Maurício A. Melo

Energetic features of copper and lead sorption by innovative aminoalcohol-functionalized cobalt phyllosilicates

Inorganic-organic cobalt phyllosilicate hybrids were synthesized by the sol-gel procedure under mild non-hydrothermal conditions with a silicon precursor, formed through individual reactions between the silane 3-glycidoxypropyltriethoxysilane and the aminoalcohols ethanol- or diethanolamine. These procedures generated talc-like phyllosilicates containing pendant organic chains with nitrogen and oxygen basic centres located in the interlamellar region. For organofunctionalized phyllosilicates the lamellar structure obtained through the sol-gel method was confirmed by X-ray powder diffraction, while elemental analysis indicated that the densities of the organic groups attached to the new matrices were 3.31 ± 0.05 and 3.08 ± 0.07 mmol g(-1) for hybrids functionalized with ethanol- and diethanolamines, respectively. Infrared spectroscopy and nuclear magnetic resonance in the solid state for (13)C and (29)Si showed that the organic groups are indeed covalently bonded to the inorganic structures and the process of functionalization did not affect the original structures of the silylating agents employed. The thermally stable hybrids presented well-formed particles with a homogeneous distribution of cobalt and nitrogen atoms. Their abilities for copper removal from aqueous solutions gave maximum capacities of sorption of 2.01 ± 0.11 and 2.55 ± 0.15 mmol g(-1) for phyllosilicates containing ethanol- and diethanolamine groups, respectively. For lead sorption the values of 2.59 ± 0.11 and 2.43 ± 0.12 mmol g(-1) were found for this same sequence. These sorption data were adjusted to the non-linear regression of the Langmuir equation. Energetic features related to the interactions between the cations and the pendant basic centres were determined through calorimetric titrations. The acid-basic interactions reflect the spontaneity of the reactions, which are also enthalpically and entropically favourable for these chelating processes at the solid-liquid interface.

Preparação de nanopartículas de prata e ouro: um método simples para a introdução da nanociência em laboratório de ensino

A new practical experiment involving silver and gold nanoparticle syntheses was introduced in an inorganic chemistry laboratory course for undergraduate students at the Institute of Chemistry, UNICAMP. The nanoparticles were synthesized by the reduction of silver nitrate and tetrachloroauric acid with sodium borohydride and sodium citrate in an aqueous medium. Stabilities of the suspensions were tested using several different reactants including sodium chloride, polyvinylpyrrolidone, polyvinyl alcohol and cistamine. Changes in optical properties were observed by electronic spectra and also by transmission electronic microscopy, which also yielded data for estimating particle size.

Useful aminoalcohol molecules incorporated in an epoxide silylating agent for silica organofunctionalization and thermodynamics of copper removal

Ethanolamine (E) and diethanolamine (D) molecules were incorporated onto a precursor 3-glycidoxypropyltrimethoxysilane (G) agent, followed by organofunctionalization of the silica gel through homogeneous (A) and heterogeneous (B) routes, to yield SiEA, SiDA, SiEB and SiDB hybrids. These chemically organofunctionalized silicas were characterized by several techniques, including infrared spectroscopy (FTIR), solid-state nuclear magnetic resonance (13C and 29Si NMR using CP/MAS), thermogravimetry and elemental analysis. Based on the obtained nitrogen percentages, the amount of pendant organic groups attached to the inorganic matrices SiEB, SiEA, SiDB and SiDA were calculated as 0.22 ± 0.04, 1.05 ± 0.03, 0.29 ± 0.02 and 0.89 ± 0.01, respectively. The infrared spectra presented characteristic bands attributed either to the inorganic framework or to the immobilized chains. Solid-state 13C NMR results clearly demonstrated that the organic moieties are covalently bonded to the inorganic framework, while 29Si NMR revealed the silicon atoms with distinct environments, Q and T, in agreement with the covalent attachment of organic moieties. The basic centers attached on the pendant groups have the ability to sorb copper from aqueous solution. This process was adjusted to the Langmuir model, to obtain maximum sorption values of 0.28 ± 0.02, 0.40 ± 0.02, 0.29 ± 0.03, 0.76 ± 0.02 mmol g−1 for SiEB, SiEA, SiDB and SiDA hybrids, respectively, from the isotherms. Thermodynamic data obtained from calorimetric titrations reflected the spontaneity of the reactions, which are also enthalpically and entropically favorable for the proposed cation/basic center interactions for these chelating processes at the solid/liquid interface.

Surface Photovoltage Measurements on a Particle Tandem Photocatalyst for Overall Water Splitting

Surface photovoltage spectroscopy (SPS) is used to measure the photopotential across a Ru-SrTiO3:Rh/BiVO4 particle tandem overall water splitting photocatalyst. The tandem is synthesized from Ru-modified SrTiO3:Rh nanocrystals and BiVO4 microcrystals by electrostatic assembly followed by thermal annealing. It splits water into H2 and O2 with an apparent quantum efficiency of 1.29% at 435 nm and a solar to hydrogen conversion efficiency of 0.028%. According to SPS, a photovoltage develops above 2.20 eV, the effective band gap of the tandem, and reaches its maximal value of -2.45 V at 435 nm (2.44 mW cm-2), which corresponds to 96% of the theoretical limit of the photocatalyst film on the fluorine-doped tin-oxide-coated glass (FTO) substrate. Charge separation is 82% reversible with 18% of charge carriers being trapped in defect states. The unusually strong light intensity dependence of the photovoltage (1.16 V per decade) is attributed to depletion layer changes inside of the BiVO4 microcrystals. These findings promote the understanding of solar energy conversion with inorganic particle photocatalysts.

Use of surface photovoltage spectroscopy to probe energy levels and charge carrier dynamics in transition metal (Ni, Cu, Fe, Mn, Rh) doped SrTiO3 photocatalysts for H2 evolution from water

Doping with transition metal ions is widely employed to adjust the optical and photocatalytic properties of wide band semiconductors, however, quantitative information about the energetics and charge transfer dynamics of the impurity states is often difficult to obtain. Here we use surface photovoltage spectroscopy (SPS), optical spectroscopy, and irradiation experiments to study the effect of several dopants on the ability of SrTiO3 nanocrystals to generate a photovoltage and to catalyse H2 evolution from aqueous methanol. Phase pure SrTiO3:TM nanocrystals with TM = Ni, Cu, Fe, Mn, Rh were synthesized by hydrothermal reaction of TiO2, Sr(OH)2, KOH, and transition metal chlorides and nitrates in water. SPS data was obtained on thin films of these nanocrystals on fluorine doped tin oxide substrates under vacuum atmosphere. All samples are n-type, which can be gauged from the negative photovoltage caused by the transfer of electrons into the FTO substrate. All dopants produce sub-bandgap states in the SrTiO3 lattice, whose energetic positions can be determined from the photovoltage onset energy in SPS and from optical absorption spectra. The reversibility and size of the photovoltage provide information about the photohole dynamics and their ability to oxidize sacrificial electron donors at the nanocrystal surface. Overall, this work provides an explanation for the inability of Ni, Cu, Fe, Mn dopants to enhance visible light photocatalytic activity in SrTiO3, and it establishes SPS as a useful tool to map the energetics and photochemistry of impurity states in metal oxide nanocrystals.

Pillaring and NiOx co-catalyst loading as alternatives for the photoactivity enhancement of K2Ti4O9 towards water splitting

Structural improvements of efficient and abundant photocatalysts and the employment of less expensive co-catalysts are appealing routes to promote the development of economically viable solar fuel production. We herein report a significant enhancement of the H2 production from photocatalytic water splitting over K2Ti4O9 after pillaring with TiO2 pillars and loading of NiOx as a co-catalyst. The pillaring favored the charge carrier separation within the photocatalyst framework under illumination, whereas the NiOx co-catalyst, composed of nickel metal nanoparticles and nanostructured NiO, intensified the electron–hole separation at the surface. These are unprecedented results indicated by surface photovoltage spectroscopy, which was applied for the first time in this kind of photocatalyst composite. All modifications enhanced the H2 evolution from 14.9 μmol, for the pristine K2Ti4O9, to 982.0 μmol, for the final product, after 7 h of irradiation tests using methanol as a hole scavenger. This boost was also favored by the mesoporosity and high surface area created by the pillaring process. Moreover, NiOx as the co-catalyst proved to be as effective as platinum and more effective than Ag and Au in this particular system.

CÉLULAS SOLARES SENSIBILIZADAS POR CORANTES NATURAIS: UM EXPERIMENTO INTRODUTÓRIO SOBRE ENERGIA RENOVÁVEL PARA ALUNOS DE GRADUAÇÃO

An interesting practical experiment about the preparation of dye–sensitized solar cells (DSSC) using natural dyes were carried out by the undergraduate students in the chemistry course at UNICAMP . Natural dyes were extracted from blueberries (Vaccinium myrtillus L.), jabuticabas (Myrciaria cauliflora), raw and cooked beets (Beta vulgaris L.), and annattos (Bixa orellana L.), which were used to sensitize TiO2 films that composed the photoanode in the DSSC. A polymer electrolyte containing an iodide/triiodide redox couple was used in lieu of the use of liquid solutions to prevent any leakage in the devices. A maximum solar-to-electric energy conversion of 0.26 ± 0.02% was obtained for the solar cell prepared with annatto extracts. This experiment was an effective way to illustrate to the undergraduate students how to apply some of the chemical concepts that they learned during their chemistry course to produce electric energy from a clean and renewable energy source. Teachers could also exploit the basics of the electronic transitions in inorganic and organic compounds (e.g., metal-to-ligand charge transfer and ϖ-ϖ* transitions), thermodynamics (e.g., Gibbs free energy), acid–base reactions in the oxide solid surface and electrolyte, and band theory (i.e., the importance of the Fermi level energy).