Light induced metastable modification of optical properties in CH3NH3PbI3−xBrx perovskite films: Two-step mechanism

We have used photoluminescence (PL) and photomodulation (PM) spectroscopy to investigate the reversible spectral changes of PL in CH₃NH₃PbI_3−xBr_x films, where x is 1.7. In an as-prepared film, the peak of PL spectra shifts from ∼640 nm near bandedge to ∼750 nm after excitation by a continuous wave (CW) or a pulsed laser with high repetition rate, but keeps at 640 nm excited by same pulsed laser with the repetition rate smaller than 500 Hz. The PM spectroscopy also shows the formation of sub bandgap states after illumination which is responsible for the red shift of PL. The light induced modification of optical properties is reversible after keeping the film out of illumination for several hours at room temperature. We analyze the photoinduced modification to be two-steps processes: the temporary sub bandgap states were first photogenerated in perovskite film, if those states interacting with more coming photons within their lifetimes, light induced metastable states responsible for red-shift of PL will be formed. This instability reduces the electronic bandgap and generates more traps which will degrade the performance of the related photovoltaic devices.     

Structural and Catalytic Characterization of Solid Acids Based on Zirconia Modified by Tungsten Oxide

Tungsten oxide species form strong acid sites on ZrO₂supports and inhibit ZrO₂crystallite sintering and tetragonal to monoclinic structural transformations. W-L_IX-ray absorption near-edge spectra suggest that the W centers are in a distorted octahedral oxygen environment, even after dehydration at 673 K, in all WO_x–ZrO₂samples (2–21 wt.% W) oxidized at 1073 K. Maximumo-xylene isomerization turnover rates (per W atom) on WO_x–ZrO₂solids occur at WO_xsurface densities (10 W nm⁻²) that exceed the theoretical monolayer capacity of ZrO₂. Similar turnover rates are obtained on WO_x–ZrO₂samples with similar WO_xsurface densities (W nm⁻²) over a large range of oxidation temperatures (773–1223 K) and WO_xconcentrations (5–21 wt.% W). UV–visible spectra suggest an increase in WO_xdomain size with increasing surface density. High isomerization turnover rates appear to require the presence of WO_xdomains of intermediate size on ZrO₂surfaces. WO_xdomains of intermediate size appear to provide a compromise between reducibility and accessibility of WO_xcenters. These domains are necessary to delocalize a temporary charge imbalance that forms Brønsted acid sites in the presence of H₂and stabilizes carbocation intermediates. The presence of H₂duringo-xylene isomerization increases turnover rates and prevents rapid deactivation. Slow D₂/o-xylene exchange reactions indicate that H atoms from H₂are not frequently involved in the activation or desorption of xylenes. H₂is required, however, in order to reverse the occasional desorption of H atoms duringo-xylene isomerization reactions. These desorption processes lead to the destruction of Brønsted acid sites by the formation of strongly adsorbed unsaturated species in the absence of H₂. After promotion with Pt (0.3 wt.%), WO_x–ZrO₂solids catalyzen-heptane isomerization in the presence of H₂at 400–500 K with much higher selectivity than sulfated oxides or zeolitic acids at similar turnover rates. On Pt/WO_x–ZrO₂, efficient hydrogen transfer steps prevent extensive cracking of adsorbed carbocations by limiting their surface lifetimes.     

TC4 合金微弧氧化层的耐磨性和耐蚀性

目的对比不同电解液体系中制备的陶瓷膜层的耐磨损和耐腐蚀性能,判断实验条件下陶瓷膜性能最优的电解液体系。方法在相同的电参数工艺下,分别在Na Al O2,(Na PO3)6和Na2Si O3电解液体系中对TC4合金进行微弧氧化处理,处理时间为15 min。分析陶瓷层的表面形貌、成分和相结构。进行干摩擦条件下的摩擦磨损实验,对比TC4合金及三种陶瓷膜的耐磨性。通过测试极化曲线,对比TC4合金及三种陶瓷膜的耐蚀性。结果在Na Al O2,(Na PO3)6,Na2Si O3电解液体系中获得的陶瓷层表面呈现出多孔和局部凸起的相似表面特征,但相组成存在差异,主要相分别为Al2Ti O5,Al PO4和Ti O2。摩擦磨损实验表明,在10 N载荷下,以Si3N4陶瓷球作为摩擦配副,陶瓷层的磨损失重相对基材均显著减小,其中(Na PO3)6-陶瓷层失重约为基材的1/22。极化曲线分析表明,在模拟油田采出液作为腐蚀液的条件下,与TC4合金相比,陶瓷层的Ecorr显著正移,Jcorr明显减小,其中(Na PO3)6-陶瓷层的Ecorr从-0.311 V正移至0.777 V,Jcorr从9.634×10-7A/cm2减小到2.595×10-8A/cm2。结论微弧氧化处理能够显著改善TC4合金的耐磨性和耐蚀性,其中(Na PO3)6-陶瓷层的综合性能较好,有望满足TC4合金服役于油田环境时的要求。