these printed samples were sintered at 550 degrees C for 2 h in air. The photocurrent of each device under different light sources was measured respectively using a high-throughput screening system. The largest photocurrent was observed when the mole ratio of TiO(2)/WO(3) was 2/8 in the composite system. X-ray diffraction (XRD) was used to investigate the phase structure of the powder which had excellent photoelectric response.

Characterization of Photoelectric Properties and Composition Effect of TiO2/ZnO/Fe2O3 Composite by Combinatorial Methodology

On the basis of combinatorial methodology and the idea of an equilateral ingredient triangle, the TiO(2)/ZnO/Fe(2)O(3) composite system including 66 ingredient points was designed. The photocurrents under different light sources and bias voltages were measured, and the photocurrent amplitude at 300 s was chosen as a parameter to evaluate the photoelectric response of the composite. To appraise the composition effect of the composite compared with pure materials, the quantitative formula of the composition effect has been provided for the first time in this paper. We found that not all the ingredient points demonstrated the enhanced composition effect in the as-designed ingredient triangle material library. The reasons of different composition effect for different ingredient points have been discussed in detail. X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) were used to investigate the phase structure and the grain morphology of the composite.

The coupled effect of oxygen vacancies and Pt on the photoelectric response of tungsten trioxide films

In this paper, four kinds of WO3 films, namely, Pt-loaded hydrogenated WO3, Pt-loaded WO3, and hydrogen-treated and untreated WO3 films were synthesized and their photoelectric properties were investigated at room temperature. The quantitative results showed that the gas-sensitized WO3 film in formaldehyde exhibited much higher photocurrent than that in air. In addition, the sensitivity of Pt-loaded hydrogenated WO3 to formaldehyde reached 15.8 which was nearly 15 times higher than that of the others. Moreover, Pt-loaded hydrogenated WO3 shows excellent electrical response towards formaldehyde in the dark. The intriguing performance of the Pt-loaded hydrogenated WO3 film indicates an efficient coupled effect of oxygen vacancies and Pt. The results provide the potential for improving the efficiency of photoelectric sensing devices by coupling two modification mechanisms. To explain the response characteristics of the four kinds of WO3 films, a schematic diagram of the band bending and spill over model are proposed.

Photoconductivity and trap-related decay in porous TiO2/ZnO nanocomposites

Photoconductivity and trap-related decay were investigated in porous TiO2/ZnO nanocomposites. Photoconductivity responses of TiO2 and ZnO were completely different, which were attributed to electron-scavenging effect and hole trapping effect, respectively. When the mole ratio of TiO2:ZnO was from 9:1 to 6:4, the photoconductivity responses were consistent with TiO2. On the contrary, when the mole ratio of TiO2:ZnO was from 4:6 to 1:9, the photoconductivity responses were controlled by ZnO. Time constants were obtained by fitting the experiment data with an exponential function. We found that they tended to get larger with the percentage of ZnO while a turning point appeared at TiO2:ZnO = 1:9. The pattern was assigned to different carrier trapping mechanisms as well as carrier separation. Composition effect was defined by a quantitative formula to evaluate the recombination processes of composite materials. A mechanism was proposed to explain this phenomenon.

Bio-templated fabrication of hierarchically porous WO3 microspheres from lotus pollens for NO gas sensing at low temperatures

Lotus pollens were used as templates to prepare WO3 microspheres; the intriguing structural features of the pollens, e.g. hollow sphere with highly porous double shells, were perfectly inherited by the WO3 microspheres. The hierarchically porous structure of the WO3 microspheres was ideal for gas sensing. The WO3 microsphere-based sensor exhibited a high sensitivity (S = 46.2) to 100 ppm NO gas with a pretty fast response and recovery speed (62 s/223 s) at 200 °C. Compared with NO sensors reported in the literature so far, the WO3 microsphere-based sensor has among the highest sensitivity and fastest response/recovery.

Hierarchical and Hollow Fe2O3 Nanoboxes Derived from Metal–Organic Frameworks with Excellent Sensitivity to H2S

Hierarchical and hollow porous Fe2O3 nanoboxes (with an average edge length of ∼500 nm) were derived from metal-organic frameworks (MOFs) and the gas sensing characteristics were investigated. Sensors based on Fe2O3 nanoboxes exhibited a response (resistance ratio) of 1.23 to 0.25 ppm (ppm) hydrogen sulfide (H2S) at 200 °C, the response/recovery speed is fast and the selectivity to H2S is excellent. Remarkably, the sensor showed fully reversible response to 5 ppm of H2S at 50 °C, demonstrating its promise for operating at near room temperature, which is favorable for medical diagnosis and indoor/outdoor environment monitoring. The excellent performance of the Fe2O3 nanoboxes can be ascribed to the unique morphology with high specific surface area (SSA) and porous nanostructure.

LaCoO3-based sensors with high sensitivity to carbon monoxide

LaCoO3 nanoparticles with particle sizes of ∼82 nm were prepared by co-precipitation, and mesoporous LaCoO3 thick films with a thickness of ∼7 μm were fabricated by screen printing the nanoparticles on Al2O3 substrates. The CO sensing properties of the LaCoO3 thick films were characterized in the temperature range of 100 to 550 °C. Under 5000 ppm CO at 500 °C, the thick film sensor achieved a high sensing response of ∼279.86, with response and recovery periods of 181 and 311 s, respectively. Under 25 ppm CO at 500 °C, a reasonable response of 1.04 was also achieved. Moreover, the sensor demonstrated reliable dynamic response-and-recovery at a temperature as low as 100 °C, therefore, LaCoO3 is very promising for CO sensing at low temperatures. The high response to CO could be ascribed to the high content of O22−/O− species in LaCoO3.

A Method of Feature Extraction From the Desorption Part of MOX's Response Curves to Gases

The desorption part of MOXs response curves to gases represents the dynamic information of gas desorption and oxide recovery on the sensitive layer. It contains abundance information and lacks to be well investigated. This paper proposes a feature extraction method from the desorption part of response curves in phase-space. The method extracts only one feature, min(dS t/dt)/max(S t), which is found to be able to represent the recovery profile. This feature could be extracted in the front part of the recovery process. Besides, it is also found that this feature does not change with the reacting time (of the reaction between a sensor and a sample) changing. A dataset was used in the paper, which was consisted of 189 measurements of 9 VOCs by 5 TGS gas sensors at three concentrations. A common feature extraction method that extracted integral in phase-space was used as a comparison in sort classification with the proposed method. The correct recognition rates of Fisher Discriminant Analysis (FDA) with the proposed method and the phase-space integral extracting method were 91.5% and 79.9%, respectively. The results show that the proposed method is an efficient feature extraction method of the recovery profile of response curves.