NT2.1: Photocatalysis and Water Splittiing
Tuesday PM, March 29, 2016
PCC North, 100 Level, Room 132 AB
2:30 PM - NT2.1.01
Nano-Topographical, Electrical and Mechanical Studies of the Terrace and Edge Sites on WSe2
Zhuangqun Huang 1,Jesus Velazquez 1,Jimmy John 1,Adam Pieterick 1,Xinghao Zhou 1,Manuel Soriaga 1,Hans Lewerenz 1,Thomas Mueller 2,Bruce Brunschwig 1,Nathan Lewis 1
2 Bruker Nano Surfaces Goleta United States,1 Joint Center for Artificial Photosynthesis California Institute of Technology Pasadena United States,1 Joint Center for Artificial Photosynthesis California Institute of Technology Pasadena United States2 Bruker Nano Surfaces Goleta United StatesShow Abstract
Innovative routes for the fabrication of semiconductors that are both stable and able to efficiently convert solar energy to molecular Hydrogen under the conditions required for the safe and operation of a solar-fuel generator will be imperative to the development of the next generation of such devices. Layered Transition-Metal Dichalcogenides (TMDCs) are suitable candidates for efficient and stable photoelectrosynthetic hydrogen generation in acidic and alkaline media, with inherent stability in harsh electrochemical environments. Furthermore, TMDCs are capable of dual functionality in such devices, having demonstrated both efficient photoconversion and electrocatalysis. Tungsten diselenide (WSe2) is an attractive TMDC material for this purpose, owning to its high light absorption coeffiecnt, 1.2 eV band gap and anisotropic properties. We investigate fundamental insights into the factors that determine the photoelectrical properties of WSe2. The present work focuses on nano- topographical, electrical and mechanical studies of the WSe2 in the absence and presence of catalysts, including the density of edge sites, surface potential profile, local current mapping and the nano lithography. These studies were performed mostly based on the PeakForce-based tapping AFM mode. Our studies show that the surface work functions and stiffness of the edge sites are different from the basal plan. The edge sites are also more conductive while showing inhomogeneous conductivity. Hence, WSe2 displays distinct chemical and physical differences when terrace and edge sites are compared. Therefore, performance optimization of the WSe2 photoelectrode requires the control of both density and electrical/chemical properties of the edge sites.
Acknowledgement: This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993. Research was in part carried out at the Molecular Materials Research Center of the Beckman Institute of the California Institute of Technology.
 Velazquez et. al. “A Scanning Probe Investigation of the Role of Surface Motifs in the Behavior of p-WSe2 Photocathodes”, Energy & Environmental Science, 2015, (Accepted)
2:45 PM - *NT2.1.02
Metal Oxide Nanosheets for Light-Induced Water Splitting
Kazuhiko Maeda 1
1 Tokyo Institute of Technology Tokyo Japan,Show Abstract
Semiconductor oxide nanosheets derived from layered metal oxides have attracted attention in various fields due to their unique structural properties. The anisotropic feature of nanosheets, which have a thickness of ~1 nm and lateral dimensions ranging from several hundred nanometers to a few micrometer, seems to be advantageous for heterogeneous photocatalysts, as the diffusion length of photogenerated electrons and holes to the surface is shortened, resulting in higher activity.
For application in photocatalysis, the composition of a metal oxide determines the band structure of the material, which has a strong impact on photocatalytic activity, as the reactivity of electrons and holes for surface redox reactions is determined by the band-edge potentials. Therefore, precise control of energy band structure is essential to designing a highly efficient photocatalyst. To date, however, such band-structure-controlled nanosheets have not been devised, and how the band gap structure affect photocatalytic activity remains little explored. We prepared perovskite nanosheets of HCa2–xSrxNb3–yTayO10, and found that the conduction band-edge potentials were successfully controlled by cationic substitution. These nanosheets functioned as highly efficient heterogeneous photocatalysts for H2 evolution from an aqueous methanol solution under band-gap irradiation. The activity was found to depend on the composition. The highest activity was obtained with HCa2Nb2TaO10 nanosheets, giving an apparent quantum yield (AQY) of ~80% at 300 nm. This is the highest value among nanosheet-based photocatalysts reported so far.
It is also known that certain nanoparticulate metals or metal oxides on a semiconductor photocatalyst work as cocatalysts to promote water reduction and/or oxidation. In heterogeneous photocatalysis, the effect of cocatalyst size on the water-splitting performance had not been examined at sizes smaller than 1 nm due to the lack of an effective preparation method and a suitable photocatalyst. We have very recently demonstrated that metal nanoclusters (such as Pt) of <1 nm in size could be deposited on the interlayer nanospace of KCa2Nb3O10 using the electrostatic attraction between a cationic metal complex and a negatively charged Ca2Nb3O10– sheet, without the aid of any additional reagent. The material obtained exhibited 8 times greater photocatalytic activity for overall water splitting under band-gap irradiation than the previously reported analog using a RuO2 promoter. This study highlighted the superior functionality of < 1 nm Pt nanoclusters for photocatalytic overall water splitting
3:30 PM - NT2.1.04
Hierarchically Nanostructured Core-Shell MoO2-MoS2 Nanofibers and Their Application toward Hydrogen Evolution
Yosep Han 1,Navaneet Ramabadran 2,Youngin Lee 3,Kyu Hwan Lee 4,Seil Kim 5,Yong-Ho Choa 5,Youngwoo Rheem 2,Nosang Myung 2
1 Chemical and Environmental Engineering University of California-Riverside Riverside United States,2 Materials Science and Engineering Program University of California-Riverside Riverside United States3 Department of Materials Science and Engineering Seoul Korea (the Republic of)4 Korea Institute of Materials Science Changwon Korea (the Republic of)5 Department of Fusion Chemical Engineering Hanyang University Gyeonggi-do Korea (the Republic of)1 Chemical and Environmental Engineering University of California-Riverside Riverside United States,2 Materials Science and Engineering Program University of California-Riverside Riverside United StatesShow Abstract
Currently, hydrocarbon reformation accounts for 96% of commercially available hydrogen, usually with high concentrations of carbon-rich molecules. This poses a problem for achieving carbon neutrality in the plethora of hydrogen-dependent processes. Transition metal chalcogenides (TMCs) (e.g., MoS2, WS2, and WSe2) have been investigated as earth abundant catalyst for hydrogen gas evolution to replace precious Pt-based catalyst.
In this work, hierarchically core-shell MoO2-MoS2 nanofibers with controlled morphology and composition were fabricated by combining various fabrication techniques including electrospinning, rapid thermal annealing, and CVD sulfurization to enhance HER properties. Design of Experiment (DOE) and dimensionless analysis will be utilized during electrospinning to optimize nanofiber diameter and morphology utilizing physical properties of both precursor solutions and electrospinning parameters. Several complementary techniques, including FE-SEM, HR-TEM, XRD, nitrogen physisorption isotherm and XPS were used to systemically evaluate the material and physical properties of the nanofibers. Lastly, the HER experiments were conducted und