Hello, A new project on the LCRC cluster has been requested. Please forward the information on to the LCRC Allocation sub-committee. Applicant's name: Liang Li Applicant's institution: ANL Applicant's division: NST Project Name: CataAgCl Project title: First-principles Investigation of Redox Reactions of Ag/AgCl Heterogeneous Photocatalyst Associated funding: Integrated Imaging Strategic Initiative LDRD, DOE Other Systems: Internal: 500,000 core-hours, Carbon cluster at Center for Nanoscale Materials Science: Ag/AgCl heterogeneous plasmonic photocatalyst has attracted lots of research interests due to its unique structural and electronic characteristics that make it a promising candidate for utilization of solar energy in photocatalytic reactions. Upon photoexcitation of the catalyst surface, the generated electrons and holes act essentially as reaction agents to initiate the corresponding reduction or oxidation processes. However, these electron-hole pairs may easily recombine with each other radiatively and deteriorate the performance of the catalyst. Thus, the ability of spatially separating the generated charge carriers is crucial for rational photocatalyst design. One important feature of Ag/Agcl heterostructure is that a Schottky junction is naturally formed at the metallic Ag nanoparticle and semiconductor AgCl interface, which effectively suppress the recombination of photogenerated charge carriers. Recent studies have focused on the electron transfer mechanism between Ag and AgCl domains during the oxidation process. However, no direct observation has been made at atomic scale to understand the redox reaction of Ag/AgCl nanocatalyst from the structural dynamics point of view. By utilizing Transmission Electron Microscopy (TEM), our collaborators recently achieved a direct visualization of redox dynamics of a single Ag/AgCl nanocatalyst at atomic scale. It is found that the Ag/AgCl heterojunction is unstable upon exposure to the electron beam, and a two-step reaction process involving the reduction of AgCl to metallic Ag, which is then further oxidized to Ag2O, is observed. Herein, we propose a computational study, performed in conjunction with experimental work by collaborators, to systematically investigate the energetics and kinetics involved in the oxidation of Ag/AgCl heterostructure. The insights obtained from this work will be important to understand both the fund amental and practical properties of Ag/AgCl heterogeneous plasmonic photocatalyst. Project description: The proposed work involves Density Functional Theory (DFT) calculations to understand the driving force and reaction mechanism of the transformation of AgCl into Ag and then Ag2O. In particular, the goals of this work includes: (a) identifying the energetically favored defect types in AgCl structure under particular experimental conditions. Six types of point defects are identified in the bulk AgCl structure, and each defect can occur in multiple charge states.We will first perform systematic calculations to determine the formation energies for each relevant charge state and the stable charge state of a specific defect type is determined as the one with the lowest formation energy at the given experimental condition. To minimize the spurious electrostatic interactions between adjacent charged defects, a sufficiently large supercell is necessary in our DFT simulations. In this study, we use a 3x3x3 AgClsupercell, which consists of 216 atoms. The calculations of the formation energiesgo over 6 defect types with 5 different charge states (ranging from -2 to +2), and on average each of them requires 12 hours on 96 cores.Image charge correction and potential alignment are crucial in order to model a trul y isolated charged defect in a limited supercell, which cause additional calculation of at least 8 hours on 96 cores for each charged defect. In addition, 20,000 core hours are needed in order to calculate the structural and electronic properties of pristine AgCl, such as the dielectric constants and formation enthalpy. Therefore, (12*96+8*96)*6*5+20,000~78,000 core hours are requested. (b) understanding why the formation of Ag2O prefers a two-step process, instead of direct replacement of Cl by oxygen. The energy cost associated with oxygen substitution will be calculated for various substitution sites and the feasibility of direct oxygen replacement of Cl atoms will be discussed. In particular, the substitutional energy costs of oxygen atoms at 5 different concentrations will be calculated. To maintain the charge neutrality of the system, two Cl atoms in the original AgCl lattice are replaced by substitutional oxygen, and 10 different combinations of Cl atom positions will be tested in order to indentify themost favorable structure. The energy calculation requires high cut-off energy because of oxygen, and the relaxation time is long due to large lattice distortion.As a result, each calculation requires 32 hours on 96 cores, which results in 5*10*32*96=154,000 core hours. (c) kinetically modeling the ionic transportation occurs at the Ag/AgCl interface. The structural model of the experimentally observed Ag(111)/AgCl(100) interface will be constructed and the kinetic barriers for the ionic transportations both in bulk phaseand on the Ag(111) surface will be quantitatively calculate by Nudged Elastic Band (NEB) method. The ionic transportation behavior of all defect types will be studied, each requiring 20 hours on 96 cores. The construction of interface model requires at least 200 atoms, and there are 3 possible reaction paths for Ag atom diffusion in the interfacial area. 5 different sizes of interface model will be tested, to account for the effect of different concentrations of mobile atom. Each NEB calculation for the interface structure is expected to take 36 hours on 96 cores. In total, 20*96*6+36*96*5*3~63,000 core hours in requested. All calculations will be carried out by the plane wave code VASP. The PBE exchange correlation functional will be used. Industry partnership: Project URL: http://www.anl.gov/imaging/project/photo-integrated-imaging-understand-and-a... Requested allocation: 300000 Q1: 0 Q2: 0 Q3: 0 Q4: 300000 Justification: Storage requirements: The requester has used undetermined amount hours of their initial startup project. In addition to approving an initial amount, please specify a Category and Subcategory for this project. For a list of the current selection of approved categories, please see: https://wiki.lcrc.anl.gov/wiki/Processes/Categories Once the Allocation committee has approved the project, please go to the Project Management page to create it: https://accounts.lcrc.anl.gov/projects.php Thank You, The LCRC Accounts System