Effects of surface stability on the morphological transformation of metals and metal oxides as investigated by first-principles calculations

Morphology is a key property of materials. Owing to their precise structure and morphology, crystals and nanocrystals provide excellent model systems for joint experimental and theoretical investigations into surface-related properties. Faceted polyhedral crystals and nanocrystals expose well-defined crystallographic planes depending on the synthesis method, which allow for thoughtful investigations into structure-reactivity relationships under practical conditions. This feature article introduces recent work, based on the combined use of experimental findings and first-principles calculations, to provide deeper knowledge of the electronic, structural, and energetic properties controlling the morphology and the transformation mechanisms of different metals and metal oxides: Ag, anatase TiO2, BaZrO3, and α-Ag2WO4. According to the Wulff theorem, the equilibrium shapes of these systems are obtained from the values of their respective surface energies. These investigations are useful to gain further understanding of how to achieve morphological control of complex three-dimensional crystals by tuning the ratio of the surface energy values of the different facets. This strategy allows the prediction of possible morphologies for a crystal and/or nanocrystal by controlling the relative values of surface energies.The authors are grateful to FAPESP (2013/07296-2, 2012/ 14468-1, 2013/26671-9 and 2014/04350-9), CAPES (process A104/2013 and 99999.002998/2014-09), CNPq INCTMN 573636/2008-7, PrometeoII/2014/022 and ACOMP/2014/270 projects (Generalitat Valenciana), Ministerio de Economia y Competitividad (Spain), CTQ2012- 36253-C03-02 and the Spanish Brazilian program (PHB2009-0065-PC) for financially supporting this research. We also acknowledge the Servei Informática, Universitat Jaume I for a generous allotment of computer time

Theoretical and experimental analysis of semiconductors

The rapid modernization of industrial and technological poles spurs the development of new materials, always in search of efficiency and low costs, given that the expression "new materials" does not relate just to the new classes or compounds never seen before but also to the structural modifications in materials already studied. In this context, research using theoretical and experimental models tends Researches using combined theoretical and experimental methods tend to be more and more commonly due to the better correlation between system characteristics and properties. However, a better interpretation can assists in the planning and developing of new materials. In the present study, a set of theoretical and experimental models were built in order to explain the properties and phenomena of semiconductors. The chosen materials were the α-Ag2WO4, β-Ag2MoO4, Co3O4, α- Fe2O3 and In2O3. Characterization techniques of X-Ray difraction, Rietveld refinament, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy were related with quantum mechanical calculations to a better understanding of the observed results. Theoretical results of band structure and density of states are in agreement with the experimental band gaps.The data showed a creation of new eletronic states on the forbiden region on the band gap due to the criation of structural defects, by means of a impurity or structural changes. The final models were used to a better explanation of the photoluminescent properties modifications. Quantum mechanical calculations were also used to the explanation of a phenomenon caused by electron bombing, which was observed in Ag2WO4 and Ag2MoO4 crystals. In this model, the Ag reduction results in its removal of the clusters generating the Ag metallic filaments on the material surface. Finally, it was created a new approach about the mapping and prediction of crystals morphology that serves as an experimental reference to the comparison between surface and final properties. The presented papers consist in an interdisciplinary research, from fundamental to applications and show how quantum chemical and other theoretical computational means can be used for an understanding of physical and chemical properties of materials for searching a rational materials design.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)A rápida modernização dos polos industriais e tecnológicos faz com que o desenvolvimento de novos materiais seja fundamental, sempre pesquisando maiores eficiências e menores custos. Por outro lado, a expressão “novos materiais” não está somente relacionada à uma nova classe ou composto obtidos pela primeira vez, mas também às modificações estruturais de materiais já conhecidos, incluindo o efeito do tamanho, forma e incorporações sinergéticas. Neste contexto, pesquisas usando modelos teórico-experimentais tendem a ser cada vez mais comuns devido à melhor correlação entre características do sistema e propriedades. Por tanto, interpretações mais claras e precisas podem auxiliar no planejamento e desenvolvimento de novos materiais. No presente trabalho, um conjunto de modelos teórico-experimentais foi construído a fim de explicar propriedades e fenômenos de semicondutores. Os semicondutores escolhidos foram: α-Ag2WO4, β-Ag2MoO4, Co3O4, α-Fe2O3 e In2O3. Técnicas de caracterização de Difração de Raios-X, refinamento Rietveld, espectroscopia Raman, microscopia de varredura e microscopia de transmissão foram vinculadas com cálculos mecânico-quânticos para um melhor entendimento dos resultados observados. Resultados teóricos de estrutura de bandas e densidade de orbitais corroboraram com os “band gaps” experimentais. Os dados mostraram a criação de novos estados na região proibida do “band gap” devido à criação de defeitos, por meio de dopagem ou mudanças estruturais. Os modelos finais foram utilizados para melhor explicar as modificações das propriedades fotoluminescentes. Cálculos mecânico-quânticos também foram utilizados para explicar o fenômeno de crescimento de prata em cristais de α-Ag2WO4 e β-Ag2MoO4 com a adição de elétrons. Neste modelo, a redução da prata resulta em seu afastamento dos “clusters” gerando o aparecimento de filamentos de prata metálica na superfície dos materiais. Por fim, foi criada uma nova abordagem sobre o mapeamento e previsão de morfologias de cristais, servindo como um referencial experimental para a comparação entre superfícies e propriedades finais. Os trabalhos presentes consistem em estudos interdiciplinares, do fundamento até a aplicação de diversos materiais, e mostram como a química quântica e outros meios teóricos podem ser usados para o estendimento de propriedades para o “design” de novos materiais.2012/14468-

Estudo das propriedades fotoluminescentes do ZnS e ZnS:Eu obtidos pelo método solvotérmico assistido por microondas

This paper is about the synthesis, characterization and studies of nanostructures of ZnS and ZnS:Eu3+ prepared by microwave-assisted solvothermal method. The influence of the synthesis time in the microwave on the samples of ZnS was investigated in order to have the most proper sample for doping. Therefore, different concentrations of Europium (dopante) were added to the chosen sample. The characterizations of the samples was made by X-ray diffraction, Ultraviolet visible spectroscopy, scanning electron microscopy, transmission electron microscopy and photoluminescence spectroscopy. The method used to the synthesis of ZnS is very efficient because it synthesizes the nanocrystals in shor time and in a low temperature. The study of the X-ray diffraction, Ultraviolet-visible spectroscopy, photoluminescence and transmission electron microscopy showed structural changes in the samples due to the low processing time and the percentage of europium added to the system. The results indicated an increase of the organization with the increase of the time of synthesis. and a disorganization when the europium is added. These structural differences were responsible for the modifications of the photoluminescence profile of the materials due to the different intermediate states. More specifically, in the synthesis of pure ZnS, the organization of the structure with the increase of the time of synthesis was responsible for a photoluminescence band with a higher contribution of the blue region. However, the disorganization due to the addition of Eu3+ resulted in a photoluminescence band with a higher contribution in the orange region.Financiadora de Estudos e ProjetosEste trabalho ocupa-se da síntese, caracterização e estudos de nanoestruturas de ZnS e ZnS:Eu3+ sintetizadas por meio de processamento solvotérmico assistido por microondas. No trabalho foi investigada a influência do tempo de síntese no microondas nas amostras de ZnS. Entre os tempos, o de 16 minutos foi escolhido como padrão para a dopagem com Európio. As caracterizações das amostras foram realizadas por intermédio de difratometria de raios-X, espectroscopia de absorção na região do Ultravioleta-Visível, microscopia eletrônica de varredura, microscopia eletrônica de transmissão e espectroscopia de fotoluminescência. O método de síntese mostrou-se muito eficiente, pois obteve-se nanocristais de ZnS em tempos curtos e utilizando baixa temperatura. Os métodos de caracterização mostraram diferenças estruturais nas amostras em função do tempo de processamento no microondas e à porcentagem de Európio adicionada no sistema. Os resultados indicam um aumento da organização em amostras com maiores tempos de síntese e desorganização no ZnS quando o Európio é adicionado. Essas diferenças estruturais foram responsáveis pelas modificações dos perfis fotoluminescentes dos materiais devido aos diferentes estados intermediários gerados. Mais especificamente, na síntese de ZnS puro, a organização da estrutura com o aumento do tempo de síntese foi responsável por uma banda de fotoluminescência com maior contribuição da região do azul. Já a desorganização devido à adição de Eu3+ resultou em uma banda de fotoluminescência com maior contribuição da região do laranja

Optical and electrical features of calcium molybdate scheelite solar cells

Calcium molybdate (CMO) is a material used in several technological applications. In this work, we explored the correlation between the optical and electrical properties of CMO in solar cell photoanodes. Six samples were prepared by a microwave-assisted hydrothermal method with pH values of 4, 7, and 10 associated with temperatures of 100 °C and 140 °C. These samples were used as a replacement for titanium dioxide TiO 2 in Graetzel solar cells. A thin blocking layer (BL), a dense and translucent film, was deposited over a CMO layer using a doctor-blade method, to create a heterojunction. We show that a strict correlation between pH, temperature, processing time, and photovoltaic response exists in CMO scheelite and needs to be considered to achieve optimal photovoltaic behavior. Almost all samples achieved typical solar cell responses, except that synthesized with pH 4 at 100 °C, which shows an anomalous behavior. Among these samples, the one synthesized with pH 10 at 100 °C was identified as the most suitable candidate for down-converter materials in solar energy applications, due to its typical diode-like properties, with an upper J sc = 180 μA cm −2, V oc = 607 mV and FF = 0.45. Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.QN/Greplová La

Effects of chemical substitution on the structural and optical properties of α-Ag 2− 2x Ni x WO 4 (0≤ x≤ 0.08) solid solutions.

In this work, we investigated the effects of chemical substitution on the structural, electronic, and optical properties of α-Ag2−2xNixWO4 (0 ≤ x ≤ 0.08) solid solutions prepared by a facile microwave-assisted hydrothermal method. The results showed that the increase of Ni concentration in α-Ag2WO4 microcrystals as a host matrix caused a morphological transformation and a shift of the electronic and optical properties. Based on first principles calculations and using Wulff's construction, particle shapes and their transformations in α-Ag2WO4 and α-Ag2−2xNixWO4 can be affected by controlling the ratios of surface energy values between the different facets. In addition, theoretical calculations revealed that Ni substitution in α-Ag2WO4 is more favorable in the Ag2 and Ag4 positions, in which the local coordination of Ag atoms corresponds to clusters with coordination numbers of seven and four, respectively. This behavior could be related to the degree of medium-range structural disorder in α-Ag2−2xNixWO4 crystals. The experimental results were correlated with theoretical simulations to achieve a deeper understanding of the relationship between morphology and properties. These results provide the basis for a rational design for the compositional modulation of structural and optical properties.The authors acknowledge the financial support of the following Brazilian research funding institutions: CDMF 2008/57872-1, FAPESP 2015/11917-8; 2013/07296-2; 2013/23995-8; 2014/04350- 9; 2012/14468-1, CNPq 153299/2015-0; 147001/2013-7, INCTMN 573636/2008-7, CAPES and PNPD 1268069. J. A. and L. G. also acknowledge the financial support of the Spanish research funding projects: PrometeoII/2014/022 and ACOMP/2014/270 projects (Generalitat-Valenciana), Ministerio de Economia y Competitividad (CTQ2012-36253-C03-02) and Programa de Cooperacio´n Cientifı´ca con Iberoamerica (Brasil), of Ministerio de Educacio´n (PHBP149-00020). We also acknowledge the Servei Informa´tica, Universitat Jaume I for the generous allotment of computer time. We also wish to thank Ju´lio Sczancoski, Rorivaldo Camargo, Ana Lucia Oliveira, Maximo S. Li, and Madalena Tursi for their technical and scientific contributions