DISCUSIÓN DE RESULTADOS

Porous metal oxides have a wide range of potential applications such as catalysis, nanoreactors, electronic devices and gas sensors due to their large surface area, large pore volume and ordered pore network. The porous rutile TiO2 and anatase nanocrystal-silica composites were synthesised and examined for its properties on proton conductivity, Li storage and photocatalysis, respectively.

2.5.1 Proton conductivity

The proton conductivity of the porous rutile TiO2 was measured by a.c. impedance spectroscopy. The pellets for measuring were 13 mm in diameter and 3-4 mm in thickness and were obtained by pressing powders under 5×103 kg cm-2. PTFE bonded

Experimental

carbon black (Carbot Vulcan 72R) and carbon paper (Torory TGPH-090) were used as the electrode and the current collector, respectively. The whole assembly was then pressed between two pieces of carbon paper for current collection. The detailed construction is schematically shown in Figure 2-10. The impedance data were collected from room temperature to 100 ˚C after holding for 1 hour at each temperature to reach equilibrium. A Schlumberger Solartron 1255 frequency response analyser coupled with a 1287 electrochemical interface controlled by Z-plot electrochemical impedance software was used. The frequency range was from 1 MHz to 100 mHz at 100 mV r.m.s. The humidity effect on the proton conductivity of porous rutile TiO2 was carried out via controlling vapour flow obtained at different temperature.199

Figure 2-10. Schematic diagram of the specimen preparation for measuring the proton conductivity of porous rutile TiO2.

2.5.2 Li ion storage

For electrochemical characterisation, electrode materials were prepared as ‘Bellcore’ type electrodes, and assembled into coin cells, as previously described.211 A slurry was made by grinding 11.6 % by weight of the dry active material and 1.4 % by weight Super S carbon together, and then adding 5.5 % by weight polyvinylidene fluoride (PVDF), 9.5 % by weight propylene carbonate (PC) and 72 % by weight acetone. The slurry was stirred for 4 h in a 50 ˚C water bath and then doctor bladed onto a glass plate, to form a self-supporting sheet, typically 100 – 200 µm thick after the evaporation of the acetone. The electrode sheet was cut into discs, and the trapped PC was leached out using ether, until the individual electrodes reached constant weight. The resulting porous electrodes were dried under vacuum and transferred to

Experimental

an argon-filled glove box. A MacPile (Bio-logic) battery testing system was used to collect the electrochemical data. Galvanostatic cycling was recorded between set potential limits, typically 0.8 and 3.5 V, after the first reduction sweep. There was 1 h relaxation time at the endpoints. Potentiostatic data were recorded with 1 h relaxation time at the endpoints (typically 0.8 and 3.5 V with a step rate of 20 mV/h).199

2.5.3 Photocatalysis

Photoactivity of the porous anatase nanocrystal-silica composite was evaluated on the basis of the decomposition of methylene blue (MB) in an aqueous solution. 0.1 g of sample powder was suspended in 200 mL of a 5 ×10-5 M MB solution (0.5g/L) in the 250 mL round bottom flask by air bubbling. This mixture was first suspended in the dark for 30 minutes to reach the adsorption equilibrium before irradiation with a high intensity discharge 250 W iron doped metal halide UV bulb (UV Light Technology Ltd., Birmingham, UK) equipped with UV cutoff filter (an equipment which can filter out ultraviolet rays), The mixture temperature was controlled at about 25 ˚C using a water bath for infrared radiation and lamp heating removal. After irradiation, 2 mL of the mixture was collected and centrifuged at the irradiation time intervals (hourly). The photoactivity was examined by monitoring the reduction of the absorbance at 665 nm. The spectrophotometric measurements were carried out using Perkin Elmer Lambda35 UV/Vis spectrometer.200

Solid-liquid method

3. Solid-liquid method

The most important process of the synthesis of porous metal oxides templated by mesoporous silicas is the impregnation of metal-containing precursor inside the pores of silica. Four methods of impregnation have been developed in the last five years: the so-called surface modification method,120,165 dual-solvent method,121 one-step nanocasting method 144 and evaporation method,122,124 respectively. The evaporation method was used usually due to its simple process. In detail, the mesoporous silica template was mixed with a selected metal nitrate in ethanol, and the nitrate precursor was generally accepted to migrate into the pores by a capillary action during the evaporation process.

Table 1. Melting point (MP) and decomposition temperature (DT) of the nitrate precursors for porous transition metal oxides.

Precursor MP(˚C) DT(˚C) Oxide Co(NO3)2·6H2O 55 >74 Co3O4 (>150˚C) Cr(NO3)3·9H2O 66 >100 Cr2O3 (>350˚C) Ni(NO3)2·6H2O 56.7 >110 NiO

Ce(NO3)3·6H2O 96 200 CeO2

In(NO3)3·xH2O 100 >100 In2O3 H3PO40W12·xH2O 107 >385 WO3

Pb(NO3)2 205~223 PbO

(NH4)2Cr2O7 180 Cr2O3 (>180˚C) Generally, a metal nitrate has a low melting point and its decomposition temperature is usually higher than its melting point. Therefore the solid-liquid method 198 _{was developed based on these characteristics of metal nitrate. In detail, a} metal-containing precursor was ground with the mesoporous silica template, and was expected to migrate into the pores of silica after melting when it was heated up to a temperature above its melting point. The liquid precursor would then decompose to generate the metal oxide inside the silica pores when the temperature was increased above its decomposition temperature. The advantage of this synthetic method is that it is convenient and solvent-free. Its limitation is that the precursor must have a melting point lower than its decomposition temperature. Table 1 shows the melting point and decomposition temperature of some metal nitrates used as the precursors (H3PO40W12·xH2O was used as the W-precursor). Mesoporous silicas of SBA-15 and KIT-6 were selected as hard templates.

Solid-liquid method