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257. Direct Imaging of Highly Anisotropic Photogenerated Charge Separations on Different Facets of a Single BiVO4 Photocatalyst.

Author

Zhu, Jian, Fan, Fengtao, Chen, Ruotian, An, Hongyu, Feng, Zhaochi, and Li, Can

Subjects
PHOTOCATALYSIS, PHOTOCATALYSTS, PHOTOCATALYTIC air purification, SEPARATION (Technology), INDUSTRIAL chemistry
Abstract

Spatially resolved surface photovoltage spectroscopy (SRSPS) was employed to obtain direct evidence for highly anisotropic photogenerated charge separation on different facets of a single BiVO4 photocatalyst. Through the controlled synthesis of a single crystal with preferentially exposed {010} facets, highly anisotropic photogenerated hole transfer to the {011} facet of single BiVO4 crystals was observed. The surface photovoltage signal intensity on the {011} facet was 70 times stronger than that on the {010} facets. The influence of the built-in electric field in the space charge region of different facets on the anisotropic photoinduced charge transfer in a single semiconductor crystal is revealed. [ABSTRACT FROM AUTHOR]

Published
2015
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278. Photoluminescence Spectroscopic Studies on TiO2 Photocatalyst.

Author

Shi, Jianying, Wang, Xiuli, Feng, Zhaochi, Chen, Tao, Chen, Jun, and Li, Can

Abstract

Photoluminescence is a powerful technique in the study of semiconductor photocatalysts. This chapter deals with the application of photoluminescence techniques to the study of TiO2 in relation to its photocatalytic performance. The assignment of the visible and the near-infrared luminescence characteristics of TiO2 are discussed. The influence of the adsorbed molecules, such as H2O, O2, H2, unsaturated hydrocarbons and Pt loaded on TiO2, on the photoluminescence characteristics of TiO2 is also discussed. The relationship between the photoluminescence features of TiO2 and the photo-assisted reaction of water and methanol mixture is also summarized. [ABSTRACT FROM AUTHOR]

Published
2010
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279. UV Raman Spectroscopic Studies on Titania: Phase Transformation and Significance of Surface Phase in Photocatalysis.

Author

Zhang, Jing, Xu, Qian, Feng, Zhaochi, and Li, Can

Abstract

Phase transformation of TiO2 from anatase to rutile was studied by UV Raman spectroscopy with the excitation lines at 325 and 244 nm, visible Raman spectroscopy with the excitation line at 532 nm, X-ray diffraction (XRD) and transmission electron microscopy (TEM). It is found that UV Raman spectroscopy is more sensitive to the surface region of TiO2 than visible Raman spectroscopy and XRD because TiO2 strongly absorbs UV light. The anatase phase can be detected by UV Raman spectroscopy for the sample calcined at higher temperatures compared with that detected by visible Raman spectroscopy and XRD. It is suggested that the rutile phase nucleates at the interfaces of the contacting anatase particles; namely, for the agglomerated TiO2 particles, the anatase phase in the inner region is easier to change into the rutile phase than that in the outer surface region. When the anatase particles are covered with highly dispersed La2O3, the anatase phase can be stabilized both in the bulk and at the surface region even calcination at 900°C, owing to avoiding the direct contact of the anatase particles and occupying the surface defect sites of the anatase particles by La2O3. Additionally, the La2O3 impregnation could effectively inhibit the growth of TiO2 particles. The photocatalytic performance of TiO2 samples with different surface phase structures was investigated. The surface-phase junction formed between the anatase nanoparticles and rutile particles can greatly enhance the photocatalytic activity for H2 production. [ABSTRACT FROM AUTHOR]

Published
2010
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289. A tetragonal tungsten bronze-type photocatalyst: Ferro-paraelectric phase transition and photocatalysis

Author

Fan, Dayong, Chong, Ruifeng, Fan, Fengtao, Wang, Xiuli, Li, Can, and Feng, Zhaochi

Abstract

Although ferroelectrics have potential applications in photocatalysis due to their highly efficient charge separation, their mechanism of charge separation is still unknown. A ferroelectric Sr0.7Ba0.3Nb2O6(SBN-70) semiconductor with a low ferro-paraelectric phase transition (65 °C) was studied. The photocatalytic activity for H2production by ferroelectric and paraelectric SBN-70 was examined. The spontaneous polarization in the ferroelectric phase strongly affected the photocatalytic performance and parallel ferroelectric domains significantly promoted photogenerated charge separation to result in better photocatalytic H2production. This knowledge provides an important basis for the fabrication of ferroelectric photocatalysts with improved charge separation ability.

Published
2016
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290. Dual Extraction of Photogenerated Electrons and Holes from a Ferroelectric Sr0.5Ba0.5Nb2O6Semiconductor

Author

Fan, Dayong, Zhu, Jian, Wang, Xiuli, Wang, Shengyang, Liu, Yong, Chen, Ruotian, Feng, Zhaochi, Fan, Fengtao, and Li, Can

Abstract

The separation of photogenerated charges is a critical factor in photocatalysis. Recently, anomalous photovoltaic (APV) field effects (Voc∼ 103V/cm) in ferroelectrics, with their strong driving force for charge separation, have attracted much attention in photocatalysis and photoelectrocatalysis. However, it is still unknown whether photogenerated electrons and holes can be simultaneously extracted by the strong driving force toward the surface of ferroelectrics and can become available for surface reactions. This issue becomes critically important in photocatalysis because the surface reaction utilizes both the electrons and holes that reach the surface. In this work, a model lateral symmetric structure, metal/Sr0.5Ba0.5Nb2O6/metal (metal = Ag or Pt), as an electrode was fabricated. The dual extractions of photogenerated electrons and holes on the two opposite metal electrodes were achieved, as revealed by photovoltaic and ferroelectrical hysteresis measurements and photoassisted Kelvin probe force microscopy (KPFM). It was found that the high Schottky barriers of the two opposite Sr0.5Ba0.5Nb2O6–Pt electrodes are key factors that alter the two space charge regions (SCRs) by a poling effect. The resulting built-in electrical fields with parallel directions near both electrodes significantly enhance the charge separation ability. Our model unravels the driving force of charge separation in ferroelectric semiconductors, thus demonstrating the potential for highly efficient charge separation in photocatalysis.

Published
2016
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291. Fabrication of a monoclinic/hexagonal junction in WO3and its enhanced photocatalytic degradation of rhodamine B

Author

Lu, Yuanyuan, Liu, Guo, Zhang, Jing, Feng, Zhaochi, Li, Can, and Li, Zhi

Abstract

A series of WO3samples with different crystalline phases were prepared by the thermal decomposition method from ammonium tungstate hydrate. X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy, and N2adsorption-desorption were used to characterize the crystalline phase, morphology, particle size, chemical composition, and surface area of the WO3samples. The formation of hexagonal (h-WO3) and monoclinic (m-WO3) crystal structures of WO3at different temperatures or different times was confirmed by XRD. m-WO3is formed at 600 °C, while m-WO3starts to transform into h-WO3at 800 °C. However, h-WO3, which forms at 800 °C, may transform into m-WO3by increasing the calcination temperature to 1000 °C. SEM results indicate that m-WO3particles exhibit a bulky shape with heavy aggregates, while h-WO3particles exhibit a rod-like shape. Moreover, m-WO3crystals are sporadically patched on the surface of the h-WO3rod-like particles, resulting in the exposure of both m-WO3and h-WO3on the surface. It is observed that the monoclinic phase (m-WO3)/hexagonal phase (h-WO3) junction was fabricated by tuning the calcination temperature and calcination time. The relative ratios between m-WO3and h-WO3in the phase junction can readily be tailored by control of the calcination time. The photocatalytic activities of WO3with different crystalline phases were evaluated by the photocatalytic degradation of rhodamine B as a model pollutant. A higher photocatalytic activity was observed in the WO3sample with the m-WO3/h-WO3junction as compared with the sample with only m-WO3. The improvement of photocatalytic activity can be attributed to the reduction of the electron-hole recombination rate owing to the formation of the phase junction, whose presence has been confirmed by HRTEM and photoluminescence spectra.

Published
2016
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292. Mechanism of alkane H/D exchange over zeolite H-ZSM-5 at low temperature: a combined computational and experimental studyElectronic supplementary information (ESI) available. See DOI: 10.1039/c6cy00467a

Author

ChuThese authors contributed equally to this work., Yueying, Xue, Nianhua, Xu, Bolian, Ding, Qian, Feng, Zhaochi, Zheng, Anmin, and Deng, Feng

Abstract

Theoretical calculations have provided fundamental insights into the possible pathways for the H/D exchange of isobutane with H-ZSM-5 zeolite at room temperature. It is theoretically demonstrated that neither the direct exchange mechanism nor the indirect bimolecular hydride transfer mechanism is an efficient route for isobutane activation due to high activation barriers, which possibly prohibit H/D exchange at low temperatures. It is revealed that a trace of olefin impurities can considerably accelerate the formation of an alkoxyl intermediate which is involved in the bimolecular hydride transfer mechanism. Once the alkoxyl intermediate is generated from the olefin impurities, the catalytic cycle is self-sustaining. On the other hand, the theoretical calculations also illustrate that the extra-framework aluminum (EFAl) species in the dealuminated zeolite has no obvious promotional effect on the H/D exchange. Furthermore, our calculations are also consistent with the experimental results.

Published
2016
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297. Effect of the Nature and Location of Copper Species on the Catalytic Nitric Oxide Selective Catalytic Reduction Performance of the Copper/SSZ-13 Zeolite.

Author

Guo, Qiang, Fan, Fengtao, Ligthart, D. A. J. Michel, Li, Guanna, Feng, Zhaochi, Hensen, Emiel J. M., and Li, Can

Subjects
COPPER, RAMAN spectroscopy, ZEOLITES, CHABAZITE, COPPER ions, RAMAN spectra
Abstract

The nature and location of copper in Cu/SSZ-13 zeolites synthesized by using a one-pot hydrothermal approach with Cu-tetraethylenepentamine as a template and by the ion exchange of SSZ-13 were investigated by applying H2-temperature-programmed reduction, FTIR, EPR, and in situ Raman spectroscopic techniques. The one-pot synthesized Cu/SSZ-13 zeolite contains predominantly isolated copper ions in the large cages, whereas copper species in the ion-exchanged Cu/SSZ-13 zeolite occupy sites in the large cages of the chabazite (CHA) structure and the six-membered rings of the CHA structure. If the one-pot synthesized Cu/SSZ-13 zeolite is exchanged with the NH4NO3 solution in addition to the removal of a part of copper ions, the remaining copper ions in the CHA structure relocated from the large cages to the six-membered rings. Isolated copper dominated in all Cu/SSZ-13 zeolites. The in situ Raman spectra demonstrated that CuOCu dimers form at higher copper content. The bis-μ-oxo dicopper(III) complex is observed only in the ion-exchanged sample upon dehydration. The higher NO selective catalytic reduction activity of the one-pot synthesized sample in a wide temperature range appears to be due to the predominance of isolated Cu2+ sites in the large cages, and their higher reactivity is possibly owing to the lower stability of Cu2+ at these sites. [ABSTRACT FROM AUTHOR]

Published
2014
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300. Relation between the Photocatalytic and Photoelectrocatalytic Performance for the Particulate Semiconductor-Based Photoconversion Systems with Surface Phase Junction Structure

Author

Wang, Xiang, Jin, Shaoqing, An, Hongyu, Wang, Xiuli, Feng, Zhaochi, and Li, Can

Abstract

Although the surface phase junction can efficiently boost photocatalytic (PC) reactions, its role in photoelectrocatalytic (PEC) reactions has not been well-understood yet. In this study, we investigated the effect of the surface phase junction on the PEC performance of photoelectrodes fabricated with Ga2O3and TiO2particles. The surface phase junctions beneficial for PC reactions show a negative effect on PEC performance, which is mainly due to the significant influence of charge transportation between semiconductor particles by the surface phase junction. Photogenerated charge separation is promoted by the surface phase junction for both PC and PEC reactions, but the much more severe interfacial recombination occurs in the PEC reaction when charge carriers migrate across semiconductor particles to reach a conducting substrate. The opposite effects of the surface phase junction on PC and PEC performance reported here indicates that the fabrication of a heterojunction or phase junction in the right structure and sequence is vital to PEC systems based on junction, which will be helpful for the development of highly efficient photoconversion systems.

Published
2015
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