ACS PRF | ACS
All e-Annual Reports
42520-AC10,5
Single Molecule Interfacial Electron Transfer
Interfacial electron transfer (ET) between molecular adsorbates and semiconductor nanoparticles is a fundamental process that is relevant to many applications of semiconductor nanoparticles, such as solar energy conversion and photo-catalytic waste degradation. Previous studies of this process by ensemble average techniques revealed highly heterogeneous distribution of rates. The goal of the proposed study is to examine ET on the single molecule/nanoparticle junction level and to gain insight into the origin of its hetrogeneous distribution. We have accomplished the following:
1. Demonstration of single molecule interfacial ET. We have shown that fluorescence lifetime of single dye molecules on ET active substrates, such as Rhodamine B on Sb:SnO2(ATO), can be measured. From the shorten fluorescence lifetime, ET rate on the single molecule level can be obtained. One of the challenges for single molecule interfacial ET study is that molecules undergoing faster ET have lower fluorescence quantum yields, hindering a complete sampling of all ET active molecules. We also showed that photo-bleaching rate increases in ET active substrates, limiting the length of observation time and presenting another challenge for single molecule ET.
2. Controlling interfacial ET rate. To overcome the first challenge, we have investigated the following approaches for controlling interfacial ET rate:
1) Band edge of semiconductor: The band edges of metal oxide semiconductors are pH dependent, increased by 59 mV per pH unit. We showed that by raising the pH of the solution, the ET rate can be lowered due to the more negative conduction band edge position.
2) Oxide over layer: It has also been shown that metal oxides, such as TiO2 and SnO2, could be coated with layers of insulating oxides such as ZrO2 and Al2O3. We showed that ET rate decreased with the number of insulating over layers. However, the coating was not uniform, adding to the heterogeneous distribution of ET rates. We are exploring ways to make more uniform coatings.
3) Anchoring group and molecular spacers: We have also examined how injection rate depends on the nature of the anchoring group and the length of molecular spacers. We found that the rate of injection varies by a factor of 10 between carboxylate and phosphonate group. We observed that electron transfer from Rhodamine B to TiO2 was slowed down by at least 1000 times with NH2-(CH2)3-SiO3 spacer.
3. Increasing the back ET rate. The enhanced photobleaching of Rhodamine on ET active substrates (TiO2 and SnO2) may be a result of slow back electron transfer. We have investigated the effect of n-doping on interfacial ET rate. We showed that for Sb:SnO2 with 0-10% Sb doping, the injection rate is similar to that of SnO2, but the recombination rate increases at higher doing levels. The doped substrate provides an approach to speed up the regeneration of photo-oxidized chromophores and will be used for single molecule ET studies.
4. Measuring interfacial ET rate. We have started to measure single molecule ET with a modified Rhodamin that contains a silane spacer. Preliminary results show a slowing down of interfacial ET rate on the single molecule level. Ongoing studies are examining this and other modified Rhodamine molecules with slower ET rates that will allow a more complete sampling of all ET active molecules on the single molecule level.
