Cobalt-containing layered or zeolitic silicates as photocatalysts for hydrogen generation

Layered magadiite and zeolites Y containing framework Co or small CoO clusters in the pores have been synthesized and tested as photocatalysts for water splitting, in the absence and presence of methanol, upon UV or simulated sunlight irradiation; the best performing material was Co-magadiite.This work was supported by the Spanish Ministry of Economy and Competitiveness (Severo Ochea and CTQ-2012-32315) and the Marie Curie project PIEF-GA-2011-298740, and the Generalidad Valenciana (Prometeo 2012/2013).Neatu, S.; Puche Panadero, M.; Fornes Seguí, V.; García Gómez, H. (2014). Cobalt-containing layered or zeolitic silicates as photocatalysts for hydrogen generation. Chemical Communications. 50(93):14643-14646. https://doi.org/10.1039/c4cc05931jS14643146465093Chen, X., Shen, S., Guo, L., & Mao, S. S. (2010). Semiconductor-based Photocatalytic Hydrogen Generation. Chemical Reviews, 110(11), 6503-6570. doi:10.1021/cr1001645Mallouk, T. E. (2010). The Emerging Technology of Solar Fuels. The Journal of Physical Chemistry Letters, 1(18), 2738-2739. doi:10.1021/jz101161sKudo, A., & Miseki, Y. (2009). Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev., 38(1), 253-278. doi:10.1039/b800489gLiao, L., Zhang, Q., Su, Z., Zhao, Z., Wang, Y., Li, Y., … Bao, J. (2013). Efficient solar water-splitting using a nanocrystalline CoO photocatalyst. Nature Nanotechnology, 9(1), 69-73. doi:10.1038/nnano.2013.272Asahi, R. (2001). Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides. Science, 293(5528), 269-271. doi:10.1126/science.1061051Khan, S. U. M. (2002). Efficient Photochemical Water Splitting by a Chemically Modified n-TiO2. Science, 297(5590), 2243-2245. doi:10.1126/science.1075035Chen, X., Liu, L., Yu, P. Y., & Mao, S. S. (2011). Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science, 331(6018), 746-750. doi:10.1126/science.1200448Garcés, J. M. (1988). Hypothetical Structures of Magadiite and Sodium Octosilicate and Structural Relationships Between the Layered Alkali Metal Silicates and the Mordenite- and Pentasil-Group Zeolites1. Clays and Clay Minerals, 36(5), 409-418. doi:10.1346/ccmn.1988.0360505Pinnavaia, T. J., Johnson, I. D., & Lipsicas, M. (1986). A 29Si MAS NMR study of tetrahedral site distributions in the layered silicic acid H+-magadiite (H2Si14O29 · nH2O) and in Na+-magadiite (Na2Si14O29 · nH2O). Journal of Solid State Chemistry, 63(1), 118-121. doi:10.1016/0022-4596(86)90159-3Barea, E. M., Fornés, V., Corma, A., Bourges, P., Guillon, E., & Puntes, V. F. (2004). A new synthetic route to produce metal zeolites with subnanometric magnetic clusters. Chem. Commun., (17), 1974-1975. doi:10.1039/b407225aR. M. Barrer , Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982Cundy, C. S., & Cox, P. A. (2003). The Hydrothermal Synthesis of Zeolites:  History and Development from the Earliest Days to the Present Time. Chemical Reviews, 103(3), 663-702. doi:10.1021/cr020060iShimizu, S., Kiyozumi, Y., Maeda, K., Mizukami, F., Pál-Borbély, G., Mihályi, R. M., & Beyer, H. K. (1996). Transformation of intercalated layered silicates to zeolites in the solid state. Advanced Materials, 8(9), 759-762. doi:10.1002/adma.19960080913Nigro, E., Testa, F., Aiello, R., Lentz, P., Fonseca, A., Oszko, A., … Nagy, J. B. (2001). Synthesis and characterization of Co-containing zeolites of MFI structure. Oxide-based Systems at the Crossroads of Chemistry - Second International Workshop October 8-11, 2000, Como, Italy, 353-360. doi:10.1016/s0167-2991(01)80164-6Verberckmoes, A. A., Uytterhoeven, M. G., & Schoonheydt, R. A. (1997). Framework and extra-framework Co2+ in CoAPO-5 by diffuse reflectance spectroscopy. Zeolites, 19(2-3), 180-189. doi:10.1016/s0144-2449(97)00068-7Verberckmoes, A. A., Weckhuysen, B. M., & Schoonheydt, R. A. (1998). Spectroscopy and coordination chemistry of cobalt in molecular sieves. Microporous and Mesoporous Materials, 22(1-3), 165-178. doi:10.1016/s1387-1811(98)00091-2Frost, D. C., McDowell, C. A., & Woolsey, I. S. (1974). X-ray photoelectron spectra of cobalt compounds. Molecular Physics, 27(6), 1473-1489. doi:10.1080/00268977400101251Weckhuysen, B. M., Rao, R. R., A. Martens, J., & Schoonheydt, R. A. (1999). Transition Metal Ions in Microporous Crystalline Aluminophosphates: Isomorphous Substitution. European Journal of Inorganic Chemistry, 1999(4), 565-577. doi:10.1002/(sici)1099-0682(199904)1999:43.0.co;2-yP. A. Wright and J. A.Conner, Microporous Framework Solids, The Royal Society of Chemistry, Cambridge, 2007Tang, Q., Zhang, Q., Wang, P., Wang, Y., & Wan, H. (2004). Characterizations of Cobalt Oxide Nanoparticles within Faujasite Zeolites and the Formation of Metallic Cobalt. Chemistry of Materials, 16(10), 1967-1976. doi:10.1021/cm030626zZ. 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Gold-copper nanoalloys supported on TiO2 as photocatalysts for CO2 reduction by water

Commercial P25 modified by AuCu alloy nanoparticles as thin film exhibits, for CO2 reduction by water under sun simulated light, a rate of methane production above 2000 mu mol (g of photocatalyst)(-1) h(1). Although evolution of hydrogen is observed and O-2 and ethane detected, the selectivity of conduction band electrons for methane formation is almost complete, about 97%. This photocatalytic behavior is completely different from that measured for Au/P25 (hydrogen evolution) and Cu/P25 (lower activity, but similar methane selectivity). Characterization by TEM, XPS, and UVvis spectroscopy shows that Au and Cu are alloyed in the nanoparticles. FT-IR spectroscopy and chemical analysis have allowed one to detect on the photocatalyst surface the presence of CO2 center dot-, CuCO, and elemental C. Accordingly, a mechanism in which the role of Au is to respond under visible light and Cu binds to CO and directs the reduction pathway is proposed.This work has been supported by the Marie Curie project PIEF-GA-2011-298740. Financial support by the Spanish Ministry of Economy and Competitiveness (Severo Ochoa CTQ 2012-32315) and the Generalidad Valenciana (Prometeo 2012/2013) is gratefully acknowledged. J.A.M.-A. acknowledges the assistance of the CSIC for their award of a Postdoctoral JAE-Doc contract.Neatu, S.; Maciá Agulló, JA.; Concepción Heydorn, P.; García Gómez, H. (2014). Gold-copper nanoalloys supported on TiO2 as photocatalysts for CO2 reduction by water. Journal of the American Chemical Society. 136(45):15969-15976. doi:10.1021/ja506433kS15969159761364

Influence of gold particle size on the photocatalytic activity for acetone oxidation of Au/TiO(2) catalysts prepared by dc-magnetron sputtering

Two series of Au/TiO2 materials with different gold content have been prepared by dc-magnetron sputtering on ceramic shaped pure anatase or Degussa P25 TiO2. The time of deposition was varied between 1 and 20 min in order to obtain different thickness and nanoparticle size of the gold films. For comparison samples with Au loadings in the range 0.3–0.9 wt% were prepared using the deposition–precipitation methodology. The obtained materials were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), DR-UV-Vis and atomic force and scanning electron microscopy techniques. The photocatalytic activity was checked in the photo-oxidation of acetone under both UV and visible irradiation conditions. Several factors were found to influence the photoactivity. An optimal Au content corresponding to a maximum activity is observed and attributed to the occurrence of adequate titania surface coverage and Au particle size. The support plays an important role and it was found that pristine anatase on which gold (average particle size 7.7 nm) was deposed by dc-reactive sputtering exhibits the maximum of the activity. Samples prepared by dc-sputtering were more active than samples prepared by deposition–precipitation method. Also, the photocatalysts prepared using dc-reactive sputtering showed activity under both UV and visible light irradiation, while those prepared using the deposition–precipitation technique are active only under UV light. The most likely mechanism of gold activation of titania is that upon light absorption, gold nanoparticles inject electrons into the titania conduction band.The authors kindly acknowledge NATO's Scientific Affairs Division in the framework of the Science for Peace Programme Sfp 981476 for the financial support. Bogdan Cojocaru wish to thank CNCSIS PNII PD 13/2010 for financial support.Cojocarua, B.; Neatu, S.; Sacaliuc-Pârvulescu, E.; Lévy, F.; Pârvulescua, VI.; García Gómez, H. (2011). Influence of gold particle size on the photocatalytic activity for acetone oxidation of Au/TiO(2) catalysts prepared by dc-magnetron sputtering. Applied Catalysis B: Environmental. 107(1):140-149. doi:10.1016/j.apcatb.2011.07.007S140149107

Understanding the Origin of the Photocatalytic CO2 Reduction by Au- and Cu-Loaded TiO2: A Microsecond Transient Absorption Spectroscopy Study

Recent photocatalytic data for CO2 reduction by H2O using simulated sunlight have shown that, while TiO2 Evonik P25 containing Au nanoparticles (NPs; Au/P25) generates considerably higher amounts of hydrogen than methane, when P25 contains Au-Cu alloy NPs the selectivity toward methane increases dramatically. To gain insight into this photocatalytic behavior, in the present work we have performed a transient absorption spectroscopy study in the microsecond time scale of three samples, namely, Au/P25, Cu/P25, and (Au, Cu)/P25 using 355 (UV) and SR nm (visible) lasers. The transient spectra exhibit as common features a narrower peak at about 320 nm and a broad band from 400 to 800 run. Using oxygen as electron quencher and methanol as hole quencher, the transient signals have been assigned to charge separation. Several cases were observed, including: (i) absence of quenching attributed to the lack of accessibility of the quencher to the site, (ii) quenching of the signal, or (iii) increase of the transient signal intensity attributed to less charge recombination by removal of one of the charge carriers. Of relevance to understand the origin of the photo catalytic CO2 reduction by H2O is the quenching of the charge separated state by these two reagents. In this way, it was observed that H2O exerts a remarkable influence to the transient signal, quenching its intensity in the three samples at the two irradiation wavelengths, except for (Au, Cu)/P25 upon 532 run excitation. Importantly, the distinctive behavior due to the presence of Cu has been attributed to the observed quenching by CO2 of the broad 400-800 nm band when excitation is performed with UV 355 nm light.Financial support by the Spanish Ministry of Economy and Competitiveness (Severo Ochoa Grant CTQ2012-32315), the Deanship of Scientific Research (DSR), King Abdulaziz University, under Grant No. 75-130-35-HiCi, and Marie Curie Project PIEF-GA-2011-298740 is gratefully acknowledged. The authors acknowledge technical and financial support of KAU. We also thank the Generalidad Valenciana for postgraduate research contract to H.G.B. (Prometeo 2012/2013).Baldovi, HG.; Neatu, S.; Khan, A.; Asiri, AM.; Kosa, SA.; García Gómez, H. (2015). Understanding the Origin of the Photocatalytic CO2 Reduction by Au- and Cu-Loaded TiO2: A Microsecond Transient Absorption Spectroscopy Study. Journal of Physical Chemistry C. 119(12):6819-6827. doi:10.1021/jp5106136S681968271191

Accidental Impurities in Epitaxial Pb(Zr0.2Ti0.8)O3 Thin Films Grown by Pulsed Laser Deposition and Their Impact on the Macroscopic Electric Properties

Structural and electrical properties of epitaxial Pb(Zr0.2Ti0.8)O3 films grown by pulsed laser deposition from targets with different purities are investigated in this study. One target was produced in-house by using high purity precursor oxides (at least 99.99%), and the other target was a commercial product (99.9% purity). It was found that the out-of-plane lattice constant is about 0.15% larger and the a domains amount is lower for the film grown from the commercial target. The polarization value is slightly lower, the dielectric constant is larger, and the height of the potential barrier at the electrode interfaces is larger for the film deposited from the pure target. The differences are attributed to the accidental impurities, with a larger amount in the commercial target as revealed by composition analysis using inductive coupling plasma-mass spectrometry. The heterovalent impurities can act as donors or acceptors, modifying the electronic characteristics. Thus, mastering impurities is a prerequisite for obtaining reliable and reproducible properties and advancing towards all ferroelectric devices

Effect of strain and stoichiometry on the ferroelectric and pyroelectric properties of the epitaxial Pb(Zr0.2Ti0.8)O-3 films deposited on Si wafers

International audienceProperties of epitaxial PbZr0.2Ti0.8O3 (PZT) films deposited on Si substrates were investigated for integration in the present CMOS technology. Polarization is downward oriented, in association with the presence of an internal electric field, and has a lower value compared to the PZT films deposited on single crystal perovskite SrTiO3 (STO) substrates (40 mu C/cm(2) versus 80 mu C/cm(2)), while the dielectric constant is larger (180 versus 120). Large value for the pyroelectric coefficient was also found, 1.22 x 10(-3)C/m(2)K, as for PZT grown on single crystal STO. The macroscopic ferroelectric and pyroelectric properties appear to be affected by the structural quality and stoichiometry of the PZT film. The changes in the electric properties are an effect of the strain gradients induced by the large difference between the thermal expansion coefficients of PZT and Si substrate, leading in turn to Pb oxidation and antisite defect formation compared to PZT films deposited on STO substrates