Enhancement of photoactivity by synthesizing nanotube-nanoparticle composites of titanium dioxide and cadmium sulfide for generation of hydrogen via splitting of water

A steady increase in the global energy consumption coupled with decreasing fossil fuel supplies, has led to the consideration of hydrogen as a possible alternative and inexhaustible fuel. The reason for considering hydrogen as a good choice for an alternative fuel is due to its potential to be a clean and sustainable energy carrier that could play a major role in a new energy infrastructure providing power to homes, vehicles and industries. In this investigation, efficient photoelectrochemical generation of hydrogen from water using solar energy was carried out by designing photoelectrodes with good absorption as well as charge transport properties. One dimensional, self-organized titania (TiO2) nanotubes are known to have excellent charge transport properties, and nanoparticles, in general, have excellent photon absorption properties. In this work, a variety of composite photocatalysts, (with TiO 2 as the substrate) were synthesized, by combining the above two properties of TiO2 nanotubes and TiO2 and/or cadmium sulfide (CdS) nanoparticles with different morphologies.;TiO2 nanoparticles (500--700 nm) were synthesized from titanium tetrachloride (TiCl4) precursor on TiO2 nanotubular arrays (∼80 nm diameter and ∼550 nm length), synthesized by the sonoelectrochemical anodization method. This TiO2 nanotubenanoparticle composite photoanode has enabled enhanced photocurrent density (2.2 mA/cm2) to be obtained as compared to nanotubes (0.9 mA/cm2) and nanoparticles (0.65 mA/cm2) alone.;As a continuation of this work to prepare nanotube/nanoparticle hybrids, a coupled semicontuctor material was prepared by filling one dimensional (1D) titania nanotubes with cadmium sulfide nanoparticles. Self-assembled TiO 2 nanotubes (length--550 nm, diameter--80 nm) were prepared by the sonoelectrochemical anodization method. These nanotubes were functionalized with CdS nanoparticles by a single step potentiostatic electrodeposition method. This new material harvested solar light in both the UV and the visible light (up to 510 nm) region. The visible light components (&lgr;≥420 nm) contributed about 68% of the total photocurrent generated from this new composite. An eight to nine fold enhancement in photoactivity was observed using CdS functionalized TiO2 nanotubes (9.5 mA/cm2) compared to pure TiO 2 nanotubes (1.21 mA/cm2) and commercial P25 nanoparticles (1.07 mA/cm2). This methodology will be very useful for designing multi-junction semiconductor materials confined within 1D nanochannels.;The concept of combining TiO2 nanotubes and nanoparticles to obtain enhanced photocurrent density and thus increased hydrogen generation was also applied to CdS bulk particles. A composite photoanode comprising of self-assembled titania nanotubes functionalized with CdS particles was synthesized, characterized and tested for photoelectrochemical hydrogen generation from an aqueous sulfide-sulfite (S2-/SO3 2-) solution. The heterostructural composite photoanode exhibited an enhanced photocurrent density of 8.2 mA/cm2 compared to TiO2 nanotubes (∼55 nm diameter and ∼1.2 mum length, 2.89 mA/cm2). It was also observed that more than 63% of the activity was contributed from the visible components of the solar spectrum in comparison to 13% from only TiO2 nanotubes. The results proved that the combined light absorption (both UV and visible regions) of CdS-TiO 2 material, and the high charge transport properties of self-assembled 1D nanotube arrays make the composite promising.