Bismuth, tungsten and silver oxides electrodes for the photocatalytic reduction of CO2

Orientador: Claudia LongoDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de QuímicaResumo: Eletrodos de filmes porosos de um óxido misto de bismuto, tungstênio e prata (ABW) foram utilizados como foto-catodo para redução de CO2. As amostras do ABW foram obtidas a partir da mistura de soluções aquosas de AgNO3 e Na2WO4 a uma solução de Bi5H9N4O22 dissolvido em ácido nítrico e etilenoglicol; a suspensão resultante foi colocada em autoclave e mantida a 200°C por 24 h. Após lavagem com água, as partículas de cor cinza obtidas foram misturadas a uma solução de polietilenoglicol (PEG), resultando em uma suspensão aquosa com quantidades relativas de PEG:ABW:H2O de 10:30:60 (em massa). Os eletrodos foram preparados pela deposição de um filme da suspensão de ABW sobre titânio metálico ou vidro revestido com SnO2:F, vidro-FTO, seguido por aquecimento (350 °C, 30 min e 450°C, 30 min). Este tratamento térmico garante a eliminação gradual do PEG, resultando em um filme poroso de partículas do óxido depositadas sobre o substrato. A caracterização de amostras em pó do ABW, obtidas a partir do tratamento térmico da suspensão, foi realizada por Difração de Raios-X e Reflectância Difusa na região do UV-Vis. Nos difratogramas, identificou-se AgBiW2O8 , além da presença das fases cristalinas de Bi2WO6 e prata metálica. O espectro de reflectância difusa indicou considerável absorbância em toda a região do visível, intensificada para lambda < 420 nm. A caracterização dos eletrodos incluiu análises por Microscopia Eletrônica de Varredura (MEV), que revelaram excelente cobertura do substrato por um filme poroso, constituído por partículas aglomeradas com diâmetro variando entre 50 e 100 nm. As propriedades eletroquímicas dos eletrodos (área ativa de 1,0 cm2) foram avaliadas em solução aquosa de Na2SO4 0,1 M como eletrólito de suporte, contendo ou não 20 mM de imidazol, agente que atua como catalisador no processo de redução eletroquímica de CO2. A comparação dos resultados obtidos na ausência de luz e sob irradiação proveniente de um simulador solar revelou que o eletrodo de ABW apresenta comportamento de semicondutor tipo-p; observou-se também maior corrente catódica para eletrodos irradiados em solução saturada com CO2. Os eletrodos foram então utilizados em processos de eletrólise sob irradiação (controle potenciostático), em meio aquoso e em líquidos iônicos saturados com CO2. Em meio aquoso, identificou-se a formação de metano como produto na fase gasosa. Em líquidos iônicos, observou-se uma diminuição significativa do sobrepotencial necessário para a redução do CO2. Em um sistema contendo 8 mL de líquido iônico, a polarização do eletrodo de ABW a -1,15 V (vs Ag/Ag+) durante 2 h resultou na formação de CO e ácido fórmico, que foi quantificado em 12,2 ?mol e 14,4 ?mol para sistemas na ausência de luz e sob irradiação, respectivamente. Os resultados obtidos revelam que o eletrodo de ABW é um material promissor para ser utilizado como foto-catodo para redução de CO2Abstract: Porous film electrodes based on bismuth, tungsten and silver mixed oxide (ABW) were used as photocathodes for the CO2 reduction. ABW samples were obtained from aqueous solutions of AgNO3 and Na2WO4 mixed with Bi5H9N4O22 solution in nitric acid and ethyleneglycol; the resulting suspension was transferred to an autoclave and kept at 200°C for 24 hours. After H2O washing, the gray¿coloured particles obtained were mixed with a PEG aqueous solution, resulting in a suspension with PEG:ABW:H2O mass ratio of 10:30:60. Electrodes were prepared by the deposition of ABW suspension film over metallic titanium or SnO2:F-coated glass, glass-FTO, followed by heating (350°C, 30min and 450°C, 30min). The thermal treatment allows the slow removal of PEG, resulting in a porous film of the oxide over the substrate. Characterisation of ABW powder samples, obtained by heat treatment, was performed by X-ray diffraction and UV-Vis diffuse reflectance. XRD analysis revealed AgBiW2O8, plus the presence of Bi2WO6 and metallic silver phasis. Diffuse reflectance spectrum indicates high absorbance values across the whole visible region, intensified for lambda?< 420 nm. Electrodes characterisation included Scanning Electron Microscopy (SEM), which revealed excelent coating of the substrate by a porous film, constituted of agglomerated particles with diameter in the 50 ¿ 100nm range. Electrochemical properties of the electrodes (active area of 1,0 cm²) were evaluated in 0,1M aqueous solution of Na2SO4 as supporting electrolyte, in the presence and absence of 20 mM imidazole, which acts as a catalyst in CO2 electrochemical reduction process. Comparison between the obtained results in the dark and under illumination with solar simulator revealed p-type semiconductor behavior for the ABW electrodes; also, increased cathodic current was observed at CO2-saturated solution. The electrodes were then utilised in electrolyses experiments under illumination (potentiostatic control), in aqueous medium and ionic liquids saturated with CO2. In aqueous medium, methane formation was identified as gas phase product. In ionic liquids, a great diminishment of the required overpotential for CO2 reduction was observed. In 8mL ionic liquid system, ABW electrode polarization at -1,15 V (vs Ag/Ag+) for 2 hours resulted in the formation of carbon monoxide and formic acid, the later being quantified: 12,2 and 14,4 umol for the dark and illuminated systems, respectively. The obtained results revealed that ABW electrodes are a promising material as a photocathode for the reduction of CO2MestradoFísico-QuímicaMestre em Química147153/2013-1CNP

Complex Oxides Based on Silver, Bismuth and Tungsten: Syntheses, Characterization and Photoelectrochemical Behavior

Silver-containing complex oxides present interesting light absorption and electronic properties pointing to their applicability for solar fuel generation and environmental remediation. In this vein, the syntheses, characterization, and photoelectrochemical properties of the Aurivillius oxide, AgBiW₂O₈, and a triphasic material, AgBiW₂O₈/Bi₂WO₆/Ag, are described herein. From UV–vis diffuse reflectance analysis, both oxides’ nanoparticles exhibited intense absorption at wavelengths <455 nm, with an additional band centered at ∼500 nm for the triphasic material, attributed to surface plasmon resonance of the metallic Ag phase. The triphasic material was further characterized by a variety of structural and spectroscopic probes, indicating a defect-rich crystalline structure. Voltammetric measurements were performed in the dark and under simulated solar irradiation with polarization at potentials more negative than the rest potential. Thin film electrodes of both materials showed p-type semiconductor behavior, with enhanced cathodic photocurrent in the presence of electron acceptors in aqueous media

White-light generation from all-solution-processed OLEDs using a benzothiazole–salophen derivative reactive to the ESIPT process

CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORA salicylidene derivative, N,N′-bis(salicylidene)-(2-(3′,4′-diaminophenyl)benzothiazole) (BTS), reactive in the Excited State Intramolecular Proton Transfer (ESIPT) process, was synthesized and its photophysical properties were evaluated, presenting an em21311721182CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORsem informação2013/16245-2sem informaçãoThe authors are thankful for financial support and scholarships from CNPq, FAPESP 2013/16245-2, FAEPEX and INEO. This research was supported in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001 an

Arc Synthesis, Crystal Structure, and Photoelectrochemistry of Copper(I) Tungstate

A little-studied p-type ternary oxide semiconductor, copper(I) tungstate (Cu2WO4), was assessed by a combined theoretical/experimental approach. A detailed computational study was performed to solve the long-standing debate on the space group of Cu2WO4, which was determined to be triclinic P1. Cu2WO4 was synthesized by a time-efficient, arc-melting method, and the crystalline reddish particulate product showed broad-band absorption in the UV–visible spectral region, thermal stability up to ∼260 °C, and cathodic photoelectrochemical activity. Controlled thermal oxidation of copper from the Cu(I) to Cu(II) oxidation state showed that the crystal lattice could accommodate Cu2+ cations up to ∼260 °C, beyond which the compound was converted to CuO and CuWO4. This process was monitored by powder X-ray diffraction and X-ray photoelectron spectroscopy. The electronic band structure of Cu2WO4 was contrasted with that of the Cu(II) counterpart, CuWO4 using spin-polarized density functional theory (DFT). Finally, the compound Cu2WO4 was determined to have a high-lying (negative potential) conduction band edge underlining its promise for driving energetic photoredox reactions

Arc Synthesis, Crystal Structure, and Photoelectrochemistry of Copper(I) Tungstate

A little-studied p-type ternary oxide semiconductor, copper(I) tungstate (Cu2WO4), was assessed by a combined theoretical/experimental approach. A detailed computational study was performed to solve the long-standing debate on the space group of Cu2WO4, which was determined to be triclinic P1. Cu2WO4 was synthesized by a time-efficient, arc-melting method, and the crystalline reddish particulate product showed broad-band absorption in the UV–visible spectral region, thermal stability up to ∼260 °C, and cathodic photoelectrochemical activity. Controlled thermal oxidation of copper from the Cu(I) to Cu(II) oxidation state showed that the crystal lattice could accommodate Cu2+ cations up to ∼260 °C, beyond which the compound was converted to CuO and CuWO4. This process was monitored by powder X-ray diffraction and X-ray photoelectron spectroscopy. The electronic band structure of Cu2WO4 was contrasted with that of the Cu(II) counterpart, CuWO4 using spin-polarized density functional theory (DFT). Finally, the compound Cu2WO4 was determined to have a high-lying (negative potential) conduction band edge underlining its promise for driving energetic photoredox reactions