Perovskite quantum dots (QDs) as a new type of colloidal nanocrystals (NCs) have gained significant attention for both fundamental research and applications of optoelectronic devices owing to their appealing optoelectronic properties and excellent chemical processability. For their wide range of potential applications, synthesizing colloidal QDs with high crystal quality and stability is of crucial importance. However, like most common QD systems, those reported perovskite QDs still suffer from a certain density of trapping defects, giving rise to detrimental non-radiative recombination centers and thus quenching luminescence. Very recently, we have suceeded in synthesis of phase stable and less defect preovksite QDs, including APbX3 NCs (A: FA, MA, Cs; X: I, Br, Cl) and Sn-Pb alloyed NCs [1-5]. We have demonstrated that a high room-temperature photoluminescence quantum yield (PL QY) of close to 100% can be obtained in the APbX3 perovskite QDs, signifying the achievement of ignorable less trapping defects in the APbX3 QDs. Ultrafast kinetic analysis with time-resolved transient absorption spectroscopy evidences the negligible electron or hole trapping pathways in our QDs, which explains such a high quantum efficiency. In addtion, photoexcited hot and cold carrier dynamics as well as charge transfer at the heterojunction of QD/metal oxide were systematically investigated [4]. Solar cells based on these high-quality perovskite QDs exhibit power conversion efficiency of over 12%, showing great promise for practical application. On the other hand, through incorporation of alkali ion Na+, we have realized for the first time efficient near-infrared emission from highly defective Sn-Pb perovskite QDs with substantially improved PL QY from ~0.3% to 28% [5]. Our findings provide new insights into the materials design strategies for improved optoelectronic properties of Sn-containing perovskites. We anticipate their use in near-infrared devices is very promising if issues of the sustainability of PL QY can be fully addressed in the near future.   

Over the past years we have developed various approaches to fabricate materials with sophisticated chemical and structural complexity. We have focused on synthesis in non-aqueous solution since this approach is not limited to one particular class of materials. Thus, it gives us flexibility to tailor the composition and properties of materials in respect to the application, for examples carbon dioxide sensors and photo-electrochemical devices.

In this talk, I will present how X-ray synchrotron methods, far from merely providing new tools, are extending the ways we study, understand and design such complex structures. A combination of spectroscopic, scattering and microscopic X-ray methods and rapid data acquisition help to uncover the complex chemical world behind the synthesis of functional materials. It gives complementary information about chemical reaction in solution and nucleation, growth and crystal phase transition of nanoparticles. [1-4] Moreover, on the selected examples, I will discuss how the possibility to select with high-energy resolution the incident and emission hard X-ray energies offers unprecedented site selectivity and give access to determine structure – function relationship of photo-, electroactive materials and devices. [5-6] Finally, will discuss the advantages and the pitfalls of the synchrotron methods for in situ and operando studies.

Solar-assisted carbon dioxide reduction reaction (CO2RR) offers the possibility to use atmospheric CO2
and sunlight to produce highly-valuable hydrocarbons such as methane, ethylene, and ethanol. So far, the
majority of the CO2RR investigations were carried out in water environments. Although this approach is
environmental-friendly and constitutes a promising strategy for future implementation with solar electro-
lyzers, three main drawbacks exist: the limited solubility of CO2 in water (~ 35 mmol/L at room tempera-
ture) (i), the competitive hydrogen evolution reaction (HER) occurring when performing CO2RR in water
that limits the overall Faradaic efficiency (ii), and the difficulty to use molecular homogeneous (pho-
to)catalysts that boost the unless poor selectivity of the reaction (iii). Therefore, the mid-term strategy is
to develop a double-compartment photoelectrochemical device where the anolyte and catholyte sides are
separated by polymeric membranes. Here, solar-driven oxygen evolution reaction (OER) occurs at the
water-based anolyte side, while CO2RR undergoes at the non-aqueous catholyte side of the cell. In this talk, we will present our recent in situ hard X-ray ambient pressure photoelectron spectroscopy
(AP-HAXPES) investigations of electrified solid/liquid interfaces. The experiments were performed at the
new SpAnTeX (Spectroscopic Analysis with Tender X-rays) end-station operating at the BESSY II syn-
chrotron facility. The seminar will focus on three activities performed at the SpAnTeX end-station:
• First, we will show how the chemistry of BiVO4 photoanode surfaces can be tuned when the sem-
iconducting material is in contact with an aqueous phosphate buffer solution;
• Second, we will present the results obtained on a gold polycrystalline surface/N,N-
dimethylformamide interface, as a function of the applied overpotential. At such an interface, io-
dine electrooxidation and CO2RR were studied under in situ conditions;
• Third, we will show some recent results obtained on newly-developed in situ cell for multimodal
in situ investigations, equipped with a gas diffusion layer and a polymeric membrane. We will conclude this contribution by discussing about future perspectives and technical implementations
for multimodal in situ investigations of (photo)electrocatalytic processes.

While the performance of catalysts for electrochemical CO₂ reduction reaction (CO₂RR) strongly depends on the nature of electrode-electrolyte interfaces, it remains a key challenge to spatially resolve interfacial physical-chemical properties under liquid phase reaction conditions. In this talk, recent advances in electrochemical atomic force microscopy (EC-AFM) to decipher in situ an electrocatalyst’s morphology, structure and  reaction-rate determining local charge transfer variations will be discussed. Selected results include deciphering the potential-dependent morphology of copper-based electrodes during CO₂RR. Distinct surface morphologies are revealed for a copper(100) electrode over wide potential ranges upon electrode exposure to electrolytes and during CO₂RR, which can be understood by surface mobility and specific adsorption. Decisively different behavior of copper electrocatalysts in the presence of halides compared to plain electrolytes is observed. Conductive AFM will be demonstrated as a correlative tool for revealing simultaneously the nanoscale electric current, morphology and friction force of electrocatalysts in relevant aqueous electrolytes. The broader relevance of implemented in situ EC-AFM methods will be discussed in the context of establishing reliable structure-property relationships for the rational design of advanced catalysts and electrochemical interfaces.

Hydrogen production through water splitting has attracted great interest in recent years due to its potential of generating energy without causing pollution. One of the best candidates for water splitting is nickel oxyhydroxide with iron content (Ni 1− x FexOOH) that has excellent efficiency at alkaline conditions and is now studied widely. But pure NiOOH has poor efficiency unless doped with Fe and Co. We explore the influence of co-doping on the efficiency of NiOOH in the process of water oxidation by using Density Functional Theory +U (DFT+U). We also test the effect of strain on catalytic efficiency by modeling water oxidation on expanded and contracted surfaces of NiFeCoOOH. We find that several doping locations of Co have a similar result as if NiFeOOH had no Co content. Iron is responsible for the high activity at the Fe active site due to the low energy required for charge extraction. Yet, the valence band edge includes Fe, Co and Ni states hybridized which allows better charge extraction during deprotonation. The valence band edge position is higher upon Co-doping, which should allow better hole transport toward the surface. Hence, the presence of Ni and Co atoms surrounding the active site is vital for better efficiency. Moreover, we found that applying strain does not improve the efficiency and therefore a substrate with minimal mismatch should be used for NiFeCoOOH electrocatalysis.

Machine learning accelerated catalyst discovery efforts have seen much progress in the last few years. Datasets of computational calculations have improved, models to connect surface structure with electronic structure or adsorption energies have gotten more sophisticated, and active learning exploration strategies are becoming routine in discovery efforts. However, there are several large challenges that remain: to date, models have had trouble generalizing to new materials or reaction intermediates and applying these methods requires significant training. I will briefly introduce the Open Catalyst Project and the Open Catalyst 2020 dataset, a collaborative project to span surface composition, structure, and chemistry and enable a new generation of deep machine learning models for catalysis. I will then discuss initial results for state-of-the-art deep graph convolutional models and significant recent progress from others in the community, many of which are likely to improve models in related materials science areas. 

Transition metal oxides are promising photoelectrode materials for solar-to-fuel conversion applications. However, their performance is limited by the low carrier mobility (especially electron mobility) due to the formation of small polarons. Recent experimental studies have shown improved carrier mobility and conductivity by atomic doping; however the underlying mechanism is not understood. A fundamental atomistic-level understanding of the effects on small polaron transport is critical to future material design with high conductivity. In this talk, we will discuss the effect of small polaron formation on optical and carrier transport properties of transition metal oxides from first-principles calculations.

First, we resolve the conflicting findings that have been reported on the optical gap of a well-known catalysis Co₃O₄ as an example[1]. We confirm that the formation of small hole polarons significantly influences the optical absorption spectra and introduces extra spectroscopic signature below the intrinsic band gap, leading to a 0.8 eV transition that is often misinterpreted as the band edge that defines the fundamental gap.

Then we discuss the formation of small polarons' effect on carrier concentration, by resolving the controversy of nature of "shallow" or "deep" impurities of intrinsic oxygen vacancies in BiVO₄ as an example[2], i.e. how to unify different experiments with the correct definition of ionization energy in polaronic oxides. We further discuss why certain dopants can have very low optimal concentrations (or very early doping bottleneck) in polaronic oxides such as Fe₂O₃, through a novel "electric-multipole" clustering between dopants and polarons[3]. These multipoles can be very stable at room temperature and are difficult to fully ionize compared to separate dopants, and thus they are detrimental to carrier concentration improvement. This allows us to uncover mysteries of the doping bottleneck in hematite and provide guidance for optimizing doping and carrier conductivity in polaronic oxides toward highly efficient energy conversion applications. In addition, we show the importance of synthesis condition such as synthesis temperature and oxygen partial pressure on dopant and polaron concentrations, and how to optimize the synthesis condition based on theoretical predictions.

At the end, we show different theoretical models for polaron mobility calculations from a macroscopic dielectric continuum picture with an example of spin polarons in CuO[4] and a microscopic polaron hopping picture by combining generalized Landau-Zener theory and kinetic Monte-Carlo samplings for doped oxides[5].

Our first-principles calculations provide important insights and suggest design principles for optimal optical and transport properties of polaronic oxides.

Interfaces play an essential role in nearly all aspects of life and are critical for electrochemistry. Electrochemical systems ranging from high-temperature solid oxide fuel cells (SOFC) to batteries to capacitors have a wide range of important interfaces between solids, liquids, and gases which play a pivotal role in how energy is stored, transferred, and/or converted. This talk will focus on our use of ambient pressure XPS (APXPS) to directly probe the solid/gas and solid/liquid electrochemical interface. APXPS is a photon-in/electron-out process that can provide both atomic concentration and chemical-specific information at pressures greater than 20 Torr. Using synchrotron X-rays at Lawrence Berkeley Nation Laboratory, the Advanced Light Source has several beamlines dedicated to APXPS endstations that are outfitted with various in situ/operando features such as heating to temperatures > 500 °C, pressures greater than 20 Torr to support solid/liquid experiments and electrical leads to support applying electrical potentials support the ability to collect XPS data of actual electrochemical devices while it's operating in near ambient pressures. This talk will introduce APXPS and provide several solid/gas and solid/liquid interface electrochemistry examples using in situ and operando APXPS including the probing a Pt metal electrode undergoing a water-splitting reaction to generate oxygen[1], utilization of theory and experiment to understand CO₂’s interaction with Cu and Ag metal surfaces [2], [3], [4], and the ability to probe the electrochemical double layer (EDL) [5]. Gaining new insight to guide the design and control of future electrochemical interfaces.

The properties of emerging non-precious metal (oxy)hydroxide electrocatalysts for the oxygen evolution reaction (OER) evolve dynamically with voltage during operation. Understanding how material properties govern the kinetics of electron and ion transfer during the OER requires integrating electrochemical cells into advanced characterization methods to study reactivity during operation. Further, the sub-particle spatial heterogeneity of electrocatalysts during operation requires that these operando methods must be able to image the local chemical, physical, and electronic structure at the nanoscale and link these properties to the local electrochemical activity.

In this talk, I will describe a suite of correlative operando microscopy techniques we have developed to study the OER on faceted Co (oxy)hydroxide nanoplatelets.^[1] Operando Scanning X-Ray Transmission Microscopy (STXM) and Electrochemical Atomic Force Microscopy (EC-AFM) are used to map the local Co oxidation state and particle morphology across voltages spanning the bulk ion insertion reactions and OER regimes. These results are correlated to local electrochemical activity maps obtained through Scanning Electrochemical Cell Microscopy (SECCM) providing insight into where and why oxygen is evolved on the Co (oxy)hydroxide platelets. These experimental material properties and local reactivity measurements are integrated into a computational model that combines thermodynamic adsorption energetics of OER surface intermediates and microkinetic modeling to derive the experimental electrochemical Tafel behavior through first principles.^[2] The results from our multi-modal correlative approach alter long standing assumptions about the OER-active properties of the Co (oxy)hydroxide system and provide an improved methodology towards developing predictive electrocatalytic theories based on material properties that govern reactivity away from open circuit conditions.

Some Cu(I)-based oxides with delafossite structure have attracted attention as promising photocathode materials for photoelectrochemical water splitting due to their intrinsic p-type conductivity and good stability against photo-corrosion.  Beside CuFeO₂, which has been intensely studied and found to have some limitations, CuRhO2 is also of interest for its considerable stability and favorable energy gap.  To obtain insight into the fundamental bulk and surface properties of CuRhO2 that are relevant to such application, we have studied the bulk and majority (001) surface of CuRhO₂ in vacuum and electrochemical environment using first principles calculations.  We examine the bulk electronic structure and intrinsic defects, the surface stability diagram and band alignment at the oxide-water interface, and the energetics of the hydrogen evolution reaction at the interface. Based on these results, a comparative analysis of the properties of CuRhO₂ and CuFeO₂ is presented.

Polymer blends, which harness desirable properties from two or more complementary materials, have numerous applications in aerospace, pharmaceuticals, medical devices, electronics, and clean energy. In this work we investigate how polymer-polymer and polymer-solvent interactions affect the phase morphology and performance of polymer composites comprised of a majority non-conjugated polymer matrix component and a dispersed minority conjugated (semi-conductive) polymer additive. These blends are of interest for numerous organic electronic device applications as the matrix component can add mechanical durability, environmental stability and, in some cases, ionic conductivity. Meanwhile the minority conjugated component has been shown to retain optoelectronic performance at loading fractions as low as 1-10 wt%. Small angle and ultra-small angle neutron and X-ray scattering are particularly useful to characterize these composite blends and the self-assembled conjugated polymer components over length scales spanning from micrometers to fractions of a nanometer. We use small angle X-ray and neutron scattering and electrochemical analysis to develop a deep understanding of how blend morphology affects performance as a function of processing conditions and composition. This talk first provides a review of past work focused on blends of commodity homopolymers (i.e. polystyrene) and conjugated polymers (i.e. polythiophene derivatives). It will then cover more recent work focused on structured composite blends where the non-conjugated component is a block copolymer capable of self-assembly into mesoscale structures with long range structural order. These new structured composites include thermoplastic elastomers with high elasticity as well as water soluble block copolymers capable of forming templated hydrogel organic mixed ionic electronic conductors (OMIECs). In these cases, scattering experiments demonstrate that the block-copolymer templates can induce the organization of the conjugated polymer component over long length scales at significant loadings (up to 5wt%).

The emergence of electrically conductive metal-organic frameworks (MOFs) has been one of the most paradoxical developments in the field in the last few years. Indeed, how can one transport charges through a material that is mostly “empty” space? In this sense, MOFs made from layers of organic ligands connected by (typically) square-planar metal ions have shown particularly good electrical conductivity. However, a precise mechanism for charge transport is still the subject of debate, with various experimental and computational reports describing these materials as metals, semiconductors, semimetals, or even borderline insulators. Most of the discussion on this point has been focused on the effects of in-plane metal-ligand conjugation and the efficiency of in-plane transport. This lecture will describe the latest efforts from our group to understand the intrinsic properties of these materials, especially as related to single-crystal electrical measurement studies, and will discuss in particular the unexpectedly large influence of out-of-plane transport. Time allowing, I will discuss unexpected results stemming from the behavior of these materials as 1D metals, and applications in energy storage and conversion.