Atomic Layer Deposition of TiO 2 as Stabilization Technique

Long term photolysis of the FTO|nanoITO|TiO2|RuP2,IrOx assemblies demonstrated a

significant decrease in photocurrent over time. This is suspected to reflect the instability of the RuP2 chromophore’s immobilization at the higher pH of the NaSiF6 buffer used in these

experiments. As observed earlier for phosphonate-derivatized assemblies on TiO2, surface

147

hydrolysis.19,20 Further evidence of this is the loss in color of the slide and the increase in color of the solution after prolonged photolysis. The Meyer and Hupp laboratories have

previously reported on atomic layer deposition (ALD) as a means to stabilize the chromophores on metal oxide surfaces.21,24,26,30,31 _{A cartoon of this ALD stabilization layer on the assembly is}

depicted in Figure 5.11. Figure 5.12 presents results that support this finding on the core/shell electrode with 6.6 nm TiO2 thickness. The effect of an added 10 cycle TiO2 overlayer (less than

1 nm in thickness) added to stabilize -RuP2 before addition of the IrOx NPs is also shown in

Figure 5.12. After two hours of photolysis with a 300 mV applied bias vs. Ag/AgCl, the

photocurrent from the unstabilized assembly has fallen appreciably, to a level that is well below that of the dark current (- 97 µA/cm2). The stabilized electrode sustains a photocurrent of 110

µA/cm2 over the photolysis period, indicating significant improvement in stability. Compared to the photocurrent obtained from 90 s photolysis at the same potential bias (150 µA/cm2), the loss is significantly decreased with ALD stabilization.

Figure 5.11: A cartoon of the assembly after an additional ALD of ~ 1 nm of TiO2, represented by the light blue

edge of the nanoparticles. This ALD layer is added after the chromophore (RuP2) is adsorbed to the surface, but before the IrOX NPs are deposited. This creates a protective layer of TiO2 around the phosphonic acid groups of the

148

A conductive polymer, poly(methacrylic acid) (PMAA), was also tested as a means to stabilize the RuP2 chromophore in higher pH solutions. A solution of 3% PMAA was applied to

the photoanode after RuP2 loading, but before the slide was dipped in the IrOX NP solution to

load the catalyst. This was then allowed to air dry to form a protective layer. After loading the IrOx NPs via the dip-coating method, there was no apparent purple color that usually

accompanies these films. This suggested that the IrOX NP did not precipitate onto the PMAA

surface. Figure 5.13 is the 10 minute photolysis experiment performed with this assembly. The light was turned on at 60 s, as indicated by the black arrow. There is a slight spike in current when the light is turned on, but no apparent water oxidation catalysis occurs (as indicated by the lack of O2 detection at the collector electrode). This further supports the theory that the IrOX

NPs do not deposit onto the PMAA surface. This is likely due to the more hydrophobic nature of the polymer, compared to that of TiO2 and RuP2.

Figure 5.12: Photocurrent densities after 2 hours of photolysis for the RuP2-IrOX assemblies on the nanoITO/TiO2

core/shell electrodes (TiO2 thickness = 6.6 nm) with and with ALD stabilization (10 cycles) were 110 µA/cm2 and

97 µA/cm2_{, respectively. Photolysis conditions were at pH 5.8 with an applied potential bias of 300 mV vs.}

149 5.4 CONCLUSIONS

The results described here are important in building on the earlier results of Mallouk and coworkers on the use of Ru(II) polypyridyl-IrOx assemblies on semiconductor oxides to achieve

visible light water splitting.7,8,10,11,15 The use of nanoITO|TiO2 core/shell structures results in

impressive gains in photocurrent efficiency which increase with applied bias. Long term instability arising from hydrolysis and loss of the chromophore from the surface has been

addressed by use of ALD overlayers of TiO2. With the combination of enhanced efficiencies and

surface stabilization this approach to self-assembly based on IrOx nanoparticles offers promise as

a platform for achieving relatively high efficiencies with extensions to chromophores and

combinations of chromophores for extending light absorption further into the visible with surface stabilization by ALD.

Figure 5.13: Photolysis of PMAA stabilized FTO|nanoITO|TiO2|RuP2, IrOX assembly. The light (455 nm, 14.6

mW/cm2_{) is turned on at 60 s, indicated by the arrow. A potential bias of 0.4 V vs. Ag/AgCl was applied in a pH}

5.8 NaSiF6 buffer. light on

150 5.5 ACKNOWLEDGEMENT

This research is based upon work supported as part of the UNC EFRC: Center for Solar Fuels, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001011. Atomic layer deposition and syntheses of the FTO/nanoITO, FTO/TiO2 and core/shell electrodes were

performed by Leila, Alibabei. Synthesis of the RuP2 chromophore was performed by Dennis

Ashford. Assembly of the collector/receptor electrode set-up for O2 detection was performed by

Ben Sherman. We would also like to thank Alessa Gambardella for her help with starting this project and Robert Binstead for writing the SpecFit program and his assistance with using the program.

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CHAPTER 6:Electrochemical Characterization of Quaterpyridine Ruthenium