Ag2ZnSnS4 Nanocrystals Expand the Availability of RoHS Compliant Colloidal Quantum Dots

The demonstration of the quantum confinement effect in colloidal quantum dots (QDs) has been extensively studied and exploited mainly in Pb and Cd chalcogenide systems. There has been an urgent need recently for the development of non(less)-toxic colloidal QDs to warrant compliance with current safety regulations (Restriction of Hazardous Substances (RoHS) Directive 2002/95/EC). Herein, we report Pb/Cd-free, solution processed luminescent Ag2ZnSnS4 (AZTS) colloidal QDs. We present a selective and controlled amine and thiol-free synthesis of air stable luminescent AZTS QDs by the hot injection technique. By controlling the reaction conditions we obtain controlled size variation and demonstrate the quantum confinement effect that is in good agreement with the theoretically calculated values. The band gap of the AZTS QDs is size-tunable in the near-infrared from 740 to 850 nm. Finally, we passivate the surface with Zn-oleate, which yields higher quantum yield (QY), longer lifetime, and better colloidal stability.Peer ReviewedPostprint (published version

Engineering Vacancies in Bi2S3 yields sub-Bandgap Photoresponse and highly sensitive Short-Wave Infrared Photodetectors

Defects play an important role in tailoring the optoelectronic properties of materials. Here we demonstrate that sulphur vacancies are able to engineer sub-band photoresponse into the short-wave infrared range due to formation of in-gap states in Bi2S3 single crystals supported by density functional (DF) calculations. Sulfurization and subsequent refill of the vacancies results in faster response but limits the spectral range to the near infrared as determined by the bandgap of Bi2S3. A facile chemical treatment is then explored to accelerate the speed of sulphur deficient (SD)-based detectors on the order of 10 ms without sacrificing its spectral coverage into the infrared, while holding a high D* close to 10^15 Jones in the visible-near infrared range and 10^12 Jones at 1.6 um. This work also provides new insights into the role sulphur vacancies play on the electronic structure and, as a result, into sub-bandgap photoresponse enabling ultrasensitive, fast and broadband photodetectors

Towards stable single-atom catalysts: Strong binding of atomically dispersed transition metals on the surface of nanostructured ceria

The interaction of a series of different transition metal atoms with nanoparticulate CeO2 has been studied by means of density-functional calculations. Recently, we demonstrated the ability of sites exposed on {100} nanofacets of CeO2 to very strongly anchor atomic Pt, making the formed species exceptionally efficient single-atom anode catalysts for proton-exchange membrane fuel cells. Herein, we analyzed the capacity of these surface sites to accommodate all other group VIII-XI transition metal atoms M = Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Cu, Ag, and Au. The interaction of the M atoms with {100} nanofacets of ceria leads to oxidation of the former and such interaction is calculated to be stronger than the binding of the atoms in the corresponding metal nanoparticles. Comparing the stability of metal-metal and metal-oxide bonds allows one to establish which metals would more strongly resist agglomeration and hence allows the proposal of promising candidates for the design of single-atom catalysts. Indeed, the remarkable stability of these adsorption complexes (particularly for Pt, Pd, Ni, Fe, Co, and Os) strongly suggests that atomically dispersed transition metals anchored as cations on {100} facets of nanostructured ceria are stable against agglomeration into metal particles. Therefore, these sites appear to be of immediate relevance to the preparation of stable catalysts featuring the highest possible metal efficiency in nanocatalysis

Single-Exciton Gain and Stimulated Emission Across the Infrared Optical Telecom Band from Robust Heavily-doped PbS Colloidal Quantum Dots

Materials with optical gain in the infrared are of paramount importance for optical communications, medical diagnostics1 and silicon photonics2,3 . The current technology is based either on costly III-V semiconductors that are not monolithic to silicon CMOS technology or Er-doped fiber technology that does not make use of the full fiber transparency window. Colloidal quantum dots (CQD) offer a unique opportunity as an optical gain medium4 in view of their tunable bandgap, solution processability and CMOS compatibility. Their potential for narrower linewidths5 and the lower-than-bulk degeneracy6 has led to dramatic progress towards successful demonstration of optical gain4, stimulated emission7 and lasing8,9,10 in the visible part of spectrum utilizing CdSe-based CQDs. Infrared Pb-chalcogenide colloidal quantum dots however exhibit higher state degeneracy and as a result the demonstration of optical gain has imposed very high thresholds.11,12 Here we demonstrate room-temperature, infrared stimulated emission, tunable across the optical communication band, based on robust electronically doped PbS CQDs, that reach gain threshold at the single exciton regime, representing a four-fold reduction from the theoretical limit of an eight-fold degenerate system and two orders of magnitude lower than prior reports

CO oxidation activity of Pt/CeO2 catalysts below 0ºC: Platinum loading effects

Reducing the operating temperature of oxidation catalysts is important for designing energy efficient processes, extending catalyst lifetime, and abating pollutants, especially in cold climates. Herein, high CO oxidation activity at sub-ambient temperatures is reported for Pt/CeO2 catalysts with high content of Pt in the form of dispersed Pt2+ and Pt4+ centers. Whereas the reference 1 wt%Pt catalyst was active for CO oxidation only above 100ᵒC, the 8 and 20 wt%Pt catalysts converted 60 and 90 % of CO, respectively, below 0ᵒC. Ionic platinum was shown to facilitate oxygen release from ceria and lower the light-off temperature of the reaction occurring through the Mars-van-Krevelen mechanism. However, the remarkable activity observed at sub-ambient temperatures for the ≥8 wt%Pt catalysts is proposed to involve O2 and CO reactants weakly adsorbed on PtOx clusters. The synergies between ionic platinum and nanostructured ceria reported in this work advance the knowledge-driven design of catalysts for low-temperature oxidation reactions

Reactivity of atomically dispersed Pt2+ species towards H2: model Pt–CeO2 fuel cell catalyst

The reactivity of atomically dispersed Pt2+ species on the surface of nanostructured CeO2 films and the mechanism of H2 activation on these sites have been investigated by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy in combination with density functional calculations. Isolated Pt2+ sites are found to be inactive towards H2 dissociation due to high activation energy required for H–H bond scission. Trace amounts of metallic Pt are necessary to initiate H2 dissociation on Pt–CeO2 films. H2 dissociation triggers the reduction of Ce4+ cations which, in turn, is coupled with the reduction of Pt2+ species. The mechanism of Pt2+ reduction involves reverse oxygen spillover and formation of oxygen vacancies on Pt–CeO2 films. Our calculations suggest the existence of a threshold concentration of oxygen vacancies associated with the onset of Pt2+ reduction

Density functional modelling of materials for fuel cell catalysts with reduced content of critical components

[eng] The thesis entiteled “Density Functional Modelling of Materials for Fuel Cell Catalysts with Reduced Content of Critical Components” deals with the following studies: • Adsorption sites at the {100} nanofacets of ceria nanoparticles can effectively anchor a wide range of transition metal atoms in the form of Mn+ cations. Oxidation of the M centers takes place with the concomitant reduction of n Ce4+ cations to Ce3+. Higher oxidation states are favored by transition metals in later periods and in groups more to the left side of the periodic table. The preferred coordination mode of Mn+ cations at the {100} facets depends on the metal and on its oxidation state. Adsorption in each studied M1-ceria system is stronger than the binding of the corresponding M atom in a representative M79 particle. • Ceria nanoparticles show the ability to accommodate atomic Pt, Pd, Ni and Cu dopants more easily in surface positions than in bulk positions. Of the surface positions, under-coordinated corner ones are most prone to stabilizing the studied atomically dispersed transition metals. Upon partial reduction of the doped ceria nanoparticles, either via oxygen vacancy formation or homolytic dissociative adsorption of H2, surface Pt, Pd and Ni dopants feature +2 oxidation states, binding to four O atoms in square-planar fashion. Pt and Pd dopants inside ceria particles can be stabilized in the +4 state. In turn, Cu exhibits +2 or +3 state depending on its location in the ceria particle. • Pt dopants in bulk ceria feature +4 oxidation state and inherent octahedral coordination by six O atoms, leaving two more distant O atoms less-coordinated. Upon formation of a nearby oxygen vacancy bulk Pt4+ is reduced to Pt2+ and modifies its environment through a strong lattice distortion to become square- planar coordinated. Nanostructuring of Pt-doped ceria makes the +4 state of Pt energetically favorable up to two oxygen vacancies formed nearby. Formation of the third vacancy destroys the octahedral environment of Pt4+ and results in Pt2+ in the specific square-planar coordination. Nanostructuring of Pt-doped ceria also facilitates the formation of oxygen vacancies close to the dopant. Such improved oxygen storage capacity is related to the presence of surface Ce atoms accepting electrons of released O atoms more favorably than bulk Ce4+ species. • Atomic Pt2+ species adsorbed on {100} nanofacets of ceria are found to be resistant to reduction upon the formation of oxygen vacancies, increasing loading of the doping noble metal and deposition of Sn atoms. The onset of Pt2+ reduction to Pt0 is determined by the concentration of Ce3+ cations in the nanoparticle. To start the reduction, adsorption energy of the Pt2+ species needs to fall to around or below the cohesive energy of Pt metal. It is estimated to take place after formation of two oxygen vacancies per Pt adsorbate or adsorption in the vicinity of Pt2+ species of approximately three Sn atoms oxidized to Sn2+. • Our study addressing the usage of CO probe molecule for exploring Pt-CeO2 electrocatalysts for methanol oxidation revealed that the stretching frequency of on-top CO adsorbed on supported Pt particles correlates with the coordination number of the underlying Pt atom and reflects the particle size. Comparison with experimental results suggests that sub-nanometer particles of ca. 30 or fewer Pt atoms are formed at the applied electrochemical conditions. • Overall, the studies outlined in the thesis demonstrated new advantages of using dedicated nanoparticle models together with density-functional calculations to describe ceria-based nanomaterials for catalysis and related applications. This approach used in combination with experimental studies has been shown particularly successful for systems with properties strongly affected by their nanostructure and thus hardly accessible to conventional slab models.[cat] A la tesi doctoral que porta per títol: “Density Functional Modelling of Materials for Fuel Cell Catalysts with Reduced Content of Critical Components” s’han estudiat diversos temes relacionats amb el disseny se materials catalítics per la seva aplicació en catalitzadors. En primer lloc, s’ha realitzar un estudi sobre la capacitat inherent de l’òxid de ceri per dispersar atòmicament metalls de transició a llocs específics de la seva superfície per tal de maximitzar l’eficiència catalítica de la concentració de metall. S’ha comprovat que, les cares {100} presents a nanopartícules d’òxid de ceri són capaces d’adsorbir totes les espècies metàl·liques adsorbides, sent aquesta adsorció més forta que l’energia de formació d’agregats metàl·lics. S’han analitzat els possibles processos de reducció que es donen a terme per tal d’estabilitzar espècies metàl·liques tant a l’interior com a la superfície de l’òxid de ceri. S’ha trobant que aquestes espècies s’estabilitzen mitjançant processos del tipus formació de vacants d’oxigen i per dissociació de molècules d’hidrogen, donant lloc a espècies catiòniques metàl·liques. S’ha intentat entendre com afecta la presència de cations Pt4+ i Pt2+ a la capacitat d’emmagatzematge d’oxigen de l’òxid de ceri, així com quin és l’efecte de la nanoestructuració tant en la estabilitat de les espècies catiòniques de Pt com en la facilitat per formar vacants d’oxigen. A més, s’han estudiat diverses maneres de desestabilitzar les espècies catiòniques de Pt2+ per tal de formar agregats de Pt metàl·lic de mida sub-nanomètrica pel seu us en catalitzadors. Factors com la formació d’un divers nombre de vacants d’oxigen, la concentració de metall i l’adsorció d’altres espècies metàl·liques com poden ser àtoms de Sn contribueixen a la desestabilització dels cations Pt2+, donant lloc a la formació de petits agregats metàl·lics. Finalment, s’han caracteritzar films de Pt-CeO2 per mitjà del canvi en les freqüències de vibració de la molècula de CO, intentant entendre els possibles factors que propicien aquets canvis

Low-Cost RoHS Compliant Solution Processed Photovoltaics Enabled by Ambient Condition Synthesis of AgBiS2 Nanocrystals

Two major challenges exist before colloidal nanocrystal solar cells can take their place in the market: So far, these devices are based on Pb/Cd-containing nanocrystals, and second, the synthesis of these nanocrystals takes place in an inert atmosphere at elevated temperatures due to the use of air-sensitive chemicals. In this report, a room-temperature, ambient-air synthesis for nontoxic AgBiS2 nanocrystals is presented. As this method utilizes stable precursors, the need for the use of a protective environment is eliminated, enabling the large-scale production of AgBiS2 nanocrystals. The production cost of AgBiS2 NCs at room temperature and under ambient conditions reduces by ∼60% compared to prior reports based on hot injection, and the solar cells made of these nanocrystals yield a promising power conversion efficiency (PCE) of 5.5%, the highest reported to date for a colloidal nanocrystal material free of Pb or Cd synthesized at room temperature and under ambient conditions.Peer ReviewedPostprint (published version

Towards stable single-atom catalysts: Strong binding of atomically dispersed transition metals on the surface of nanostructured ceria

The interaction of a series of different transition metal atoms with nanoparticulate CeO2 has been studied by means of density-functional calculations. Recently, we demonstrated the ability of sites exposed on {100} nanofacets of CeO2 to very strongly anchor atomic Pt, making the formed species exceptionally efficient single-atom anode catalysts for proton-exchange membrane fuel cells. Herein, we analyzed the capacity of these surface sites to accommodate all other group VIII-XI transition metal atoms M = Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Cu, Ag, and Au. The interaction of the M atoms with {100} nanofacets of ceria leads to oxidation of the former and such interaction is calculated to be stronger than the binding of the atoms in the corresponding metal nanoparticles. Comparing the stability of metal-metal and metal-oxide bonds allows one to establish which metals would more strongly resist agglomeration and hence allows the proposal of promising candidates for the design of single-atom catalysts. Indeed, the remarkable stability of these adsorption complexes (particularly for Pt, Pd, Ni, Fe, Co, and Os) strongly suggests that atomically dispersed transition metals anchored as cations on {100} facets of nanostructured ceria are stable against agglomeration into metal particles. Therefore, these sites appear to be of immediate relevance to the preparation of stable catalysts featuring the highest possible metal efficiency in nanocatalysis

Engineering vacancies in Bi2S3 yielding sub-bandgap photoresponse and highly sensitive short-wave infrared photodetectors

Defects play an important role in tailoring the optoelectronic properties of materials. Here we demonstrate that sulphur vacancies are able to engineer sub-band photoresponse into the short-wave infrared range due to formation of in-gap states in Bi2S3 single crystals supported by density functional (DF) calculations. Sulfurization and subsequent refill of the vacancies results in faster response but limits the spectral range to the near infrared as determined by the bandgap of Bi2S3. A facile chemical treatment is then explored to accelerate the speed of sulphur deficient (SD)-based detectors on the order of 10 ms without sacrificing its spectral coverage into the infrared, while holding a high D* close to 10^15 Jones in the visible-near infrared range and 10^12 Jones at 1.6 um. This work also provides new insights into the role sulphur vacancies play on the electronic structure and, as a result, into sub-bandgap photoresponse enabling ultrasensitive, fast and broadband photodetectors.The authors acknowledge financial support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 725165), the Spanish Ministry of Economy and Competitiveness (MINECO), and the “Fondo Europeo de Desarrollo Regional” (FEDER) through grant TEC2017-88655-R. The authors also acknowledge financial support from Fundacio Privada Cellex, the program CERCA and from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0522). Y.Y. acknowledges support from the National Natural Science Foundation of China under grant no. 61805045. S.C. acknowledges support from Marie Curie Standard European Fellowship (“NAROBAND”, H2020-MSCA-IF-2016-750600). Furthermore, the research leading to these results has received funding from the European Union H2020 Programme under grant agreement no. 785219 Graphene FlagshipPeer reviewe