Enhancement of photoactivity of
nano-photocatalysts
Under bandgap excitation, metal oxide semiconductorparticles behave as short-circuited electrodes, withboth oxidation and reduction processes occurring ontheir surfaces. Thus two critical processesdetermine the overall quantum efficiency forinterfacial charge transfer.
These are the competitionbetween the recombination and the trapping of thecharge carriers, followed by the competition between the recombination of the trapped carriers and the interfacialcharge transfer At least in colloidal sols of TiO2, it was found that the chargecarriers undergo 90% recombination
after excitation. The authors concludedthat the quantum yields of any surface photoredox reaction
could not be greater than _10%.In order to enhance the photoactivity of bulk andcolloidal TiO2 particles, interfacial charge-transferreactions need to be enhanced. Improved charge separation
and inhibition of charge carrier recombination isessential in improving the overall quantum efficiency for interfacial charge transfer . Thiscan be achieved by modifying the properties of the particles by selective surface treatment Several approaches have been taken to achieve this
These have included surface modification of the semiconductorparticles with redox couples or noble metals. Bahnemann et al.have shown that the efficiency of charge transferat the semiconductor–electrolyte can also be improved by simultaneous scavenging of holes and electrons bysurface adsorbed redox species. Another approach has involved the coupling of two semiconductor particleswith different electronic energy levels
Metal ion dopants
Depositing or incorporating metal ion dopants intothe titanium dioxide particles can influence the performance of these photocatalysts. This affects thedynamics of electron:hole recombination and interfacial charge transfer. The largest enhancement ofphotoactivity through doping was found in nanosized particles, in which the dopant ions are located within1–2 nm of the). Also, the high surface areas characteristic of nanoparticles (100–500m2/g) appear to enhance the deposition process
And the resulting activity of the catalyst The work of Choi et al. involved a systematic study
of the effects of 21 different metal ion dopants on nanocrystalline TiO2. Chloroform oxidation and carbon
Tetrachloride reductions were used as photoreactivity tests. Their results showed that some doped Q-TiO2Particles had much greater photoactivity than their undoped counterparts.
Doping with Fe(III), Mo(V),Ru(III), Os(III), Re(V), V(IV) and Rh(III) at the0.5 at%
level in the TiO2 matrix, significantly improvedthe photoreactivity for both oxidation and . Choi used laser flash photolysis and time-resolvedreduction
microwave conductivity measurements to correlate the effects of metal ion dopants to the lifetime of the photoexcited electron. In the Fe(III), V(IV), Mo(V) and Ru(III) doped samples, the lifetime of the generated electron was found to have increased to 50 ms compared to<200 _s with the undoped Q-TiO2. Therewas also an apparent linear correlation between the oxidation quantum yield (of chloroform) and the reduction quantum yield (of carbon tetrachloride) regardless of the nature of the dopant.
This ion doping of large bandgap semiconductor colloids might not always be effective in lengthening
the lifetime of the generated charge carriers. Recently, Smith et al. (1998) showed that in Ru(III) doped
TiO2colloids, the electronic decaywas as fast as or even faster than in undoped TiO2. The difference between
the studies carried out by Smith et al. and those carried out by Choi et al., was the higher dopant level of
Ru (III) of 3 at% used by Smith et al., compared with the 0.5 at% dopant level used by Choi et al.
There could be several reasons for the variations in the reported effects of the dopant ions. One reason forthese variations is the location and co-ordination of the dopant ions
. These depend critically on the methods
of sample preparation and pre-treatment as well as the concentration of the dopant ions. The dopant
ons may be adsorbed on the surface, they may be incorporated into the interior of the particle on firing,
or they may form separate oxide phases The dopant ions can function as both hole and electron
traps or they can mediate interfacial charge transfer Once incorporated into the interior
of the TiO2, the dopant ions may occupy eitherlattice (substitutional) or interstitial sites. Their ability
to function as trap sites and/or to mediate interfacial
charge transfer will depend on these factors When incorporated in the interior of the particles
the d-electronic configuration of the dopant and its energy level within the TiO2 lattice also seem to significantly influence the photoactivity Finally, the site where the electron gets trapped greatly
influences the redox chemistry of the doped semiconductor.
A dopant ion might act as an electron trap, and this might in fact lead to a lengthening in the lifetime of
the generated charge carriers, resulting in an enhancement in photoactivity.However if an electron is trapped in a deep trapping site, it will have a longer lifetime
but it may also have a lower redox potential. This might result in a decrease in the photoreactivity
The work carried out by Zhang et al. (1998) shed a new light on the role of dopant ions and their effect
on photoactivity. Firstly, these authors provided further support for the existence of an optimum dopant concentration Their main finding however was that this optimum concentration is particle-size dependent and decreases with an increase in size. The system they studied was Fe3C doped TiO2 for the photocatalytic degradation of CHCl3. They observed that for 6 nm particles, the optimum Fe concentration was 0.2 at%,
while for 11 nm particles, the optimum concentration was 0.05 at%. They provided the following explanation for their observations. Their first explanation was with regards the existence
of an optimal Fe3C dopant concentration. Fe3C ions serve as shallow trapping sites for the charge carriers
and increase the photocatalytic efficiency by separating the arrival time of e− and hC at the surface. If
Fe3C can act as a trap for both e− and hC, at high dopant concentration, the possibility of charge trapping is high and as such, the charge carriers may recombine through
quantum tunneling. If Fe3C acts as a hC trap only, the recombination of the charge carriers is not of great concern at low dopant concentrations. At high concentrations
however, a hC may be trapped more than once as it tries to make its way to the surface. This hole which
had been ‘held back’, might then recombine with an electron which is generated by a subsequent photon
before it can reach the surface (i.e. increased incidence of volume recombination). Thus there exists an optimum Fe3C concentration whether the Fe3C acts as an e− and hC trap or as a hC trap only. With regards to the optimum concentration depending on the particle size, Zhang et al. suggested that
when the particle size becomes larger, the average path length of a charge carrier to the surface is longer. Thus, for a constant dopant concentration, the longer the path length which the charge carrier needs to travel, the higher the probability of meeting a dopant ion, and hence the greater the chance of multiple trappings. This multiple trapping leads to increased volume recombination. A lowering in the dopant concentration reduces the chance of multiple trappings for a larger particle. Thus the optimal Fe3C dopant concentration should decrease with increasing TiO2 particle size.
