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5. Revisiting heat treatment and surface activation of GaAs photocathodes: In situ studies using scanning tunneling microscopy and photoelectron spectroscopy.

Author

Biswas, Jyoti, Cen, Jiajie, Gaowei, Mengjia, Rahman, Omer, Liu, Wei, Tong, Xiao, and Wang, Erdong

Subjects
*SCANNING tunneling microscopy, *PHOTOELECTRON spectroscopy, *PHOTOCATHODES, *SURFACE preparation, *HEAT treatment, *AUDITING standards, *ELECTRON tunneling
Abstract

The lifetime of GaAs photocathodes in polarized electron guns is limited due to the delicate activation layer. An atomically clean and smooth GaAs surface is needed to deposit a robust activation layer, such as Cs 2 Te , with longer lifetime compared to traditional (Cs,O) activation. A previous experiment with Cs 2 Te activation on GaAs used heat cleaning temperatures around 400 ° C to avoid an increase in surface roughness [Bae et al., Appl. Phys. Lett. 112, 154101 (2018)]. High-temperature heat cleaning around 580 ° C , which results in a relatively contamination-free surface, could be one possible way to improve quantum efficiency. However, one should be cautious about surface roughness degradation during high-temperature heat cleaning. In this paper, we report results of surface roughness measurements on native, heat cleaned, and (Cs,O) activated GaAs photocathodes under vacuum. The results, measured by ultrahigh vacuum scanning tunneling microscopy, show that the surface roughness improves as the heat cleaning temperature is increased, by at least a factor of three for 580 ° C heat cleaning, compared to the native sample. Activation with (Cs,O) is shown to increase surface roughness by a factor of four compared to a 580 ° C heat cleaned sample. This confirms that high-temperature heat cleaning can be useful for depositing good quality robust activation layers on GaAs. We also report chemical analysis for each step of preparation for p-doped GaAs photocathodes using X-ray photoelectron spectroscopy (XPS), angle-resolved XPS, and ultraviolet photoelectron spectroscopy. Our results indicate that the (Cs,O) activation layer forms a sandwich structure consists of Cs and oxygen. We found no formation of any specific compound such as Cs 2 O or Cs 11 O 3. [ABSTRACT FROM AUTHOR]

Published
2020
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9. High Current Performance of Alkali Antimonide Photocathode in LEReC DC Gun

Author

Gaowei, Mengjia, Cen, Jiajie, Fedotov, Alexei, Kayran, Dmitry, Lehn, Daniel, Liaw, Chong-Jer, Rao, Triveni, Tuozzolo, Joseph, Walsh, John, and Wang, Erdong

Subjects
WG3: Electron sources and injectors, Accelerator Physics
Abstract

The bi-alkali antimonide photocathode are chosen as the electron source material for the Low Energy RHIC electron Cooling (LEReC) project at RHIC, BNL based on its requirement for high bunch charge and long-time beam operation. This report presents the design and operation of the cathode deposition and transportation systems for the LEReC photocathodes, the cathode performance under the high current operation in the LEReC DC gun, as well as the characterization of the damaged cathodes from the long-time operation., Proceedings of the 63th ICFA Advanced Beam Dynamics Workshop on Energy Recovery Linacs, ERL2019, Berlin, Germany

Published
2020
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13. Enhancing CO Oxidation Activity viaTuning a Charge Transfer Between Gold Nanoparticles and Supports

Author

Yang, Haotian, Cen, Jiajie, Wu, Qiyuan, Ridge, Claron J., Tong, Xiao, Zhou, Chenyu, Veerasamy, Vijayen, Su, Dong, Lindsay, C. Michael, Liu, Mingzhao, and Orlov, Alexander

Abstract

Charge transfer from the supports to nanoparticles at the interface is one of the key factors to determine the catalytic performances of supported nanoparticles. In this work, we showed in a systematic way that the charge transfer from semiconductor supports to Au nanoparticle catalysts can lower the onset temperature toward CO oxidation. For this study, a novel Au/SiO2/Si composite system synthesized by the helium droplet deposition method with precisely tuned SiO2layer thickness was fabricated to control the magnitude of interfacial charge transfer. With the support of X-ray photoelectron spectroscopy and numerical simulations, it was demonstrated that the Schottky barrier formed across the Au/SiO2/Si heterojunction led to a negative charge accumulation on the surface of Au nanoparticles. In turn, this additional charge can be transferred to the antibonding orbital of adsorbed O2molecules to activate the O–O bonds, leading to enhanced CO oxidation. In addition to the charge transfer mechanism, the role of a strong electric field arising from the formation of the Schottky barrier was also explored to explain the observed enhancement of catalytic reactivity. Overall, this work highlights an important pathway for systematically tuning metal–support interactions to accelerate catalytic reactions and designing the next generation of nanocatalysts.

Published
2022
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15. Exploring Metal–Support Interactions To Immobilize Subnanometer Co Clusters on γ–Mo2N: A Highly Selective and Stable Catalyst for CO2 Activation

Author

Yao, Siyu, primary, Lin, Lili, additional, Liao, Wenjie, additional, Rui, Ning, additional, Li, Na, additional, Liu, Zongyuan, additional, Cen, Jiajie, additional, Zhang, Feng, additional, Li, Xing, additional, Song, Liang, additional, Betancourt De Leon, Luis, additional, Su, Dong, additional, Senanayake, Sanjaya D., additional, Liu, Ping, additional, Ma, Ding, additional, Chen, Jingguang G., additional, and Rodriguez, José A., additional

Published
2019
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17. Highly Active Ceria-Supported Ru Catalyst for the Dry Reforming of Methane: In Situ Identification of Ruδ+–Ce3+ Interactions for Enhanced Conversion

Author

Liu, Zongyuan, primary, Zhang, Feng, additional, Rui, Ning, additional, Li, Xing, additional, Lin, Lili, additional, Betancourt, Luis E., additional, Su, Dong, additional, Xu, Wenqian, additional, Cen, Jiajie, additional, Attenkofer, Klaus, additional, Idriss, Hicham, additional, Rodriguez, José A., additional, and Senanayake, Sanjaya D., additional

Published
2019
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18. Conversion of CO2 on a highly active and stable Cu/FeOx/CeO2 catalyst: tuning catalytic performance by oxide-oxide interactions

Author

Lin, Lili, primary, Yao, Siyu, additional, Rui, Ning, additional, Han, Lili, additional, Zhang, Feng, additional, Gerlak, Clifford A., additional, Liu, Zongyuan, additional, Cen, Jiajie, additional, Song, Liang, additional, Senanayake, Sanjaya D., additional, Xin, Huolin L., additional, Chen, Jingguang G., additional, and Rodriguez, José A., additional

Published
2019
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23. Growth of Nanoparticles with Desired Catalytic Functions by Controlled Doping-Segregation of Metal in Oxide

Author

Wu, Qiyuan, primary, Yan, Binhang, additional, Cen, Jiajie, additional, Timoshenko, Janis, additional, Zakharov, Dmitri N., additional, Chen, Xianyin, additional, Xin, Huolin L., additional, Yao, Siyu, additional, Parise, John B., additional, Frenkel, Anatoly I., additional, Stach, Eric A., additional, Chen, Jingguang G., additional, and Orlov, Alexander, additional

Published
2018
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26. Resolving the Evolution of Atomic Layer-Deposited Thin-Film Growth by Continuous In SituX-Ray Absorption Spectroscopy

Author

Qu, Xiaohui, Yan, Danhua, Li, Ruoshui, Cen, Jiajie, Zhou, Chenyu, Zhang, Wenrui, Lu, Deyu, Attenkofer, Klaus, Stacchiola, Dario J., Hybertsen, Mark S., Stavitski, Eli, and Liu, Mingzhao

Abstract

In situsynchrotron X-ray absorption near-edge structure characterization of thin-film titania growth by atomic layer deposition (ALD) over ZnO nanowires reveals persistent low-coordinated Ti motifs leading to a new picture of ALD growth. Through the design of growth and measurement cycles, Ti K-edge spectral data are continuously recorded so as to characterize the film evolution as a function of ALD cycle number and the surface changes within the time scale of the ALD cycle. A unified set of analysis tools is developed to interpret the time-series of spectral data. A prenucleation stage of growth, a transition region, and then a steady-state growth stage are observed with distinguishable features. Multivariate curve resolution analysis, that is physically constrained, demonstrates two specific spectral components with associated, time-dependent concentrations. The bulk-film component tracks the stages of growth. The surface and interface components, present throughout the stages of growth, reveal a significant coverage of relatively isolated or loosely networked tetrahedrally coordinated Ti atomic motifs. Finally, spectral signatures for the intra-cycle growth kinetics are reconstructed at a time resolution of ∼ 1 s and demonstrate that the transient Ti motifs on the growing surface stabilize within a few seconds of the Ti precursor pulse.

Published
2021
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39. Near band edge photoluminescence of ZnO nanowires: Optimization via surface engineering.

Author

Danhua Yan, Wenrui Zhang, Cen, Jiajie, Stavitski, Eli, Sadowski, Jerzy T., Vescovo, Elio, Walter, Andrew, Attenkofer, Klaus, Stacchiola, Darío J., and Mingzhao Liu

Subjects
ZINC oxide, PHOTOLUMINESCENCE, NANOWIRES, LIGHT emitting diodes, SURFACE defects
Abstract

Zinc oxide (ZnO) nanowire arrays have potential applications for various devices such as ultraviolet light emitting diodes and lasers, where photoluminescence of intense near band edge emission without defect emissions is usually desired. Here, we demonstrate, counter-intuitively, that the near band edge emission may become dominant by introducing certain surface defects to ZnO nanowires via surface engineering. Specifically, near band edge emission (NBE) is effectively enhanced after a low pressure O2 plasma treatment that sputters off surface oxygen species to produce a reduced and oxygen vacancy-rich surface. The effect is attributed to the lowered surface valence band maximum of the reduced ZnO surface that creates an accumulative band bending, which screens the photo-generated minority carriers (holes) from reaching or being trapped by the surface defects. [ABSTRACT FROM AUTHOR]

Published
2017
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40. Conversion of CO2on a highly active and stable Cu/FeOx/CeO2catalyst: tuning catalytic performance by oxide-oxide interactionsElectronic supplementary information (ESI) available: BET surface area, EXAFS fitting results. See DOI: 10.1039/c9cy00722a

Author

Lin, Lili, Yao, Siyu, Rui, Ning, Han, Lili, Zhang, Feng, Gerlak, Clifford A., Liu, Zongyuan, Cen, Jiajie, Song, Liang, Senanayake, Sanjaya D., Xin, Huolin L., Chen, Jingguang G., and Rodriguez, José A.

Abstract

Nanoparticles of FeOxdispersed on ceria exhibit unique chemical and textural properties. They undergo a dynamic FeOx↔ Fe transformation depending on the reaction conditions. The high density Fe oxide clusters dispersed over CeO2are effective texture promoters that enhance the stability of Cu/CeO2catalyst in the high temperature reverse water gas shift reaction. At the optimal Fe loading, the deactivation rate constant of a 5Cu1.6Fe/CeO2catalyst is only 0.003 h−1, five times smaller than that of the monometallic 5Cu/CeO2catalyst. In situXRD, AP-XPS and XAFS characterizations reveal that the partially reduced Fe nanoparticles would be oxidized by CO2in the reductive atmosphere and re-disperse into high density FeOxclusters under the high temperature CO2hydrogenation process. The wetting phenomenon, maximizing the density of FeOxparticles on the CeO2support, benefits from strong oxide-oxide (Fe–O–Ce) interactions.

Published
2019
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41. Reaction of Methane with MOx/CeO2(M = Fe, Ni, and Cu) Catalysts: In Situ Studies with Time-Resolved X-ray Diffraction

Author

Zhang, Feng, Yao, Siyu, Liu, Zongyuan, Gutiérrez, Ramón A., Vovchok, Dimitriy, Cen, Jiajie, Xu, Wenqian, Ramírez, Pedro J., Kim, Taejin, Senanayake, Sanjaya D., and Rodriguez, José A.

Abstract

Changes in chemical state and structural transformations occurring in a series of MOx/CeO2(M = Cu, Ni and Fe) powder catalysts upon reaction with methane were investigated using in situ time-resolved X-ray diffraction (TR-XRD), ex situ X-ray absorption near edge structure (XANES)/extended X-ray absorption fine structure (EXAFS) spectroscopy, and X-ray photoelectron spectroscopy (XPS). XPS shows the presence of adsorbed CHxand COxspecies after exposing the powder catalysts to methane at room temperature. Temperature-programmed reduction (TPR) measurements point to reaction of the samples with methane and formation of CO, CO2, and H2O gas at temperatures as low as 100 °C. The TR-XRD results show that all of the transition-metal oxides in the as-prepared catalysts can be reduced to their metallic phase during the CH4-TPR process with the ceria support undergoing significant reduction from surface to bulk, yet the reduction temperature varies for different MOx/CeO2samples. Among these samples, CuOx/CeO2shows the lowest reduction temperature (below 260 °C) for both the oxide overlayer and the ceria support. The NiOx/CeO2and CoOx/CeO2powder catalysts also activate CH4at relatively low temperatures (below 350 °C), and the oxide overlayers undergo NiO → Ni and Co3O4→ CoO → Co transformations. In the case of FeOx/CeO2, a series of complex changes in chemical state is observed for the oxide overlayer, Fe2O3→ Fe3O4→ FeO → Fe → Fe3C. The ceria support of the three MOx/CeO2(M = Cu, Ni, and Fe) samples undergoes severe reduction and forms Ce2O3at high temperature (>650 °C) during the CH4-TPR reaction. The in situ TR-XRD results highlight a rich and complex chemistry for methane on these MOx/CeO2mixed oxide systems.

Published
2018
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44. Strain Coupling of Conversion-type Fe3O4 Thin Films for Lithium Ion Batteries.

Author

Hwang, Sooyeon, Meng, Qingping, Chen, Ping-Fan, Kisslinger, Kim, Cen, Jiajie, Orlov, Alexander, Zhu, Yimei, Stach, Eric A., Chu, Ying-Hao, and Su, Dong

Subjects
LITHIATION, THIN films, METALATION, ELECTRODES, LITHIUM-ion batteries
Abstract

Lithiation/delithiation induces significant stresses and strains into the electrodes for lithium ion batteries, which can severely degrade their cycling performance. Moreover, this electrochemically induced strain can interact with the local strain existing at solid-solid interfaces. It is not clear how this interaction affects the lithiation mechanism. The effect of this coupling on the lithiation kinetics in epitaxial Fe3O4 thin film on a Nb-doped SrTiO3 substrate is investigated. In situ and ex situ transmission electron microscopy (TEM) results show that the lithiation is suppressed by the compressive interfacial strain. At the interface between the film and substrate, the existence of Li xFe3O4 rock-salt phase during lithiation consequently restrains the film from delamination. 2D phase-field simulation verifies the effect of strain. This work provides critical insights of understanding the solid-solid interfaces of conversion-type electrodes. [ABSTRACT FROM AUTHOR]

Published
2017
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45. Photoelectrochemical water splitting with a SrTiO3:Nb/SrTiO3 n+–n homojunction structure.

Author

Cen, Jiajie, Wu, Qiyuan, Yan, Danhua, Tao, Jing, Kisslinger, Kim, Liu, Mingzhao, and Orlov, Alexander

Abstract

A very limited knowledge exists about the effect of non-uniform doping of epitaxially grown strontium titanate thin film electrodes on their photoelectrochemical performance in water splitting. In this work, water splitting photoanodes featuring an n+–n homojunction were fabricated by the pulsed laser deposition technique, where epitaxial SrTiO3 thin films were grown on Nb doped n+–SrTiO3 single crystalline substrates. Thermal diffusion of niobium from doped substrates into the deposited thin films formed an n+–n homojunction, which was profiled by angle-resolved XPS and cross-sectional STEM-EDX techniques. This homojunction was found to make a significant impact on the incident photon-to-current efficiency of photoanodes by affecting their depletion width, which was in agreement with the theoretical simulations. [ABSTRACT FROM AUTHOR]

Published
2017
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48. Understanding the Interactions of CO2 with Doped and Undoped SrTiO3.

Author

Wu, Qiyuan, Cen, Jiajie, Goodman, Kenneth R, White, Michael G., Ramakrishnan, Girish, and Orlov, Alexander

Subjects
DOPED semiconductors, PHOTOCATALYSIS, INFRARED spectroscopy, HYDROGEN oxidation, ELECTRICAL engineering
Abstract

SrTiO3 and doped SrTiO3 have a wide range of applications in different fields. For example, Rh-doped SrTiO3 has been shown to have photocatalytic activity for both hydrogen production and CO2 conversion. In this study, both undoped and Rh-doped SrTiO3 were synthesized by hydrothermal and polymerizable complex methods. Different characterizations techniques including X-ray photoelectron spectroscopy (XPS), XRD, Raman, and UV/Vis spectroscopy were utilized to establish correlations between the preparation methods and the electronic/structural properties of Rh-doped SrTiO3. The presence of dopants and oxygen vacancies substantially influenced the CO2 interactions with the surface, as revealed by the in situ infrared spectroscopic study. The presence of distinctly different adsorption sites was correlated to oxygen vacancies and oxidation states of Ti and Rh. [ABSTRACT FROM AUTHOR]

Published
2016
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49. Developing new understanding of photoelectrochemical water splitting via in-situtechniques: A review on recent progress

Author

Cen, Jiajie, Wu, Qiyuan, Liu, Mingzhao, and Orlov, Alexander

Abstract

Photoelectrochemical (PEC) water splitting is a promising technology for solar hydrogen production to build a sustainable, renewable and clean energy economy. Given the complexity of the PEC water splitting processes, it is important to note that developing in-situtechniques for studying PEC water splitting presents a formidable challenge. This review is aimed at highlighting advantages and disadvantages of each technique, while offering a pathway of potentially combining several techniques to address different aspects of interfacial processes in PEC water splitting. We reviewed recent progress in various techniques and approaches utilized to study PEC water splitting, focusing on spectroscopic and scanning-probe methods.

Published
2017
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50. A reversible ratiometric sensor for intracellular Cu2+ imaging: metal coordination-altered FRET in a dual fluorophore hybrid.

Author

Chen, Yuncong, Zhu, Chengcheng, Cen, Jiajie, Li, Jing, He, Weijiang, Jiao, Yang, and Guo, Zijian

Subjects
FLUOROPHORES, COPPER, DETECTORS, CHEMICALS, NATIVE element minerals
Abstract

ICT fluorophore benzoxadiazole with its electron-donating group modified as a Cu2+ chelator was conjugated with coumarin to construct a new ratiometric sensor with reversible intracellular Cu2+ imaging ability. [ABSTRACT FROM AUTHOR]

Published
2013
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